866 lines
24 KiB
Groff
866 lines
24 KiB
Groff
.\" -*- nroff -*-
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.\"
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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 1.44 2005/07/25 11:59:40 markus Exp $
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.\" $FreeBSD$
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.Dd September 25, 1999
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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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.Bl -tag -width Ds -compact
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.It Pa /etc/ssh/sshd_config
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.El
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.Sh DESCRIPTION
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.Nm sshd
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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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.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.
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.It Cm AddressFamily
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Specifies which address family should be used by
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.Nm sshd .
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Valid arguments are
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.Dq any ,
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.Dq inet
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(use IPv4 only) or
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.Dq inet6
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(use IPv6 only).
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The default is
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.Dq any .
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.It Cm AllowGroups
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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.
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.Ql \&*
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and
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.Ql \&?
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can be used as
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wildcards in the patterns.
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Only group names are valid; a numerical group ID is not recognized.
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By default, login is allowed for all groups.
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.It Cm AllowTcpForwarding
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Specifies whether TCP forwarding is permitted.
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The default is
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.Dq yes .
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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.
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.It Cm AllowUsers
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This keyword can be followed by a list of user name patterns, separated
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by spaces.
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If specified, login is allowed only for user names that
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match one of the patterns.
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.Ql \&*
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and
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.Ql \&?
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can be used as
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wildcards in the patterns.
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Only user names are valid; a numerical user ID is not recognized.
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By default, login is allowed for all users.
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If the pattern takes the form USER@HOST then USER and HOST
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are separately checked, restricting logins to particular
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users from particular hosts.
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.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.
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.Cm AuthorizedKeysFile
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may contain tokens of the form %T which are substituted during connection
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set-up.
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The following tokens are defined: %% is replaced by a literal '%',
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%h is replaced by the home directory of the user being authenticated and
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%u is replaced by the username of that user.
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After expansion,
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.Cm AuthorizedKeysFile
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is taken to be an absolute path or one relative to the user's home
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directory.
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The default is
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.Dq .ssh/authorized_keys .
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.It Cm Banner
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In some jurisdictions, sending a warning message before authentication
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may be relevant for getting legal protection.
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The contents of the specified file are sent to the remote user before
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authentication is allowed.
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This option is only available for protocol version 2.
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By default, no banner is displayed.
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.It Cm ChallengeResponseAuthentication
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Specifies whether challenge-response authentication is allowed.
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Specifically, in
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.Fx ,
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this controls the use of PAM (see
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.Xr pam 3 )
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for authentication.
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Note that this affects the effectiveness of the
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.Cm PasswordAuthentication
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and
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.Cm PermitRootLogin
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variables.
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The default is
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.Dq yes .
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.It Cm Ciphers
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Specifies the ciphers allowed for protocol version 2.
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Multiple ciphers must be comma-separated.
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The supported ciphers are
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.Dq 3des-cbc ,
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.Dq aes128-cbc ,
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.Dq aes192-cbc ,
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.Dq aes256-cbc ,
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.Dq aes128-ctr ,
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.Dq aes192-ctr ,
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.Dq aes256-ctr ,
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.Dq arcfour128 ,
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.Dq arcfour256 ,
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.Dq arcfour ,
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.Dq blowfish-cbc ,
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and
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.Dq cast128-cbc .
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The default is
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.Bd -literal
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``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
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arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
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aes192-ctr,aes256-ctr''
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.Ed
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.It Cm ClientAliveCountMax
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Sets the number of client alive messages (see above) which may be
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sent without
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.Nm sshd
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receiving any messages back from the client.
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If this threshold is reached while client alive messages are being sent,
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.Nm sshd
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will disconnect the client, terminating the session.
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It is important to note that the use of client alive messages is very
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different from
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.Cm TCPKeepAlive
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(below).
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The client alive messages are sent through the encrypted channel
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and therefore will not be spoofable.
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The TCP keepalive option enabled by
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.Cm TCPKeepAlive
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is spoofable.
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The client alive mechanism is valuable when the client or
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server depend on knowing when a connection has become inactive.
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.Pp
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The default value is 3.
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If
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.Cm ClientAliveInterval
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(above) is set to 15, and
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.Cm ClientAliveCountMax
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is left at the default, unresponsive ssh clients
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will be disconnected after approximately 45 seconds.
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.It Cm ClientAliveInterval
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Sets a timeout interval in seconds after which if no data has been received
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from the client,
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.Nm sshd
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will send a message through the encrypted
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channel to request a response from the client.
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The default
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is 0, indicating that these messages will not be sent to the client.
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This option applies to protocol version 2 only.
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.It Cm Compression
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Specifies whether compression is allowed, or delayed until
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the user has authenticated successfully.
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The argument must be
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.Dq yes ,
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.Dq delayed ,
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or
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.Dq no .
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The default is
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.Dq delayed .
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.It Cm DenyGroups
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This keyword can be followed by a list of group name patterns, separated
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by spaces.
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Login is disallowed for users whose primary group or supplementary
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group list matches one of the patterns.
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.Ql \&*
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and
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.Ql \&?
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can be used as
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wildcards in the patterns.
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Only group names are valid; a numerical group ID is not recognized.
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By default, login is allowed for all groups.
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.It Cm DenyUsers
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This keyword can be followed by a list of user name patterns, separated
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by spaces.
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Login is disallowed for user names that match one of the patterns.
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.Ql \&*
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and
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.Ql \&?
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can be used as wildcards in the patterns.
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Only user names are valid; a numerical user ID is not recognized.
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By default, login is allowed for all users.
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If the pattern takes the form USER@HOST then USER and HOST
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are separately checked, restricting logins to particular
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users from particular hosts.
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.It Cm GatewayPorts
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Specifies whether remote hosts are allowed to connect to ports
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forwarded for the client.
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By default,
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.Nm sshd
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binds remote port forwardings to the loopback address.
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This prevents other remote hosts from connecting to forwarded ports.
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.Cm GatewayPorts
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can be used to specify that
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.Nm sshd
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should allow remote port forwardings to bind to non-loopback addresses, thus
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allowing other hosts to connect.
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The argument may be
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.Dq no
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to force remote port forwardings to be available to the local host only,
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.Dq yes
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to force remote port forwardings to bind to the wildcard address, or
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.Dq clientspecified
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to allow the client to select the address to which the forwarding is bound.
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The default is
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.Dq no .
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.It Cm GSSAPIAuthentication
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Specifies whether user authentication based on GSSAPI is allowed.
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The default is
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.Dq no .
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Note that this option applies to protocol version 2 only.
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.It Cm GSSAPICleanupCredentials
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Specifies whether to automatically destroy the user's credentials cache
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on logout.
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The default is
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.Dq yes .
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Note that this option applies to protocol version 2 only.
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.It Cm HostbasedAuthentication
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Specifies whether rhosts or /etc/hosts.equiv authentication together
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with successful public key client host authentication is allowed
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(hostbased authentication).
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This option is similar to
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.Cm RhostsRSAAuthentication
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and applies to protocol version 2 only.
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The default is
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.Dq no .
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.It Cm HostKey
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Specifies a file containing a private host key
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used by SSH.
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The default is
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.Pa /etc/ssh/ssh_host_key
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for protocol version 1, and
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.Pa /etc/ssh/ssh_host_dsa_key
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for protocol version 2.
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Note that
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.Nm sshd
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will refuse to use a file if it is group/world-accessible.
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It is possible to have multiple host key files.
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.Dq rsa1
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keys are used for version 1 and
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.Dq dsa
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or
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.Dq rsa
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are used for version 2 of the SSH protocol.
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.It Cm IgnoreRhosts
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Specifies that
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.Pa .rhosts
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and
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.Pa .shosts
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files will not be used in
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.Cm RhostsRSAAuthentication
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or
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.Cm HostbasedAuthentication .
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.Pp
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.Pa /etc/hosts.equiv
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and
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.Pa /etc/ssh/shosts.equiv
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are still used.
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The default is
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.Dq yes .
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.It Cm IgnoreUserKnownHosts
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Specifies whether
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.Nm sshd
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should ignore the user's
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.Pa ~/.ssh/known_hosts
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during
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.Cm RhostsRSAAuthentication
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or
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.Cm HostbasedAuthentication .
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The default is
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.Dq no .
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.It Cm KerberosAuthentication
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Specifies whether the password provided by the user for
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.Cm PasswordAuthentication
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will be validated through the Kerberos KDC.
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To use this option, the server needs a
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Kerberos servtab which allows the verification of the KDC's identity.
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Default is
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.Dq no .
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.It Cm KerberosGetAFSToken
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If AFS is active and the user has a Kerberos 5 TGT, attempt to aquire
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an AFS token before accessing the user's home directory.
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Default is
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.Dq no .
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.It Cm KerberosOrLocalPasswd
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If set then if password authentication through Kerberos fails then
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the password will be validated via any additional local mechanism
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such as
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.Pa /etc/passwd .
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Default is
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.Dq yes .
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.It Cm KerberosTicketCleanup
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Specifies whether to automatically destroy the user's ticket cache
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file on logout.
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Default is
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.Dq yes .
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.It Cm KeyRegenerationInterval
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In protocol version 1, the ephemeral server key is automatically regenerated
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after this many seconds (if it has been used).
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The purpose of regeneration is to prevent
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decrypting captured sessions by later breaking into the machine and
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stealing the keys.
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The key is never stored anywhere.
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If the value is 0, the key is never regenerated.
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The default is 3600 (seconds).
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.It Cm ListenAddress
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Specifies the local addresses
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.Nm sshd
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should listen on.
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The following forms may be used:
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.Pp
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.Bl -item -offset indent -compact
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.It
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.Cm ListenAddress
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.Sm off
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.Ar host No | Ar IPv4_addr No | Ar IPv6_addr
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.Sm on
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.It
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.Cm ListenAddress
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.Sm off
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.Ar host No | Ar IPv4_addr No : Ar port
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.Sm on
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.It
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.Cm ListenAddress
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.Sm off
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.Oo
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.Ar host No | Ar IPv6_addr Oc : Ar port
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.Sm on
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.El
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.Pp
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If
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.Ar port
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is not specified,
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.Nm sshd
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will listen on the address and all prior
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.Cm Port
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options specified.
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The default is to listen on all local addresses.
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Multiple
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.Cm ListenAddress
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options are permitted.
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Additionally, any
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.Cm Port
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options must precede this option for non port qualified addresses.
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.It Cm LoginGraceTime
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The server disconnects after this time if the user has not
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successfully logged in.
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If the value is 0, there is no time limit.
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The default is 120 seconds.
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.It Cm LogLevel
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Gives the verbosity level that is used when logging messages from
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.Nm sshd .
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The possible values are:
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QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
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The default is INFO.
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DEBUG and DEBUG1 are equivalent.
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DEBUG2 and DEBUG3 each specify higher levels of debugging output.
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Logging with a DEBUG level violates the privacy of users and is not recommended.
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.It Cm MACs
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Specifies the available MAC (message authentication code) algorithms.
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The MAC algorithm is used in protocol version 2
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for data integrity protection.
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Multiple algorithms must be comma-separated.
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The default is
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.Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
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.It Cm MaxAuthTries
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Specifies the maximum number of authentication attempts permitted per
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connection.
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Once the number of failures reaches half this value,
|
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additional failures are logged.
|
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The default is 6.
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.It Cm MaxStartups
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Specifies the maximum number of concurrent unauthenticated connections to the
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.Nm sshd
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daemon.
|
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Additional connections will be dropped until authentication succeeds or the
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.Cm LoginGraceTime
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expires for a connection.
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The default is 10.
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.Pp
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Alternatively, random early drop can be enabled by specifying
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the three colon separated values
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.Dq start:rate:full
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(e.g., "10:30:60").
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.Nm sshd
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will refuse connection attempts with a probability of
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.Dq rate/100
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(30%)
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if there are currently
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.Dq start
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(10)
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unauthenticated connections.
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The probability increases linearly and all connection attempts
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are refused if the number of unauthenticated connections reaches
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.Dq full
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(60).
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.It Cm PasswordAuthentication
|
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Specifies whether password authentication is allowed.
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The default is
|
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.Dq no ,
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unless
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.Nm sshd
|
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was built without PAM support, in which case the default is
|
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.Dq yes .
|
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Note that if
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.Cm ChallengeResponseAuthentication
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is
|
|
.Dq yes ,
|
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and the PAM authentication policy for
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.Nm sshd
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includes
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.Xr pam_unix 8 ,
|
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password authentication will be allowed through the challenge-response
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mechanism regardless of the value of
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.Cm PasswordAuthentication .
|
|
.It Cm PermitEmptyPasswords
|
|
When password authentication is allowed, it specifies whether the
|
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server allows login to accounts with empty password strings.
|
|
The default is
|
|
.Dq no .
|
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.It Cm PermitRootLogin
|
|
Specifies whether root can log in using
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.Xr ssh 1 .
|
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The argument must be
|
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.Dq yes ,
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.Dq without-password ,
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.Dq forced-commands-only
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or
|
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.Dq no .
|
|
The default is
|
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.Dq no .
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Note that if
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.Cm ChallengeResponseAuthentication
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is
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.Dq yes ,
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the root user may be allowed in with its password even if
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.Cm PermitRootLogin is set to
|
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.Dq without-password .
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.Pp
|
|
If this option is set to
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.Dq without-password
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password authentication is disabled for root.
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.Pp
|
|
If this option is set to
|
|
.Dq forced-commands-only
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|
root login with public key authentication will be allowed,
|
|
but only if the
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.Ar command
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|
option has been specified
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|
(which may be useful for taking remote backups even if root login is
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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 PermitUserEnvironment
|
|
Specifies whether
|
|
.Pa ~/.ssh/environment
|
|
and
|
|
.Cm environment=
|
|
options in
|
|
.Pa ~/.ssh/authorized_keys
|
|
are processed by
|
|
.Nm sshd .
|
|
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
|
|
.Nm sshd
|
|
daemon.
|
|
The default is
|
|
.Pa /var/run/sshd.pid .
|
|
.It Cm Port
|
|
Specifies the port number that
|
|
.Nm sshd
|
|
listens on.
|
|
The default is 22.
|
|
Multiple options of this type are permitted.
|
|
See also
|
|
.Cm ListenAddress .
|
|
.It Cm PrintLastLog
|
|
Specifies whether
|
|
.Nm sshd
|
|
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
|
|
.Nm sshd
|
|
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
|
|
.Nm sshd
|
|
supports.
|
|
The possible values are
|
|
.Dq 1
|
|
and
|
|
.Dq 2 .
|
|
Multiple versions must be comma-separated.
|
|
The default is
|
|
.Dq 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 RhostsRSAAuthentication
|
|
Specifies whether rhosts or
|
|
.Pa /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 768.
|
|
.It Cm StrictModes
|
|
Specifies whether
|
|
.Nm sshd
|
|
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 .
|
|
.It Cm Subsystem
|
|
Configures an external subsystem (e.g., file transfer daemon).
|
|
Arguments should be a subsystem name and a command to execute upon subsystem
|
|
request.
|
|
The command
|
|
.Xr sftp-server 8
|
|
implements the
|
|
.Dq sftp
|
|
file transfer subsystem.
|
|
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
|
|
.Nm sshd .
|
|
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 UseDNS
|
|
Specifies whether
|
|
.Nm sshd
|
|
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 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 yes .
|
|
.It Cm UsePrivilegeSeparation
|
|
Specifies whether
|
|
.Nm sshd
|
|
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 .
|
|
.It Cm VersionAddendum
|
|
Specifies a string to append to the regular version string to identify
|
|
OS- or site-specific modifications.
|
|
The default is
|
|
.Dq FreeBSD-20050903 .
|
|
.It Cm X11DisplayOffset
|
|
Specifies the first display number available for
|
|
.Nm sshd Ns 's
|
|
X11 forwarding.
|
|
This prevents
|
|
.Nm 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 yes .
|
|
.Pp
|
|
When X11 forwarding is enabled, there may be additional exposure to
|
|
the server and to client displays if the
|
|
.Nm sshd
|
|
proxy display is configured to listen on the wildcard address (see
|
|
.Cm X11UseLocalhost
|
|
below), however 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
|
|
.Nm sshd
|
|
should bind the X11 forwarding server to the loopback address or to
|
|
the wildcard address.
|
|
By default,
|
|
.Nm 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
|
|
.Ss Time Formats
|
|
.Nm sshd
|
|
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 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
|
|
.Nm sshd .
|
|
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.
|