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NCAT(1) Ncat Reference Guide NCAT(1)
ncat - Concatenate and redirect sockets
ncat [OPTIONS...] [hostname] [port]
Ncat is a feature-packed networking utility which reads and
writes data across networks from the command line. Ncat was
written for the Nmap Project and is the culmination of the
currently splintered family of Netcat incarnations. It is
designed to be a reliable back-end tool to instantly provide
network connectivity to other applications and users. Ncat will
not only work with IPv4 and IPv6 but provides the user with a
virtually limitless number of potential uses.
Among Ncat's vast number of features there is the ability to
chain Ncats together; redirection of TCP, UDP, and SCTP ports to
other sites; SSL support; and proxy connections via SOCKS4,
SOCKS5 or HTTP proxies (with optional proxy authentication as
well). Some general principles apply to most applications and
thus give you the capability of instantly adding networking
support to software that would normally never support it.
Ncat 7.95SVN ( https://nmap.org/ncat )
Usage: ncat [options] [hostname] [port]
Options taking a time assume seconds. Append 'ms' for milliseconds,
's' for seconds, 'm' for minutes, or 'h' for hours (e.g. 500ms).
-4 Use IPv4 only
-6 Use IPv6 only
-U, --unixsock Use Unix domain sockets only
--vsock Use vsock sockets only
-C, --crlf Use CRLF for EOL sequence
-c, --sh-exec <command> Executes the given command via /bin/sh
-e, --exec <command> Executes the given command
--lua-exec <filename> Executes the given Lua script
-g hop1[,hop2,...] Loose source routing hop points (8 max)
-G <n> Loose source routing hop pointer (4, 8, 12, ...)
-m, --max-conns <n> Maximum <n> simultaneous connections
-h, --help Display this help screen
-d, --delay <time> Wait between read/writes
-o, --output <filename> Dump session data to a file
-x, --hex-dump <filename> Dump session data as hex to a file
-i, --idle-timeout <time> Idle read/write timeout
-p, --source-port port Specify source port to use
-s, --source addr Specify source address to use (doesn't affect -l)
-l, --listen Bind and listen for incoming connections
-k, --keep-open Accept multiple connections in listen mode
-n, --nodns Do not resolve hostnames via DNS
-t, --telnet Answer Telnet negotiations
-u, --udp Use UDP instead of default TCP
--sctp Use SCTP instead of default TCP
-v, --verbose Set verbosity level (can be used several times)
-w, --wait <time> Connect timeout
-z Zero-I/O mode, report connection status only
--append-output Append rather than clobber specified output files
--send-only Only send data, ignoring received; quit on EOF
--recv-only Only receive data, never send anything
--no-shutdown Continue half-duplex when receiving EOF on stdin
--allow Allow only given hosts to connect to Ncat
--allowfile A file of hosts allowed to connect to Ncat
--deny Deny given hosts from connecting to Ncat
--denyfile A file of hosts denied from connecting to Ncat
--broker Enable Ncat's connection brokering mode
--chat Start a simple Ncat chat server
--proxy <addr[:port]> Specify address of host to proxy through
--proxy-type <type> Specify proxy type ("http", "socks4", "socks5")
--proxy-auth <auth> Authenticate with HTTP or SOCKS proxy server
--proxy-dns <type> Specify where to resolve proxy destination
--ssl Connect or listen with SSL
--ssl-cert Specify SSL certificate file (PEM) for listening
--ssl-key Specify SSL private key (PEM) for listening
--ssl-verify Verify trust and domain name of certificates
--ssl-trustfile PEM file containing trusted SSL certificates
--ssl-ciphers Cipherlist containing SSL ciphers to use
--ssl-servername Request distinct server name (SNI)
--ssl-alpn ALPN protocol list to use
--version Display Ncat's version information and exit
See the ncat(1) manpage for full options, descriptions and usage examples
Ncat operates in one of two primary modes: connect mode and
listen mode. Other modes, such as the HTTP proxy server, act as
special cases of these two. In connect mode, Ncat works as a
client. In listen mode it is a server.
In connect mode, the hostname and port arguments tell what to
connect to. hostname is required, and may be a hostname or IP
address. If port is supplied, it must be a decimal port number.
If omitted, it defaults to 31337.
In listen mode, hostname and port control the address the server
will bind to. Both arguments are optional in listen mode. If
hostname is omitted, it defaults to listening on all available
addresses over IPv4 and IPv6. If port is omitted, it defaults to
31337.
-4 (IPv4 only)
Force the use of IPv4 only.
-6 (IPv6 only)
Force the use of IPv6 only.
-U, --unixsock (Use Unix domain sockets)
Use Unix domain sockets rather than network sockets. This
option may be used on its own for stream sockets, or combined
with --udp for datagram sockets. A description of -U mode is
in the section called “UNIX DOMAIN SOCKETS”.
-u, --udp (Use UDP)
Use UDP for the connection (the default is TCP).
--sctp (Use SCTP)
Use SCTP for the connection (the default is TCP). SCTP
support is implemented in TCP-compatible mode.
--vsock (Use AF_VSOCK sockets)
Use AF_VSOCK sockets rather than the default TCP sockets
(Linux only). This option may be used on its own for stream
sockets or combined with --udp for datagram sockets. A
description of --vsock mode is in the section called
“AF_VSOCK SOCKETS”.
-g hop1[,hop2,...] (Loose source routing)
Sets hops for IPv4 loose source routing. You can use -g once
with a comma-separated list of hops, use -g multiple times
with single hops to build the list, or combine the two. Hops
can be given as IP addresses or hostnames.
-G ptr (Set source routing pointer)
Sets the IPv4 source route “pointer” for use with -g. The
argument must be a multiple of 4 and no more than 28. Not all
operating systems support setting this pointer to anything
other than four.
-p port, --source-port port (Specify source port)
Set the port number for Ncat to bind to.
-s host, --source host (Specify source address)
Set the address for Ncat to bind to.
See the section called “ACCESS CONTROL OPTIONS” for information
on limiting the hosts that may connect to the listening Ncat
process.
-l, --listen (Listen for connections)
Listen for connections rather than connecting to a remote
machine
-m numconns, --max-conns numconns (Specify maximum number of
connections)
The maximum number of simultaneous connections accepted by an
Ncat instance. 100 is the default (60 on Windows).
-k, --keep-open (Accept multiple connections)
Normally a listening server accepts only one connection and
then quits when the connection is closed. This option makes
it accept multiple simultaneous connections and wait for more
connections after they have all been closed. It must be
combined with --listen. In this mode there is no way for Ncat
to know when its network input is finished, so it will keep
running until interrupted. This also means that it will never
close its output stream, so any program reading from Ncat and
looking for end-of-file will also hang.
--broker (Connection brokering)
Allow multiple parties to connect to a centralised Ncat
server and communicate with each other. Ncat can broker
communication between systems that are behind a NAT or
otherwise unable to directly connect. This option is used in
conjunction with --listen, which causes the --listen port to
have broker mode enabled.
--chat (Ad-hoc “chat server”)
The --chat option enables chat mode, intended for the
exchange of text between several users. In chat mode,
connection brokering is turned on. Ncat prefixes each message
received with an ID before relaying it to the other
connections. The ID is unique for each connected client. This
helps distinguish who sent what. Additionally, non-printing
characters such as control characters are escaped to keep
them from doing damage to a terminal.
--ssl (Use SSL)
In connect mode, this option transparently negotiates an SSL
session with an SSL server to securely encrypt the
connection. This is particularly handy for talking to SSL
enabled HTTP servers, etc.
In server mode, this option listens for incoming SSL
connections, rather than plain untunneled traffic.
In UDP mode, this option enables Datagram TLS (DTLS).
--ssl-verify (Verify server certificates)
In client mode, --ssl-verify is like --ssl except that it
also requires verification of the server certificate. Ncat
comes with a default set of trusted certificates in the file
ca-bundle.crt. Some operating systems provide a default list
of trusted certificates; these will also be used if
available. Use --ssl-trustfile to give a custom list. Use -v
one or more times to get details about verification failures.
Ncat does not check for revoked certificates.
This option has no effect in server mode.
--ssl-cert certfile.pem (Specify SSL certificate)
This option gives the location of a PEM-encoded certificate
files used to authenticate the server (in listen mode) or the
client (in connect mode). Use it in combination with
--ssl-key.
--ssl-key keyfile.pem (Specify SSL private key)
This option gives the location of the PEM-encoded private key
file that goes with the certificate named with --ssl-cert.
--ssl-trustfile cert.pem (List trusted certificates)
This option sets a list of certificates that are trusted for
purposes of certificate verification. It has no effect unless
combined with --ssl-verify. The argument to this option is
the name of a PEM file containing trusted certificates.
Typically, the file will contain certificates of
certification authorities, though it may also contain server
certificates directly. When this option is used, Ncat does
not use its default certificates.
--ssl-ciphers cipherlist (Specify SSL ciphersuites)
This option sets the list of ciphersuites that Ncat will use
when connecting to servers or when accepting SSL connections
from clients. The syntax is described in the OpenSSL
ciphers(1) man page, and defaults to
ALL:!aNULL:!eNULL:!LOW:!EXP:!RC4:!MD5:@STRENGTH
--ssl-servername name (Request distinct server name)
In client mode, this option sets the TLS SNI (Server Name
Indication) extension, which tells the server the name of the
logical server Ncat is contacting. This is important when the
target server hosts multiple virtual servers at a single
underlying network address. If the option is not provided,
the TLS SNI extension will be populated with the target
server hostname.
--ssl-alpn ALPN list (Specify ALPN protocol list)
This option allows you to specify a comma-separated list of
protocols to send via the Application-Layer Protocol
Negotiation (ALPN) TLS extension. Not supported by all
versions of OpenSSL.
--proxy host[:port] (Specify proxy address)
Requests proxying through host:port, using the protocol
specified by --proxy-type.
If no port is specified, the proxy protocol's well-known port
is used (1080 for SOCKS and 3128 for HTTP). When specifying
an IPv6 HTTP proxy server using the IP address rather than
the hostname, the square-bracket notation (for example
[2001:db8::1]:8080) MUST be used to separate the port from
the IPv6 address. If the proxy requires authentication, use
--proxy-auth.
--proxy-type proto (Specify proxy protocol)
In connect mode, this option requests the protocol proto to
connect through the proxy host specified by --proxy. In
listen mode, this option has Ncat act as a proxy server using
the specified protocol.
The currently available protocols in connect mode are http
(CONNECT), socks4 (SOCKSv4), and socks5 (SOCKSv5). The only
server currently supported is http. If this option is not
used, the default protocol is http.
--proxy-auth user[:pass] (Specify proxy credentials)
In connect mode, gives the credentials that will be used to
connect to the proxy server. In listen mode, gives the
credentials that will be required of connecting clients. For
use with --proxy-type http or --proxy-type socks5, the form
should be username:password. For --proxy-type socks4, it
should be a username only.
These credentials can be alternatively passed onto Ncat by
setting environment variable NCAT_PROXY_AUTH, which reduces
the risk of the credentials being captured in process logs.
(Option --proxy-auth takes precedence.)
--proxy-dns type (Specify where to resolve proxy destination)
In connect mode, it provides control over whether proxy
destination hostnames are resolved by the remote proxy server
or locally, by Ncat itself. Possible values for type are:
local - Hostnames are resolved locally on the Ncat host. Ncat
exits with error if the hostname cannot be resolved.
remote - Hostnames are passed directly onto the remote proxy
server. This is the default behavior.
both - Hostname resolution is first attempted on the Ncat
host. Unresolvable hostnames are passed onto the remote proxy
server.
none - Hostname resolution is completely disabled. Only a
literal IPv4 or IPv6 address can be used as the proxy
destination.
Local hostname resolution generally respects IP version
specified with options -4 or -6, except for SOCKS4, which is
incompatible with IPv6.
-e command, --exec command (Execute command)
Execute the specified command after a connection has been
established. The command must be specified as a full
pathname. All input from the remote client will be sent to
the application and responses sent back to the remote client
over the socket, thus making your command-line application
interactive over a socket. Combined with --keep-open, Ncat
will handle multiple simultaneous connections to your
specified port/application like inetd. Ncat will only accept
a maximum, definable, number of simultaneous connections
controlled by the -m option. By default this is set to 100
(60 on Windows).
-c command, --sh-exec command (Execute command via sh)
Same as -e, except it tries to execute the command via
/bin/sh. This means you don't have to specify the full path
for the command, and shell facilities like environment
variables are available.
--lua-exec file (Execute a .lua script)
Runs the specified file as a Lua script after a connection
has been established, using a built-in interpreter. Both the
script's standard input and the standard output are
redirected to the connection data streams.
All exec options add the following variables to the child's
environment:
NCAT_REMOTE_ADDR, NCAT_REMOTE_PORT
The IP address and port number of the remote host. In connect
mode, it's the target's address; in listen mode, it's the
client's address.
NCAT_LOCAL_ADDR, NCAT_LOCAL_PORT
The IP address and port number of the local end of the
connection.
NCAT_PROTO
The protocol in use: one of TCP, UDP, and SCTP.
--allow host[,host,...] (Allow connections)
The list of hosts specified will be the only hosts allowed to
connect to the Ncat process. All other connection attempts
will be disconnected. In case of a conflict between --allow
and --deny, --allow takes precedence. Host specifications
follow the same syntax used by Nmap.
--allowfile file (Allow connections from file)
This has the same functionality as --allow, except that the
allowed hosts are provided in a new-line delimited allow
file, rather than directly on the command line.
--deny host[,host,...] (Deny connections)
Issue Ncat with a list of hosts that will not be allowed to
connect to the listening Ncat process. Specified hosts will
have their session silently terminated if they try to
connect. In case of a conflict between --allow and --deny,
--allow takes precedence. Host specifications follow the same
syntax used by Nmap.
--denyfile file (Deny connections from file)
This is the same functionality as --deny, except that
excluded hosts are provided in a new-line delimited deny
file, rather than directly on the command line.
These 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.
-d time, --delay time (Specify line delay)
Set the delay interval for lines sent. This effectively
limits the number of lines that Ncat will send in the
specified period. This may be useful for low-bandwidth sites,
or have other uses such as coping with annoying iptables
--limit options.
-i time, --idle-timeout time (Specify idle timeout)
Set a fixed timeout for idle connections. If the idle timeout
is reached, the connection is terminated.
-w time, --wait time (Specify connect timeout)
Set a fixed timeout for connection attempts.
-o file, --output file (Save session data)
Dump session data to a file
-x file, --hex-dump file (Save session data in hex)
Dump session data in hex to a file.
--append-output (Append output)
Issue Ncat with --append-ouput along with -o and/or -x and it
will append the resulted output rather than truncating the
specified output files.
-v, --verbose (Be verbose)
Issue Ncat with -v and it will be verbose and display all
kinds of useful connection based information. Use more than
once (-vv, -vvv...) for greater verbosity.
-C, --crlf (Use CRLF as EOL)
This option tells Ncat to convert LF line endings to CRLF
when taking input from standard input. This is useful for
talking to some stringent servers directly from a terminal in
one of the many common plain-text protocols that use CRLF for
end-of-line.
-h, --help (Help screen)
Displays a short help screen with common options and
parameters, and then exits.
--recv-only (Only receive data)
If this option is passed, Ncat will only receive data and
will not try to send anything.
--send-only (Only send data)
If this option is passed, then Ncat will only send data and
will ignore anything received. This option also causes Ncat
to close the network connection and terminate after EOF is
received on standard input.
--no-shutdown (Do not shutdown into half-duplex mode)
If this option is passed, Ncat will not invoke shutdown on a
socket after seeing EOF on stdin. This is provided for
backward-compatibility with OpenBSD netcat, which exhibits
this behavior when executed with its '-d' option.
-n, --nodns (Do not resolve hostnames)
Completely disable hostname resolution across all Ncat
options, such as the destination, source address, source
routing hops, and the proxy. All addresses must be specified
numerically. (Note that resolution of proxy destinations is
controlled separately via option --proxy-dns.)
-t, --telnet (Answer Telnet negotiations)
Handle DO/DONT WILL/WONT Telnet negotiations. This makes it
possible to script Telnet sessions with Ncat.
--version (Display version)
Displays the Ncat version number and exits.
The -U option (same as --unixsock) causes Ncat to use Unix domain
sockets rather than network sockets. Unix domain sockets exist as
an entry in the filesystem. You must give the name of a socket to
connect to or to listen on. For example, to make a connection,
ncat -U ~/unixsock
To listen on a socket:
ncat -l -U ~/unixsock
Listen mode will create the socket if it doesn't exist. The
socket will continue to exist after the program ends.
Both stream and datagram domain sockets are supported. Use -U on
its own for stream sockets, or combine it with --udp for datagram
sockets. Datagram sockets require a source socket to connect
from. By default, a source socket with a random filename will be
created as needed, and deleted when the program ends. Use the
--source with a path to use a source socket with a specific name.
The --vsock option causes Ncat to use AF_VSOCK sockets rather
than network sockets. A CID must be given instead of a hostname
or IP address. For example, to make a connection to the host,
ncat --vsock 2 1234
To listen on a socket:
ncat -l --vsock 1234
Both stream and datagram domain sockets are supported, but socket
type availability depends on the hypervisor. Use --vsock on its
own for stream sockets, or combine it with --udp for datagram
sockets.
Connect to example.org on TCP port 8080.
ncat example.org 8080
Listen for connections on TCP port 8080.
ncat -l 8080
Redirect TCP port 8080 on the local machine to host on port 80.
ncat --sh-exec "ncat example.org 80" -l 8080 --keep-open
Bind to TCP port 8081 and attach /bin/bash for the world to
access freely.
ncat --exec "/bin/bash" -l 8081 --keep-open
Bind a shell to TCP port 8081, limit access to hosts on a local
network, and limit the maximum number of simultaneous connections
to 3.
ncat --exec "/bin/bash" --max-conns 3 --allow 192.168.0.0/24
-l 8081 --keep-open
Connect to smtphost:25 through a SOCKS4 server on port 1080.
ncat --proxy socks4host --proxy-type socks4 --proxy-auth joe
smtphost 25
Connect to smtphost:25 through a SOCKS5 server on port 1080.
ncat --proxy socks5host --proxy-type socks5 --proxy-auth
joe:secret smtphost 25
Create an HTTP proxy server on localhost port 8888.
ncat -l --proxy-type http localhost 8888
Send a file over TCP port 9899 from host2 (client) to host1
(server).
HOST1$ ncat -l 9899 > outputfile
HOST2$ ncat HOST1 9899 < inputfile
Transfer in the other direction, turning Ncat into a “one file”
server.
HOST1$ ncat -l 9899 < inputfile
HOST2$ ncat HOST1 9899 > outputfile
The exit code reflects whether a connection was made and
completed successfully. 0 means there was no error. 1 means there
was a network error of some kind, for example “Connection
refused” or “Connection reset”. 2 is reserved for all other
errors, like an invalid option or a nonexistent file.
Like its authors, Ncat isn't perfect. But you can help make it
better by sending bug reports or even writing patches. If Ncat
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 Googling the error message or
browsing the nmap-dev archives at https://seclists.org/ .
Read this full manual page as well. If nothing comes of this,
mail a bug report to <[email protected]>. Please include everything
you have learned about the problem, as well as what version of
Ncat you are running and what operating system version it is
running on. Problem reports and Ncat usage questions sent to
[email protected] are far more likely to be answered than those sent
to Fyodor directly.
Code patches to fix bugs are even better than bug reports. Basic
instructions for creating patch files with your changes are
available at https://svn.nmap.org/nmap/HACKING . Patches may be
sent to nmap-dev (recommended) or to Fyodor directly.
• Chris Gibson <[email protected]>
• Gordon Lyon (Fyodor)<[email protected]> (http://insecure.org )
• Kris Katterjohn <[email protected]>
• Mixter <[email protected]>
The original Netcat was written by *Hobbit* <[email protected]>.
While Ncat isn't built on any code from the “traditional” Netcat
(or any other implementation), Ncat is most definitely based on
Netcat in spirit and functionality.
Ncat Copyright and Licensing
Ncat is (C) 2005–2022 Nmap Software LLC. It is distributed as
free and open source software under the same license terms as our
Nmap software. Precise terms and further details are available
from https://nmap.org/man/man-legal.html .
Creative Commons License for this Ncat Guide
This Ncat Reference Guide is (C) 2005–2022 Nmap Software LLC. It
is hereby placed under version 3.0 of the Creative Commons
Attribution License[1]. 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 Ncat 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 (none have been found so far).
Source code also allows you to port Nmap (which includes Ncat) to
new platforms, fix bugs, and add new features. You are highly
encouraged to send your changes to <[email protected]> for possible
incorporation into the main distribution. By sending these
changes to Fyodor or one of the Insecure.Org development mailing
lists, it is assumed that you are offering the Nmap Project (Nmap
Software LLC) the unlimited, non-exclusive right to reuse,
modify, and relicense the code. Nmap will always be available
open source, but 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 occasionally
relicense the code to third parties as discussed in the Nmap man
page. 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. See the Nmap
Public Source License for more details at https://nmap.org/npsl/ ,
or in the LICENSE file included with Nmap.
Inappropriate Usage
Ncat 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.
Third-Party Software
This product includes software developed by the Apache Software
Foundation[2]. A modified version of the Libpcap portable packet
capture library[3] is distributed along with Ncat. The Windows
version of Ncat utilized the Libpcap-derived Npcap library[4]
instead. Certain raw networking functions use the Libdnet[5]
networking library, which was written by Dug Song. A modified
version is distributed with Ncat. Ncat can optionally link with
the OpenSSL cryptography toolkit[6] for SSL version detection
support. All of the third-party software described in this
paragraph is freely redistributable under BSD-style software
licenses.
1. Creative Commons Attribution License
http://creativecommons.org/licenses/by/3.0/
2. Apache Software Foundation
http://www.apache.org
3. Libpcap portable packet capture library
http://www.tcpdump.org
4. Npcap library
https://npcap.com
5. Libdnet
http://libdnet.sourceforge.net
6. OpenSSL cryptography toolkit
http://www.openssl.org
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]
Ncat 04/23/2024 NCAT(1)
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