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Damien Miller32aa1441999-10-29 09:15:49 +10001.\" -*- nroff -*-
2.\"
Damien Miller32aa1441999-10-29 09:15:49 +10003.\" Author: Tatu Ylonen <ylo@cs.hut.fi>
Damien Miller32aa1441999-10-29 09:15:49 +10004.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
5.\" All rights reserved
6.\"
Damien Millere4340be2000-09-16 13:29:08 +11007.\" As far as I am concerned, the code I have written for this software
8.\" can be used freely for any purpose. Any derived versions of this
9.\" software must be clearly marked as such, and if the derived work is
10.\" incompatible with the protocol description in the RFC file, it must be
11.\" called by a name other than "ssh" or "Secure Shell".
Damien Miller32aa1441999-10-29 09:15:49 +100012.\"
Ben Lindstrom92a2e382001-03-05 06:59:27 +000013.\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
14.\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
15.\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
Damien Millere4340be2000-09-16 13:29:08 +110016.\"
17.\" Redistribution and use in source and binary forms, with or without
18.\" modification, are permitted provided that the following conditions
19.\" are met:
20.\" 1. Redistributions of source code must retain the above copyright
21.\" notice, this list of conditions and the following disclaimer.
22.\" 2. Redistributions in binary form must reproduce the above copyright
23.\" notice, this list of conditions and the following disclaimer in the
24.\" documentation and/or other materials provided with the distribution.
25.\"
26.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
27.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
28.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
29.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
30.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
31.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
32.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
33.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
34.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
35.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
Damien Miller32aa1441999-10-29 09:15:49 +100036.\"
Darren Tucker6aaa58c2003-08-02 22:24:49 +100037.\" $OpenBSD: ssh.1,v 1.175 2003/07/22 13:35:22 markus Exp $
Damien Miller32aa1441999-10-29 09:15:49 +100038.Dd September 25, 1999
39.Dt SSH 1
40.Os
41.Sh NAME
42.Nm ssh
Ben Lindstromd58eb5f2001-03-07 06:07:22 +000043.Nd OpenSSH SSH client (remote login program)
Damien Miller32aa1441999-10-29 09:15:49 +100044.Sh SYNOPSIS
45.Nm ssh
46.Op Fl l Ar login_name
Ben Lindstroma10aed82001-09-12 16:43:26 +000047.Ar hostname | user@hostname
Damien Miller32aa1441999-10-29 09:15:49 +100048.Op Ar command
49.Pp
50.Nm ssh
Damien Miller495dca32003-04-01 21:42:14 +100051.Bk -words
Damien Miller44e72a72003-05-14 13:42:08 +100052.Op Fl afgknqstvxACNTVX1246
Ben Lindstrome0f88042001-04-30 13:06:24 +000053.Op Fl b Ar bind_address
Damien Miller30c3d422000-05-09 11:02:59 +100054.Op Fl c Ar cipher_spec
Damien Miller32aa1441999-10-29 09:15:49 +100055.Op Fl e Ar escape_char
56.Op Fl i Ar identity_file
57.Op Fl l Ar login_name
Ben Lindstrom06b33aa2001-02-15 03:01:59 +000058.Op Fl m Ar mac_spec
Damien Miller32aa1441999-10-29 09:15:49 +100059.Op Fl o Ar option
60.Op Fl p Ar port
Ben Lindstrom14f31ab2001-09-12 17:48:04 +000061.Op Fl F Ar configfile
Damien Miller32aa1441999-10-29 09:15:49 +100062.Oo Fl L Xo
63.Sm off
Damien Miller32aa1441999-10-29 09:15:49 +100064.Ar port :
Damien Miller396691a2000-01-20 22:44:08 +110065.Ar host :
Damien Miller32aa1441999-10-29 09:15:49 +100066.Ar hostport
67.Sm on
68.Xc
69.Oc
Damien Miller495dca32003-04-01 21:42:14 +100070.Ek
71.Bk -words
Damien Miller32aa1441999-10-29 09:15:49 +100072.Oo Fl R Xo
73.Sm off
Damien Miller32aa1441999-10-29 09:15:49 +100074.Ar port :
Damien Miller396691a2000-01-20 22:44:08 +110075.Ar host :
Damien Miller32aa1441999-10-29 09:15:49 +100076.Ar hostport
77.Sm on
78.Xc
79.Oc
Ben Lindstrom6e69d532001-09-12 17:59:59 +000080.Op Fl D Ar port
Ben Lindstroma10aed82001-09-12 16:43:26 +000081.Ar hostname | user@hostname
Damien Miller32aa1441999-10-29 09:15:49 +100082.Op Ar command
Damien Miller495dca32003-04-01 21:42:14 +100083.Ek
Damien Miller22c77262000-04-13 12:26:34 +100084.Sh DESCRIPTION
Damien Miller32aa1441999-10-29 09:15:49 +100085.Nm
Ben Lindstromd58eb5f2001-03-07 06:07:22 +000086(SSH client) is a program for logging into a remote machine and for
Damien Miller7684ee12000-03-17 23:40:15 +110087executing commands on a remote machine.
88It is intended to replace
Damien Miller32aa1441999-10-29 09:15:49 +100089rlogin and rsh, and provide secure encrypted communications between
Damien Miller7684ee12000-03-17 23:40:15 +110090two untrusted hosts over an insecure network.
91X11 connections and
Damien Miller32aa1441999-10-29 09:15:49 +100092arbitrary TCP/IP ports can also be forwarded over the secure channel.
93.Pp
94.Nm
Damien Miller22c77262000-04-13 12:26:34 +100095connects and logs into the specified
Damien Miller32aa1441999-10-29 09:15:49 +100096.Ar hostname .
97The user must prove
Damien Millere247cc42000-05-07 12:03:14 +100098his/her identity to the remote machine using one of several methods
99depending on the protocol version used:
100.Pp
101.Ss SSH protocol version 1
Damien Miller32aa1441999-10-29 09:15:49 +1000102.Pp
103First, if the machine the user logs in from is listed in
104.Pa /etc/hosts.equiv
105or
Damien Miller886c63a2000-01-20 23:13:36 +1100106.Pa /etc/shosts.equiv
Damien Miller32aa1441999-10-29 09:15:49 +1000107on the remote machine, and the user names are
108the same on both sides, the user is immediately permitted to log in.
Damien Miller22c77262000-04-13 12:26:34 +1000109Second, if
Damien Miller32aa1441999-10-29 09:15:49 +1000110.Pa \&.rhosts
111or
112.Pa \&.shosts
113exists in the user's home directory on the
114remote machine and contains a line containing the name of the client
115machine and the name of the user on that machine, the user is
Damien Miller7684ee12000-03-17 23:40:15 +1100116permitted to log in.
117This form of authentication alone is normally not
Damien Miller32aa1441999-10-29 09:15:49 +1000118allowed by the server because it is not secure.
119.Pp
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000120The second authentication method is the
Damien Miller32aa1441999-10-29 09:15:49 +1000121.Pa rhosts
122or
123.Pa hosts.equiv
Damien Miller7684ee12000-03-17 23:40:15 +1100124method combined with RSA-based host authentication.
125It means that if the login would be permitted by
Damien Millere247cc42000-05-07 12:03:14 +1000126.Pa $HOME/.rhosts ,
127.Pa $HOME/.shosts ,
Damien Miller32aa1441999-10-29 09:15:49 +1000128.Pa /etc/hosts.equiv ,
129or
Damien Miller886c63a2000-01-20 23:13:36 +1100130.Pa /etc/shosts.equiv ,
Damien Miller32aa1441999-10-29 09:15:49 +1000131and if additionally the server can verify the client's
Damien Miller22c77262000-04-13 12:26:34 +1000132host key (see
Damien Miller05eda432002-02-10 18:32:28 +1100133.Pa /etc/ssh/ssh_known_hosts
Damien Miller33e511e1999-11-11 11:43:13 +1100134and
135.Pa $HOME/.ssh/known_hosts
Damien Miller32aa1441999-10-29 09:15:49 +1000136in the
137.Sx FILES
Damien Miller7684ee12000-03-17 23:40:15 +1100138section), only then login is permitted.
139This authentication method closes security holes due to IP
140spoofing, DNS spoofing and routing spoofing.
141[Note to the administrator:
Damien Miller32aa1441999-10-29 09:15:49 +1000142.Pa /etc/hosts.equiv ,
Damien Millere247cc42000-05-07 12:03:14 +1000143.Pa $HOME/.rhosts ,
Damien Miller32aa1441999-10-29 09:15:49 +1000144and the rlogin/rsh protocol in general, are inherently insecure and should be
145disabled if security is desired.]
146.Pp
Damien Miller22c77262000-04-13 12:26:34 +1000147As a third authentication method,
Damien Miller32aa1441999-10-29 09:15:49 +1000148.Nm
149supports RSA based authentication.
150The scheme is based on public-key cryptography: there are cryptosystems
151where encryption and decryption are done using separate keys, and it
152is not possible to derive the decryption key from the encryption key.
Damien Miller7684ee12000-03-17 23:40:15 +1100153RSA is one such system.
Damien Miller22c77262000-04-13 12:26:34 +1000154The idea is that each user creates a public/private
Damien Miller7684ee12000-03-17 23:40:15 +1100155key pair for authentication purposes.
156The server knows the public key, and only the user knows the private key.
Damien Miller22c77262000-04-13 12:26:34 +1000157The file
Damien Miller32aa1441999-10-29 09:15:49 +1000158.Pa $HOME/.ssh/authorized_keys
159lists the public keys that are permitted for logging
Damien Miller7684ee12000-03-17 23:40:15 +1100160in.
161When the user logs in, the
Damien Miller32aa1441999-10-29 09:15:49 +1000162.Nm
163program tells the server which key pair it would like to use for
Damien Miller7684ee12000-03-17 23:40:15 +1100164authentication.
165The server checks if this key is permitted, and if
Damien Miller32aa1441999-10-29 09:15:49 +1000166so, sends the user (actually the
167.Nm
168program running on behalf of the user) a challenge, a random number,
Damien Miller7684ee12000-03-17 23:40:15 +1100169encrypted by the user's public key.
170The challenge can only be
171decrypted using the proper private key.
172The user's client then decrypts the
Damien Miller32aa1441999-10-29 09:15:49 +1000173challenge using the private key, proving that he/she knows the private
174key but without disclosing it to the server.
175.Pp
176.Nm
Damien Miller7684ee12000-03-17 23:40:15 +1100177implements the RSA authentication protocol automatically.
178The user creates his/her RSA key pair by running
Damien Miller32aa1441999-10-29 09:15:49 +1000179.Xr ssh-keygen 1 .
Damien Miller22c77262000-04-13 12:26:34 +1000180This stores the private key in
Damien Millere247cc42000-05-07 12:03:14 +1000181.Pa $HOME/.ssh/identity
Damien Miller32aa1441999-10-29 09:15:49 +1000182and the public key in
Damien Millere247cc42000-05-07 12:03:14 +1000183.Pa $HOME/.ssh/identity.pub
Damien Miller7684ee12000-03-17 23:40:15 +1100184in the user's home directory.
185The user should then copy the
Damien Miller32aa1441999-10-29 09:15:49 +1000186.Pa identity.pub
Damien Miller22c77262000-04-13 12:26:34 +1000187to
Damien Millere247cc42000-05-07 12:03:14 +1000188.Pa $HOME/.ssh/authorized_keys
Damien Miller22c77262000-04-13 12:26:34 +1000189in his/her home directory on the remote machine (the
Damien Miller32aa1441999-10-29 09:15:49 +1000190.Pa authorized_keys
Damien Miller22c77262000-04-13 12:26:34 +1000191file corresponds to the conventional
Damien Millere247cc42000-05-07 12:03:14 +1000192.Pa $HOME/.rhosts
Damien Miller32aa1441999-10-29 09:15:49 +1000193file, and has one key
Damien Miller7684ee12000-03-17 23:40:15 +1100194per line, though the lines can be very long).
195After this, the user can log in without giving the password.
196RSA authentication is much
Damien Miller32aa1441999-10-29 09:15:49 +1000197more secure than rhosts authentication.
198.Pp
199The most convenient way to use RSA authentication may be with an
Damien Miller7684ee12000-03-17 23:40:15 +1100200authentication agent.
201See
Damien Miller32aa1441999-10-29 09:15:49 +1000202.Xr ssh-agent 1
203for more information.
204.Pp
Damien Miller22c77262000-04-13 12:26:34 +1000205If other authentication methods fail,
Damien Miller32aa1441999-10-29 09:15:49 +1000206.Nm
Damien Miller7684ee12000-03-17 23:40:15 +1100207prompts the user for a password.
208The password is sent to the remote
Damien Miller32aa1441999-10-29 09:15:49 +1000209host for checking; however, since all communications are encrypted,
210the password cannot be seen by someone listening on the network.
211.Pp
Damien Millere247cc42000-05-07 12:03:14 +1000212.Ss SSH protocol version 2
213.Pp
Damien Miller07a2d422002-02-05 12:16:15 +1100214When a user connects using protocol version 2
215similar authentication methods are available.
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000216Using the default values for
217.Cm PreferredAuthentications ,
Ben Lindstroma9086a12001-08-06 20:58:51 +0000218the client will try to authenticate first using the hostbased method;
219if this method fails public key authentication is attempted,
220and finally if this method fails keyboard-interactive and
221password authentication are tried.
Damien Millere247cc42000-05-07 12:03:14 +1000222.Pp
223The public key method is similar to RSA authentication described
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000224in the previous section and allows the RSA or DSA algorithm to be used:
Ben Lindstrom18a82ac2001-04-11 15:59:35 +0000225The client uses his private key,
Damien Millere247cc42000-05-07 12:03:14 +1000226.Pa $HOME/.ssh/id_dsa
Ben Lindstrom18a82ac2001-04-11 15:59:35 +0000227or
228.Pa $HOME/.ssh/id_rsa ,
Damien Millere247cc42000-05-07 12:03:14 +1000229to sign the session identifier and sends the result to the server.
230The server checks whether the matching public key is listed in
Ben Lindstromf96704d2001-06-25 04:17:12 +0000231.Pa $HOME/.ssh/authorized_keys
Damien Millere247cc42000-05-07 12:03:14 +1000232and grants access if both the key is found and the signature is correct.
233The session identifier is derived from a shared Diffie-Hellman value
234and is only known to the client and the server.
235.Pp
236If public key authentication fails or is not available a password
237can be sent encrypted to the remote host for proving the user's identity.
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000238.Pp
239Additionally,
240.Nm
241supports hostbased or challenge response authentication.
Damien Millere247cc42000-05-07 12:03:14 +1000242.Pp
243Protocol 2 provides additional mechanisms for confidentiality
Damien Miller30c3d422000-05-09 11:02:59 +1000244(the traffic is encrypted using 3DES, Blowfish, CAST128 or Arcfour)
Ben Lindstromec26fb12001-03-06 01:05:23 +0000245and integrity (hmac-md5, hmac-sha1).
Damien Millere247cc42000-05-07 12:03:14 +1000246Note that protocol 1 lacks a strong mechanism for ensuring the
247integrity of the connection.
248.Pp
249.Ss Login session and remote execution
250.Pp
Damien Miller32aa1441999-10-29 09:15:49 +1000251When the user's identity has been accepted by the server, the server
252either executes the given command, or logs into the machine and gives
Damien Miller7684ee12000-03-17 23:40:15 +1100253the user a normal shell on the remote machine.
254All communication with
Damien Miller32aa1441999-10-29 09:15:49 +1000255the remote command or shell will be automatically encrypted.
256.Pp
257If a pseudo-terminal has been allocated (normal login session), the
Ben Lindstrom9ffdcb52001-04-20 12:50:51 +0000258user may use the escape characters noted below.
Damien Miller32aa1441999-10-29 09:15:49 +1000259.Pp
260If no pseudo tty has been allocated, the
261session is transparent and can be used to reliably transfer binary
Damien Miller7684ee12000-03-17 23:40:15 +1100262data.
263On most systems, setting the escape character to
Damien Miller32aa1441999-10-29 09:15:49 +1000264.Dq none
265will also make the session transparent even if a tty is used.
266.Pp
Ben Lindstroma6885612000-12-09 03:45:32 +0000267The session terminates when the command or shell on the remote
Ben Lindstromebd888d2001-03-05 05:49:29 +0000268machine exits and all X11 and TCP/IP connections have been closed.
Damien Miller32aa1441999-10-29 09:15:49 +1000269The exit status of the remote program is returned as the exit status
270of
271.Nm ssh .
272.Pp
Ben Lindstrom9ffdcb52001-04-20 12:50:51 +0000273.Ss Escape Characters
274.Pp
275When a pseudo terminal has been requested, ssh supports a number of functions
Ben Lindstrom24643222001-06-25 05:08:11 +0000276through the use of an escape character.
Ben Lindstrom9ffdcb52001-04-20 12:50:51 +0000277.Pp
278A single tilde character can be sent as
279.Ic ~~
Ben Lindstrom66007692001-07-22 20:41:59 +0000280or by following the tilde by a character other than those described below.
Ben Lindstrom9ffdcb52001-04-20 12:50:51 +0000281The escape character must always follow a newline to be interpreted as
282special.
283The escape character can be changed in configuration files using the
284.Cm EscapeChar
Ben Lindstrom24643222001-06-25 05:08:11 +0000285configuration directive or on the command line by the
Ben Lindstrom9ffdcb52001-04-20 12:50:51 +0000286.Fl e
287option.
288.Pp
289The supported escapes (assuming the default
290.Ql ~ )
291are:
292.Bl -tag -width Ds
293.It Cm ~.
294Disconnect
295.It Cm ~^Z
296Background ssh
297.It Cm ~#
298List forwarded connections
299.It Cm ~&
300Background ssh at logout when waiting for forwarded connection / X11 sessions
Damien Miller1f8dddc2001-11-12 11:02:25 +1100301to terminate
Ben Lindstrom9ffdcb52001-04-20 12:50:51 +0000302.It Cm ~?
303Display a list of escape characters
Damien Miller54c45982003-05-15 10:20:13 +1000304.It Cm ~B
Damien Miller46a7b402003-05-15 14:17:28 +1000305Send a BREAK to the remote system (only useful for SSH protocol version 2
306and if the peer supports it)
Ben Lindstrom5589f4b2002-03-22 03:24:32 +0000307.It Cm ~C
308Open command line (only useful for adding port forwardings using the
309.Fl L
310and
311.Fl R
312options)
Ben Lindstrom9ffdcb52001-04-20 12:50:51 +0000313.It Cm ~R
Ben Lindstromf8d24572001-04-22 17:11:56 +0000314Request rekeying of the connection (only useful for SSH protocol version 2
315and if the peer supports it)
Ben Lindstrom9ffdcb52001-04-20 12:50:51 +0000316.El
317.Pp
Damien Millere247cc42000-05-07 12:03:14 +1000318.Ss X11 and TCP forwarding
319.Pp
Ben Lindstrom71215072001-05-17 03:16:18 +0000320If the
321.Cm ForwardX11
322variable is set to
323.Dq yes
324(or, see the description of the
325.Fl X
326and
327.Fl x
328options described later)
329and the user is using X11 (the
Damien Miller32aa1441999-10-29 09:15:49 +1000330.Ev DISPLAY
331environment variable is set), the connection to the X11 display is
332automatically forwarded to the remote side in such a way that any X11
333programs started from the shell (or command) will go through the
334encrypted channel, and the connection to the real X server will be made
Damien Miller7684ee12000-03-17 23:40:15 +1100335from the local machine.
336The user should not manually set
Damien Miller32aa1441999-10-29 09:15:49 +1000337.Ev DISPLAY .
338Forwarding of X11 connections can be
339configured on the command line or in configuration files.
340.Pp
341The
Damien Miller22c77262000-04-13 12:26:34 +1000342.Ev DISPLAY
Damien Miller32aa1441999-10-29 09:15:49 +1000343value set by
344.Nm
345will point to the server machine, but with a display number greater
Damien Miller7684ee12000-03-17 23:40:15 +1100346than zero.
347This is normal, and happens because
Damien Miller32aa1441999-10-29 09:15:49 +1000348.Nm
349creates a
350.Dq proxy
351X server on the server machine for forwarding the
352connections over the encrypted channel.
353.Pp
354.Nm
355will also automatically set up Xauthority data on the server machine.
356For this purpose, it will generate a random authorization cookie,
357store it in Xauthority on the server, and verify that any forwarded
358connections carry this cookie and replace it by the real cookie when
Damien Miller7684ee12000-03-17 23:40:15 +1100359the connection is opened.
360The real authentication cookie is never
Damien Miller32aa1441999-10-29 09:15:49 +1000361sent to the server machine (and no cookies are sent in the plain).
362.Pp
Ben Lindstrom3541f182002-08-20 19:03:20 +0000363If the
364.Cm ForwardAgent
365variable is set to
366.Dq yes
367(or, see the description of the
368.Fl A
369and
370.Fl a
Damien Miller495dca32003-04-01 21:42:14 +1000371options described later) and
Ben Lindstrom3541f182002-08-20 19:03:20 +0000372the user is using an authentication agent, the connection to the agent
373is automatically forwarded to the remote side.
Damien Miller32aa1441999-10-29 09:15:49 +1000374.Pp
375Forwarding of arbitrary TCP/IP connections over the secure channel can
Ben Lindstrom979c9812001-07-22 20:45:39 +0000376be specified either on the command line or in a configuration file.
Damien Miller7684ee12000-03-17 23:40:15 +1100377One possible application of TCP/IP forwarding is a secure connection to an
Ben Lindstromebd888d2001-03-05 05:49:29 +0000378electronic purse; another is going through firewalls.
Damien Miller32aa1441999-10-29 09:15:49 +1000379.Pp
Damien Millere247cc42000-05-07 12:03:14 +1000380.Ss Server authentication
381.Pp
Damien Miller32aa1441999-10-29 09:15:49 +1000382.Nm
Damien Millere247cc42000-05-07 12:03:14 +1000383automatically maintains and checks a database containing
Damien Miller7684ee12000-03-17 23:40:15 +1100384identifications for all hosts it has ever been used with.
Ben Lindstromd6481ea2001-06-25 04:37:41 +0000385Host keys are stored in
Damien Millere247cc42000-05-07 12:03:14 +1000386.Pa $HOME/.ssh/known_hosts
Damien Miller7684ee12000-03-17 23:40:15 +1100387in the user's home directory.
Ben Lindstromd6481ea2001-06-25 04:37:41 +0000388Additionally, the file
Damien Miller05eda432002-02-10 18:32:28 +1100389.Pa /etc/ssh/ssh_known_hosts
Ben Lindstromd6481ea2001-06-25 04:37:41 +0000390is automatically checked for known hosts.
Damien Miller7684ee12000-03-17 23:40:15 +1100391Any new hosts are automatically added to the user's file.
392If a host's identification
Damien Miller32aa1441999-10-29 09:15:49 +1000393ever changes,
394.Nm
395warns about this and disables password authentication to prevent a
Damien Miller7684ee12000-03-17 23:40:15 +1100396trojan horse from getting the user's password.
397Another purpose of
Damien Miller32aa1441999-10-29 09:15:49 +1000398this mechanism is to prevent man-in-the-middle attacks which could
Damien Miller7684ee12000-03-17 23:40:15 +1100399otherwise be used to circumvent the encryption.
400The
Damien Miller32aa1441999-10-29 09:15:49 +1000401.Cm StrictHostKeyChecking
Ben Lindstrom9f049032002-06-21 00:59:05 +0000402option can be used to prevent logins to machines whose
Damien Miller32aa1441999-10-29 09:15:49 +1000403host key is not known or has changed.
Damien Miller0bc1bd82000-11-13 22:57:25 +1100404.Pp
405The options are as follows:
Damien Miller32aa1441999-10-29 09:15:49 +1000406.Bl -tag -width Ds
407.It Fl a
Damien Miller450a7a12000-03-26 13:04:51 +1000408Disables forwarding of the authentication agent connection.
Damien Millerb1715dc2000-05-30 13:44:51 +1000409.It Fl A
410Enables forwarding of the authentication agent connection.
411This can also be specified on a per-host basis in a configuration file.
Damien Millerb5fdfaa2002-09-12 09:52:03 +1000412.Pp
Damien Miller495dca32003-04-01 21:42:14 +1000413Agent forwarding should be enabled with caution.
414Users with the ability to bypass file permissions on the remote host
415(for the agent's Unix-domain socket)
416can access the local agent through the forwarded connection.
417An attacker cannot obtain key material from the agent,
Damien Millerb5fdfaa2002-09-12 09:52:03 +1000418however they can perform operations on the keys that enable them to
419authenticate using the identities loaded into the agent.
Ben Lindstrome0f88042001-04-30 13:06:24 +0000420.It Fl b Ar bind_address
421Specify the interface to transmit from on machines with multiple
422interfaces or aliased addresses.
Ben Lindstromffa1dd62001-09-12 16:52:28 +0000423.It Fl c Ar blowfish|3des|des
Damien Miller22c77262000-04-13 12:26:34 +1000424Selects the cipher to use for encrypting the session.
Damien Miller32aa1441999-10-29 09:15:49 +1000425.Ar 3des
Damien Miller7684ee12000-03-17 23:40:15 +1100426is used by default.
Damien Miller22c77262000-04-13 12:26:34 +1000427It is believed to be secure.
Damien Miller32aa1441999-10-29 09:15:49 +1000428.Ar 3des
429(triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
Damien Miller32aa1441999-10-29 09:15:49 +1000430.Ar blowfish
431is a fast block cipher, it appears very secure and is much faster than
Damien Miller7684ee12000-03-17 23:40:15 +1100432.Ar 3des .
Ben Lindstromffa1dd62001-09-12 16:52:28 +0000433.Ar des
434is only supported in the
435.Nm
436client for interoperability with legacy protocol 1 implementations
437that do not support the
438.Ar 3des
Damien Miller495dca32003-04-01 21:42:14 +1000439cipher.
440Its use is strongly discouraged due to cryptographic weaknesses.
Ben Lindstrom5ab6ae12001-02-10 22:08:03 +0000441.It Fl c Ar cipher_spec
Damien Miller30c3d422000-05-09 11:02:59 +1000442Additionally, for protocol version 2 a comma-separated list of ciphers can
Damien Miller874d77b2000-10-14 16:23:11 +1100443be specified in order of preference.
Ben Lindstrom5ab6ae12001-02-10 22:08:03 +0000444See
445.Cm Ciphers
446for more information.
Damien Miller32aa1441999-10-29 09:15:49 +1000447.It Fl e Ar ch|^ch|none
448Sets the escape character for sessions with a pty (default:
449.Ql ~ ) .
Damien Miller7684ee12000-03-17 23:40:15 +1100450The escape character is only recognized at the beginning of a line.
451The escape character followed by a dot
Damien Miller32aa1441999-10-29 09:15:49 +1000452.Pq Ql \&.
453closes the connection, followed
454by control-Z suspends the connection, and followed by itself sends the
Damien Miller7684ee12000-03-17 23:40:15 +1100455escape character once.
456Setting the character to
Damien Miller32aa1441999-10-29 09:15:49 +1000457.Dq none
458disables any escapes and makes the session fully transparent.
459.It Fl f
460Requests
461.Nm
Damien Miller7684ee12000-03-17 23:40:15 +1100462to go to background just before command execution.
463This is useful if
Damien Miller32aa1441999-10-29 09:15:49 +1000464.Nm
465is going to ask for passwords or passphrases, but the user
Damien Miller7684ee12000-03-17 23:40:15 +1100466wants it in the background.
Damien Miller22c77262000-04-13 12:26:34 +1000467This implies
Damien Miller32aa1441999-10-29 09:15:49 +1000468.Fl n .
469The recommended way to start X11 programs at a remote site is with
470something like
471.Ic ssh -f host xterm .
Damien Miller396691a2000-01-20 22:44:08 +1100472.It Fl g
473Allows remote hosts to connect to local forwarded ports.
Damien Miller32aa1441999-10-29 09:15:49 +1000474.It Fl i Ar identity_file
Damien Millerfae23862002-01-22 23:32:26 +1100475Selects a file from which the identity (private key) for
Damien Miller0bc1bd82000-11-13 22:57:25 +1100476RSA or DSA authentication is read.
Damien Millerfae23862002-01-22 23:32:26 +1100477The default is
Damien Millere247cc42000-05-07 12:03:14 +1000478.Pa $HOME/.ssh/identity
Damien Millerfae23862002-01-22 23:32:26 +1100479for protocol version 1, and
480.Pa $HOME/.ssh/id_rsa
481and
482.Pa $HOME/.ssh/id_dsa
483for protocol version 2.
Damien Miller7684ee12000-03-17 23:40:15 +1100484Identity files may also be specified on
485a per-host basis in the configuration file.
486It is possible to have multiple
Damien Miller32aa1441999-10-29 09:15:49 +1000487.Fl i
488options (and multiple identities specified in
489configuration files).
Ben Lindstromae996bf2001-08-06 21:27:53 +0000490.It Fl I Ar smartcard_device
Damien Millerfbf486b2003-05-23 18:44:23 +1000491Specifies which smartcard device to use.
492The argument is the device
Ben Lindstromae996bf2001-08-06 21:27:53 +0000493.Nm
494should use to communicate with a smartcard used for storing the user's
495private RSA key.
Damien Miller32aa1441999-10-29 09:15:49 +1000496.It Fl k
Darren Tucker6aaa58c2003-08-02 22:24:49 +1000497Disables forwarding of Kerberos tickets.
Damien Miller450a7a12000-03-26 13:04:51 +1000498This may also be specified on a per-host basis in the configuration file.
Damien Miller32aa1441999-10-29 09:15:49 +1000499.It Fl l Ar login_name
Damien Miller7684ee12000-03-17 23:40:15 +1100500Specifies the user to log in as on the remote machine.
501This also may be specified on a per-host basis in the configuration file.
Ben Lindstrom06b33aa2001-02-15 03:01:59 +0000502.It Fl m Ar mac_spec
503Additionally, for protocol version 2 a comma-separated list of MAC
504(message authentication code) algorithms can
505be specified in order of preference.
506See the
507.Cm MACs
508keyword for more information.
Damien Miller32aa1441999-10-29 09:15:49 +1000509.It Fl n
510Redirects stdin from
511.Pa /dev/null
512(actually, prevents reading from stdin).
513This must be used when
514.Nm
Damien Miller7684ee12000-03-17 23:40:15 +1100515is run in the background.
516A common trick is to use this to run X11 programs on a remote machine.
517For example,
Damien Miller32aa1441999-10-29 09:15:49 +1000518.Ic ssh -n shadows.cs.hut.fi emacs &
519will start an emacs on shadows.cs.hut.fi, and the X11
520connection will be automatically forwarded over an encrypted channel.
521The
522.Nm
523program will be put in the background.
524(This does not work if
525.Nm
526needs to ask for a password or passphrase; see also the
527.Fl f
528option.)
Damien Millerdcb6ecd2000-05-17 22:34:22 +1000529.It Fl N
530Do not execute a remote command.
Ben Lindstrom594e2032001-09-12 18:35:30 +0000531This is useful for just forwarding ports
Damien Millerdcb6ecd2000-05-17 22:34:22 +1000532(protocol version 2 only).
Damien Miller32aa1441999-10-29 09:15:49 +1000533.It Fl o Ar option
Ben Lindstrom14c62eb2001-08-15 23:25:46 +0000534Can be used to give options in the format used in the configuration file.
Damien Miller32aa1441999-10-29 09:15:49 +1000535This is useful for specifying options for which there is no separate
Damien Miller7684ee12000-03-17 23:40:15 +1100536command-line flag.
Damien Miller32aa1441999-10-29 09:15:49 +1000537.It Fl p Ar port
Damien Miller7684ee12000-03-17 23:40:15 +1100538Port to connect to on the remote host.
539This can be specified on a
Damien Miller32aa1441999-10-29 09:15:49 +1000540per-host basis in the configuration file.
Damien Miller32aa1441999-10-29 09:15:49 +1000541.It Fl q
Damien Miller7684ee12000-03-17 23:40:15 +1100542Quiet mode.
543Causes all warning and diagnostic messages to be suppressed.
Damien Miller832562e2001-01-30 09:30:01 +1100544.It Fl s
Damien Millerfbf486b2003-05-23 18:44:23 +1000545May be used to request invocation of a subsystem on the remote system.
546Subsystems are a feature of the SSH2 protocol which facilitate the use
547of SSH as a secure transport for other applications (eg. sftp).
548The subsystem is specified as the remote command.
Damien Miller32aa1441999-10-29 09:15:49 +1000549.It Fl t
Damien Miller7684ee12000-03-17 23:40:15 +1100550Force pseudo-tty allocation.
Damien Miller450a7a12000-03-26 13:04:51 +1000551This can be used to execute arbitrary
Damien Miller7684ee12000-03-17 23:40:15 +1100552screen-based programs on a remote machine, which can be very useful,
553e.g., when implementing menu services.
Ben Lindstrom4dccfa52000-12-28 16:40:05 +0000554Multiple
555.Fl t
556options force tty allocation, even if
557.Nm
558has no local tty.
Damien Millerdcb6ecd2000-05-17 22:34:22 +1000559.It Fl T
Ben Lindstromc72745a2000-12-02 19:03:54 +0000560Disable pseudo-tty allocation.
Damien Miller32aa1441999-10-29 09:15:49 +1000561.It Fl v
Damien Miller7684ee12000-03-17 23:40:15 +1100562Verbose mode.
563Causes
Damien Miller32aa1441999-10-29 09:15:49 +1000564.Nm
Damien Miller7684ee12000-03-17 23:40:15 +1100565to print debugging messages about its progress.
566This is helpful in
Damien Miller32aa1441999-10-29 09:15:49 +1000567debugging connection, authentication, and configuration problems.
Ben Lindstrom4dccfa52000-12-28 16:40:05 +0000568Multiple
569.Fl v
Damien Miller44e72a72003-05-14 13:42:08 +1000570options increase the verbosity.
571The maximum is 3.
572.It Fl V
573Display the version number and exit.
Damien Miller32aa1441999-10-29 09:15:49 +1000574.It Fl x
Damien Miller7684ee12000-03-17 23:40:15 +1100575Disables X11 forwarding.
Damien Miller32aa1441999-10-29 09:15:49 +1000576.It Fl X
577Enables X11 forwarding.
Damien Millerb1715dc2000-05-30 13:44:51 +1000578This can also be specified on a per-host basis in a configuration file.
Damien Millerb5fdfaa2002-09-12 09:52:03 +1000579.Pp
Damien Miller495dca32003-04-01 21:42:14 +1000580X11 forwarding should be enabled with caution.
581Users with the ability to bypass file permissions on the remote host
582(for the user's X authorization database)
583can access the local X11 display through the forwarded connection.
584An attacker may then be able to perform activities such as keystroke monitoring.
Damien Miller32aa1441999-10-29 09:15:49 +1000585.It Fl C
586Requests compression of all data (including stdin, stdout, stderr, and
Damien Miller7684ee12000-03-17 23:40:15 +1100587data for forwarded X11 and TCP/IP connections).
588The compression algorithm is the same used by
Damien Miller396691a2000-01-20 22:44:08 +1100589.Xr gzip 1 ,
590and the
Damien Miller32aa1441999-10-29 09:15:49 +1000591.Dq level
592can be controlled by the
593.Cm CompressionLevel
Damien Millerd8769622002-09-30 12:00:55 +1000594option for protocol version 1.
Damien Miller7684ee12000-03-17 23:40:15 +1100595Compression is desirable on modem lines and other
Damien Miller32aa1441999-10-29 09:15:49 +1000596slow connections, but will only slow down things on fast networks.
597The default value can be set on a host-by-host basis in the
598configuration files; see the
Ben Lindstrome2b9b062001-08-06 20:50:55 +0000599.Cm Compression
Ben Lindstrom9f049032002-06-21 00:59:05 +0000600option.
Ben Lindstrom14f31ab2001-09-12 17:48:04 +0000601.It Fl F Ar configfile
602Specifies an alternative per-user configuration file.
603If a configuration file is given on the command line,
604the system-wide configuration file
Damien Miller05eda432002-02-10 18:32:28 +1100605.Pq Pa /etc/ssh/ssh_config
Ben Lindstrom14f31ab2001-09-12 17:48:04 +0000606will be ignored.
607The default for the per-user configuration file is
608.Pa $HOME/.ssh/config .
Damien Miller32aa1441999-10-29 09:15:49 +1000609.It Fl L Ar port:host:hostport
610Specifies that the given port on the local (client) host is to be
Damien Miller7684ee12000-03-17 23:40:15 +1100611forwarded to the given host and port on the remote side.
612This works by allocating a socket to listen to
Damien Miller32aa1441999-10-29 09:15:49 +1000613.Ar port
614on the local side, and whenever a connection is made to this port, the
615connection is forwarded over the secure channel, and a connection is
616made to
Damien Miller34132e52000-01-14 15:45:46 +1100617.Ar host
618port
619.Ar hostport
Damien Miller7684ee12000-03-17 23:40:15 +1100620from the remote machine.
621Port forwardings can also be specified in the configuration file.
622Only root can forward privileged ports.
Damien Miller34132e52000-01-14 15:45:46 +1100623IPv6 addresses can be specified with an alternative syntax:
624.Ar port/host/hostport
Damien Miller32aa1441999-10-29 09:15:49 +1000625.It Fl R Ar port:host:hostport
626Specifies that the given port on the remote (server) host is to be
Damien Miller7684ee12000-03-17 23:40:15 +1100627forwarded to the given host and port on the local side.
628This works by allocating a socket to listen to
Damien Miller32aa1441999-10-29 09:15:49 +1000629.Ar port
630on the remote side, and whenever a connection is made to this port, the
631connection is forwarded over the secure channel, and a connection is
632made to
Damien Miller34132e52000-01-14 15:45:46 +1100633.Ar host
634port
635.Ar hostport
Damien Miller7684ee12000-03-17 23:40:15 +1100636from the local machine.
637Port forwardings can also be specified in the configuration file.
638Privileged ports can be forwarded only when
Damien Miller32aa1441999-10-29 09:15:49 +1000639logging in as root on the remote machine.
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000640IPv6 addresses can be specified with an alternative syntax:
641.Ar port/host/hostport
Ben Lindstrom19ceb172001-09-12 17:54:24 +0000642.It Fl D Ar port
643Specifies a local
644.Dq dynamic
645application-level port forwarding.
646This works by allocating a socket to listen to
647.Ar port
648on the local side, and whenever a connection is made to this port, the
649connection is forwarded over the secure channel, and the application
650protocol is then used to determine where to connect to from the
Damien Miller495dca32003-04-01 21:42:14 +1000651remote machine.
Darren Tucker46471c92003-07-03 13:55:19 +1000652Currently the SOCKS4 and SOCKS5 protocols are supported, and
Ben Lindstrom19ceb172001-09-12 17:54:24 +0000653.Nm
Darren Tucker46471c92003-07-03 13:55:19 +1000654will act as a SOCKS server.
Ben Lindstrom19ceb172001-09-12 17:54:24 +0000655Only root can forward privileged ports.
656Dynamic port forwardings can also be specified in the configuration file.
Ben Lindstrom5ab6ae12001-02-10 22:08:03 +0000657.It Fl 1
658Forces
659.Nm
660to try protocol version 1 only.
Damien Miller4af51302000-04-16 11:18:38 +1000661.It Fl 2
662Forces
663.Nm
Damien Millere247cc42000-05-07 12:03:14 +1000664to try protocol version 2 only.
Damien Miller34132e52000-01-14 15:45:46 +1100665.It Fl 4
666Forces
667.Nm
668to use IPv4 addresses only.
669.It Fl 6
670Forces
671.Nm
672to use IPv6 addresses only.
Damien Miller32aa1441999-10-29 09:15:49 +1000673.El
674.Sh CONFIGURATION FILES
675.Nm
Ben Lindstrom9f049032002-06-21 00:59:05 +0000676may additionally obtain configuration data from
677a per-user configuration file and a system-wide configuration file.
678The file format and configuration options are described in
679.Xr ssh_config 5 .
Damien Miller32aa1441999-10-29 09:15:49 +1000680.Sh ENVIRONMENT
681.Nm
682will normally set the following environment variables:
683.Bl -tag -width Ds
684.It Ev DISPLAY
685The
686.Ev DISPLAY
Damien Miller7684ee12000-03-17 23:40:15 +1100687variable indicates the location of the X11 server.
Damien Miller22c77262000-04-13 12:26:34 +1000688It is automatically set by
Damien Miller32aa1441999-10-29 09:15:49 +1000689.Nm
690to point to a value of the form
691.Dq hostname:n
692where hostname indicates
Damien Miller7684ee12000-03-17 23:40:15 +1100693the host where the shell runs, and n is an integer >= 1.
694.Nm
695uses this special value to forward X11 connections over the secure
696channel.
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000697The user should normally not set
698.Ev DISPLAY
699explicitly, as that
Damien Miller32aa1441999-10-29 09:15:49 +1000700will render the X11 connection insecure (and will require the user to
701manually copy any required authorization cookies).
702.It Ev HOME
703Set to the path of the user's home directory.
704.It Ev LOGNAME
705Synonym for
706.Ev USER ;
707set for compatibility with systems that use this variable.
708.It Ev MAIL
Ben Lindstrome59433d2001-09-12 16:41:37 +0000709Set to the path of the user's mailbox.
Damien Miller7684ee12000-03-17 23:40:15 +1100710.It Ev PATH
Damien Miller32aa1441999-10-29 09:15:49 +1000711Set to the default
712.Ev PATH ,
713as specified when compiling
714.Nm ssh .
Ben Lindstrom5bf5d672001-07-04 04:31:38 +0000715.It Ev SSH_ASKPASS
716If
717.Nm
718needs a passphrase, it will read the passphrase from the current
719terminal if it was run from a terminal.
720If
721.Nm
722does not have a terminal associated with it but
723.Ev DISPLAY
724and
725.Ev SSH_ASKPASS
726are set, it will execute the program specified by
727.Ev SSH_ASKPASS
728and open an X11 window to read the passphrase.
729This is particularly useful when calling
730.Nm
731from a
732.Pa .Xsession
733or related script.
734(Note that on some machines it
735may be necessary to redirect the input from
736.Pa /dev/null
737to make this work.)
Damien Miller32aa1441999-10-29 09:15:49 +1000738.It Ev SSH_AUTH_SOCK
Ben Lindstrome59433d2001-09-12 16:41:37 +0000739Identifies the path of a unix-domain socket used to communicate with the
Damien Miller32aa1441999-10-29 09:15:49 +1000740agent.
Damien Millerf37e2462002-09-19 11:47:55 +1000741.It Ev SSH_CONNECTION
742Identifies the client and server ends of the connection.
Damien Miller7684ee12000-03-17 23:40:15 +1100743The variable contains
Damien Millerf37e2462002-09-19 11:47:55 +1000744four space-separated values: client ip-address, client port number,
745server ip-address and server port number.
Ben Lindstrom4dccfa52000-12-28 16:40:05 +0000746.It Ev SSH_ORIGINAL_COMMAND
747The variable contains the original command line if a forced command
748is executed.
749It can be used to extract the original arguments.
Damien Miller32aa1441999-10-29 09:15:49 +1000750.It Ev SSH_TTY
751This is set to the name of the tty (path to the device) associated
Damien Miller7684ee12000-03-17 23:40:15 +1100752with the current shell or command.
753If the current session has no tty,
Damien Miller32aa1441999-10-29 09:15:49 +1000754this variable is not set.
755.It Ev TZ
756The timezone variable is set to indicate the present timezone if it
Damien Miller942da032000-08-18 13:59:06 +1000757was set when the daemon was started (i.e., the daemon passes the value
Damien Miller32aa1441999-10-29 09:15:49 +1000758on to new connections).
759.It Ev USER
760Set to the name of the user logging in.
761.El
762.Pp
Damien Miller22c77262000-04-13 12:26:34 +1000763Additionally,
Damien Miller32aa1441999-10-29 09:15:49 +1000764.Nm
Damien Miller22c77262000-04-13 12:26:34 +1000765reads
766.Pa $HOME/.ssh/environment ,
Damien Miller32aa1441999-10-29 09:15:49 +1000767and adds lines of the format
768.Dq VARNAME=value
Ben Lindstromdc7adf22002-08-20 18:38:02 +0000769to the environment if the file exists and if users are allowed to
770change their environment.
Ben Lindstrombd9bf382002-08-20 18:54:20 +0000771See the
Ben Lindstromdc7adf22002-08-20 18:38:02 +0000772.Cm PermitUserEnvironment
Ben Lindstrombd9bf382002-08-20 18:54:20 +0000773option in
Ben Lindstromdc7adf22002-08-20 18:38:02 +0000774.Xr sshd_config 5 .
Damien Miller32aa1441999-10-29 09:15:49 +1000775.Sh FILES
Damien Miller98c7ad62000-03-09 21:27:49 +1100776.Bl -tag -width Ds
Ben Lindstromd6481ea2001-06-25 04:37:41 +0000777.It Pa $HOME/.ssh/known_hosts
Ben Lindstrome59433d2001-09-12 16:41:37 +0000778Records host keys for all hosts the user has logged into that are not
Damien Miller32aa1441999-10-29 09:15:49 +1000779in
Damien Miller05eda432002-02-10 18:32:28 +1100780.Pa /etc/ssh/ssh_known_hosts .
Damien Miller32aa1441999-10-29 09:15:49 +1000781See
782.Xr sshd 8 .
Ben Lindstrom18a82ac2001-04-11 15:59:35 +0000783.It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa
784Contains the authentication identity of the user.
785They are for protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively.
Damien Millere247cc42000-05-07 12:03:14 +1000786These files
787contain sensitive data and should be readable by the user but not
Damien Miller32aa1441999-10-29 09:15:49 +1000788accessible by others (read/write/execute).
789Note that
790.Nm
Damien Millere247cc42000-05-07 12:03:14 +1000791ignores a private key file if it is accessible by others.
Damien Miller32aa1441999-10-29 09:15:49 +1000792It is possible to specify a passphrase when
793generating the key; the passphrase will be used to encrypt the
794sensitive part of this file using 3DES.
Ben Lindstrom18a82ac2001-04-11 15:59:35 +0000795.It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub
Damien Miller32aa1441999-10-29 09:15:49 +1000796Contains the public key for authentication (public part of the
Damien Miller7684ee12000-03-17 23:40:15 +1100797identity file in human-readable form).
Damien Millere247cc42000-05-07 12:03:14 +1000798The contents of the
799.Pa $HOME/.ssh/identity.pub
800file should be added to
Damien Miller32aa1441999-10-29 09:15:49 +1000801.Pa $HOME/.ssh/authorized_keys
802on all machines
Ben Lindstrom594e2032001-09-12 18:35:30 +0000803where the user wishes to log in using protocol version 1 RSA authentication.
Damien Millere247cc42000-05-07 12:03:14 +1000804The contents of the
805.Pa $HOME/.ssh/id_dsa.pub
Ben Lindstrom18a82ac2001-04-11 15:59:35 +0000806and
807.Pa $HOME/.ssh/id_rsa.pub
Damien Millere247cc42000-05-07 12:03:14 +1000808file should be added to
Ben Lindstromf96704d2001-06-25 04:17:12 +0000809.Pa $HOME/.ssh/authorized_keys
Damien Millere247cc42000-05-07 12:03:14 +1000810on all machines
Ben Lindstrom594e2032001-09-12 18:35:30 +0000811where the user wishes to log in using protocol version 2 DSA/RSA authentication.
Damien Millere247cc42000-05-07 12:03:14 +1000812These files are not
Damien Miller7684ee12000-03-17 23:40:15 +1100813sensitive and can (but need not) be readable by anyone.
Damien Millere247cc42000-05-07 12:03:14 +1000814These files are
Kevin Steves3c034ae2001-02-05 13:47:11 +0000815never used automatically and are not necessary; they are only provided for
Damien Miller32aa1441999-10-29 09:15:49 +1000816the convenience of the user.
817.It Pa $HOME/.ssh/config
Damien Miller7684ee12000-03-17 23:40:15 +1100818This is the per-user configuration file.
Ben Lindstrom9f049032002-06-21 00:59:05 +0000819The file format and configuration options are described in
820.Xr ssh_config 5 .
Damien Miller32aa1441999-10-29 09:15:49 +1000821.It Pa $HOME/.ssh/authorized_keys
Ben Lindstromf96704d2001-06-25 04:17:12 +0000822Lists the public keys (RSA/DSA) that can be used for logging in as this user.
Damien Miller7684ee12000-03-17 23:40:15 +1100823The format of this file is described in the
Damien Miller32aa1441999-10-29 09:15:49 +1000824.Xr sshd 8
Damien Miller7684ee12000-03-17 23:40:15 +1100825manual page.
Ben Lindstromd6481ea2001-06-25 04:37:41 +0000826In the simplest form the format is the same as the .pub
827identity files.
Damien Millere247cc42000-05-07 12:03:14 +1000828This file is not highly sensitive, but the recommended
829permissions are read/write for the user, and not accessible by others.
Damien Miller05eda432002-02-10 18:32:28 +1100830.It Pa /etc/ssh/ssh_known_hosts
Damien Miller7684ee12000-03-17 23:40:15 +1100831Systemwide list of known host keys.
Ben Lindstromd6481ea2001-06-25 04:37:41 +0000832This file should be prepared by the
Damien Miller32aa1441999-10-29 09:15:49 +1000833system administrator to contain the public host keys of all machines in the
Damien Miller7684ee12000-03-17 23:40:15 +1100834organization.
835This file should be world-readable.
836This file contains
Damien Miller32aa1441999-10-29 09:15:49 +1000837public keys, one per line, in the following format (fields separated
Ben Lindstromd6481ea2001-06-25 04:37:41 +0000838by spaces): system name, public key and optional comment field.
Damien Miller7684ee12000-03-17 23:40:15 +1100839When different names are used
Damien Miller32aa1441999-10-29 09:15:49 +1000840for the same machine, all such names should be listed, separated by
Damien Miller7684ee12000-03-17 23:40:15 +1100841commas.
842The format is described on the
Damien Miller32aa1441999-10-29 09:15:49 +1000843.Xr sshd 8
844manual page.
845.Pp
846The canonical system name (as returned by name servers) is used by
847.Xr sshd 8
848to verify the client host when logging in; other names are needed because
849.Nm
850does not convert the user-supplied name to a canonical name before
851checking the key, because someone with access to the name servers
852would then be able to fool host authentication.
Damien Miller05eda432002-02-10 18:32:28 +1100853.It Pa /etc/ssh/ssh_config
Damien Miller7684ee12000-03-17 23:40:15 +1100854Systemwide configuration file.
Ben Lindstrom9f049032002-06-21 00:59:05 +0000855The file format and configuration options are described in
856.Xr ssh_config 5 .
Damien Miller05eda432002-02-10 18:32:28 +1100857.It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key
Damien Miller705499b2001-11-12 11:05:38 +1100858These three files contain the private parts of the host keys
859and are used for
860.Cm RhostsRSAAuthentication
861and
862.Cm HostbasedAuthentication .
Ben Lindstrom5cac4232002-06-11 15:45:02 +0000863If the protocol version 1
864.Cm RhostsRSAAuthentication
Ben Lindstromcb72e4f2002-06-21 00:41:51 +0000865method is used,
Damien Miller705499b2001-11-12 11:05:38 +1100866.Nm
Ben Lindstrom5cac4232002-06-11 15:45:02 +0000867must be setuid root, since the host key is readable only by root.
868For protocol version 2,
869.Nm
870uses
871.Xr ssh-keysign 8
872to access the host keys for
873.Cm HostbasedAuthentication .
874This eliminates the requirement that
875.Nm
876be setuid root when that authentication method is used.
877By default
878.Nm
879is not setuid root.
Damien Miller32aa1441999-10-29 09:15:49 +1000880.It Pa $HOME/.rhosts
881This file is used in
882.Pa \&.rhosts
883authentication to list the
Damien Miller7684ee12000-03-17 23:40:15 +1100884host/user pairs that are permitted to log in.
885(Note that this file is
Damien Miller32aa1441999-10-29 09:15:49 +1000886also used by rlogin and rsh, which makes using this file insecure.)
887Each line of the file contains a host name (in the canonical form
888returned by name servers), and then a user name on that host,
Damien Miller7684ee12000-03-17 23:40:15 +1100889separated by a space.
Ben Lindstromebd888d2001-03-05 05:49:29 +0000890On some machines this file may need to be
Damien Miller32aa1441999-10-29 09:15:49 +1000891world-readable if the user's home directory is on a NFS partition,
892because
893.Xr sshd 8
Damien Miller7684ee12000-03-17 23:40:15 +1100894reads it as root.
895Additionally, this file must be owned by the user,
896and must not have write permissions for anyone else.
897The recommended
Damien Miller32aa1441999-10-29 09:15:49 +1000898permission for most machines is read/write for the user, and not
899accessible by others.
900.Pp
901Note that by default
902.Xr sshd 8
903will be installed so that it requires successful RSA host
Damien Miller7684ee12000-03-17 23:40:15 +1100904authentication before permitting \s+2.\s0rhosts authentication.
Ben Lindstrom594e2032001-09-12 18:35:30 +0000905If the server machine does not have the client's host key in
Damien Miller05eda432002-02-10 18:32:28 +1100906.Pa /etc/ssh/ssh_known_hosts ,
Ben Lindstrom594e2032001-09-12 18:35:30 +0000907it can be stored in
Damien Miller32aa1441999-10-29 09:15:49 +1000908.Pa $HOME/.ssh/known_hosts .
909The easiest way to do this is to
910connect back to the client from the server machine using ssh; this
Damien Millere247cc42000-05-07 12:03:14 +1000911will automatically add the host key to
Damien Miller32aa1441999-10-29 09:15:49 +1000912.Pa $HOME/.ssh/known_hosts .
913.It Pa $HOME/.shosts
914This file is used exactly the same way as
915.Pa \&.rhosts .
916The purpose for
917having this file is to be able to use rhosts authentication with
918.Nm
919without permitting login with
Ben Lindstromc57bbf12002-05-15 21:36:45 +0000920.Nm rlogin
Damien Miller32aa1441999-10-29 09:15:49 +1000921or
922.Xr rsh 1 .
923.It Pa /etc/hosts.equiv
924This file is used during
Damien Millerf1ce5052003-06-11 22:04:39 +1000925.Pa \&.rhosts
926authentication.
Damien Miller7684ee12000-03-17 23:40:15 +1100927It contains
Damien Miller32aa1441999-10-29 09:15:49 +1000928canonical hosts names, one per line (the full format is described on
929the
930.Xr sshd 8
Damien Miller7684ee12000-03-17 23:40:15 +1100931manual page).
932If the client host is found in this file, login is
Damien Miller32aa1441999-10-29 09:15:49 +1000933automatically permitted provided client and server user names are the
Damien Miller7684ee12000-03-17 23:40:15 +1100934same.
935Additionally, successful RSA host authentication is normally
936required.
937This file should only be writable by root.
Damien Miller886c63a2000-01-20 23:13:36 +1100938.It Pa /etc/shosts.equiv
Damien Miller22c77262000-04-13 12:26:34 +1000939This file is processed exactly as
Damien Miller32aa1441999-10-29 09:15:49 +1000940.Pa /etc/hosts.equiv .
941This file may be useful to permit logins using
942.Nm
943but not using rsh/rlogin.
Damien Miller05eda432002-02-10 18:32:28 +1100944.It Pa /etc/ssh/sshrc
Damien Miller32aa1441999-10-29 09:15:49 +1000945Commands in this file are executed by
946.Nm
947when the user logs in just before the user's shell (or command) is started.
948See the
949.Xr sshd 8
950manual page for more information.
951.It Pa $HOME/.ssh/rc
952Commands in this file are executed by
953.Nm
954when the user logs in just before the user's shell (or command) is
955started.
Damien Miller22c77262000-04-13 12:26:34 +1000956See the
Damien Miller32aa1441999-10-29 09:15:49 +1000957.Xr sshd 8
958manual page for more information.
Damien Miller4f0fa561999-12-26 14:24:41 +1100959.It Pa $HOME/.ssh/environment
960Contains additional definitions for environment variables, see section
961.Sx ENVIRONMENT
962above.
Damien Miller7b28dc52000-09-05 13:34:53 +1100963.El
Damien Miller07a2d422002-02-05 12:16:15 +1100964.Sh DIAGNOSTICS
965.Nm
966exits with the exit status of the remote command or with 255
967if an error occurred.
Damien Miller32aa1441999-10-29 09:15:49 +1000968.Sh SEE ALSO
Damien Miller32aa1441999-10-29 09:15:49 +1000969.Xr rsh 1 ,
970.Xr scp 1 ,
Damien Miller33804262001-02-04 23:20:18 +1100971.Xr sftp 1 ,
Damien Miller32aa1441999-10-29 09:15:49 +1000972.Xr ssh-add 1 ,
973.Xr ssh-agent 1 ,
974.Xr ssh-keygen 1 ,
975.Xr telnet 1 ,
Ben Lindstrombf69e3b2002-06-23 00:31:24 +0000976.Xr ssh_config 5 ,
Ben Lindstromc001cd32002-06-23 00:32:11 +0000977.Xr ssh-keysign 8 ,
Ben Lindstrom5ab6ae12001-02-10 22:08:03 +0000978.Xr sshd 8
Ben Lindstrom160ec622001-04-22 17:17:46 +0000979.Rs
980.%A T. Ylonen
981.%A T. Kivinen
982.%A M. Saarinen
983.%A T. Rinne
984.%A S. Lehtinen
985.%T "SSH Protocol Architecture"
Ben Lindstromf1813842002-03-27 17:18:31 +0000986.%N draft-ietf-secsh-architecture-12.txt
987.%D January 2002
Ben Lindstrom160ec622001-04-22 17:17:46 +0000988.%O work in progress material
989.Re
Damien Millerf1ce5052003-06-11 22:04:39 +1000990.Sh AUTHORS
991OpenSSH is a derivative of the original and free
992ssh 1.2.12 release by Tatu Ylonen.
993Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
994Theo de Raadt and Dug Song
995removed many bugs, re-added newer features and
996created OpenSSH.
997Markus Friedl contributed the support for SSH
998protocol versions 1.5 and 2.0.