1274 lines
36 KiB
Groff
1274 lines
36 KiB
Groff
.\"
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.\" Author: Tatu Ylonen <ylo@cs.hut.fi>
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.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
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.\" All rights reserved
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.\"
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.\" As far as I am concerned, the code I have written for this software
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.\" can be used freely for any purpose. Any derived versions of this
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.\" software must be clearly marked as such, and if the derived work is
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.\" incompatible with the protocol description in the RFC file, it must be
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.\" called by a name other than "ssh" or "Secure Shell".
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.\"
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.\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
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.\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
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.\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
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.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
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.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
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.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
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.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
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.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
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.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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.\"
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.\" $OpenBSD: sshd_config.5,v 1.149 2012/11/04 11:09:15 djm Exp $
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.Dd $Mdocdate: November 4 2012 $
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.Dt SSHD_CONFIG 5
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.Os
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.Sh NAME
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.Nm sshd_config
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.Nd OpenSSH SSH daemon configuration file
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.Sh SYNOPSIS
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.Nm /etc/ssh/sshd_config
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.Sh DESCRIPTION
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.Xr sshd 8
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reads configuration data from
|
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.Pa /etc/ssh/sshd_config
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(or the file specified with
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.Fl f
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on the command line).
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The file contains keyword-argument pairs, one per line.
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Lines starting with
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.Ql #
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and empty lines are interpreted as comments.
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Arguments may optionally be enclosed in double quotes
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.Pq \&"
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in order to represent arguments containing spaces.
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.Pp
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|
The possible
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keywords and their meanings are as follows (note that
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keywords are case-insensitive and arguments are case-sensitive):
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.Bl -tag -width Ds
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|
.It Cm AcceptEnv
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Specifies what environment variables sent by the client will be copied into
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the session's
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.Xr environ 7 .
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See
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.Cm SendEnv
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in
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.Xr ssh_config 5
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for how to configure the client.
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Note that environment passing is only supported for protocol 2.
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Variables are specified by name, which may contain the wildcard characters
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.Ql *
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and
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|
.Ql \&? .
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Multiple environment variables may be separated by whitespace or spread
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across multiple
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.Cm AcceptEnv
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directives.
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Be warned that some environment variables could be used to bypass restricted
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user environments.
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|
For this reason, care should be taken in the use of this directive.
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|
The default is not to accept any environment variables.
|
|
.It Cm AddressFamily
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|
Specifies which address family should be used by
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|
.Xr sshd 8 .
|
|
Valid arguments are
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|
.Dq any ,
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|
.Dq inet
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|
(use IPv4 only), or
|
|
.Dq inet6
|
|
(use IPv6 only).
|
|
The default is
|
|
.Dq any .
|
|
.It Cm AllowAgentForwarding
|
|
Specifies whether
|
|
.Xr ssh-agent 1
|
|
forwarding is permitted.
|
|
The default is
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|
.Dq yes .
|
|
Note that disabling agent forwarding does not improve security
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|
unless users are also denied shell access, as they can always install
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|
their own forwarders.
|
|
.It Cm AllowGroups
|
|
This keyword can be followed by a list of group name patterns, separated
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|
by spaces.
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|
If specified, login is allowed only for users whose primary
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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 ,
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|
.Cm AllowUsers ,
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|
.Cm DenyGroups ,
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|
and finally
|
|
.Cm AllowGroups .
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|
.Pp
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|
See
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.Sx PATTERNS
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in
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|
.Xr ssh_config 5
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|
for more information on patterns.
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|
.It Cm AllowTcpForwarding
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|
Specifies whether TCP forwarding is permitted.
|
|
The default is
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|
.Dq yes .
|
|
Note that disabling TCP forwarding does not improve security unless
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users are also denied shell access, as they can always install their
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|
own forwarders.
|
|
.It Cm AllowUsers
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|
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
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|
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 ,
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|
.Cm DenyGroups ,
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|
and finally
|
|
.Cm AllowGroups .
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|
.Pp
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|
See
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|
.Sx PATTERNS
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in
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|
.Xr ssh_config 5
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|
for more information on patterns.
|
|
.It Cm AuthenticationMethods
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|
Specifies the authentication methods that must be successfully completed
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|
for a user to be granted access.
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|
This option must be followed by one or more comma-separated lists of
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authentication method names.
|
|
Successful authentication requires completion of every method in at least
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one of these lists.
|
|
.Pp
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|
For example, an argument of
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.Dq publickey,password publickey,keyboard-interactive
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|
would require the user to complete public key authentication, followed by
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either password or keyboard interactive authentication.
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|
Only methods that are next in one or more lists are offered at each stage,
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|
so for this example, it would not be possible to attempt password or
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|
keyboard-interactive authentication before public key.
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|
.Pp
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This option is only available for SSH protocol 2 and will yield a fatal
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error if enabled if protocol 1 is also enabled.
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|
Note that each authentication method listed should also be explicitly enabled
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in the configuration.
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|
The default is not to require multiple authentication; successful completion
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of a single authentication method is sufficient.
|
|
.It Cm AuthorizedKeysCommand
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Specifies a program to be used to look up the user's public keys.
|
|
The program will be invoked with a single argument of the username
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being authenticated, and should produce on standard output zero or
|
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more lines of authorized_keys output (see
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.Sx AUTHORIZED_KEYS
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|
in
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.Xr sshd 8 ) .
|
|
If a key supplied by AuthorizedKeysCommand does not successfully authenticate
|
|
and authorize the user then public key authentication continues using the usual
|
|
.Cm AuthorizedKeysFile
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|
files.
|
|
By default, no AuthorizedKeysCommand is run.
|
|
.It Cm AuthorizedKeysCommandUser
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|
Specifies the user under whose account the AuthorizedKeysCommand is run.
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It is recommended to use a dedicated user that has no other role on the host
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than running authorized keys commands.
|
|
.It Cm AuthorizedKeysFile
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Specifies the file that contains the public keys that can be used
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|
for user authentication.
|
|
The format is described in the
|
|
.Sx AUTHORIZED_KEYS FILE FORMAT
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|
section of
|
|
.Xr sshd 8 .
|
|
.Cm AuthorizedKeysFile
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|
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.
|
|
Multiple files may be listed, separated by whitespace.
|
|
The default is
|
|
.Dq .ssh/authorized_keys .ssh/authorized_keys2 .
|
|
.It Cm AuthorizedPrincipalsFile
|
|
Specifies a file that lists principal names that are accepted for
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|
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
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|
.Xr sshd 8 ) .
|
|
Empty lines and comments starting with
|
|
.Ql #
|
|
are ignored.
|
|
.Pp
|
|
.Cm AuthorizedPrincipalsFile
|
|
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 AuthorizedPrincipalsFile
|
|
is taken to be an absolute path or one relative to the user's home
|
|
directory.
|
|
.Pp
|
|
The default is
|
|
.Dq 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.
|
|
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
|
|
.Dq none
|
|
then no banner is displayed.
|
|
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 (e.g. via
|
|
PAM or though authentication styles supported in
|
|
.Xr login.conf 5 )
|
|
The default is
|
|
.Dq yes .
|
|
.It Cm ChrootDirectory
|
|
Specifies the pathname of a directory to
|
|
.Xr chroot 2
|
|
to after authentication.
|
|
All components of the pathname must be root-owned directories that 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.
|
|
.Pp
|
|
The pathname may contain the following tokens that are expanded at runtime once
|
|
the connecting user has been authenticated: %% 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.
|
|
.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 ,
|
|
.Xr arandom 4
|
|
and
|
|
.Xr tty 4
|
|
devices.
|
|
For file transfer sessions using
|
|
.Dq sftp ,
|
|
no additional configuration of the environment is necessary if the
|
|
in-process sftp server is used,
|
|
though sessions which use logging do require
|
|
.Pa /dev/log
|
|
inside the chroot directory (see
|
|
.Xr sftp-server 8
|
|
for details).
|
|
.Pp
|
|
The default is not to
|
|
.Xr chroot 2 .
|
|
.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-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
|
|
aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
|
|
aes256-cbc,arcfour
|
|
.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 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
|
|
.Dq internal-sftp
|
|
will force the use of an in-process sftp server that requires no support
|
|
files when used with
|
|
.Cm ChrootDirectory .
|
|
.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 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 default is
|
|
.Pa /etc/ssh/ssh_host_key
|
|
for protocol version 1, and
|
|
.Pa /etc/ssh/ssh_host_dsa_key ,
|
|
.Pa /etc/ssh/ssh_host_ecdsa_key
|
|
and
|
|
.Pa /etc/ssh/ssh_host_rsa_key
|
|
for protocol version 2.
|
|
Note that
|
|
.Xr sshd 8
|
|
will refuse to use a file if it is group/world-accessible.
|
|
It is possible to have multiple host key files.
|
|
.Dq rsa1
|
|
keys are used for version 1 and
|
|
.Dq dsa ,
|
|
.Dq ecdsa
|
|
or
|
|
.Dq rsa
|
|
are used for version 2 of the SSH protocol.
|
|
.It Cm IgnoreRhosts
|
|
Specifies that
|
|
.Pa .rhosts
|
|
and
|
|
.Pa .shosts
|
|
files will not be used in
|
|
.Cm RhostsRSAAuthentication
|
|
or
|
|
.Cm HostbasedAuthentication .
|
|
.Pp
|
|
.Pa /etc/hosts.equiv
|
|
and
|
|
.Pa /etc/shosts.equiv
|
|
are still used.
|
|
The default is
|
|
.Dq yes .
|
|
.It Cm IgnoreUserKnownHosts
|
|
Specifies whether
|
|
.Xr sshd 8
|
|
should ignore the user's
|
|
.Pa ~/.ssh/known_hosts
|
|
during
|
|
.Cm RhostsRSAAuthentication
|
|
or
|
|
.Cm HostbasedAuthentication .
|
|
The default is
|
|
.Dq no .
|
|
.It Cm IPQoS
|
|
Specifies the IPv4 type-of-service or DSCP class for the connection.
|
|
Accepted values are
|
|
.Dq af11 ,
|
|
.Dq af12 ,
|
|
.Dq af13 ,
|
|
.Dq af21 ,
|
|
.Dq af22 ,
|
|
.Dq af23 ,
|
|
.Dq af31 ,
|
|
.Dq af32 ,
|
|
.Dq af33 ,
|
|
.Dq af41 ,
|
|
.Dq af42 ,
|
|
.Dq af43 ,
|
|
.Dq cs0 ,
|
|
.Dq cs1 ,
|
|
.Dq cs2 ,
|
|
.Dq cs3 ,
|
|
.Dq cs4 ,
|
|
.Dq cs5 ,
|
|
.Dq cs6 ,
|
|
.Dq cs7 ,
|
|
.Dq ef ,
|
|
.Dq lowdelay ,
|
|
.Dq throughput ,
|
|
.Dq reliability ,
|
|
or a numeric value.
|
|
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
|
|
.Dq lowdelay
|
|
for interactive sessions and
|
|
.Dq throughput
|
|
for non-interactive sessions.
|
|
.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
|
|
.Dq 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
|
|
.Dq 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
|
|
.Dq yes .
|
|
.It Cm KerberosTicketCleanup
|
|
Specifies whether to automatically destroy the user's ticket cache
|
|
file on logout.
|
|
The default is
|
|
.Dq yes .
|
|
.It Cm KexAlgorithms
|
|
Specifies the available KEX (Key Exchange) algorithms.
|
|
Multiple algorithms must be comma-separated.
|
|
The default is
|
|
.Dq ecdh-sha2-nistp256 ,
|
|
.Dq ecdh-sha2-nistp384 ,
|
|
.Dq ecdh-sha2-nistp521 ,
|
|
.Dq diffie-hellman-group-exchange-sha256 ,
|
|
.Dq diffie-hellman-group-exchange-sha1 ,
|
|
.Dq diffie-hellman-group14-sha1 ,
|
|
.Dq diffie-hellman-group1-sha1 .
|
|
.It Cm KeyRegenerationInterval
|
|
In protocol version 1, the ephemeral server key is automatically regenerated
|
|
after this many seconds (if it has been used).
|
|
The purpose of regeneration is to prevent
|
|
decrypting captured sessions by later breaking into the machine and
|
|
stealing the keys.
|
|
The key is never stored anywhere.
|
|
If the value is 0, the key is never regenerated.
|
|
The default is 3600 (seconds).
|
|
.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 host No | Ar IPv4_addr No | Ar IPv6_addr
|
|
.Sm on
|
|
.It
|
|
.Cm ListenAddress
|
|
.Sm off
|
|
.Ar host No | Ar IPv4_addr No : Ar port
|
|
.Sm on
|
|
.It
|
|
.Cm ListenAddress
|
|
.Sm off
|
|
.Oo
|
|
.Ar host No | Ar IPv6_addr Oc : Ar port
|
|
.Sm on
|
|
.El
|
|
.Pp
|
|
If
|
|
.Ar port
|
|
is not specified,
|
|
sshd will listen on the address and all prior
|
|
.Cm Port
|
|
options specified.
|
|
The default is to listen on all local addresses.
|
|
Multiple
|
|
.Cm ListenAddress
|
|
options are permitted.
|
|
Additionally, any
|
|
.Cm Port
|
|
options must precede this option for non-port qualified addresses.
|
|
.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 in protocol version 2
|
|
for data integrity protection.
|
|
Multiple algorithms must be comma-separated.
|
|
The default is:
|
|
.Bd -literal -offset indent
|
|
hmac-md5,hmac-sha1,umac-64@openssh.com,umac-128@openssh.com,
|
|
hmac-sha2-256,hmac-sha2-512,hmac-ripemd160,
|
|
hmac-sha1-96,hmac-md5-96
|
|
.Ed
|
|
.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.
|
|
.Pp
|
|
The arguments to
|
|
.Cm Match
|
|
are one or more criteria-pattern pairs.
|
|
The available criteria are
|
|
.Cm User ,
|
|
.Cm Group ,
|
|
.Cm Host ,
|
|
.Cm LocalAddress ,
|
|
.Cm LocalPort ,
|
|
and
|
|
.Cm Address .
|
|
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, e.g.\&
|
|
.Dq 192.0.2.0/24
|
|
or
|
|
.Dq 3ffe:ffff::/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,
|
|
.Dq 192.0.2.0/33
|
|
and
|
|
.Dq 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 AllowTcpForwarding ,
|
|
.Cm AllowUsers ,
|
|
.Cm AuthenticationMethods ,
|
|
.Cm AuthorizedKeysCommand ,
|
|
.Cm AuthorizedKeysCommandUser ,
|
|
.Cm AuthorizedKeysFile ,
|
|
.Cm AuthorizedPrincipalsFile ,
|
|
.Cm Banner ,
|
|
.Cm ChrootDirectory ,
|
|
.Cm DenyGroups ,
|
|
.Cm DenyUsers ,
|
|
.Cm ForceCommand ,
|
|
.Cm GatewayPorts ,
|
|
.Cm GSSAPIAuthentication ,
|
|
.Cm HostbasedAuthentication ,
|
|
.Cm HostbasedUsesNameFromPacketOnly ,
|
|
.Cm KbdInteractiveAuthentication ,
|
|
.Cm KerberosAuthentication ,
|
|
.Cm MaxAuthTries ,
|
|
.Cm MaxSessions ,
|
|
.Cm PasswordAuthentication ,
|
|
.Cm PermitEmptyPasswords ,
|
|
.Cm PermitOpen ,
|
|
.Cm PermitRootLogin ,
|
|
.Cm PermitTunnel ,
|
|
.Cm PubkeyAuthentication ,
|
|
.Cm RhostsRSAAuthentication ,
|
|
.Cm RSAAuthentication ,
|
|
.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 sessions permitted per network connection.
|
|
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.
|
|
.Pp
|
|
Alternatively, random early drop can be enabled by specifying
|
|
the three colon separated values
|
|
.Dq start:rate:full
|
|
(e.g. "10:30:60").
|
|
.Xr sshd 8
|
|
will refuse connection attempts with a probability of
|
|
.Dq rate/100
|
|
(30%)
|
|
if there are currently
|
|
.Dq start
|
|
(10)
|
|
unauthenticated connections.
|
|
The probability increases linearly and all connection attempts
|
|
are refused if the number of unauthenticated connections reaches
|
|
.Dq full
|
|
(60).
|
|
.It Cm PasswordAuthentication
|
|
Specifies whether password authentication is allowed.
|
|
The default is
|
|
.Dq 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
|
|
.Dq no .
|
|
.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
|
|
.Dq any
|
|
can be used to remove all restrictions and permit any forwarding requests.
|
|
An argument of
|
|
.Dq none
|
|
can be used to prohibit all forwarding requests.
|
|
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
|
|
.Dq yes ,
|
|
.Dq without-password ,
|
|
.Dq forced-commands-only ,
|
|
or
|
|
.Dq no .
|
|
The default is
|
|
.Dq yes .
|
|
.Pp
|
|
If this option is set to
|
|
.Dq without-password ,
|
|
password authentication is disabled for root.
|
|
.Pp
|
|
If this option is set to
|
|
.Dq 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
|
|
.Dq no ,
|
|
root is not allowed to log in.
|
|
.It Cm PermitTunnel
|
|
Specifies whether
|
|
.Xr tun 4
|
|
device forwarding is allowed.
|
|
The argument must be
|
|
.Dq yes ,
|
|
.Dq point-to-point
|
|
(layer 3),
|
|
.Dq ethernet
|
|
(layer 2), or
|
|
.Dq no .
|
|
Specifying
|
|
.Dq yes
|
|
permits both
|
|
.Dq point-to-point
|
|
and
|
|
.Dq ethernet .
|
|
The default is
|
|
.Dq no .
|
|
.It Cm PermitUserEnvironment
|
|
Specifies whether
|
|
.Pa ~/.ssh/environment
|
|
and
|
|
.Cm environment=
|
|
options in
|
|
.Pa ~/.ssh/authorized_keys
|
|
are processed by
|
|
.Xr sshd 8 .
|
|
The default is
|
|
.Dq no .
|
|
Enabling environment processing may enable users to bypass access
|
|
restrictions in some configurations using mechanisms such as
|
|
.Ev LD_PRELOAD .
|
|
.It Cm PidFile
|
|
Specifies the file that contains the process ID of the
|
|
SSH daemon.
|
|
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
|
|
.Dq 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
|
|
.Dq yes .
|
|
.It Cm Protocol
|
|
Specifies the protocol versions
|
|
.Xr sshd 8
|
|
supports.
|
|
The possible values are
|
|
.Sq 1
|
|
and
|
|
.Sq 2 .
|
|
Multiple versions must be comma-separated.
|
|
The default is
|
|
.Sq 2 .
|
|
Note that the order of the protocol list does not indicate preference,
|
|
because the client selects among multiple protocol versions offered
|
|
by the server.
|
|
Specifying
|
|
.Dq 2,1
|
|
is identical to
|
|
.Dq 1,2 .
|
|
.It Cm PubkeyAuthentication
|
|
Specifies whether public key authentication is allowed.
|
|
The default is
|
|
.Dq yes .
|
|
Note that this option applies to protocol version 2 only.
|
|
.It Cm RevokedKeys
|
|
Specifies a list of revoked public keys.
|
|
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.
|
|
.It Cm RhostsRSAAuthentication
|
|
Specifies whether rhosts or /etc/hosts.equiv authentication together
|
|
with successful RSA host authentication is allowed.
|
|
The default is
|
|
.Dq no .
|
|
This option applies to protocol version 1 only.
|
|
.It Cm RSAAuthentication
|
|
Specifies whether pure RSA authentication is allowed.
|
|
The default is
|
|
.Dq yes .
|
|
This option applies to protocol version 1 only.
|
|
.It Cm ServerKeyBits
|
|
Defines the number of bits in the ephemeral protocol version 1 server key.
|
|
The minimum value is 512, and the default is 1024.
|
|
.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
|
|
.Dq 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
|
|
.Xr sftp-server 8
|
|
implements the
|
|
.Dq sftp
|
|
file transfer subsystem.
|
|
.Pp
|
|
Alternately the name
|
|
.Dq internal-sftp
|
|
implements an in-process
|
|
.Dq sftp
|
|
server.
|
|
This may simplify configurations using
|
|
.Cm ChrootDirectory
|
|
to force a different filesystem root on clients.
|
|
.Pp
|
|
By default no subsystems are defined.
|
|
Note that this option applies to protocol version 2 only.
|
|
.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
|
|
.Dq ghost
|
|
users and consuming server resources.
|
|
.Pp
|
|
The default is
|
|
.Dq 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
|
|
.Dq no .
|
|
.It Cm TrustedUserCAKeys
|
|
Specifies a file containing public keys of certificate authorities that are
|
|
trusted to sign user certificates for authentication.
|
|
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
|
|
.Sx CERTIFICATES
|
|
section in
|
|
.Xr ssh-keygen 1 .
|
|
.It Cm UseDNS
|
|
Specifies whether
|
|
.Xr sshd 8
|
|
should look up the remote host name and check that
|
|
the resolved host name for the remote IP address maps back to the
|
|
very same IP address.
|
|
The default is
|
|
.Dq yes .
|
|
.It Cm UseLogin
|
|
Specifies whether
|
|
.Xr login 1
|
|
is used for interactive login sessions.
|
|
The default is
|
|
.Dq no .
|
|
Note that
|
|
.Xr login 1
|
|
is never used for remote command execution.
|
|
Note also, that if this is enabled,
|
|
.Cm X11Forwarding
|
|
will be disabled because
|
|
.Xr login 1
|
|
does not know how to handle
|
|
.Xr xauth 1
|
|
cookies.
|
|
If
|
|
.Cm UsePrivilegeSeparation
|
|
is specified, it will be disabled after authentication.
|
|
.It Cm UsePAM
|
|
Enables the Pluggable Authentication Module interface.
|
|
If set to
|
|
.Dq 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
|
|
.Dq no .
|
|
.It Cm UsePrivilegeSeparation
|
|
Specifies whether
|
|
.Xr sshd 8
|
|
separates privileges by creating an unprivileged child process
|
|
to deal with incoming network traffic.
|
|
After successful authentication, another process will be created that has
|
|
the privilege of the authenticated user.
|
|
The goal of privilege separation is to prevent privilege
|
|
escalation by containing any corruption within the unprivileged processes.
|
|
The default is
|
|
.Dq yes .
|
|
If
|
|
.Cm UsePrivilegeSeparation
|
|
is set to
|
|
.Dq sandbox
|
|
then the pre-authentication unprivileged process is subject to additional
|
|
restrictions.
|
|
.It Cm VersionAddendum
|
|
Optionally specifies additional text to append to the SSH protocol banner
|
|
sent by the server upon connection.
|
|
The default is
|
|
.Dq 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
|
|
.Dq yes
|
|
or
|
|
.Dq no .
|
|
The default is
|
|
.Dq 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
|
|
below), 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
|
|
.Dq no
|
|
setting.
|
|
.Pp
|
|
Note that disabling X11 forwarding does not prevent users from
|
|
forwarding X11 traffic, as users can always install their own forwarders.
|
|
X11 forwarding is automatically disabled if
|
|
.Cm UseLogin
|
|
is enabled.
|
|
.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
|
|
.Dq 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
|
|
.Dq no
|
|
to specify that the forwarding server should be bound to the wildcard
|
|
address.
|
|
The argument must be
|
|
.Dq yes
|
|
or
|
|
.Dq no .
|
|
The default is
|
|
.Dq yes .
|
|
.It Cm XAuthLocation
|
|
Specifies the full pathname of the
|
|
.Xr xauth 1
|
|
program.
|
|
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 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 sshd 8
|
|
.Sh 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.
|