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NAME | DESCRIPTION | TIME FORMATS | TOKENS | FILES | SEE ALSO | AUTHORS | COLOPHON |
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SSHD_CONFIG(5) File Formats Manual SSHD_CONFIG(5)
sshd_config — OpenSSH daemon configuration file
sshd(8) reads configuration data from /etc/ssh/sshd_config (or
the file specified with -f on the command line). The file
contains keyword-argument pairs, one per line. Unless noted
otherwise, for each keyword, the first obtained value will be
used. Lines starting with ‘#’ and empty lines are interpreted as
comments. Arguments may optionally be enclosed in double quotes
(") in order to represent arguments containing spaces.
The possible keywords and their meanings are as follows (note
that keywords are case-insensitive and arguments are case-
sensitive):
AcceptEnv
Specifies what environment variables sent by the client
will be copied into the session's environ(7). See
SendEnv and SetEnv in ssh_config(5) for how to configure
the client. The TERM environment variable is always
accepted whenever the client requests a pseudo-terminal
as it is required by the protocol. Variables are
specified by name, which may contain the wildcard
characters ‘*’ and ‘?’. Multiple environment variables
may be separated by whitespace or spread across multiple
AcceptEnv directives. Be warned that some environment
variables could be used to bypass restricted user
environments. For this reason, care should be taken in
the use of this directive. The default is not to accept
any environment variables.
AddressFamily
Specifies which address family should be used by sshd(8).
Valid arguments are any (the default), inet (use IPv4
only), or inet6 (use IPv6 only).
AllowAgentForwarding
Specifies whether ssh-agent(1) forwarding is permitted.
The default is yes. Note that disabling agent forwarding
does not improve security unless users are also denied
shell access, as they can always install their own
forwarders.
AllowGroups
This keyword can be followed by a list of group name
patterns, separated by spaces. If specified, login is
allowed only for users whose primary group or
supplementary group list matches one of the patterns.
Only group names are valid; a numerical group ID is not
recognized. By default, login is allowed for all groups.
The allow/deny groups directives are processed in the
following order: DenyGroups, AllowGroups.
See PATTERNS in ssh_config(5) for more information on
patterns. This keyword may appear multiple times in
sshd_config with each instance appending to the list.
AllowStreamLocalForwarding
Specifies whether StreamLocal (Unix-domain socket)
forwarding is permitted. The available options are yes
(the default) or all to allow StreamLocal forwarding, no
to prevent all StreamLocal forwarding, local to allow
local (from the perspective of ssh(1)) forwarding only or
remote to allow remote forwarding only. Note that
disabling StreamLocal forwarding does not improve
security unless users are also denied shell access, as
they can always install their own forwarders.
AllowTcpForwarding
Specifies whether TCP forwarding is permitted. The
available options are yes (the default) or all to allow
TCP forwarding, no to prevent all TCP forwarding, local
to allow local (from the perspective of ssh(1))
forwarding only or remote to allow remote forwarding
only. Note that disabling TCP forwarding does not
improve security unless users are also denied shell
access, as they can always install their own forwarders.
AllowUsers
This keyword can be followed by a list of user name
patterns, separated by spaces. If specified, login is
allowed only for user names that match one of the
patterns. Only user names are valid; a numerical user ID
is not recognized. By default, login is allowed for all
users. If the pattern takes the form USER@HOST then USER
and HOST are separately checked, restricting logins to
particular users from particular hosts. HOST criteria
may additionally contain addresses to match in CIDR
address/masklen format. The allow/deny users directives
are processed in the following order: DenyUsers,
AllowUsers.
See PATTERNS in ssh_config(5) for more information on
patterns. This keyword may appear multiple times in
sshd_config with each instance appending to the list.
AuthenticationMethods
Specifies the authentication methods that must be
successfully completed for a user to be granted access.
This option must be followed by one or more lists of
comma-separated authentication method names, or by the
single string any to indicate the default behaviour of
accepting any single authentication method. If the
default is overridden, then successful authentication
requires completion of every method in at least one of
these lists.
For example, "publickey,password
publickey,keyboard-interactive" would require the user to
complete public key authentication, followed by either
password or keyboard interactive authentication. Only
methods that are next in one or more lists are offered at
each stage, so for this example it would not be possible
to attempt password or keyboard-interactive
authentication before public key.
For keyboard interactive authentication it is also
possible to restrict authentication to a specific device
by appending a colon followed by the device identifier
bsdauth or pam. depending on the server configuration.
For example, "keyboard-interactive:bsdauth" would
restrict keyboard interactive authentication to the
bsdauth device.
If the publickey method is listed more than once, sshd(8)
verifies that keys that have been used successfully are
not reused for subsequent authentications. For example,
"publickey,publickey" requires successful authentication
using two different public keys.
Note that each authentication method listed should also
be explicitly enabled in the configuration.
The available authentication methods are:
"gssapi-with-mic", "hostbased", "keyboard-interactive",
"none" (used for access to password-less accounts when
PermitEmptyPasswords is enabled), "password" and
"publickey".
AuthorizedKeysCommand
Specifies a program to be used to look up the user's
public keys. The program must be owned by root, not
writable by group or others and specified by an absolute
path. Arguments to AuthorizedKeysCommand accept the
tokens described in the “TOKENS” section. If no
arguments are specified then the username of the target
user is used.
The program should produce on standard output zero or
more lines of authorized_keys output (see
“AUTHORIZED_KEYS” in sshd(8)). AuthorizedKeysCommand is
tried after the usual AuthorizedKeysFile files and will
not be executed if a matching key is found there. By
default, no AuthorizedKeysCommand is run.
AuthorizedKeysCommandUser
Specifies the user under whose account the
AuthorizedKeysCommand is run. It is recommended to use a
dedicated user that has no other role on the host than
running authorized keys commands. If
AuthorizedKeysCommand is specified but
AuthorizedKeysCommandUser is not, then sshd(8) will
refuse to start.
AuthorizedKeysFile
Specifies the file that contains the public keys used for
user authentication. The format is described in the
AUTHORIZED_KEYS FILE FORMAT section of sshd(8).
Arguments to AuthorizedKeysFile accept the tokens
described in the “TOKENS” section. After expansion,
AuthorizedKeysFile is taken to be an absolute path or one
relative to the user's home directory. Multiple files
may be listed, separated by whitespace. Alternately this
option may be set to none to skip checking for user keys
in files. The default is ".ssh/authorized_keys
.ssh/authorized_keys2".
AuthorizedPrincipalsCommand
Specifies a program to be used to generate the list of
allowed certificate principals as per
AuthorizedPrincipalsFile. The program must be owned by
root, not writable by group or others and specified by an
absolute path. Arguments to AuthorizedPrincipalsCommand
accept the tokens described in the “TOKENS” section. If
no arguments are specified then the username of the
target user is used.
The program should produce on standard output zero or
more lines of AuthorizedPrincipalsFile output. If either
AuthorizedPrincipalsCommand or AuthorizedPrincipalsFile
is specified, then certificates offered by the client for
authentication must contain a principal that is listed.
By default, no AuthorizedPrincipalsCommand is run.
AuthorizedPrincipalsCommandUser
Specifies the user under whose account the
AuthorizedPrincipalsCommand is run. It is recommended to
use a dedicated user that has no other role on the host
than running authorized principals commands. If
AuthorizedPrincipalsCommand is specified but
AuthorizedPrincipalsCommandUser is not, then sshd(8) will
refuse to start.
AuthorizedPrincipalsFile
Specifies a file that lists principal names that are
accepted for certificate authentication. When using
certificates signed by a key listed in TrustedUserCAKeys,
this file lists names, one of which must appear in the
certificate for it to be accepted for authentication.
Names are listed one per line preceded by key options (as
described in “AUTHORIZED_KEYS FILE FORMAT” in sshd(8)).
Empty lines and comments starting with ‘#’ are ignored.
Arguments to AuthorizedPrincipalsFile accept the tokens
described in the “TOKENS” section. After expansion,
AuthorizedPrincipalsFile is taken to be an absolute path
or one relative to the user's home directory. The
default is none, i.e. not to use a principals file – in
this case, the username of the user must appear in a
certificate's principals list for it to be accepted.
Note that AuthorizedPrincipalsFile is only used when
authentication proceeds using a CA listed in
TrustedUserCAKeys and is not consulted for certification
authorities trusted via ~/.ssh/authorized_keys, though
the principals= key option offers a similar facility (see
sshd(8) for details).
Banner The contents of the specified file are sent to the remote
user before authentication is allowed. If the argument
is none then no banner is displayed. By default, no
banner is displayed.
CASignatureAlgorithms
Specifies which algorithms are allowed for signing of
certificates by certificate authorities (CAs). The
default is:
ssh-ed25519,ecdsa-sha2-nistp256,
ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
[email protected],
[email protected],
rsa-sha2-512,rsa-sha2-256
If the specified list begins with a ‘+’ character, then
the specified algorithms will be appended to the default
set instead of replacing them. If the specified list
begins with a ‘-’ character, then the specified
algorithms (including wildcards) will be removed from the
default set instead of replacing them.
Certificates signed using other algorithms will not be
accepted for public key or host-based authentication.
ChannelTimeout
Specifies whether and how quickly sshd(8) should close
inactive channels. Timeouts are specified as one or more
“type=interval” pairs separated by whitespace, where the
“type” must be the special keyword “global” or a channel
type name from the list below, optionally containing
wildcard characters.
The timeout value “interval” is specified in seconds or
may use any of the units documented in the “TIME FORMATS”
section. For example, “session=5m” would cause
interactive sessions to terminate after five minutes of
inactivity. Specifying a zero value disables the
inactivity timeout.
The special timeout “global” applies to all active
channels, taken together. Traffic on any active channel
will reset the timeout, but when the timeout expires then
all open channels will be closed. Note that this global
timeout is not matched by wildcards and must be specified
explicitly.
The available channel type names include:
agent-connection
Open connections to ssh-agent(1).
direct-tcpip, [email protected]
Open TCP or Unix socket (respectively)
connections that have been established from a
ssh(1) local forwarding, i.e. LocalForward or
DynamicForward.
forwarded-tcpip, [email protected]
Open TCP or Unix socket (respectively)
connections that have been established to a
sshd(8) listening on behalf of a ssh(1) remote
forwarding, i.e. RemoteForward.
session
The interactive main session, including shell
session, command execution, scp(1), sftp(1), etc.
tun-connection
Open TunnelForward connections.
x11-connection
Open X11 forwarding sessions.
Note that in all the above cases, terminating an inactive
session does not guarantee to remove all resources
associated with the session, e.g. shell processes or X11
clients relating to the session may continue to execute.
Moreover, terminating an inactive channel or session does
not necessarily close the SSH connection, nor does it
prevent a client from requesting another channel of the
same type. In particular, expiring an inactive
forwarding session does not prevent another identical
forwarding from being subsequently created.
The default is not to expire channels of any type for
inactivity.
ChrootDirectory
Specifies the pathname of a directory to chroot(2) to
after authentication. At session startup sshd(8) checks
that all components of the pathname are root-owned
directories which are not writable by group or others.
After the chroot, sshd(8) changes the working directory
to the user's home directory. Arguments to
ChrootDirectory accept the tokens described in the
“TOKENS” section.
The ChrootDirectory must contain the necessary files and
directories to support the user's session. For an
interactive session this requires at least a shell,
typically sh(1), and basic /dev nodes such as null(4),
zero(4), stdin(4), stdout(4), stderr(4), and tty(4)
devices. For file transfer sessions using SFTP no
additional configuration of the environment is necessary
if the in-process sftp-server is used, though sessions
which use logging may require /dev/log inside the chroot
directory on some operating systems (see sftp-server(8)
for details).
For safety, it is very important that the directory
hierarchy be prevented from modification by other
processes on the system (especially those outside the
jail). Misconfiguration can lead to unsafe environments
which sshd(8) cannot detect.
The default is none, indicating not to chroot(2).
Ciphers
Specifies the ciphers allowed. Multiple ciphers must be
comma-separated. If the specified list begins with a ‘+’
character, then the specified ciphers will be appended to
the default set instead of replacing them. If the
specified list begins with a ‘-’ character, then the
specified ciphers (including wildcards) will be removed
from the default set instead of replacing them. If the
specified list begins with a ‘^’ character, then the
specified ciphers will be placed at the head of the
default set.
The supported ciphers are:
3des-cbc
aes128-cbc
aes192-cbc
aes256-cbc
aes128-ctr
aes192-ctr
aes256-ctr
[email protected]
[email protected]
[email protected]
The default is:
[email protected],
aes128-ctr,aes192-ctr,aes256-ctr,
[email protected],[email protected]
The list of available ciphers may also be obtained using
"ssh -Q cipher".
ClientAliveCountMax
Sets the number of client alive messages which may be
sent without sshd(8) receiving any messages back from the
client. If this threshold is reached while client alive
messages are being sent, sshd will disconnect the client,
terminating the session. It is important to note that
the use of client alive messages is very different from
TCPKeepAlive. The client alive messages are sent through
the encrypted channel and therefore will not be
spoofable. The TCP keepalive option enabled by
TCPKeepAlive is spoofable. The client alive mechanism is
valuable when the client or server depend on knowing when
a connection has become unresponsive.
The default value is 3. If ClientAliveInterval is set to
15, and ClientAliveCountMax is left at the default,
unresponsive SSH clients will be disconnected after
approximately 45 seconds. Setting a zero
ClientAliveCountMax disables connection termination.
ClientAliveInterval
Sets a timeout interval in seconds after which if no data
has been received from the client, sshd(8) will send a
message through the encrypted channel to request a
response from the client. The default is 0, indicating
that these messages will not be sent to the client.
Compression
Specifies whether compression is enabled after the user
has authenticated successfully. The argument must be
yes, delayed (a legacy synonym for yes) or no. The
default is yes.
DenyGroups
This keyword can be followed by a list of group name
patterns, separated by spaces. Login is disallowed for
users whose primary group or supplementary group list
matches one of the patterns. Only group names are valid;
a numerical group ID is not recognized. By default,
login is allowed for all groups. The allow/deny groups
directives are processed in the following order:
DenyGroups, AllowGroups.
See PATTERNS in ssh_config(5) for more information on
patterns. This keyword may appear multiple times in
sshd_config with each instance appending to the list.
DenyUsers
This keyword can be followed by a list of user name
patterns, separated by spaces. Login is disallowed for
user names that match one of the patterns. Only user
names are valid; a numerical user ID is not recognized.
By default, login is allowed for all users. If the
pattern takes the form USER@HOST then USER and HOST are
separately checked, restricting logins to particular
users from particular hosts. HOST criteria may
additionally contain addresses to match in CIDR
address/masklen format. The allow/deny users directives
are processed in the following order: DenyUsers,
AllowUsers.
See PATTERNS in ssh_config(5) for more information on
patterns. This keyword may appear multiple times in
sshd_config with each instance appending to the list.
DisableForwarding
Disables all forwarding features, including X11,
ssh-agent(1), TCP and StreamLocal. This option overrides
all other forwarding-related options and may simplify
restricted configurations.
ExposeAuthInfo
Writes a temporary file containing a list of
authentication methods and public credentials (e.g. keys)
used to authenticate the user. The location of the file
is exposed to the user session through the SSH_USER_AUTH
environment variable. The default is no.
FingerprintHash
Specifies the hash algorithm used when logging key
fingerprints. Valid options are: md5 and sha256. The
default is sha256.
ForceCommand
Forces the execution of the command specified by
ForceCommand, ignoring any command supplied by the client
and ~/.ssh/rc if present. The command is invoked by
using the user's login shell with the -c option. This
applies to shell, command, or subsystem execution. It is
most useful inside a Match block. The command originally
supplied by the client is available in the
SSH_ORIGINAL_COMMAND environment variable. Specifying a
command of internal-sftp will force the use of an in-
process SFTP server that requires no support files when
used with ChrootDirectory. The default is none.
GatewayPorts
Specifies whether remote hosts are allowed to connect to
ports forwarded for the client. By default, sshd(8)
binds remote port forwardings to the loopback address.
This prevents other remote hosts from connecting to
forwarded ports. GatewayPorts can be used to specify
that sshd should allow remote port forwardings to bind to
non-loopback addresses, thus allowing other hosts to
connect. The argument may be no to force remote port
forwardings to be available to the local host only, yes
to force remote port forwardings to bind to the wildcard
address, or clientspecified to allow the client to select
the address to which the forwarding is bound. The
default is no.
GSSAPIAuthentication
Specifies whether user authentication based on GSSAPI is
allowed. The default is no.
GSSAPICleanupCredentials
Specifies whether to automatically destroy the user's
credentials cache on logout. The default is yes.
GSSAPIStrictAcceptorCheck
Determines whether to be strict about the identity of the
GSSAPI acceptor a client authenticates against. If set
to yes then the client must authenticate against the host
service on the current hostname. If set to no then the
client may authenticate against any service key stored in
the machine's default store. This facility is provided
to assist with operation on multi homed machines. The
default is yes.
HostbasedAcceptedAlgorithms
Specifies the signature algorithms that will be accepted
for hostbased authentication as a list of comma-separated
patterns. Alternately if the specified list begins with
a ‘+’ character, then the specified signature algorithms
will be appended to the default set instead of replacing
them. If the specified list begins with a ‘-’ character,
then the specified signature algorithms (including
wildcards) will be removed from the default set instead
of replacing them. If the specified list begins with a
‘^’ character, then the specified signature algorithms
will be placed at the head of the default set. The
default for this option is:
[email protected],
[email protected],
[email protected],
[email protected],
[email protected],
[email protected],
[email protected],
[email protected],
ssh-ed25519,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
[email protected],
[email protected],
rsa-sha2-512,rsa-sha2-256
The list of available signature algorithms may also be
obtained using "ssh -Q HostbasedAcceptedAlgorithms".
This was formerly named HostbasedAcceptedKeyTypes.
HostbasedAuthentication
Specifies whether rhosts or /etc/hosts.equiv
authentication together with successful public key client
host authentication is allowed (host-based
authentication). The default is no.
HostbasedUsesNameFromPacketOnly
Specifies whether or not the server will attempt to
perform a reverse name lookup when matching the name in
the ~/.shosts, ~/.rhosts, and /etc/hosts.equiv files
during HostbasedAuthentication. A setting of yes means
that sshd(8) uses the name supplied by the client rather
than attempting to resolve the name from the TCP
connection itself. The default is no.
HostCertificate
Specifies a file containing a public host certificate.
The certificate's public key must match a private host
key already specified by HostKey. The default behaviour
of sshd(8) is not to load any certificates.
HostKey
Specifies a file containing a private host key used by
SSH. The defaults are /etc/ssh/ssh_host_ecdsa_key,
/etc/ssh/ssh_host_ed25519_key and
/etc/ssh/ssh_host_rsa_key.
Note that sshd(8) will refuse to use a file if it is
group/world-accessible and that the HostKeyAlgorithms
option restricts which of the keys are actually used by
sshd(8).
It is possible to have multiple host key files. It is
also possible to specify public host key files instead.
In this case operations on the private key will be
delegated to an ssh-agent(1).
HostKeyAgent
Identifies the UNIX-domain socket used to communicate
with an agent that has access to the private host keys.
If the string "SSH_AUTH_SOCK" is specified, the location
of the socket will be read from the SSH_AUTH_SOCK
environment variable.
HostKeyAlgorithms
Specifies the host key signature algorithms that the
server offers. The default for this option is:
[email protected],
[email protected],
[email protected],
[email protected],
[email protected],
[email protected],
[email protected],
[email protected],
ssh-ed25519,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
[email protected],
[email protected],
rsa-sha2-512,rsa-sha2-256
The list of available signature algorithms may also be
obtained using "ssh -Q HostKeyAlgorithms".
IgnoreRhosts
Specifies whether to ignore per-user .rhosts and .shosts
files during HostbasedAuthentication. The system-wide
/etc/hosts.equiv and /etc/shosts.equiv are still used
regardless of this setting.
Accepted values are yes (the default) to ignore all per-
user files, shosts-only to allow the use of .shosts but
to ignore .rhosts or no to allow both .shosts and rhosts.
IgnoreUserKnownHosts
Specifies whether sshd(8) should ignore the user's
~/.ssh/known_hosts during HostbasedAuthentication and use
only the system-wide known hosts file
/etc/ssh/ssh_known_hosts. The default is “no”.
Include
Include the specified configuration file(s). Multiple
pathnames may be specified and each pathname may contain
glob(7) wildcards that will be expanded and processed in
lexical order. Files without absolute paths are assumed
to be in /etc/ssh. An Include directive may appear
inside a Match block to perform conditional inclusion.
IPQoS Specifies the IPv4 type-of-service or DSCP class for the
connection. Accepted values are af11, af12, af13, af21,
af22, af23, af31, af32, af33, af41, af42, af43, cs0, cs1,
cs2, cs3, cs4, cs5, cs6, cs7, ef, le, lowdelay,
throughput, reliability, a numeric value, or none to use
the operating system default. This option may take one
or two arguments, separated by whitespace. If one
argument is specified, it is used as the packet class
unconditionally. If two values are specified, the first
is automatically selected for interactive sessions and
the second for non-interactive sessions. The default is
af21 (Low-Latency Data) for interactive sessions and cs1
(Lower Effort) for non-interactive sessions.
KbdInteractiveAuthentication
Specifies whether to allow keyboard-interactive
authentication. All authentication styles from
login.conf(5) are supported. The default is yes. The
argument to this keyword must be yes or no.
ChallengeResponseAuthentication is a deprecated alias for
this.
KerberosAuthentication
Specifies whether the password provided by the user for
PasswordAuthentication will be validated through the
Kerberos KDC. To use this option, the server needs a
Kerberos servtab which allows the verification of the
KDC's identity. The default is no.
KerberosGetAFSToken
If AFS is active and the user has a Kerberos 5 TGT,
attempt to acquire an AFS token before accessing the
user's home directory. The default is no.
KerberosOrLocalPasswd
If password authentication through Kerberos fails then
the password will be validated via any additional local
mechanism such as /etc/passwd. The default is yes.
KerberosTicketCleanup
Specifies whether to automatically destroy the user's
ticket cache file on logout. The default is yes.
KexAlgorithms
Specifies the available KEX (Key Exchange) algorithms.
Multiple algorithms must be comma-separated. Alternately
if the specified list begins with a ‘+’ character, then
the specified algorithms will be appended to the default
set instead of replacing them. If the specified list
begins with a ‘-’ character, then the specified
algorithms (including wildcards) will be removed from the
default set instead of replacing them. If the specified
list begins with a ‘^’ character, then the specified
algorithms will be placed at the head of the default set.
The supported algorithms are:
curve25519-sha256
[email protected]
diffie-hellman-group1-sha1
diffie-hellman-group14-sha1
diffie-hellman-group14-sha256
diffie-hellman-group16-sha512
diffie-hellman-group18-sha512
diffie-hellman-group-exchange-sha1
diffie-hellman-group-exchange-sha256
ecdh-sha2-nistp256
ecdh-sha2-nistp384
ecdh-sha2-nistp521
[email protected]
The default is:
[email protected],
curve25519-sha256,[email protected],
ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
diffie-hellman-group-exchange-sha256,
diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,
diffie-hellman-group14-sha256
The list of available key exchange algorithms may also be
obtained using "ssh -Q KexAlgorithms".
ListenAddress
Specifies the local addresses sshd(8) should listen on.
The following forms may be used:
ListenAddress hostname|address [rdomain domain]
ListenAddress hostname:port [rdomain domain]
ListenAddress IPv4_address:port [rdomain domain]
ListenAddress [hostname|address]:port [rdomain
domain]
The optional rdomain qualifier requests sshd(8) listen in
an explicit routing domain. If port is not specified,
sshd will listen on the address and all Port options
specified. The default is to listen on all local
addresses on the current default routing domain.
Multiple ListenAddress options are permitted. For more
information on routing domains, see rdomain(4).
LoginGraceTime
The server disconnects after this time if the user has
not successfully logged in. If the value is 0, there is
no time limit. The default is 120 seconds.
LogLevel
Gives the verbosity level that is used when logging
messages from sshd(8). The possible values are: QUIET,
FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and
DEBUG3. The default is INFO. DEBUG and DEBUG1 are
equivalent. DEBUG2 and DEBUG3 each specify higher levels
of debugging output. Logging with a DEBUG level violates
the privacy of users and is not recommended.
LogVerbose
Specify one or more overrides to LogLevel. An override
consists of a pattern lists that matches the source file,
function and line number to force detailed logging for.
For example, an override pattern of:
kex.c:*:1000,*:kex_exchange_identification():*,packet.c:*
would enable detailed logging for line 1000 of kex.c,
everything in the kex_exchange_identification() function,
and all code in the packet.c file. This option is
intended for debugging and no overrides are enabled by
default.
MACs Specifies the available MAC (message authentication code)
algorithms. The MAC algorithm is used for data integrity
protection. Multiple algorithms must be comma-separated.
If the specified list begins with a ‘+’ character, then
the specified algorithms will be appended to the default
set instead of replacing them. If the specified list
begins with a ‘-’ character, then the specified
algorithms (including wildcards) will be removed from the
default set instead of replacing them. If the specified
list begins with a ‘^’ character, then the specified
algorithms will be placed at the head of the default set.
The algorithms that contain "-etm" calculate the MAC
after encryption (encrypt-then-mac). These are
considered safer and their use recommended. The
supported MACs are:
hmac-md5
hmac-md5-96
hmac-sha1
hmac-sha1-96
hmac-sha2-256
hmac-sha2-512
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
The default is:
[email protected],[email protected],
[email protected],[email protected],
[email protected],
[email protected],[email protected],
hmac-sha2-256,hmac-sha2-512,hmac-sha1
The list of available MAC algorithms may also be obtained
using "ssh -Q mac".
Match Introduces a conditional block. If all of the criteria
on the Match line are satisfied, the keywords on the
following lines override those set in the global section
of the config file, until either another Match line or
the end of the file. If a keyword appears in multiple
Match blocks that are satisfied, only the first instance
of the keyword is applied.
The arguments to Match are one or more criteria-pattern
pairs or the single token All which matches all criteria.
The available criteria are User, Group, Host,
LocalAddress, LocalPort, RDomain, and Address (with
RDomain representing the rdomain(4) on which the
connection was received).
The match patterns may consist of single entries or
comma-separated lists and may use the wildcard and
negation operators described in the “PATTERNS” section of
ssh_config(5).
The patterns in an Address criteria may additionally
contain addresses to match in CIDR address/masklen
format, such as 192.0.2.0/24 or 2001:db8::/32. Note that
the mask length provided must be consistent with the
address - it is an error to specify a mask length that is
too long for the address or one with bits set in this
host portion of the address. For example, 192.0.2.0/33
and 192.0.2.0/8, respectively.
Only a subset of keywords may be used on the lines
following a Match keyword. Available keywords are
AcceptEnv, AllowAgentForwarding, AllowGroups,
AllowStreamLocalForwarding, AllowTcpForwarding,
AllowUsers, AuthenticationMethods, AuthorizedKeysCommand,
AuthorizedKeysCommandUser, AuthorizedKeysFile,
AuthorizedPrincipalsCommand,
AuthorizedPrincipalsCommandUser,
AuthorizedPrincipalsFile, Banner, CASignatureAlgorithms,
ChannelTimeout, ChrootDirectory, ClientAliveCountMax,
ClientAliveInterval, DenyGroups, DenyUsers,
DisableForwarding, ExposeAuthInfo, ForceCommand,
GatewayPorts, GSSAPIAuthentication,
HostbasedAcceptedAlgorithms, HostbasedAuthentication,
HostbasedUsesNameFromPacketOnly, IgnoreRhosts, Include,
IPQoS, KbdInteractiveAuthentication,
KerberosAuthentication, LogLevel, MaxAuthTries,
MaxSessions, PasswordAuthentication,
PermitEmptyPasswords, PermitListen, PermitOpen,
PermitRootLogin, PermitTTY, PermitTunnel, PermitUserRC,
PubkeyAcceptedAlgorithms, PubkeyAuthentication,
PubkeyAuthOptions, RekeyLimit, RevokedKeys, RDomain,
SetEnv, StreamLocalBindMask, StreamLocalBindUnlink,
TrustedUserCAKeys, UnusedConnectionTimeout,
X11DisplayOffset, X11Forwarding and X11UseLocalhost.
MaxAuthTries
Specifies the maximum number of authentication attempts
permitted per connection. Once the number of failures
reaches half this value, additional failures are logged.
The default is 6.
MaxSessions
Specifies the maximum number of open shell, login or
subsystem (e.g. sftp) sessions permitted per network
connection. Multiple sessions may be established by
clients that support connection multiplexing. Setting
MaxSessions to 1 will effectively disable session
multiplexing, whereas setting it to 0 will prevent all
shell, login and subsystem sessions while still
permitting forwarding. The default is 10.
MaxStartups
Specifies the maximum number of concurrent
unauthenticated connections to the SSH daemon.
Additional connections will be dropped until
authentication succeeds or the LoginGraceTime expires for
a connection. The default is 10:30:100.
Alternatively, random early drop can be enabled by
specifying the three colon separated values
start:rate:full (e.g. "10:30:60"). sshd(8) will refuse
connection attempts with a probability of rate/100 (30%)
if there are currently start (10) unauthenticated
connections. The probability increases linearly and all
connection attempts are refused if the number of
unauthenticated connections reaches full (60).
ModuliFile
Specifies the moduli(5) file that contains the Diffie-
Hellman groups used for the
“diffie-hellman-group-exchange-sha1” and
“diffie-hellman-group-exchange-sha256” key exchange
methods. The default is /etc/moduli.
PasswordAuthentication
Specifies whether password authentication is allowed.
The default is yes.
PermitEmptyPasswords
When password authentication is allowed, it specifies
whether the server allows login to accounts with empty
password strings. The default is no.
PermitListen
Specifies the addresses/ports on which a remote TCP port
forwarding may listen. The listen specification must be
one of the following forms:
PermitListen port
PermitListen host:port
Multiple permissions may be specified by separating them
with whitespace. An argument of any can be used to
remove all restrictions and permit any listen requests.
An argument of none can be used to prohibit all listen
requests. The host name may contain wildcards as
described in the PATTERNS section in ssh_config(5). The
wildcard ‘*’ can also be used in place of a port number
to allow all ports. By default all port forwarding
listen requests are permitted. Note that the
GatewayPorts option may further restrict which addresses
may be listened on. Note also that ssh(1) will request a
listen host of “localhost” if no listen host was
specifically requested, and this name is treated
differently to explicit localhost addresses of
“127.0.0.1” and “::1”.
PermitOpen
Specifies the destinations to which TCP port forwarding
is permitted. The forwarding specification must be one
of the following forms:
PermitOpen host:port
PermitOpen IPv4_addr:port
PermitOpen [IPv6_addr]:port
Multiple forwards may be specified by separating them
with whitespace. An argument of any can be used to
remove all restrictions and permit any forwarding
requests. An argument of none can be used to prohibit
all forwarding requests. The wildcard ‘*’ can be used
for host or port to allow all hosts or ports
respectively. Otherwise, no pattern matching or address
lookups are performed on supplied names. By default all
port forwarding requests are permitted.
PermitRootLogin
Specifies whether root can log in using ssh(1). The
argument must be yes, prohibit-password,
forced-commands-only, or no. The default is
prohibit-password.
If this option is set to prohibit-password (or its
deprecated alias, without-password), password and
keyboard-interactive authentication are disabled for
root.
If this option is set to forced-commands-only, root login
with public key authentication will be allowed, but only
if the command option has been specified (which may be
useful for taking remote backups even if root login is
normally not allowed). All other authentication methods
are disabled for root.
If this option is set to no, root is not allowed to log
in.
PermitTTY
Specifies whether pty(4) allocation is permitted. The
default is yes.
PermitTunnel
Specifies whether tun(4) device forwarding is allowed.
The argument must be yes, point-to-point (layer 3),
ethernet (layer 2), or no. Specifying yes permits both
point-to-point and ethernet. The default is no.
Independent of this setting, the permissions of the
selected tun(4) device must allow access to the user.
PermitUserEnvironment
Specifies whether ~/.ssh/environment and environment=
options in ~/.ssh/authorized_keys are processed by
sshd(8). Valid options are yes, no or a pattern-list
specifying which environment variable names to accept
(for example "LANG,LC_*"). The default is no. Enabling
environment processing may enable users to bypass access
restrictions in some configurations using mechanisms such
as LD_PRELOAD.
PermitUserRC
Specifies whether any ~/.ssh/rc file is executed. The
default is yes.
PerSourceMaxStartups
Specifies the number of unauthenticated connections
allowed from a given source address, or “none” if there
is no limit. This limit is applied in addition to
MaxStartups, whichever is lower. The default is none.
PerSourceNetBlockSize
Specifies the number of bits of source address that are
grouped together for the purposes of applying
PerSourceMaxStartups limits. Values for IPv4 and
optionally IPv6 may be specified, separated by a colon.
The default is 32:128, which means each address is
considered individually.
PidFile
Specifies the file that contains the process ID of the
SSH daemon, or none to not write one. The default is
/var/run/sshd.pid.
Port Specifies the port number that sshd(8) listens on. The
default is 22. Multiple options of this type are
permitted. See also ListenAddress.
PrintLastLog
Specifies whether sshd(8) should print the date and time
of the last user login when a user logs in interactively.
The default is yes.
PrintMotd
Specifies whether sshd(8) should print /etc/motd when a
user logs in interactively. (On some systems it is also
printed by the shell, /etc/profile, or equivalent.) The
default is yes.
PubkeyAcceptedAlgorithms
Specifies the signature algorithms that will be accepted
for public key authentication as a list of comma-
separated patterns. Alternately if the specified list
begins with a ‘+’ character, then the specified
algorithms will be appended to the default set instead of
replacing them. If the specified list begins with a ‘-’
character, then the specified algorithms (including
wildcards) will be removed from the default set instead
of replacing them. If the specified list begins with a
‘^’ character, then the specified algorithms will be
placed at the head of the default set. The default for
this option is:
[email protected],
[email protected],
[email protected],
[email protected],
[email protected],
[email protected],
[email protected],
[email protected],
ssh-ed25519,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
[email protected],
[email protected],
rsa-sha2-512,rsa-sha2-256
The list of available signature algorithms may also be
obtained using "ssh -Q PubkeyAcceptedAlgorithms".
PubkeyAuthOptions
Sets one or more public key authentication options. The
supported keywords are: none (the default; indicating no
additional options are enabled), touch-required and
verify-required.
The touch-required option causes public key
authentication using a FIDO authenticator algorithm (i.e.
ecdsa-sk or ed25519-sk) to always require the signature
to attest that a physically present user explicitly
confirmed the authentication (usually by touching the
authenticator). By default, sshd(8) requires user
presence unless overridden with an authorized_keys
option. The touch-required flag disables this override.
The verify-required option requires a FIDO key signature
attest that the user was verified, e.g. via a PIN.
Neither the touch-required or verify-required options
have any effect for other, non-FIDO, public key types.
PubkeyAuthentication
Specifies whether public key authentication is allowed.
The default is yes.
RekeyLimit
Specifies the maximum amount of data that may be
transmitted or received before the session key is
renegotiated, optionally followed by a maximum amount of
time that may pass before the session key is
renegotiated. The first argument is specified in bytes
and may have a suffix of ‘K’, ‘M’, or ‘G’ to indicate
Kilobytes, Megabytes, or Gigabytes, respectively. The
default is between ‘1G’ and ‘4G’, depending on the
cipher. The optional second value is specified in
seconds and may use any of the units documented in the
“TIME FORMATS” section. The default value for RekeyLimit
is default none, which means that rekeying is performed
after the cipher's default amount of data has been sent
or received and no time based rekeying is done.
RequiredRSASize
Specifies the minimum RSA key size (in bits) that sshd(8)
will accept. User and host-based authentication keys
smaller than this limit will be refused. The default is
1024 bits. Note that this limit may only be raised from
the default.
RevokedKeys
Specifies revoked public keys file, or none to not use
one. Keys listed in this file will be refused for public
key authentication. Note that if this file is not
readable, then public key authentication will be refused
for all users. Keys may be specified as a text file,
listing one public key per line, or as an OpenSSH Key
Revocation List (KRL) as generated by ssh-keygen(1). For
more information on KRLs, see the KEY REVOCATION LISTS
section in ssh-keygen(1).
RDomain
Specifies an explicit routing domain that is applied
after authentication has completed. The user session, as
well as any forwarded or listening IP sockets, will be
bound to this rdomain(4). If the routing domain is set
to %D, then the domain in which the incoming connection
was received will be applied.
SecurityKeyProvider
Specifies a path to a library that will be used when
loading FIDO authenticator-hosted keys, overriding the
default of using the built-in USB HID support.
SetEnv Specifies one or more environment variables to set in
child sessions started by sshd(8) as “NAME=VALUE”. The
environment value may be quoted (e.g. if it contains
whitespace characters). Environment variables set by
SetEnv override the default environment and any variables
specified by the user via AcceptEnv or
PermitUserEnvironment.
StreamLocalBindMask
Sets the octal file creation mode mask (umask) used when
creating a Unix-domain socket file for local or remote
port forwarding. This option is only used for port
forwarding to a Unix-domain socket file.
The default value is 0177, which creates a Unix-domain
socket file that is readable and writable only by the
owner. Note that not all operating systems honor the
file mode on Unix-domain socket files.
StreamLocalBindUnlink
Specifies whether to remove an existing Unix-domain
socket file for local or remote port forwarding before
creating a new one. If the socket file already exists
and StreamLocalBindUnlink is not enabled, sshd will be
unable to forward the port to the Unix-domain socket
file. This option is only used for port forwarding to a
Unix-domain socket file.
The argument must be yes or no. The default is no.
StrictModes
Specifies whether sshd(8) should check file modes and
ownership of the user's files and home directory before
accepting login. This is normally desirable because
novices sometimes accidentally leave their directory or
files world-writable. The default is yes. Note that
this does not apply to ChrootDirectory, whose permissions
and ownership are checked unconditionally.
Subsystem
Configures an external subsystem (e.g. file transfer
daemon). Arguments should be a subsystem name and a
command (with optional arguments) to execute upon
subsystem request.
The command sftp-server implements the SFTP file transfer
subsystem.
Alternately the name internal-sftp implements an in-
process SFTP server. This may simplify configurations
using ChrootDirectory to force a different filesystem
root on clients. It accepts the same command line
arguments as sftp-server and even though it is in-
process, settings such as LogLevel or SyslogFacility do
not apply to it and must be set explicitly via command
line arguments.
By default no subsystems are defined.
SyslogFacility
Gives the facility code that is used when logging
messages from sshd(8). The possible values are: DAEMON,
USER, AUTH, LOCAL0, LOCAL1, LOCAL2, LOCAL3, LOCAL4,
LOCAL5, LOCAL6, LOCAL7. The default is AUTH.
TCPKeepAlive
Specifies whether the system should send TCP keepalive
messages to the other side. If they are sent, death of
the connection or crash of one of the machines will be
properly noticed. However, this means that connections
will die if the route is down temporarily, and some
people find it annoying. On the other hand, if TCP
keepalives are not sent, sessions may hang indefinitely
on the server, leaving "ghost" users and consuming server
resources.
The default is yes (to send TCP keepalive messages), and
the server will notice if the network goes down or the
client host crashes. This avoids infinitely hanging
sessions.
To disable TCP keepalive messages, the value should be
set to no.
TrustedUserCAKeys
Specifies a file containing public keys of certificate
authorities that are trusted to sign user certificates
for authentication, or none to not use one. Keys are
listed one per line; empty lines and comments starting
with ‘#’ are allowed. If a certificate is presented for
authentication and has its signing CA key listed in this
file, then it may be used for authentication for any user
listed in the certificate's principals list. Note that
certificates that lack a list of principals will not be
permitted for authentication using TrustedUserCAKeys.
For more details on certificates, see the CERTIFICATES
section in ssh-keygen(1).
UnusedConnectionTimeout
Specifies whether and how quickly sshd(8) should close
client connections with no open channels. Open channels
include active shell, command execution or subsystem
sessions, connected network, socket, agent or X11
forwardings. Forwarding listeners, such as those from
the ssh(1) -R flag, are not considered as open channels
and do not prevent the timeout. The timeout value is
specified in seconds or may use any of the units
documented in the “TIME FORMATS” section.
Note that this timeout starts when the client connection
completes user authentication but before the client has
an opportunity to open any channels. Caution should be
used when using short timeout values, as they may not
provide sufficient time for the client to request and
open its channels before terminating the connection.
The default none is to never expire connections for
having no open channels. This option may be useful in
conjunction with ChannelTimeout.
UseDNS Specifies whether sshd(8) should look up the remote host
name, and to check that the resolved host name for the
remote IP address maps back to the very same IP address.
If this option is set to no (the default) then only
addresses and not host names may be used in
~/.ssh/authorized_keys from and sshd_config Match Host
directives.
UsePAM Enables the Pluggable Authentication Module interface.
If set to yes this will enable PAM authentication using
KbdInteractiveAuthentication and PasswordAuthentication
in addition to PAM account and session module processing
for all authentication types.
Because PAM keyboard-interactive authentication usually
serves an equivalent role to password authentication, you
should disable either PasswordAuthentication or
KbdInteractiveAuthentication.
If UsePAM is enabled, you will not be able to run sshd(8)
as a non-root user. The default is no.
VersionAddendum
Optionally specifies additional text to append to the SSH
protocol banner sent by the server upon connection. The
default is none.
X11DisplayOffset
Specifies the first display number available for
sshd(8)'s X11 forwarding. This prevents sshd from
interfering with real X11 servers. The default is 10.
X11Forwarding
Specifies whether X11 forwarding is permitted. The
argument must be yes or no. The default is no.
When X11 forwarding is enabled, there may be additional
exposure to the server and to client displays if the
sshd(8) proxy display is configured to listen on the
wildcard address (see X11UseLocalhost), though this is
not the default. Additionally, the authentication
spoofing and authentication data verification and
substitution occur on the client side. The security risk
of using X11 forwarding is that the client's X11 display
server may be exposed to attack when the SSH client
requests forwarding (see the warnings for ForwardX11 in
ssh_config(5)). A system administrator may have a stance
in which they want to protect clients that may expose
themselves to attack by unwittingly requesting X11
forwarding, which can warrant a no setting.
Note that disabling X11 forwarding does not prevent users
from forwarding X11 traffic, as users can always install
their own forwarders.
X11UseLocalhost
Specifies whether sshd(8) should bind the X11 forwarding
server to the loopback address or to the wildcard
address. By default, sshd binds the forwarding server to
the loopback address and sets the hostname part of the
DISPLAY environment variable to localhost. This prevents
remote hosts from connecting to the proxy display.
However, some older X11 clients may not function with
this configuration. X11UseLocalhost may be set to no to
specify that the forwarding server should be bound to the
wildcard address. The argument must be yes or no. The
default is yes.
XAuthLocation
Specifies the full pathname of the xauth(1) program, or
none to not use one. The default is
/usr/X11R6/bin/xauth.
sshd(8) command-line arguments and configuration file options
that specify time may be expressed using a sequence of the form:
time[qualifier], where time is a positive integer value and
qualifier is one of the following:
⟨none⟩ seconds
s | S seconds
m | M minutes
h | H hours
d | D days
w | W weeks
Each member of the sequence is added together to calculate the
total time value.
Time format examples:
600 600 seconds (10 minutes)
10m 10 minutes
1h30m 1 hour 30 minutes (90 minutes)
Arguments to some keywords can make use of tokens, which are
expanded at runtime:
%% A literal ‘%’.
%C Identifies the connection endpoints, containing four
space-separated values: client address, client port
number, server address, and server port number.
%D The routing domain in which the incoming connection
was received.
%F The fingerprint of the CA key.
%f The fingerprint of the key or certificate.
%h The home directory of the user.
%i The key ID in the certificate.
%K The base64-encoded CA key.
%k The base64-encoded key or certificate for
authentication.
%s The serial number of the certificate.
%T The type of the CA key.
%t The key or certificate type.
%U The numeric user ID of the target user.
%u The username.
AuthorizedKeysCommand accepts the tokens %%, %C, %D, %f, %h, %k,
%t, %U, and %u.
AuthorizedKeysFile accepts the tokens %%, %h, %U, and %u.
AuthorizedPrincipalsCommand accepts the tokens %%, %C, %D, %F,
%f, %h, %i, %K, %k, %s, %T, %t, %U, and %u.
AuthorizedPrincipalsFile accepts the tokens %%, %h, %U, and %u.
ChrootDirectory accepts the tokens %%, %h, %U, and %u.
RoutingDomain accepts the token %D.
/etc/ssh/sshd_config
Contains configuration data for sshd(8). This file
should be writable by root only, but it is recommended
(though not necessary) that it be world-readable.
sftp-server(8), sshd(8)
OpenSSH is a derivative of the original and free ssh 1.2.12
release by Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl,
Niels Provos, Theo de Raadt and Dug Song removed many bugs, re-
added newer features and created OpenSSH. Markus Friedl
contributed the support for SSH protocol versions 1.5 and 2.0.
Niels Provos and Markus Friedl contributed support for privilege
separation.
This page is part of the openssh (Portable OpenSSH) project.
Information about the project can be found at
http://www.openssh.com/portable.html. If you have a bug report
for this manual page, see ⟨http://www.openssh.com/report.html⟩.
This page was obtained from the tarball openssh-9.7p1.tar.gz
fetched from
⟨http://ftp.eu.openbsd.org/pub/OpenBSD/OpenSSH/portable/⟩ on
2024-06-14. 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]
GNU February 21, 2024 SSHD_CONFIG(5)
Pages that refer to this page: userdbctl(1)
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