SSHD_CONFIG(5) - File Formats Manual

SSHD_CONFIG(5) - File Formats Manual #

SSHD_CONFIG(5) - File Formats Manual

NAME #

sshd_config - OpenSSH daemon configuration file

DESCRIPTION #

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)](/man/man8/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**,
**pam**,
or
**skey**,
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)](/man/man8/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)](/man/man8/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)](/man/man8/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)](/man/man8/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)](/man/man8/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)](/man/man8/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)](/man/man8/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,
	sk-ssh-ed25519@openssh.com,
	sk-ecdsa-sha2-nistp256@openssh.com,
	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)](/man/man8/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**, **direct-streamlocal@openssh.com**

	Open TCP or Unix socket (respectively) connections that have
	been established from a
	ssh(1)
	local forwarding, i.e.
	**LocalForward**
	or
	**DynamicForward**.

**forwarded-tcpip**, **forwarded-streamlocal@openssh.com**

	Open TCP or Unix socket (respectively) connections that have been
	established to a
	[sshd(8)](/man/man8/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)](/man/man8/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)](/man/man8/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)](/man/man8/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)](/man/man8/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  
aes128-gcm@openssh.com  
aes256-gcm@openssh.com  
chacha20-poly1305@openssh.com

The default is:

	chacha20-poly1305@openssh.com,
	aes128-ctr,aes192-ctr,aes256-ctr,
	aes128-gcm@openssh.com,aes256-gcm@openssh.com

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)](/man/man8/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)](/man/man8/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)](/man/man8/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:

	ssh-ed25519-cert-v01@openssh.com,
	ecdsa-sha2-nistp256-cert-v01@openssh.com,
	ecdsa-sha2-nistp384-cert-v01@openssh.com,
	ecdsa-sha2-nistp521-cert-v01@openssh.com,
	sk-ssh-ed25519-cert-v01@openssh.com,
	sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
	rsa-sha2-512-cert-v01@openssh.com,
	rsa-sha2-256-cert-v01@openssh.com,
	ssh-ed25519,
	ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
	sk-ssh-ed25519@openssh.com,
	sk-ecdsa-sha2-nistp256@openssh.com,
	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)](/man/man8/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)](/man/man8/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)](/man/man8/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)](/man/man8/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:

	ssh-ed25519-cert-v01@openssh.com,
	ecdsa-sha2-nistp256-cert-v01@openssh.com,
	ecdsa-sha2-nistp384-cert-v01@openssh.com,
	ecdsa-sha2-nistp521-cert-v01@openssh.com,
	sk-ssh-ed25519-cert-v01@openssh.com,
	sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
	rsa-sha2-512-cert-v01@openssh.com,
	rsa-sha2-256-cert-v01@openssh.com,
	ssh-ed25519,
	ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
	sk-ssh-ed25519@openssh.com,
	sk-ecdsa-sha2-nistp256@openssh.com,
	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)](/man/man8/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  
curve25519-sha256@libssh.org  
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  
sntrup761x25519-sha512@openssh.com

The default is:

	sntrup761x25519-sha512@openssh.com,
	curve25519-sha256,curve25519-sha256@libssh.org,
	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)](/man/man8/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)](/man/man8/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)](/man/man8/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  
umac-64@openssh.com  
umac-128@openssh.com  
hmac-md5-etm@openssh.com  
hmac-md5-96-etm@openssh.com  
hmac-sha1-etm@openssh.com  
hmac-sha1-96-etm@openssh.com  
hmac-sha2-256-etm@openssh.com  
hmac-sha2-512-etm@openssh.com  
umac-64-etm@openssh.com  
umac-128-etm@openssh.com

The default is:

	umac-64-etm@openssh.com,umac-128-etm@openssh.com,
	hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
	hmac-sha1-etm@openssh.com,
	umac-64@openssh.com,umac-128@openssh.com,
	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)](/man/man8/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)](/man/man8/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)](/man/man8/sshd.8)
listens on.
The default is 22.
Multiple options of this type are permitted.
See also
**ListenAddress**.

PrintLastLog

Specifies whether
[sshd(8)](/man/man8/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)](/man/man8/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:

	ssh-ed25519-cert-v01@openssh.com,
	ecdsa-sha2-nistp256-cert-v01@openssh.com,
	ecdsa-sha2-nistp384-cert-v01@openssh.com,
	ecdsa-sha2-nistp521-cert-v01@openssh.com,
	sk-ssh-ed25519-cert-v01@openssh.com,
	sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
	rsa-sha2-512-cert-v01@openssh.com,
	rsa-sha2-256-cert-v01@openssh.com,
	ssh-ed25519,
	ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
	sk-ssh-ed25519@openssh.com,
	sk-ecdsa-sha2-nistp256@openssh.com,
	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)](/man/man8/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)](/man/man8/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)](/man/man8/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)](/man/man8/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)](/man/man8/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)](/man/man8/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)](/man/man8/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.

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)](/man/man8/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)](/man/man8/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)](/man/man8/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*.

TIME FORMATS #

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)

TOKENS #

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.

FILES #

/etc/ssh/sshd_config

Contains configuration data for
[sshd(8)](/man/man8/sshd.8).
This file should be writable by root only, but it is recommended
(though not necessary) that it be world-readable.

SEE ALSO #

sftp-server(8), sshd(8)

AUTHORS #

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.

OpenBSD 7.5 - February 21, 2024