path: root/sshd_config.5

.\"  -*- nroff -*-
.\"
.\" 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.
.\"
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.\" $OpenBSD: sshd_config.5,v 1.71 2007/01/02 09:57:25 jmc Exp $
.Dd September 25, 1999
.Dt SSHD_CONFIG 5
.Os
.Sh NAME
.Nm sshd_config
.Nd OpenSSH SSH daemon configuration file
.Sh SYNOPSIS
.Nm /etc/ssh/sshd_config
.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.
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
in
.Xr ssh_config 5
for how to configure the client.
Note that environment passing is only supported for protocol 2.
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
.Dq any ,
.Dq inet
(use IPv4 only), or
.Dq inet6
(use IPv6 only).
The default is
.Dq any .
.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
.Sx PATTERNS
in
.Xr ssh_config 5
for more information on patterns.
.It Cm AllowTcpForwarding
Specifies whether TCP forwarding is permitted.
The default is
.Dq yes .
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.
The allow/deny directives are processed in the following order:
.Cm DenyUsers ,
.Cm AllowUsers ,
.Cm DenyGroups ,
and finally
.Cm AllowGroups .
.Pp
See
.Sx PATTERNS
in
.Xr ssh_config 5
for more information on patterns.
.It Cm AuthorizedKeysFile
Specifies the file that contains the public keys that can be used
for user authentication.
.Cm AuthorizedKeysFile
may contain tokens of the form %T which are substituted during connection
setup.
The following tokens are defined: %% is replaced by a literal '%',
%h is replaced by the home directory of the user being authenticated, and
%u is replaced by the username of that user.
After expansion,
.Cm AuthorizedKeysFile
is taken to be an absolute path or one relative to the user's home
directory.
The default is
.Dq .ssh/authorized_keys .
.It Cm Banner
In some jurisdictions, sending a warning message before authentication
may be relevant for getting legal protection.
The contents of the specified file are sent to the remote user before
authentication is allowed.
This option is only available for protocol version 2.
By default, no banner is displayed.
.It Cm ChallengeResponseAuthentication
Specifies whether challenge-response authentication is allowed.
All authentication styles from
.Xr login.conf 5
are supported.
The default is
.Dq yes .
.It Cm Ciphers
Specifies the ciphers allowed for protocol version 2.
Multiple ciphers must be comma-separated.
The supported ciphers are
.Dq 3des-cbc ,
.Dq aes128-cbc ,
.Dq aes192-cbc ,
.Dq aes256-cbc ,
.Dq aes128-ctr ,
.Dq aes192-ctr ,
.Dq aes256-ctr ,
.Dq arcfour128 ,
.Dq arcfour256 ,
.Dq arcfour ,
.Dq blowfish-cbc ,
and
.Dq cast128-cbc .
The default is:
.Bd -literal -offset 3n
aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
aes192-ctr,aes256-ctr
.Ed
.It Cm ClientAliveCountMax
Sets the number of client alive messages (see below) 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
(below).
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 inactive.
.Pp
The default value is 3.
If
.Cm ClientAliveInterval
(see below) is set to 15, and
.Cm ClientAliveCountMax
is left at the default, unresponsive SSH clients
will be disconnected after approximately 45 seconds.
This option applies to protocol version 2 only.
.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.
This option applies to protocol version 2 only.
.It Cm Compression
Specifies whether compression is allowed, or delayed until
the user has authenticated successfully.
The argument must be
.Dq yes ,
.Dq delayed ,
or
.Dq no .
The default is
.Dq delayed .
.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
.Sx 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.
The allow/deny directives are processed in the following order:
.Cm DenyUsers ,
.Cm AllowUsers ,
.Cm DenyGroups ,
and finally
.Cm AllowGroups .
.Pp
See
.Sx PATTERNS
in
.Xr ssh_config 5
for more information on patterns.
.It Cm ForceCommand
Forces the execution of the command specified by
.Cm ForceCommand ,
ignoring any command supplied by the client.
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.
.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
.Dq no
to force remote port forwardings to be available to the local host only,
.Dq yes
to force remote port forwardings to bind to the wildcard address, or
.Dq clientspecified
to allow the client to select the address to which the forwarding is bound.
The default is
.Dq no .
.It Cm GSSAPIAuthentication
Specifies whether user authentication based on GSSAPI is allowed.
The default is
.Dq no .
Note that this option applies to protocol version 2 only.
.It Cm GSSAPICleanupCredentials
Specifies whether to automatically destroy the user's credentials cache
on logout.
The default is
.Dq yes .
Note that this option applies to protocol version 2 only.
.It Cm HostbasedAuthentication
Specifies whether rhosts or /etc/hosts.equiv authentication together
with successful public key client host authentication is allowed
(host-based authentication).
This option is similar to
.Cm RhostsRSAAuthentication
and applies to protocol version 2 only.
The default is
.Dq 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
.Dq 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
.Dq no .
.It Cm HostKey
Specifies a file containing a private host key
used by SSH.
The default is
.Pa /etc/ssh/ssh_host_key
for protocol version 1, and
.Pa /etc/ssh/ssh_host_rsa_key
and
.Pa /etc/ssh/ssh_host_dsa_key
for protocol version 2.
Note that
.Xr sshd 8
will refuse to use a file if it is group/world-accessible.
It