b7aa600c41
I have worked hard to reduce diffs against the vendor branch. One notable change in that respect is that we no longer prefer DSA over RSA - the reasons for doing so went away years ago. This may cause some surprises, as ssh will warn about unknown host keys even for hosts whose keys haven't changed. MFC after: 6 weeks
1085 lines
30 KiB
Groff
1085 lines
30 KiB
Groff
.\" -*- nroff -*-
|
|
.\"
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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
|
|
.\" notice, this list of conditions and the following disclaimer.
|
|
.\" 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 1.96 2008/07/02 02:24:18 djm Exp $
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.\" $FreeBSD$
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.Dd $Mdocdate: July 2 2008 $
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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
|
|
directives.
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|
Be warned that some environment variables could be used to bypass restricted
|
|
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
|
|
Specifies which address family should be used by
|
|
.Xr sshd 8 .
|
|
Valid arguments are
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|
.Dq any ,
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|
.Dq inet
|
|
(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
|
|
.Dq yes .
|
|
Note that disabling agent forwarding does not improve security
|
|
unless users are also denied shell access, as they can always install
|
|
their own forwarders.
|
|
.It Cm AllowGroups
|
|
This keyword can be followed by a list of group name patterns, separated
|
|
by spaces.
|
|
If specified, login is allowed only for users whose primary
|
|
group or supplementary group list matches one of the patterns.
|
|
Only group names are valid; a numerical group ID is not recognized.
|
|
By default, login is allowed for all groups.
|
|
The allow/deny directives are processed in the following order:
|
|
.Cm DenyUsers ,
|
|
.Cm AllowUsers ,
|
|
.Cm DenyGroups ,
|
|
and finally
|
|
.Cm AllowGroups .
|
|
.Pp
|
|
See
|
|
.Sx PATTERNS
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|
in
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|
.Xr ssh_config 5
|
|
for more information on patterns.
|
|
.It Cm AllowTcpForwarding
|
|
Specifies whether TCP forwarding is permitted.
|
|
The default is
|
|
.Dq yes .
|
|
Note that disabling TCP forwarding does not improve security unless
|
|
users are also denied shell access, as they can always install their
|
|
own forwarders.
|
|
.It Cm AllowUsers
|
|
This keyword can be followed by a list of user name patterns, separated
|
|
by spaces.
|
|
If specified, login is allowed only for user names that
|
|
match one of the patterns.
|
|
Only user names are valid; a numerical user ID is not recognized.
|
|
By default, login is allowed for all users.
|
|
If the pattern takes the form USER@HOST then USER and HOST
|
|
are separately checked, restricting logins to particular
|
|
users from particular hosts.
|
|
The allow/deny directives are processed in the following order:
|
|
.Cm DenyUsers ,
|
|
.Cm AllowUsers ,
|
|
.Cm DenyGroups ,
|
|
and finally
|
|
.Cm AllowGroups .
|
|
.Pp
|
|
See
|
|
.Sx PATTERNS
|
|
in
|
|
.Xr ssh_config 5
|
|
for more information on patterns.
|
|
.It Cm AuthorizedKeysFile
|
|
Specifies the file that contains the public keys that can be used
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|
for user authentication.
|
|
.Cm AuthorizedKeysFile
|
|
may contain tokens of the form %T which are substituted during connection
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|
setup.
|
|
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.
|
|
After expansion,
|
|
.Cm AuthorizedKeysFile
|
|
is taken to be an absolute path or one relative to the user's home
|
|
directory.
|
|
The default is
|
|
.Dq .ssh/authorized_keys .
|
|
.It Cm Banner
|
|
The contents of the specified file are sent to the remote user before
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|
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.
|
|
See also
|
|
.Cm UsePAM .
|
|
The default is
|
|
.Dq yes .
|
|
.It Cm ChrootDirectory
|
|
Specifies a path to
|
|
.Xr chroot 2
|
|
to after authentication.
|
|
This path, and all its components, must be root-owned directories that are
|
|
not writable by any other user or group.
|
|
.Pp
|
|
The path may contain the following tokens that are expanded at runtime once
|
|
the connecting user has been authenticated: %% 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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|
.Pp
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|
The
|
|
.Cm ChrootDirectory
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|
must contain the necessary files and directories to support the
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|
users' session.
|
|
For an interactive session this requires at least a shell, typically
|
|
.Xr sh 1 ,
|
|
and basic
|
|
.Pa /dev
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|
nodes such as
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|
.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 (see
|
|
.Cm Subsystem
|
|
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-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
|
|
arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
|
|
aes192-ctr,aes256-ctr
|
|
.Ed
|
|
.It Cm ClientAliveCountMax
|
|
Sets the number of client alive messages (see below) which may be
|
|
sent without
|
|
.Xr sshd 8
|
|
receiving any messages back from the client.
|
|
If this threshold is reached while client alive messages are being sent,
|
|
sshd will disconnect the client, terminating the session.
|
|
It is important to note that the use of client alive messages is very
|
|
different from
|
|
.Cm TCPKeepAlive
|
|
(below).
|
|
The client alive messages are sent through the encrypted channel
|
|
and therefore will not be spoofable.
|
|
The TCP keepalive option enabled by
|
|
.Cm TCPKeepAlive
|
|
is spoofable.
|
|
The client alive mechanism is valuable when the client or
|
|
server depend on knowing when a connection has become inactive.
|
|
.Pp
|
|
The default value is 3.
|
|
If
|
|
.Cm ClientAliveInterval
|
|
(see below) is set to 15, and
|
|
.Cm ClientAliveCountMax
|
|
is left at the default, unresponsive SSH clients
|
|
will be disconnected after approximately 45 seconds.
|
|
This option applies to protocol version 2 only.
|
|
.It Cm ClientAliveInterval
|
|
Sets a timeout interval in seconds after which if no data has been received
|
|
from the client,
|
|
.Xr sshd 8
|
|
will send a message through the encrypted
|
|
channel to request a response from the client.
|
|
The default
|
|
is 0, indicating that these messages will not be sent to the client.
|
|
This option applies to protocol version 2 only.
|
|
.It Cm Compression
|
|
Specifies whether compression is allowed, or delayed until
|
|
the user has authenticated successfully.
|
|
The argument must be
|
|
.Dq yes ,
|
|
.Dq delayed ,
|
|
or
|
|
.Dq no .
|
|
The default is
|
|
.Dq delayed .
|
|
.It Cm DenyGroups
|
|
This keyword can be followed by a list of group name patterns, separated
|
|
by spaces.
|
|
Login is disallowed for users whose primary group or supplementary
|
|
group list matches one of the patterns.
|
|
Only group names are valid; a numerical group ID is not recognized.
|
|
By default, login is allowed for all groups.
|
|
The allow/deny directives are processed in the following order:
|
|
.Cm DenyUsers ,
|
|
.Cm AllowUsers ,
|
|
.Cm DenyGroups ,
|
|
and finally
|
|
.Cm AllowGroups .
|
|
.Pp
|
|
See
|
|
.Sx PATTERNS
|
|
in
|
|
.Xr ssh_config 5
|
|
for more information on patterns.
|
|
.It Cm DenyUsers
|
|
This keyword can be followed by a list of user name patterns, separated
|
|
by spaces.
|
|
Login is disallowed for user names that match one of the patterns.
|
|
Only user names are valid; a numerical user ID is not recognized.
|
|
By default, login is allowed for all users.
|
|
If the pattern takes the form USER@HOST then USER and HOST
|
|
are separately checked, restricting logins to particular
|
|
users from particular hosts.
|
|
The allow/deny directives are processed in the following order:
|
|
.Cm DenyUsers ,
|
|
.Cm AllowUsers ,
|
|
.Cm DenyGroups ,
|
|
and finally
|
|
.Cm AllowGroups .
|
|
.Pp
|
|
See
|
|
.Sx PATTERNS
|
|
in
|
|
.Xr ssh_config 5
|
|
for more information on patterns.
|
|
.It Cm ForceCommand
|
|
Forces the execution of the command specified by
|
|
.Cm ForceCommand ,
|
|
ignoring any command supplied by the client 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 HostKey
|
|
Specifies a file containing a private host key
|
|
used by SSH.
|
|
The default is
|
|
.Pa /etc/ssh/ssh_host_key
|
|
for protocol version 1, and
|
|
.Pa /etc/ssh/ssh_host_rsa_key
|
|
and
|
|
.Pa /etc/ssh/ssh_host_dsa_key
|
|
for protocol version 2.
|
|
Note that
|
|
.Xr sshd 8
|
|
will refuse to use a file if it is group/world-accessible.
|
|
It is possible to have multiple host key files.
|
|
.Dq rsa1
|
|
keys are used for version 1 and
|
|
.Dq dsa
|
|
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/ssh/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 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 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,
|
|
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 ,
|
|
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 AllowTcpForwarding ,
|
|
.Cm Banner ,
|
|
.Cm ChrootDirectory ,
|
|
.Cm ForceCommand ,
|
|
.Cm GatewayPorts ,
|
|
.Cm GSSAPIAuthentication ,
|
|
.Cm HostbasedAuthentication ,
|
|
.Cm KbdInteractiveAuthentication ,
|
|
.Cm KerberosAuthentication ,
|
|
.Cm MaxAuthTries ,
|
|
.Cm MaxSessions ,
|
|
.Cm PasswordAuthentication ,
|
|
.Cm PermitOpen ,
|
|
.Cm PermitRootLogin ,
|
|
.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.
|
|
See also
|
|
.Cm UsePAM .
|
|
The default is
|
|
.Dq no .
|
|
.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.
|
|
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 no .
|
|
Note that if
|
|
.Cm ChallengeResponseAuthentication
|
|
is
|
|
.Dq yes ,
|
|
the root user may be allowed in with its password even if
|
|
.Cm PermitRootLogin is set to
|
|
.Dq without-password .
|
|
.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
|
|
.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 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 .
|
|
.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 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 yes .
|
|
.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 .
|
|
.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-20080801 .
|
|
.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 yes .
|
|
.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/local/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.
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Niels Provos and Markus Friedl contributed support
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for privilege separation.
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