sshd_config.5

.\"
.\" Author: Tatu Ylonen <ylo@cs.hut.fi>
.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
.\"                    All rights reserved
.\"
.\" As far as I am concerned, the code I have written for this software
.\" can be used freely for any purpose.  Any derived versions of this
.\" software must be clearly marked as such, and if the derived work is
.\" incompatible with the protocol description in the RFC file, it must be
.\" called by a name other than "ssh" or "Secure Shell".
.\"
.\" Copyright (c) 1999,2000 Markus Friedl.  All rights reserved.
.\" Copyright (c) 1999 Aaron Campbell.  All rights reserved.
.\" Copyright (c) 1999 Theo de Raadt.  All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
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.\"
.\" $OpenBSD: sshd_config.5,v 1.291 2019/11/07 08:38:38 naddy Exp $
.Dd $Mdocdate: November 7 2019 $
.Dt SSHD_CONFIG 5
.Os
.Sh NAME
.Nm sshd_config
.Nd OpenSSH SSH daemon configuration file
.Sh DESCRIPTION
.Xr sshd 8
reads configuration data from
.Pa /etc/ssh/sshd_config
(or the file specified with
.Fl f
on the command line).
The file contains keyword-argument pairs, one per line.
For each keyword, the first obtained value will be used.
Lines starting with
.Ql #
and empty lines are interpreted as comments.
Arguments may optionally be enclosed in double quotes
.Pq \&"
in order to represent arguments containing spaces.
.Pp
The possible
keywords and their meanings are as follows (note that
keywords are case-insensitive and arguments are case-sensitive):
.Bl -tag -width Ds
.It Cm AcceptEnv
Specifies what environment variables sent by the client will be copied into
the session's
.Xr environ 7 .
See
.Cm SendEnv
and
.Cm SetEnv
in
.Xr ssh_config 5
for how to configure the client.
The
.Ev 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
.Ql *
and
.Ql \&? .
Multiple environment variables may be separated by whitespace or spread
across multiple
.Cm 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.
.It Cm AddressFamily
Specifies which address family should be used by
.Xr sshd 8 .
Valid arguments are
.Cm any
(the default),
.Cm inet
(use IPv4 only), or
.Cm inet6
(use IPv6 only).
.It Cm AllowAgentForwarding
Specifies whether
.Xr ssh-agent 1
forwarding is permitted.
The default is
.Cm 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.
.It Cm 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 directives are processed in the following order:
.Cm DenyUsers ,
.Cm AllowUsers ,
.Cm DenyGroups ,
and finally
.Cm AllowGroups .
.Pp
See PATTERNS in
.Xr ssh_config 5
for more information on patterns.
.It Cm AllowStreamLocalForwarding
Specifies whether StreamLocal (Unix-domain socket) forwarding is permitted.
The available options are
.Cm yes
(the default)
or
.Cm all
to allow StreamLocal forwarding,
.Cm no
to prevent all StreamLocal forwarding,
.Cm local
to allow local (from the perspective of
.Xr ssh 1 )
forwarding only or
.Cm 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.
.It Cm AllowTcpForwarding
Specifies whether TCP forwarding is permitted.
The available options are
.Cm yes
(the default)
or
.Cm all
to allow TCP forwarding,
.Cm no
to prevent all TCP forwarding,
.Cm local
to allow local (from the perspective of
.Xr ssh 1 )
forwarding only or
.Cm 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.
.It Cm 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 directives are processed in the following order:
.Cm DenyUsers ,
.Cm AllowUsers ,
.Cm DenyGroups ,
and finally
.Cm AllowGroups .
.Pp
See PATTERNS in
.Xr ssh_config 5
for more information on patterns.
.It Cm 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
.Cm 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.
.Pp
For example,
.Qq 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.
.Pp
For keyboard interactive authentication it is also possible to
restrict authentication to a specific device by appending a
colon followed by the device identifier
.Cm bsdauth
or
.Cm pam .
depending on the server configuration.
For example,
.Qq keyboard-interactive:bsdauth
would restrict keyboard interactive authentication to the
.Cm bsdauth
device.
.Pp
If the publickey method is listed more than once,
.Xr sshd 8
verifies that keys that have been used successfully are not reused for
subsequent authentications.
For example,
.Qq publickey,publickey
requires successful authentication using two different public keys.
.Pp
Note that each authentication method listed should also be explicitly enabled
in the configuration.
.Pp
The available authentication methods are:
.Qq gssapi-with-mic ,
.Qq hostbased ,
.Qq keyboard-interactive ,
.Qq none
(used for access to password-less accounts when
.Cm PermitEmptyPasswords
is enabled),
.Qq password
and
.Qq publickey .
.It Cm 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
.Cm AuthorizedKeysCommand
accept the tokens described in the
.Sx TOKENS
section.
If no arguments are specified then the username of the target user is used.
.Pp
The program should produce on standard output zero or
more lines of authorized_keys output (see
.Sx AUTHORIZED_KEYS
in
.Xr sshd 8 ) .
If a key supplied by
.Cm AuthorizedKeysCommand
does not successfully authenticate
and authorize the user then public key authentication continues using the usual
.Cm AuthorizedKeysFile
files.
By default, no
.Cm AuthorizedKeysCommand
is run.
.It Cm AuthorizedKeysCommandUser
Specifies the user under whose account the
.Cm 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
.Cm AuthorizedKeysCommand
is specified but
.Cm AuthorizedKeysCommandUser
is not, then
.Xr sshd 8
will refuse to start.
.It Cm 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
.Xr sshd 8 .
Arguments to
.Cm AuthorizedKeysFile
accept the tokens described in the
.Sx TOKENS
section.
After expansion,
.Cm 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
.Cm none
to skip checking for user keys in files.
The default is
.Qq .ssh/authorized_keys .ssh/authorized_keys2 .
.It Cm AuthorizedPrincipalsCommand
Specifies a program to be used to generate the list of allowed
certificate principals as per
.Cm AuthorizedPrincipalsFile .
The program must be owned by root, not writable by group or others and
specified by an absolute path.
Arguments to
.Cm AuthorizedPrincipalsCommand
accept the tokens described in the
.Sx TOKENS
section.
If no arguments are specified then the username of the target user is used.
.Pp
The program should produce on standard output zero or
more lines of
.Cm AuthorizedPrincipalsFile
output.
If either
.Cm AuthorizedPrincipalsCommand
or
.Cm AuthorizedPrincipalsFile
is specified, then certificates offered by the client for authentication
must contain a principal that is listed.
By default, no
.Cm AuthorizedPrincipalsCommand
is run.
.It Cm AuthorizedPrincipalsCommandUser
Specifies the user under whose account the
.Cm 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
.Cm AuthorizedPrincipalsCommand
is specified but
.Cm AuthorizedPrincipalsCommandUser
is not, then
.Xr sshd 8
will refuse to start.
.It Cm AuthorizedPrincipalsFile
Specifies a file that lists principal names that are accepted for
certificate authentication.
When using certificates signed by a key listed in
.Cm 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
.Sx AUTHORIZED_KEYS FILE FORMAT
in
.Xr sshd 8 ) .
Empty lines and comments starting with
.Ql #
are ignored.
.Pp
Arguments to
.Cm AuthorizedPrincipalsFile
accept the tokens described in the
.Sx TOKENS
section.
After expansion,
.Cm AuthorizedPrincipalsFile
is taken to be an absolute path or one relative to the user's home directory.
The default is
.Cm none ,
i.e. not to use a principals file \(en in this case, the username
of the user must appear in a certificate's principals list for it to be
accepted.
.Pp
Note that
.Cm AuthorizedPrincipalsFile
is only used when authentication proceeds using a CA listed in
.Cm TrustedUserCAKeys
and is not consulted for certification authorities trusted via
.Pa ~/.ssh/authorized_keys ,
though the
.Cm principals=
key option offers a similar facility (see
.Xr sshd 8
for details).
.It Cm Banner
The contents of the specified file are sent to the remote user before
authentication is allowed.
If the argument is
.Cm none
then no banner is displayed.
By default, no banner is displayed.
.It Cm CASignatureAlgorithms
Specifies which algorithms are allowed for signing of certificates
by certificate authorities (CAs).
The default is:
.Bd -literal -offset indent
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
.Ed
.Pp
Certificates signed using other algorithms will not be accepted for
public key or host-based authentication.
.It Cm ChallengeResponseAuthentication
Specifies whether challenge-response authentication is allowed (e.g. via
PAM or through authentication styles supported in
.Xr login.conf 5 )
The default is
.Cm yes .
.It Cm ChrootDirectory
Specifies the pathname of a directory to
.Xr chroot 2
to after authentication.
At session startup
.Xr sshd 8
checks that all components of the pathname are root-owned directories
which are not writable by any other user or group.
After the chroot,
.Xr sshd 8
changes the working directory to the user's home directory.
Arguments to
.Cm ChrootDirectory
accept the tokens described in the
.Sx TOKENS
section.
.Pp
The
.Cm 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
.Xr sh 1 ,
and basic
.Pa /dev
nodes such as
.Xr null 4 ,
.Xr zero 4 ,
.Xr stdin 4 ,
.Xr stdout 4 ,
.Xr stderr 4 ,
and
.Xr 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
.Pa /dev/log
inside the chroot directory on some operating systems (see
.Xr sftp-server 8
for details).
.Pp
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
.Xr sshd 8
cannot detect.
.Pp
The default is
.Cm none ,
indicating not to
.Xr chroot 2 .
.It Cm Ciphers
Specifies the ciphers allowed.
Multiple ciphers must be comma-separated.
If the specified list begins with a
.Sq +
character, then the specified ciphers will be appended to the default set
instead of replacing them.
If the specified list begins with a
.Sq -
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
.Sq ^
character, then the specified ciphers will be placed at the head of the
default set.
.Pp
The supported ciphers are:
.Pp
.Bl -item -compact -offset indent
.It
3des-cbc
.It
aes128-cbc
.It
aes192-cbc
.It
aes256-cbc
.It
aes128-ctr
.It
aes192-ctr
.It
aes256-ctr
.It
aes128-gcm@openssh.com
.It
aes256-gcm@openssh.com
.It
chacha20-poly1305@openssh.com
.El
.Pp
The default is:
.Bd -literal -offset indent
chacha20-poly1305@openssh.com,
aes128-ctr,aes192-ctr,aes256-ctr,
aes128-gcm@openssh.com,aes256-gcm@openssh.com
.Ed
.Pp
The list of available ciphers may also be obtained using
.Qq ssh -Q cipher .
.It Cm ClientAliveCountMax
Sets the number of client alive messages which may be sent without
.Xr 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
.Cm TCPKeepAlive .
The client alive messages are sent through the encrypted channel
and therefore will not be spoofable.
The TCP keepalive option enabled by
.Cm TCPKeepAlive
is spoofable.
The client alive mechanism is valuable when the client or
server depend on knowing when a connection has become unresponsive.
.Pp
The default value is 3.
If
.Cm ClientAliveInterval
is set to 15, and
.Cm ClientAliveCountMax
is left at the default, unresponsive SSH clients
will be disconnected after approximately 45 seconds.
.It Cm ClientAliveInterval
Sets a timeout interval in seconds after which if no data has been received
from the client,
.Xr 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.
.It Cm Compression
Specifies whether compression is enabled after
the user has authenticated successfully.
The argument must be
.Cm yes ,
.Cm delayed
(a legacy synonym for
.Cm yes )
or
.Cm no .
The default is
.Cm yes .
.It Cm 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 directives are processed in the following order:
.Cm DenyUsers ,
.Cm AllowUsers ,
.Cm DenyGroups ,
and finally
.Cm AllowGroups .
.Pp
See PATTERNS in
.Xr ssh_config 5
for more information on patterns.
.It Cm 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 directives are processed in the following order:
.Cm DenyUsers ,
.Cm AllowUsers ,
.Cm DenyGroups ,
and finally
.Cm AllowGroups .
.Pp
See PATTERNS in
.Xr ssh_config 5
for more information on patterns.
.It Cm DisableForwarding
Disables all forwarding features, including X11,
.Xr ssh-agent 1 ,
TCP and StreamLocal.
This option overrides all other forwarding-related options and may
simplify restricted configurations.
.It Cm 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
.Ev SSH_USER_AUTH
environment variable.
The default is
.Cm no .
.It Cm FingerprintHash
Specifies the hash algorithm used when logging key fingerprints.
Valid options are:
.Cm md5
and
.Cm sha256 .
The default is
.Cm sha256 .
.It Cm ForceCommand
Forces the execution of the command specified by
.Cm ForceCommand ,
ignoring any command supplied by the client and
.Pa ~/.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
.Cm Match
block.
The command originally supplied by the client is available in the
.Ev SSH_ORIGINAL_COMMAND
environment variable.
Specifying a command of
.Cm internal-sftp
will force the use of an in-process SFTP server that requires no support
files when used with
.Cm ChrootDirectory .
The default is
.Cm none .
.It Cm GatewayPorts
Specifies whether remote hosts are allowed to connect to ports
forwarded for the client.
By default,
.Xr sshd 8
binds remote port forwardings to the loopback address.
This prevents other remote hosts from connecting to forwarded ports.
.Cm 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
.Cm no
to force remote port forwardings to be available to the local host only,
.Cm yes
to force remote port forwardings to bind to the wildcard address, or
.Cm clientspecified
to allow the client to select the address to which the forwarding is bound.
The default is
.Cm no .
.It Cm GSSAPIAuthentication
Specifies whether user authentication based on GSSAPI is allowed.
The default is
.Cm no .
.It Cm GSSAPICleanupCredentials
Specifies whether to automatically destroy the user's credentials cache
on logout.
The default is
.Cm yes .
.It Cm GSSAPIStrictAcceptorCheck
Determines whether to be strict about the identity of the GSSAPI acceptor
a client authenticates against.
If set to
.Cm yes
then the client must authenticate against the host
service on the current hostname.
If set to
.Cm 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
.Cm yes .
.It Cm HostbasedAcceptedKeyTypes
Specifies the key types that will be accepted for hostbased authentication
as a list of comma-separated patterns.
Alternately if the specified list begins with a
.Sq +
character, then the specified key types will be appended to the default set
instead of replacing them.
If the specified list begins with a
.Sq -
character, then the specified key types (including wildcards) will be removed
from the default set instead of replacing them.
If the specified list begins with a
.Sq ^
character, then the specified key types will be placed at the head of the
default set.
The default for this option is:
.Bd -literal -offset 3n
ecdsa-sha2-nistp256-cert-v01@openssh.com,
ecdsa-sha2-nistp384-cert-v01@openssh.com,
ecdsa-sha2-nistp521-cert-v01@openssh.com,
ssh-ed25519-cert-v01@openssh.com,
rsa-sha2-512-cert-v01@openssh.com,
rsa-sha2-256-cert-v01@openssh.com,
ssh-rsa-cert-v01@openssh.com,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
.Ed
.Pp
The list of available key types may also be obtained using
.Qq ssh -Q key .
.It Cm 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
.Cm no .
.It Cm HostbasedUsesNameFromPacketOnly
Specifies whether or not the server will attempt to perform a reverse
name lookup when matching the name in the
.Pa ~/.shosts ,
.Pa ~/.rhosts ,
and
.Pa /etc/hosts.equiv
files during
.Cm HostbasedAuthentication .
A setting of
.Cm yes
means that
.Xr sshd 8
uses the name supplied by the client rather than
attempting to resolve the name from the TCP connection itself.
The default is
.Cm no .
.It Cm HostCertificate
Specifies a file containing a public host certificate.
The certificate's public key must match a private host key already specified
by
.Cm HostKey .
The default behaviour of
.Xr sshd 8
is not to load any certificates.
.It Cm HostKey
Specifies a file containing a private host key
used by SSH.
The defaults are
.Pa /etc/ssh/ssh_host_ecdsa_key ,
.Pa /etc/ssh/ssh_host_ed25519_key
and
.Pa /etc/ssh/ssh_host_rsa_key .
.Pp
Note that
.Xr sshd 8
will refuse to use a file if it is group/world-accessible
and that the
.Cm HostKeyAlgorithms
option restricts which of the keys are actually used by
.Xr sshd 8 .
.Pp
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
.Xr ssh-agent 1 .
.It Cm HostKeyAgent
Identifies the UNIX-domain socket used to communicate
with an agent that has access to the private host keys.
If the string
.Qq SSH_AUTH_SOCK
is specified, the location of the socket will be read from the
.Ev SSH_AUTH_SOCK
environment variable.
.It Cm HostKeyAlgorithms
Specifies the host key algorithms
that the server offers.
The default for this option is:
.Bd -literal -offset 3n
ecdsa-sha2-nistp256-cert-v01@openssh.com,
ecdsa-sha2-nistp384-cert-v01@openssh.com,
ecdsa-sha2-nistp521-cert-v01@openssh.com,
ssh-ed25519-cert-v01@openssh.com,
rsa-sha2-512-cert-v01@openssh.com,
rsa-sha2-256-cert-v01@openssh.com,
ssh-rsa-cert-v01@openssh.com,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
.Ed
.Pp
The list of available key types may also be obtained using
.Qq ssh -Q key .
.It Cm IgnoreRhosts
Specifies that
.Pa .rhosts
and
.Pa .shosts
files will not be used in
.Cm HostbasedAuthentication .
.Pp
.Pa /etc/hosts.equiv
and
.Pa /etc/shosts.equiv
are still used.
The default is
.Cm yes .
.It Cm IgnoreUserKnownHosts
Specifies whether
.Xr sshd 8
should ignore the user's
.Pa ~/.ssh/known_hosts
during
.Cm HostbasedAuthentication
and use only the system-wide known hosts file
.Pa /etc/ssh/known_hosts .
The default is
.Cm no .
.It Cm IPQoS
Specifies the IPv4 type-of-service or DSCP class for the connection.
Accepted values are
.Cm af11 ,
.Cm af12 ,
.Cm af13 ,
.Cm af21 ,
.Cm af22 ,
.Cm af23 ,
.Cm af31 ,
.Cm af32 ,
.Cm af33 ,
.Cm af41 ,
.Cm af42 ,
.Cm af43 ,
.Cm cs0 ,
.Cm cs1 ,
.Cm cs2 ,
.Cm cs3 ,
.Cm cs4 ,
.Cm cs5 ,
.Cm cs6 ,
.Cm cs7 ,
.Cm ef ,
.Cm lowdelay ,
.Cm throughput ,
.Cm reliability ,
a numeric value, or
.Cm 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
.Cm af21
(Low-Latency Data)
for interactive sessions and
.Cm cs1
(Lower Effort)
for non-interactive sessions.
.It Cm KbdInteractiveAuthentication
Specifies whether to allow keyboard-interactive authentication.
The argument to this keyword must be
.Cm yes
or
.Cm no .
The default is to use whatever value
.Cm ChallengeResponseAuthentication
is set to
(by default
.Cm yes ) .
.It Cm KerberosAuthentication
Specifies whether the password provided by the user for
.Cm 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
.Cm no .
.It Cm 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
.Cm no .
.It Cm KerberosOrLocalPasswd
If password authentication through Kerberos fails then
the password will be validated via any additional local mechanism
such as
.Pa /etc/passwd .
The default is
.Cm yes .
.It Cm KerberosTicketCleanup
Specifies whether to automatically destroy the user's ticket cache
file on logout.
The default is
.Cm yes .
.It Cm KexAlgorithms
Specifies the available KEX (Key Exchange) algorithms.
Multiple algorithms must be comma-separated.
Alternately if the specified list begins with a
.Sq +
character, then the specified methods will be appended to the default set
instead of replacing them.
If the specified list begins with a
.Sq -
character, then the specified methods (including wildcards) will be removed
from the default set instead of replacing them.
If the specified list begins with a
.Sq ^
character, then the specified methods will be placed at the head of the
default set.
The supported algorithms are:
.Pp
.Bl -item -compact -offset indent
.It
curve25519-sha256
.It
curve25519-sha256@libssh.org
.It
diffie-hellman-group1-sha1
.It
diffie-hellman-group14-sha1
.It
diffie-hellman-group14-sha256
.It
diffie-hellman-group16-sha512
.It
diffie-hellman-group18-sha512
.It
diffie-hellman-group-exchange-sha1
.It
diffie-hellman-group-exchange-sha256
.It
ecdh-sha2-nistp256
.It
ecdh-sha2-nistp384
.It
ecdh-sha2-nistp521
.El
.Pp
The default is:
.Bd -literal -offset indent
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,diffie-hellman-group14-sha1
.Ed
.Pp
The list of available key exchange algorithms may also be obtained using
.Qq ssh -Q kex .
.It Cm ListenAddress
Specifies the local addresses
.Xr sshd 8
should listen on.
The following forms may be used:
.Pp
.Bl -item -offset indent -compact
.It
.Cm ListenAddress
.Sm off
.Ar hostname | address
.Sm on
.Op Cm rdomain Ar domain
.It
.Cm ListenAddress
.Sm off
.Ar hostname : port
.Sm on
.Op Cm rdomain Ar domain
.It
.Cm ListenAddress
.Sm off
.Ar IPv4_address : port
.Sm on
.Op Cm rdomain Ar domain
.It
.Cm ListenAddress
.Sm off
.Oo Ar hostname | address Oc : Ar port
.Sm on
.Op Cm rdomain Ar domain
.El
.Pp
The optional
.Cm rdomain
qualifier requests
.Xr sshd 8
listen in an explicit routing domain.
If
.Ar port
is not specified,
sshd will listen on the address and all
.Cm Port
options specified.
The default is to listen on all local addresses on the current default
routing domain.
Multiple
.Cm ListenAddress
options are permitted.
For more information on routing domains, see
.Xr rdomain 4 .
.It Cm 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.
.It Cm LogLevel
Gives the verbosity level that is used when logging messages from
.Xr 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.
.It Cm 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
.Sq +
character, then the specified algorithms will be appended to the default set
instead of replacing them.
If the specified list begins with a
.Sq -
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
.Sq ^
character, then the specified algorithms will be placed at the head of the
default set.
.Pp
The algorithms that contain
.Qq -etm
calculate the MAC after encryption (encrypt-then-mac).
These are considered safer and their use recommended.
The supported MACs are:
.Pp
.Bl -item -compact -offset indent
.It
hmac-md5
.It
hmac-md5-96
.It
hmac-sha1
.It
hmac-sha1-96
.It
hmac-sha2-256
.It
hmac-sha2-512
.It
umac-64@openssh.com
.It
umac-128@openssh.com
.It
hmac-md5-etm@openssh.com
.It
hmac-md5-96-etm@openssh.com
.It
hmac-sha1-etm@openssh.com
.It
hmac-sha1-96-etm@openssh.com
.It
hmac-sha2-256-etm@openssh.com
.It
hmac-sha2-512-etm@openssh.com
.It
umac-64-etm@openssh.com
.It
umac-128-etm@openssh.com
.El
.Pp
The default is:
.Bd -literal -offset indent
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
.Ed
.Pp
The list of available MAC algorithms may also be obtained using
.Qq ssh -Q mac .
.It Cm Match
Introduces a conditional block.
If all of the criteria on the
.Cm Match
line are satisfied, the keywords on the following lines override those
set in the global section of the config file, until either another
.Cm Match
line or the end of the file.
If a keyword appears in multiple
.Cm Match
blocks that are satisfied, only the first instance of the keyword is
applied.
.Pp
The arguments to
.Cm Match
are one or more criteria-pattern pairs or the single token
.Cm All
which matches all criteria.
The available criteria are
.Cm User ,
.Cm Group ,
.Cm Host ,
.Cm LocalAddress ,
.Cm LocalPort ,
.Cm RDomain ,
and
.Cm Address
(with
.Cm RDomain
representing the
.Xr rdomain 4
on which the connection was received).
.Pp
The match patterns may consist of single entries or comma-separated
lists and may use the wildcard and negation operators described in the
.Sx PATTERNS
section of
.Xr ssh_config 5 .
.Pp
The patterns in an
.Cm 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.
.Pp
Only a subset of keywords may be used on the lines following a
.Cm Match
keyword.
Available keywords are
.Cm AcceptEnv ,
.Cm AllowAgentForwarding ,
.Cm AllowGroups ,
.Cm AllowStreamLocalForwarding ,
.Cm AllowTcpForwarding ,
.Cm AllowUsers ,
.Cm AuthenticationMethods ,
.Cm AuthorizedKeysCommand ,
.Cm AuthorizedKeysCommandUser ,
.Cm AuthorizedKeysFile ,
.Cm AuthorizedPrincipalsCommand ,
.Cm AuthorizedPrincipalsCommandUser ,
.Cm AuthorizedPrincipalsFile ,
.Cm Banner ,
.Cm ChrootDirectory ,
.Cm ClientAliveCountMax ,
.Cm ClientAliveInterval ,
.Cm DenyGroups ,
.Cm DenyUsers ,
.Cm ForceCommand ,
.Cm GatewayPorts ,
.Cm GSSAPIAuthentication ,
.Cm HostbasedAcceptedKeyTypes ,
.Cm HostbasedAuthentication ,
.Cm HostbasedUsesNameFromPacketOnly ,
.Cm IPQoS ,
.Cm KbdInteractiveAuthentication ,
.Cm KerberosAuthentication ,
.Cm LogLevel ,
.Cm MaxAuthTries ,
.Cm MaxSessions ,
.Cm PasswordAuthentication ,
.Cm PermitEmptyPasswords ,
.Cm PermitListen ,
.Cm PermitOpen ,
.Cm PermitRootLogin ,
.Cm PermitTTY ,
.Cm PermitTunnel ,
.Cm PermitUserRC ,
.Cm PubkeyAcceptedKeyTypes ,
.Cm PubkeyAuthentication ,
.Cm RekeyLimit ,
.Cm RevokedKeys ,
.Cm RDomain ,
.Cm SetEnv ,
.Cm StreamLocalBindMask ,
.Cm StreamLocalBindUnlink ,
.Cm TrustedUserCAKeys ,
.Cm X11DisplayOffset ,
.Cm X11Forwarding
and
.Cm X11UseLocalhost .
.It Cm 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.
.It Cm 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
.Cm 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.
.It Cm MaxStartups
Specifies the maximum number of concurrent unauthenticated connections to the
SSH daemon.
Additional connections will be dropped until authentication succeeds or the
.Cm LoginGraceTime
expires for a connection.
The default is 10:30:100.
.Pp
Alternatively, random early drop can be enabled by specifying
the three colon separated values
start:rate:full (e.g. "10:30:60").
.Xr 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).
.It Cm PasswordAuthentication
Specifies whether password authentication is allowed.
The default is
.Cm yes .
.It Cm PermitEmptyPasswords
When password authentication is allowed, it specifies whether the
server allows login to accounts with empty password strings.
The default is
.Cm no .
.It Cm PermitListen
Specifies the addresses/ports on which a remote TCP port forwarding may listen.
The listen specification must be one of the following forms:
.Pp
.Bl -item -offset indent -compact
.It
.Cm PermitListen
.Sm off
.Ar port
.Sm on
.It
.Cm PermitListen
.Sm off
.Ar host : port
.Sm on
.El
.Pp
Multiple permissions may be specified by separating them with whitespace.
An argument of
.Cm any
can be used to remove all restrictions and permit any listen requests.
An argument of
.Cm none
can be used to prohibit all listen requests.
The host name may contain wildcards as described in the PATTERNS section in
.Xr ssh_config 5 .
The wildcard
.Sq *
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
.Cm GatewayPorts
option may further restrict which addresses may be listened on.
Note also that
.Xr ssh 1
will request a listen host of
.Dq localhost
if no listen host was specifically requested, and this name is
treated differently to explicit localhost addresses of
.Dq 127.0.0.1
and
.Dq ::1 .
.It Cm PermitOpen
Specifies the destinations to which TCP port forwarding is permitted.
The forwarding specification must be one of the following forms:
.Pp
.Bl -item -offset indent -compact
.It
.Cm PermitOpen
.Sm off
.Ar host : port
.Sm on
.It
.Cm PermitOpen
.Sm off
.Ar IPv4_addr : port
.Sm on
.It
.Cm PermitOpen
.Sm off
.Ar \&[ IPv6_addr \&] : port
.Sm on
.El
.Pp
Multiple forwards may be specified by separating them with whitespace.
An argument of
.Cm any
can be used to remove all restrictions and permit any forwarding requests.
An argument of
.Cm none
can be used to prohibit all forwarding requests.
The wildcard
.Sq *
can be used for host or port to allow all hosts or ports, respectively.
By default all port forwarding requests are permitted.
.It Cm PermitRootLogin
Specifies whether root can log in using
.Xr ssh 1 .
The argument must be
.Cm yes ,
.Cm prohibit-password ,
.Cm forced-commands-only ,
or
.Cm no .
The default is
.Cm prohibit-password .
.Pp
If this option is set to
.Cm prohibit-password
(or its deprecated alias,
.Cm without-password ) ,
password and keyboard-interactive authentication are disabled for root.
.Pp
If this option is set to
.Cm forced-commands-only ,
root login with public key authentication will be allowed,
but only if the
.Ar 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.
.Pp
If this option is set to
.Cm no ,
root is not allowed to log in.
.It Cm PermitTTY
Specifies whether
.Xr pty 4
allocation is permitted.
The default is
.Cm yes .
.It Cm PermitTunnel
Specifies whether
.Xr tun 4
device forwarding is allowed.
The argument must be
.Cm yes ,
.Cm point-to-point
(layer 3),
.Cm ethernet
(layer 2), or
.Cm no .
Specifying
.Cm yes
permits both
.Cm point-to-point
and
.Cm ethernet .
The default is
.Cm no .
.Pp
Independent of this setting, the permissions of the selected
.Xr tun 4
device must allow access to the user.
.It Cm PermitUserEnvironment
Specifies whether
.Pa ~/.ssh/environment
and
.Cm environment=
options in
.Pa ~/.ssh/authorized_keys
are processed by
.Xr sshd 8 .
Valid options are
.Cm yes ,
.Cm no
or a pattern-list specifying which environment variable names to accept
(for example
.Qq LANG,LC_* ) .
The default is
.Cm no .
Enabling environment processing may enable users to bypass access
restrictions in some configurations using mechanisms such as
.Ev LD_PRELOAD .
.It Cm PermitUserRC
Specifies whether any
.Pa ~/.ssh/rc
file is executed.
The default is
.Cm yes .
.It Cm PidFile
Specifies the file that contains the process ID of the
SSH daemon, or
.Cm none
to not write one.
The default is
.Pa /var/run/sshd.pid .
.It Cm Port
Specifies the port number that
.Xr sshd 8
listens on.
The default is 22.
Multiple options of this type are permitted.
See also
.Cm ListenAddress .
.It Cm PrintLastLog
Specifies whether
.Xr sshd 8
should print the date and time of the last user login when a user logs
in interactively.
The default is
.Cm yes .
.It Cm PrintMotd
Specifies whether
.Xr sshd 8
should print
.Pa /etc/motd
when a user logs in interactively.
(On some systems it is also printed by the shell,
.Pa /etc/profile ,
or equivalent.)
The default is
.Cm yes .
.It Cm PubkeyAcceptedKeyTypes
Specifies the key types that will be accepted for public key authentication
as a list of comma-separated patterns.
Alternately if the specified list begins with a
.Sq +
character, then the specified key types will be appended to the default set
instead of replacing them.
If the specified list begins with a
.Sq -
character, then the specified key types (including wildcards) will be removed
from the default set instead of replacing them.
If the specified list begins with a
.Sq ^
character, then the specified key types will be placed at the head of the
default set.
The default for this option is:
.Bd -literal -offset 3n
sk-ecdsa-sha2-nistp256-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,
ssh-ed25519-cert-v01@openssh.com,
rsa-sha2-512-cert-v01@openssh.com,
rsa-sha2-256-cert-v01@openssh.com,
ssh-rsa-cert-v01@openssh.com,
sk-ecdsa-sha2-nistp256@openssh.com,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
.Ed
.Pp
The list of available key types may also be obtained using
.Qq ssh -Q key .
.It Cm PubkeyAuthentication
Specifies whether public key authentication is allowed.
The default is
.Cm yes .
.It Cm RekeyLimit
Specifies the maximum amount of data that may be transmitted before the
session key is renegotiated, optionally followed 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
.Sq K ,
.Sq M ,
or
.Sq G
to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
The default is between
.Sq 1G
and
.Sq 4G ,
depending on the cipher.
The optional second value is specified in seconds and may use any of the
units documented in the
.Sx TIME FORMATS
section.
The default value for
.Cm RekeyLimit
is
.Cm 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.
.It Cm RevokedKeys
Specifies revoked public keys file, or
.Cm 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
.Xr ssh-keygen 1 .
For more information on KRLs, see the KEY REVOCATION LISTS section in
.Xr ssh-keygen 1 .
.It Cm RDomain
Specifies an explicit routing domain that is applied after authentication
has completed.
The user session, as well and any forwarded or listening IP sockets,
will be bound to this
.Xr rdomain 4 .
If the routing domain is set to
.Cm \&%D ,
then the domain in which the incoming connection was received will be applied.
.It Cm SetEnv
Specifies one or more environment variables to set in child sessions started
by
.Xr sshd 8
as
.Dq NAME=VALUE .
The environment value may be quoted (e.g. if it contains whitespace
characters).
Environment variables set by
.Cm SetEnv
override the default environment and any variables specified by the user
via
.Cm AcceptEnv
or
.Cm PermitUserEnvironment .
.It Cm StreamLocalBindMask
Sets the octal file creation mode mask
.Pq 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.
.Pp
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.
.It Cm 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
.Cm StreamLocalBindUnlink
is not enabled,
.Nm 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.
.Pp
The argument must be
.Cm yes
or
.Cm no .
The default is
.Cm no .
.It Cm StrictModes
Specifies whether
.Xr 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
.Cm yes .
Note that this does not apply to
.Cm ChrootDirectory ,
whose permissions and ownership are checked unconditionally.
.It Cm 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.
.Pp
The command
.Cm sftp-server
implements the SFTP file transfer subsystem.
.Pp
Alternately the name
.Cm internal-sftp
implements an in-process SFTP server.
This may simplify configurations using
.Cm ChrootDirectory
to force a different filesystem root on clients.
.Pp
By default no subsystems are defined.
.It Cm SyslogFacility
Gives the facility code that is used when logging messages from
.Xr sshd 8 .
The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
The default is AUTH.
.It Cm 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
.Qq ghost
users and consuming server resources.
.Pp
The default is
.Cm 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.
.Pp
To disable TCP keepalive messages, the value should be set to
.Cm no .
.It Cm TrustedUserCAKeys
Specifies a file containing public keys of certificate authorities that are
trusted to sign user certificates for authentication, or
.Cm none
to not use one.
Keys are listed one per line; empty lines and comments starting with
.Ql #
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
.Cm TrustedUserCAKeys .
For more details on certificates, see the CERTIFICATES section in
.Xr ssh-keygen 1 .
.It Cm UseDNS
Specifies whether
.Xr 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.
.Pp
If this option is set to
.Cm no
(the default) then only addresses and not host names may be used in
.Pa ~/.ssh/authorized_keys
.Cm from
and
.Nm
.Cm Match
.Cm Host
directives.
.It Cm UsePAM
Enables the Pluggable Authentication Module interface.
If set to
.Cm yes
this will enable PAM authentication using
.Cm ChallengeResponseAuthentication
and
.Cm PasswordAuthentication
in addition to PAM account and session module processing for all
authentication types.
.Pp
Because PAM challenge-response authentication usually serves an equivalent
role to password authentication, you should disable either
.Cm PasswordAuthentication
or
.Cm ChallengeResponseAuthentication.
.Pp
If
.Cm UsePAM
is enabled, you will not be able to run
.Xr sshd 8
as a non-root user.
The default is
.Cm no .
.It Cm VersionAddendum
Optionally specifies additional text to append to the SSH protocol banner
sent by the server upon connection.
The default is
.Cm none .
.It Cm X11DisplayOffset
Specifies the first display number available for
.Xr sshd 8 Ns 's
X11 forwarding.
This prevents sshd from interfering with real X11 servers.
The default is 10.
.It Cm X11Forwarding
Specifies whether X11 forwarding is permitted.
The argument must be
.Cm yes
or
.Cm no .
The default is
.Cm no .
.Pp
When X11 forwarding is enabled, there may be additional exposure to
the server and to client displays if the
.Xr sshd 8
proxy display is configured to listen on the wildcard address (see
.Cm 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
.Cm ForwardX11
in
.Xr 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
.Cm no
setting.
.Pp
Note that disabling X11 forwarding does not prevent users from
forwarding X11 traffic, as users can always install their own forwarders.
.It Cm X11UseLocalhost
Specifies whether
.Xr 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
.Ev DISPLAY
environment variable to
.Cm localhost .
This prevents remote hosts from connecting to the proxy display.
However, some older X11 clients may not function with this
configuration.
.Cm X11UseLocalhost
may be set to
.Cm no
to specify that the forwarding server should be bound to the wildcard
address.
The argument must be
.Cm yes
or
.Cm no .
The default is
.Cm yes .
.It Cm XAuthLocation
Specifies the full pathname of the
.Xr xauth 1
program, or
.Cm none
to not use one.
The default is
.Pa /usr/X11R6/bin/xauth .
.El
.Sh TIME FORMATS
.Xr sshd 8
command-line arguments and configuration file options that specify time
may be expressed using a sequence of the form:
.Sm off
.Ar time Op Ar qualifier ,
.Sm on
where
.Ar time
is a positive integer value and
.Ar qualifier
is one of the following:
.Pp
.Bl -tag -width Ds -compact -offset indent
.It Aq Cm none
seconds
.It Cm s | Cm S
seconds
.It Cm m | Cm M
minutes
.It Cm h | Cm H
hours
.It Cm d | Cm D
days
.It Cm w | Cm W
weeks
.El
.Pp
Each member of the sequence is added together to calculate
the total time value.
.Pp
Time format examples:
.Pp
.Bl -tag -width Ds -compact -offset indent
.It 600
600 seconds (10 minutes)
.It 10m
10 minutes
.It 1h30m
1 hour 30 minutes (90 minutes)
.El
.Sh TOKENS
Arguments to some keywords can make use of tokens,
which are expanded at runtime:
.Pp
.Bl -tag -width XXXX -offset indent -compact
.It %%
A literal
.Sq % .
.It \&%D
The routing domain in which the incoming connection was received.
.It %F
The fingerprint of the CA key.
.It %f
The fingerprint of the key or certificate.
.It %h
The home directory of the user.
.It %i
The key ID in the certificate.
.It %K
The base64-encoded CA key.
.It %k
The base64-encoded key or certificate for authentication.
.It %s
The serial number of the certificate.
.It \&%T
The type of the CA key.
.It %t
The key or certificate type.
.It \&%U
The numeric user ID of the target user.
.It %u
The username.
.El
.Pp
.Cm AuthorizedKeysCommand
accepts the tokens %%, %f, %h, %k, %t, %U, and %u.
.Pp
.Cm AuthorizedKeysFile
accepts the tokens %%, %h, %U, and %u.
.Pp
.Cm AuthorizedPrincipalsCommand
accepts the tokens %%, %F, %f, %h, %i, %K, %k, %s, %T, %t, %U, and %u.
.Pp
.Cm AuthorizedPrincipalsFile
accepts the tokens %%, %h, %U, and %u.
.Pp
.Cm ChrootDirectory
accepts the tokens %%, %h, %U, and %u.
.Pp
.Cm RoutingDomain
accepts the token %D.
.Sh FILES
.Bl -tag -width Ds
.It Pa /etc/ssh/sshd_config
Contains configuration data for
.Xr sshd 8 .
This file should be writable by root only, but it is recommended
(though not necessary) that it be world-readable.
.El
.Sh SEE ALSO
.Xr sftp-server 8 ,
.Xr sshd 8
.Sh AUTHORS
.An -nosplit
OpenSSH is a derivative of the original and free
ssh 1.2.12 release by
.An Tatu Ylonen .
.An Aaron Campbell , Bob Beck , Markus Friedl , Niels Provos ,
.An Theo de Raadt
and
.An Dug Song
removed many bugs, re-added newer features and
created OpenSSH.
.An Markus Friedl
contributed the support for SSH protocol versions 1.5 and 2.0.
.An Niels Provos
and
.An Markus Friedl
contributed support for privilege separation.