|
HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|
NMAP(1) Nmap Reference Guide NMAP(1)
nmap - Network exploration tool and security / port scanner
nmap [Scan Type...] [Options] {target specification}
Nmap (“Network Mapper”) is an open source tool for network
exploration and security auditing. It was designed to rapidly
scan large networks, although it works fine against single hosts.
Nmap uses raw IP packets in novel ways to determine what hosts
are available on the network, what services (application name and
version) those hosts are offering, what operating systems (and OS
versions) they are running, what type of packet filters/firewalls
are in use, and dozens of other characteristics. While Nmap is
commonly used for security audits, many systems and network
administrators find it useful for routine tasks such as network
inventory, managing service upgrade schedules, and monitoring
host or service uptime.
The output from Nmap is a list of scanned targets, with
supplemental information on each depending on the options used.
Key among that information is the “interesting ports table”.
That table lists the port number and protocol, service name, and
state. The state is either open, filtered, closed, or unfiltered.
Open means that an application on the target machine is listening
for connections/packets on that port. Filtered means that a
firewall, filter, or other network obstacle is blocking the port
so that Nmap cannot tell whether it is open or closed. Closed
ports have no application listening on them, though they could
open up at any time. Ports are classified as unfiltered when they
are responsive to Nmap's probes, but Nmap cannot determine
whether they are open or closed. Nmap reports the state
combinations open|filtered and closed|filtered when it cannot
determine which of the two states describe a port. The port table
may also include software version details when version detection
has been requested. When an IP protocol scan is requested (-sO),
Nmap provides information on supported IP protocols rather than
listening ports.
In addition to the interesting ports table, Nmap can provide
further information on targets, including reverse DNS names,
operating system guesses, device types, and MAC addresses.
A typical Nmap scan is shown in Example 1. The only Nmap
arguments used in this example are -A, to enable OS and version
detection, script scanning, and traceroute; -T4 for faster
execution; and then the hostname.
Example 1. A representative Nmap scan
# nmap -A -T4 scanme.nmap.org
Nmap scan report for scanme.nmap.org (74.207.244.221)
Host is up (0.029s latency).
rDNS record for 74.207.244.221: li86-221.members.linode.com
Not shown: 995 closed ports
PORT STATE SERVICE VERSION
22/tcp open ssh OpenSSH 5.3p1 Debian 3ubuntu7 (protocol 2.0)
| ssh-hostkey: 1024 8d:60:f1:7c:ca:b7:3d:0a:d6:67:54:9d:69:d9:b9:dd (DSA)
|_2048 79:f8:09:ac:d4:e2:32:42:10:49:d3:bd:20:82:85:ec (RSA)
80/tcp open http Apache httpd 2.2.14 ((Ubuntu))
|_http-title: Go ahead and ScanMe!
646/tcp filtered ldp
1720/tcp filtered H.323/Q.931
9929/tcp open nping-echo Nping echo
Device type: general purpose
Running: Linux 2.6.X
OS CPE: cpe:/o:linux:linux_kernel:2.6.39
OS details: Linux 2.6.39
Network Distance: 11 hops
Service Info: OS: Linux; CPE: cpe:/o:linux:kernel
TRACEROUTE (using port 53/tcp)
HOP RTT ADDRESS
[Cut first 10 hops for brevity]
11 17.65 ms li86-221.members.linode.com (74.207.244.221)
Nmap done: 1 IP address (1 host up) scanned in 14.40 seconds
The newest version of Nmap can be obtained from https://nmap.org .
The newest version of this man page is available at
https://nmap.org/book/man.html . It is also included as a chapter
of Nmap Network Scanning: The Official Nmap Project Guide to
Network Discovery and Security Scanning (see
https://nmap.org/book/ ).
This options summary is printed when Nmap is run with no
arguments, and the latest version is always available at
https://svn.nmap.org/nmap/docs/nmap.usage.txt . It helps people
remember the most common options, but is no substitute for the
in-depth documentation in the rest of this manual. Some obscure
options aren't even included here.
Nmap 7.95SVN ( https://nmap.org )
Usage: nmap [Scan Type(s)] [Options] {target specification}
TARGET SPECIFICATION:
Can pass hostnames, IP addresses, networks, etc.
Ex: scanme.nmap.org, microsoft.com/24, 192.168.0.1; 10.0.0-255.1-254
-iL <inputfilename>: Input from list of hosts/networks
-iR <num hosts>: Choose random targets
--exclude <host1[,host2][,host3],...>: Exclude hosts/networks
--excludefile <exclude_file>: Exclude list from file
HOST DISCOVERY:
-sL: List Scan - simply list targets to scan
-sn: Ping Scan - disable port scan
-Pn: Treat all hosts as online -- skip host discovery
-PS/PA/PU/PY[portlist]: TCP SYN, TCP ACK, UDP or SCTP discovery to given ports
-PE/PP/PM: ICMP echo, timestamp, and netmask request discovery probes
-PO[protocol list]: IP Protocol Ping
-n/-R: Never do DNS resolution/Always resolve [default: sometimes]
--dns-servers <serv1[,serv2],...>: Specify custom DNS servers
--system-dns: Use OS's DNS resolver
--traceroute: Trace hop path to each host
SCAN TECHNIQUES:
-sS/sT/sA/sW/sM: TCP SYN/Connect()/ACK/Window/Maimon scans
-sU: UDP Scan
-sN/sF/sX: TCP Null, FIN, and Xmas scans
--scanflags <flags>: Customize TCP scan flags
-sI <zombie host[:probeport]>: Idle scan
-sY/sZ: SCTP INIT/COOKIE-ECHO scans
-sO: IP protocol scan
-b <FTP relay host>: FTP bounce scan
PORT SPECIFICATION AND SCAN ORDER:
-p <port ranges>: Only scan specified ports
Ex: -p22; -p1-65535; -p U:53,111,137,T:21-25,80,139,8080,S:9
--exclude-ports <port ranges>: Exclude the specified ports from scanning
-F: Fast mode - Scan fewer ports than the default scan
-r: Scan ports sequentially - don't randomize
--top-ports <number>: Scan <number> most common ports
--port-ratio <ratio>: Scan ports more common than <ratio>
SERVICE/VERSION DETECTION:
-sV: Probe open ports to determine service/version info
--version-intensity <level>: Set from 0 (light) to 9 (try all probes)
--version-light: Limit to most likely probes (intensity 2)
--version-all: Try every single probe (intensity 9)
--version-trace: Show detailed version scan activity (for debugging)
SCRIPT SCAN:
-sC: equivalent to --script=default
--script=<Lua scripts>: <Lua scripts> is a comma separated list of
directories, script-files or script-categories
--script-args=<n1=v1,[n2=v2,...]>: provide arguments to scripts
--script-args-file=filename: provide NSE script args in a file
--script-trace: Show all data sent and received
--script-updatedb: Update the script database.
--script-help=<Lua scripts>: Show help about scripts.
<Lua scripts> is a comma-separated list of script-files or
script-categories.
OS DETECTION:
-O: Enable OS detection
--osscan-limit: Limit OS detection to promising targets
--osscan-guess: Guess OS more aggressively
TIMING AND PERFORMANCE:
Options which take <time> are in seconds, or append 'ms' (milliseconds),
's' (seconds), 'm' (minutes), or 'h' (hours) to the value (e.g. 30m).
-T<0-5>: Set timing template (higher is faster)
--min-hostgroup/max-hostgroup <size>: Parallel host scan group sizes
--min-parallelism/max-parallelism <numprobes>: Probe parallelization
--min-rtt-timeout/max-rtt-timeout/initial-rtt-timeout <time>: Specifies
probe round trip time.
--max-retries <tries>: Caps number of port scan probe retransmissions.
--host-timeout <time>: Give up on target after this long
--scan-delay/--max-scan-delay <time>: Adjust delay between probes
--min-rate <number>: Send packets no slower than <number> per second
--max-rate <number>: Send packets no faster than <number> per second
FIREWALL/IDS EVASION AND SPOOFING:
-f; --mtu <val>: fragment packets (optionally w/given MTU)
-D <decoy1,decoy2[,ME],...>: Cloak a scan with decoys
-S <IP_Address>: Spoof source address
-e <iface>: Use specified interface
-g/--source-port <portnum>: Use given port number
--proxies <url1,[url2],...>: Relay connections through HTTP/SOCKS4 proxies
--data <hex string>: Append a custom payload to sent packets
--data-string <string>: Append a custom ASCII string to sent packets
--data-length <num>: Append random data to sent packets
--ip-options <options>: Send packets with specified ip options
--ttl <val>: Set IP time-to-live field
--spoof-mac <mac address/prefix/vendor name>: Spoof your MAC address
--badsum: Send packets with a bogus TCP/UDP/SCTP checksum
OUTPUT:
-oN/-oX/-oS/-oG <file>: Output scan in normal, XML, s|<rIpt kIddi3,
and Grepable format, respectively, to the given filename.
-oA <basename>: Output in the three major formats at once
-v: Increase verbosity level (use -vv or more for greater effect)
-d: Increase debugging level (use -dd or more for greater effect)
--reason: Display the reason a port is in a particular state
--open: Only show open (or possibly open) ports
--packet-trace: Show all packets sent and received
--iflist: Print host interfaces and routes (for debugging)
--append-output: Append to rather than clobber specified output files
--resume <filename>: Resume an aborted scan
--noninteractive: Disable runtime interactions via keyboard
--stylesheet <path/URL>: XSL stylesheet to transform XML output to HTML
--webxml: Reference stylesheet from Nmap.Org for more portable XML
--no-stylesheet: Prevent associating of XSL stylesheet w/XML output
MISC:
-6: Enable IPv6 scanning
-A: Enable OS detection, version detection, script scanning, and traceroute
--datadir <dirname>: Specify custom Nmap data file location
--send-eth/--send-ip: Send using raw ethernet frames or IP packets
--privileged: Assume that the user is fully privileged
--unprivileged: Assume the user lacks raw socket privileges
-V: Print version number
-h: Print this help summary page.
EXAMPLES:
nmap -v -A scanme.nmap.org
nmap -v -sn 192.168.0.0/16 10.0.0.0/8
nmap -v -iR 10000 -Pn -p 80
SEE THE MAN PAGE (https://nmap.org/book/man.html) FOR MORE OPTIONS AND EXAMPLES
Everything on the Nmap command-line that isn't an option (or
option argument) is treated as a target host specification. The
simplest case is to specify a target IP address or hostname for
scanning.
When a hostname is given as a target, it is resolved via the
Domain Name System (DNS) to determine the IP address to scan. If
the name resolves to more than one IP address, only the first one
will be scanned. To make Nmap scan all the resolved addresses
instead of only the first one, use the --resolve-all option.
Sometimes you wish to scan a whole network of adjacent hosts. For
this, Nmap supports CIDR-style addressing. You can append
/numbits to an IP address or hostname and Nmap will scan every IP
address for which the first numbits are the same as for the
reference IP or hostname given. For example, 192.168.10.0/24
would scan the 256 hosts between 192.168.10.0 (binary: 11000000
10101000 00001010 00000000) and 192.168.10.255 (binary: 11000000
10101000 00001010 11111111), inclusive. 192.168.10.40/24 would
scan exactly the same targets. Given that the host
scanme.nmap.org is at the IP address 64.13.134.52, the
specification scanme.nmap.org/16 would scan the 65,536 IP
addresses between 64.13.0.0 and 64.13.255.255. The smallest
allowed value is /0, which targets the whole Internet. The
largest value for IPv4 is /32, which scans just the named host or
IP address because all address bits are fixed. The largest value
for IPv6 is /128, which does the same thing.
CIDR notation is short but not always flexible enough. For
example, you might want to scan 192.168.0.0/16 but skip any IPs
ending with .0 or .255 because they may be used as subnet network
and broadcast addresses. Nmap supports this through octet range
addressing. Rather than specify a normal IP address, you can
specify a comma-separated list of numbers or ranges for each
octet. For example, 192.168.0-255.1-254 will skip all addresses
in the range that end in .0 or .255, and 192.168.3-5,7.1 will
scan the four addresses 192.168.3.1, 192.168.4.1, 192.168.5.1,
and 192.168.7.1. Either side of a range may be omitted; the
default values are 0 on the left and 255 on the right. Using - by
itself is the same as 0-255, but remember to use 0- in the first
octet so the target specification doesn't look like a
command-line option. Ranges need not be limited to the final
octets: the specifier 0-255.0-255.13.37 will perform an
Internet-wide scan for all IP addresses ending in 13.37. This
sort of broad sampling can be useful for Internet surveys and
research.
IPv6 addresses can be specified by their fully qualified IPv6
address or hostname or with CIDR notation for subnets. Octet
ranges aren't yet supported for IPv6.
IPv6 addresses with non-global scope need to have a zone ID
suffix. On Unix systems, this is a percent sign followed by an
interface name; a complete address might be
fe80::a8bb:ccff:fedd:eeff%eth0. On Windows, use an interface
index number in place of an interface name:
fe80::a8bb:ccff:fedd:eeff%1. You can see a list of interface
indexes by running the command netsh.exe interface ipv6 show
interface.
Nmap accepts multiple host specifications on the command line,
and they don't need to be the same type. The command nmap
scanme.nmap.org 192.168.0.0/8 10.0.0,1,3-7.- does what you would
expect.
While targets are usually specified on the command lines, the
following options are also available to control target selection:
-iL inputfilename (Input from list)
Reads target specifications from inputfilename. Passing a
huge list of hosts is often awkward on the command line, yet
it is a common desire. For example, your DHCP server might
export a list of 10,000 current leases that you wish to scan.
Or maybe you want to scan all IP addresses except for those
to locate hosts using unauthorized static IP addresses.
Simply generate the list of hosts to scan and pass that
filename to Nmap as an argument to the -iL option. Entries
can be in any of the formats accepted by Nmap on the command
line (IP address, hostname, CIDR, IPv6, or octet ranges).
Each entry must be separated by one or more spaces, tabs, or
newlines. You can specify a hyphen (-) as the filename if you
want Nmap to read hosts from standard input rather than an
actual file.
The input file may contain comments that start with # and
extend to the end of the line.
-iR num hosts (Choose random targets)
For Internet-wide surveys and other research, you may want to
choose targets at random. The num hosts argument tells Nmap
how many IPs to generate. Undesirable IPs such as those in
certain private, multicast, or unallocated address ranges are
automatically skipped. The argument 0 can be specified for a
never-ending scan. Keep in mind that some network
administrators bristle at unauthorized scans of their
networks and may complain. Use this option at your own risk!
If you find yourself really bored one rainy afternoon, try
the command nmap -Pn -sS -p 80 -iR 0 --open to locate random
web servers for browsing.
--exclude host1[,host2[,...]] (Exclude hosts/networks)
Specifies a comma-separated list of targets to be excluded
from the scan even if they are part of the overall network
range you specify. The list you pass in uses normal Nmap
syntax, so it can include hostnames, CIDR netblocks, octet
ranges, etc. This can be useful when the network you wish to
scan includes untouchable mission-critical servers, systems
that are known to react adversely to port scans, or subnets
administered by other people.
--excludefile exclude_file (Exclude list from file)
This offers the same functionality as the --exclude option,
except that the excluded targets are provided in a newline-,
space-, or tab-delimited exclude_file rather than on the
command line.
The exclude file may contain comments that start with # and
extend to the end of the line.
-n (No DNS resolution)
Tells Nmap to never do reverse DNS resolution on the active
IP addresses it finds. Since DNS can be slow even with Nmap's
built-in parallel stub resolver, this option can slash
scanning times.
-R (DNS resolution for all targets)
Tells Nmap to always do reverse DNS resolution on the target
IP addresses. Normally reverse DNS is only performed against
responsive (online) hosts.
--resolve-all (Scan each resolved address)
If a hostname target resolves to more than one address, scan
all of them. The default behavior is to only scan the first
resolved address. Regardless, only addresses in the
appropriate address family will be scanned: IPv4 by default,
IPv6 with -6.
--unique (Scan each address only once)
Scan each IP address only once. The default behavior is to
scan each address as many times as it is specified in the
target list, such as when network ranges overlap or different
hostnames resolve to the same address.
--system-dns (Use system DNS resolver)
By default, Nmap reverse-resolves IP addresses by sending
queries directly to the name servers configured on your host
and then listening for responses. Many requests (often
dozens) are performed in parallel to improve performance.
Specify this option to use your system resolver instead (one
IP at a time via the getnameinfo call). This is slower and
rarely useful unless you find a bug in the Nmap parallel
resolver (please let us know if you do). The system resolver
is always used for forward lookups (getting an IP address
from a hostname).
--dns-servers server1[,server2[,...]] (Servers to use for
reverse DNS queries)
By default, Nmap determines your DNS servers (for rDNS
resolution) from your resolv.conf file (Unix) or the Registry
(Win32). Alternatively, you may use this option to specify
alternate servers. This option is not honored if you are
using --system-dns. Using multiple DNS servers is often
faster, especially if you choose authoritative servers for
your target IP space. This option can also improve stealth,
as your requests can be bounced off just about any recursive
DNS server on the Internet.
This option also comes in handy when scanning private
networks. Sometimes only a few name servers provide proper
rDNS information, and you may not even know where they are.
You can scan the network for port 53 (perhaps with version
detection), then try Nmap list scans (-sL) specifying each
name server one at a time with --dns-servers until you find
one which works.
This option might not be honored if the DNS response exceeds
the size of a UDP packet. In such a situation our DNS
resolver will make the best effort to extract a response from
the truncated packet, and if not successful it will fall back
to using the system resolver. Also, responses that contain
CNAME aliases will fall back to the system resolver.
One of the very first steps in any network reconnaissance mission
is to reduce a (sometimes huge) set of IP ranges into a list of
active or interesting hosts. Scanning every port of every single
IP address is slow and usually unnecessary. Of course what makes
a host interesting depends greatly on the scan purposes. Network
administrators may only be interested in hosts running a certain
service, while security auditors may care about every single
device with an IP address. An administrator may be comfortable
using just an ICMP ping to locate hosts on his internal network,
while an external penetration tester may use a diverse set of
dozens of probes in an attempt to evade firewall restrictions.
Because host discovery needs are so diverse, Nmap offers a wide
variety of options for customizing the techniques used. Host
discovery is sometimes called ping scan, but it goes well beyond
the simple ICMP echo request packets associated with the
ubiquitous ping tool. Users can skip the discovery step entirely
with a list scan (-sL) or by disabling host discovery (-Pn), or
engage the network with arbitrary combinations of multi-port TCP
SYN/ACK, UDP, SCTP INIT and ICMP probes. The goal of these probes
is to solicit responses which demonstrate that an IP address is
actually active (is being used by a host or network device). On
many networks, only a small percentage of IP addresses are active
at any given time. This is particularly common with private
address space such as 10.0.0.0/8. That network has 16 million
IPs, but I have seen it used by companies with less than a
thousand machines. Host discovery can find those machines in a
sparsely allocated sea of IP addresses.
If no host discovery options are given, Nmap sends an ICMP echo
request, a TCP SYN packet to port 443, a TCP ACK packet to port
80, and an ICMP timestamp request. (For IPv6, the ICMP timestamp
request is omitted because it is not part of ICMPv6.) These
defaults are equivalent to the -PE -PS443 -PA80 -PP options. The
exceptions to this are the ARP (for IPv4) and Neighbor Discovery
(for IPv6) scans which are used for any targets on a local
ethernet network. For unprivileged Unix shell users, the default
probes are a SYN packet to ports 80 and 443 using the connect
system call. This host discovery is often sufficient when
scanning local networks, but a more comprehensive set of
discovery probes is recommended for security auditing.
The -P* options (which select ping types) can be combined. You
can increase your odds of penetrating strict firewalls by sending
many probe types using different TCP ports/flags and ICMP codes.
Also note that ARP/Neighbor Discovery is done by default against
targets on a local Ethernet network even if you specify other -P*
options, because it is almost always faster and more effective.
By default, Nmap does host discovery and then performs a port
scan against each host it determines is online. This is true even
if you specify non-default host discovery types such as UDP
probes (-PU). Read about the -sn option to learn how to perform
only host discovery, or use -Pn to skip host discovery and port
scan all target addresses. The following options control host
discovery:
-sL (List Scan)
The list scan is a degenerate form of host discovery that
simply lists each host of the network(s) specified, without
sending any packets to the target hosts. By default, Nmap
still does reverse-DNS resolution on the hosts to learn their
names. It is often surprising how much useful information
simple hostnames give out. For example, fw.chi is the name of
one company's Chicago firewall.
Nmap also reports the total number of IP addresses at the
end. The list scan is a good sanity check to ensure that you
have proper IP addresses for your targets. If the hosts sport
domain names you do not recognize, it is worth investigating
further to prevent scanning the wrong company's network.
Since the idea is to simply print a list of target hosts,
options for higher level functionality such as port scanning,
OS detection, or host discovery cannot be combined with this.
If you wish to disable host discovery while still performing
such higher level functionality, read up on the -Pn (skip
host discovery) option.
-sn (No port scan)
This option tells Nmap not to do a port scan after host
discovery, and only print out the available hosts that
responded to the host discovery probes. This is often known
as a “ping scan”, but you can also request that traceroute
and NSE host scripts be run. This is by default one step more
intrusive than the list scan, and can often be used for the
same purposes. It allows light reconnaissance of a target
network without attracting much attention. Knowing how many
hosts are up is more valuable to attackers than the list
provided by list scan of every single IP and host name.
Systems administrators often find this option valuable as
well. It can easily be used to count available machines on a
network or monitor server availability. This is often called
a ping sweep, and is more reliable than pinging the broadcast
address because many hosts do not reply to broadcast queries.
The default host discovery done with -sn consists of an ICMP
echo request, TCP SYN to port 443, TCP ACK to port 80, and an
ICMP timestamp request by default. When executed by an
unprivileged user, only SYN packets are sent (using a connect
call) to ports 80 and 443 on the target. When a privileged
user tries to scan targets on a local ethernet network, ARP
requests are used unless --send-ip was specified. The -sn
option can be combined with any of the discovery probe types
(the -P* options) for greater flexibility. If any of those
probe type and port number options are used, the default
probes are overridden. When strict firewalls are in place
between the source host running Nmap and the target network,
using those advanced techniques is recommended. Otherwise
hosts could be missed when the firewall drops probes or their
responses.
In previous releases of Nmap, -sn was known as -sP.
-Pn (No ping)
This option skips the host discovery stage altogether.
Normally, Nmap uses this stage to determine active machines
for heavier scanning and to gauge the speed of the network.
By default, Nmap only performs heavy probing such as port
scans, version detection, or OS detection against hosts that
are found to be up. Disabling host discovery with -Pn causes
Nmap to attempt the requested scanning functions against
every target IP address specified. So if a /16 sized network
is specified on the command line, all 65,536 IP addresses are
scanned. Proper host discovery is skipped as with the list
scan, but instead of stopping and printing the target list,
Nmap continues to perform requested functions as if each
target IP is active. Default timing parameters are used,
which may result in slower scans. To skip host discovery and
port scan, while still allowing NSE to run, use the two
options -Pn -sn together.
For machines on a local ethernet network, ARP scanning will
still be performed (unless --disable-arp-ping or --send-ip is
specified) because Nmap needs MAC addresses to further scan
target hosts. In previous versions of Nmap, -Pn was -P0 and
-PN.
-PS port list (TCP SYN Ping)
This option sends an empty TCP packet with the SYN flag set.
The default destination port is 80 (configurable at compile
time by changing DEFAULT_TCP_PROBE_PORT_SPEC in nmap.h).
Alternate ports can be specified as a parameter. The syntax
is the same as for the -p except that port type specifiers
like T: are not allowed. Examples are -PS22 and
-PS22-25,80,113,1050,35000. Note that there can be no space
between -PS and the port list. If multiple probes are
specified they will be sent in parallel.
The SYN flag suggests to the remote system that you are
attempting to establish a connection. Normally the
destination port will be closed, and a RST (reset) packet
sent back. If the port happens to be open, the target will
take the second step of a TCP three-way-handshake by
responding with a SYN/ACK TCP packet. The machine running
Nmap then tears down the nascent connection by responding
with a RST rather than sending an ACK packet which would
complete the three-way-handshake and establish a full
connection. The RST packet is sent by the kernel of the
machine running Nmap in response to the unexpected SYN/ACK,
not by Nmap itself.
Nmap does not care whether the port is open or closed. Either
the RST or SYN/ACK response discussed previously tell Nmap
that the host is available and responsive.
On Unix boxes, only the privileged user root is generally
able to send and receive raw TCP packets. For unprivileged
users, a workaround is automatically employed whereby the
connect system call is initiated against each target port.
This has the effect of sending a SYN packet to the target
host, in an attempt to establish a connection. If connect
returns with a quick success or an ECONNREFUSED failure, the
underlying TCP stack must have received a SYN/ACK or RST and
the host is marked available. If the connection attempt is
left hanging until a timeout is reached, the host is marked
as down.
-PA port list (TCP ACK Ping)
The TCP ACK ping is quite similar to the just-discussed SYN
ping. The difference, as you could likely guess, is that the
TCP ACK flag is set instead of the SYN flag. Such an ACK
packet purports to be acknowledging data over an established
TCP connection, but no such connection exists. So remote
hosts should always respond with a RST packet, disclosing
their existence in the process.
The -PA option uses the same default port as the SYN probe
(80) and can also take a list of destination ports in the
same format. If an unprivileged user tries this, the connect
workaround discussed previously is used. This workaround is
imperfect because connect is actually sending a SYN packet
rather than an ACK.
The reason for offering both SYN and ACK ping probes is to
maximize the chances of bypassing firewalls. Many
administrators configure routers and other simple firewalls
to block incoming SYN packets except for those destined for
public services like the company web site or mail server.
This prevents other incoming connections to the organization,
while allowing users to make unobstructed outgoing
connections to the Internet. This non-stateful approach takes
up few resources on the firewall/router and is widely
supported by hardware and software filters. The Linux
Netfilter/iptables firewall software offers the --syn
convenience option to implement this stateless approach. When
stateless firewall rules such as this are in place, SYN ping
probes (-PS) are likely to be blocked when sent to closed
target ports. In such cases, the ACK probe shines as it cuts
right through these rules.
Another common type of firewall uses stateful rules that drop
unexpected packets. This feature was initially found mostly
on high-end firewalls, though it has become much more common
over the years. The Linux Netfilter/iptables system supports
this through the --state option, which categorizes packets
based on connection state. A SYN probe is more likely to work
against such a system, as unexpected ACK packets are
generally recognized as bogus and dropped. A solution to this
quandary is to send both SYN and ACK probes by specifying -PS
and -PA.
-PU port list (UDP Ping)
Another host discovery option is the UDP ping, which sends a
UDP packet to the given ports. For most ports, the packet
will be empty, though some use a protocol-specific payload
that is more likely to elicit a response.
The payloads are the same probes used in service and version
detection and are defined in the nmap-service-probes
file. Packet content can also be affected with the --data,
--data-string, and --data-length options.
The port list takes the same format as with the previously
discussed -PS and -PA options. If no ports are specified, the
default is 40125. This default can be configured at
compile-time by changing DEFAULT_UDP_PROBE_PORT_SPEC in
nmap.h. A highly uncommon port is used by default because
sending to open ports is often undesirable for this
particular scan type.
Upon hitting a closed port on the target machine, the UDP
probe should elicit an ICMP port unreachable packet in
return. This signifies to Nmap that the machine is up and
available. Many other types of ICMP errors, such as
host/network unreachables or TTL exceeded are indicative of a
down or unreachable host. A lack of response is also
interpreted this way. If an open port is reached, most
services simply ignore the empty packet and fail to return
any response. This is why the default probe port is 40125,
which is highly unlikely to be in use. A few services, such
as the Character Generator (chargen) protocol, will respond
to an empty UDP packet, and thus disclose to Nmap that the
machine is available.
The primary advantage of this scan type is that it bypasses
firewalls and filters that only screen TCP. For example, I
once owned a Linksys BEFW11S4 wireless broadband router. The
external interface of this device filtered all TCP ports by
default, but UDP probes would still elicit port unreachable
messages and thus give away the device.
-PY port list (SCTP INIT Ping)
This option sends an SCTP packet containing a minimal INIT
chunk. The default destination port is 80 (configurable at
compile time by changing DEFAULT_SCTP_PROBE_PORT_SPEC in
nmap.h). Alternate ports can be specified as a parameter. The
syntax is the same as for the -p except that port type
specifiers like S: are not allowed. Examples are -PY22 and
-PY22,80,179,5060. Note that there can be no space between
-PY and the port list. If multiple probes are specified they
will be sent in parallel.
The INIT chunk suggests to the remote system that you are
attempting to establish an association. Normally the
destination port will be closed, and an ABORT chunk will be
sent back. If the port happens to be open, the target will
take the second step of an SCTP four-way-handshake by
responding with an INIT-ACK chunk. If the machine running
Nmap has a functional SCTP stack, then it tears down the
nascent association by responding with an ABORT chunk rather
than sending a COOKIE-ECHO chunk which would be the next step
in the four-way-handshake. The ABORT packet is sent by the
kernel of the machine running Nmap in response to the
unexpected INIT-ACK, not by Nmap itself.
Nmap does not care whether the port is open or closed. Either
the ABORT or INIT-ACK response discussed previously tell Nmap
that the host is available and responsive.
On Unix boxes, only the privileged user root is generally
able to send and receive raw SCTP packets. Using SCTP INIT
Pings is currently not possible for unprivileged users.
-PE; -PP; -PM (ICMP Ping Types)
In addition to the unusual TCP, UDP and SCTP host discovery
types discussed previously, Nmap can send the standard
packets sent by the ubiquitous ping program. Nmap sends an
ICMP type 8 (echo request) packet to the target IP addresses,
expecting a type 0 (echo reply) in return from available
hosts. Unfortunately for network explorers, many hosts and
firewalls now block these packets, rather than responding as
required by RFC 1122[2]. For this reason, ICMP-only scans
are rarely reliable enough against unknown targets over the
Internet. But for system administrators monitoring an
internal network, they can be a practical and efficient
approach. Use the -PE option to enable this echo request
behavior.
While echo request is the standard ICMP ping query, Nmap does
not stop there. The ICMP standards (RFC 792[3] and RFC 950[4]
) also specify timestamp request, information request, and
address mask request packets as codes 13, 15, and 17,
respectively. While the ostensible purpose for these queries
is to learn information such as address masks and current
times, they can easily be used for host discovery. A system
that replies is up and available. Nmap does not currently
implement information request packets, as they are not widely
supported. RFC 1122 insists that “a host SHOULD NOT implement
these messages”. Timestamp and address mask queries can be
sent with the -PP and -PM options, respectively. A timestamp
reply (ICMP code 14) or address mask reply (code 18)
discloses that the host is available. These two queries can
be valuable when administrators specifically block echo
request packets while forgetting that other ICMP queries can
be used for the same purpose.
-PO protocol list (IP Protocol Ping)
One of the newer host discovery options is the IP protocol
ping, which sends IP packets with the specified protocol
number set in their IP header. The protocol list takes the
same format as do port lists in the previously discussed TCP,
UDP and SCTP host discovery options. If no protocols are
specified, the default is to send multiple IP packets for
ICMP (protocol 1), IGMP (protocol 2), and IP-in-IP (protocol
4). The default protocols can be configured at compile-time
by changing DEFAULT_PROTO_PROBE_PORT_SPEC in nmap.h. Note
that for the ICMP, IGMP, TCP (protocol 6), UDP (protocol 17)
and SCTP (protocol 132), the packets are sent with the proper
protocol headers while other protocols are sent with no
additional data beyond the IP header (unless any of --data,
--data-string, or --data-length options are specified).
This host discovery method looks for either responses using
the same protocol as a probe, or ICMP protocol unreachable
messages which signify that the given protocol isn't
supported on the destination host. Either type of response
signifies that the target host is alive.
--disable-arp-ping (No ARP or ND Ping)
Nmap normally does ARP or IPv6 Neighbor Discovery (ND)
discovery of locally connected ethernet hosts, even if other
host discovery options such as -Pn or -PE are used. To
disable this implicit behavior, use the --disable-arp-ping
option.
The default behavior is normally faster, but this option is
useful on networks using proxy ARP, in which a router
speculatively replies to all ARP requests, making every
target appear to be up according to ARP scan.
--discovery-ignore-rst
In some cases, firewalls may spoof TCP reset (RST) replies in
response to probes to unoccupied or disallowed addresses.
Since Nmap ordinarily considers RST replies to be proof that
the target is up, this can lead to wasted time scanning
targets that aren't there. Using the --discovery-ignore-rst
will prevent Nmap from considering these replies during host
discovery. You may need to select extra host discovery
options to ensure you don't miss targets in this case.
--traceroute (Trace path to host)
Traceroutes are performed post-scan using information from
the scan results to determine the port and protocol most
likely to reach the target. It works with all scan types
except connect scans (-sT) and idle scans (-sI). All traces
use Nmap's dynamic timing model and are performed in
parallel.
Traceroute works by sending packets with a low TTL
(time-to-live) in an attempt to elicit ICMP Time Exceeded
messages from intermediate hops between the scanner and the
target host. Standard traceroute implementations start with a
TTL of 1 and increment the TTL until the destination host is
reached. Nmap's traceroute starts with a high TTL and then
decrements the TTL until it reaches zero. Doing it backwards
lets Nmap employ clever caching algorithms to speed up traces
over multiple hosts. On average Nmap sends 5–10 fewer packets
per host, depending on network conditions. If a single subnet
is being scanned (i.e. 192.168.0.0/24) Nmap may only have to
send two packets to most hosts.
While Nmap has grown in functionality over the years, it began as
an efficient port scanner, and that remains its core function.
The simple command nmap target scans 1,000 TCP ports on the host
target. While many port scanners have traditionally lumped all
ports into the open or closed states, Nmap is much more granular.
It divides ports into six states: open, closed, filtered,
unfiltered, open|filtered, or closed|filtered.
These states are not intrinsic properties of the port itself, but
describe how Nmap sees them. For example, an Nmap scan from the
same network as the target may show port 135/tcp as open, while a
scan at the same time with the same options from across the
Internet might show that port as filtered.
The six port states recognized by Nmap
open
An application is actively accepting TCP connections, UDP
datagrams or SCTP associations on this port. Finding these is
often the primary goal of port scanning. Security-minded
people know that each open port is an avenue for attack.
Attackers and pen-testers want to exploit the open ports,
while administrators try to close or protect them with
firewalls without thwarting legitimate users. Open ports are
also interesting for non-security scans because they show
services available for use on the network.
closed
A closed port is accessible (it receives and responds to Nmap
probe packets), but there is no application listening on it.
They can be helpful in showing that a host is up on an IP
address (host discovery, or ping scanning), and as part of OS
detection. Because closed ports are reachable, it may be
worth scanning later in case some open up. Administrators may
want to consider blocking such ports with a firewall. Then
they would appear in the filtered state, discussed next.
filtered
Nmap cannot determine whether the port is open because packet
filtering prevents its probes from reaching the port. The
filtering could be from a dedicated firewall device, router
rules, or host-based firewall software. These ports frustrate
attackers because they provide so little information.
Sometimes they respond with ICMP error messages such as type
3 code 13 (destination unreachable: communication
administratively prohibited), but filters that simply drop
probes without responding are far more common. This forces
Nmap to retry several times just in case the probe was
dropped due to network congestion rather than filtering. This
slows down the scan dramatically.
unfiltered
The unfiltered state means that a port is accessible, but
Nmap is unable to determine whether it is open or closed.
Only the ACK scan, which is used to map firewall rulesets,
classifies ports into this state. Scanning unfiltered ports
with other scan types such as Window scan, SYN scan, or FIN
scan, may help resolve whether the port is open.
open|filtered
Nmap places ports in this state when it is unable to
determine whether a port is open or filtered. This occurs for
scan types in which open ports give no response. The lack of
response could also mean that a packet filter dropped the
probe or any response it elicited. So Nmap does not know for
sure whether the port is open or being filtered. The UDP, IP
protocol, FIN, NULL, and Xmas scans classify ports this way.
closed|filtered
This state is used when Nmap is unable to determine whether a
port is closed or filtered. It is only used for the IP ID
idle scan.
As a novice performing automotive repair, I can struggle for
hours trying to fit my rudimentary tools (hammer, duct tape,
wrench, etc.) to the task at hand. When I fail miserably and tow
my jalopy to a real mechanic, he invariably fishes around in a
huge tool chest until pulling out the perfect gizmo which makes
the job seem effortless. The art of port scanning is similar.
Experts understand the dozens of scan techniques and choose the
appropriate one (or combination) for a given task. Inexperienced
users and script kiddies, on the other hand, try to solve every
problem with the default SYN scan. Since Nmap is free, the only
barrier to port scanning mastery is knowledge. That certainly
beats the automotive world, where it may take great skill to
determine that you need a strut spring compressor, then you still
have to pay thousands of dollars for it.
Most of the scan types are only available to privileged users.
This is because they send and receive raw packets, which requires
root access on Unix systems. Using an administrator account on
Windows is recommended, though Nmap sometimes works for
unprivileged users on that platform when Npcap has already been
loaded into the OS. Requiring root privileges was a serious
limitation when Nmap was released in 1997, as many users only had
access to shared shell accounts. Now, the world is different.
Computers are cheaper, far more people have always-on direct
Internet access, and desktop Unix systems (including Linux and
Mac OS X) are prevalent. A Windows version of Nmap is now
available, allowing it to run on even more desktops. For all
these reasons, users have less need to run Nmap from limited
shared shell accounts. This is fortunate, as the privileged
options make Nmap far more powerful and flexible.
While Nmap attempts to produce accurate results, keep in mind
that all of its insights are based on packets returned by the
target machines (or firewalls in front of them). Such hosts may
be untrustworthy and send responses intended to confuse or
mislead Nmap. Much more common are non-RFC-compliant hosts that
do not respond as they should to Nmap probes. FIN, NULL, and Xmas
scans are particularly susceptible to this problem. Such issues
are specific to certain scan types and so are discussed in the
individual scan type entries.
This section documents the dozen or so port scan techniques
supported by Nmap. Only one method may be used at a time, except
that UDP scan (-sU) and any one of the SCTP scan types (-sY, -sZ)
may be combined with any one of the TCP scan types. As a memory
aid, port scan type options are of the form -sC, where C is a
prominent character in the scan name, usually the first. The one
exception to this is the deprecated FTP bounce scan (-b). By
default, Nmap performs a SYN Scan, though it substitutes a
connect scan if the user does not have proper privileges to send
raw packets (requires root access on Unix). Of the scans listed
in this section, unprivileged users can only execute connect and
FTP bounce scans.
-sS (TCP SYN scan)
SYN scan is the default and most popular scan option for good
reasons. It can be performed quickly, scanning thousands of
ports per second on a fast network not hampered by
restrictive firewalls. It is also relatively unobtrusive and
stealthy since it never completes TCP connections. SYN scan
works against any compliant TCP stack rather than depending
on idiosyncrasies of specific platforms as Nmap's
FIN/NULL/Xmas, Maimon and idle scans do. It also allows
clear, reliable differentiation between the open, closed, and
filtered states.
This technique is often referred to as half-open scanning,
because you don't open a full TCP connection. You send a SYN
packet, as if you are going to open a real connection and
then wait for a response. A SYN/ACK indicates the port is
listening (open), while a RST (reset) is indicative of a
non-listener. If no response is received after several
retransmissions, the port is marked as filtered. The port is
also marked filtered if an ICMP unreachable error (type 3,
code 0, 1, 2, 3, 9, 10, or 13) is received. The port is also
considered open if a SYN packet (without the ACK flag) is
received in response. This can be due to an extremely rare
TCP feature known as a simultaneous open or split handshake
connection (see https://nmap.org/misc/split-handshake.pdf ).
-sT (TCP connect scan)
TCP connect scan is the default TCP scan type when SYN scan
is not an option. This is the case when a user does not have
raw packet privileges. Instead of writing raw packets as most
other scan types do, Nmap asks the underlying operating
system to establish a connection with the target machine and
port by issuing the connect system call. This is the same
high-level system call that web browsers, P2P clients, and
most other network-enabled applications use to establish a
connection. It is part of a programming interface known as
the Berkeley Sockets API. Rather than read raw packet
responses off the wire, Nmap uses this API to obtain status
information on each connection attempt.
When SYN scan is available, it is usually a better choice.
Nmap has less control over the high level connect call than
with raw packets, making it less efficient. The system call
completes connections to open target ports rather than
performing the half-open reset that SYN scan does. Not only
does this take longer and require more packets to obtain the
same information, but target machines are more likely to log
the connection. A decent IDS will catch either, but most
machines have no such alarm system. Many services on your
average Unix system will add a note to syslog, and sometimes
a cryptic error message, when Nmap connects and then closes
the connection without sending data. Truly pathetic services
crash when this happens, though that is uncommon. An
administrator who sees a bunch of connection attempts in her
logs from a single system should know that she has been
connect scanned.
-sU (UDP scans)
While most popular services on the Internet run over the TCP
protocol, UDP[5] services are widely deployed. DNS, SNMP, and
DHCP (registered ports 53, 161/162, and 67/68) are three of
the most common. Because UDP scanning is generally slower and
more difficult than TCP, some security auditors ignore these
ports. This is a mistake, as exploitable UDP services are
quite common and attackers certainly don't ignore the whole
protocol. Fortunately, Nmap can help inventory UDP ports.
UDP scan is activated with the -sU option. It can be combined
with a TCP scan type such as SYN scan (-sS) to check both
protocols during the same run.
UDP scan works by sending a UDP packet to every targeted
port. For some common ports such as 53 and 161, a
protocol-specific payload is sent to increase response rate,
but for most ports the packet is empty unless the --data,
--data-string, or --data-length options are specified. If an
ICMP port unreachable error (type 3, code 3) is returned, the
port is closed. Other ICMP unreachable errors (type 3, codes
0, 1, 2, 9, 10, or 13) mark the port as filtered.
Occasionally, a service will respond with a UDP packet,
proving that it is open. If no response is received after
retransmissions, the port is classified as open|filtered.
This means that the port could be open, or perhaps packet
filters are blocking the communication. Version detection
(-sV) can be used to help differentiate the truly open ports
from the filtered ones.
A big challenge with UDP scanning is doing it quickly. Open
and filtered ports rarely send any response, leaving Nmap to
time out and then conduct retransmissions just in case the
probe or response were lost. Closed ports are often an even
bigger problem. They usually send back an ICMP port
unreachable error. But unlike the RST packets sent by closed
TCP ports in response to a SYN or connect scan, many hosts
rate limit ICMP port unreachable messages by default. Linux
and Solaris are particularly strict about this. For example,
the Linux 2.4.20 kernel limits destination unreachable
messages to one per second (in net/ipv4/icmp.c).
Nmap detects rate limiting and slows down accordingly to
avoid flooding the network with useless packets that the
target machine will drop. Unfortunately, a Linux-style limit
of one packet per second makes a 65,536-port scan take more
than 18 hours. Ideas for speeding your UDP scans up include
scanning more hosts in parallel, doing a quick scan of just
the popular ports first, scanning from behind the firewall,
and using --host-timeout to skip slow hosts.
-sY (SCTP INIT scan)
SCTP[6] is a relatively new alternative to the TCP and UDP
protocols, combining most characteristics of TCP and UDP, and
also adding new features like multi-homing and
multi-streaming. It is mostly being used for SS7/SIGTRAN
related services but has the potential to be used for other
applications as well. SCTP INIT scan is the SCTP equivalent
of a TCP SYN scan. It can be performed quickly, scanning
thousands of ports per second on a fast network not hampered
by restrictive firewalls. Like SYN scan, INIT scan is
relatively unobtrusive and stealthy, since it never completes
SCTP associations. It also allows clear, reliable
differentiation between the open, closed, and filtered
states.
This technique is often referred to as half-open scanning,
because you don't open a full SCTP association. You send an
INIT chunk, as if you are going to open a real association
and then wait for a response. An INIT-ACK chunk indicates the
port is listening (open), while an ABORT chunk is indicative
of a non-listener. If no response is received after several
retransmissions, the port is marked as filtered. The port is
also marked filtered if an ICMP unreachable error (type 3,
code 0, 1, 2, 3, 9, 10, or 13) is received.
-sN; -sF; -sX (TCP NULL, FIN, and Xmas scans)
These three scan types (even more are possible with the
--scanflags option described in the next section) exploit a
subtle loophole in the TCP RFC[7] to differentiate between
open and closed ports. Page 65 of RFC 793 says that “if the
[destination] port state is CLOSED .... an incoming segment
not containing a RST causes a RST to be sent in response.”
Then the next page discusses packets sent to open ports
without the SYN, RST, or ACK bits set, stating that: “you are
unlikely to get here, but if you do, drop the segment, and
return.”
When scanning systems compliant with this RFC text, any
packet not containing SYN, RST, or ACK bits will result in a
returned RST if the port is closed and no response at all if
the port is open. As long as none of those three bits are
included, any combination of the other three (FIN, PSH, and
URG) are OK. Nmap exploits this with three scan types:
Null scan (-sN)
Does not set any bits (TCP flag header is 0)
FIN scan (-sF)
Sets just the TCP FIN bit.
Xmas scan (-sX)
Sets the FIN, PSH, and URG flags, lighting the packet up
like a Christmas tree.
These three scan types are exactly the same in behavior
except for the TCP flags set in probe packets. If a RST
packet is received, the port is considered closed, while no
response means it is open|filtered. The port is marked
filtered if an ICMP unreachable error (type 3, code 0, 1, 2,
3, 9, 10, or 13) is received.
The key advantage to these scan types is that they can sneak
through certain non-stateful firewalls and packet filtering
routers. Another advantage is that these scan types are a
little more stealthy than even a SYN scan. Don't count on
this though—most modern IDS products can be configured to
detect them. The big downside is that not all systems follow
RFC 793 to the letter. A number of systems send RST responses
to the probes regardless of whether the port is open or not.
This causes all of the ports to be labeled closed. Major
operating systems that do this are Microsoft Windows, many
Cisco devices, BSDI, and IBM OS/400. This scan does work
against most Unix-based systems though. Another downside of
these scans is that they can't distinguish open ports from
certain filtered ones, leaving you with the response
open|filtered.
-sA (TCP ACK scan)
This scan is different than the others discussed so far in
that it never determines open (or even open|filtered) ports.
It is used to map out firewall rulesets, determining whether
they are stateful or not and which ports are filtered.
The ACK scan probe packet has only the ACK flag set (unless
you use --scanflags). When scanning unfiltered systems, open
and closed ports will both return a RST packet. Nmap then
labels them as unfiltered, meaning that they are reachable by
the ACK packet, but whether they are open or closed is
undetermined. Ports that don't respond, or send certain ICMP
error messages back (type 3, code 0, 1, 2, 3, 9, 10, or 13),
are labeled filtered.
-sW (TCP Window scan)
Window scan is exactly the same as ACK scan except that it
exploits an implementation detail of certain systems to
differentiate open ports from closed ones, rather than always
printing unfiltered when a RST is returned. It does this by
examining the TCP Window field of the RST packets returned.
On some systems, open ports use a positive window size (even
for RST packets) while closed ones have a zero window. So
instead of always listing a port as unfiltered when it
receives a RST back, Window scan lists the port as open or
closed if the TCP Window value in that reset is positive or
zero, respectively.
This scan relies on an implementation detail of a minority of
systems out on the Internet, so you can't always trust it.
Systems that don't support it will usually return all ports
closed. Of course, it is possible that the machine really has
no open ports. If most scanned ports are closed but a few
common port numbers (such as 22, 25, 53) are filtered, the
system is most likely susceptible. Occasionally, systems will
even show the exact opposite behavior. If your scan shows
1,000 open ports and three closed or filtered ports, then
those three may very well be the truly open ones.
-sM (TCP Maimon scan)
The Maimon scan is named after its discoverer, Uriel Maimon.
He described the technique in Phrack Magazine issue #49
(November 1996). Nmap, which included this technique, was
released two issues later. This technique is exactly the same
as NULL, FIN, and Xmas scans, except that the probe is
FIN/ACK. According to RFC 793[7] (TCP), a RST packet should
be generated in response to such a probe whether the port is
open or closed. However, Uriel noticed that many BSD-derived
systems simply drop the packet if the port is open.
--scanflags (Custom TCP scan)
Truly advanced Nmap users need not limit themselves to the
canned scan types offered. The --scanflags option allows you
to design your own scan by specifying arbitrary TCP flags.
Let your creative juices flow, while evading intrusion
detection systems whose vendors simply paged through the Nmap
man page adding specific rules!
The --scanflags argument can be a numerical flag value such
as 9 (PSH and FIN), but using symbolic names is easier. Just
mash together any combination of URG, ACK, PSH, RST, SYN, and
FIN. For example, --scanflags URGACKPSHRSTSYNFIN sets
everything, though it's not very useful for scanning. The
order these are specified in is irrelevant.
In addition to specifying the desired flags, you can specify
a TCP scan type (such as -sA or -sF). That base type tells
Nmap how to interpret responses. For example, a SYN scan
considers no-response to indicate a filtered port, while a
FIN scan treats the same as open|filtered. Nmap will behave
the same way it does for the base scan type, except that it
will use the TCP flags you specify instead. If you don't
specify a base type, SYN scan is used.
-sZ (SCTP COOKIE ECHO scan)
SCTP COOKIE ECHO scan is a more advanced SCTP scan. It takes
advantage of the fact that SCTP implementations should
silently drop packets containing COOKIE ECHO chunks on open
ports, but send an ABORT if the port is closed. The advantage
of this scan type is that it is not as obvious a port scan
than an INIT scan. Also, there may be non-stateful firewall
rulesets blocking INIT chunks, but not COOKIE ECHO chunks.
Don't be fooled into thinking that this will make a port scan
invisible; a good IDS will be able to detect SCTP COOKIE ECHO
scans too. The downside is that SCTP COOKIE ECHO scans cannot
differentiate between open and filtered ports, leaving you
with the state open|filtered in both cases.
-sI zombie host[:probeport] (idle scan)
This advanced scan method allows for a truly blind TCP port
scan of the target (meaning no packets are sent to the target
from your real IP address). Instead, a unique side-channel
attack exploits predictable IP fragmentation ID sequence
generation on the zombie host to glean information about the
open ports on the target. IDS systems will display the scan
as coming from the zombie machine you specify (which must be
up and meet certain criteria). This fascinating scan type is
too complex to fully describe in this reference guide, so I
wrote and posted an informal paper with full details at
https://nmap.org/book/idlescan.html .
Besides being extraordinarily stealthy (due to its blind
nature), this scan type permits mapping out IP-based trust
relationships between machines. The port listing shows open
ports from the perspective of the zombie host. So you can
try scanning a target using various zombies that you think
might be trusted (via router/packet filter rules).
You can add a colon followed by a port number to the zombie
host if you wish to probe a particular port on the zombie for
IP ID changes. Otherwise Nmap will use the port it uses by
default for TCP pings (80).
-sO (IP protocol scan)
IP protocol scan allows you to determine which IP protocols
(TCP, ICMP, IGMP, etc.) are supported by target machines.
This isn't technically a port scan, since it cycles through
IP protocol numbers rather than TCP or UDP port numbers. Yet
it still uses the -p option to select scanned protocol
numbers, reports its results within the normal port table
format, and even uses the same underlying scan engine as the
true port scanning methods. So it is close enough to a port
scan that it belongs here.
Besides being useful in its own right, protocol scan
demonstrates the power of open-source software. While the
fundamental idea is pretty simple, I had not thought to add
it nor received any requests for such functionality. Then in
the summer of 2000, Gerhard Rieger conceived the idea, wrote
an excellent patch implementing it, and sent it to the
announce mailing list (then called nmap-hackers). I
incorporated that patch into the Nmap tree and released a new
version the next day. Few pieces of commercial software have
users enthusiastic enough to design and contribute their own
improvements!
Protocol scan works in a similar fashion to UDP scan. Instead
of iterating through the port number field of a UDP packet,
it sends IP packet headers and iterates through the eight-bit
IP protocol field. The headers are usually empty, containing
no data and not even the proper header for the claimed
protocol. The exceptions are TCP, UDP, ICMP, SCTP, and IGMP.
A proper protocol header for those is included since some
systems won't send them otherwise and because Nmap already
has functions to create them. Instead of watching for ICMP
port unreachable messages, protocol scan is on the lookout
for ICMP protocol unreachable messages. If Nmap receives any
response in any protocol from the target host, Nmap marks
that protocol as open. An ICMP protocol unreachable error
(type 3, code 2) causes the protocol to be marked as closed
while port unreachable (type 3, code 3) marks the protocol
open. Other ICMP unreachable errors (type 3, code 0, 1, 9,
10, or 13) cause the protocol to be marked filtered (though
they prove that ICMP is open at the same time). If no
response is received after retransmissions, the protocol is
marked open|filtered
-b FTP relay host (FTP bounce scan)
An interesting feature of the FTP protocol (RFC 959[8]) is
support for so-called proxy FTP connections. This allows a
user to connect to one FTP server, then ask that files be
sent to a third-party server. Such a feature is ripe for
abuse on many levels, so most servers have ceased supporting
it. One of the abuses this feature allows is causing the FTP
server to port scan other hosts. Simply ask the FTP server to
send a file to each interesting port of a target host in
turn. The error message will describe whether the port is
open or not. This is a good way to bypass firewalls because
organizational FTP servers are often placed where they have
more access to other internal hosts than any old Internet
host would. Nmap supports FTP bounce scan with the -b option.
It takes an argument of the form
username:password@server:port. Server is the name or IP
address of a vulnerable FTP server. As with a normal URL, you
may omit username:password, in which case anonymous login
credentials (user: anonymous password:-wwwuser@) are used.
The port number (and preceding colon) may be omitted as well,
in which case the default FTP port (21) on server is used.
This vulnerability was widespread in 1997 when Nmap was
released, but has largely been fixed. Vulnerable servers are
still around, so it is worth trying when all else fails. If
bypassing a firewall is your goal, scan the target network
for port 21 (or even for any FTP services if you scan all
ports with version detection) and use the ftp-bounce NSE
script. Nmap will tell you whether the host is vulnerable or
not. If you are just trying to cover your tracks, you don't
need to (and, in fact, shouldn't) limit yourself to hosts on
the target network. Before you go scanning random Internet
addresses for vulnerable FTP servers, consider that sysadmins
may not appreciate you abusing their servers in this way.
In addition to all of the scan methods discussed previously, Nmap
offers options for specifying which ports are scanned and whether
the scan order is randomized or sequential. By default, Nmap
scans the most common 1,000 ports for each protocol.
-p port ranges (Only scan specified ports)
This option specifies which ports you want to scan and
overrides the default. Individual port numbers are OK, as are
ranges separated by a hyphen (e.g. 1-1023). The beginning
and/or end values of a range may be omitted, causing Nmap to
use 1 and 65535, respectively. So you can specify -p- to scan
ports from 1 through 65535. Scanning port zero is allowed if
you specify it explicitly. For IP protocol scanning (-sO),
this option specifies the protocol numbers you wish to scan
for (0–255).
When scanning a combination of protocols (e.g. TCP and UDP),
you can specify a particular protocol by preceding the port
numbers by T: for TCP, U: for UDP, S: for SCTP, or P: for IP
Protocol. The qualifier lasts until you specify another
qualifier. For example, the argument -p
U:53,111,137,T:21-25,80,139,8080 would scan UDP ports 53,
111,and 137, as well as the listed TCP ports. Note that to
scan both UDP and TCP, you have to specify -sU and at least
one TCP scan type (such as -sS, -sF, or -sT). If no protocol
qualifier is given, the port numbers are added to all
protocol lists. Ports can also be specified by name
according to what the port is referred to in the
nmap-services. You can even use the wildcards * and ? with
the names. For example, to scan FTP and all ports whose names
begin with “http”, use -p ftp,http*. Be careful about shell
expansions and quote the argument to -p if unsure.
Ranges of ports can be surrounded by square brackets to
indicate ports inside that range that appear in
nmap-services. For example, the following will scan all ports
in nmap-services equal to or below 1024: -p [-1024]. Be
careful with shell expansions and quote the argument to -p if
unsure.
--exclude-ports port ranges (Exclude the specified ports from
scanning)
This option specifies which ports you do want Nmap to exclude
from scanning. The port ranges are specified similar to -p.
For IP protocol scanning (-sO), this option specifies the
protocol numbers you wish to exclude (0–255).
When ports are asked to be excluded, they are excluded from
all types of scans (i.e. they will not be scanned under any
circumstances). This also includes the discovery phase.
-F (Fast (limited port) scan)
Specifies that you wish to scan fewer ports than the default.
Normally Nmap scans the most common 1,000 ports for each
scanned protocol. With -F, this is reduced to 100.
Nmap needs an nmap-services file with frequency information
in order to know which ports are the most common. If port
frequency information isn't available, perhaps because of the
use of a custom nmap-services file, Nmap scans all named
ports plus ports 1-1024. In that case, -F means to scan only
ports that are named in the services file.
-r (Don't randomize ports)
By default, Nmap randomizes the scanned port order (except
that certain commonly accessible ports are moved near the
beginning for efficiency reasons). This randomization is
normally desirable, but you can specify -r for sequential
(sorted from lowest to highest) port scanning instead.
--port-ratio ratio<decimal number between 0 and 1>
Scans all ports in nmap-services file with a ratio greater
than the one given. ratio must be between 0.0 and 1.0.
--top-ports n
Scans the n highest-ratio ports found in nmap-services file
after excluding all ports specified by --exclude-ports. n
must be 1 or greater.
Point Nmap at a remote machine and it might tell you that ports
25/tcp, 80/tcp, and 53/udp are open. Using its nmap-services
database of about 2,200 well-known services, Nmap would report
that those ports probably correspond to a mail server (SMTP), web
server (HTTP), and name server (DNS) respectively. This lookup is
usually accurate—the vast majority of daemons listening on TCP
port 25 are, in fact, mail servers. However, you should not bet
your security on this! People can and do run services on strange
ports.
Even if Nmap is right, and the hypothetical server above is
running SMTP, HTTP, and DNS servers, that is not a lot of
information. When doing vulnerability assessments (or even simple
network inventories) of your companies or clients, you really
want to know which mail and DNS servers and versions are running.
Having an accurate version number helps dramatically in
determining which exploits a server is vulnerable to. Version
detection helps you obtain this information.
After TCP and/or UDP ports are discovered using one of the other
scan methods, version detection interrogates those ports to
determine more about what is actually running. The
nmap-service-probes database contains probes for querying various
services and match expressions to recognize and parse responses.
Nmap tries to determine the service protocol (e.g. FTP, SSH,
Telnet, HTTP), the application name (e.g. ISC BIND, Apache httpd,
Solaris telnetd), the version number, hostname, device type (e.g.
printer, router), the OS family (e.g. Windows, Linux). When
possible, Nmap also gets the Common Platform Enumeration (CPE)
representation of this information. Sometimes miscellaneous
details like whether an X server is open to connections, the SSH
protocol version, or the KaZaA user name, are available. Of
course, most services don't provide all of this information. If
Nmap was compiled with OpenSSL support, it will connect to SSL
servers to deduce the service listening behind that encryption
layer. Some UDP ports are left in the open|filtered state after
a UDP port scan is unable to determine whether the port is open
or filtered. Version detection will try to elicit a response from
these ports (just as it does with open ports), and change the
state to open if it succeeds. open|filtered TCP ports are
treated the same way. Note that the Nmap -A option enables
version detection among other things. A paper documenting the
workings, usage, and customization of version detection is
available at https://nmap.org/book/vscan.html .
When RPC services are discovered, the Nmap RPC grinder is
automatically used to determine the RPC program and version
numbers. It takes all the TCP/UDP ports detected as RPC and
floods them with SunRPC program NULL commands in an attempt to
determine whether they are RPC ports, and if so, what program and
version number they serve up. Thus you can effectively obtain the
same info as rpcinfo -p even if the target's portmapper is behind
a firewall (or protected by TCP wrappers). Decoys do not
currently work with RPC scan.
When Nmap receives responses from a service but cannot match them
to its database, it prints out a special fingerprint and a URL
for you to submit it to if you know for sure what is running on
the port. Please take a couple minutes to make the submission so
that your find can benefit everyone. Thanks to these submissions,
Nmap has about 6,500 pattern matches for more than 650 protocols
such as SMTP, FTP, HTTP, etc.
Version detection is enabled and controlled with the following
options:
-sV (Version detection)
Enables version detection, as discussed above. Alternatively,
you can use -A, which enables version detection among other
things.
-sR is an alias for -sV. Prior to March 2011, it was used to
active the RPC grinder separately from version detection, but
now these options are always combined.
--allports (Don't exclude any ports from version detection)
By default, Nmap version detection skips TCP port 9100
because some printers simply print anything sent to that
port, leading to dozens of pages of HTTP GET requests, binary
SSL session requests, etc. This behavior can be changed by
modifying or removing the Exclude directive in
nmap-service-probes, or you can specify --allports to scan
all ports regardless of any Exclude directive.
--version-intensity intensity (Set version scan intensity)
When performing a version scan (-sV), Nmap sends a series of
probes, each of which is assigned a rarity value between one
and nine. The lower-numbered probes are effective against a
wide variety of common services, while the higher-numbered
ones are rarely useful. The intensity level specifies which
probes should be applied. The higher the number, the more
likely it is the service will be correctly identified.
However, high intensity scans take longer. The intensity must
be between 0 and 9. The default is 7. When a probe is
registered to the target port via the nmap-service-probes
ports directive, that probe is tried regardless of intensity
level. This ensures that the DNS probes will always be
attempted against any open port 53, the SSL probe will be
done against 443, etc.
--version-light (Enable light mode)
This is a convenience alias for --version-intensity 2. This
light mode makes version scanning much faster, but it is
slightly less likely to identify services.
--version-all (Try every single probe)
An alias for --version-intensity 9, ensuring that every
single probe is attempted against each port.
--version-trace (Trace version scan activity)
This causes Nmap to print out extensive debugging info about
what version scanning is doing. It is a subset of what you
get with --packet-trace.
One of Nmap's best-known features is remote OS detection using
TCP/IP stack fingerprinting. Nmap sends a series of TCP and UDP
packets to the remote host and examines practically every bit in
the responses. After performing dozens of tests such as TCP ISN
sampling, TCP options support and ordering, IP ID sampling, and
the initial window size check, Nmap compares the results to its
nmap-os-db database of more than 2,600 known OS fingerprints and
prints out the OS details if there is a match. Each fingerprint
includes a freeform textual description of the OS, and a
classification which provides the vendor name (e.g. Sun),
underlying OS (e.g. Solaris), OS generation (e.g. 10), and device
type (general purpose, router, switch, game console, etc). Most
fingerprints also have a Common Platform Enumeration (CPE)
representation, like cpe:/o:linux:linux_kernel:2.6.
If Nmap is unable to guess the OS of a machine, and conditions
are good (e.g. at least one open port and one closed port were
found), Nmap will provide a URL you can use to submit the
fingerprint if you know (for sure) the OS running on the machine.
By doing this you contribute to the pool of operating systems
known to Nmap and thus it will be more accurate for everyone.
OS detection enables some other tests which make use of
information that is gathered during the process anyway. One of
these is TCP Sequence Predictability Classification. This
measures approximately how hard it is to establish a forged TCP
connection against the remote host. It is useful for exploiting
source-IP based trust relationships (rlogin, firewall filters,
etc) or for hiding the source of an attack. This sort of spoofing
is rarely performed any more, but many machines are still
vulnerable to it. The actual difficulty number is based on
statistical sampling and may fluctuate. It is generally better to
use the English classification such as “worthy challenge” or
“trivial joke”. This is only reported in normal output in verbose
(-v) mode. When verbose mode is enabled along with -O, IP ID
sequence generation is also reported. Most machines are in the
“incremental” class, which means that they increment the ID field
in the IP header for each packet they send. This makes them
vulnerable to several advanced information gathering and spoofing
attacks.
Another bit of extra information enabled by OS detection is a
guess at a target's uptime. This uses the TCP timestamp option
(RFC 1323[9]) to guess when a machine was last rebooted. The
guess can be inaccurate due to the timestamp counter not being
initialized to zero or the counter overflowing and wrapping
around, so it is printed only in verbose mode.
A paper documenting the workings, usage, and customization of OS
detection is available at https://nmap.org/book/osdetect.html .
OS detection is enabled and controlled with the following
options:
-O (Enable OS detection)
Enables OS detection, as discussed above. Alternatively, you
can use -A to enable OS detection along with other things.
--osscan-limit (Limit OS detection to promising targets)
OS detection is far more effective if at least one open and
one closed TCP port are found. Set this option and Nmap will
not even try OS detection against hosts that do not meet this
criteria. This can save substantial time, particularly on -Pn
scans against many hosts. It only matters when OS detection
is requested with -O or -A.
--osscan-guess; --fuzzy (Guess OS detection results)
When Nmap is unable to detect a perfect OS match, it
sometimes offers up near-matches as possibilities. The match
has to be very close for Nmap to do this by default. Either
of these (equivalent) options make Nmap guess more
aggressively. Nmap will still tell you when an imperfect
match is printed and display its confidence level
(percentage) for each guess.
--max-os-tries (Set the maximum number of OS detection tries
against a target)
When Nmap performs OS detection against a target and fails to
find a perfect match, it usually repeats the attempt. By
default, Nmap tries five times if conditions are favorable
for OS fingerprint submission, and twice when conditions
aren't so good. Specifying a lower --max-os-tries value (such
as 1) speeds Nmap up, though you miss out on retries which
could potentially identify the OS. Alternatively, a high
value may be set to allow even more retries when conditions
are favorable. This is rarely done, except to generate better
fingerprints for submission and integration into the Nmap OS
database.
The Nmap Scripting Engine (NSE) is one of Nmap's most powerful
and flexible features. It allows users to write (and share)
simple scripts (using the Lua programming language[10]
) to automate a wide variety of networking tasks. Those scripts
are executed in parallel with the speed and efficiency you expect
from Nmap. Users can rely on the growing and diverse set of
scripts distributed with Nmap, or write their own to meet custom
needs.
Tasks we had in mind when creating the system include network
discovery, more sophisticated version detection, vulnerability
detection. NSE can even be used for vulnerability exploitation.
To reflect those different uses and to simplify the choice of
which scripts to run, each script contains a field associating it
with one or more categories. Currently defined categories are
auth, broadcast, default. discovery, dos, exploit, external,
fuzzer, intrusive, malware, safe, version, and vuln. These are
all described at
https://nmap.org/book/nse-usage.html#nse-categories .
Scripts are not run in a sandbox and thus could accidentally or
maliciously damage your system or invade your privacy. Never run
scripts from third parties unless you trust the authors or have
carefully audited the scripts yourself.
The Nmap Scripting Engine is described in detail at
https://nmap.org/book/nse.html
and is controlled by the following options:
-sC
Performs a script scan using the default set of scripts. It
is equivalent to --script=default. Some of the scripts in
this category are considered intrusive and should not be run
against a target network without permission.
--script filename|category|directory/|expression[,...]
Runs a script scan using the comma-separated list of
filenames, script categories, and directories. Each element
in the list may also be a Boolean expression describing a
more complex set of scripts. Each element is interpreted
first as an expression, then as a category, and finally as a
file or directory name.
There are two special features for advanced users only. One
is to prefix script names and expressions with + to force
them to run even if they normally wouldn't (e.g. the relevant
service wasn't detected on the target port). The other is
that the argument all may be used to specify every script in
Nmap's database. Be cautious with this because NSE contains
dangerous scripts such as exploits, brute force
authentication crackers, and denial of service attacks.
File and directory names may be relative or absolute.
Absolute names are used directly. Relative paths are looked
for in the scripts of each of the following places until
found:
--datadir
$NMAPDIR
~/.nmap (not searched on Windows)
APPDATA\nmap (only on Windows)
the directory containing the nmap executable
the directory containing the nmap executable, followed by
../share/nmap (not searched on Windows)
NMAPDATADIR (not searched on Windows)
the current directory.
When a directory name ending in / is given, Nmap loads every
file in the directory whose name ends with .nse. All other
files are ignored and directories are not searched
recursively. When a filename is given, it does not have to
have the .nse extension; it will be added automatically if
necessary. Nmap scripts are stored in a scripts subdirectory
of the Nmap data directory by default (see
https://nmap.org/book/data-files.html ).
For efficiency, scripts are indexed in a database stored in
scripts/script.db, which lists the category or categories in
which each script belongs. When referring to scripts from
script.db by name, you can use a shell-style ‘*’ wildcard.
nmap --script "http-*"
Loads all scripts whose name starts with http-, such as
http-auth and http-open-proxy. The argument to --script
had to be in quotes to protect the wildcard from the
shell.
More complicated script selection can be done using the and,
or, and not operators to build Boolean expressions. The
operators have the same precedence[11] as in Lua: not is the
highest, followed by and and then or. You can alter
precedence by using parentheses. Because expressions contain
space characters it is necessary to quote them.
nmap --script "not intrusive"
Loads every script except for those in the intrusive
category.
nmap --script "default or safe"
This is functionally equivalent to nmap --script
"default,safe". It loads all scripts that are in the
default category or the safe category or both.
nmap --script "default and safe"
Loads those scripts that are in both the default and safe
categories.
nmap --script "(default or safe or intrusive) and not http-*"
Loads scripts in the default, safe, or intrusive
categories, except for those whose names start with
http-.
--script-args n1=v1,n2={n3=v3},n4={v4,v5}
Lets you provide arguments to NSE scripts. Arguments are a
comma-separated list of name=value pairs. Names and values
may be strings not containing whitespace or the characters
‘{’, ‘}’, ‘=’, or ‘,’. To include one of these characters in
a string, enclose the string in single or double quotes.
Within a quoted string, ‘\’ escapes a quote. A backslash is
only used to escape quotation marks in this special case; in
all other cases a backslash is interpreted literally. Values
may also be tables enclosed in {}, just as in Lua. A table
may contain simple string values or more name-value pairs,
including nested tables. Many scripts qualify their arguments
with the script name, as in xmpp-info.server_name. You may
use that full qualified version to affect just the specified
script, or you may pass the unqualified version (server_name
in this case) to affect all scripts using that argument name.
A script will first check for its fully qualified argument
name (the name specified in its documentation) before it
accepts an unqualified argument name. A complex example of
script arguments is --script-args
'user=foo,pass=",{}=bar",whois={whodb=nofollow+ripe},xmpp-info.server_name=localhost'.
The online NSE Documentation Portal at
https://nmap.org/nsedoc/ lists the arguments that each script
accepts.
--script-args-file filename
Lets you load arguments to NSE scripts from a file. Any
arguments on the command line supersede ones in the file. The
file can be an absolute path, or a path relative to Nmap's
usual search path (NMAPDIR, etc.) Arguments can be
comma-separated or newline-separated, but otherwise follow
the same rules as for --script-args, without requiring
special quoting and escaping, since they are not parsed by
the shell.
--script-help filename|category|directory|expression|all[,...]
Shows help about scripts. For each script matching the given
specification, Nmap prints the script name, its categories,
and its description. The specifications are the same as those
accepted by --script; so for example if you want help about
the ftp-anon script, you would run nmap --script-help
ftp-anon. In addition to getting help for individual scripts,
you can use this as a preview of what scripts will be run for
a specification, for example with nmap --script-help default.
--script-trace
This option does what --packet-trace does, just one ISO layer
higher. If this option is specified all incoming and outgoing
communication performed by a script is printed. The displayed
information includes the communication protocol, the source,
the target and the transmitted data. If more than 5% of all
transmitted data is not printable, then the trace output is
in a hex dump format. Specifying --packet-trace enables
script tracing too.
--script-updatedb
This option updates the script database found in
scripts/script.db which is used by Nmap to determine the
available default scripts and categories. It is only
necessary to update the database if you have added or removed
NSE scripts from the default scripts directory or if you have
changed the categories of any script. This option is
generally used by itself: nmap --script-updatedb.
One of my highest Nmap development priorities has always been
performance. A default scan (nmap hostname) of a host on my local
network takes a fifth of a second. That is barely enough time to
blink, but adds up when you are scanning hundreds or thousands of
hosts. Moreover, certain scan options such as UDP scanning and
version detection can increase scan times substantially. So can
certain firewall configurations, particularly response rate
limiting. While Nmap utilizes parallelism and many advanced
algorithms to accelerate these scans, the user has ultimate
control over how Nmap runs. Expert users carefully craft Nmap
commands to obtain only the information they care about while
meeting their time constraints.
Techniques for improving scan times include omitting non-critical
tests, and upgrading to the latest version of Nmap (performance
enhancements are made frequently). Optimizing timing parameters
can also make a substantial difference. Those options are listed
below.
Some options accept a time parameter. This is specified in
seconds by default, though you can append ‘ms’, ‘s’, ‘m’, or ‘h’
to the value to specify milliseconds, seconds, minutes, or hours.
So the --host-timeout arguments 900000ms, 900, 900s, and 15m all
do the same thing.
--min-hostgroup numhosts; --max-hostgroup numhosts (Adjust
parallel scan group sizes)
Nmap has the ability to port scan or version scan multiple
hosts in parallel. Nmap does this by dividing the target IP
space into groups and then scanning one group at a time. In
general, larger groups are more efficient. The downside is
that host results can't be provided until the whole group is
finished. So if Nmap started out with a group size of 50, the
user would not receive any reports (except for the updates
offered in verbose mode) until the first 50 hosts are
completed.
By default, Nmap takes a compromise approach to this
conflict. It starts out with a group size as low as five so
the first results come quickly and then increases the
groupsize to as high as 1024. The exact default numbers
depend on the options given. For efficiency reasons, Nmap
uses larger group sizes for UDP or few-port TCP scans.
When a maximum group size is specified with --max-hostgroup,
Nmap will never exceed that size. Specify a minimum size with
--min-hostgroup and Nmap will try to keep group sizes above
that level. Nmap may have to use smaller groups than you
specify if there are not enough target hosts left on a given
interface to fulfill the specified minimum. Both may be set
to keep the group size within a specific range, though this
is rarely desired.
These options do not have an effect during the host discovery
phase of a scan. This includes plain ping scans (-sn). Host
discovery always works in large groups of hosts to improve
speed and accuracy.
The primary use of these options is to specify a large
minimum group size so that the full scan runs more quickly. A
common choice is 256 to scan a network in /24 sized chunks.
For a scan with many ports, exceeding that number is unlikely
to help much. For scans of just a few port numbers, host
group sizes of 2048 or more may be helpful.
--min-parallelism numprobes; --max-parallelism numprobes (Adjust
probe parallelization)
These options control the total number of probes that may be
outstanding for a host group. They are used for port scanning
and host discovery. By default, Nmap calculates an
ever-changing ideal parallelism based on network performance.
If packets are being dropped, Nmap slows down and allows
fewer outstanding probes. The ideal probe number slowly rises
as the network proves itself worthy. These options place
minimum or maximum bounds on that variable. By default, the
ideal parallelism can drop to one if the network proves
unreliable and rise to several hundred in perfect conditions.
The most common usage is to set --min-parallelism to a number
higher than one to speed up scans of poorly performing hosts
or networks. This is a risky option to play with, as setting
it too high may affect accuracy. Setting this also reduces
Nmap's ability to control parallelism dynamically based on
network conditions. A value of 10 might be reasonable, though
I only adjust this value as a last resort.
The --max-parallelism option is sometimes set to one to
prevent Nmap from sending more than one probe at a time to
hosts. The --scan-delay option, discussed later, is another
way to do this.
--min-rtt-timeout time, --max-rtt-timeout time,
--initial-rtt-timeout time (Adjust probe timeouts)
Nmap maintains a running timeout value for determining how
long it will wait for a probe response before giving up or
retransmitting the probe. This is calculated based on the
response times of previous probes.
If the network latency shows itself to be significant and
variable, this timeout can grow to several seconds. It also
starts at a conservative (high) level and may stay that way
for a while when Nmap scans unresponsive hosts.
Specifying a lower --max-rtt-timeout and
--initial-rtt-timeout than the defaults can cut scan times
significantly. This is particularly true for pingless (-Pn)
scans, and those against heavily filtered networks. Don't get
too aggressive though. The scan can end up taking longer if
you specify such a low value that many probes are timing out
and retransmitting while the response is in transit.
If all the hosts are on a local network, 100 milliseconds
(--max-rtt-timeout 100ms) is a reasonable aggressive value.
If routing is involved, ping a host on the network first with
the ICMP ping utility, or with a custom packet crafter such
as Nping that is more likely to get through a firewall. Look
at the maximum round trip time out of ten packets or so. You
might want to double that for the --initial-rtt-timeout and
triple or quadruple it for the --max-rtt-timeout. I generally
do not set the maximum RTT below 100 ms, no matter what the
ping times are. Nor do I exceed 1000 ms.
--min-rtt-timeout is a rarely used option that could be
useful when a network is so unreliable that even Nmap's
default is too aggressive. Since Nmap only reduces the
timeout down to the minimum when the network seems to be
reliable, this need is unusual and should be reported as a
bug to the nmap-dev mailing list.
--max-retries numtries (Specify the maximum number of port scan
probe retransmissions)
When Nmap receives no response to a port scan probe, it could
mean the port is filtered. Or maybe the probe or response was
simply lost on the network. It is also possible that the
target host has rate limiting enabled that temporarily
blocked the response. So Nmap tries again by retransmitting
the initial probe. If Nmap detects poor network reliability,
it may try many more times before giving up on a port. While
this benefits accuracy, it also lengthens scan times. When
performance is critical, scans may be sped up by limiting the
number of retransmissions allowed. You can even specify
--max-retries 0 to prevent any retransmissions, though that
is only recommended for situations such as informal surveys
where occasional missed ports and hosts are acceptable.
The default (with no -T template) is to allow ten
retransmissions. If a network seems reliable and the target
hosts aren't rate limiting, Nmap usually only does one
retransmission. So most target scans aren't even affected by
dropping --max-retries to a low value such as three. Such
values can substantially speed scans of slow (rate limited)
hosts. You usually lose some information when Nmap gives up
on ports early, though that may be preferable to letting the
--host-timeout expire and losing all information about the
target.
--host-timeout time (Give up on slow target hosts)
Some hosts simply take a long time to scan. This may be due
to poorly performing or unreliable networking hardware or
software, packet rate limiting, or a restrictive firewall.
The slowest few percent of the scanned hosts can eat up a
majority of the scan time. Sometimes it is best to cut your
losses and skip those hosts initially. Specify --host-timeout
with the maximum amount of time you are willing to wait. For
example, specify 30m to ensure that Nmap doesn't waste more
than half an hour on a single host. Note that Nmap may be
scanning other hosts at the same time during that half an
hour, so it isn't a complete loss. A host that times out is
skipped. No port table, OS detection, or version detection
results are printed for that host.
The special value 0 can be used to mean “no timeout”, which
can be used to override the T5 timing template, which sets
the host timeout to 15 minutes.
--script-timeout time
While some scripts complete in fractions of a second, others
can take hours or more depending on the nature of the script,
arguments passed in, network and application conditions, and
more. The --script-timeout option sets a ceiling on script
execution time. Any script instance which exceeds that time
will be terminated and no output will be shown. If debugging
(-d) is enabled, Nmap will report on each timeout. For host
and service scripts, a script instance only scans a single
target host or port and the timeout period will be reset for
the next instance.
The special value 0 can be used to mean “no timeout”, which
can be used to override the T5 timing template, which sets
the script timeout to 10 minutes.
--scan-delay time; --max-scan-delay time (Adjust delay between
probes)
This option causes Nmap to wait at least the given amount of
time between each probe it sends to a given host. This is
particularly useful in the case of rate limiting. Solaris
machines (among many others) will usually respond to UDP scan
probe packets with only one ICMP message per second. Any more
than that sent by Nmap will be wasteful. A --scan-delay of 1s
will keep Nmap at that slow rate. Nmap tries to detect rate
limiting and adjust the scan delay accordingly, but it
doesn't hurt to specify it explicitly if you already know
what rate works best.
When Nmap adjusts the scan delay upward to cope with rate
limiting, the scan slows down dramatically. The
--max-scan-delay option specifies the largest delay that Nmap
will allow. A low --max-scan-delay can speed up Nmap, but it
is risky. Setting this value too low can lead to wasteful
packet retransmissions and possible missed ports when the
target implements strict rate limiting.
Another use of --scan-delay is to evade threshold based
intrusion detection and prevention systems (IDS/IPS).
--min-rate number; --max-rate number (Directly control the
scanning rate)
Nmap's dynamic timing does a good job of finding an
appropriate speed at which to scan. Sometimes, however, you
may happen to know an appropriate scanning rate for a
network, or you may have to guarantee that a scan will be
finished by a certain time. Or perhaps you must keep Nmap
from scanning too quickly. The --min-rate and --max-rate
options are designed for these situations.
When the --min-rate option is given Nmap will do its best to
send packets as fast as or faster than the given rate. The
argument is a positive real number representing a packet rate
in packets per second. For example, specifying --min-rate 300
means that Nmap will try to keep the sending rate at or above
300 packets per second. Specifying a minimum rate does not
keep Nmap from going faster if conditions warrant.
Likewise, --max-rate limits a scan's sending rate to a given
maximum. Use --max-rate 100, for example, to limit sending to
100 packets per second on a fast network. Use --max-rate 0.1
for a slow scan of one packet every ten seconds. Use
--min-rate and --max-rate together to keep the rate inside a
certain range.
These two options are global, affecting an entire scan, not
individual hosts. They only affect port scans and host
discovery scans. Other features like OS detection implement
their own timing.
There are two conditions when the actual scanning rate may
fall below the requested minimum. The first is if the minimum
is faster than the fastest rate at which Nmap can send, which
is dependent on hardware. In this case Nmap will simply send
packets as fast as possible, but be aware that such high
rates are likely to cause a loss of accuracy. The second case
is when Nmap has nothing to send, for example at the end of a
scan when the last probes have been sent and Nmap is waiting
for them to time out or be responded to. It's normal to see
the scanning rate drop at the end of a scan or in between
hostgroups. The sending rate may temporarily exceed the
maximum to make up for unpredictable delays, but on average
the rate will stay at or below the maximum.
Specifying a minimum rate should be done with care. Scanning
faster than a network can support may lead to a loss of
accuracy. In some cases, using a faster rate can make a scan
take longer than it would with a slower rate. This is because
Nmap's adaptive retransmission algorithms will detect the
network congestion caused by an excessive scanning rate and
increase the number of retransmissions in order to improve
accuracy. So even though packets are sent at a higher rate,
more packets are sent overall. Cap the number of
retransmissions with the --max-retries option if you need to
set an upper limit on total scan time.
--defeat-rst-ratelimit
Many hosts have long used rate limiting to reduce the number
of ICMP error messages (such as port-unreachable errors) they
send. Some systems now apply similar rate limits to the RST
(reset) packets they generate. This can slow Nmap down
dramatically as it adjusts its timing to reflect those rate
limits. You can tell Nmap to ignore those rate limits (for
port scans such as SYN scan which don't treat non-responsive
ports as open) by specifying --defeat-rst-ratelimit.
Using this option can reduce accuracy, as some ports will
appear non-responsive because Nmap didn't wait long enough
for a rate-limited RST response. With a SYN scan, the
non-response results in the port being labeled filtered
rather than the closed state we see when RST packets are
received. This option is useful when you only care about open
ports, and distinguishing between closed and filtered ports
isn't worth the extra time.
--defeat-icmp-ratelimit
Similar to --defeat-rst-ratelimit, the
--defeat-icmp-ratelimit option trades accuracy for speed,
increasing UDP scanning speed against hosts that rate-limit
ICMP error messages. Because this option causes Nmap to not
delay in order to receive the port unreachable messages, a
non-responsive port will be labeled closed|filtered instead
of the default open|filtered. This has the effect of only
treating ports which actually respond via UDP as open. Since
many UDP services do not respond in this way, the chance for
inaccuracy is greater with this option than with
--defeat-rst-ratelimit.
--nsock-engine iocp|epoll|kqueue|poll|select
Enforce use of a given nsock IO multiplexing engine. Only the
select(2)-based fallback engine is guaranteed to be available
on your system. Engines are named after the name of the IO
management facility they leverage. Engines currently
implemented are epoll, kqueue, poll, and select, but not all
will be present on any platform. By default, Nmap will use
the "best" engine, i.e. the first one in this list that is
supported. Use nmap -V to see which engines are supported on
your platform.
-T paranoid|sneaky|polite|normal|aggressive|insane (Set a timing
template)
While the fine-grained timing controls discussed in the
previous section are powerful and effective, some people find
them confusing. Moreover, choosing the appropriate values can
sometimes take more time than the scan you are trying to
optimize. Fortunately, Nmap offers a simpler approach, with
six timing templates. You can specify them with the -T option
and their number (0–5) or their name. The template names are
paranoid (0), sneaky (1), polite (2), normal (3),
aggressive (4), and insane (5). The first two are for IDS
evasion. Polite mode slows down the scan to use less
bandwidth and target machine resources. Normal mode is the
default and so -T3 does nothing. Aggressive mode speeds scans
up by making the assumption that you are on a reasonably fast
and reliable network. Finally insane mode assumes that you
are on an extraordinarily fast network or are willing to
sacrifice some accuracy for speed.
These templates allow the user to specify how aggressive they
wish to be, while leaving Nmap to pick the exact timing
values. The templates also make some minor speed adjustments
for which fine-grained control options do not currently
exist. For example, -T4 prohibits the dynamic scan delay from
exceeding 10 ms for TCP ports and -T5 caps that value at
5 ms. Templates can be used in combination with fine-grained
controls, and the fine-grained controls that you specify will
take precedence over the timing template default for that
parameter. I recommend using -T4 when scanning reasonably
modern and reliable networks. Keep that option even when you
add fine-grained controls so that you benefit from those
extra minor optimizations that it enables.
If you are on a decent broadband or ethernet connection, I
would recommend always using -T4. Some people love -T5 though
it is too aggressive for my taste. People sometimes specify
-T2 because they think it is less likely to crash hosts or
because they consider themselves to be polite in general.
They often don't realize just how slow -T polite really is.
Their scan may take ten times longer than a default scan.
Machine crashes and bandwidth problems are rare with the
default timing options (-T3) and so I normally recommend that
for cautious scanners. Omitting version detection is far more
effective than playing with timing values at reducing these
problems.
While -T0 and -T1 may be useful for avoiding IDS alerts, they
will take an extraordinarily long time to scan thousands of
machines or ports. For such a long scan, you may prefer to
set the exact timing values you need rather than rely on the
canned -T0 and -T1 values.
The main effects of T0 are serializing the scan so only one
port is scanned at a time, and waiting five minutes between
sending each probe. T1 and T2 are similar but they only wait
15 seconds and 0.4 seconds, respectively, between probes. T3
is Nmap's default behavior, which includes parallelization.
-T4 does the equivalent of
--max-rtt-timeout 1250ms --min-rtt-timeout 100ms
--initial-rtt-timeout 500ms --max-retries 6 and sets the
maximum TCP and SCTP scan delay to 10ms. T5 does the
equivalent of
--max-rtt-timeout 300ms --min-rtt-timeout 50ms
--initial-rtt-timeout 250ms --max-retries 2 --host-timeout
15m --script-timeout 10m as well as setting the maximum TCP
and SCTP scan delay to 5ms. Maximum UDP scan delay is not set
by T4 or T5, but it can be set with the --max-scan-delay
option.
Many Internet pioneers envisioned a global open network with a
universal IP address space allowing virtual connections between
any two nodes. This allows hosts to act as true peers, serving
and retrieving information from each other. People could access
all of their home systems from work, changing the climate control
settings or unlocking the doors for early guests. This vision of
universal connectivity has been stifled by address space
shortages and security concerns. In the early 1990s,
organizations began deploying firewalls for the express purpose
of reducing connectivity. Huge networks were cordoned off from
the unfiltered Internet by application proxies, network address
translation, and packet filters. The unrestricted flow of
information gave way to tight regulation of approved
communication channels and the content that passes over them.
Network obstructions such as firewalls can make mapping a network
exceedingly difficult. It will not get any easier, as stifling
casual reconnaissance is often a key goal of implementing the
devices. Nevertheless, Nmap offers many features to help
understand these complex networks, and to verify that filters are
working as intended. It even supports mechanisms for bypassing
poorly implemented defenses. One of the best methods of
understanding your network security posture is to try to defeat
it. Place yourself in the mind-set of an attacker, and deploy
techniques from this section against your networks. Launch an FTP
bounce scan, idle scan, fragmentation attack, or try to tunnel
through one of your own proxies.
In addition to restricting network activity, companies are
increasingly monitoring traffic with intrusion detection systems
(IDS). All of the major IDSs ship with rules designed to detect
Nmap scans because scans are sometimes a precursor to attacks.
Many of these products have recently morphed into intrusion
prevention systems (IPS) that actively block traffic deemed
malicious. Unfortunately for network administrators and IDS
vendors, reliably detecting bad intentions by analyzing packet
data is a tough problem. Attackers with patience, skill, and the
help of certain Nmap options can usually pass by IDSs undetected.
Meanwhile, administrators must cope with large numbers of false
positive results where innocent activity is misdiagnosed and
alerted on or blocked.
Occasionally people suggest that Nmap should not offer features
for evading firewall rules or sneaking past IDSs. They argue that
these features are just as likely to be misused by attackers as
used by administrators to enhance security. The problem with this
logic is that these methods would still be used by attackers, who
would just find other tools or patch the functionality into Nmap.
Meanwhile, administrators would find it that much harder to do
their jobs. Deploying only modern, patched FTP servers is a far
more powerful defense than trying to prevent the distribution of
tools implementing the FTP bounce attack.
There is no magic bullet (or Nmap option) for detecting and
subverting firewalls and IDS systems. It takes skill and
experience. A tutorial is beyond the scope of this reference
guide, which only lists the relevant options and describes what
they do.
-f (fragment packets); --mtu (using the specified MTU)
The -f option causes the requested scan (including host
discovery scans) to use tiny fragmented IP packets. The idea
is to split up the TCP header over several packets to make it
harder for packet filters, intrusion detection systems, and
other annoyances to detect what you are doing. Be careful
with this! Some programs have trouble handling these tiny
packets. The old-school sniffer named Sniffit segmentation
faulted immediately upon receiving the first fragment.
Specify this option once, and Nmap splits the packets into
eight bytes or less after the IP header. So a 20-byte TCP
header would be split into three packets. Two with eight
bytes of the TCP header, and one with the final four. Of
course each fragment also has an IP header. Specify -f again
to use 16 bytes per fragment (reducing the number of
fragments). Or you can specify your own offset size with the
--mtu option. Don't also specify -f if you use --mtu. The
offset must be a multiple of eight. While fragmented packets
won't get by packet filters and firewalls that queue all IP
fragments, such as the CONFIG_IP_ALWAYS_DEFRAG option in the
Linux kernel, some networks can't afford the performance hit
this causes and thus leave it disabled. Others can't enable
this because fragments may take different routes into their
networks. Some source systems defragment outgoing packets in
the kernel. Linux with the iptables connection tracking
module is one such example. Do a scan while a sniffer such as
Wireshark is running to ensure that sent packets are
fragmented. If your host OS is causing problems, try the
--send-eth option to bypass the IP layer and send raw
ethernet frames.
Fragmentation is only supported for Nmap's raw packet
features, which includes TCP and UDP port scans (except
connect scan and FTP bounce scan) and OS detection. Features
such as version detection and the Nmap Scripting Engine
generally don't support fragmentation because they rely on
your host's TCP stack to communicate with target services.
-D decoy1[,decoy2][,ME][,...] (Cloak a scan with decoys)
Causes a decoy scan to be performed, which makes it appear to
the remote host that the host(s) you specify as decoys are
scanning the target network too. Thus their IDS might report
5–10 port scans from unique IP addresses, but they won't know
which IP was scanning them and which were innocent decoys.
While this can be defeated through router path tracing,
response-dropping, and other active mechanisms, it is
generally an effective technique for hiding your IP address.
Separate each decoy host with commas, and you can optionally
use ME as one of the decoys to represent the position for
your real IP address. If you put ME in the sixth position or
later, some common port scan detectors (such as Solar
Designer's excellent Scanlogd) are unlikely to show your IP
address at all. If you don't use ME, Nmap will put you in a
random position. You can also use RND to generate a random,
non-reserved IP address, or RND:number to generate number
addresses.
Note that the hosts you use as decoys should be up or you
might accidentally SYN flood your targets. Also it will be
pretty easy to determine which host is scanning if only one
is actually up on the network. You might want to use IP
addresses instead of names (so the decoy networks don't see
you in their nameserver logs). Right now random IP address
generation is only supported with IPv4
Decoys are used both in the initial host discovery scan
(using ICMP, SYN, ACK, or whatever) and during the actual
port scanning phase. Decoys are also used during remote OS
detection (-O). Decoys do not work with version detection or
TCP connect scan. When a scan delay is in effect, the delay
is enforced between each batch of spoofed probes, not between
each individual probe. Because decoys are sent as a batch all
at once, they may temporarily violate congestion control
limits.
It is worth noting that using too many decoys may slow your
scan and potentially even make it less accurate. Also, some
ISPs will filter out your spoofed packets, but many do not
restrict spoofed IP packets at all.
-S IP_Address (Spoof source address)
In some circumstances, Nmap may not be able to determine your
source address (Nmap will tell you if this is the case). In
this situation, use -S with the IP address of the interface
you wish to send packets through.
Another possible use of this flag is to spoof the scan to
make the targets think that someone else is scanning them.
Imagine a company being repeatedly port scanned by a
competitor! The -e option and -Pn are generally required for
this sort of usage. Note that you usually won't receive reply
packets back (they will be addressed to the IP you are
spoofing), so Nmap won't produce useful reports.
-e interface (Use specified interface)
Tells Nmap what interface to send and receive packets on.
Nmap should be able to detect this automatically, but it will
tell you if it cannot.
--source-port portnumber; -g portnumber (Spoof source port
number)
One surprisingly common misconfiguration is to trust traffic
based only on the source port number. It is easy to
understand how this comes about. An administrator will set up
a shiny new firewall, only to be flooded with complaints from
ungrateful users whose applications stopped working. In
particular, DNS may be broken because the UDP DNS replies
from external servers can no longer enter the network. FTP is
another common example. In active FTP transfers, the remote
server tries to establish a connection back to the client to
transfer the requested file.
Secure solutions to these problems exist, often in the form
of application-level proxies or protocol-parsing firewall
modules. Unfortunately there are also easier, insecure
solutions. Noting that DNS replies come from port 53 and
active FTP from port 20, many administrators have fallen into
the trap of simply allowing incoming traffic from those
ports. They often assume that no attacker would notice and
exploit such firewall holes. In other cases, administrators
consider this a short-term stop-gap measure until they can
implement a more secure solution. Then they forget the
security upgrade.
Overworked network administrators are not the only ones to
fall into this trap. Numerous products have shipped with
these insecure rules. Even Microsoft has been guilty. The
IPsec filters that shipped with Windows 2000 and Windows XP
contain an implicit rule that allows all TCP or UDP traffic
from port 88 (Kerberos). In another well-known case, versions
of the Zone Alarm personal firewall up to 2.1.25 allowed any
incoming UDP packets with the source port 53 (DNS) or 67
(DHCP).
Nmap offers the -g and --source-port options (they are
equivalent) to exploit these weaknesses. Simply provide a
port number and Nmap will send packets from that port where
possible. Most scanning operations that use raw sockets,
including SYN and UDP scans, support the option completely.
The option notably doesn't have an effect for any operations
that use normal operating system sockets, including DNS
requests, TCP connect scan, version detection, and script
scanning. Setting the source port also doesn't work for OS
detection, because Nmap must use different port numbers for
certain OS detection tests to work properly.
--data hex string (Append custom binary data to sent packets)
This option lets you include binary data as payload in sent
packets. hex string may be specified in any of the following
formats: 0xAABBCCDDEEFF..., AABBCCDDEEFF... or
\xAA\xBB\xCC\xDD\xEE\xFF.... Examples of use are --data
0xdeadbeef and --data \xCA\xFE\x09. Note that if you specify
a number like 0x00ff no byte-order conversion is performed.
Make sure you specify the information in the byte order
expected by the receiver.
--data-string string (Append custom string to sent packets)
This option lets you include a regular string as payload in
sent packets. string can contain any string. However, note
that some characters may depend on your system's locale and
the receiver may not see the same information. Also, make
sure you enclose the string in double quotes and escape any
special characters from the shell. Examples: --data-string
"Scan conducted by Security Ops, extension 7192" or
--data-string "Ph34r my l33t skills". Keep in mind that
nobody is likely to actually see any comments left by this
option unless they are carefully monitoring the network with
a sniffer or custom IDS rules.
--data-length number (Append random data to sent packets)
Normally Nmap sends minimalist packets containing only a
header. So its TCP packets are generally 40 bytes and ICMP
echo requests are just 28. Some UDP ports and IP protocols
get a custom payload by default. This option tells Nmap to
append the given number of random bytes to most of the
packets it sends, and not to use any protocol-specific
payloads. (Use --data-length 0 for no random or
protocol-specific payloads. OS detection (-O) packets are
not affected because accuracy there requires probe
consistency, but most pinging and portscan packets support
this. It slows things down a little, but can make a scan
slightly less conspicuous.
--ip-options R|S [route]|L [route]|T|U ... ; --ip-options hex
string (Send packets with specified ip options)
The IP protocol[12] offers several options which may be
placed in packet headers. Unlike the ubiquitous TCP options,
IP options are rarely seen due to practicality and security
concerns. In fact, many Internet routers block the most
dangerous options such as source routing. Yet options can
still be useful in some cases for determining and
manipulating the network route to target machines. For
example, you may be able to use the record route option to
determine a path to a target even when more traditional
traceroute-style approaches fail. Or if your packets are
being dropped by a certain firewall, you may be able to
specify a different route with the strict or loose source
routing options.
The most powerful way to specify IP options is to simply pass
in values as the argument to --ip-options. Precede each hex
number with \x then the two digits. You may repeat certain
characters by following them with an asterisk and then the
number of times you wish them to repeat. For example,
\x01\x07\x04\x00*36\x01 is a hex string containing 36 NUL
bytes.
Nmap also offers a shortcut mechanism for specifying options.
Simply pass the letter R, T, or U to request record-route,
record-timestamp, or both options together, respectively.
Loose or strict source routing may be specified with an L or
S followed by a space and then a space-separated list of IP
addresses.
If you wish to see the options in packets sent and received,
specify --packet-trace. For more information and examples of
using IP options with Nmap, see
https://seclists.org/nmap-dev/2006/q3/52 .
--ttl value (Set IP time-to-live field)
Sets the IPv4 time-to-live field in sent packets to the given
value.
--randomize-hosts (Randomize target host order)
Tells Nmap to shuffle each group of up to 16384 hosts before
it scans them. This can make the scans less obvious to
various network monitoring systems, especially when you
combine it with slow timing options. If you want to randomize
over larger group sizes, increase PING_GROUP_SZ in nmap.h and
recompile. An alternative solution is to generate the target
IP list with a list scan (-sL -n -oN filename), randomize it
with a Perl script, then provide the whole list to Nmap with
-iL.
--spoof-mac MAC address, prefix, or vendor name (Spoof MAC
address)
Asks Nmap to use the given MAC address
for all of the raw ethernet frames it sends. This option
implies --send-eth to ensure that Nmap actually sends
ethernet-level packets. The MAC given can take several
formats. If it is simply the number 0, Nmap chooses a
completely random MAC address for the session. If the given
string is an even number of hex digits (with the pairs
optionally separated by a colon), Nmap will use those as the
MAC. If fewer than 12 hex digits are provided, Nmap fills in
the remainder of the six bytes with random values. If the
argument isn't a zero or hex string, Nmap looks through
nmap-mac-prefixes to find a vendor name containing the given
string (it is case insensitive). If a match is found, Nmap
uses the vendor's OUI (three-byte prefix) and fills out the
remaining three bytes randomly. Valid --spoof-mac argument
examples are Apple, 0, 01:02:03:04:05:06, deadbeefcafe,
0020F2, and Cisco. This option only affects raw packet scans
such as SYN scan or OS detection, not connection-oriented
features such as version detection or the Nmap Scripting
Engine.
--proxies Comma-separated list of proxy URLs (Relay TCP
connections through a chain of proxies)
Asks Nmap to establish TCP connections with a final target
through supplied chain of one or more HTTP or SOCKS4 proxies.
Proxies can help hide the true source of a scan or evade
certain firewall restrictions, but they can hamper scan
performance by increasing latency. Users may need to adjust
Nmap timeouts and other scan parameters accordingly. In
particular, a lower --max-parallelism may help because some
proxies refuse to handle as many concurrent connections as
Nmap opens by default.
This option takes a list of proxies as argument, expressed as
URLs in the format proto://host:port. Use commas to separate
node URLs in a chain. No authentication is supported yet.
Valid protocols are HTTP and SOCKS4.
Warning: this feature is still under development and has
limitations. It is implemented within the nsock library and
thus has no effect on the ping, port scanning and OS
discovery phases of a scan. Only NSE and version scan benefit
from this option so far—other features may disclose your true
address. SSL connections are not yet supported, nor is
proxy-side DNS resolution (hostnames are always resolved by
Nmap).
--badsum (Send packets with bogus TCP/UDP checksums)
Asks Nmap to use an invalid TCP, UDP or SCTP checksum for
packets sent to target hosts. Since virtually all host IP
stacks properly drop these packets, any responses received
are likely coming from a firewall or IDS that didn't bother
to verify the checksum. For more details on this technique,
see https://nmap.org/p60-12.html
--adler32 (Use deprecated Adler32 instead of CRC32C for SCTP
checksums)
Asks Nmap to use the deprecated Adler32 algorithm for
calculating the SCTP checksum. If --adler32 is not given,
CRC-32C (Castagnoli) is used. RFC 2960[13] originally
defined Adler32 as checksum algorithm for SCTP; RFC 4960[6]
later redefined the SCTP checksums to use CRC-32C. Current
SCTP implementations should be using CRC-32C, but in order to
elicit responses from old, legacy SCTP implementations, it
may be preferable to use Adler32.
Any security tool is only as useful as the output it generates.
Complex tests and algorithms are of little value if they aren't
presented in an organized and comprehensible fashion. Given the
number of ways Nmap is used by people and other software, no
single format can please everyone. So Nmap offers several
formats, including the interactive mode for humans to read
directly and XML for easy parsing by software.
In addition to offering different output formats, Nmap provides
options for controlling the verbosity of output as well as
debugging messages. Output types may be sent to standard output
or to named files, which Nmap can append to or clobber. Output
files may also be used to resume aborted scans.
Nmap makes output available in five different formats. The
default is called interactive output, and it is sent to standard
output (stdout). There is also normal output, which is similar
to interactive except that it displays less runtime information
and warnings since it is expected to be analyzed after the scan
completes rather than interactively.
XML output is one of the most important output types, as it can
be converted to HTML, easily parsed by programs such as Nmap
graphical user interfaces, or imported into databases.
The two remaining output types are the simple grepable output
which includes most information for a target host on a single
line, and sCRiPt KiDDi3 0utPUt for users who consider themselves
|<-r4d.
While interactive output is the default and has no associated
command-line options, the other four format options use the same
syntax. They take one argument, which is the filename that
results should be stored in. Multiple formats may be specified,
but each format may only be specified once. For example, you may
wish to save normal output for your own review while saving XML
of the same scan for programmatic analysis. You might do this
with the options -oX myscan.xml -oN myscan.nmap. While this
chapter uses the simple names like myscan.xml for brevity, more
descriptive names are generally recommended. The names chosen are
a matter of personal preference, though I use long ones that
incorporate the scan date and a word or two describing the scan,
placed in a directory named after the company I'm scanning.
While these options save results to files, Nmap still prints
interactive output to stdout as usual. For example, the command
nmap -oX myscan.xml target prints XML to myscan.xml and fills
standard output with the same interactive results it would have
printed if -oX wasn't specified at all. You can change this by
passing a hyphen character as the argument to one of the format
types. This causes Nmap to deactivate interactive output, and
instead print results in the format you specified to the standard
output stream. So the command nmap -oX - target will send only
XML output to stdout. Serious errors may still be printed to the
normal error stream, stderr.
Unlike some Nmap arguments, the space between the logfile option
flag (such as -oX) and the filename or hyphen is mandatory. If
you omit the flags and give arguments such as -oG- or
-oXscan.xml, a backwards compatibility feature of Nmap will cause
the creation of normal format output files named G- and Xscan.xml
respectively.
All of these arguments support strftime-like conversions in the
filename. %H, %M, %S, %m, %d, %y, and %Y are all exactly the
same as in strftime. %T is the same as %H%M%S, %R is the same as
%H%M, and %D is the same as %m%d%y. A % followed by any other
character just yields that character (%% gives you a percent
symbol). So -oX 'scan-%T-%D.xml' will use an XML file with a name
in the form of scan-144840-121307.xml.
Nmap also offers options to control scan verbosity and to append
to output files rather than clobbering them. All of these options
are described below.
Nmap Output Formats
-oN filespec (normal output)
Requests that normal output be directed to the given
filename. As discussed above, this differs slightly from
interactive output.
-oX filespec (XML output)
Requests that XML output be directed to the given filename.
Nmap includes a document type definition (DTD) which allows
XML parsers to validate Nmap XML output. While it is
primarily intended for programmatic use, it can also help
humans interpret Nmap XML output. The DTD defines the legal
elements of the format, and often enumerates the attributes
and values they can take on. The latest version is always
available from https://svn.nmap.org/nmap/docs/nmap.dtd .
XML offers a stable format that is easily parsed by software.
Free XML parsers are available for all major computer
languages, including C/C++, Perl, Python, and Java. People
have even written bindings for most of these languages to
handle Nmap output and execution specifically. Examples are
Nmap::Scanner[14] and Nmap::Parser[15] in Perl CPAN. In
almost all cases that a non-trivial application interfaces
with Nmap, XML is the preferred format.
The XML output references an XSL stylesheet which can be used
to format the results as HTML. The easiest way to use this is
simply to load the XML output in a web browser such as
Firefox or IE. By default, this will only work on the machine
you ran Nmap on (or a similarly configured one) due to the
hard-coded nmap.xsl filesystem path. Use the --webxml or
--stylesheet options to create portable XML files that render
as HTML on any web-connected machine.
-oS filespec (ScRipT KIdd|3 oUTpuT)
Script kiddie output is like interactive output, except that
it is post-processed to better suit the l33t HaXXorZ who
previously looked down on Nmap due to its consistent
capitalization and spelling. Humor impaired people should
note that this option is making fun of the script kiddies
before flaming me for supposedly “helping them”.
-oG filespec (grepable output)
This output format is covered last because it is deprecated.
The XML output format is far more powerful, and is nearly as
convenient for experienced users. XML is a standard for which
dozens of excellent parsers are available, while grepable
output is my own simple hack. XML is extensible to support
new Nmap features as they are released, while I often must
omit those features from grepable output for lack of a place
to put them.
Nevertheless, grepable output is still quite popular. It is a
simple format that lists each host on one line and can be
trivially searched and parsed with standard Unix tools such
as grep, awk, cut, sed, diff, and Perl. Even I usually use it
for one-off tests done at the command line. Finding all the
hosts with the SSH port open or that are running Solaris
takes only a simple grep to identify the hosts, piped to an
awk or cut command to print the desired fields.
Grepable output consists of comments (lines starting with a
pound (#)) and target lines. A target line includes a
combination of six labeled fields, separated by tabs and
followed with a colon. The fields are Host, Ports, Protocols,
Ignored State, OS, Seq Index, IP ID, and Status.
The most important of these fields is generally Ports, which
gives details on each interesting port. It is a comma
separated list of port entries. Each port entry represents
one interesting port, and takes the form of seven slash (/)
separated subfields. Those subfields are: Port number, State,
Protocol, Owner, Service, SunRPC info, and Version info.
As with XML output, this man page does not allow for
documenting the entire format. A more detailed look at the
Nmap grepable output format is available from
https://nmap.org/book/output-formats-grepable-output.html .
-oA basename (Output to all formats)
As a convenience, you may specify -oA basename to store scan
results in normal, XML, and grepable formats at once. They
are stored in basename.nmap, basename.xml, and
basename.gnmap, respectively. As with most programs, you can
prefix the filenames with a directory path, such as
~/nmaplogs/foocorp/ on Unix or c:\hacking\sco on Windows.
Verbosity and debugging options
-v (Increase verbosity level), -vlevel (Set verbosity level)
Increases the verbosity level, causing Nmap to print more
information about the scan in progress. Open ports are shown
as they are found and completion time estimates are provided
when Nmap thinks a scan will take more than a few minutes.
Use it twice or more for even greater verbosity: -vv, or give
a verbosity level directly, for example -v3.
Most changes only affect interactive output, and some also
affect normal and script kiddie output. The other output
types are meant to be processed by machines, so Nmap can give
substantial detail by default in those formats without
fatiguing a human user. However, there are a few changes in
other modes where output size can be reduced substantially by
omitting some detail. For example, a comment line in the
grepable output that provides a list of all ports scanned is
only printed in verbose mode because it can be quite long.
-d (Increase debugging level), -dlevel (Set debugging level)
When even verbose mode doesn't provide sufficient data for
you, debugging is available to flood you with much more! As
with the verbosity option (-v), debugging is enabled with a
command-line flag (-d) and the debug level can be increased
by specifying it multiple times, as in -dd, or by setting a
level directly. For example, -d9 sets level nine. That is the
highest effective level and will produce thousands of lines
unless you run a very simple scan with very few ports and
targets.
Debugging output is useful when a bug is suspected in Nmap,
or if you are simply confused as to what Nmap is doing and
why. As this feature is mostly intended for developers, debug
lines aren't always self-explanatory. You may get something
like: Timeout vals: srtt: -1 rttvar: -1 to: 1000000 delta
14987 ==> srtt: 14987 rttvar: 14987 to: 100000. If you don't
understand a line, your only recourses are to ignore it, look
it up in the source code, or request help from the
development list (nmap-dev). Some lines are self
explanatory, but the messages become more obscure as the
debug level is increased.
--reason (Host and port state reasons)
Shows the reason each port is set to a specific state and the
reason each host is up or down. This option displays the type
of the packet that determined a port or hosts state. For
example, A RST packet from a closed port or an echo reply
from an alive host. The information Nmap can provide is
determined by the type of scan or ping. The SYN scan and SYN
ping (-sS and -PS) are very detailed, but the TCP connect
scan (-sT) is limited by the implementation of the connect
system call. This feature is automatically enabled by the
debug option (-d) and the results are stored in XML log files
even if this option is not specified.
--stats-every time (Print periodic timing stats)
Periodically prints a timing status message after each
interval of time. The time is a specification of the kind
described in the section called “TIMING AND PERFORMANCE”; so
for example, use --stats-every 10s to get a status update
every 10 seconds. Updates are printed to interactive output
(the screen) and XML output.
--packet-trace (Trace packets and data sent and received)
Causes Nmap to print a summary of every packet sent or
received. This is often used for debugging, but is also a
valuable way for new users to understand exactly what Nmap is
doing under the covers. To avoid printing thousands of lines,
you may want to specify a limited number of ports to scan,
such as -p20-30. If you only care about the goings on of the
version detection subsystem, use --version-trace instead. If
you only care about script tracing, specify --script-trace.
With --packet-trace, you get all of the above.
--open (Show only open (or possibly open) ports)
Sometimes you only care about ports you can actually connect
to (open ones), and don't want results cluttered with closed,
filtered, and closed|filtered ports. Output customization is
normally done after the scan using tools such as grep, awk,
and Perl, but this feature was added due to overwhelming
requests. Specify --open to only see hosts with at least one
open, open|filtered, or unfiltered port, and only see ports
in those states. These three states are treated just as they
normally are, which means that open|filtered and unfiltered
may be condensed into counts if there are an overwhelming
number of them.
Beginning with Nmap 7.40, the --open option implies
--defeat-rst-ratelimit, because that option only affects
closed and filtered ports, which are hidden by --open.
--iflist (List interfaces and routes)
Prints the interface list and system routes as detected by
Nmap and quits. This is useful for debugging routing problems
or device mischaracterization (such as Nmap treating a PPP
connection as ethernet).
Miscellaneous output options
--append-output (Append to rather than clobber output files)
When you specify a filename to an output format flag such as
-oX or -oN, that file is overwritten by default. If you
prefer to keep the existing content of the file and append
the new results, specify the --append-output option. All
output filenames specified in that Nmap execution will then
be appended to rather than clobbered. This doesn't work well
for XML (-oX) scan data as the resultant file generally won't
parse properly until you fix it up by hand.
--resume filename (Resume aborted scan)
Some extensive Nmap runs take a very long time—on the order
of days. Such scans don't always run to completion.
Restrictions may prevent Nmap from being run during working
hours, the network could go down, the machine Nmap is running
on might suffer a planned or unplanned reboot, or Nmap itself
could crash. The administrator running Nmap could cancel it
for any other reason as well, by pressing ctrl-C. Restarting
the whole scan from the beginning may be undesirable.
Fortunately, if scan output files were kept, the user can ask
Nmap to resume scanning with the target it was working on
when execution ceased. Simply specify the --resume option and
pass the output file as its argument. No other arguments are
permitted, as Nmap parses the output file to use the same
ones specified previously. Simply call Nmap as nmap --resume
logfilename. Nmap will append new results to the data files
specified in the previous execution. Scans can be resumed
from any of the 3 major output formats: Normal, Grepable, or
XML
--noninteractive (Disable runtime interactions)
At times, such as when running Nmap in a shell background, it
might be undesirable for Nmap to monitor and respond to user
keyboard input when running. (See the section called “RUNTIME
INTERACTION” about how to control Nmap during a scan.) Use
option --noninteractive to prevent Nmap taking control of the
terminal.
--stylesheet path or URL (Set XSL stylesheet to transform XML
output)
Nmap ships with an XSL stylesheet named nmap.xsl for viewing
or translating XML output to HTML. The XML output includes
an xml-stylesheet directive which points to nmap.xml where it
was initially installed by Nmap. Run the XML file through an
XSLT processor such as xsltproc[16] to produce an HTML file.
Directly opening the XML file in a browser no longer works
well because modern browsers limit the locations a stylesheet
may be loaded from. If you wish to use a different
stylesheet, specify it as the argument to --stylesheet. You
must pass the full pathname or URL. One common invocation is
--stylesheet https://nmap.org/svn/docs/nmap.xsl. This tells
an XSLT processor to load the latest version of the
stylesheet from Nmap.Org. The --webxml option does the same
thing with less typing and memorization. Loading the XSL from
Nmap.Org makes it easier to view results on a machine that
doesn't have Nmap (and thus nmap.xsl) installed. So the URL
is often more useful, but the local filesystem location of
nmap.xsl is used by default for privacy reasons.
--webxml (Load stylesheet from Nmap.Org)
This is a convenience option, nothing more than an alias for
--stylesheet https://nmap.org/svn/docs/nmap.xsl.
--no-stylesheet (Omit XSL stylesheet declaration from XML)
Specify this option to prevent Nmap from associating any XSL
stylesheet with its XML output. The xml-stylesheet directive
is omitted.
This section describes some important (and not-so-important)
options that don't really fit anywhere else.
-6 (Enable IPv6 scanning)
Nmap has IPv6 support for its most popular features. Ping
scanning, port scanning, version detection, and the Nmap
Scripting Engine all support IPv6. The command syntax is the
same as usual except that you also add the -6 option. Of
course, you must use IPv6 syntax if you specify an address
rather than a hostname. An address might look like
3ffe:7501:4819:2000:210:f3ff:fe03:14d0, so hostnames are
recommended. The output looks the same as usual, with the
IPv6 address on the “interesting ports” line being the only
IPv6 giveaway.
While IPv6 hasn't exactly taken the world by storm, it gets
significant use in some (usually Asian) countries and most
modern operating systems support it. To use Nmap with IPv6,
both the source and target of your scan must be configured
for IPv6. If your ISP (like most of them) does not allocate
IPv6 addresses to you, free tunnel brokers are widely
available and work fine with Nmap. I use the free IPv6 tunnel
broker service at http://www.tunnelbroker.net . Other tunnel
brokers are listed at Wikipedia[17]. 6to4 tunnels are another
popular, free approach.
On Windows, raw-socket IPv6 scans are supported only on
ethernet devices (not tunnels), and only on Windows Vista and
later. Use the --unprivileged option in other situations.
-A (Aggressive scan options)
This option enables additional advanced and aggressive
options. Presently this enables OS detection (-O), version
scanning (-sV), script scanning (-sC) and traceroute
(--traceroute). More features may be added in the future.
The point is to enable a comprehensive set of scan options
without people having to remember a large set of flags.
However, because script scanning with the default set is
considered intrusive, you should not use -A against target
networks without permission. This option only enables
features, and not timing options (such as -T4) or verbosity
options (-v) that you might want as well. Options which
require privileges (e.g. root access) such as OS detection
and traceroute will only be enabled if those privileges are
available.
--datadir directoryname (Specify custom Nmap data file location)
Nmap obtains some special data at runtime in files named
nmap-service-probes, nmap-services, nmap-protocols, nmap-rpc,
nmap-mac-prefixes, and nmap-os-db. If the location of any of
these files has been specified (using the --servicedb or
--versiondb options), that location is used for that file.
After that, Nmap searches these files in the directory
specified with the --datadir option (if any). Any files not
found there, are searched for in the directory specified by
the NMAPDIR environment variable. Next comes ~/.nmap for real
and effective UIDs; or on Windows, HOME\AppData\Roaming\nmap
(where HOME is the user's home directory, like
C:\Users\user). This is followed by the location of the nmap
executable and the same location with ../share/nmap appended.
Then a compiled-in location such as /usr/local/share/nmap or
/usr/share/nmap.
--servicedb services file (Specify custom services file)
Asks Nmap to use the specified services file rather than the
nmap-services data file that comes with Nmap. Using this
option also causes a fast scan (-F) to be used. See the
description for --datadir for more information on Nmap's data
files.
--versiondb service probes file (Specify custom service probes
file)
Asks Nmap to use the specified service probes file rather
than the nmap-service-probes data file that comes with Nmap.
See the description for --datadir for more information on
Nmap's data files.
--send-eth (Use raw ethernet sending)
Asks Nmap to send packets at the raw ethernet (data link)
layer rather than the higher IP (network) layer. By default,
Nmap chooses the one which is generally best for the platform
it is running on. Raw sockets (IP layer) are generally most
efficient for Unix machines, while ethernet frames are
required for Windows operation since Microsoft disabled raw
socket support. Nmap still uses raw IP packets on Unix
despite this option when there is no other choice (such as
non-ethernet connections).
--send-ip (Send at raw IP level)
Asks Nmap to send packets via raw IP sockets rather than
sending lower level ethernet frames. It is the complement to
the --send-eth option discussed previously.
--privileged (Assume that the user is fully privileged)
Tells Nmap to simply assume that it is privileged enough to
perform raw socket sends, packet sniffing, and similar
operations that usually require root privileges on Unix
systems. By default Nmap quits if such operations are
requested but geteuid is not zero. --privileged is useful
with Linux kernel capabilities and similar systems that may
be configured to allow unprivileged users to perform
raw-packet scans. Be sure to provide this option flag before
any flags for options that require privileges (SYN scan, OS
detection, etc.). The NMAP_PRIVILEGED environment variable
may be set as an equivalent alternative to --privileged.
--unprivileged (Assume that the user lacks raw socket privileges)
This option is the opposite of --privileged. It tells Nmap to
treat the user as lacking network raw socket and sniffing
privileges. This is useful for testing, debugging, or when
the raw network functionality of your operating system is
somehow broken. The NMAP_UNPRIVILEGED environment variable
may be set as an equivalent alternative to --unprivileged.
--release-memory (Release memory before quitting)
This option is only useful for memory-leak debugging. It
causes Nmap to release allocated memory just before it quits
so that actual memory leaks are easier to spot. Normally Nmap
skips this as the OS does this anyway upon process
termination.
-V; --version (Print version number)
Prints the Nmap version number and exits.
-h; --help (Print help summary page)
Prints a short help screen with the most common command
flags. Running Nmap without any arguments does the same
thing.
During the execution of Nmap, all key presses are captured. This
allows you to interact with the program without aborting and
restarting it. Certain special keys will change options, while
any other keys will print out a status message telling you about
the scan. The convention is that lowercase letters increase the
amount of printing, and uppercase letters decrease the printing.
You may also press ‘?’ for help.
v / V
Increase / decrease the verbosity level
d / D
Increase / decrease the debugging Level
p / P
Turn on / off packet tracing
?
Print a runtime interaction help screen
Anything else
Print out a status message like this:
Stats: 0:00:07 elapsed; 20 hosts completed (1 up), 1 undergoing Service Scan
Service scan Timing: About 33.33% done; ETC: 20:57 (0:00:12 remaining)
Here are some Nmap usage examples, from the simple and routine to
a little more complex and esoteric. Some actual IP addresses and
domain names are used to make things more concrete. In their
place you should substitute addresses/names from your own
network. While I don't think port scanning other networks is or
should be illegal, some network administrators don't appreciate
unsolicited scanning of their networks and may complain. Getting
permission first is the best approach.
For testing purposes, you have permission to scan the host
scanme.nmap.org. This permission only includes scanning via Nmap
and not testing exploits or denial of service attacks. To
conserve bandwidth, please do not initiate more than a dozen
scans against that host per day. If this free scanning target
service is abused, it will be taken down and Nmap will report
Failed to resolve given hostname/IP: scanme.nmap.org. These
permissions also apply to the hosts scanme2.nmap.org,
scanme3.nmap.org, and so on, though those hosts do not currently
exist.
nmap -v scanme.nmap.org
This option scans all reserved TCP ports on the machine
scanme.nmap.org . The -v option enables verbose mode.
nmap -sS -O scanme.nmap.org/24
Launches a stealth SYN scan against each machine that is up out
of the 256 IPs on the /24 sized network where Scanme resides. It
also tries to determine what operating system is running on each
host that is up and running. This requires root privileges
because of the SYN scan and OS detection.
nmap -sV -p 22,53,110,143,4564 198.116.0-255.1-127
Launches host enumeration and a TCP scan at the first half of
each of the 255 possible eight-bit subnets in the 198.116.0.0/16
address space. This tests whether the systems run SSH, DNS, POP3,
or IMAP on their standard ports, or anything on port 4564. For
any of these ports found open, version detection is used to
determine what application is running.
nmap -v -iR 100000 -Pn -p 80
Asks Nmap to choose 100,000 hosts at random and scan them for web
servers (port 80). Host enumeration is disabled with -Pn since
first sending a couple probes to determine whether a host is up
is wasteful when you are only probing one port on each target
host anyway.
nmap -Pn -p80 -oX logs/pb-port80scan.xml -oG
logs/pb-port80scan.gnmap 216.163.128.20/20
This scans 4096 IPs for any web servers (without pinging them)
and saves the output in grepable and XML formats.
While this reference guide details all material Nmap options, it
can't fully demonstrate how to apply those features to quickly
solve real-world tasks. For that, we released Nmap Network
Scanning: The Official Nmap Project Guide to Network Discovery
and Security Scanning. Topics include subverting firewalls and
intrusion detection systems, optimizing Nmap performance, and
automating common networking tasks with the Nmap Scripting
Engine. Hints and instructions are provided for common Nmap tasks
such as taking network inventory, penetration testing, detecting
rogue wireless access points, and quashing network worm
outbreaks. Examples and diagrams show actual communication on the
wire. More than half of the book is available free online. See
https://nmap.org/book for more information.
Like its author, Nmap isn't perfect. But you can help make it
better by sending bug reports or even writing patches. If Nmap
doesn't behave the way you expect, first upgrade to the latest
version available from https://nmap.org . If the problem persists,
do some research to determine whether it has already been
discovered and addressed. Try searching for the problem or error
message on Google since that aggregates so many forums. If
nothing comes of this, create an Issue on our tracker (‐
http://issues.nmap.org ) and/or mail a bug report to
<[email protected]>. If you subscribe to the nmap-dev list before
posting, your message will bypass moderation and get through more
quickly. Subscribe at https://nmap.org/mailman/listinfo/dev .
Please include everything you have learned about the problem, as
well as what version of Nmap you are using and what operating
system version it is running on. Other suggestions for improving
Nmap may be sent to the Nmap dev mailing list as well.
If you are able to write a patch improving Nmap or fixing a bug,
that is even better! Instructions for submitting patches or git
pull requests are available from
https://github.com/nmap/nmap/blob/master/CONTRIBUTING.md
Particularly sensitive issues such as a security reports may be
sent directly to Nmap's author Fyodor directly at
<[email protected]>. All other reports and comments should use the
dev list or issue tracker instead because more people read,
follow, and respond to those.
Gordon “Fyodor” Lyon <[email protected]> wrote and released Nmap in
1997. Since then, hundreds of people have made valuable
contributions, as detailed in the CHANGELOG file distributed with
Nmap and also available from https://nmap.org/changelog.html .
David Fifield and Daniel Miller deserve special recognition for
their enormous multi-year contributions!
Nmap Copyright and Licensing
The Nmap Security Scanner is (C) 1996–2022 Nmap Software LLC
("The Nmap Project"). Nmap is also a registered trademark of the
Nmap Project. It is published under the Nmap Public Source
License[18]. This generally allows end users to download and use
Nmap for free. It doesn't allow Nmap to be used and redistributed
within commercial software or hardware products (including
appliances, virtual machines, and traditional applications). We
fund the project by selling a special Nmap OEM Edition for this
purpose, as described at https://nmap.org/oem . Hundreds of large
and small software vendors have already purchased OEM licenses to
embed Nmap technology such as host discovery, port scanning, OS
detection, version detection, and the Nmap Scripting Engine
within their products.
The Nmap Project has permission to redistribute Npcap, a packet
capturing driver and library for the Microsoft Windows platform.
Npcap is a separate work with it's own license rather than this
Nmap license. Since the Npcap license does not permit
redistribution without special permission, our Nmap Windows
binary packages which contain Npcap may not be redistributed
without special permission.
Even though the NPSL is based on GPLv2, it contains different
provisions and is not directly compatible. It is incompatible
with some other open source licenses as well. In some cases we
can relicense portions of Nmap or grant special permissions to
use it in other open source software. Please contact
[email protected] with any such requests. Similarly, we don't
incorporate incompatible open source software into Nmap without
special permission from the copyright holders.
If you have received a written license agreement or contract for
Nmap (such as an Nmap OEM license[19]) stating terms other than
these, you may choose to use and redistribute Nmap under those
terms instead.
Creative Commons License for this Nmap Guide
This Nmap Reference Guide is (C) 2005–2022 Nmap Software LLC. It
is hereby placed under version 3.0 of the Creative Commons
Attribution License[20]. This allows you redistribute and modify
the work as you desire, as long as you credit the original
source. Alternatively, you may choose to treat this document as
falling under the same license as Nmap itself (discussed
previously).
Source Code Availability and Community Contributions
Source is provided to this software because we believe users have
a right to know exactly what a program is going to do before they
run it. This also allows you to audit the software for security
holes.
Source code also allows you to port Nmap to new platforms, fix
bugs, and add new features. You are highly encouraged to submit
your changes as Github Pull Requests (PR) or send them to
<[email protected]> for possible incorporation into the main
distribution. By submitting such changes, it is assumed that you
are offering the Nmap Project the unlimited, non-exclusive right
to reuse, modify, and relicense the code. This is important
because the inability to relicense code has caused devastating
problems for other Free Software projects (such as KDE and NASM).
We also sell commercial licenses to Nmap OEM[21]. If you wish to
specify special license conditions of your contributions, just
say so when you send them.
No Warranty
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
It should also be noted that Nmap has occasionally been known to
crash poorly written applications, TCP/IP stacks, and even
operating systems. While this is extremely rare, it is important
to keep in mind. Nmap should never be run against mission
critical systems unless you are prepared to suffer downtime. We
acknowledge here that Nmap may crash your systems or networks and
we disclaim all liability for any damage or problems Nmap could
cause.
Inappropriate Usage
Because of the slight risk of crashes and because a few black
hats like to use Nmap for reconnaissance prior to attacking
systems, there are administrators who become upset and may
complain when their system is scanned. Thus, it is often
advisable to request permission before doing even a light scan of
a network.
Nmap should never be installed with special privileges (e.g. suid
root). That would open up a major security vulnerability as
other users on the system (or attackers) could use it for
privilege escalation.
Nmap is not designed, manufactured, or intended for use in
hazardous environments requiring fail- safe performance where the
failure of the software could lead directly to death, personal
injury, or significant physical or environmental damage.
Third-Party Software and Funding Notices
This product includes software developed by the Apache Software
Foundation[22]. A modified version of the Libpcap portable packet
capture library[23] is distributed along with Nmap. The Windows
version of Nmap utilizes the Libpcap-derived Ncap library[24]
instead. Regular expression support is provided by the PCRE
library[25], which is open-source software, written by Philip
Hazel. Certain raw networking functions use the Libdnet[26]
networking library, which was written by Dug Song. A modified
version is distributed with Nmap. Nmap can optionally link with
the OpenSSL cryptography toolkit[27] for SSL version detection
support. The Nmap Scripting Engine uses an embedded version of
the Lua programming language[10]. The Liblinear linear
classification library[28] is used for our IPv6 OS detection
machine learning techniques[29].
All of the third-party software described in this paragraph is
freely redistributable under BSD-style software licenses.
Binary packages for Windows and Mac OS X include support
libraries necessary to run Zenmap and Ndiff with Python and
PyGTK. (Unix platforms commonly make these libraries easy to
install, so they are not part of the packages.) A listing of
these support libraries and their licenses is included in the
LICENSES files.
This software was supported in part through the Google Summer of
Code[30] and the DARPA CINDER program[31] (DARPA-BAA-10-84).
United States Export Control
Nmap only uses encryption when compiled with the optional OpenSSL
support and linked with OpenSSL. When compiled without OpenSSL
support, the Nmap Project believes that Nmap is not subject to
U.S. Export Administration Regulations (EAR)[32] export control.
As such, there is no applicable ECCN (export control
classification number) and exportation does not require any
special license, permit, or other governmental authorization.
When compiled with OpenSSL support or distributed as source code,
the Nmap Project believes that Nmap falls under U.S. ECCN
5D002[33] (“Information Security Software”). We distribute Nmap
under the TSU exception for publicly available encryption
software defined in EAR 740.13(e)[34].
1. Nmap Network Scanning: The Official Nmap Project Guide to
Network Discovery and Security Scanning
https://nmap.org/book/
2. RFC 1122
http://www.rfc-editor.org/rfc/rfc1122.txt
3. RFC 792
http://www.rfc-editor.org/rfc/rfc792.txt
4. RFC 950
http://www.rfc-editor.org/rfc/rfc950.txt
5. UDP
http://www.rfc-editor.org/rfc/rfc768.txt
6. SCTP
http://www.rfc-editor.org/rfc/rfc4960.txt
7. TCP RFC
http://www.rfc-editor.org/rfc/rfc793.txt
8. RFC 959
http://www.rfc-editor.org/rfc/rfc959.txt
9. RFC 1323
http://www.rfc-editor.org/rfc/rfc1323.txt
10. Lua programming language
https://lua.org
11. precedence
http://www.lua.org/manual/5.4/manual.html#3.4.8
12. IP protocol
http://www.rfc-editor.org/rfc/rfc791.txt
13. RFC 2960
http://www.rfc-editor.org/rfc/rfc2960.txt
14. Nmap::Scanner
http://sourceforge.net/projects/nmap-scanner/
15. Nmap::Parser
http://nmapparser.wordpress.com/
16. xsltproc
http://xmlsoft.org/XSLT/
17. listed at Wikipedia
http://en.wikipedia.org/wiki/List_of_IPv6_tunnel_brokers
18. Nmap Public Source License
https://nmap.org/npsl
19. Nmap OEM license
https://nmap.org/oem/
20. Creative Commons Attribution License
http://creativecommons.org/licenses/by/3.0/
21. Nmap OEM
https://nmap.org/oem
22. Apache Software Foundation
https://www.apache.org
23. Libpcap portable packet capture library
https://www.tcpdump.org
24. Ncap library
https://npcap.com
25. PCRE library
https://pcre.org
26. Libdnet
http://libdnet.sourceforge.net
27. OpenSSL cryptography toolkit
https://openssl.org
28. Liblinear linear classification library
https://www.csie.ntu.edu.tw/~cjlin/liblinear/
29. IPv6 OS detection machine learning techniques
https://nmap.org/book/osdetect-guess.html#osdetect-guess-ipv6
30. Google Summer of Code
https://nmap.org/soc/
31. DARPA CINDER program
https://www.fbo.gov/index?s=opportunity&mode=form&id=585e02a51f77af5cb3c9e06b9cc82c48&tab=core&_cview=1
32. Export Administration Regulations (EAR)
https://www.bis.doc.gov/index.php/regulations/export-administration-regulations-ear
33. 5D002
https://www.bis.doc.gov/index.php/documents/regulations-docs/federal-register-notices/federal-register-2014/951-ccl5-pt2/file
34. EAR 740.13(e)
https://www.bis.doc.gov/index.php/documents/regulations-docs/2341-740-2/file
This page is part of the nmap (a network scanner) project.
Information about the project can be found at ⟨http://nmap.org/⟩.
If you have a bug report for this manual page, send it to
[email protected]. This page was obtained from the project's upstream
Git mirror of the Subversion repository
⟨https://github.com/nmap/nmap⟩ on 2024-06-14. (At that time, the
date of the most recent commit that was found in the repository
was 2024-06-13.) If you discover any rendering problems in this
HTML version of the page, or you believe there is a better or
more up-to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
[email protected]
Nmap 04/23/2024 NMAP(1)
|
HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|