1071 lines
32 KiB
Groff
1071 lines
32 KiB
Groff
.\" -*- nroff -*-
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.\"
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.\" Author: Tatu Ylonen <ylo@cs.hut.fi>
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.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
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.\" All rights reserved
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.\"
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.\" As far as I am concerned, the code I have written for this software
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.\" can be used freely for any purpose. Any derived versions of this
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.\" software must be clearly marked as such, and if the derived work is
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.\" incompatible with the protocol description in the RFC file, it must be
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.\" called by a name other than "ssh" or "Secure Shell".
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.\"
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.\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
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.\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
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.\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
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.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
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.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
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.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
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.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
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.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
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.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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.\"
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.\" $OpenBSD: ssh.1,v 1.181 2003/12/16 15:49:51 markus Exp $
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.\" $FreeBSD$
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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 SSH client (remote login program)
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.Sh SYNOPSIS
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.Nm ssh
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.Op Fl 1246AaCfgkNnqsTtVvXxY
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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 Ar port
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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 identity_file
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.Bk -words
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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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.Ek
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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 option
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.Bk -words
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.Op Fl p Ar port
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.Ek
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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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.Oo Ar user Ns @ Oc Ns Ar hostname
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.Op Ar command
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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 replace rlogin and rsh,
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and provide secure encrypted communications between
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two untrusted hosts over an insecure network.
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X11 connections and arbitrary TCP/IP ports
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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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(with optional
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.Ar user
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name).
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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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If
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.Ar command
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is specified,
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.Ar command
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is executed on the remote host instead of a login shell.
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.Ss SSH protocol version 1
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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 authentication method is the
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.Em rhosts
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or
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.Em 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 is login 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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.Pp
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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 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 so,
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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 decrypted using the proper private key.
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The user's client then decrypts the challenge using the private key,
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proving that he/she knows the private key
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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 stores 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 more secure than
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.Em rhosts
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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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.Ss SSH protocol version 2
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When a user connects using protocol version 2,
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similar authentication methods are available.
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Using the default values for
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.Cm PreferredAuthentications ,
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the client will try to authenticate first using the hostbased method;
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if this method fails, public key authentication is attempted,
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and finally if this method fails, keyboard-interactive and
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password authentication are 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 and allows the RSA or DSA algorithm to be used:
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The client uses his private key,
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.Pa $HOME/.ssh/id_dsa
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or
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.Pa $HOME/.ssh/id_rsa ,
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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_keys
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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 to prove the user's identity.
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.Pp
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Additionally,
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.Nm
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supports hostbased or challenge response 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-md5, hmac-sha1).
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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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.Ss Login session and remote execution
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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 may use the escape characters noted below.
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.Pp
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If no pseudo-tty has been allocated,
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the session is transparent and can be used to reliably transfer binary 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 on the remote
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machine exits 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 of
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.Nm ssh .
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.Ss Escape Characters
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When a pseudo-terminal has been requested,
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.Nm
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supports a number of functions through the use of an escape character.
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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 below.
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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 using the
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.Cm EscapeChar
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configuration directive or on the command line by the
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.Fl e
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option.
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.Pp
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The supported escapes (assuming the default
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.Ql ~ )
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are:
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.Bl -tag -width Ds
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.It Cm ~.
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Disconnect.
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.It Cm ~^Z
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Background
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.Nm ssh .
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.It Cm ~#
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List forwarded connections.
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.It Cm ~&
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Background
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.Nm
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at logout when waiting for forwarded connection / X11 sessions to terminate.
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.It Cm ~?
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Display a list of escape characters.
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.It Cm ~B
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Send a BREAK to the remote system
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(only useful for SSH protocol version 2 and if the peer supports it).
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.It Cm ~C
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Open command line (only useful for adding port forwardings using the
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.Fl L
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and
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.Fl R
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options).
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.It Cm ~R
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Request rekeying of the connection
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(only useful for SSH protocol version 2 and if the peer supports it).
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.El
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.Ss X11 and TCP forwarding
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If the
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.Cm ForwardX11
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variable is set to
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.Dq yes
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(or see the description of the
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.Fl X
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and
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.Fl x
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options described later)
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and 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 is
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automatically 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 can be
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configured on the command line or in configuration files.
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Take note that X11 forwarding can represent a security hazard.
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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 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
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.Cm ForwardAgent
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variable is set to
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.Dq yes
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(or see the description of the
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.Fl A
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and
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.Fl a
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options described later) and
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the user is using an authentication agent, the connection to the agent
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is automatically forwarded to the remote side.
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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 the 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 through firewalls.
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.Ss Server authentication
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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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Host keys are stored in
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.Pa $HOME/.ssh/known_hosts
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in the user's home directory.
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Additionally, the file
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.Pa /etc/ssh/ssh_known_hosts
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is 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 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 this mechanism is to prevent man-in-the-middle attacks
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which could otherwise be used to circumvent the encryption.
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The
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.Cm StrictHostKeyChecking
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option 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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.Pp
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The options are as follows:
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.Bl -tag -width Ds
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.It Fl 1
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Forces
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.Nm
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to try protocol version 1 only.
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.It Fl 2
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Forces
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.Nm
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to try protocol version 2 only.
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.It Fl 4
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Forces
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.Nm
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to use IPv4 addresses only.
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.It Fl 6
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Forces
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.Nm
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to use IPv6 addresses only.
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.It Fl 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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.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 Unix-domain socket)
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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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.It Fl a
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Disables forwarding of the authentication agent connection.
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.It Fl b Ar bind_address
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Specify the interface to transmit from on machines with multiple
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interfaces or aliased addresses.
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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).
|
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The compression algorithm is the same used by
|
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.Xr gzip 1 ,
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and the
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.Dq level
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can be controlled by the
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.Cm CompressionLevel
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option for protocol version 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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.It Fl c Ar blowfish | 3des | des
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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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.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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.Ar des
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is only supported in the
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.Nm
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client for interoperability with legacy protocol 1 implementations
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that do not support the
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.Ar 3des
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cipher.
|
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Its use is strongly discouraged due to cryptographic weaknesses.
|
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.It Fl c Ar cipher_spec
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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.
|
|
See
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.Cm Ciphers
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for more information.
|
|
.It Fl D Ar port
|
|
Specifies a local
|
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.Dq dynamic
|
|
application-level port forwarding.
|
|
This works by allocating a socket to listen to
|
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.Ar port
|
|
on the local side, and whenever a connection is made to this port, the
|
|
connection is forwarded over the secure channel, and the application
|
|
protocol is then used to determine where to connect to from the
|
|
remote machine.
|
|
Currently the SOCKS4 and SOCKS5 protocols are supported, and
|
|
.Nm
|
|
will act as a SOCKS server.
|
|
Only root can forward privileged ports.
|
|
Dynamic port forwardings can also be specified in the configuration file.
|
|
.It Fl e Ar ch | ^ch | none
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Sets the escape character for sessions with a pty (default:
|
|
.Ql ~ ) .
|
|
The escape character is only recognized at the beginning of a line.
|
|
The escape character followed by a dot
|
|
.Pq Ql \&.
|
|
closes the connection;
|
|
followed by control-Z suspends the connection;
|
|
and followed by itself sends the escape character once.
|
|
Setting the character to
|
|
.Dq none
|
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disables any escapes and makes the session fully transparent.
|
|
.It Fl F Ar configfile
|
|
Specifies an alternative per-user configuration file.
|
|
If a configuration file is given on the command line,
|
|
the system-wide configuration file
|
|
.Pq Pa /etc/ssh/ssh_config
|
|
will be ignored.
|
|
The default for the per-user configuration file is
|
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.Pa $HOME/.ssh/config .
|
|
.It Fl f
|
|
Requests
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.Nm
|
|
to go to background just before command execution.
|
|
This is useful if
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.Nm
|
|
is going to ask for passwords or passphrases, but the user
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|
wants it in the background.
|
|
This implies
|
|
.Fl n .
|
|
The recommended way to start X11 programs at a remote site is with
|
|
something like
|
|
.Ic ssh -f host xterm .
|
|
.It Fl g
|
|
Allows remote hosts to connect to local forwarded ports.
|
|
.It Fl I Ar smartcard_device
|
|
Specifies which smartcard device to use.
|
|
The argument is the device
|
|
.Nm
|
|
should use to communicate with a smartcard used for storing the user's
|
|
private RSA key.
|
|
.It Fl i Ar identity_file
|
|
Selects a file from which the identity (private key) for
|
|
RSA or DSA authentication is read.
|
|
The default is
|
|
.Pa $HOME/.ssh/identity
|
|
for protocol version 1, and
|
|
.Pa $HOME/.ssh/id_rsa
|
|
and
|
|
.Pa $HOME/.ssh/id_dsa
|
|
for protocol version 2.
|
|
Identity files may also be specified on
|
|
a per-host basis in the configuration file.
|
|
It is possible to have multiple
|
|
.Fl i
|
|
options (and multiple identities specified in
|
|
configuration files).
|
|
.It Fl k
|
|
Disables forwarding (delegation) of GSSAPI credentials to the server.
|
|
.It Fl L Xo
|
|
.Sm off
|
|
.Ar port : host : hostport
|
|
.Sm on
|
|
.Xc
|
|
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:
|
|
.Sm off
|
|
.Xo
|
|
.Ar port No / Ar host No /
|
|
.Ar hostport .
|
|
.Xc
|
|
.Sm on
|
|
.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.
|
|
.It Fl m Ar mac_spec
|
|
Additionally, for protocol version 2 a comma-separated list of MAC
|
|
(message authentication code) algorithms can
|
|
be specified in order of preference.
|
|
See the
|
|
.Cm MACs
|
|
keyword for more information.
|
|
.It Fl N
|
|
Do not execute a remote command.
|
|
This is useful for just forwarding ports
|
|
(protocol version 2 only).
|
|
.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.)
|
|
.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 AddressFamily
|
|
.It BatchMode
|
|
.It BindAddress
|
|
.It ChallengeResponseAuthentication
|
|
.It CheckHostIP
|
|
.It Cipher
|
|
.It Ciphers
|
|
.It ClearAllForwardings
|
|
.It Compression
|
|
.It CompressionLevel
|
|
.It ConnectionAttempts
|
|
.It ConnectionTimeout
|
|
.It DynamicForward
|
|
.It EscapeChar
|
|
.It ForwardAgent
|
|
.It ForwardX11
|
|
.It ForwardX11Trusted
|
|
.It GatewayPorts
|
|
.It GlobalKnownHostsFile
|
|
.It GSSAPIAuthentication
|
|
.It GSSAPIDelegateCredentials
|
|
.It Host
|
|
.It HostbasedAuthentication
|
|
.It HostKeyAlgorithms
|
|
.It HostKeyAlias
|
|
.It HostName
|
|
.It IdentityFile
|
|
.It LocalForward
|
|
.It LogLevel
|
|
.It MACs
|
|
.It NoHostAuthenticationForLocalhost
|
|
.It NumberOfPasswordPrompts
|
|
.It PasswordAuthentication
|
|
.It Port
|
|
.It PreferredAuthentications
|
|
.It Protocol
|
|
.It ProxyCommand
|
|
.It PubkeyAuthentication
|
|
.It RemoteForward
|
|
.It RhostsRSAAuthentication
|
|
.It RSAAuthentication
|
|
.It ServerAliveInterval
|
|
.It ServerAliveCountMax
|
|
.It SmartcardDevice
|
|
.It StrictHostKeyChecking
|
|
.It TCPKeepAlive
|
|
.It UsePrivilegedPort
|
|
.It User
|
|
.It UserKnownHostsFile
|
|
.It VerifyHostKeyDNS
|
|
.It XAuthLocation
|
|
.El
|
|
.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.
|
|
.It Fl q
|
|
Quiet mode.
|
|
Causes all warning and diagnostic messages to be suppressed.
|
|
.It Fl R Xo
|
|
.Sm off
|
|
.Ar port : host : hostport
|
|
.Sm on
|
|
.Xc
|
|
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.
|
|
IPv6 addresses can be specified with an alternative syntax:
|
|
.Sm off
|
|
.Xo
|
|
.Ar port No / Ar host No /
|
|
.Ar hostport .
|
|
.Xc
|
|
.Sm on
|
|
.It Fl s
|
|
May be used to request invocation of a subsystem on the remote system.
|
|
Subsystems are a feature of the SSH2 protocol which facilitate the use
|
|
of SSH as a secure transport for other applications (eg.\&
|
|
.Xr sftp 1 ) .
|
|
The subsystem is specified as the remote command.
|
|
.It Fl T
|
|
Disable pseudo-tty allocation.
|
|
.It Fl t
|
|
Force pseudo-tty 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.
|
|
.It Fl V
|
|
Display the version number and exit.
|
|
.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.
|
|
.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.
|
|
.It Fl x
|
|
Disables X11 forwarding.
|
|
.It Fl Y
|
|
Enables trusted X11 forwarding.
|
|
.El
|
|
.Sh CONFIGURATION FILES
|
|
.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 ENVIRONMENT
|
|
.Nm
|
|
will normally set the following environment variables:
|
|
.Bl -tag -width LOGNAME
|
|
.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 \*(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 ssh .
|
|
.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_AUTH_SOCK
|
|
Identifies the path of a unix-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
|
|
The 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 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 if the file exists and if 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
|
|
.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, $HOME/.ssh/id_rsa
|
|
Contains the authentication identity of the user.
|
|
They are for protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively.
|
|
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, $HOME/.ssh/id_rsa.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 the file
|
|
.Pa $HOME/.ssh/authorized_keys
|
|
on all machines
|
|
where the user wishes to log in using protocol version 1 RSA authentication.
|
|
The contents of the
|
|
.Pa $HOME/.ssh/id_dsa.pub
|
|
and
|
|
.Pa $HOME/.ssh/id_rsa.pub
|
|
file should be added to
|
|
.Pa $HOME/.ssh/authorized_keys
|
|
on all machines
|
|
where the user wishes to log in using protocol version 2 DSA/RSA 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 file format and configuration options are described in
|
|
.Xr ssh_config 5 .
|
|
.It Pa $HOME/.ssh/authorized_keys
|
|
Lists the public keys (RSA/DSA) 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
|
|
.Pa .pub
|
|
identity files.
|
|
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
|
|
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.
|
|
This file should be world-readable.
|
|
This file contains
|
|
public keys, one per line, in the following format (fields separated
|
|
by spaces): system name, public key 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 in 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.
|
|
The file format and configuration options are described in
|
|
.Xr ssh_config 5 .
|
|
.It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key
|
|
These three files contain the private parts of the host keys
|
|
and are used for
|
|
.Cm RhostsRSAAuthentication
|
|
and
|
|
.Cm HostbasedAuthentication .
|
|
If the protocol version 1
|
|
.Cm RhostsRSAAuthentication
|
|
method is used,
|
|
.Nm
|
|
must be setuid root, since the host key is readable only by root.
|
|
For protocol version 2,
|
|
.Nm
|
|
uses
|
|
.Xr ssh-keysign 8
|
|
to access the host keys for
|
|
.Cm HostbasedAuthentication .
|
|
This eliminates the requirement that
|
|
.Nm
|
|
be setuid root when that authentication method is used.
|
|
By default
|
|
.Nm
|
|
is not setuid root.
|
|
.It Pa $HOME/.rhosts
|
|
This file is used in
|
|
.Em 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.
|
|
On 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
|
|
.Em rhosts
|
|
authentication.
|
|
If the server machine does not have the client's host key in
|
|
.Pa /etc/ssh/ssh_known_hosts ,
|
|
it can be stored 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
|
|
or
|
|
.Xr rsh 1 .
|
|
.It Pa /etc/hosts.equiv
|
|
This file is used during
|
|
.Em rhosts
|
|
authentication.
|
|
It contains
|
|
canonical hosts names, one per line (the full format is described in 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.
|
|
.El
|
|
.Sh DIAGNOSTICS
|
|
.Nm
|
|
exits with the exit status of the remote command or with 255
|
|
if an error occurred.
|
|
.Sh SEE ALSO
|
|
.Xr gzip 1 ,
|
|
.Xr rsh 1 ,
|
|
.Xr scp 1 ,
|
|
.Xr sftp 1 ,
|
|
.Xr ssh-add 1 ,
|
|
.Xr ssh-agent 1 ,
|
|
.Xr ssh-keygen 1 ,
|
|
.Xr telnet 1 ,
|
|
.Xr hosts.equiv 5 ,
|
|
.Xr ssh_config 5 ,
|
|
.Xr ssh-keysign 8 ,
|
|
.Xr sshd 8
|
|
.Rs
|
|
.%A T. Ylonen
|
|
.%A T. Kivinen
|
|
.%A M. Saarinen
|
|
.%A T. Rinne
|
|
.%A S. Lehtinen
|
|
.%T "SSH Protocol Architecture"
|
|
.%N draft-ietf-secsh-architecture-12.txt
|
|
.%D January 2002
|
|
.%O work in progress material
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.Re
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.Sh AUTHORS
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OpenSSH is a derivative of the original and free
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ssh 1.2.12 release by Tatu Ylonen.
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Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
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Theo de Raadt and Dug Song
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removed many bugs, re-added newer features and
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created OpenSSH.
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Markus Friedl contributed the support for SSH
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protocol versions 1.5 and 2.0.
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