994 lines
31 KiB
Groff
994 lines
31 KiB
Groff
.\"
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.\" Author: Tatu Ylonen <ylo@cs.hut.fi>
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.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
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.\" All rights reserved
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.\"
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.\" As far as I am concerned, the code I have written for this software
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.\" can be used freely for any purpose. Any derived versions of this
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.\" software must be clearly marked as such, and if the derived work is
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.\" incompatible with the protocol description in the RFC file, it must be
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.\" called by a name other than "ssh" or "Secure Shell".
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.\"
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.\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
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.\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
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.\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
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.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
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.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
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.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
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.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
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.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
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.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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.\"
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.\" $OpenBSD: sshd.8,v 1.273 2013/12/07 11:58:46 naddy Exp $
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.\" $FreeBSD$
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.Dd December 7, 2013
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.Dt SSHD 8
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.Os
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.Sh NAME
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.Nm sshd
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.Nd OpenSSH SSH daemon
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.Sh SYNOPSIS
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.Nm sshd
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.Bk -words
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.Op Fl 46DdeiqTt
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.Op Fl b Ar bits
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.Op Fl C Ar connection_spec
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.Op Fl c Ar host_certificate_file
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.Op Fl E Ar log_file
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.Op Fl f Ar config_file
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.Op Fl g Ar login_grace_time
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.Op Fl h Ar host_key_file
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.Op Fl k Ar key_gen_time
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.Op Fl o Ar option
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.Op Fl p Ar port
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.Op Fl u Ar len
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.Ek
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.Sh DESCRIPTION
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.Nm
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(OpenSSH Daemon) is the daemon program for
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.Xr ssh 1 .
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Together these programs replace
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.Xr rlogin 1
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and
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.Xr rsh 1 ,
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and provide secure encrypted communications between two untrusted hosts
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over an insecure network.
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.Pp
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.Nm
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listens for connections from clients.
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It is normally started at boot from
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.Pa /etc/rc.d/sshd .
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It forks a new
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daemon for each incoming connection.
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The forked daemons handle
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key exchange, encryption, authentication, command execution,
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and data exchange.
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.Pp
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.Nm
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can be configured using command-line options or a configuration file
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(by default
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.Xr sshd_config 5 ) ;
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command-line options override values specified in the
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configuration file.
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.Nm
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rereads its configuration file when it receives a hangup signal,
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.Dv SIGHUP ,
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by executing itself with the name and options it was started with, e.g.\&
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.Pa /usr/sbin/sshd .
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.Pp
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The options are as follows:
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.Bl -tag -width Ds
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.It Fl 4
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Forces
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.Nm
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to use IPv4 addresses only.
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.It Fl 6
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Forces
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.Nm
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to use IPv6 addresses only.
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.It Fl b Ar bits
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Specifies the number of bits in the ephemeral protocol version 1
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server key (default 1024).
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.It Fl C Ar connection_spec
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Specify the connection parameters to use for the
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.Fl T
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extended test mode.
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If provided, any
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.Cm Match
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directives in the configuration file
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that would apply to the specified user, host, and address will be set before
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the configuration is written to standard output.
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The connection parameters are supplied as keyword=value pairs.
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The keywords are
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.Dq user ,
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.Dq host ,
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.Dq laddr ,
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.Dq lport ,
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and
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.Dq addr .
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All are required and may be supplied in any order, either with multiple
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.Fl C
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options or as a comma-separated list.
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.It Fl c Ar host_certificate_file
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Specifies a path to a certificate file to identify
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.Nm
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during key exchange.
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The certificate file must match a host key file specified using the
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.Fl h
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option or the
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.Cm HostKey
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configuration directive.
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.It Fl D
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When this option is specified,
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.Nm
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will not detach and does not become a daemon.
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This allows easy monitoring of
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.Nm sshd .
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.It Fl d
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Debug mode.
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The server sends verbose debug output to standard error,
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and does not put itself in the background.
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The server also will not fork and will only process one connection.
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This option is only intended for debugging for the server.
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Multiple
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.Fl d
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options increase the debugging level.
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Maximum is 3.
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.It Fl E Ar log_file
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Append debug logs to
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.Ar log_file
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instead of the system log.
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.It Fl e
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Write debug logs to standard error instead of the system log.
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.It Fl f Ar config_file
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Specifies the name of the configuration file.
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The default is
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.Pa /etc/ssh/sshd_config .
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.Nm
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refuses to start if there is no configuration file.
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.It Fl g Ar login_grace_time
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Gives the grace time for clients to authenticate themselves (default
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120 seconds).
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If the client fails to authenticate the user within
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this many seconds, the server disconnects and exits.
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A value of zero indicates no limit.
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.It Fl h Ar host_key_file
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Specifies a file from which a host key is read.
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This option must be given if
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.Nm
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is not run as root (as the normal
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host key files are normally not readable by anyone but root).
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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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.Pa /etc/ssh/ssh_host_ecdsa_key .
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.Pa /etc/ssh/ssh_host_ed25519_key
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and
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.Pa /etc/ssh/ssh_host_rsa_key
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for protocol version 2.
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It is possible to have multiple host key files for
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the different protocol versions and host key algorithms.
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.It Fl i
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Specifies that
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.Nm
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is being run from
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.Xr inetd 8 .
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.Nm
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is normally not run
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from inetd because it needs to generate the server key before it can
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respond to the client, and this may take tens of seconds.
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Clients would have to wait too long if the key was regenerated every time.
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However, with small key sizes (e.g. 512) using
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.Nm
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from inetd may
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be feasible.
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.It Fl k Ar key_gen_time
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Specifies how often the ephemeral protocol version 1 server key is
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regenerated (default 3600 seconds, or one hour).
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The motivation for regenerating the key fairly
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often is that the key is not stored anywhere, and after about an hour
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it becomes impossible to recover the key for decrypting intercepted
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communications even if the machine is cracked into or physically
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seized.
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A value of zero indicates that the key will never be regenerated.
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.It Fl o Ar option
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Can be used to give options in the format used in the configuration file.
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This is useful for specifying options for which there is no separate
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command-line flag.
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For full details of the options, and their values, see
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.Xr sshd_config 5 .
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.It Fl p Ar port
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Specifies the port on which the server listens for connections
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(default 22).
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Multiple port options are permitted.
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Ports specified in the configuration file with the
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.Cm Port
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option are ignored when a command-line port is specified.
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Ports specified using the
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.Cm ListenAddress
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option override command-line ports.
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.It Fl q
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Quiet mode.
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Nothing is sent to the system log.
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Normally the beginning,
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authentication, and termination of each connection is logged.
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.It Fl T
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Extended test mode.
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Check the validity of the configuration file, output the effective configuration
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to stdout and then exit.
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Optionally,
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.Cm Match
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rules may be applied by specifying the connection parameters using one or more
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.Fl C
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options.
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.It Fl t
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Test mode.
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Only check the validity of the configuration file and sanity of the keys.
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This is useful for updating
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.Nm
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reliably as configuration options may change.
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.It Fl u Ar len
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This option is used to specify the size of the field
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in the
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.Li utmp
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structure that holds the remote host name.
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If the resolved host name is longer than
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.Ar len ,
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the dotted decimal value will be used instead.
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This allows hosts with very long host names that
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overflow this field to still be uniquely identified.
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Specifying
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.Fl u0
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indicates that only dotted decimal addresses
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should be put into the
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.Pa utmp
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file.
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.Fl u0
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may also be used to prevent
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.Nm
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from making DNS requests unless the authentication
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mechanism or configuration requires it.
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Authentication mechanisms that may require DNS include
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.Cm RhostsRSAAuthentication ,
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.Cm HostbasedAuthentication ,
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and using a
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.Cm from="pattern-list"
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option in a key file.
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Configuration options that require DNS include using a
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USER@HOST pattern in
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.Cm AllowUsers
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or
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.Cm DenyUsers .
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.El
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.Sh AUTHENTICATION
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The OpenSSH SSH daemon supports SSH protocols 1 and 2.
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The default is to use protocol 2 only,
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though this can be changed via the
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.Cm Protocol
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option in
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.Xr sshd_config 5 .
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Protocol 2 supports DSA, ECDSA, ED25519 and RSA keys;
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protocol 1 only supports RSA keys.
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For both protocols,
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each host has a host-specific key,
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normally 2048 bits,
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used to identify the host.
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.Pp
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Forward security for protocol 1 is provided through
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an additional server key,
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normally 768 bits,
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generated when the server starts.
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This key is normally regenerated every hour if it has been used, and
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is never stored on disk.
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Whenever a client connects, the daemon responds with its public
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host and server keys.
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The client compares the
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RSA host key against its own database to verify that it has not changed.
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The client then generates a 256-bit random number.
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It encrypts this
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random number using both the host key and the server key, and sends
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the encrypted number to the server.
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Both sides then use this
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random number as a session key which is used to encrypt all further
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communications in the session.
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The rest of the session is encrypted
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using a conventional cipher, currently Blowfish or 3DES, with 3DES
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being used by default.
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The client selects the encryption algorithm
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to use from those offered by the server.
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.Pp
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For protocol 2,
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forward security is provided through a Diffie-Hellman key agreement.
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This key agreement results in a shared session key.
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The rest of the session is encrypted using a symmetric cipher, currently
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128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES.
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The client selects the encryption algorithm
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to use from those offered by the server.
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Additionally, session integrity is provided
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through a cryptographic message authentication code
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(hmac-md5, hmac-sha1, umac-64, umac-128, hmac-ripemd160,
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hmac-sha2-256 or hmac-sha2-512).
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.Pp
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Finally, the server and the client enter an authentication dialog.
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The client tries to authenticate itself using
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host-based authentication,
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public key authentication,
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challenge-response authentication,
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or password authentication.
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.Pp
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Regardless of the authentication type, the account is checked to
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ensure that it is accessible. An account is not accessible if it is
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locked, listed in
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.Cm DenyUsers
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or its group is listed in
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.Cm DenyGroups
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\&. The definition of a locked account is system dependant. Some platforms
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have their own account database (eg AIX) and some modify the passwd field (
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.Ql \&*LK\&*
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on Solaris and UnixWare,
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.Ql \&*
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on HP-UX, containing
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.Ql Nologin
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on Tru64,
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a leading
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.Ql \&*LOCKED\&*
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on FreeBSD and a leading
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.Ql \&!
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on most Linuxes).
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If there is a requirement to disable password authentication
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for the account while allowing still public-key, then the passwd field
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should be set to something other than these values (eg
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.Ql NP
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or
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.Ql \&*NP\&*
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).
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.Pp
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If the client successfully authenticates itself, a dialog for
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preparing the session is entered.
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At this time the client may request
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things like allocating a pseudo-tty, forwarding X11 connections,
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forwarding TCP connections, or forwarding the authentication agent
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connection over the secure channel.
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.Pp
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After this, the client either requests a shell or execution of a command.
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The sides then enter session mode.
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In this mode, either side may send
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data at any time, and such data is forwarded to/from the shell or
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command on the server side, and the user terminal in the client side.
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.Pp
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When the user program terminates and all forwarded X11 and other
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connections have been closed, the server sends command exit status to
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the client, and both sides exit.
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.Sh LOGIN PROCESS
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When a user successfully logs in,
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.Nm
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does the following:
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.Bl -enum -offset indent
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.It
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If the login is on a tty, and no command has been specified,
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prints last login time and
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.Pa /etc/motd
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(unless prevented in the configuration file or by
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.Pa ~/.hushlogin ;
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see the
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.Sx FILES
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section).
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.It
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If the login is on a tty, records login time.
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.It
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Checks
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.Pa /etc/nologin and
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.Pa /var/run/nologin ;
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if one exists, it prints the contents and quits
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(unless root).
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.It
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Changes to run with normal user privileges.
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.It
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Sets up basic environment.
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.It
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Reads the file
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.Pa ~/.ssh/environment ,
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if it exists, and users are allowed to change their environment.
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See the
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.Cm PermitUserEnvironment
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option in
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.Xr sshd_config 5 .
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.It
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Changes to user's home directory.
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.It
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If
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.Pa ~/.ssh/rc
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exists, runs it; else if
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.Pa /etc/ssh/sshrc
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exists, runs
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it; otherwise runs
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.Xr xauth 1 .
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The
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.Dq rc
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files are given the X11
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authentication protocol and cookie in standard input.
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See
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.Sx SSHRC ,
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below.
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.It
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Runs user's shell or command.
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.El
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.Sh SSHRC
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If the file
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.Pa ~/.ssh/rc
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exists,
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.Xr sh 1
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runs it after reading the
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environment files but before starting the user's shell or command.
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It must not produce any output on stdout; stderr must be used
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instead.
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If X11 forwarding is in use, it will receive the "proto cookie" pair in
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its standard input (and
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.Ev DISPLAY
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in its environment).
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The script must call
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.Xr xauth 1
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because
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.Nm
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will not run xauth automatically to add X11 cookies.
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.Pp
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The primary purpose of this file is to run any initialization routines
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which may be needed before the user's home directory becomes
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accessible; AFS is a particular example of such an environment.
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.Pp
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This file will probably contain some initialization code followed by
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something similar to:
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.Bd -literal -offset 3n
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if read proto cookie && [ -n "$DISPLAY" ]; then
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if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
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# X11UseLocalhost=yes
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echo add unix:`echo $DISPLAY |
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cut -c11-` $proto $cookie
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else
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# X11UseLocalhost=no
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echo add $DISPLAY $proto $cookie
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fi | xauth -q -
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fi
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.Ed
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.Pp
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If this file does not exist,
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.Pa /etc/ssh/sshrc
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is run, and if that
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does not exist either, xauth is used to add the cookie.
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.Sh AUTHORIZED_KEYS FILE FORMAT
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.Cm AuthorizedKeysFile
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specifies the files containing public keys for
|
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public key authentication;
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if none is specified, the default is
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.Pa ~/.ssh/authorized_keys
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and
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.Pa ~/.ssh/authorized_keys2 .
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Each line of the file contains one
|
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key (empty lines and lines starting with a
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.Ql #
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are ignored as
|
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comments).
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Protocol 1 public keys consist of the following space-separated fields:
|
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options, bits, exponent, modulus, comment.
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Protocol 2 public key consist of:
|
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options, keytype, base64-encoded key, comment.
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The options field is optional;
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its presence is determined by whether the line starts
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with a number or not (the options field never starts with a number).
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The bits, exponent, modulus, and comment fields give the RSA key for
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protocol version 1; the
|
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comment field is not used for anything (but may be convenient for the
|
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user to identify the key).
|
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For protocol version 2 the keytype is
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.Dq ecdsa-sha2-nistp256 ,
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.Dq ecdsa-sha2-nistp384 ,
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.Dq ecdsa-sha2-nistp521 ,
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.Dq ssh-ed25519 ,
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.Dq ssh-dss
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or
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.Dq ssh-rsa .
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.Pp
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Note that lines in this file are usually several hundred bytes long
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(because of the size of the public key encoding) up to a limit of
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8 kilobytes, which permits DSA keys up to 8 kilobits and RSA
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keys up to 16 kilobits.
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You don't want to type them in; instead, copy the
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.Pa identity.pub ,
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.Pa id_dsa.pub ,
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.Pa id_ecdsa.pub ,
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.Pa id_ed25519.pub ,
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or the
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.Pa id_rsa.pub
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file and edit it.
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.Pp
|
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.Nm
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|
enforces a minimum RSA key modulus size for protocol 1
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and protocol 2 keys of 768 bits.
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.Pp
|
|
The options (if present) consist of comma-separated option
|
|
specifications.
|
|
No spaces are permitted, except within double quotes.
|
|
The following option specifications are supported (note
|
|
that option keywords are case-insensitive):
|
|
.Bl -tag -width Ds
|
|
.It Cm cert-authority
|
|
Specifies that the listed key is a certification authority (CA) that is
|
|
trusted to validate signed certificates for user authentication.
|
|
.Pp
|
|
Certificates may encode access restrictions similar to these key options.
|
|
If both certificate restrictions and key options are present, the most
|
|
restrictive union of the two is applied.
|
|
.It Cm command="command"
|
|
Specifies that the command is executed whenever this key is used for
|
|
authentication.
|
|
The command supplied by the user (if any) is ignored.
|
|
The command is run on a pty if the client requests a pty;
|
|
otherwise it is run without a tty.
|
|
If an 8-bit clean channel is required,
|
|
one must not request a pty or should specify
|
|
.Cm no-pty .
|
|
A quote may be included in the command by quoting it with a backslash.
|
|
This option might be useful
|
|
to restrict certain public keys to perform just a specific operation.
|
|
An example might be a key that permits remote backups but nothing else.
|
|
Note that the client may specify TCP and/or X11
|
|
forwarding unless they are explicitly prohibited.
|
|
The command originally supplied by the client is available in the
|
|
.Ev SSH_ORIGINAL_COMMAND
|
|
environment variable.
|
|
Note that this option applies to shell, command or subsystem execution.
|
|
Also note that this command may be superseded by either a
|
|
.Xr sshd_config 5
|
|
.Cm ForceCommand
|
|
directive or a command embedded in a certificate.
|
|
.It Cm environment="NAME=value"
|
|
Specifies that the string is to be added to the environment when
|
|
logging in using this key.
|
|
Environment variables set this way
|
|
override other default environment values.
|
|
Multiple options of this type are permitted.
|
|
Environment processing is disabled by default and is
|
|
controlled via the
|
|
.Cm PermitUserEnvironment
|
|
option.
|
|
This option is automatically disabled if
|
|
.Cm UseLogin
|
|
is enabled.
|
|
.It Cm from="pattern-list"
|
|
Specifies that in addition to public key authentication, either the canonical
|
|
name of the remote host or its IP address must be present in the
|
|
comma-separated list of patterns.
|
|
See PATTERNS in
|
|
.Xr ssh_config 5
|
|
for more information on patterns.
|
|
.Pp
|
|
In addition to the wildcard matching that may be applied to hostnames or
|
|
addresses, a
|
|
.Cm from
|
|
stanza may match IP addresses using CIDR address/masklen notation.
|
|
.Pp
|
|
The purpose of this option is to optionally increase security: public key
|
|
authentication by itself does not trust the network or name servers or
|
|
anything (but the key); however, if somebody somehow steals the key, the key
|
|
permits an intruder to log in from anywhere in the world.
|
|
This additional option makes using a stolen key more difficult (name
|
|
servers and/or routers would have to be compromised in addition to
|
|
just the key).
|
|
.It Cm no-agent-forwarding
|
|
Forbids authentication agent forwarding when this key is used for
|
|
authentication.
|
|
.It Cm no-port-forwarding
|
|
Forbids TCP forwarding when this key is used for authentication.
|
|
Any port forward requests by the client will return an error.
|
|
This might be used, e.g. in connection with the
|
|
.Cm command
|
|
option.
|
|
.It Cm no-pty
|
|
Prevents tty allocation (a request to allocate a pty will fail).
|
|
.It Cm no-user-rc
|
|
Disables execution of
|
|
.Pa ~/.ssh/rc .
|
|
.It Cm no-X11-forwarding
|
|
Forbids X11 forwarding when this key is used for authentication.
|
|
Any X11 forward requests by the client will return an error.
|
|
.It Cm permitopen="host:port"
|
|
Limit local
|
|
.Li ``ssh -L''
|
|
port forwarding such that it may only connect to the specified host and
|
|
port.
|
|
IPv6 addresses can be specified by enclosing the address in square brackets.
|
|
Multiple
|
|
.Cm permitopen
|
|
options may be applied separated by commas.
|
|
No pattern matching is performed on the specified hostnames,
|
|
they must be literal domains or addresses.
|
|
A port specification of
|
|
.Cm *
|
|
matches any port.
|
|
.It Cm principals="principals"
|
|
On a
|
|
.Cm cert-authority
|
|
line, specifies allowed principals for certificate authentication as a
|
|
comma-separated list.
|
|
At least one name from the list must appear in the certificate's
|
|
list of principals for the certificate to be accepted.
|
|
This option is ignored for keys that are not marked as trusted certificate
|
|
signers using the
|
|
.Cm cert-authority
|
|
option.
|
|
.It Cm tunnel="n"
|
|
Force a
|
|
.Xr tun 4
|
|
device on the server.
|
|
Without this option, the next available device will be used if
|
|
the client requests a tunnel.
|
|
.El
|
|
.Pp
|
|
An example authorized_keys file:
|
|
.Bd -literal -offset 3n
|
|
# Comments allowed at start of line
|
|
ssh-rsa AAAAB3Nza...LiPk== user@example.net
|
|
from="*.sales.example.net,!pc.sales.example.net" ssh-rsa
|
|
AAAAB2...19Q== john@example.net
|
|
command="dump /home",no-pty,no-port-forwarding ssh-dss
|
|
AAAAC3...51R== example.net
|
|
permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss
|
|
AAAAB5...21S==
|
|
tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...==
|
|
jane@example.net
|
|
.Ed
|
|
.Sh SSH_KNOWN_HOSTS FILE FORMAT
|
|
The
|
|
.Pa /etc/ssh/ssh_known_hosts
|
|
and
|
|
.Pa ~/.ssh/known_hosts
|
|
files contain host public keys for all known hosts.
|
|
The global file should
|
|
be prepared by the administrator (optional), and the per-user file is
|
|
maintained automatically: whenever the user connects from an unknown host,
|
|
its key is added to the per-user file.
|
|
.Pp
|
|
Each line in these files contains the following fields: markers (optional),
|
|
hostnames, bits, exponent, modulus, comment.
|
|
The fields are separated by spaces.
|
|
.Pp
|
|
The marker is optional, but if it is present then it must be one of
|
|
.Dq @cert-authority ,
|
|
to indicate that the line contains a certification authority (CA) key,
|
|
or
|
|
.Dq @revoked ,
|
|
to indicate that the key contained on the line is revoked and must not ever
|
|
be accepted.
|
|
Only one marker should be used on a key line.
|
|
.Pp
|
|
Hostnames is a comma-separated list of patterns
|
|
.Pf ( Ql *
|
|
and
|
|
.Ql \&?
|
|
act as
|
|
wildcards); each pattern in turn is matched against the canonical host
|
|
name (when authenticating a client) or against the user-supplied
|
|
name (when authenticating a server).
|
|
A pattern may also be preceded by
|
|
.Ql \&!
|
|
to indicate negation: if the host name matches a negated
|
|
pattern, it is not accepted (by that line) even if it matched another
|
|
pattern on the line.
|
|
A hostname or address may optionally be enclosed within
|
|
.Ql \&[
|
|
and
|
|
.Ql \&]
|
|
brackets then followed by
|
|
.Ql \&:
|
|
and a non-standard port number.
|
|
.Pp
|
|
Alternately, hostnames may be stored in a hashed form which hides host names
|
|
and addresses should the file's contents be disclosed.
|
|
Hashed hostnames start with a
|
|
.Ql |
|
|
character.
|
|
Only one hashed hostname may appear on a single line and none of the above
|
|
negation or wildcard operators may be applied.
|
|
.Pp
|
|
Bits, exponent, and modulus are taken directly from the RSA host key; they
|
|
can be obtained, for example, from
|
|
.Pa /etc/ssh/ssh_host_key.pub .
|
|
The optional comment field continues to the end of the line, and is not used.
|
|
.Pp
|
|
Lines starting with
|
|
.Ql #
|
|
and empty lines are ignored as comments.
|
|
.Pp
|
|
When performing host authentication, authentication is accepted if any
|
|
matching line has the proper key; either one that matches exactly or,
|
|
if the server has presented a certificate for authentication, the key
|
|
of the certification authority that signed the certificate.
|
|
For a key to be trusted as a certification authority, it must use the
|
|
.Dq @cert-authority
|
|
marker described above.
|
|
.Pp
|
|
The known hosts file also provides a facility to mark keys as revoked,
|
|
for example when it is known that the associated private key has been
|
|
stolen.
|
|
Revoked keys are specified by including the
|
|
.Dq @revoked
|
|
marker at the beginning of the key line, and are never accepted for
|
|
authentication or as certification authorities, but instead will
|
|
produce a warning from
|
|
.Xr ssh 1
|
|
when they are encountered.
|
|
.Pp
|
|
It is permissible (but not
|
|
recommended) to have several lines or different host keys for the same
|
|
names.
|
|
This will inevitably happen when short forms of host names
|
|
from different domains are put in the file.
|
|
It is possible
|
|
that the files contain conflicting information; authentication is
|
|
accepted if valid information can be found from either file.
|
|
.Pp
|
|
Note that the lines in these files are typically hundreds of characters
|
|
long, and you definitely don't want to type in the host keys by hand.
|
|
Rather, generate them by a script,
|
|
.Xr ssh-keyscan 1
|
|
or by taking
|
|
.Pa /etc/ssh/ssh_host_key.pub
|
|
and adding the host names at the front.
|
|
.Xr ssh-keygen 1
|
|
also offers some basic automated editing for
|
|
.Pa ~/.ssh/known_hosts
|
|
including removing hosts matching a host name and converting all host
|
|
names to their hashed representations.
|
|
.Pp
|
|
An example ssh_known_hosts file:
|
|
.Bd -literal -offset 3n
|
|
# Comments allowed at start of line
|
|
closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net
|
|
cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....=
|
|
# A hashed hostname
|
|
|1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
|
|
AAAA1234.....=
|
|
# A revoked key
|
|
@revoked * ssh-rsa AAAAB5W...
|
|
# A CA key, accepted for any host in *.mydomain.com or *.mydomain.org
|
|
@cert-authority *.mydomain.org,*.mydomain.com ssh-rsa AAAAB5W...
|
|
.Ed
|
|
.Sh FILES
|
|
.Bl -tag -width Ds -compact
|
|
.It Pa ~/.hushlogin
|
|
This file is used to suppress printing the last login time and
|
|
.Pa /etc/motd ,
|
|
if
|
|
.Cm PrintLastLog
|
|
and
|
|
.Cm PrintMotd ,
|
|
respectively,
|
|
are enabled.
|
|
It does not suppress printing of the banner specified by
|
|
.Cm Banner .
|
|
.Pp
|
|
.It Pa ~/.rhosts
|
|
This file is used for host-based authentication (see
|
|
.Xr ssh 1
|
|
for more information).
|
|
On some machines this file may need to be
|
|
world-readable if the user's home directory is on an NFS partition,
|
|
because
|
|
.Nm
|
|
reads it as root.
|
|
Additionally, this file must be owned by the user,
|
|
and must not have write permissions for anyone else.
|
|
The recommended
|
|
permission for most machines is read/write for the user, and not
|
|
accessible by others.
|
|
.Pp
|
|
.It Pa ~/.shosts
|
|
This file is used in exactly the same way as
|
|
.Pa .rhosts ,
|
|
but allows host-based authentication without permitting login with
|
|
rlogin/rsh.
|
|
.Pp
|
|
.It Pa ~/.ssh/
|
|
This directory is the default location for all user-specific configuration
|
|
and authentication information.
|
|
There is no general requirement to keep the entire contents of this directory
|
|
secret, but the recommended permissions are read/write/execute for the user,
|
|
and not accessible by others.
|
|
.Pp
|
|
.It Pa ~/.ssh/authorized_keys
|
|
Lists the public keys (DSA, ECDSA, ED25519, RSA)
|
|
that can be used for logging in as this user.
|
|
The format of this file is described above.
|
|
The content of the file is not highly sensitive, but the recommended
|
|
permissions are read/write for the user, and not accessible by others.
|
|
.Pp
|
|
If this file, the
|
|
.Pa ~/.ssh
|
|
directory, or the user's home directory are writable
|
|
by other users, then the file could be modified or replaced by unauthorized
|
|
users.
|
|
In this case,
|
|
.Nm
|
|
will not allow it to be used unless the
|
|
.Cm StrictModes
|
|
option has been set to
|
|
.Dq no .
|
|
.Pp
|
|
.It Pa ~/.ssh/environment
|
|
This file is read into the environment at login (if it exists).
|
|
It can only contain empty lines, comment lines (that start with
|
|
.Ql # ) ,
|
|
and assignment lines of the form name=value.
|
|
The file should be writable
|
|
only by the user; it need not be readable by anyone else.
|
|
Environment processing is disabled by default and is
|
|
controlled via the
|
|
.Cm PermitUserEnvironment
|
|
option.
|
|
.Pp
|
|
.It Pa ~/.ssh/known_hosts
|
|
Contains a list of host keys for all hosts the user has logged into
|
|
that are not already in the systemwide list of known host keys.
|
|
The format of this file is described above.
|
|
This file should be writable only by root/the owner and
|
|
can, but need not be, world-readable.
|
|
.Pp
|
|
.It Pa ~/.ssh/rc
|
|
Contains initialization routines to be run before
|
|
the user's home directory becomes accessible.
|
|
This file should be writable only by the user, and need not be
|
|
readable by anyone else.
|
|
.Pp
|
|
.It Pa /etc/hosts.allow
|
|
.It Pa /etc/hosts.deny
|
|
Access controls that should be enforced by tcp-wrappers are defined here.
|
|
Further details are described in
|
|
.Xr hosts_access 5 .
|
|
.Pp
|
|
.It Pa /etc/hosts.equiv
|
|
This file is for host-based authentication (see
|
|
.Xr ssh 1 ) .
|
|
It should only be writable by root.
|
|
.Pp
|
|
.It Pa /etc/moduli
|
|
Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
|
|
The file format is described in
|
|
.Xr moduli 5 .
|
|
.Pp
|
|
.It Pa /etc/motd
|
|
See
|
|
.Xr motd 5 .
|
|
.Pp
|
|
.It Pa /etc/nologin
|
|
If this file exists,
|
|
.Nm
|
|
refuses to let anyone except root log in.
|
|
The contents of the file
|
|
are displayed to anyone trying to log in, and non-root connections are
|
|
refused.
|
|
The file should be world-readable.
|
|
.Pp
|
|
.It Pa /etc/shosts.equiv
|
|
This file is used in exactly the same way as
|
|
.Pa hosts.equiv ,
|
|
but allows host-based authentication without permitting login with
|
|
rlogin/rsh.
|
|
.Pp
|
|
.It Pa /etc/ssh/ssh_host_key
|
|
.It Pa /etc/ssh/ssh_host_dsa_key
|
|
.It Pa /etc/ssh/ssh_host_ecdsa_key
|
|
.It Pa /etc/ssh/ssh_host_ed25519_key
|
|
.It Pa /etc/ssh/ssh_host_rsa_key
|
|
These files contain the private parts of the host keys.
|
|
These files should only be owned by root, readable only by root, and not
|
|
accessible to others.
|
|
Note that
|
|
.Nm
|
|
does not start if these files are group/world-accessible.
|
|
.Pp
|
|
.It Pa /etc/ssh/ssh_host_key.pub
|
|
.It Pa /etc/ssh/ssh_host_dsa_key.pub
|
|
.It Pa /etc/ssh/ssh_host_ecdsa_key.pub
|
|
.It Pa /etc/ssh/ssh_host_ed25519_key.pub
|
|
.It Pa /etc/ssh/ssh_host_rsa_key.pub
|
|
These files contain the public parts of the host keys.
|
|
These files should be world-readable but writable only by
|
|
root.
|
|
Their contents should match the respective private parts.
|
|
These files are not
|
|
really used for anything; they are provided for the convenience of
|
|
the user so their contents can be copied to known hosts files.
|
|
These files are created using
|
|
.Xr ssh-keygen 1 .
|
|
.Pp
|
|
.It Pa /etc/ssh/ssh_known_hosts
|
|
Systemwide list of known host keys.
|
|
This file should be prepared by the
|
|
system administrator to contain the public host keys of all machines in the
|
|
organization.
|
|
The format of this file is described above.
|
|
This file should be writable only by root/the owner and
|
|
should be world-readable.
|
|
.Pp
|
|
.It Pa /etc/ssh/sshd_config
|
|
Contains configuration data for
|
|
.Nm sshd .
|
|
The file format and configuration options are described in
|
|
.Xr sshd_config 5 .
|
|
.Pp
|
|
.It Pa /etc/ssh/sshrc
|
|
Similar to
|
|
.Pa ~/.ssh/rc ,
|
|
it can be used to specify
|
|
machine-specific login-time initializations globally.
|
|
This file should be writable only by root, and should be world-readable.
|
|
.Pp
|
|
.It Pa /var/empty
|
|
.Xr chroot 2
|
|
directory used by
|
|
.Nm
|
|
during privilege separation in the pre-authentication phase.
|
|
The directory should not contain any files and must be owned by root
|
|
and not group or world-writable.
|
|
.Pp
|
|
.It Pa /var/run/sshd.pid
|
|
Contains the process ID of the
|
|
.Nm
|
|
listening for connections (if there are several daemons running
|
|
concurrently for different ports, this contains the process ID of the one
|
|
started last).
|
|
The content of this file is not sensitive; it can be world-readable.
|
|
.El
|
|
.Sh SEE ALSO
|
|
.Xr scp 1 ,
|
|
.Xr sftp 1 ,
|
|
.Xr ssh 1 ,
|
|
.Xr ssh-add 1 ,
|
|
.Xr ssh-agent 1 ,
|
|
.Xr ssh-keygen 1 ,
|
|
.Xr ssh-keyscan 1 ,
|
|
.Xr chroot 2 ,
|
|
.Xr hosts_access 5 ,
|
|
.Xr login.conf 5 ,
|
|
.Xr moduli 5 ,
|
|
.Xr sshd_config 5 ,
|
|
.Xr inetd 8 ,
|
|
.Xr sftp-server 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.
|
|
.Sh CAVEATS
|
|
System security is not improved unless
|
|
.Nm rshd ,
|
|
.Nm rlogind ,
|
|
and
|
|
.Nm rexecd
|
|
are disabled (thus completely disabling
|
|
.Xr rlogin
|
|
and
|
|
.Xr rsh
|
|
into the machine).
|