1273 lines
39 KiB
Groff
1273 lines
39 KiB
Groff
.\" -*- nroff -*-
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.\"
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.\" Author: Tatu Ylonen <ylo@cs.hut.fi>
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.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
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.\" All rights reserved
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.\"
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.\" As far as I am concerned, the code I have written for this software
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.\" can be used freely for any purpose. Any derived versions of this
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.\" software must be clearly marked as such, and if the derived work is
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.\" incompatible with the protocol description in the RFC file, it must be
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.\" called by a name other than "ssh" or "Secure Shell".
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.\"
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.\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
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.\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
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.\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
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.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
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.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
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.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
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.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
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.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
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.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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.\"
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.\" $OpenBSD: sshd.8,v 1.120 2001/04/22 23:58:36 markus Exp $
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.\" $FreeBSD$
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.\"
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.Dd September 25, 1999
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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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.Op Fl deiqD46
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.Op Fl b Ar bits
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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 p Ar port
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.Op Fl u Ar len
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.Op Fl V Ar client_protocol_id
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.Sh DESCRIPTION
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.Nm
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(SSH Daemon) is the daemon program for
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.Xr ssh 1 .
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Together these programs replace rlogin and rsh, and
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provide secure encrypted communications between two untrusted hosts
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over an insecure network.
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The programs are intended to be as easy to
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install and use as possible.
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.Pp
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.Nm
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is the daemon that listens for connections from clients.
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It is normally started at boot from
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.Pa /etc/rc.network .
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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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This implementation of
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.Nm
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supports both SSH protocol version 1 and 2 simultaneously.
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.Nm
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works as follows.
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.Pp
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.Ss SSH protocol version 1
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.Pp
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Each host has a host-specific RSA key
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(normally 1024 bits) used to identify the host.
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Additionally, when
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the daemon starts, it generates a server RSA key (normally 768 bits).
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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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.Pp
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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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Next, the server and the client enter an authentication dialog.
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The client tries to authenticate itself using
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.Pa .rhosts
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authentication,
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.Pa .rhosts
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authentication combined with RSA host
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authentication, RSA challenge-response authentication, or password
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based authentication.
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.Pp
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Rhosts authentication is normally disabled
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because it is fundamentally insecure, but can be enabled in the server
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configuration file if desired.
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System security is not improved unless
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.Xr rshd 8 ,
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.Xr rlogind 8 ,
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.Xr rexecd 8 ,
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and
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.Xr rexd 8
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are disabled (thus completely disabling
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.Xr rlogin 1
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and
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.Xr rsh 1
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into the machine).
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.Pp
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.Ss SSH protocol version 2
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.Pp
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Version 2 works similarly:
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Each host has a host-specific DSA key used to identify the host.
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However, when the daemon starts, it does not generate a server key.
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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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.Pp
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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-sha1 or hmac-md5).
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.Pp
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Protocol version 2 provides a public key based
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user (PubkeyAuthentication) or
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client host (HostbasedAuthentication) authentication method,
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conventional password authentication and challenge response based methods.
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.Pp
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.Ss Command execution and data forwarding
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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/IP connections, or forwarding the authentication agent
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connection over the secure channel.
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.Pp
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Finally, 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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.Pp
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.Nm
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can be configured using command-line options or a configuration
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file.
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Command-line options override values specified in the
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configuration file.
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.Pp
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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 it was started as, ie.
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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 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 768).
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.Pp
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.It Fl d
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Debug mode.
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The server sends verbose debug output to the system
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log, 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 -d options increase the debugging level.
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Maximum is 3.
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.It Fl e
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When this option is specified,
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.Nm
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will send the output to the standard error instead of the system log.
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.It Fl f Ar configuration_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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600 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 the file from which the host key is read (default
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.Pa /etc/ssh/ssh_host_key ) .
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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 file is normally not readable by anyone but root).
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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 inetd.
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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 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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.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 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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.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 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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.El
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.Sh CONFIGURATION FILE
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.Nm
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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-value pairs, one per line.
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Lines starting with
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.Ql #
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and empty lines are interpreted as comments.
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.Pp
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The following keywords are possible.
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.Bl -tag -width Ds
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.It Cm AFSTokenPassing
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Specifies whether an AFS token may be forwarded to the server.
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Default is
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.Dq yes .
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.It Cm AllowGroups
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This keyword can be followed by a list of group names, separated
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by spaces.
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If specified, login is allowed only for users whose primary
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group or supplementary group list matches one of the patterns.
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.Ql \&*
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and
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.Ql ?
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can be used as
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wildcards in the patterns.
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Only group names are valid; a numerical group ID isn't recognized.
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By default login is allowed regardless of the group list.
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.Pp
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.It Cm AllowTcpForwarding
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Specifies whether TCP forwarding is permitted.
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The default is
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.Dq yes .
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Note that disabling TCP forwarding does not improve security unless
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users are also denied shell access, as they can always install their
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own forwarders.
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.Pp
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.It Cm AllowUsers
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This keyword can be followed by a list of user names, separated
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by spaces.
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If specified, login is allowed only for users names that
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match one of the patterns.
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.Ql \&*
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and
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.Ql ?
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can be used as
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wildcards in the patterns.
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Only user names are valid; a numerical user ID isn't recognized.
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By default login is allowed regardless of the user name.
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.Pp
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.It Cm Banner
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In some jurisdictions, sending a warning message before authentication
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may be relevant for getting legal protection.
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The contents of the specified file are sent to the remote user before
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authentication is allowed.
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This option is only available for protocol version 2.
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.Pp
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.It Cm ChallengeResponseAuthentication
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Specifies whether
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challenge response
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authentication is allowed.
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Currently there is only support for
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.Xr skey 1
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authentication.
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The default is
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.Dq yes .
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.It Cm Ciphers
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Specifies the ciphers allowed for protocol version 2.
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Multiple ciphers must be comma-separated.
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The default is
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.Dq aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour.
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.It Cm CheckMail
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Specifies whether
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.Nm
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should check for new mail for interactive logins.
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The default is
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.Dq yes .
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.It Cm ClientAliveInterval
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Sets a timeout interval in seconds after which if no data has been received
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from the client,
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.Nm
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will send a message through the encrypted
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channel to request a response from the client.
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The default
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is 0, indicating that these messages will not be sent to the client.
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This option applies to protocol version 2 only.
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.It Cm ClientAliveCountMax
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Sets the number of client alive messages (see above) which may be
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sent without
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.Nm
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receiving any messages back from the client. If this threshold is
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reached while client alive messages are being sent,
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.Nm
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will disconnect the client, terminating the session. It is important
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to note that the use of client alive messages is very different from
|
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.Cm Keepalive
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(below). The client alive messages are sent through the
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encrypted channel and therefore will not be spoofable. The TCP keepalive
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option enabled by
|
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.Cm Keepalive
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is spoofable. You want to use the client
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alive mechanism when you are basing something important on
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clients having an active connection to the server.
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.Pp
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The default value is 3. If you set
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.Cm ClientAliveInterval
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(above) to 15, and leave this value at the default, unresponsive ssh clients
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will be disconnected after approximately 45 seconds.
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.It Cm DenyGroups
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This keyword can be followed by a number of group names, separated
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by spaces.
|
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Users whose primary group or supplementary group list matches
|
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one of the patterns aren't allowed to log in.
|
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.Ql \&*
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and
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.Ql ?
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can be used as
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wildcards in the patterns.
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Only group names are valid; a numerical group ID isn't recognized.
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By default login is allowed regardless of the group list.
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.Pp
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.It Cm DenyUsers
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This keyword can be followed by a number of user names, separated
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by spaces.
|
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Login is disallowed for user names that match one of the patterns.
|
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.Ql \&*
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and
|
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.Ql ?
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can be used as wildcards in the patterns.
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Only user names are valid; a numerical user ID isn't recognized.
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By default login is allowed regardless of the user name.
|
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.It Cm GatewayPorts
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Specifies whether remote hosts are allowed to connect to ports
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forwarded for the client.
|
|
The argument must be
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.Dq yes
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or
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.Dq no .
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The default is
|
|
.Dq no .
|
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.It Cm HostbasedAuthentication
|
|
Specifies whether rhosts or /etc/hosts.equiv authentication together
|
|
with successful public key client host authentication is allowed
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(hostbased authentication).
|
|
This option is similar to
|
|
.Cm RhostsRSAAuthentication
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|
and applies to protocol version 2 only.
|
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The default is
|
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.Dq no .
|
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.It Cm HostKey
|
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Specifies the file containing the private host keys (default
|
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.Pa /etc/ssh/ssh_host_key )
|
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used by SSH protocol versions 1 and 2.
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Note that
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.Nm
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will refuse to use a file if it is group/world-accessible.
|
|
It is possible to have multiple host key files.
|
|
.Dq rsa1
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|
keys are used for version 1 and
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.Dq dsa
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or
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.Dq rsa
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are used for version 2 of the SSH protocol.
|
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.It Cm IgnoreRhosts
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Specifies that
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.Pa .rhosts
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and
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.Pa .shosts
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files will not be used in
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.Cm RhostsAuthentication ,
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.Cm RhostsRSAAuthentication
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or
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.Cm HostbasedAuthentication .
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.Pp
|
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.Pa /etc/hosts.equiv
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and
|
|
.Pa /etc/ssh/shosts.equiv
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are still used.
|
|
The default is
|
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.Dq yes .
|
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.It Cm IgnoreUserKnownHosts
|
|
Specifies whether
|
|
.Nm
|
|
should ignore the user's
|
|
.Pa $HOME/.ssh/known_hosts
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|
during
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|
.Cm RhostsRSAAuthentication
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|
or
|
|
.Cm HostbasedAuthentication .
|
|
The default is
|
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.Dq no .
|
|
.It Cm KeepAlive
|
|
Specifies whether the system should send 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 keepalives are not sent,
|
|
sessions may hang indefinitely on the server, leaving
|
|
.Dq ghost
|
|
users and consuming server resources.
|
|
.Pp
|
|
The default is
|
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.Dq yes
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(to send keepalives), and the server will notice
|
|
if the network goes down or the client host reboots.
|
|
This avoids infinitely hanging sessions.
|
|
.Pp
|
|
To disable keepalives, the value should be set to
|
|
.Dq no
|
|
in both the server and the client configuration files.
|
|
.It Cm KerberosAuthentication
|
|
Specifies whether Kerberos authentication is allowed.
|
|
This can be in the form of a Kerberos ticket, or if
|
|
.Cm PasswordAuthentication
|
|
is yes, the password provided by the user 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.
|
|
Default is
|
|
.Dq yes .
|
|
.It Cm KerberosOrLocalPasswd
|
|
If set then if password authentication through Kerberos fails then
|
|
the password will be validated via any additional local mechanism
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|
such as
|
|
.Pa /etc/passwd .
|
|
Default is
|
|
.Dq yes .
|
|
.It Cm KerberosTgtPassing
|
|
Specifies whether a Kerberos TGT may be forwarded to the server.
|
|
Default is
|
|
.Dq no ,
|
|
as this only works when the Kerberos KDC is actually an AFS kaserver.
|
|
.It Cm KerberosTicketCleanup
|
|
Specifies whether to automatically destroy the user's ticket cache
|
|
file on logout.
|
|
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
|
|
.Nm
|
|
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,
|
|
.Nm
|
|
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
|
|
.Nm sshd .
|
|
The possible values are:
|
|
QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
|
|
The default is INFO.
|
|
Logging with level DEBUG 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
|
|
.Pp
|
|
.Bd -literal
|
|
``hmac-md5,hmac-sha1,hmac-ripemd160,hmac-ripemd160@openssh.com,
|
|
hmac-sha1-96,hmac-md5-96''
|
|
.Ed
|
|
.It Cm MaxStartups
|
|
Specifies the maximum number of concurrent unauthenticated connections to the
|
|
.Nm
|
|
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").
|
|
.Nm
|
|
will refuse connection attempts with a probability of
|
|
.Dq rate/100
|
|
(30%)
|
|
if there are currently
|
|
.Dq start
|
|
(10)
|
|
unauthenticated connections.
|
|
The probability increases linearly and all connection attempts
|
|
are refused if the number of unauthenticated connections reaches
|
|
.Dq full
|
|
(60).
|
|
.It Cm PasswordAuthentication
|
|
Specifies whether password authentication is allowed.
|
|
The default is
|
|
.Dq yes .
|
|
.It Cm PermitEmptyPasswords
|
|
When password authentication is allowed, it specifies whether the
|
|
server allows login to accounts with empty password strings.
|
|
The default is
|
|
.Dq no .
|
|
.It Cm PermitRootLogin
|
|
Specifies whether root can login 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 .
|
|
.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 login.
|
|
.It Cm PidFile
|
|
Specifies the file that contains the process identifier of the
|
|
.Nm
|
|
daemon.
|
|
The default is
|
|
.Pa /var/run/sshd.pid .
|
|
.It Cm Port
|
|
Specifies the port number that
|
|
.Nm
|
|
listens on.
|
|
The default is 22.
|
|
Multiple options of this type are permitted.
|
|
See also
|
|
.Cm ListenAddress .
|
|
.It Cm PrintLastLog
|
|
Specifies whether
|
|
.Nm
|
|
should print the date and time when the user last logged in.
|
|
The default is
|
|
.Dq yes .
|
|
.It Cm PrintMotd
|
|
Specifies whether
|
|
.Nm
|
|
should print
|
|
.Pa /etc/motd
|
|
when a user logs in interactively.
|
|
(On some systems it is also printed by the shell,
|
|
.Pa /etc/profile ,
|
|
or equivalent.)
|
|
The default is
|
|
.Dq yes .
|
|
.It Cm Protocol
|
|
Specifies the protocol versions
|
|
.Nm
|
|
should support.
|
|
The possible values are
|
|
.Dq 1
|
|
and
|
|
.Dq 2 .
|
|
Multiple versions must be comma-separated.
|
|
The default is
|
|
.Dq 2,1 .
|
|
.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 ReverseMappingCheck
|
|
Specifies whether
|
|
.Nm
|
|
should try to verify 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 no .
|
|
.It Cm RhostsAuthentication
|
|
Specifies whether authentication using rhosts or
|
|
.Pa /etc/hosts.equiv
|
|
files is sufficient.
|
|
Normally, this method should not be permitted because it is insecure.
|
|
.Cm RhostsRSAAuthentication
|
|
should be used
|
|
instead, because it performs RSA-based host authentication in addition
|
|
to normal rhosts or
|
|
.Pa /etc/hosts.equiv
|
|
authentication.
|
|
The default is
|
|
.Dq no .
|
|
This option applies to protocol version 1 only.
|
|
.It Cm RhostsRSAAuthentication
|
|
Specifies whether rhosts or
|
|
.Pa /etc/hosts.equiv
|
|
authentication together
|
|
with successful RSA host authentication is allowed.
|
|
The default is
|
|
.Dq no .
|
|
This option applies to protocol version 1 only.
|
|
.It Cm RSAAuthentication
|
|
Specifies whether pure RSA authentication is allowed.
|
|
The default is
|
|
.Dq yes .
|
|
This option applies to protocol version 1 only.
|
|
.It Cm ServerKeyBits
|
|
Defines the number of bits in the ephemeral protocol version 1 server key.
|
|
The minimum value is 512, and the default is 768.
|
|
.It Cm SkeyAuthentication
|
|
Specifies whether
|
|
.Xr skey 1
|
|
authentication is allowed.
|
|
The default is
|
|
.Dq yes .
|
|
Note that OPIE authentication is enabled only if
|
|
.Cm PasswordAuthentication
|
|
is allowed, too.
|
|
.It Cm StrictModes
|
|
Specifies whether
|
|
.Nm
|
|
should check file modes and ownership of the
|
|
user's files and home directory before accepting login.
|
|
This is normally desirable because novices sometimes accidentally leave their
|
|
directory or files world-writable.
|
|
The default is
|
|
.Dq yes .
|
|
.It Cm Subsystem
|
|
Configures an external subsystem (e.g., file transfer daemon).
|
|
Arguments should be a subsystem name and a command to execute upon subsystem
|
|
request.
|
|
The command
|
|
.Xr sftp-server 8
|
|
implements the
|
|
.Dq sftp
|
|
file transfer subsystem.
|
|
By default no subsystems are defined.
|
|
Note that this option applies to protocol version 2 only.
|
|
.It Cm SyslogFacility
|
|
Gives the facility code that is used when logging messages from
|
|
.Nm sshd .
|
|
The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
|
|
LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
|
|
The default is AUTH.
|
|
.It Cm UseLogin
|
|
Specifies whether
|
|
.Xr login 1
|
|
is used for interactive login sessions.
|
|
Note that
|
|
.Xr login 1
|
|
is never used for remote command execution.
|
|
The default is
|
|
.Dq no .
|
|
.It Cm X11DisplayOffset
|
|
Specifies the first display number available for
|
|
.Nm sshd Ns 's
|
|
X11 forwarding.
|
|
This prevents
|
|
.Nm
|
|
from interfering with real X11 servers.
|
|
The default is 10.
|
|
.It Cm X11Forwarding
|
|
Specifies whether X11 forwarding is permitted.
|
|
The default is
|
|
.Dq no .
|
|
Note that disabling X11 forwarding does not improve security in any
|
|
way, as users can always install their own forwarders.
|
|
.It Cm XAuthLocation
|
|
Specifies the location of the
|
|
.Xr xauth 1
|
|
program.
|
|
The default is
|
|
.Pa /usr/X11R6/bin/xauth .
|
|
.El
|
|
.Sh LOGIN PROCESS
|
|
When a user successfully logs in,
|
|
.Nm
|
|
does the following:
|
|
.Bl -enum -offset indent
|
|
.It
|
|
If the login is on a tty, and no command has been specified,
|
|
prints last login time and
|
|
.Pa /etc/motd
|
|
(unless prevented in the configuration file or by
|
|
.Pa $HOME/.hushlogin ;
|
|
see the
|
|
.Sx FILES
|
|
section).
|
|
.It
|
|
If the login is on a tty, records login time.
|
|
.It
|
|
Checks
|
|
.Pa /etc/nologin and
|
|
.Pa /var/run/nologin ;
|
|
if one exists, it prints the contents and quits
|
|
(unless root).
|
|
.It
|
|
Changes to run with normal user privileges.
|
|
.It
|
|
Sets up basic environment.
|
|
.It
|
|
Reads
|
|
.Pa $HOME/.ssh/environment
|
|
if it exists.
|
|
.It
|
|
Changes to user's home directory.
|
|
.It
|
|
If
|
|
.Pa $HOME/.ssh/rc
|
|
exists, runs it; else if
|
|
.Pa /etc/ssh/sshrc
|
|
exists, runs
|
|
it; otherwise runs
|
|
.Xr xauth 1 .
|
|
The
|
|
.Dq rc
|
|
files are given the X11
|
|
authentication protocol and cookie (if applicable) in standard input.
|
|
.It
|
|
Runs user's shell or command.
|
|
.El
|
|
.Sh AUTHORIZED_KEYS FILE FORMAT
|
|
The
|
|
.Pa $HOME/.ssh/authorized_keys
|
|
file lists the RSA keys that are
|
|
permitted for RSA authentication in protocol version 1
|
|
Similarly, the
|
|
.Pa $HOME/.ssh/authorized_keys2
|
|
file lists the DSA and RSA keys that are
|
|
permitted for public key authentication (PubkeyAuthentication)
|
|
in protocol version 2.
|
|
.Pp
|
|
Each line of the file contains one
|
|
key (empty lines and lines starting with a
|
|
.Ql #
|
|
are ignored as
|
|
comments).
|
|
Each RSA public key consists of the following fields, separated by
|
|
spaces: options, bits, exponent, modulus, comment.
|
|
Each protocol version 2 public key consists of:
|
|
options, keytype, base64 encoded key, comment.
|
|
The options fields
|
|
are optional; its presence is determined by whether the line starts
|
|
with a number or not (the option field never starts with a number).
|
|
The bits, exponent, modulus and comment fields give the RSA key for
|
|
protocol version 1; the
|
|
comment field is not used for anything (but may be convenient for the
|
|
user to identify the key).
|
|
For protocol version 2 the keytype is
|
|
.Dq ssh-dss
|
|
or
|
|
.Dq ssh-rsa .
|
|
.Pp
|
|
Note that lines in this file are usually several hundred bytes long
|
|
(because of the size of the RSA key modulus).
|
|
You don't want to type them in; instead, copy the
|
|
.Pa identity.pub ,
|
|
.Pa id_dsa.pub
|
|
or the
|
|
.Pa id_rsa.pub
|
|
file and edit it.
|
|
.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:
|
|
.Bl -tag -width Ds
|
|
.It Cm from="pattern-list"
|
|
Specifies that in addition to RSA authentication, the canonical name
|
|
of the remote host must be present in the comma-separated list of
|
|
patterns
|
|
.Pf ( Ql *
|
|
and
|
|
.Ql ?
|
|
serve as wildcards).
|
|
The list may also contain
|
|
patterns negated by prefixing them with
|
|
.Ql ! ;
|
|
if the canonical host name matches a negated pattern, the key is not accepted.
|
|
The purpose
|
|
of this option is to optionally increase security: RSA 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 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 connection requests a pty;
|
|
otherwise it is run without a tty.
|
|
Note that if you want a 8-bit clean channel,
|
|
you 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 RSA 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/IP and/or X11
|
|
forwarding unless they are explicitly prohibited.
|
|
.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.
|
|
.It Cm no-port-forwarding
|
|
Forbids TCP/IP 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-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 no-agent-forwarding
|
|
Forbids authentication agent forwarding when this key is used for
|
|
authentication.
|
|
.It Cm no-pty
|
|
Prevents tty allocation (a request to allocate a pty will fail).
|
|
.It Cm permitopen="host:port"
|
|
Limit local
|
|
.Li ``ssh -L''
|
|
port forwarding such that it may only connect to the specified host and
|
|
port. 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.
|
|
.El
|
|
.Ss Examples
|
|
1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
|
|
.Pp
|
|
from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
|
|
.Pp
|
|
command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
|
|
.Pp
|
|
permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323
|
|
.Sh SSH_KNOWN_HOSTS FILE FORMAT
|
|
The
|
|
.Pa /etc/ssh/ssh_known_hosts ,
|
|
.Pa /etc/ssh/ssh_known_hosts2 ,
|
|
.Pa $HOME/.ssh/known_hosts ,
|
|
and
|
|
.Pa $HOME/.ssh/known_hosts2
|
|
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: hostnames,
|
|
bits, exponent, modulus, comment.
|
|
The fields are separated by spaces.
|
|
.Pp
|
|
Hostnames is a comma-separated list of patterns ('*' and '?' 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.
|
|
.Pp
|
|
Bits, exponent, and modulus are taken directly from the RSA host key; they
|
|
can be obtained, e.g., 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.
|
|
It is thus 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
|
|
or by taking
|
|
.Pa /etc/ssh/ssh_host_key.pub
|
|
and adding the host names at the front.
|
|
.Ss Examples
|
|
.Bd -literal
|
|
closenet,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
|
|
cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
|
|
.Ed
|
|
.Sh FILES
|
|
.Bl -tag -width Ds
|
|
.It Pa /etc/ssh/sshd_config
|
|
Contains configuration data for
|
|
.Nm sshd .
|
|
This file should be writable by root only, but it is recommended
|
|
(though not necessary) that it be world-readable.
|
|
.It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key
|
|
These three 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 this file is group/world-accessible.
|
|
.It Pa /etc/ssh/ssh_host_key.pub, /etc/ssh/ssh_host_dsa_key.pub, /etc/ssh/ssh_host_rsa_key.pub
|
|
These three 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 .
|
|
.It Pa /etc/primes
|
|
Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
|
|
.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 pid of the one
|
|
started last).
|
|
The content of this file is not sensitive; it can be world-readable.
|
|
.It Pa $HOME/.ssh/authorized_keys
|
|
Lists the RSA keys that can be used to log into the user's account.
|
|
This file must be readable by root (which may on some machines imply
|
|
it being world-readable if the user's home directory resides on an NFS
|
|
volume).
|
|
It is recommended that it not be accessible by others.
|
|
The format of this file is described above.
|
|
Users will place the contents of their
|
|
.Pa identity.pub
|
|
files into this file, as described in
|
|
.Xr ssh-keygen 1 .
|
|
.It Pa $HOME/.ssh/authorized_keys2
|
|
Lists the public keys (RSA or DSA) that can be used to log into the user's account.
|
|
This file must be readable by root (which may on some machines imply
|
|
it being world-readable if the user's home directory resides on an NFS
|
|
volume).
|
|
It is recommended that it not be accessible by others.
|
|
The format of this file is described above.
|
|
Users will place the contents of their
|
|
.Pa id_dsa.pub
|
|
and/or
|
|
.Pa id_rsa.pub
|
|
files into this file, as described in
|
|
.Xr ssh-keygen 1 .
|
|
.It Pa "/etc/ssh/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
|
|
These files are consulted when using rhosts with RSA host
|
|
authentication to check the public key of the host.
|
|
The key must be listed in one of these files to be accepted.
|
|
The client uses the same files
|
|
to verify that it is connecting to the correct remote host.
|
|
These files should be writable only by root/the owner.
|
|
.Pa /etc/ssh/ssh_known_hosts
|
|
should be world-readable, and
|
|
.Pa $HOME/.ssh/known_hosts
|
|
can but need not be world-readable.
|
|
.It Pa "/etc/ssh_known_hosts2" and "$HOME/.ssh/known_hosts2"
|
|
These files are consulted when using protocol version 2 hostbased
|
|
authentication to check the public key of the host.
|
|
The key must be listed in one of these files to be accepted.
|
|
The client uses the same files
|
|
to verify that it is connecting to the correct remote host.
|
|
These files should be writable only by root/the owner.
|
|
.Pa /etc/ssh_known_hosts2
|
|
should be world-readable, and
|
|
.Pa $HOME/.ssh/known_hosts2
|
|
can but need not be world-readable.
|
|
.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.
|
|
.It Pa /etc/hosts.allow
|
|
If compiled with
|
|
.Sy LIBWRAP
|
|
support, tcp-wrappers access controls may be defined here as described in
|
|
.Xr hosts_access 5 .
|
|
.It Pa $HOME/.rhosts
|
|
This file contains host-username pairs, separated by a space, one per
|
|
line.
|
|
The given user on the corresponding host is permitted to log in
|
|
without password.
|
|
The same file is used by rlogind and rshd.
|
|
The file must
|
|
be writable only by the user; it is recommended that it not be
|
|
accessible by others.
|
|
.Pp
|
|
If is also possible to use netgroups in the file.
|
|
Either host or user
|
|
name may be of the form +@groupname to specify all hosts or all users
|
|
in the group.
|
|
.It Pa $HOME/.shosts
|
|
For ssh,
|
|
this file is exactly the same as for
|
|
.Pa .rhosts .
|
|
However, this file is
|
|
not used by rlogin and rshd, so using this permits access using SSH only.
|
|
.It Pa /etc/hosts.equiv
|
|
This file is used during
|
|
.Pa .rhosts
|
|
authentication.
|
|
In the simplest form, this file contains host names, one per line.
|
|
Users on
|
|
those hosts are permitted to log in without a password, provided they
|
|
have the same user name on both machines.
|
|
The host name may also be
|
|
followed by a user name; such users are permitted to log in as
|
|
.Em any
|
|
user on this machine (except root).
|
|
Additionally, the syntax
|
|
.Dq +@group
|
|
can be used to specify netgroups.
|
|
Negated entries start with
|
|
.Ql \&- .
|
|
.Pp
|
|
If the client host/user is successfully matched in this file, login is
|
|
automatically permitted provided the client and server user names are the
|
|
same.
|
|
Additionally, successful RSA host authentication is normally required.
|
|
This file must be writable only by root; it is recommended
|
|
that it be world-readable.
|
|
.Pp
|
|
.Sy "Warning: It is almost never a good idea to use user names in"
|
|
.Pa hosts.equiv .
|
|
Beware that it really means that the named user(s) can log in as
|
|
.Em anybody ,
|
|
which includes bin, daemon, adm, and other accounts that own critical
|
|
binaries and directories.
|
|
Using a user name practically grants the user root access.
|
|
The only valid use for user names that I can think
|
|
of is in negative entries.
|
|
.Pp
|
|
Note that this warning also applies to rsh/rlogin.
|
|
.It Pa /etc/ssh/shosts.equiv
|
|
This is processed exactly as
|
|
.Pa /etc/hosts.equiv .
|
|
However, this file may be useful in environments that want to run both
|
|
rsh/rlogin and ssh.
|
|
.It Pa $HOME/.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.
|
|
.It Pa $HOME/.ssh/rc
|
|
If this file exists, it is run with
|
|
.Pa /bin/sh
|
|
after reading the
|
|
environment files but before starting the user's shell or command.
|
|
If X11 spoofing is in use, this will receive the "proto cookie" pair in
|
|
standard input (and
|
|
.Ev DISPLAY
|
|
in environment).
|
|
This must call
|
|
.Xr xauth 1
|
|
in that case.
|
|
.Pp
|
|
The primary purpose of this file is to run any initialization routines
|
|
which may be needed before the user's home directory becomes
|
|
accessible; AFS is a particular example of such an environment.
|
|
.Pp
|
|
This file will probably contain some initialization code followed by
|
|
something similar to:
|
|
.Bd -literal -offset indent
|
|
if [ -n "$DISPLAY" ] && read proto cookie; then
|
|
echo add "$DISPLAY" "$proto" "$cookie" | xauth -q -
|
|
fi
|
|
.Ed
|
|
.Pp
|
|
If this file does not exist,
|
|
.Pa /etc/ssh/sshrc
|
|
is run, and if that
|
|
does not exist either,
|
|
.Xr xauth 1
|
|
is used to store the cookie.
|
|
.Pp
|
|
This file should be writable only by the user, and need not be
|
|
readable by anyone else.
|
|
.It Pa /etc/ssh/sshrc
|
|
Like
|
|
.Pa $HOME/.ssh/rc .
|
|
This can be used to specify
|
|
machine-specific login-time initializations globally.
|
|
This file should be writable only by root, and should be world-readable.
|
|
.El
|
|
.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.
|
|
.Sh SEE ALSO
|
|
.Xr scp 1 ,
|
|
.Xr sftp 1 ,
|
|
.Xr sftp-server 8 ,
|
|
.Xr ssh 1 ,
|
|
.Xr ssh-add 1 ,
|
|
.Xr ssh-agent 1 ,
|
|
.Xr ssh-keygen 1 ,
|
|
.Xr rlogin 1 ,
|
|
.Xr rsh 1
|
|
.Rs
|
|
.%A T. Ylonen
|
|
.%A T. Kivinen
|
|
.%A M. Saarinen
|
|
.%A T. Rinne
|
|
.%A S. Lehtinen
|
|
.%T "SSH Protocol Architecture"
|
|
.%N draft-ietf-secsh-architecture-07.txt
|
|
.%D January 2001
|
|
.%O work in progress material
|
|
.Re
|
|
.Rs
|
|
.%A M. Friedl
|
|
.%A N. Provos
|
|
.%A W. A. Simpson
|
|
.%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol"
|
|
.%N draft-ietf-secsh-dh-group-exchange-00.txt
|
|
.%D January 2001
|
|
.%O work in progress material
|
|
.Re
|