path: root/ssh.1

.\"  -*- 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 1.1.1.1 1999/10/27 03:42:45 damien 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 afgknqtvxCPX
.Op Fl c Ar blowfish | 3des
.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 host :
.Ar port :
.Ar hostport
.Sm on
.Xc
.Oc
.Oo Fl R Xo
.Sm off
.Ar host :
.Ar port :
.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.
.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 \&.rhosts ,
.Pa \&.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
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 \&.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 \&.ssh/identity
and the public key in
.Pa \&.ssh/identity.pub
in the user's home directory.  The user should then
copy the 
.Pa identity.pub
to 
.Pa \&.ssh/authorized_keys
in his/her home directory on the remote machine (the 
.Pa authorized_keys
file corresponds to the conventional 
.Pa \&.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
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 can disconnect with
.Ic ~. ,
and suspend
.Nm
with
.Ic ~^Z .
All forwarded connections can be listed with
.Ic ~# 
and if
the session blocks waiting for forwarded X11 or TCP/IP
connections to terminate, it can be backgrounded with
.Ic ~&
(this should not be used while the user shell is active, as it can cause the
shell to hang).  All available escapes can be listed with
.Ic ~? .
.Pp
A single tilde character can be sent as
.Ic ~~
(or by following the tilde by a character other than those described above).
The escape character must always follow a newline to be interpreted as
special.  The escape character can be changed in configuration files
or on the command line.  
.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 in on the remote
machine exists 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
If 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 user is using an authentication agent, the connection to the agent
is automatically forwarded to the remote side unless disabled on
command line or in a configuration file.
.Pp
Forwarding of arbitrary TCP/IP connections over the secure channel can
be specified either on command line or in a configuration file.  One
possible application of TCP/IP forwarding is a secure connection to an
electronic purse; another is going trough firewalls.
.Pp
.Nm
automatically maintains and checks a database containing RSA-based
identifications for all hosts it has ever been used with.  The
database is stored in 
.Pa \&.ssh/known_hosts
in the user's home directory.  Additionally, the file 
.Pa /etc/ssh_known_hosts
is automatically checked for known hosts.  Any new hosts are
automatically added to the user's file.  If a host's identification
ever changes,
.Nm
warns about this and disables password authentication to prevent a
trojan horse from getting the user's password.  Another purpose of
this mechanism is to prevent man-in-the-middle attacks which could
otherwise be used to circumvent the encryption.  The
.Cm StrictHostKeyChecking
option (see below) can be used to prevent logins to machines whose
host key is not known or has changed.
.Sh OPTIONS
.Bl -tag -width Ds
.It Fl a
Disables forwarding of the authentication agent connection. This may
also be specified on a per-host basis in the configuration file.
.It Fl c Ar blowfish|3des
Selects the cipher to use for encrypting the session. 
.Ar 3des
is used by default.  It is believed to be secure. 
.Ar 3des
(triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
It is presumably more secure than the
.Ar des
cipher which is no longer supported in ssh.
.Ar blowfish
is a fast block cipher, it appears very secure and is much faster than
.Ar 3des .  
.It Fl e Ar ch|^ch|none
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
disables any escapes and makes the session fully transparent.
.It Fl f
Requests
.Nm
to go to background after authentication.  This is useful
if
.Nm
is going to ask for passwords or passphrases, but the user
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 i Ar identity_file
Selects the file from which the identity (private key) for 
RSA authentication is read.  Default is 
.Pa \&.ssh/identity
in the user's home directory.  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 g
Allows remote hosts to connect to local forwarded ports.
.It Fl k
Disables forwarding of Kerberos tickets and AFS tokens. This may
also be specified on a per-host basis in the configuration file.
.It Fl l Ar login_name
Specifies the user to log in as on the remote machine.  This may also
be specified on a per-host basis in the configuration file.
.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 in 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 config file.
This is useful for specifying options for which there is no separate
command-line flag.  The option has the same format as a line in the
configuration file.
.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 P
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 of
.Cm RhostsAuthentication
and
.Cm RhostsRSAAuthentication .
.It Fl q
Quiet mode.  Causes all warning and diagnostic messages to be
suppressed.  Only fatal errors are displayed.
.It Fl t
Force pseudo-tty allocation.  This can be used to execute arbitary
screen-based programs on a remote machine, which can be very useful
e.g. when implementing menu services.
.It Fl v
Verbose mode.  Causes
.Nm
to print debugging messages about its progress.  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
Disables X11 forwarding.  This can also be specified on a per-host
basis in a configuration file.
.It Fl X
Enables X11 forwarding.
.It Fl