1263 lines
38 KiB
Groff
1263 lines
38 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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.\" $FreeBSD$
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.\"
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.Dd September 25, 1999
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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 secure shell client (remote login program)
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.Sh SYNOPSIS
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.Nm ssh
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.Op Fl l Ar login_name
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.Op Ar hostname | user@hostname
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.Op Ar command
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.Pp
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.Nm ssh
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.Op Fl afgknqtvxACNPTX246
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.Op Fl c Ar cipher_spec
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.Op Fl e Ar escape_char
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.Op Fl i Ar identity_file
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.Op Fl l Ar login_name
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.Op Fl o Ar option
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.Op Fl p Ar port
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.Oo Fl L Xo
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.Sm off
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.Ar port :
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.Ar host :
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.Ar hostport
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.Sm on
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.Xc
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.Oc
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.Oo Fl R Xo
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.Sm off
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.Ar port :
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.Ar host :
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.Ar hostport
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.Sm on
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.Xc
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.Oc
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.Op Ar hostname | user@hostname
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.Op Ar command
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.Sh DESCRIPTION
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.Nm
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(Secure Shell) 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 replace
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rlogin and rsh, and provide secure encrypted communications between
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two untrusted hosts over an insecure network.
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X11 connections and
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arbitrary TCP/IP ports 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 hostname .
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The user must prove
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his/her identity to the remote machine using one of several methods
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depending on the protocol version used:
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.Pp
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.Ss SSH protocol version 1
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.Pp
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First, if the machine the user logs in from is listed in
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.Pa /etc/hosts.equiv
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or
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.Pa /etc/ssh/shosts.equiv
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on the remote machine, and the user names are
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the same on both sides, the user is immediately permitted to log in.
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Second, if
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.Pa \&.rhosts
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or
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.Pa \&.shosts
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exists in the user's home directory on the
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remote machine and contains a line containing the name of the client
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machine and the name of the user on that machine, the user is
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permitted to log in.
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This form of authentication alone is normally not
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allowed by the server because it is not secure.
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.Pp
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The second (and primary) authentication method is the
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.Pa rhosts
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or
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.Pa hosts.equiv
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method combined with RSA-based host authentication.
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It means that if the login would be permitted by
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.Pa $HOME/.rhosts ,
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.Pa $HOME/.shosts ,
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.Pa /etc/hosts.equiv ,
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or
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.Pa /etc/ssh/shosts.equiv ,
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and if additionally the server can verify the client's
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host key (see
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.Pa /etc/ssh/ssh_known_hosts
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and
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.Pa $HOME/.ssh/known_hosts
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in the
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.Sx FILES
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section), only then login is permitted.
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This authentication method closes security holes due to IP
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spoofing, DNS spoofing and routing spoofing.
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[Note to the administrator:
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.Pa /etc/hosts.equiv ,
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.Pa $HOME/.rhosts ,
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and the rlogin/rsh protocol in general, are inherently insecure and should be
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disabled if security is desired.]
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.Pp
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As a third authentication method,
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.Nm
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supports RSA based authentication.
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The scheme is based on public-key cryptography: there are cryptosystems
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where encryption and decryption are done using separate keys, and it
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is not possible to derive the decryption key from the encryption key.
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RSA is one such system.
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The idea is that each user creates a public/private
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key pair for authentication purposes.
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The server knows the public key, and only the user knows the private key.
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The file
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.Pa $HOME/.ssh/authorized_keys
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lists the public keys that are permitted for logging
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in.
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When the user logs in, the
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.Nm
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program tells the server which key pair it would like to use for
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authentication.
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The server checks if this key is permitted, and if
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so, sends the user (actually the
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.Nm
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program running on behalf of the user) a challenge, a random number,
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encrypted by the user's public key.
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The challenge can only be
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decrypted using the proper private key.
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The user's client then decrypts the
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challenge using the private key, proving that he/she knows the private
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key but without disclosing it to the server.
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.Pp
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.Nm
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implements the RSA authentication protocol automatically.
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The user creates his/her RSA key pair by running
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.Xr ssh-keygen 1 .
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This stores the private key in
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.Pa $HOME/.ssh/identity
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and the public key in
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.Pa $HOME/.ssh/identity.pub
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in the user's home directory.
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The user should then copy the
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.Pa identity.pub
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to
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.Pa $HOME/.ssh/authorized_keys
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in his/her home directory on the remote machine (the
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.Pa authorized_keys
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file corresponds to the conventional
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.Pa $HOME/.rhosts
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file, and has one key
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per line, though the lines can be very long).
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After this, the user can log in without giving the password.
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RSA authentication is much
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more secure than rhosts authentication.
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.Pp
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The most convenient way to use RSA authentication may be with an
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authentication agent.
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See
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.Xr ssh-agent 1
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for more information.
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.Pp
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If other authentication methods fail,
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.Nm
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prompts the user for a password.
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The password is sent to the remote
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host for checking; however, since all communications are encrypted,
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the password cannot be seen by someone listening on the network.
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.Pp
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.Ss SSH protocol version 2
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.Pp
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When a user connects using the protocol version 2
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different authentication methods are available:
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At first, the client attempts to authenticate using the public key method.
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If this method fails password authentication is tried.
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.Pp
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The public key method is similar to RSA authentication described
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in the previous section except that the DSA algorithm is used
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instead of the patented RSA algorithm.
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The client uses his private DSA key
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.Pa $HOME/.ssh/id_dsa
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to sign the session identifier and sends the result to the server.
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The server checks whether the matching public key is listed in
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.Pa $HOME/.ssh/authorized_keys2
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and grants access if both the key is found and the signature is correct.
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The session identifier is derived from a shared Diffie-Hellman value
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and is only known to the client and the server.
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.Pp
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If public key authentication fails or is not available a password
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can be sent encrypted to the remote host for proving the user's identity.
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This protocol 2 implementation does not yet support Kerberos or
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OPIE authentication.
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.Pp
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Protocol 2 provides additional mechanisms for confidentiality
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(the traffic is encrypted using 3DES, Blowfish, CAST128 or Arcfour)
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and integrity (hmac-sha1, hmac-md5).
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Note that protocol 1 lacks a strong mechanism for ensuring the
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integrity of the connection.
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.Pp
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.Ss Login session and remote execution
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.Pp
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When the user's identity has been accepted by the server, the server
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either executes the given command, or logs into the machine and gives
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the user a normal shell on the remote machine.
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All communication with
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the remote command or shell will be automatically encrypted.
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.Pp
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If a pseudo-terminal has been allocated (normal login session), the
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user can disconnect with
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.Ic ~. ,
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and suspend
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.Nm
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with
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.Ic ~^Z .
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All forwarded connections can be listed with
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.Ic ~#
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and if
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the session blocks waiting for forwarded X11 or TCP/IP
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connections to terminate, it can be backgrounded with
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.Ic ~&
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(this should not be used while the user shell is active, as it can cause the
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shell to hang).
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All available escapes can be listed with
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.Ic ~? .
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.Pp
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A single tilde character can be sent as
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.Ic ~~
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(or by following the tilde by a character other than those described above).
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The escape character must always follow a newline to be interpreted as
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special.
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The escape character can be changed in configuration files
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or on the command line.
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.Pp
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If no pseudo tty has been allocated, the
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session is transparent and can be used to reliably transfer binary
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data.
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On most systems, setting the escape character to
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.Dq none
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will also make the session transparent even if a tty is used.
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.Pp
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The session terminates when the command or shell in on the remote
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machine exists and all X11 and TCP/IP connections have been closed.
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The exit status of the remote program is returned as the exit status
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of
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.Nm ssh .
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.Pp
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.Ss X11 and TCP forwarding
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.Pp
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If the user is using X11 (the
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.Ev DISPLAY
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environment variable is set), the connection to the X11 display can
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be forwarded to the remote side in such a way that any X11
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programs started from the shell (or command) will go through the
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encrypted channel, and the connection to the real X server will be made
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from the local machine.
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The user should not manually set
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.Ev DISPLAY .
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Forwarding of X11 connections weakens the security of ssh and is
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disabled by default. X11 forwarding can be enabled on the command line
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or in configuration files.
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.Pp
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The
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.Ev DISPLAY
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value set by
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.Nm
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will point to the server machine, but with a display number greater
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than zero.
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This is normal, and happens because
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.Nm
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creates a
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.Dq proxy
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X server on the server machine for forwarding the
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connections over the encrypted channel.
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.Pp
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.Nm
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will also automatically set up Xauthority data on the server machine.
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For this purpose, it will generate a random authorization cookie,
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store it in Xauthority on the server, and verify that any forwarded
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connections carry this cookie and replace it by the real cookie when
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the connection is opened.
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The real authentication cookie is never
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sent to the server machine (and no cookies are sent in the plain).
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.Pp
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If the user is using an authentication agent, the connection to the agent
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is automatically forwarded to the remote side unless disabled on
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command line or in a configuration file.
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.Pp
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Forwarding of arbitrary TCP/IP connections over the secure channel can
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be specified either on command line or in a configuration file.
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One possible application of TCP/IP forwarding is a secure connection to an
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electronic purse; another is going trough firewalls.
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.Pp
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.Ss Server authentication
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.Pp
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.Nm
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automatically maintains and checks a database containing
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identifications for all hosts it has ever been used with.
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RSA host keys are stored in
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.Pa $HOME/.ssh/known_hosts
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and
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DSA host keys are stored in
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.Pa $HOME/.ssh/known_hosts2
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in the user's home directory.
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Additionally, the files
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.Pa /etc/ssh/ssh_known_hosts
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and
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.Pa /etc/ssh/ssh_known_hosts2
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are automatically checked for known hosts.
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Any new hosts are automatically added to the user's file.
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If a host's identification
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ever changes,
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.Nm
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warns about this and disables password authentication to prevent a
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trojan horse from getting the user's password.
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Another purpose of
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this mechanism is to prevent man-in-the-middle attacks which could
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otherwise be used to circumvent the encryption.
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The
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.Cm StrictHostKeyChecking
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option (see below) can be used to prevent logins to machines whose
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host key is not known or has changed.
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.Sh OPTIONS
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.Bl -tag -width Ds
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.It Fl a
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Disables forwarding of the authentication agent connection.
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.It Fl A
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Enables forwarding of the authentication agent connection.
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This can also be specified on a per-host basis in a configuration file.
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.It Fl c Ar blowfish|3des
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Selects the cipher to use for encrypting the session.
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.Ar 3des
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is used by default.
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It is believed to be secure.
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.Ar 3des
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(triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
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It is presumably more secure than the
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.Ar des
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cipher which is no longer supported in
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.Nm ssh .
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.Ar blowfish
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is a fast block cipher, it appears very secure and is much faster than
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.Ar 3des .
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.It Fl c Ar "3des-cbc,blowfish-cbc,arcfour,cast128-cbc"
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Additionally, for protocol version 2 a comma-separated list of ciphers can
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be specified in order of preference. Protocol version 2 supports
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3DES, Blowfish and CAST128 in CBC mode and Arcfour.
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.It Fl e Ar ch|^ch|none
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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, followed
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by control-Z suspends the connection, and followed by itself sends the
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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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.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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.It Fl g
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Allows remote hosts to connect to local forwarded ports.
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.It Fl i Ar identity_file
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Selects the file from which the identity (private key) for
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RSA authentication is read.
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Default is
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.Pa $HOME/.ssh/identity
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in the user's home directory.
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Identity files may also be specified on
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a per-host basis in the configuration file.
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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).
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.It Fl k
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Disables forwarding of Kerberos tickets and AFS tokens.
|
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This may also be specified on a per-host basis in the configuration file.
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.It Fl l Ar login_name
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Specifies the user to log in as on the remote machine.
|
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This also may be specified on a per-host basis in the configuration file.
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.It Fl n
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Redirects stdin from
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.Pa /dev/null
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(actually, prevents reading from stdin).
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This must be used when
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.Nm
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is run in the background.
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A common trick is to use this to run X11 programs on a remote machine.
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For example,
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.Ic ssh -n shadows.cs.hut.fi emacs &
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will start an emacs on shadows.cs.hut.fi, and the X11
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|
connection will be automatically forwarded over an encrypted channel.
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The
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.Nm
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program will be put in the background.
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(This does not work if
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.Nm
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needs to ask for a password or passphrase; see also the
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.Fl f
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option.)
|
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.It Fl N
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Do not execute a remote command.
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This is usefull if you just want to forward ports
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(protocol version 2 only).
|
|
.It Fl o Ar option
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|
Can be used to give options in the format used in the config file.
|
|
This is useful for specifying options for which there is no separate
|
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command-line flag.
|
|
The option has the same format as a line in the configuration file.
|
|
.It Fl p Ar port
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Port to connect to on the remote host.
|
|
This can be specified on a
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|
per-host basis in the configuration file.
|
|
.It Fl P
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Use a non-privileged port for outgoing connections.
|
|
This can be used if your firewall does
|
|
not permit connections from privileged ports.
|
|
Note that this option turns off
|
|
.Cm RhostsAuthentication
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|
and
|
|
.Cm RhostsRSAAuthentication .
|
|
.It Fl q
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|
Quiet mode.
|
|
Causes all warning and diagnostic messages to be suppressed.
|
|
Only fatal errors are displayed.
|
|
.It Fl t
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|
Force pseudo-tty allocation.
|
|
This can be used to execute arbitrary
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screen-based programs on a remote machine, which can be very useful,
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|
e.g., when implementing menu services.
|
|
.It Fl T
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Disable pseudo-tty allocation (protocol version 2 only).
|
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.It Fl v
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Verbose mode.
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Causes
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.Nm
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to print debugging messages about its progress.
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|
This is helpful in
|
|
debugging connection, authentication, and configuration problems.
|
|
The verbose mode is also used to display
|
|
.Xr skey 1
|
|
challenges, if the user entered "s/key" as password.
|
|
.It Fl x
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|
Disables X11 forwarding.
|
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.It Fl X
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|
Enables X11 forwarding.
|
|
This can also be specified on a per-host basis in a configuration file.
|
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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 and TCP/IP connections).
|
|
The compression algorithm is the same used by
|
|
.Xr gzip 1 ,
|
|
and the
|
|
.Dq level
|
|
can be controlled by the
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|
.Cm CompressionLevel
|
|
option (see below).
|
|
Compression is desirable on modem lines and other
|
|
slow connections, but will only slow down things on fast networks.
|
|
The default value can be set on a host-by-host basis in the
|
|
configuration files; see the
|
|
.Cm Compress
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|
option below.
|
|
.It Fl L Ar port:host:hostport
|
|
Specifies that the given port on the local (client) host is to be
|
|
forwarded to the given host and port on the remote side.
|
|
This works by allocating a socket to listen to
|
|
.Ar port
|
|
on the local side, and whenever a connection is made to this port, the
|
|
connection is forwarded over the secure channel, and a connection is
|
|
made to
|
|
.Ar host
|
|
port
|
|
.Ar hostport
|
|
from the remote machine.
|
|
Port forwardings can also be specified in the configuration file.
|
|
Only root can forward privileged ports.
|
|
IPv6 addresses can be specified with an alternative syntax:
|
|
.Ar port/host/hostport
|
|
.It Fl R Ar port:host:hostport
|
|
Specifies that the given port on the remote (server) host is to be
|
|
forwarded to the given host and port on the local side.
|
|
This works by allocating a socket to listen to
|
|
.Ar port
|
|
on the remote side, and whenever a connection is made to this port, the
|
|
connection is forwarded over the secure channel, and a connection is
|
|
made to
|
|
.Ar host
|
|
port
|
|
.Ar hostport
|
|
from the local machine.
|
|
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.
|
|
.It Fl 2
|
|
Forces
|
|
.Nm
|
|
to try protocol version 2 only.
|
|
.It Fl 4
|
|
Forces
|
|
.Nm
|
|
to use IPv4 addresses only.
|
|
.It Fl 6
|
|
Forces
|
|
.Nm
|
|
to use IPv6 addresses only.
|
|
.El
|
|
.Sh CONFIGURATION FILES
|
|
.Nm
|
|
obtains configuration data from the following sources (in this order):
|
|
command line options, user's configuration file
|
|
.Pq Pa $HOME/.ssh/config ,
|
|
and system-wide configuration file
|
|
.Pq Pa /etc/ssh/ssh_config .
|
|
For each parameter, the first obtained value
|
|
will be used.
|
|
The configuration files contain sections bracketed by
|
|
.Dq Host
|
|
specifications, and that section is only applied for hosts that
|
|
match one of the patterns given in the specification.
|
|
The matched host name is the one given on the command line.
|
|
.Pp
|
|
Since the first obtained value for each parameter is used, more
|
|
host-specific declarations should be given near the beginning of the
|
|
file, and general defaults at the end.
|
|
.Pp
|
|
The configuration file has the following format:
|
|
.Pp
|
|
Empty lines and lines starting with
|
|
.Ql #
|
|
are comments.
|
|
.Pp
|
|
Otherwise a line is of the format
|
|
.Dq keyword arguments .
|
|
The possible
|
|
keywords and their meanings are as follows (note that the
|
|
configuration files are case-sensitive):
|
|
.Bl -tag -width Ds
|
|
.It Cm Host
|
|
Restricts the following declarations (up to the next
|
|
.Cm Host
|
|
keyword) to be only for those hosts that match one of the patterns
|
|
given after the keyword.
|
|
.Ql \&*
|
|
and
|
|
.Ql ?
|
|
can be used as wildcards in the
|
|
patterns.
|
|
A single
|
|
.Ql \&*
|
|
as a pattern can be used to provide global
|
|
defaults for all hosts.
|
|
The host is the
|
|
.Ar hostname
|
|
argument given on the command line (i.e., the name is not converted to
|
|
a canonicalized host name before matching).
|
|
.It Cm AFSTokenPassing
|
|
Specifies whether to pass AFS tokens to remote host.
|
|
The argument to this keyword must be
|
|
.Dq yes
|
|
or
|
|
.Dq no .
|
|
.It Cm BatchMode
|
|
If set to
|
|
.Dq yes ,
|
|
passphrase/password querying will be disabled.
|
|
This option is useful in scripts and other batch jobs where you have no
|
|
user to supply the password.
|
|
The argument must be
|
|
.Dq yes
|
|
or
|
|
.Dq no .
|
|
.It Cm CheckHostIP
|
|
If this flag is set to
|
|
.Dq yes ,
|
|
ssh will additionally check the host ip address in the
|
|
.Pa known_hosts
|
|
file.
|
|
This allows ssh to detect if a host key changed due to DNS spoofing.
|
|
If the option is set to
|
|
.Dq no ,
|
|
the check will not be executed.
|
|
.It Cm Cipher
|
|
Specifies the cipher to use for encrypting the session.
|
|
Currently,
|
|
.Dq blowfish ,
|
|
and
|
|
.Dq 3des
|
|
are supported.
|
|
The default is
|
|
.Dq 3des .
|
|
.It Cm Ciphers
|
|
Specifies the ciphers allowed for protocol version 2
|
|
in order of preference.
|
|
Multiple ciphers must be comma-separated.
|
|
The default is
|
|
.Dq 3des-cbc,blowfish-cbc,arcfour,cast128-cbc .
|
|
.It Cm Compression
|
|
Specifies whether to use compression.
|
|
The argument must be
|
|
.Dq yes
|
|
or
|
|
.Dq no .
|
|
.It Cm CompressionLevel
|
|
Specifies the compression level to use if compression is enable.
|
|
The argument must be an integer from 1 (fast) to 9 (slow, best).
|
|
The default level is 6, which is good for most applications.
|
|
The meaning of the values is the same as in
|
|
.Xr gzip 1 .
|
|
.It Cm ConnectionAttempts
|
|
Specifies the number of tries (one per second) to make before falling
|
|
back to rsh or exiting.
|
|
The argument must be an integer.
|
|
This may be useful in scripts if the connection sometimes fails.
|
|
.It Cm DSAAuthentication
|
|
Specifies whether to try DSA authentication.
|
|
The argument to this keyword must be
|
|
.Dq yes
|
|
or
|
|
.Dq no .
|
|
DSA authentication will only be
|
|
attempted if a DSA identity file exists.
|
|
Note that this option applies to protocol version 2 only.
|
|
.It Cm EscapeChar
|
|
Sets the escape character (default:
|
|
.Ql ~ ) .
|
|
The escape character can also
|
|
be set on the command line.
|
|
The argument should be a single character,
|
|
.Ql ^
|
|
followed by a letter, or
|
|
.Dq none
|
|
to disable the escape
|
|
character entirely (making the connection transparent for binary
|
|
data).
|
|
.It Cm FallBackToRsh
|
|
Specifies that if connecting via
|
|
.Nm
|
|
fails due to a connection refused error (there is no
|
|
.Xr sshd 8
|
|
listening on the remote host),
|
|
.Xr rsh 1
|
|
should automatically be used instead (after a suitable warning about
|
|
the session being unencrypted).
|
|
The argument must be
|
|
.Dq yes
|
|
or
|
|
.Dq no .
|
|
.It Cm ForwardAgent
|
|
Specifies whether the connection to the authentication agent (if any)
|
|
will be forwarded to the remote machine.
|
|
The argument must be
|
|
.Dq yes
|
|
or
|
|
.Dq no .
|
|
The default is
|
|
.Dq no .
|
|
.It Cm ForwardX11
|
|
Specifies whether X11 connections will be automatically redirected
|
|
over the secure channel and
|
|
.Ev DISPLAY
|
|
set.
|
|
The argument must be
|
|
.Dq yes
|
|
or
|
|
.Dq no .
|
|
The default is
|
|
.Dq no .
|
|
.It Cm GatewayPorts
|
|
Specifies whether remote hosts are allowed to connect to local
|
|
forwarded ports.
|
|
The argument must be
|
|
.Dq yes
|
|
or
|
|
.Dq no .
|
|
The default is
|
|
.Dq no .
|
|
.It Cm GlobalKnownHostsFile
|
|
Specifies a file to use instead of
|
|
.Pa /etc/ssh/ssh_known_hosts .
|
|
.It Cm HostName
|
|
Specifies the real host name to log into.
|
|
This can be used to specify nicknames or abbreviations for hosts.
|
|
Default is the name given on the command line.
|
|
Numeric IP addresses are also permitted (both on the command line and in
|
|
.Cm HostName
|
|
specifications).
|
|
.It Cm IdentityFile
|
|
Specifies the file from which the user's RSA authentication identity
|
|
is read (default
|
|
.Pa $HOME/.ssh/identity
|
|
in the user's home directory).
|
|
Additionally, any identities represented by the authentication agent
|
|
will be used for authentication.
|
|
The file name may use the tilde
|
|
syntax to refer to a user's home directory.
|
|
It is possible to have
|
|
multiple identity files specified in configuration files; all these
|
|
identities will be tried in sequence.
|
|
.It Cm IdentityFile2
|
|
Specifies the file from which the user's DSA authentication identity
|
|
is read (default
|
|
.Pa $HOME/.ssh/id_dsa
|
|
in the user's home directory).
|
|
The file name may use the tilde
|
|
syntax to refer to a user's home directory.
|
|
It is possible to have
|
|
multiple identity files specified in configuration files; all these
|
|
identities will be tried in sequence.
|
|
.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.
|
|
.Pp
|
|
The default is
|
|
.Dq yes
|
|
(to send keepalives), and the client will notice
|
|
if the network goes down or the remote host dies.
|
|
This is important in scripts, and many users want it too.
|
|
.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 will be used.
|
|
The argument to this keyword must be
|
|
.Dq yes
|
|
or
|
|
.Dq no .
|
|
.It Cm KerberosTgtPassing
|
|
Specifies whether a Kerberos TGT will be forwarded to the server.
|
|
This will only work if the Kerberos server is actually an AFS kaserver.
|
|
The argument to this keyword must be
|
|
.Dq yes
|
|
or
|
|
.Dq no .
|
|
.It Cm LocalForward
|
|
Specifies that a TCP/IP port on the local machine be forwarded over
|
|
the secure channel to given host:port from the remote machine.
|
|
The first argument must be a port number, and the second must be
|
|
host:port.
|
|
Multiple forwardings may be specified, and additional
|
|
forwardings can be given on the command line.
|
|
Only the superuser can forward privileged ports.
|
|
.It Cm LogLevel
|
|
Gives the verbosity level that is used when logging messages from
|
|
.Nm ssh .
|
|
The possible values are:
|
|
QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
|
|
The default is INFO.
|
|
.It Cm NumberOfPasswordPrompts
|
|
Specifies the number of password prompts before giving up.
|
|
The argument to this keyword must be an integer.
|
|
Default is 3.
|
|
.It Cm PasswordAuthentication
|
|
Specifies whether to use password authentication.
|
|
The argument to this keyword must be
|
|
.Dq yes
|
|
or
|
|
.Dq no .
|
|
Note that this option applies to both protocol version 1 and 2.
|
|
.It Cm Port
|
|
Specifies the port number to connect on the remote host.
|
|
Default is 22.
|
|
.It Cm Protocol
|
|
Specifies the protocol versions
|
|
.Nm
|
|
should support in order of preference.
|
|
The possible values are
|
|
.Dq 1
|
|
and
|
|
.Dq 2 .
|
|
Multiple versions must be comma-separated.
|
|
The default is
|
|
.Dq 1,2 .
|
|
This means that
|
|
.Nm
|
|
tries version 1 and falls back to version 2
|
|
if version 1 is not available.
|
|
.It Cm ProxyCommand
|
|
Specifies the command to use to connect to the server.
|
|
The command
|
|
string extends to the end of the line, and is executed with
|
|
.Pa /bin/sh .
|
|
In the command string,
|
|
.Ql %h
|
|
will be substituted by the host name to
|
|
connect and
|
|
.Ql %p
|
|
by the port.
|
|
The command can be basically anything,
|
|
and should read from its standard input and write to its standard output.
|
|
It should eventually connect an
|
|
.Xr sshd 8
|
|
server running on some machine, or execute
|
|
.Ic sshd -i
|
|
somewhere.
|
|
Host key management will be done using the
|
|
HostName of the host being connected (defaulting to the name typed by
|
|
the user).
|
|
Note that
|
|
.Cm CheckHostIP
|
|
is not available for connects with a proxy command.
|
|
.Pp
|
|
.It Cm RemoteForward
|
|
Specifies that a TCP/IP port on the remote machine be forwarded over
|
|
the secure channel to given host:port from the local machine.
|
|
The first argument must be a port number, and the second must be
|
|
host:port.
|
|
Multiple forwardings may be specified, and additional
|
|
forwardings can be given on the command line.
|
|
Only the superuser can forward privileged ports.
|
|
.It Cm RhostsAuthentication
|
|
Specifies whether to try rhosts based authentication.
|
|
Note that this
|
|
declaration only affects the client side and has no effect whatsoever
|
|
on security.
|
|
Disabling rhosts authentication may reduce
|
|
authentication time on slow connections when rhosts authentication is
|
|
not used.
|
|
Most servers do not permit RhostsAuthentication because it
|
|
is not secure (see RhostsRSAAuthentication).
|
|
The argument to this keyword must be
|
|
.Dq yes
|
|
or
|
|
.Dq no .
|
|
.It Cm RhostsRSAAuthentication
|
|
Specifies whether to try rhosts based authentication with RSA host
|
|
authentication.
|
|
This is the primary authentication method for most sites.
|
|
The argument must be
|
|
.Dq yes
|
|
or
|
|
.Dq no .
|
|
.It Cm RSAAuthentication
|
|
Specifies whether to try RSA authentication.
|
|
The argument to this keyword must be
|
|
.Dq yes
|
|
or
|
|
.Dq no .
|
|
RSA authentication will only be
|
|
attempted if the identity file exists, or an authentication agent is
|
|
running.
|
|
Note that this option applies to protocol version 1 only.
|
|
.It Cm SkeyAuthentication
|
|
Specifies whether to use
|
|
.Xr skey 1
|
|
authentication.
|
|
The argument to this keyword must be
|
|
.Dq yes
|
|
or
|
|
.Dq no .
|
|
The default is
|
|
.Dq no .
|
|
.It Cm StrictHostKeyChecking
|
|
If this flag is set to
|
|
.Dq yes ,
|
|
.Nm
|
|
ssh will never automatically add host keys to the
|
|
.Pa $HOME/.ssh/known_hosts
|
|
and
|
|
.Pa $HOME/.ssh/known_hosts2
|
|
files, and refuses to connect hosts whose host key has changed.
|
|
This provides maximum protection against trojan horse attacks.
|
|
However, it can be somewhat annoying if you don't have good
|
|
.Pa /etc/ssh/ssh_known_hosts
|
|
and
|
|
.Pa /etc/ssh/ssh_known_hosts2
|
|
files installed and frequently
|
|
connect new hosts.
|
|
Basically this option forces the user to manually
|
|
add any new hosts.
|
|
Normally this option is disabled, and new hosts
|
|
will automatically be added to the known host files.
|
|
The host keys of
|
|
known hosts will be verified automatically in either case.
|
|
The argument must be
|
|
.Dq yes
|
|
or
|
|
.Dq no .
|
|
.It Cm UsePrivilegedPort
|
|
Specifies whether to use a privileged port for outgoing connections.
|
|
The argument must be
|
|
.Dq yes
|
|
or
|
|
.Dq no .
|
|
The default is
|
|
.Dq yes .
|
|
Note that setting this option to
|
|
.Dq no
|
|
turns off
|
|
.Cm RhostsAuthentication
|
|
and
|
|
.Cm RhostsRSAAuthentication .
|
|
.It Cm User
|
|
Specifies the user to log in as.
|
|
This can be useful if you have a different user name on different machines.
|
|
This saves the trouble of
|
|
having to remember to give the user name on the command line.
|
|
.It Cm UserKnownHostsFile
|
|
Specifies a file to use instead of
|
|
.Pa $HOME/.ssh/known_hosts .
|
|
.It Cm UseRsh
|
|
Specifies that rlogin/rsh should be used for this host.
|
|
It is possible that the host does not at all support the
|
|
.Nm
|
|
protocol.
|
|
This causes
|
|
.Nm
|
|
to immediately execute
|
|
.Xr rsh 1 .
|
|
All other options (except
|
|
.Cm HostName )
|
|
are ignored if this has been specified.
|
|
The argument must be
|
|
.Dq yes
|
|
or
|
|
.Dq no .
|
|
.It Cm XAuthLocation
|
|
Specifies the location of the
|
|
.Xr xauth 1
|
|
program.
|
|
The default is
|
|
.Pa /usr/X11R6/bin/xauth .
|
|
.El
|
|
.Sh ENVIRONMENT
|
|
.Nm
|
|
will normally set the following environment variables:
|
|
.Bl -tag -width Ds
|
|
.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 hostname indicates
|
|
the host where the shell runs, and n is an integer \*(>= 1.
|
|
.Nm
|
|
uses this special value to forward X11 connections over the secure
|
|
channel.
|
|
The user should normally not set 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 point the user's mailbox.
|
|
.It Ev PATH
|
|
Set to the default
|
|
.Ev PATH ,
|
|
as specified when compiling
|
|
.Nm ssh .
|
|
.It Ev SSH_AUTH_SOCK
|
|
indicates the path of a unix-domain socket used to communicate with the
|
|
agent.
|
|
.It Ev SSH_CLIENT
|
|
Identifies the client end of the connection.
|
|
The variable contains
|
|
three space-separated values: client ip-address, client port number,
|
|
and server port number.
|
|
.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 TZ
|
|
The timezone variable is set to indicate the present timezone 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 $HOME/.ssh/environment ,
|
|
and adds lines of the format
|
|
.Dq VARNAME=value
|
|
to the environment.
|
|
.Sh FILES
|
|
.Bl -tag -width Ds
|
|
.It Pa $HOME/.ssh/known_hosts
|
|
Records host keys for all hosts the user has logged into (that are not
|
|
in
|
|
.Pa /etc/ssh/ssh_known_hosts ) .
|
|
See
|
|
.Xr sshd 8 .
|
|
.It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa
|
|
Contains the RSA and the DSA authentication identity of the user.
|
|
These files
|
|
contain sensitive data and should be readable by the user but not
|
|
accessible by others (read/write/execute).
|
|
Note that
|
|
.Nm
|
|
ignores a private key file if it is accessible by others.
|
|
It is possible to specify a passphrase when
|
|
generating the key; the passphrase will be used to encrypt the
|
|
sensitive part of this file using 3DES.
|
|
.It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub
|
|
Contains the public key for authentication (public part of the
|
|
identity file in human-readable form).
|
|
The contents of the
|
|
.Pa $HOME/.ssh/identity.pub
|
|
file should be added to
|
|
.Pa $HOME/.ssh/authorized_keys
|
|
on all machines
|
|
where you wish to log in using RSA authentication.
|
|
The contents of the
|
|
.Pa $HOME/.ssh/id_dsa.pub
|
|
file should be added to
|
|
.Pa $HOME/.ssh/authorized_keys2
|
|
on all machines
|
|
where you wish to log in using DSA authentication.
|
|
These files are not
|
|
sensitive and can (but need not) be readable by anyone.
|
|
These files are
|
|
never used automatically and are not necessary; they are only provided for
|
|
the convenience of the user.
|
|
.It Pa $HOME/.ssh/config
|
|
This is the per-user configuration file.
|
|
The format of this file is described above.
|
|
This file is used by the
|
|
.Nm
|
|
client.
|
|
This file does not usually contain any sensitive information,
|
|
but the recommended permissions are read/write for the user, and not
|
|
accessible by others.
|
|
.It Pa $HOME/.ssh/authorized_keys
|
|
Lists the RSA keys that can be used for logging in as this user.
|
|
The format of this file is described in the
|
|
.Xr sshd 8
|
|
manual page.
|
|
In the simplest form the format is the same as the .pub
|
|
identity files (that is, each line contains the number of bits in
|
|
modulus, public exponent, modulus, and comment fields, separated by
|
|
spaces).
|
|
This file is not highly sensitive, but the recommended
|
|
permissions are read/write for the user, and not accessible by others.
|
|
.It Pa $HOME/.ssh/authorized_keys2
|
|
Lists the DSA keys that can be used for logging in as this user.
|
|
This file is not highly sensitive, but the recommended
|
|
permissions are read/write for the user, and not accessible by others.
|
|
.It Pa /etc/ssh/ssh_known_hosts, /etc/ssh/ssh_known_hosts2
|
|
Systemwide list of known host keys.
|
|
.Pa /etc/ssh_known_hosts
|
|
contains RSA and
|
|
.Pa /etc/ssh_known_hosts2
|
|
contains DSA keys.
|
|
These files should be prepared by the
|
|
system administrator to contain the public host keys of all machines in the
|
|
organization.
|
|
This file should be world-readable.
|
|
This file contains
|
|
public keys, one per line, in the following format (fields separated
|
|
by spaces): system name, number of bits in modulus, public exponent,
|
|
modulus, and optional comment field.
|
|
When different names are used
|
|
for the same machine, all such names should be listed, separated by
|
|
commas.
|
|
The format is described on the
|
|
.Xr sshd 8
|
|
manual page.
|
|
.Pp
|
|
The canonical system name (as returned by name servers) is used by
|
|
.Xr sshd 8
|
|
to verify the client host when logging in; other names are needed because
|
|
.Nm
|
|
does not convert the user-supplied name to a canonical name before
|
|
checking the key, because someone with access to the name servers
|
|
would then be able to fool host authentication.
|
|
.It Pa /etc/ssh/ssh_config
|
|
Systemwide configuration file.
|
|
This file provides defaults for those
|
|
values that are not specified in the user's configuration file, and
|
|
for those users who do not have a configuration file.
|
|
This file must be world-readable.
|
|
.It Pa $HOME/.rhosts
|
|
This file is used in
|
|
.Pa \&.rhosts
|
|
authentication to list the
|
|
host/user pairs that are permitted to log in.
|
|
(Note that this file is
|
|
also used by rlogin and rsh, which makes using this file insecure.)
|
|
Each line of the file contains a host name (in the canonical form
|
|
returned by name servers), and then a user name on that host,
|
|
separated by a space.
|
|
One some machines this file may need to be
|
|
world-readable if the user's home directory is on a 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
|
|
Note that by default
|
|
.Xr sshd 8
|
|
will be installed so that it requires successful RSA host
|
|
authentication before permitting \s+2.\s0rhosts authentication.
|
|
If your server machine does not have the client's host key in
|
|
.Pa /etc/ssh/ssh_known_hosts ,
|
|
you can store it in
|
|
.Pa $HOME/.ssh/known_hosts .
|
|
The easiest way to do this is to
|
|
connect back to the client from the server machine using ssh; this
|
|
will automatically add the host key to
|
|
.Pa $HOME/.ssh/known_hosts .
|
|
.It Pa $HOME/.shosts
|
|
This file is used exactly the same way as
|
|
.Pa \&.rhosts .
|
|
The purpose for
|
|
having this file is to be able to use rhosts authentication with
|
|
.Nm
|
|
without permitting login with
|
|
.Xr rlogin 1
|
|
or
|
|
.Xr rsh 1 .
|
|
.It Pa /etc/hosts.equiv
|
|
This file is used during
|
|
.Pa \&.rhosts
|
|
authentication.
|
|
It contains
|
|
canonical hosts names, one per line (the full format is described on
|
|
the
|
|
.Xr sshd 8
|
|
manual page).
|
|
If the client host is found in this file, login is
|
|
automatically permitted provided client and server user names are the
|
|
same.
|
|
Additionally, successful RSA host authentication is normally
|
|
required.
|
|
This file should only be writable by root.
|
|
.It Pa /etc/ssh/shosts.equiv
|
|
This file is processed exactly as
|
|
.Pa /etc/hosts.equiv .
|
|
This file may be useful to permit logins using
|
|
.Nm
|
|
but not using rsh/rlogin.
|
|
.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.
|
|
.It Pa $HOME/.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.
|
|
.It Pa $HOME/.ssh/environment
|
|
Contains additional definitions for environment variables, see section
|
|
.Sx ENVIRONMENT
|
|
above.
|
|
.It Pa libcrypto.so.X.1
|
|
A version of this library which includes support for the RSA algorithm
|
|
is required for proper operation.
|
|
.El
|
|
.Sh AUTHOR
|
|
OpenSSH
|
|
is a derivative of the original (free) ssh 1.2.12 release by Tatu Ylonen,
|
|
but with bugs removed and newer features re-added.
|
|
Rapidly after the
|
|
1.2.12 release, newer versions of the original ssh bore successively
|
|
more restrictive licenses, and thus demand for a free version was born.
|
|
.Pp
|
|
This version of OpenSSH
|
|
.Bl -bullet
|
|
.It
|
|
has all components of a restrictive nature (i.e., patents, see
|
|
.Xr ssl 8 )
|
|
directly removed from the source code; any licensed or patented components
|
|
are chosen from
|
|
external libraries.
|
|
.It
|
|
has been updated to support SSH protocol 1.5 and 2, making it compatible with
|
|
all other SSH clients and servers.
|
|
.It
|
|
contains added support for
|
|
.Xr kerberos 8
|
|
authentication and ticket passing.
|
|
.It
|
|
supports one-time password authentication with
|
|
.Xr skey 1 .
|
|
.El
|
|
.Pp
|
|
OpenSSH has been created by Aaron Campbell, Bob Beck, Markus Friedl,
|
|
Niels Provos, Theo de Raadt, and Dug Song.
|
|
.Pp
|
|
The support for SSH protocol 2 was written by Markus Friedl.
|
|
.Sh SEE ALSO
|
|
.Xr rlogin 1 ,
|
|
.Xr rsh 1 ,
|
|
.Xr scp 1 ,
|
|
.Xr ssh-add 1 ,
|
|
.Xr ssh-agent 1 ,
|
|
.Xr ssh-keygen 1 ,
|
|
.Xr telnet 1 ,
|
|
.Xr sshd 8 ,
|
|
.Xr ssl 8
|