.\" -*- nroff -*-
.\"
.\" ssh.1.in
.\"
.\" Author: Tatu Ylonen <ylo@cs.hut.fi>
.\"
.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
.\" All rights reserved
.\"
.\" Created: Sat Apr 22 21:55:14 1995 ylo
.\"
.\" $Id: ssh.1,v 1.28 2000/06/07 09:55:44 djm Exp $
.\"
.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 would like to use for
authentication.
The server checks if this key is permitted, and if
so, sends the user (actually the
.Nm
program running on behalf of the user) a challenge, a random number,
encrypted by the user's public key.
The challenge can only be
decrypted using the proper private key.
The user's client then decrypts the
challenge using the private key, proving that he/she knows the private
key but without disclosing it to the server.
.Pp
.Nm
implements the RSA authentication protocol automatically.
The user creates his/her RSA key pair by running
.Xr ssh-keygen 1 .
This stores the private key in
.Pa $HOME/.ssh/identity
and the public key in
.Pa $HOME/.ssh/identity.pub
in the user's home directory.
The user should then copy the
.Pa identity.pub
to
.Pa $HOME/.ssh/authorized_keys
in his/her home directory on the remote machine (the
.Pa authorized_keys
file corresponds to the conventional
.Pa $HOME/.rhosts
file, and has one key
per line, though the lines can be very long).
After this, the user can log in without giving the password.
RSA authentication is much
more secure than rhosts authentication.
.Pp
The most convenient way to use RSA authentication may be with an
authentication agent.
See
.Xr ssh-agent 1
for more information.
.Pp
If other authentication methods fail,
.Nm
prompts the user for a password.
The password is sent to the remote
host for checking; however, since all communications are encrypted,
the password cannot be seen by someone listening on the network.
.Pp
.Ss SSH protocol version 2
.Pp
When a user connects using the protocol version 2
different authentication methods are available:
At first, the client attempts to authenticate using the public key method.
If this method fails password authentication is tried.
.Pp
The public key method is similar to RSA authentication described
in the previous section except that the DSA algorithm is used
instead of the patented RSA algorithm.
The client uses his private DSA key
.Pa $HOME/.ssh/id_dsa
to sign the session identifier and sends the result to the server.
The server checks whether the matching public key is listed in
.Pa $HOME/.ssh/authorized_keys2
and grants access if both the key is found and the signature is correct.
The session identifier is derived from a shared Diffie-Hellman value
and is only known to the client and the server.
.Pp
If public key authentication fails or is not available a password
can be sent encrypted to the remote host for proving the user's identity.
This protocol 2 implementation does not yet support Kerberos or
S/Key authentication.
.Pp
Protocol 2 provides additional mechanisms for confidentiality
(the traffic is encrypted using 3DES, Blowfish, CAST128 or Arcfour)
and integrity (hmac-sha1, hmac-md5).
Note that protocol 1 lacks a strong mechanism for ensuring the
integrity of the connection.
.Pp
.Ss Login session and remote execution
.Pp
When the user's identity has been accepted by th