.\" -*- nroff -*-
.\"
.\" Author: Tatu Ylonen <ylo@cs.hut.fi>
.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
.\" All rights reserved
.\"
.\" As far as I am concerned, the code I have written for this software
.\" can be used freely for any purpose. Any derived versions of this
.\" software must be clearly marked as such, and if the derived work is
.\" incompatible with the protocol description in the RFC file, it must be
.\" called by a name other than "ssh" or "Secure Shell".
.\"
.\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
.\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
.\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\" notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\" notice, this list of conditions and the following disclaimer in the
.\" documentation and/or other materials provided with the distribution.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
.\"
.Dd September 25, 1999
.Dt SSH 1
.Os
.Sh NAME
.Nm ssh
.Nd OpenSSH secure shell client (remote login program)
.Sh SYNOPSIS
.Nm ssh
.Op Fl l Ar login_name
.Op Ar hostname | user@hostname
.Op Ar command
.Pp
.Nm ssh
.Op Fl afgknqtvxACNPTX246
.Op Fl c Ar cipher_spec
.Op Fl e Ar escape_char
.Op Fl i Ar identity_file
.Op Fl l Ar login_name
.Op Fl o Ar option
.Op Fl p Ar port
.Oo Fl L Xo
.Sm off
.Ar port :
.Ar host :
.Ar hostport
.Sm on
.Xc
.Oc
.Oo Fl R Xo
.Sm off
.Ar port :
.Ar host :
.Ar hostport
.Sm on
.Xc
.Oc
.Op Ar hostname | user@hostname
.Op Ar command
.Sh DESCRIPTION
.Nm
(Secure Shell) is a program for logging into a remote machine and for
executing commands on a remote machine.
It is intended to replace
rlogin and rsh, and provide secure encrypted communications between
two untrusted hosts over an insecure network.
X11 connections and
arbitrary TCP/IP ports can also be forwarded over the secure channel.
.Pp
.Nm
connects and logs into the specified
.Ar hostname .
The user must prove
his/her identity to the remote machine using one of several methods
depending on the protocol version used:
.Pp
.Ss SSH protocol version 1
.Pp
First, if the machine the user logs in from is listed in
.Pa /etc/hosts.equiv
or
.Pa /etc/shosts.equiv
on the remote machine, and the user names are
the same on both sides, the user is immediately permitted to log in.
Second, if
.Pa \&.rhosts
or
.Pa \&.shosts
exists in the user's home directory on the
remote machine and contains a line containing the name of the client
machine and the name of the user on that machine, the user is
permitted to log in.
This form of authentication alone is normally not
allowed by the server because it is not secure.
.Pp
The second (and primary) authentication method is the
.Pa rhosts
or
.Pa hosts.equiv
method combined with RSA-based host authentication.
It means that if the login would be permitted by
.Pa $HOME/.rhosts ,
.Pa $HOME/.shosts ,
.Pa /etc/hosts.equiv ,
or
.Pa /etc/shosts.equiv ,
and if additionally the server can verify the client's
host key (see
.Pa /etc/ssh_known_hosts
and
.Pa $HOME/.ssh/known_hosts
in the
.Sx FILES
section), only then login is permitted.
This authentication method closes security holes due to IP
spoofing, DNS spoofing and routing spoofing.
[Note to the administrator:
.Pa /etc/hosts.equiv ,
.Pa $HOME/.rhosts ,
and the rlogin/rsh protocol in general, are inherently insecure and should be
disabled if security is desired.]
.Pp
As a third authentication method,
.Nm
supports RSA based authentication.
The scheme is based on public-key cryptography: there are cryptosystems
where encryption and decryption are done using separate keys, and it
is not possible to derive the decryption key from the encryption key.
RSA is one such system.
The idea is that each user creates a public/private
key pair for authentication purposes.
The server knows the public key, and only the user knows the private key.
The file
.Pa $HOME/.ssh/authorized_keys
lists the public keys that are permitted for logging
in.
When the user logs in, the
.Nm
program tells the server which key pair it wou