mirror of git://anongit.mindrot.org/openssh.git
1779 lines
46 KiB
Groff
1779 lines
46 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: ssh.1,v 1.429 2022/02/06 00:29:03 jsg Exp $
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.Dd $Mdocdate: February 6 2022 $
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.Dt SSH 1
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.Os
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.Sh NAME
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.Nm ssh
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.Nd OpenSSH remote login client
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.Sh SYNOPSIS
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.Nm ssh
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.Op Fl 46AaCfGgKkMNnqsTtVvXxYy
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.Op Fl B Ar bind_interface
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.Op Fl b Ar bind_address
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.Op Fl c Ar cipher_spec
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.Op Fl D Oo Ar bind_address : Oc Ns Ar port
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.Op Fl E Ar log_file
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.Op Fl e Ar escape_char
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.Op Fl F Ar configfile
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.Op Fl I Ar pkcs11
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.Op Fl i Ar identity_file
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.Op Fl J Ar destination
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.Op Fl L Ar address
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.Op Fl l Ar login_name
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.Op Fl m Ar mac_spec
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.Op Fl O Ar ctl_cmd
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.Op Fl o Ar option
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.Op Fl p Ar port
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.Op Fl Q Ar query_option
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.Op Fl R Ar address
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.Op Fl S Ar ctl_path
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.Op Fl W Ar host : Ns Ar port
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.Op Fl w Ar local_tun Ns Op : Ns Ar remote_tun
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.Ar destination
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.Op Ar command Op Ar argument ...
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.Sh DESCRIPTION
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.Nm
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(SSH client) is a program for logging into a remote machine and for
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executing commands on a remote machine.
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It is intended to provide secure encrypted communications between
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two untrusted hosts over an insecure network.
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X11 connections, arbitrary TCP ports and
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.Ux Ns -domain
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sockets can also be forwarded over the secure channel.
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.Pp
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.Nm
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connects and logs into the specified
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.Ar destination ,
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which may be specified as either
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.Sm off
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.Oo user @ Oc hostname
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.Sm on
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or a URI of the form
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.Sm off
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.No ssh:// Oo user @ Oc hostname Op : port .
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.Sm on
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The user must prove
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their identity to the remote machine using one of several methods
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(see below).
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.Pp
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If a
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.Ar command
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is specified,
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it will be executed on the remote host instead of a login shell.
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A complete command line may be specified as
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.Ar command ,
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or it may have additional arguments.
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If supplied, the arguments will be appended to the command, separated by
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spaces, before it is sent to the server to be executed.
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.Pp
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The options are as follows:
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.Pp
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.Bl -tag -width Ds -compact
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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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.Pp
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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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.Pp
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.It Fl A
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Enables forwarding of connections from an authentication agent such as
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.Xr ssh-agent 1 .
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This can also be specified on a per-host basis in a configuration file.
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.Pp
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Agent forwarding should be enabled with caution.
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|
Users with the ability to bypass file permissions on the remote host
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(for the agent's
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.Ux Ns -domain
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socket) can access the local agent through the forwarded connection.
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An attacker cannot obtain key material from the agent,
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|
however they can perform operations on the keys that enable them to
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authenticate using the identities loaded into the agent.
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A safer alternative may be to use a jump host
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(see
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.Fl J ) .
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.Pp
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.It Fl a
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Disables forwarding of the authentication agent connection.
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.Pp
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.It Fl B Ar bind_interface
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Bind to the address of
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.Ar bind_interface
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before attempting to connect to the destination host.
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This is only useful on systems with more than one address.
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.Pp
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.It Fl b Ar bind_address
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Use
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.Ar bind_address
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on the local machine as the source address
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of the connection.
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Only useful on systems with more than one address.
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.Pp
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.It Fl C
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Requests compression of all data (including stdin, stdout, stderr, and
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data for forwarded X11, TCP and
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.Ux Ns -domain
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connections).
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The compression algorithm is the same used by
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.Xr gzip 1 .
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Compression is desirable on modem lines and other
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slow connections, but will only slow down things on fast networks.
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The default value can be set on a host-by-host basis in the
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configuration files; see the
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.Cm Compression
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option.
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.Pp
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.It Fl c Ar cipher_spec
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Selects the cipher specification for encrypting the session.
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.Ar cipher_spec
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is a comma-separated list of ciphers
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listed in order of preference.
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See the
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.Cm Ciphers
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keyword in
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.Xr ssh_config 5
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for more information.
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.Pp
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.It Fl D Xo
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.Sm off
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.Oo Ar bind_address : Oc
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.Ar port
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.Sm on
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.Xc
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Specifies a local
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.Dq dynamic
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application-level port forwarding.
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This works by allocating a socket to listen to
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.Ar port
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on the local side, optionally bound to the specified
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.Ar bind_address .
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Whenever a connection is made to this port, the
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connection is forwarded over the secure channel, and the application
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protocol is then used to determine where to connect to from the
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remote machine.
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Currently the SOCKS4 and SOCKS5 protocols are supported, and
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.Nm
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|
will act as a SOCKS server.
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Only root can forward privileged ports.
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Dynamic port forwardings can also be specified in the configuration file.
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.Pp
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IPv6 addresses can be specified by enclosing the address in square brackets.
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Only the superuser can forward privileged ports.
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|
By default, the local port is bound in accordance with the
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.Cm GatewayPorts
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setting.
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|
However, an explicit
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.Ar bind_address
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|
may be used to bind the connection to a specific address.
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|
The
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.Ar bind_address
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of
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.Dq localhost
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|
indicates that the listening port be bound for local use only, while an
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|
empty address or
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.Sq *
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|
indicates that the port should be available from all interfaces.
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.Pp
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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 standard error.
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|
.Pp
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|
.It Fl e Ar escape_char
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|
Sets the escape character for sessions with a pty (default:
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.Ql ~ ) .
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|
The escape character is only recognized at the beginning of a line.
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|
The escape character followed by a dot
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|
.Pq Ql \&.
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|
closes the connection;
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|
followed by control-Z suspends the connection;
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|
and followed by itself sends the escape character once.
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|
Setting the character to
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.Dq none
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disables any escapes and makes the session fully transparent.
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.Pp
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.It Fl F Ar configfile
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Specifies an alternative per-user configuration file.
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|
If a configuration file is given on the command line,
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|
the system-wide configuration file
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|
.Pq Pa /etc/ssh/ssh_config
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|
will be ignored.
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|
The default for the per-user configuration file is
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|
.Pa ~/.ssh/config .
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|
If set to
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|
.Dq none ,
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|
no configuration files will be read.
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.Pp
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.It Fl f
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Requests
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.Nm
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to go to background just before command execution.
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This is useful if
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.Nm
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is going to ask for passwords or passphrases, but the user
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|
wants it in the background.
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This implies
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.Fl n .
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|
The recommended way to start X11 programs at a remote site is with
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|
something like
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.Ic ssh -f host xterm .
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|
.Pp
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|
If the
|
|
.Cm ExitOnForwardFailure
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|
configuration option is set to
|
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.Dq yes ,
|
|
then a client started with
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.Fl f
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|
will wait for all remote port forwards to be successfully established
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|
before placing itself in the background.
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|
Refer to the description of
|
|
.Cm ForkAfterAuthentication
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|
in
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.Xr ssh_config 5
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for details.
|
|
.Pp
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.It Fl G
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Causes
|
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.Nm
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|
to print its configuration after evaluating
|
|
.Cm Host
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|
and
|
|
.Cm Match
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|
blocks and exit.
|
|
.Pp
|
|
.It Fl g
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|
Allows remote hosts to connect to local forwarded ports.
|
|
If used on a multiplexed connection, then this option must be specified
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|
on the master process.
|
|
.Pp
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|
.It Fl I Ar pkcs11
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|
Specify the PKCS#11 shared library
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.Nm
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|
should use to communicate with a PKCS#11 token providing keys for user
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authentication.
|
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.Pp
|
|
.It Fl i Ar identity_file
|
|
Selects a file from which the identity (private key) for
|
|
public key authentication is read.
|
|
You can also specify a public key file to use the corresponding
|
|
private key that is loaded in
|
|
.Xr ssh-agent 1
|
|
when the private key file is not present locally.
|
|
The default is
|
|
.Pa ~/.ssh/id_rsa ,
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|
.Pa ~/.ssh/id_ecdsa ,
|
|
.Pa ~/.ssh/id_ecdsa_sk ,
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|
.Pa ~/.ssh/id_ed25519 ,
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|
.Pa ~/.ssh/id_ed25519_sk
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|
and
|
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.Pa ~/.ssh/id_dsa .
|
|
Identity files may also be specified on
|
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a per-host basis in the configuration file.
|
|
It is possible to have multiple
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.Fl i
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options (and multiple identities specified in
|
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configuration files).
|
|
If no certificates have been explicitly specified by the
|
|
.Cm CertificateFile
|
|
directive,
|
|
.Nm
|
|
will also try to load certificate information from the filename obtained
|
|
by appending
|
|
.Pa -cert.pub
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|
to identity filenames.
|
|
.Pp
|
|
.It Fl J Ar destination
|
|
Connect to the target host by first making a
|
|
.Nm
|
|
connection to the jump host described by
|
|
.Ar destination
|
|
and then establishing a TCP forwarding to the ultimate destination from
|
|
there.
|
|
Multiple jump hops may be specified separated by comma characters.
|
|
This is a shortcut to specify a
|
|
.Cm ProxyJump
|
|
configuration directive.
|
|
Note that configuration directives supplied on the command-line generally
|
|
apply to the destination host and not any specified jump hosts.
|
|
Use
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|
.Pa ~/.ssh/config
|
|
to specify configuration for jump hosts.
|
|
.Pp
|
|
.It Fl K
|
|
Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
|
|
credentials to the server.
|
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.Pp
|
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.It Fl k
|
|
Disables forwarding (delegation) of GSSAPI credentials to the server.
|
|
.Pp
|
|
.It Fl L Xo
|
|
.Sm off
|
|
.Oo Ar bind_address : Oc
|
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.Ar port : host : hostport
|
|
.Sm on
|
|
.Xc
|
|
.It Fl L Xo
|
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.Sm off
|
|
.Oo Ar bind_address : Oc
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|
.Ar port : remote_socket
|
|
.Sm on
|
|
.Xc
|
|
.It Fl L Xo
|
|
.Sm off
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|
.Ar local_socket : host : hostport
|
|
.Sm on
|
|
.Xc
|
|
.It Fl L Xo
|
|
.Sm off
|
|
.Ar local_socket : remote_socket
|
|
.Sm on
|
|
.Xc
|
|
Specifies that connections to the given TCP port or Unix socket on the local
|
|
(client) host are to be forwarded to the given host and port, or Unix socket,
|
|
on the remote side.
|
|
This works by allocating a socket to listen to either a TCP
|
|
.Ar port
|
|
on the local side, optionally bound to the specified
|
|
.Ar bind_address ,
|
|
or to a Unix socket.
|
|
Whenever a connection is made to the local port or socket, the
|
|
connection is forwarded over the secure channel, and a connection is
|
|
made to either
|
|
.Ar host
|
|
port
|
|
.Ar hostport ,
|
|
or the Unix socket
|
|
.Ar remote_socket ,
|
|
from the remote machine.
|
|
.Pp
|
|
Port forwardings can also be specified in the configuration file.
|
|
Only the superuser can forward privileged ports.
|
|
IPv6 addresses can be specified by enclosing the address in square brackets.
|
|
.Pp
|
|
By default, the local port is bound in accordance with the
|
|
.Cm GatewayPorts
|
|
setting.
|
|
However, an explicit
|
|
.Ar bind_address
|
|
may be used to bind the connection to a specific address.
|
|
The
|
|
.Ar bind_address
|
|
of
|
|
.Dq localhost
|
|
indicates that the listening port be bound for local use only, while an
|
|
empty address or
|
|
.Sq *
|
|
indicates that the port should be available from all interfaces.
|
|
.Pp
|
|
.It Fl l Ar login_name
|
|
Specifies the user to log in as on the remote machine.
|
|
This also may be specified on a per-host basis in the configuration file.
|
|
.Pp
|
|
.It Fl M
|
|
Places the
|
|
.Nm
|
|
client into
|
|
.Dq master
|
|
mode for connection sharing.
|
|
Multiple
|
|
.Fl M
|
|
options places
|
|
.Nm
|
|
into
|
|
.Dq master
|
|
mode but with confirmation required using
|
|
.Xr ssh-askpass 1
|
|
before each operation that changes the multiplexing state
|
|
(e.g. opening a new session).
|
|
Refer to the description of
|
|
.Cm ControlMaster
|
|
in
|
|
.Xr ssh_config 5
|
|
for details.
|
|
.Pp
|
|
.It Fl m Ar mac_spec
|
|
A comma-separated list of MAC (message authentication code) algorithms,
|
|
specified in order of preference.
|
|
See the
|
|
.Cm MACs
|
|
keyword for more information.
|
|
.Pp
|
|
.It Fl N
|
|
Do not execute a remote command.
|
|
This is useful for just forwarding ports.
|
|
Refer to the description of
|
|
.Cm SessionType
|
|
in
|
|
.Xr ssh_config 5
|
|
for details.
|
|
.Pp
|
|
.It Fl n
|
|
Redirects stdin from
|
|
.Pa /dev/null
|
|
(actually, prevents reading from stdin).
|
|
This must be used when
|
|
.Nm
|
|
is run in the background.
|
|
A common trick is to use this to run X11 programs on a remote machine.
|
|
For example,
|
|
.Ic ssh -n shadows.cs.hut.fi emacs &
|
|
will start an emacs on shadows.cs.hut.fi, and the X11
|
|
connection will be automatically forwarded over an encrypted channel.
|
|
The
|
|
.Nm
|
|
program will be put in the background.
|
|
(This does not work if
|
|
.Nm
|
|
needs to ask for a password or passphrase; see also the
|
|
.Fl f
|
|
option.)
|
|
Refer to the description of
|
|
.Cm StdinNull
|
|
in
|
|
.Xr ssh_config 5
|
|
for details.
|
|
.Pp
|
|
.It Fl O Ar ctl_cmd
|
|
Control an active connection multiplexing master process.
|
|
When the
|
|
.Fl O
|
|
option is specified, the
|
|
.Ar ctl_cmd
|
|
argument is interpreted and passed to the master process.
|
|
Valid commands are:
|
|
.Dq check
|
|
(check that the master process is running),
|
|
.Dq forward
|
|
(request forwardings without command execution),
|
|
.Dq cancel
|
|
(cancel forwardings),
|
|
.Dq exit
|
|
(request the master to exit), and
|
|
.Dq stop
|
|
(request the master to stop accepting further multiplexing requests).
|
|
.Pp
|
|
.It Fl o Ar option
|
|
Can be used to give options in the format used in the configuration file.
|
|
This is useful for specifying options for which there is no separate
|
|
command-line flag.
|
|
For full details of the options listed below, and their possible values, see
|
|
.Xr ssh_config 5 .
|
|
.Pp
|
|
.Bl -tag -width Ds -offset indent -compact
|
|
.It AddKeysToAgent
|
|
.It AddressFamily
|
|
.It BatchMode
|
|
.It BindAddress
|
|
.It CanonicalDomains
|
|
.It CanonicalizeFallbackLocal
|
|
.It CanonicalizeHostname
|
|
.It CanonicalizeMaxDots
|
|
.It CanonicalizePermittedCNAMEs
|
|
.It CASignatureAlgorithms
|
|
.It CertificateFile
|
|
.It CheckHostIP
|
|
.It Ciphers
|
|
.It ClearAllForwardings
|
|
.It Compression
|
|
.It ConnectionAttempts
|
|
.It ConnectTimeout
|
|
.It ControlMaster
|
|
.It ControlPath
|
|
.It ControlPersist
|
|
.It DynamicForward
|
|
.It EscapeChar
|
|
.It ExitOnForwardFailure
|
|
.It FingerprintHash
|
|
.It ForkAfterAuthentication
|
|
.It ForwardAgent
|
|
.It ForwardX11
|
|
.It ForwardX11Timeout
|
|
.It ForwardX11Trusted
|
|
.It GatewayPorts
|
|
.It GlobalKnownHostsFile
|
|
.It GSSAPIAuthentication
|
|
.It GSSAPIDelegateCredentials
|
|
.It HashKnownHosts
|
|
.It Host
|
|
.It HostbasedAcceptedAlgorithms
|
|
.It HostbasedAuthentication
|
|
.It HostKeyAlgorithms
|
|
.It HostKeyAlias
|
|
.It Hostname
|
|
.It IdentitiesOnly
|
|
.It IdentityAgent
|
|
.It IdentityFile
|
|
.It IPQoS
|
|
.It KbdInteractiveAuthentication
|
|
.It KbdInteractiveDevices
|
|
.It KexAlgorithms
|
|
.It KnownHostsCommand
|
|
.It LocalCommand
|
|
.It LocalForward
|
|
.It LogLevel
|
|
.It MACs
|
|
.It Match
|
|
.It NoHostAuthenticationForLocalhost
|
|
.It NumberOfPasswordPrompts
|
|
.It PasswordAuthentication
|
|
.It PermitLocalCommand
|
|
.It PermitRemoteOpen
|
|
.It PKCS11Provider
|
|
.It Port
|
|
.It PreferredAuthentications
|
|
.It ProxyCommand
|
|
.It ProxyJump
|
|
.It ProxyUseFdpass
|
|
.It PubkeyAcceptedAlgorithms
|
|
.It PubkeyAuthentication
|
|
.It RekeyLimit
|
|
.It RemoteCommand
|
|
.It RemoteForward
|
|
.It RequestTTY
|
|
.It SendEnv
|
|
.It ServerAliveInterval
|
|
.It ServerAliveCountMax
|
|
.It SessionType
|
|
.It SetEnv
|
|
.It StdinNull
|
|
.It StreamLocalBindMask
|
|
.It StreamLocalBindUnlink
|
|
.It StrictHostKeyChecking
|
|
.It TCPKeepAlive
|
|
.It Tunnel
|
|
.It TunnelDevice
|
|
.It UpdateHostKeys
|
|
.It User
|
|
.It UserKnownHostsFile
|
|
.It VerifyHostKeyDNS
|
|
.It VisualHostKey
|
|
.It XAuthLocation
|
|
.El
|
|
.Pp
|
|
.It Fl p Ar port
|
|
Port to connect to on the remote host.
|
|
This can be specified on a
|
|
per-host basis in the configuration file.
|
|
.Pp
|
|
.It Fl Q Ar query_option
|
|
Queries for the algorithms supported by one of the following features:
|
|
.Ar cipher
|
|
(supported symmetric ciphers),
|
|
.Ar cipher-auth
|
|
(supported symmetric ciphers that support authenticated encryption),
|
|
.Ar help
|
|
(supported query terms for use with the
|
|
.Fl Q
|
|
flag),
|
|
.Ar mac
|
|
(supported message integrity codes),
|
|
.Ar kex
|
|
(key exchange algorithms),
|
|
.Ar key
|
|
(key types),
|
|
.Ar key-cert
|
|
(certificate key types),
|
|
.Ar key-plain
|
|
(non-certificate key types),
|
|
.Ar key-sig
|
|
(all key types and signature algorithms),
|
|
.Ar protocol-version
|
|
(supported SSH protocol versions), and
|
|
.Ar sig
|
|
(supported signature algorithms).
|
|
Alternatively, any keyword from
|
|
.Xr ssh_config 5
|
|
or
|
|
.Xr sshd_config 5
|
|
that takes an algorithm list may be used as an alias for the corresponding
|
|
query_option.
|
|
.Pp
|
|
.It Fl q
|
|
Quiet mode.
|
|
Causes most warning and diagnostic messages to be suppressed.
|
|
.Pp
|
|
.It Fl R Xo
|
|
.Sm off
|
|
.Oo Ar bind_address : Oc
|
|
.Ar port : host : hostport
|
|
.Sm on
|
|
.Xc
|
|
.It Fl R Xo
|
|
.Sm off
|
|
.Oo Ar bind_address : Oc
|
|
.Ar port : local_socket
|
|
.Sm on
|
|
.Xc
|
|
.It Fl R Xo
|
|
.Sm off
|
|
.Ar remote_socket : host : hostport
|
|
.Sm on
|
|
.Xc
|
|
.It Fl R Xo
|
|
.Sm off
|
|
.Ar remote_socket : local_socket
|
|
.Sm on
|
|
.Xc
|
|
.It Fl R Xo
|
|
.Sm off
|
|
.Oo Ar bind_address : Oc
|
|
.Ar port
|
|
.Sm on
|
|
.Xc
|
|
Specifies that connections to the given TCP port or Unix socket on the remote
|
|
(server) host are to be forwarded to the local side.
|
|
.Pp
|
|
This works by allocating a socket to listen to either a TCP
|
|
.Ar port
|
|
or to a Unix socket on the remote side.
|
|
Whenever a connection is made to this port or Unix socket, the
|
|
connection is forwarded over the secure channel, and a connection
|
|
is made from the local machine to either an explicit destination specified by
|
|
.Ar host
|
|
port
|
|
.Ar hostport ,
|
|
or
|
|
.Ar local_socket ,
|
|
or, if no explicit destination was specified,
|
|
.Nm
|
|
will act as a SOCKS 4/5 proxy and forward connections to the destinations
|
|
requested by the remote SOCKS client.
|
|
.Pp
|
|
Port forwardings can also be specified in the configuration file.
|
|
Privileged ports can be forwarded only when
|
|
logging in as root on the remote machine.
|
|
IPv6 addresses can be specified by enclosing the address in square brackets.
|
|
.Pp
|
|
By default, TCP listening sockets on the server will be bound to the loopback
|
|
interface only.
|
|
This may be overridden by specifying a
|
|
.Ar bind_address .
|
|
An empty
|
|
.Ar bind_address ,
|
|
or the address
|
|
.Ql * ,
|
|
indicates that the remote socket should listen on all interfaces.
|
|
Specifying a remote
|
|
.Ar bind_address
|
|
will only succeed if the server's
|
|
.Cm GatewayPorts
|
|
option is enabled (see
|
|
.Xr sshd_config 5 ) .
|
|
.Pp
|
|
If the
|
|
.Ar port
|
|
argument is
|
|
.Ql 0 ,
|
|
the listen port will be dynamically allocated on the server and reported
|
|
to the client at run time.
|
|
When used together with
|
|
.Ic -O forward
|
|
the allocated port will be printed to the standard output.
|
|
.Pp
|
|
.It Fl S Ar ctl_path
|
|
Specifies the location of a control socket for connection sharing,
|
|
or the string
|
|
.Dq none
|
|
to disable connection sharing.
|
|
Refer to the description of
|
|
.Cm ControlPath
|
|
and
|
|
.Cm ControlMaster
|
|
in
|
|
.Xr ssh_config 5
|
|
for details.
|
|
.Pp
|
|
.It Fl s
|
|
May be used to request invocation of a subsystem on the remote system.
|
|
Subsystems facilitate the use of SSH
|
|
as a secure transport for other applications (e.g.\&
|
|
.Xr sftp 1 ) .
|
|
The subsystem is specified as the remote command.
|
|
Refer to the description of
|
|
.Cm SessionType
|
|
in
|
|
.Xr ssh_config 5
|
|
for details.
|
|
.Pp
|
|
.It Fl T
|
|
Disable pseudo-terminal allocation.
|
|
.Pp
|
|
.It Fl t
|
|
Force pseudo-terminal allocation.
|
|
This can be used to execute arbitrary
|
|
screen-based programs on a remote machine, which can be very useful,
|
|
e.g. when implementing menu services.
|
|
Multiple
|
|
.Fl t
|
|
options force tty allocation, even if
|
|
.Nm
|
|
has no local tty.
|
|
.Pp
|
|
.It Fl V
|
|
Display the version number and exit.
|
|
.Pp
|
|
.It Fl v
|
|
Verbose mode.
|
|
Causes
|
|
.Nm
|
|
to print debugging messages about its progress.
|
|
This is helpful in
|
|
debugging connection, authentication, and configuration problems.
|
|
Multiple
|
|
.Fl v
|
|
options increase the verbosity.
|
|
The maximum is 3.
|
|
.Pp
|
|
.It Fl W Ar host : Ns Ar port
|
|
Requests that standard input and output on the client be forwarded to
|
|
.Ar host
|
|
on
|
|
.Ar port
|
|
over the secure channel.
|
|
Implies
|
|
.Fl N ,
|
|
.Fl T ,
|
|
.Cm ExitOnForwardFailure
|
|
and
|
|
.Cm ClearAllForwardings ,
|
|
though these can be overridden in the configuration file or using
|
|
.Fl o
|
|
command line options.
|
|
.Pp
|
|
.It Fl w Xo
|
|
.Ar local_tun Ns Op : Ns Ar remote_tun
|
|
.Xc
|
|
Requests
|
|
tunnel
|
|
device forwarding with the specified
|
|
.Xr tun 4
|
|
devices between the client
|
|
.Pq Ar local_tun
|
|
and the server
|
|
.Pq Ar remote_tun .
|
|
.Pp
|
|
The devices may be specified by numerical ID or the keyword
|
|
.Dq any ,
|
|
which uses the next available tunnel device.
|
|
If
|
|
.Ar remote_tun
|
|
is not specified, it defaults to
|
|
.Dq any .
|
|
See also the
|
|
.Cm Tunnel
|
|
and
|
|
.Cm TunnelDevice
|
|
directives in
|
|
.Xr ssh_config 5 .
|
|
.Pp
|
|
If the
|
|
.Cm Tunnel
|
|
directive is unset, it will be set to the default tunnel mode, which is
|
|
.Dq point-to-point .
|
|
If a different
|
|
.Cm Tunnel
|
|
forwarding mode it desired, then it should be specified before
|
|
.Fl w .
|
|
.Pp
|
|
.It Fl X
|
|
Enables X11 forwarding.
|
|
This can also be specified on a per-host basis in a configuration file.
|
|
.Pp
|
|
X11 forwarding should be enabled with caution.
|
|
Users with the ability to bypass file permissions on the remote host
|
|
(for the user's X authorization database)
|
|
can access the local X11 display through the forwarded connection.
|
|
An attacker may then be able to perform activities such as keystroke monitoring.
|
|
.Pp
|
|
For this reason, X11 forwarding is subjected to X11 SECURITY extension
|
|
restrictions by default.
|
|
Refer to the
|
|
.Nm
|
|
.Fl Y
|
|
option and the
|
|
.Cm ForwardX11Trusted
|
|
directive in
|
|
.Xr ssh_config 5
|
|
for more information.
|
|
.Pp
|
|
.It Fl x
|
|
Disables X11 forwarding.
|
|
.Pp
|
|
.It Fl Y
|
|
Enables trusted X11 forwarding.
|
|
Trusted X11 forwardings are not subjected to the X11 SECURITY extension
|
|
controls.
|
|
.Pp
|
|
.It Fl y
|
|
Send log information using the
|
|
.Xr syslog 3
|
|
system module.
|
|
By default this information is sent to stderr.
|
|
.El
|
|
.Pp
|
|
.Nm
|
|
may additionally obtain configuration data from
|
|
a per-user configuration file and a system-wide configuration file.
|
|
The file format and configuration options are described in
|
|
.Xr ssh_config 5 .
|
|
.Sh AUTHENTICATION
|
|
The OpenSSH SSH client supports SSH protocol 2.
|
|
.Pp
|
|
The methods available for authentication are:
|
|
GSSAPI-based authentication,
|
|
host-based authentication,
|
|
public key authentication,
|
|
keyboard-interactive authentication,
|
|
and password authentication.
|
|
Authentication methods are tried in the order specified above,
|
|
though
|
|
.Cm PreferredAuthentications
|
|
can be used to change the default order.
|
|
.Pp
|
|
Host-based authentication works as follows:
|
|
If the machine the user logs in from is listed in
|
|
.Pa /etc/hosts.equiv
|
|
or
|
|
.Pa /etc/shosts.equiv
|
|
on the remote machine, the user is non-root and the user names are
|
|
the same on both sides, or if the files
|
|
.Pa ~/.rhosts
|
|
or
|
|
.Pa ~/.shosts
|
|
exist in the user's home directory on the
|
|
remote machine and contain a line containing the name of the client
|
|
machine and the name of the user on that machine, the user is
|
|
considered for login.
|
|
Additionally, the server
|
|
.Em must
|
|
be able to verify the client's
|
|
host key (see the description of
|
|
.Pa /etc/ssh/ssh_known_hosts
|
|
and
|
|
.Pa ~/.ssh/known_hosts ,
|
|
below)
|
|
for login to be permitted.
|
|
This authentication method closes security holes due to IP
|
|
spoofing, DNS spoofing, and routing spoofing.
|
|
[Note to the administrator:
|
|
.Pa /etc/hosts.equiv ,
|
|
.Pa ~/.rhosts ,
|
|
and the rlogin/rsh protocol in general, are inherently insecure and should be
|
|
disabled if security is desired.]
|
|
.Pp
|
|
Public key authentication works as follows:
|
|
The scheme is based on public-key cryptography,
|
|
using cryptosystems
|
|
where encryption and decryption are done using separate keys,
|
|
and it is unfeasible to derive the decryption key from the encryption key.
|
|
The idea is that each user creates a public/private
|
|
key pair for authentication purposes.
|
|
The server knows the public key, and only the user knows the private key.
|
|
.Nm
|
|
implements public key authentication protocol automatically,
|
|
using one of the DSA, ECDSA, Ed25519 or RSA algorithms.
|
|
The HISTORY section of
|
|
.Xr ssl 8
|
|
contains a brief discussion of the DSA and RSA algorithms.
|
|
.Pp
|
|
The file
|
|
.Pa ~/.ssh/authorized_keys
|
|
lists the public keys that are permitted for logging in.
|
|
When the user logs in, the
|
|
.Nm
|
|
program tells the server which key pair it would like to use for
|
|
authentication.
|
|
The client proves that it has access to the private key
|
|
and the server checks that the corresponding public key
|
|
is authorized to accept the account.
|
|
.Pp
|
|
The server may inform the client of errors that prevented public key
|
|
authentication from succeeding after authentication completes using a
|
|
different method.
|
|
These may be viewed by increasing the
|
|
.Cm LogLevel
|
|
to
|
|
.Cm DEBUG
|
|
or higher (e.g. by using the
|
|
.Fl v
|
|
flag).
|
|
.Pp
|
|
The user creates their key pair by running
|
|
.Xr ssh-keygen 1 .
|
|
This stores the private key in
|
|
.Pa ~/.ssh/id_dsa
|
|
(DSA),
|
|
.Pa ~/.ssh/id_ecdsa
|
|
(ECDSA),
|
|
.Pa ~/.ssh/id_ecdsa_sk
|
|
(authenticator-hosted ECDSA),
|
|
.Pa ~/.ssh/id_ed25519
|
|
(Ed25519),
|
|
.Pa ~/.ssh/id_ed25519_sk
|
|
(authenticator-hosted Ed25519),
|
|
or
|
|
.Pa ~/.ssh/id_rsa
|
|
(RSA)
|
|
and stores the public key in
|
|
.Pa ~/.ssh/id_dsa.pub
|
|
(DSA),
|
|
.Pa ~/.ssh/id_ecdsa.pub
|
|
(ECDSA),
|
|
.Pa ~/.ssh/id_ecdsa_sk.pub
|
|
(authenticator-hosted ECDSA),
|
|
.Pa ~/.ssh/id_ed25519.pub
|
|
(Ed25519),
|
|
.Pa ~/.ssh/id_ed25519_sk.pub
|
|
(authenticator-hosted Ed25519),
|
|
or
|
|
.Pa ~/.ssh/id_rsa.pub
|
|
(RSA)
|
|
in the user's home directory.
|
|
The user should then copy the public key
|
|
to
|
|
.Pa ~/.ssh/authorized_keys
|
|
in their home directory on the remote machine.
|
|
The
|
|
.Pa authorized_keys
|
|
file corresponds to the conventional
|
|
.Pa ~/.rhosts
|
|
file, and has one key
|
|
per line, though the lines can be very long.
|
|
After this, the user can log in without giving the password.
|
|
.Pp
|
|
A variation on public key authentication
|
|
is available in the form of certificate authentication:
|
|
instead of a set of public/private keys,
|
|
signed certificates are used.
|
|
This has the advantage that a single trusted certification authority
|
|
can be used in place of many public/private keys.
|
|
See the CERTIFICATES section of
|
|
.Xr ssh-keygen 1
|
|
for more information.
|
|
.Pp
|
|
The most convenient way to use public key or certificate authentication
|
|
may be with an authentication agent.
|
|
See
|
|
.Xr ssh-agent 1
|
|
and (optionally) the
|
|
.Cm AddKeysToAgent
|
|
directive in
|
|
.Xr ssh_config 5
|
|
for more information.
|
|
.Pp
|
|
Keyboard-interactive authentication works as follows:
|
|
The server sends an arbitrary
|
|
.Qq challenge
|
|
text and prompts for a response, possibly multiple times.
|
|
Examples of keyboard-interactive authentication include
|
|
.Bx
|
|
Authentication (see
|
|
.Xr login.conf 5 )
|
|
and PAM (some
|
|
.Pf non- Ox
|
|
systems).
|
|
.Pp
|
|
Finally, if other authentication methods fail,
|
|
.Nm
|
|
prompts the user for a password.
|
|
The password is sent to the remote
|
|
host for checking; however, since all communications are encrypted,
|
|
the password cannot be seen by someone listening on the network.
|
|
.Pp
|
|
.Nm
|
|
automatically maintains and checks a database containing
|
|
identification for all hosts it has ever been used with.
|
|
Host keys are stored in
|
|
.Pa ~/.ssh/known_hosts
|
|
in the user's home directory.
|
|
Additionally, the file
|
|
.Pa /etc/ssh/ssh_known_hosts
|
|
is automatically checked for known hosts.
|
|
Any new hosts are automatically added to the user's file.
|
|
If a host's identification ever changes,
|
|
.Nm
|
|
warns about this and disables password authentication to prevent
|
|
server spoofing or man-in-the-middle attacks,
|
|
which could otherwise be used to circumvent the encryption.
|
|
The
|
|
.Cm StrictHostKeyChecking
|
|
option can be used to control logins to machines whose
|
|
host key is not known or has changed.
|
|
.Pp
|
|
When the user's identity has been accepted by the server, the server
|
|
either executes the given command in a non-interactive session or,
|
|
if no command has been specified, logs into the machine and gives
|
|
the user a normal shell as an interactive session.
|
|
All communication with
|
|
the remote command or shell will be automatically encrypted.
|
|
.Pp
|
|
If an interactive session is requested
|
|
.Nm
|
|
by default will only request a pseudo-terminal (pty) for interactive
|
|
sessions when the client has one.
|
|
The flags
|
|
.Fl T
|
|
and
|
|
.Fl t
|
|
can be used to override this behaviour.
|
|
.Pp
|
|
If a pseudo-terminal has been allocated the
|
|
user may use the escape characters noted below.
|
|
.Pp
|
|
If no pseudo-terminal has been allocated,
|
|
the session is transparent and can be used to reliably transfer binary data.
|
|
On most systems, setting the escape character to
|
|
.Dq none
|
|
will also make the session transparent even if a tty is used.
|
|
.Pp
|
|
The session terminates when the command or shell on the remote
|
|
machine exits and all X11 and TCP connections have been closed.
|
|
.Sh ESCAPE CHARACTERS
|
|
When a pseudo-terminal has been requested,
|
|
.Nm
|
|
supports a number of functions through the use of an escape character.
|
|
.Pp
|
|
A single tilde character can be sent as
|
|
.Ic ~~
|
|
or by following the tilde by a character other than those described below.
|
|
The escape character must always follow a newline to be interpreted as
|
|
special.
|
|
The escape character can be changed in configuration files using the
|
|
.Cm EscapeChar
|
|
configuration directive or on the command line by the
|
|
.Fl e
|
|
option.
|
|
.Pp
|
|
The supported escapes (assuming the default
|
|
.Ql ~ )
|
|
are:
|
|
.Bl -tag -width Ds
|
|
.It Cm ~.
|
|
Disconnect.
|
|
.It Cm ~^Z
|
|
Background
|
|
.Nm .
|
|
.It Cm ~#
|
|
List forwarded connections.
|
|
.It Cm ~&
|
|
Background
|
|
.Nm
|
|
at logout when waiting for forwarded connection / X11 sessions to terminate.
|
|
.It Cm ~?
|
|
Display a list of escape characters.
|
|
.It Cm ~B
|
|
Send a BREAK to the remote system
|
|
(only useful if the peer supports it).
|
|
.It Cm ~C
|
|
Open command line.
|
|
Currently this allows the addition of port forwardings using the
|
|
.Fl L ,
|
|
.Fl R
|
|
and
|
|
.Fl D
|
|
options (see above).
|
|
It also allows the cancellation of existing port-forwardings
|
|
with
|
|
.Sm off
|
|
.Fl KL Oo Ar bind_address : Oc Ar port
|
|
.Sm on
|
|
for local,
|
|
.Sm off
|
|
.Fl KR Oo Ar bind_address : Oc Ar port
|
|
.Sm on
|
|
for remote and
|
|
.Sm off
|
|
.Fl KD Oo Ar bind_address : Oc Ar port
|
|
.Sm on
|
|
for dynamic port-forwardings.
|
|
.Ic !\& Ns Ar command
|
|
allows the user to execute a local command if the
|
|
.Ic PermitLocalCommand
|
|
option is enabled in
|
|
.Xr ssh_config 5 .
|
|
Basic help is available, using the
|
|
.Fl h
|
|
option.
|
|
.It Cm ~R
|
|
Request rekeying of the connection
|
|
(only useful if the peer supports it).
|
|
.It Cm ~V
|
|
Decrease the verbosity
|
|
.Pq Ic LogLevel
|
|
when errors are being written to stderr.
|
|
.It Cm ~v
|
|
Increase the verbosity
|
|
.Pq Ic LogLevel
|
|
when errors are being written to stderr.
|
|
.El
|
|
.Sh TCP FORWARDING
|
|
Forwarding of arbitrary TCP connections over a secure channel
|
|
can be specified either on the command line or in a configuration file.
|
|
One possible application of TCP forwarding is a secure connection to a
|
|
mail server; another is going through firewalls.
|
|
.Pp
|
|
In the example below, we look at encrypting communication for an IRC client,
|
|
even though the IRC server it connects to does not directly
|
|
support encrypted communication.
|
|
This works as follows:
|
|
the user connects to the remote host using
|
|
.Nm ,
|
|
specifying the ports to be used to forward the connection.
|
|
After that it is possible to start the program locally,
|
|
and
|
|
.Nm
|
|
will encrypt and forward the connection to the remote server.
|
|
.Pp
|
|
The following example tunnels an IRC session from the client
|
|
to an IRC server at
|
|
.Dq server.example.com ,
|
|
joining channel
|
|
.Dq #users ,
|
|
nickname
|
|
.Dq pinky ,
|
|
using the standard IRC port, 6667:
|
|
.Bd -literal -offset 4n
|
|
$ ssh -f -L 6667:localhost:6667 server.example.com sleep 10
|
|
$ irc -c '#users' pinky IRC/127.0.0.1
|
|
.Ed
|
|
.Pp
|
|
The
|
|
.Fl f
|
|
option backgrounds
|
|
.Nm
|
|
and the remote command
|
|
.Dq sleep 10
|
|
is specified to allow an amount of time
|
|
(10 seconds, in the example)
|
|
to start the program which is going to use the tunnel.
|
|
If no connections are made within the time specified,
|
|
.Nm
|
|
will exit.
|
|
.Sh X11 FORWARDING
|
|
If the
|
|
.Cm ForwardX11
|
|
variable is set to
|
|
.Dq yes
|
|
(or see the description of the
|
|
.Fl X ,
|
|
.Fl x ,
|
|
and
|
|
.Fl Y
|
|
options above)
|
|
and the user is using X11 (the
|
|
.Ev DISPLAY
|
|
environment variable is set), the connection to the X11 display is
|
|
automatically forwarded to the remote side in such a way that any X11
|
|
programs started from the shell (or command) will go through the
|
|
encrypted channel, and the connection to the real X server will be made
|
|
from the local machine.
|
|
The user should not manually set
|
|
.Ev DISPLAY .
|
|
Forwarding of X11 connections can be
|
|
configured on the command line or in configuration files.
|
|
.Pp
|
|
The
|
|
.Ev DISPLAY
|
|
value set by
|
|
.Nm
|
|
will point to the server machine, but with a display number greater than zero.
|
|
This is normal, and happens because
|
|
.Nm
|
|
creates a
|
|
.Dq proxy
|
|
X server on the server machine for forwarding the
|
|
connections over the encrypted channel.
|
|
.Pp
|
|
.Nm
|
|
will also automatically set up Xauthority data on the server machine.
|
|
For this purpose, it will generate a random authorization cookie,
|
|
store it in Xauthority on the server, and verify that any forwarded
|
|
connections carry this cookie and replace it by the real cookie when
|
|
the connection is opened.
|
|
The real authentication cookie is never
|
|
sent to the server machine (and no cookies are sent in the plain).
|
|
.Pp
|
|
If the
|
|
.Cm ForwardAgent
|
|
variable is set to
|
|
.Dq yes
|
|
(or see the description of the
|
|
.Fl A
|
|
and
|
|
.Fl a
|
|
options above) and
|
|
the user is using an authentication agent, the connection to the agent
|
|
is automatically forwarded to the remote side.
|
|
.Sh VERIFYING HOST KEYS
|
|
When connecting to a server for the first time,
|
|
a fingerprint of the server's public key is presented to the user
|
|
(unless the option
|
|
.Cm StrictHostKeyChecking
|
|
has been disabled).
|
|
Fingerprints can be determined using
|
|
.Xr ssh-keygen 1 :
|
|
.Pp
|
|
.Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
|
|
.Pp
|
|
If the fingerprint is already known, it can be matched
|
|
and the key can be accepted or rejected.
|
|
If only legacy (MD5) fingerprints for the server are available, the
|
|
.Xr ssh-keygen 1
|
|
.Fl E
|
|
option may be used to downgrade the fingerprint algorithm to match.
|
|
.Pp
|
|
Because of the difficulty of comparing host keys
|
|
just by looking at fingerprint strings,
|
|
there is also support to compare host keys visually,
|
|
using
|
|
.Em random art .
|
|
By setting the
|
|
.Cm VisualHostKey
|
|
option to
|
|
.Dq yes ,
|
|
a small ASCII graphic gets displayed on every login to a server, no matter
|
|
if the session itself is interactive or not.
|
|
By learning the pattern a known server produces, a user can easily
|
|
find out that the host key has changed when a completely different pattern
|
|
is displayed.
|
|
Because these patterns are not unambiguous however, a pattern that looks
|
|
similar to the pattern remembered only gives a good probability that the
|
|
host key is the same, not guaranteed proof.
|
|
.Pp
|
|
To get a listing of the fingerprints along with their random art for
|
|
all known hosts, the following command line can be used:
|
|
.Pp
|
|
.Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
|
|
.Pp
|
|
If the fingerprint is unknown,
|
|
an alternative method of verification is available:
|
|
SSH fingerprints verified by DNS.
|
|
An additional resource record (RR),
|
|
SSHFP,
|
|
is added to a zonefile
|
|
and the connecting client is able to match the fingerprint
|
|
with that of the key presented.
|
|
.Pp
|
|
In this example, we are connecting a client to a server,
|
|
.Dq host.example.com .
|
|
The SSHFP resource records should first be added to the zonefile for
|
|
host.example.com:
|
|
.Bd -literal -offset indent
|
|
$ ssh-keygen -r host.example.com.
|
|
.Ed
|
|
.Pp
|
|
The output lines will have to be added to the zonefile.
|
|
To check that the zone is answering fingerprint queries:
|
|
.Pp
|
|
.Dl $ dig -t SSHFP host.example.com
|
|
.Pp
|
|
Finally the client connects:
|
|
.Bd -literal -offset indent
|
|
$ ssh -o "VerifyHostKeyDNS ask" host.example.com
|
|
[...]
|
|
Matching host key fingerprint found in DNS.
|
|
Are you sure you want to continue connecting (yes/no)?
|
|
.Ed
|
|
.Pp
|
|
See the
|
|
.Cm VerifyHostKeyDNS
|
|
option in
|
|
.Xr ssh_config 5
|
|
for more information.
|
|
.Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
|
|
.Nm
|
|
contains support for Virtual Private Network (VPN) tunnelling
|
|
using the
|
|
.Xr tun 4
|
|
network pseudo-device,
|
|
allowing two networks to be joined securely.
|
|
The
|
|
.Xr sshd_config 5
|
|
configuration option
|
|
.Cm PermitTunnel
|
|
controls whether the server supports this,
|
|
and at what level (layer 2 or 3 traffic).
|
|
.Pp
|
|
The following example would connect client network 10.0.50.0/24
|
|
with remote network 10.0.99.0/24 using a point-to-point connection
|
|
from 10.1.1.1 to 10.1.1.2,
|
|
provided that the SSH server running on the gateway to the remote network,
|
|
at 192.168.1.15, allows it.
|
|
.Pp
|
|
On the client:
|
|
.Bd -literal -offset indent
|
|
# ssh -f -w 0:1 192.168.1.15 true
|
|
# ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
|
|
# route add 10.0.99.0/24 10.1.1.2
|
|
.Ed
|
|
.Pp
|
|
On the server:
|
|
.Bd -literal -offset indent
|
|
# ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
|
|
# route add 10.0.50.0/24 10.1.1.1
|
|
.Ed
|
|
.Pp
|
|
Client access may be more finely tuned via the
|
|
.Pa /root/.ssh/authorized_keys
|
|
file (see below) and the
|
|
.Cm PermitRootLogin
|
|
server option.
|
|
The following entry would permit connections on
|
|
.Xr tun 4
|
|
device 1 from user
|
|
.Dq jane
|
|
and on tun device 2 from user
|
|
.Dq john ,
|
|
if
|
|
.Cm PermitRootLogin
|
|
is set to
|
|
.Dq forced-commands-only :
|
|
.Bd -literal -offset 2n
|
|
tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
|
|
tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
|
|
.Ed
|
|
.Pp
|
|
Since an SSH-based setup entails a fair amount of overhead,
|
|
it may be more suited to temporary setups,
|
|
such as for wireless VPNs.
|
|
More permanent VPNs are better provided by tools such as
|
|
.Xr ipsecctl 8
|
|
and
|
|
.Xr isakmpd 8 .
|
|
.Sh ENVIRONMENT
|
|
.Nm
|
|
will normally set the following environment variables:
|
|
.Bl -tag -width "SSH_ORIGINAL_COMMAND"
|
|
.It Ev DISPLAY
|
|
The
|
|
.Ev DISPLAY
|
|
variable indicates the location of the X11 server.
|
|
It is automatically set by
|
|
.Nm
|
|
to point to a value of the form
|
|
.Dq hostname:n ,
|
|
where
|
|
.Dq hostname
|
|
indicates the host where the shell runs, and
|
|
.Sq n
|
|
is an integer \*(Ge 1.
|
|
.Nm
|
|
uses this special value to forward X11 connections over the secure
|
|
channel.
|
|
The user should normally not set
|
|
.Ev DISPLAY
|
|
explicitly, as that
|
|
will render the X11 connection insecure (and will require the user to
|
|
manually copy any required authorization cookies).
|
|
.It Ev HOME
|
|
Set to the path of the user's home directory.
|
|
.It Ev LOGNAME
|
|
Synonym for
|
|
.Ev USER ;
|
|
set for compatibility with systems that use this variable.
|
|
.It Ev MAIL
|
|
Set to the path of the user's mailbox.
|
|
.It Ev PATH
|
|
Set to the default
|
|
.Ev PATH ,
|
|
as specified when compiling
|
|
.Nm .
|
|
.It Ev SSH_ASKPASS
|
|
If
|
|
.Nm
|
|
needs a passphrase, it will read the passphrase from the current
|
|
terminal if it was run from a terminal.
|
|
If
|
|
.Nm
|
|
does not have a terminal associated with it but
|
|
.Ev DISPLAY
|
|
and
|
|
.Ev SSH_ASKPASS
|
|
are set, it will execute the program specified by
|
|
.Ev SSH_ASKPASS
|
|
and open an X11 window to read the passphrase.
|
|
This is particularly useful when calling
|
|
.Nm
|
|
from a
|
|
.Pa .xsession
|
|
or related script.
|
|
(Note that on some machines it
|
|
may be necessary to redirect the input from
|
|
.Pa /dev/null
|
|
to make this work.)
|
|
.It Ev SSH_ASKPASS_REQUIRE
|
|
Allows further control over the use of an askpass program.
|
|
If this variable is set to
|
|
.Dq never
|
|
then
|
|
.Nm
|
|
will never attempt to use one.
|
|
If it is set to
|
|
.Dq prefer ,
|
|
then
|
|
.Nm
|
|
will prefer to use the askpass program instead of the TTY when requesting
|
|
passwords.
|
|
Finally, if the variable is set to
|
|
.Dq force ,
|
|
then the askpass program will be used for all passphrase input regardless
|
|
of whether
|
|
.Ev DISPLAY
|
|
is set.
|
|
.It Ev SSH_AUTH_SOCK
|
|
Identifies the path of a
|
|
.Ux Ns -domain
|
|
socket used to communicate with the agent.
|
|
.It Ev SSH_CONNECTION
|
|
Identifies the client and server ends of the connection.
|
|
The variable contains
|
|
four space-separated values: client IP address, client port number,
|
|
server IP address, and server port number.
|
|
.It Ev SSH_ORIGINAL_COMMAND
|
|
This variable contains the original command line if a forced command
|
|
is executed.
|
|
It can be used to extract the original arguments.
|
|
.It Ev SSH_TTY
|
|
This is set to the name of the tty (path to the device) associated
|
|
with the current shell or command.
|
|
If the current session has no tty,
|
|
this variable is not set.
|
|
.It Ev SSH_TUNNEL
|
|
Optionally set by
|
|
.Xr sshd 8
|
|
to contain the interface names assigned if tunnel forwarding was
|
|
requested by the client.
|
|
.It Ev SSH_USER_AUTH
|
|
Optionally set by
|
|
.Xr sshd 8 ,
|
|
this variable may contain a pathname to a file that lists the authentication
|
|
methods successfully used when the session was established, including any
|
|
public keys that were used.
|
|
.It Ev TZ
|
|
This variable is set to indicate the present time zone if it
|
|
was set when the daemon was started (i.e. the daemon passes the value
|
|
on to new connections).
|
|
.It Ev USER
|
|
Set to the name of the user logging in.
|
|
.El
|
|
.Pp
|
|
Additionally,
|
|
.Nm
|
|
reads
|
|
.Pa ~/.ssh/environment ,
|
|
and adds lines of the format
|
|
.Dq VARNAME=value
|
|
to the environment if the file exists and users are allowed to
|
|
change their environment.
|
|
For more information, see the
|
|
.Cm PermitUserEnvironment
|
|
option in
|
|
.Xr sshd_config 5 .
|
|
.Sh FILES
|
|
.Bl -tag -width Ds -compact
|
|
.It Pa ~/.rhosts
|
|
This file is used for host-based authentication (see above).
|
|
On some machines this file may need to be
|
|
world-readable if the user's home directory is on an NFS partition,
|
|
because
|
|
.Xr sshd 8
|
|
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 in the
|
|
.Xr sshd 8
|
|
manual page.
|
|
This file is not highly sensitive, but the recommended
|
|
permissions are read/write for the user, and not accessible by others.
|
|
.Pp
|
|
.It Pa ~/.ssh/config
|
|
This is the per-user configuration file.
|
|
The file format and configuration options are described in
|
|
.Xr ssh_config 5 .
|
|
Because of the potential for abuse, this file must have strict permissions:
|
|
read/write for the user, and not writable by others.
|
|
.Pp
|
|
.It Pa ~/.ssh/environment
|
|
Contains additional definitions for environment variables; see
|
|
.Sx ENVIRONMENT ,
|
|
above.
|
|
.Pp
|
|
.It Pa ~/.ssh/id_dsa
|
|
.It Pa ~/.ssh/id_ecdsa
|
|
.It Pa ~/.ssh/id_ecdsa_sk
|
|
.It Pa ~/.ssh/id_ed25519
|
|
.It Pa ~/.ssh/id_ed25519_sk
|
|
.It Pa ~/.ssh/id_rsa
|
|
Contains the private key for authentication.
|
|
These files
|
|
contain sensitive data and should be readable by the user but not
|
|
accessible by others (read/write/execute).
|
|
.Nm
|
|
will simply ignore a private key file if it is accessible by others.
|
|
It is possible to specify a passphrase when
|
|
generating the key which will be used to encrypt the
|
|
sensitive part of this file using AES-128.
|
|
.Pp
|
|
.It Pa ~/.ssh/id_dsa.pub
|
|
.It Pa ~/.ssh/id_ecdsa.pub
|
|
.It Pa ~/.ssh/id_ecdsa_sk.pub
|
|
.It Pa ~/.ssh/id_ed25519.pub
|
|
.It Pa ~/.ssh/id_ed25519_sk.pub
|
|
.It Pa ~/.ssh/id_rsa.pub
|
|
Contains the public key for authentication.
|
|
These files are not
|
|
sensitive and can (but need not) be readable by anyone.
|
|
.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.
|
|
See
|
|
.Xr sshd 8
|
|
for further details of the format of this file.
|
|
.Pp
|
|
.It Pa ~/.ssh/rc
|
|
Commands in this file are executed by
|
|
.Nm
|
|
when the user logs in, just before the user's shell (or command) is
|
|
started.
|
|
See the
|
|
.Xr sshd 8
|
|
manual page for more information.
|
|
.Pp
|
|
.It Pa /etc/hosts.equiv
|
|
This file is for host-based authentication (see above).
|
|
It should only be writable by root.
|
|
.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_config
|
|
Systemwide configuration file.
|
|
The file format and configuration options are described in
|
|
.Xr ssh_config 5 .
|
|
.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
|
|
and are used for host-based authentication.
|
|
.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.
|
|
It should be world-readable.
|
|
See
|
|
.Xr sshd 8
|
|
for further details of the format of this file.
|
|
.Pp
|
|
.It Pa /etc/ssh/sshrc
|
|
Commands in this file are executed by
|
|
.Nm
|
|
when the user logs in, just before the user's shell (or command) is started.
|
|
See the
|
|
.Xr sshd 8
|
|
manual page for more information.
|
|
.El
|
|
.Sh EXIT STATUS
|
|
.Nm
|
|
exits with the exit status of the remote command or with 255
|
|
if an error occurred.
|
|
.Sh SEE ALSO
|
|
.Xr scp 1 ,
|
|
.Xr sftp 1 ,
|
|
.Xr ssh-add 1 ,
|
|
.Xr ssh-agent 1 ,
|
|
.Xr ssh-keygen 1 ,
|
|
.Xr ssh-keyscan 1 ,
|
|
.Xr tun 4 ,
|
|
.Xr ssh_config 5 ,
|
|
.Xr ssh-keysign 8 ,
|
|
.Xr sshd 8
|
|
.Sh STANDARDS
|
|
.Rs
|
|
.%A S. Lehtinen
|
|
.%A C. Lonvick
|
|
.%D January 2006
|
|
.%R RFC 4250
|
|
.%T The Secure Shell (SSH) Protocol Assigned Numbers
|
|
.Re
|
|
.Pp
|
|
.Rs
|
|
.%A T. Ylonen
|
|
.%A C. Lonvick
|
|
.%D January 2006
|
|
.%R RFC 4251
|
|
.%T The Secure Shell (SSH) Protocol Architecture
|
|
.Re
|
|
.Pp
|
|
.Rs
|
|
.%A T. Ylonen
|
|
.%A C. Lonvick
|
|
.%D January 2006
|
|
.%R RFC 4252
|
|
.%T The Secure Shell (SSH) Authentication Protocol
|
|
.Re
|
|
.Pp
|
|
.Rs
|
|
.%A T. Ylonen
|
|
.%A C. Lonvick
|
|
.%D January 2006
|
|
.%R RFC 4253
|
|
.%T The Secure Shell (SSH) Transport Layer Protocol
|
|
.Re
|
|
.Pp
|
|
.Rs
|
|
.%A T. Ylonen
|
|
.%A C. Lonvick
|
|
.%D January 2006
|
|
.%R RFC 4254
|
|
.%T The Secure Shell (SSH) Connection Protocol
|
|
.Re
|
|
.Pp
|
|
.Rs
|
|
.%A J. Schlyter
|
|
.%A W. Griffin
|
|
.%D January 2006
|
|
.%R RFC 4255
|
|
.%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
|
|
.Re
|
|
.Pp
|
|
.Rs
|
|
.%A F. Cusack
|
|
.%A M. Forssen
|
|
.%D January 2006
|
|
.%R RFC 4256
|
|
.%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
|
|
.Re
|
|
.Pp
|
|
.Rs
|
|
.%A J. Galbraith
|
|
.%A P. Remaker
|
|
.%D January 2006
|
|
.%R RFC 4335
|
|
.%T The Secure Shell (SSH) Session Channel Break Extension
|
|
.Re
|
|
.Pp
|
|
.Rs
|
|
.%A M. Bellare
|
|
.%A T. Kohno
|
|
.%A C. Namprempre
|
|
.%D January 2006
|
|
.%R RFC 4344
|
|
.%T The Secure Shell (SSH) Transport Layer Encryption Modes
|
|
.Re
|
|
.Pp
|
|
.Rs
|
|
.%A B. Harris
|
|
.%D January 2006
|
|
.%R RFC 4345
|
|
.%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
|
|
.Re
|
|
.Pp
|
|
.Rs
|
|
.%A M. Friedl
|
|
.%A N. Provos
|
|
.%A W. Simpson
|
|
.%D March 2006
|
|
.%R RFC 4419
|
|
.%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
|
|
.Re
|
|
.Pp
|
|
.Rs
|
|
.%A J. Galbraith
|
|
.%A R. Thayer
|
|
.%D November 2006
|
|
.%R RFC 4716
|
|
.%T The Secure Shell (SSH) Public Key File Format
|
|
.Re
|
|
.Pp
|
|
.Rs
|
|
.%A D. Stebila
|
|
.%A J. Green
|
|
.%D December 2009
|
|
.%R RFC 5656
|
|
.%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
|
|
.Re
|
|
.Pp
|
|
.Rs
|
|
.%A A. Perrig
|
|
.%A D. Song
|
|
.%D 1999
|
|
.%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
|
|
.%T Hash Visualization: a New Technique to improve Real-World Security
|
|
.Re
|
|
.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.
|