path: root/ssh.1

.\"  -*- 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.
.\"
.\" $OpenBSD: ssh.1,v 1.167 2002/09/27 15:46:21 stevesk Exp $
.Dd September 25, 1999
.Dt SSH 1
.Os
.Sh NAME
.Nm ssh
.Nd OpenSSH SSH client (remote login program)
.Sh SYNOPSIS
.Nm ssh
.Op Fl l Ar login_name
.Ar hostname | user@hostname
.Op Ar command
.Pp
.Nm ssh
.Op Fl afgknqstvxACNTX1246
.Op Fl b Ar bind_address
.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 m Ar mac_spec
.Op Fl o Ar option
.Op Fl p Ar port
.Op Fl F Ar configfile
.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 Fl D Ar port
.Ar hostname | user@hostname
.Op Ar command
.Sh DESCRIPTION
.Nm
(SSH client) 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 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/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 protocol version 2
similar authentication methods are available.
Using the default values for
.Cm PreferredAuthentications ,
the client will try to authenticate first using the hostbased method;
if this method fails public key authentication is attempted,
and finally if this method fails keyboard-interactive and
password authentication are tried.
.Pp
The public key method is similar to RSA authentication described
in the previous section and allows the RSA or DSA algorithm to be used:
The client uses his private key,
.Pa $HOME/.ssh/id_dsa
or
.Pa $HOME/.ssh/id_rsa ,
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_keys
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.
.Pp
Additionally,
.Nm
supports hostbased or challenge response authentication.
.Pp
Protocol 2 provides additional mechanisms for confidentiality
(the traffic is encrypted using 3DES, Blowfish, CAST128 or Arcfour)
and integrity (hmac-md5, hmac-sha1).
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 the server, the server
either executes the given command, or logs into the machine and gives
the user a normal shell on the remote machine.
All communication with
the remote command or shell will be automatically encrypted.
.Pp
If a pseudo-terminal has been allocated (normal login session), the
user may use the escape characters noted below.
.Pp
If no pseudo tty has been allocated, the
session is transparent and can be used to reliably transfer binary
data.
On most systems, setting the escape character to
.Dq none
will also make the session transparent even if a tty is used.
.Pp
The session terminates when the command or shell on the remote
machine exits and all X11 and TCP/IP connections have been closed.
The exit status of the remote program is returned as the exit status
of
.Nm ssh .
.Pp
.Ss Escape Characters
.Pp
When a pseudo terminal has been requested, ssh supports a number of functions
through the use of an escape character.
.Pp
A single tilde character can be sent as
.Ic ~~
or by following the tilde by a character other than those described below.
The escape character must always follow a newline to be interpreted as
special.
The escape character can be changed in configuration files using the
.Cm EscapeChar
configuration directive or on the command line by the
.Fl e
option.
.Pp
The supported escapes (assuming the default
.Ql ~ )
are:
.Bl -tag -width Ds
.It Cm ~.
Disconnect
.It Cm ~^Z
Background ssh
.It Cm ~#
List forwarded connections
.It Cm ~&
Background ssh at logout when waiting for forwarded connection / X11 sessions
to terminate
.It Cm ~?
Display a list of escape characters
.It Cm ~C
Open command line (only useful for adding port forwardings using the
.Fl L
and
.Fl R
options)
.It Cm ~R
Request rekeying of the connection (only useful for SSH protocol version 2
and if the peer supports it)
.El
.Pp
.Ss X11 and TCP forwarding
.Pp
If the
.Cm ForwardX11
variable is set to
.Dq yes
(or, see the description of the
.Fl X
and
.Fl x
options described later)
and the user is using X11 (the
.Ev DISPLAY
environment variable is set), the connection to the X11 display is
automatically forwarded to the remote side in such a way that any X11
programs started from the shell (or command) will go through the
encrypted channel, and the connection to the real X server will be made
from the local machine.
The user should not manually set
.Ev DISPLAY .
Forwarding of X11 connections can be
configured on the command line or in configuration files.
.Pp
The
.Ev DISPLAY
value set by
.Nm
will point to the server machine, but with a display number greater
than zero.
This is normal, and happens because
.Nm
creates a
.Dq proxy
X server on the server machine for forwarding the
connections over the encrypted channel.
.Pp
.Nm
will also automatically set up Xauthority data on the server machine.
For this purpose, it will generate a random authorization cookie,
store it in Xauthority on the server, and verify that any forwarded
connections carry this cookie and replace it by the real cookie when
the connection is opened.
The real authentication cookie is never
sent to the server machine (and no cookies are sent in the plain).
.Pp
If the
.Cm ForwardAgent
variable is set to
.Dq yes
(or, see the description of the
.Fl