blob: b6fe0550f42f73493f90e5b011da59ba1b004333 [file] [log] [blame]
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.\"
Ben Lindstrom020a8692001-08-06 21:38:10 +000037.\" $OpenBSD: ssh.1,v 1.126 2001/08/01 22:16:45 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
47.Op Ar hostname | user@hostname
48.Op Ar command
49.Pp
50.Nm ssh
Ben Lindstrom5ab6ae12001-02-10 22:08:03 +000051.Op Fl afgknqstvxACNPTX1246
Ben Lindstrome0f88042001-04-30 13:06:24 +000052.Op Fl b Ar bind_address
Damien Miller30c3d422000-05-09 11:02:59 +100053.Op Fl c Ar cipher_spec
Damien Miller32aa1441999-10-29 09:15:49 +100054.Op Fl e Ar escape_char
55.Op Fl i Ar identity_file
56.Op Fl l Ar login_name
Ben Lindstrom06b33aa2001-02-15 03:01:59 +000057.Op Fl m Ar mac_spec
Damien Miller32aa1441999-10-29 09:15:49 +100058.Op Fl o Ar option
59.Op Fl p Ar port
60.Oo Fl L Xo
61.Sm off
Damien Miller32aa1441999-10-29 09:15:49 +100062.Ar port :
Damien Miller396691a2000-01-20 22:44:08 +110063.Ar host :
Damien Miller32aa1441999-10-29 09:15:49 +100064.Ar hostport
65.Sm on
66.Xc
67.Oc
68.Oo Fl R Xo
69.Sm off
Damien Miller32aa1441999-10-29 09:15:49 +100070.Ar port :
Damien Miller396691a2000-01-20 22:44:08 +110071.Ar host :
Damien Miller32aa1441999-10-29 09:15:49 +100072.Ar hostport
73.Sm on
74.Xc
75.Oc
76.Op Ar hostname | user@hostname
77.Op Ar command
Damien Miller22c77262000-04-13 12:26:34 +100078.Sh DESCRIPTION
Damien Miller32aa1441999-10-29 09:15:49 +100079.Nm
Ben Lindstromd58eb5f2001-03-07 06:07:22 +000080(SSH client) is a program for logging into a remote machine and for
Damien Miller7684ee12000-03-17 23:40:15 +110081executing commands on a remote machine.
82It is intended to replace
Damien Miller32aa1441999-10-29 09:15:49 +100083rlogin and rsh, and provide secure encrypted communications between
Damien Miller7684ee12000-03-17 23:40:15 +110084two untrusted hosts over an insecure network.
85X11 connections and
Damien Miller32aa1441999-10-29 09:15:49 +100086arbitrary TCP/IP ports can also be forwarded over the secure channel.
87.Pp
88.Nm
Damien Miller22c77262000-04-13 12:26:34 +100089connects and logs into the specified
Damien Miller32aa1441999-10-29 09:15:49 +100090.Ar hostname .
91The user must prove
Damien Millere247cc42000-05-07 12:03:14 +100092his/her identity to the remote machine using one of several methods
93depending on the protocol version used:
94.Pp
95.Ss SSH protocol version 1
Damien Miller32aa1441999-10-29 09:15:49 +100096.Pp
97First, if the machine the user logs in from is listed in
98.Pa /etc/hosts.equiv
99or
Damien Miller886c63a2000-01-20 23:13:36 +1100100.Pa /etc/shosts.equiv
Damien Miller32aa1441999-10-29 09:15:49 +1000101on the remote machine, and the user names are
102the same on both sides, the user is immediately permitted to log in.
Damien Miller22c77262000-04-13 12:26:34 +1000103Second, if
Damien Miller32aa1441999-10-29 09:15:49 +1000104.Pa \&.rhosts
105or
106.Pa \&.shosts
107exists in the user's home directory on the
108remote machine and contains a line containing the name of the client
109machine and the name of the user on that machine, the user is
Damien Miller7684ee12000-03-17 23:40:15 +1100110permitted to log in.
111This form of authentication alone is normally not
Damien Miller32aa1441999-10-29 09:15:49 +1000112allowed by the server because it is not secure.
113.Pp
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000114The second authentication method is the
Damien Miller32aa1441999-10-29 09:15:49 +1000115.Pa rhosts
116or
117.Pa hosts.equiv
Damien Miller7684ee12000-03-17 23:40:15 +1100118method combined with RSA-based host authentication.
119It means that if the login would be permitted by
Damien Millere247cc42000-05-07 12:03:14 +1000120.Pa $HOME/.rhosts ,
121.Pa $HOME/.shosts ,
Damien Miller32aa1441999-10-29 09:15:49 +1000122.Pa /etc/hosts.equiv ,
123or
Damien Miller886c63a2000-01-20 23:13:36 +1100124.Pa /etc/shosts.equiv ,
Damien Miller32aa1441999-10-29 09:15:49 +1000125and if additionally the server can verify the client's
Damien Miller22c77262000-04-13 12:26:34 +1000126host key (see
Damien Miller886c63a2000-01-20 23:13:36 +1100127.Pa /etc/ssh_known_hosts
Damien Miller33e511e1999-11-11 11:43:13 +1100128and
129.Pa $HOME/.ssh/known_hosts
Damien Miller32aa1441999-10-29 09:15:49 +1000130in the
131.Sx FILES
Damien Miller7684ee12000-03-17 23:40:15 +1100132section), only then login is permitted.
133This authentication method closes security holes due to IP
134spoofing, DNS spoofing and routing spoofing.
135[Note to the administrator:
Damien Miller32aa1441999-10-29 09:15:49 +1000136.Pa /etc/hosts.equiv ,
Damien Millere247cc42000-05-07 12:03:14 +1000137.Pa $HOME/.rhosts ,
Damien Miller32aa1441999-10-29 09:15:49 +1000138and the rlogin/rsh protocol in general, are inherently insecure and should be
139disabled if security is desired.]
140.Pp
Damien Miller22c77262000-04-13 12:26:34 +1000141As a third authentication method,
Damien Miller32aa1441999-10-29 09:15:49 +1000142.Nm
143supports RSA based authentication.
144The scheme is based on public-key cryptography: there are cryptosystems
145where encryption and decryption are done using separate keys, and it
146is not possible to derive the decryption key from the encryption key.
Damien Miller7684ee12000-03-17 23:40:15 +1100147RSA is one such system.
Damien Miller22c77262000-04-13 12:26:34 +1000148The idea is that each user creates a public/private
Damien Miller7684ee12000-03-17 23:40:15 +1100149key pair for authentication purposes.
150The server knows the public key, and only the user knows the private key.
Damien Miller22c77262000-04-13 12:26:34 +1000151The file
Damien Miller32aa1441999-10-29 09:15:49 +1000152.Pa $HOME/.ssh/authorized_keys
153lists the public keys that are permitted for logging
Damien Miller7684ee12000-03-17 23:40:15 +1100154in.
155When the user logs in, the
Damien Miller32aa1441999-10-29 09:15:49 +1000156.Nm
157program tells the server which key pair it would like to use for
Damien Miller7684ee12000-03-17 23:40:15 +1100158authentication.
159The server checks if this key is permitted, and if
Damien Miller32aa1441999-10-29 09:15:49 +1000160so, sends the user (actually the
161.Nm
162program running on behalf of the user) a challenge, a random number,
Damien Miller7684ee12000-03-17 23:40:15 +1100163encrypted by the user's public key.
164The challenge can only be
165decrypted using the proper private key.
166The user's client then decrypts the
Damien Miller32aa1441999-10-29 09:15:49 +1000167challenge using the private key, proving that he/she knows the private
168key but without disclosing it to the server.
169.Pp
170.Nm
Damien Miller7684ee12000-03-17 23:40:15 +1100171implements the RSA authentication protocol automatically.
172The user creates his/her RSA key pair by running
Damien Miller32aa1441999-10-29 09:15:49 +1000173.Xr ssh-keygen 1 .
Damien Miller22c77262000-04-13 12:26:34 +1000174This stores the private key in
Damien Millere247cc42000-05-07 12:03:14 +1000175.Pa $HOME/.ssh/identity
Damien Miller32aa1441999-10-29 09:15:49 +1000176and the public key in
Damien Millere247cc42000-05-07 12:03:14 +1000177.Pa $HOME/.ssh/identity.pub
Damien Miller7684ee12000-03-17 23:40:15 +1100178in the user's home directory.
179The user should then copy the
Damien Miller32aa1441999-10-29 09:15:49 +1000180.Pa identity.pub
Damien Miller22c77262000-04-13 12:26:34 +1000181to
Damien Millere247cc42000-05-07 12:03:14 +1000182.Pa $HOME/.ssh/authorized_keys
Damien Miller22c77262000-04-13 12:26:34 +1000183in his/her home directory on the remote machine (the
Damien Miller32aa1441999-10-29 09:15:49 +1000184.Pa authorized_keys
Damien Miller22c77262000-04-13 12:26:34 +1000185file corresponds to the conventional
Damien Millere247cc42000-05-07 12:03:14 +1000186.Pa $HOME/.rhosts
Damien Miller32aa1441999-10-29 09:15:49 +1000187file, and has one key
Damien Miller7684ee12000-03-17 23:40:15 +1100188per line, though the lines can be very long).
189After this, the user can log in without giving the password.
190RSA authentication is much
Damien Miller32aa1441999-10-29 09:15:49 +1000191more secure than rhosts authentication.
192.Pp
193The most convenient way to use RSA authentication may be with an
Damien Miller7684ee12000-03-17 23:40:15 +1100194authentication agent.
195See
Damien Miller32aa1441999-10-29 09:15:49 +1000196.Xr ssh-agent 1
197for more information.
198.Pp
Damien Miller22c77262000-04-13 12:26:34 +1000199If other authentication methods fail,
Damien Miller32aa1441999-10-29 09:15:49 +1000200.Nm
Damien Miller7684ee12000-03-17 23:40:15 +1100201prompts the user for a password.
202The password is sent to the remote
Damien Miller32aa1441999-10-29 09:15:49 +1000203host for checking; however, since all communications are encrypted,
204the password cannot be seen by someone listening on the network.
205.Pp
Damien Millere247cc42000-05-07 12:03:14 +1000206.Ss SSH protocol version 2
207.Pp
208When a user connects using the protocol version 2
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000209different authentication methods are available.
210Using the default values for
211.Cm PreferredAuthentications ,
Ben Lindstroma9086a12001-08-06 20:58:51 +0000212the client will try to authenticate first using the hostbased method;
213if this method fails public key authentication is attempted,
214and finally if this method fails keyboard-interactive and
215password authentication are tried.
Damien Millere247cc42000-05-07 12:03:14 +1000216.Pp
217The public key method is similar to RSA authentication described
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000218in the previous section and allows the RSA or DSA algorithm to be used:
Ben Lindstrom18a82ac2001-04-11 15:59:35 +0000219The client uses his private key,
Damien Millere247cc42000-05-07 12:03:14 +1000220.Pa $HOME/.ssh/id_dsa
Ben Lindstrom18a82ac2001-04-11 15:59:35 +0000221or
222.Pa $HOME/.ssh/id_rsa ,
Damien Millere247cc42000-05-07 12:03:14 +1000223to sign the session identifier and sends the result to the server.
224The server checks whether the matching public key is listed in
Ben Lindstromf96704d2001-06-25 04:17:12 +0000225.Pa $HOME/.ssh/authorized_keys
Damien Millere247cc42000-05-07 12:03:14 +1000226and grants access if both the key is found and the signature is correct.
227The session identifier is derived from a shared Diffie-Hellman value
228and is only known to the client and the server.
229.Pp
230If public key authentication fails or is not available a password
231can be sent encrypted to the remote host for proving the user's identity.
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000232.Pp
233Additionally,
234.Nm
235supports hostbased or challenge response authentication.
Damien Millere247cc42000-05-07 12:03:14 +1000236.Pp
237Protocol 2 provides additional mechanisms for confidentiality
Damien Miller30c3d422000-05-09 11:02:59 +1000238(the traffic is encrypted using 3DES, Blowfish, CAST128 or Arcfour)
Ben Lindstromec26fb12001-03-06 01:05:23 +0000239and integrity (hmac-md5, hmac-sha1).
Damien Millere247cc42000-05-07 12:03:14 +1000240Note that protocol 1 lacks a strong mechanism for ensuring the
241integrity of the connection.
242.Pp
243.Ss Login session and remote execution
244.Pp
Damien Miller32aa1441999-10-29 09:15:49 +1000245When the user's identity has been accepted by the server, the server
246either executes the given command, or logs into the machine and gives
Damien Miller7684ee12000-03-17 23:40:15 +1100247the user a normal shell on the remote machine.
248All communication with
Damien Miller32aa1441999-10-29 09:15:49 +1000249the remote command or shell will be automatically encrypted.
250.Pp
251If a pseudo-terminal has been allocated (normal login session), the
Ben Lindstrom9ffdcb52001-04-20 12:50:51 +0000252user may use the escape characters noted below.
Damien Miller32aa1441999-10-29 09:15:49 +1000253.Pp
254If no pseudo tty has been allocated, the
255session is transparent and can be used to reliably transfer binary
Damien Miller7684ee12000-03-17 23:40:15 +1100256data.
257On most systems, setting the escape character to
Damien Miller32aa1441999-10-29 09:15:49 +1000258.Dq none
259will also make the session transparent even if a tty is used.
260.Pp
Ben Lindstroma6885612000-12-09 03:45:32 +0000261The session terminates when the command or shell on the remote
Ben Lindstromebd888d2001-03-05 05:49:29 +0000262machine exits and all X11 and TCP/IP connections have been closed.
Damien Miller32aa1441999-10-29 09:15:49 +1000263The exit status of the remote program is returned as the exit status
264of
265.Nm ssh .
266.Pp
Ben Lindstrom9ffdcb52001-04-20 12:50:51 +0000267.Ss Escape Characters
268.Pp
269When a pseudo terminal has been requested, ssh supports a number of functions
Ben Lindstrom24643222001-06-25 05:08:11 +0000270through the use of an escape character.
Ben Lindstrom9ffdcb52001-04-20 12:50:51 +0000271.Pp
272A single tilde character can be sent as
273.Ic ~~
Ben Lindstrom66007692001-07-22 20:41:59 +0000274or by following the tilde by a character other than those described below.
Ben Lindstrom9ffdcb52001-04-20 12:50:51 +0000275The escape character must always follow a newline to be interpreted as
276special.
277The escape character can be changed in configuration files using the
278.Cm EscapeChar
Ben Lindstrom24643222001-06-25 05:08:11 +0000279configuration directive or on the command line by the
Ben Lindstrom9ffdcb52001-04-20 12:50:51 +0000280.Fl e
281option.
282.Pp
283The supported escapes (assuming the default
284.Ql ~ )
285are:
286.Bl -tag -width Ds
287.It Cm ~.
288Disconnect
289.It Cm ~^Z
290Background ssh
291.It Cm ~#
292List forwarded connections
293.It Cm ~&
294Background ssh at logout when waiting for forwarded connection / X11 sessions
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000295to terminate (protocol version 1 only)
Ben Lindstrom9ffdcb52001-04-20 12:50:51 +0000296.It Cm ~?
297Display a list of escape characters
298.It Cm ~R
Ben Lindstromf8d24572001-04-22 17:11:56 +0000299Request rekeying of the connection (only useful for SSH protocol version 2
300and if the peer supports it)
Ben Lindstrom9ffdcb52001-04-20 12:50:51 +0000301.El
302.Pp
Damien Millere247cc42000-05-07 12:03:14 +1000303.Ss X11 and TCP forwarding
304.Pp
Ben Lindstrom71215072001-05-17 03:16:18 +0000305If the
306.Cm ForwardX11
307variable is set to
308.Dq yes
309(or, see the description of the
310.Fl X
311and
312.Fl x
313options described later)
314and the user is using X11 (the
Damien Miller32aa1441999-10-29 09:15:49 +1000315.Ev DISPLAY
316environment variable is set), the connection to the X11 display is
317automatically forwarded to the remote side in such a way that any X11
318programs started from the shell (or command) will go through the
319encrypted channel, and the connection to the real X server will be made
Damien Miller7684ee12000-03-17 23:40:15 +1100320from the local machine.
321The user should not manually set
Damien Miller32aa1441999-10-29 09:15:49 +1000322.Ev DISPLAY .
323Forwarding of X11 connections can be
324configured on the command line or in configuration files.
325.Pp
326The
Damien Miller22c77262000-04-13 12:26:34 +1000327.Ev DISPLAY
Damien Miller32aa1441999-10-29 09:15:49 +1000328value set by
329.Nm
330will point to the server machine, but with a display number greater
Damien Miller7684ee12000-03-17 23:40:15 +1100331than zero.
332This is normal, and happens because
Damien Miller32aa1441999-10-29 09:15:49 +1000333.Nm
334creates a
335.Dq proxy
336X server on the server machine for forwarding the
337connections over the encrypted channel.
338.Pp
339.Nm
340will also automatically set up Xauthority data on the server machine.
341For this purpose, it will generate a random authorization cookie,
342store it in Xauthority on the server, and verify that any forwarded
343connections carry this cookie and replace it by the real cookie when
Damien Miller7684ee12000-03-17 23:40:15 +1100344the connection is opened.
345The real authentication cookie is never
Damien Miller32aa1441999-10-29 09:15:49 +1000346sent to the server machine (and no cookies are sent in the plain).
347.Pp
348If the user is using an authentication agent, the connection to the agent
349is automatically forwarded to the remote side unless disabled on
Ben Lindstrom979c9812001-07-22 20:45:39 +0000350the command line or in a configuration file.
Damien Miller32aa1441999-10-29 09:15:49 +1000351.Pp
352Forwarding of arbitrary TCP/IP connections over the secure channel can
Ben Lindstrom979c9812001-07-22 20:45:39 +0000353be specified either on the command line or in a configuration file.
Damien Miller7684ee12000-03-17 23:40:15 +1100354One possible application of TCP/IP forwarding is a secure connection to an
Ben Lindstromebd888d2001-03-05 05:49:29 +0000355electronic purse; another is going through firewalls.
Damien Miller32aa1441999-10-29 09:15:49 +1000356.Pp
Damien Millere247cc42000-05-07 12:03:14 +1000357.Ss Server authentication
358.Pp
Damien Miller32aa1441999-10-29 09:15:49 +1000359.Nm
Damien Millere247cc42000-05-07 12:03:14 +1000360automatically maintains and checks a database containing
Damien Miller7684ee12000-03-17 23:40:15 +1100361identifications for all hosts it has ever been used with.
Ben Lindstromd6481ea2001-06-25 04:37:41 +0000362Host keys are stored in
Damien Millere247cc42000-05-07 12:03:14 +1000363.Pa $HOME/.ssh/known_hosts
Damien Miller7684ee12000-03-17 23:40:15 +1100364in the user's home directory.
Ben Lindstromd6481ea2001-06-25 04:37:41 +0000365Additionally, the file
Damien Miller886c63a2000-01-20 23:13:36 +1100366.Pa /etc/ssh_known_hosts
Ben Lindstromd6481ea2001-06-25 04:37:41 +0000367is automatically checked for known hosts.
Damien Miller7684ee12000-03-17 23:40:15 +1100368Any new hosts are automatically added to the user's file.
369If a host's identification
Damien Miller32aa1441999-10-29 09:15:49 +1000370ever changes,
371.Nm
372warns about this and disables password authentication to prevent a
Damien Miller7684ee12000-03-17 23:40:15 +1100373trojan horse from getting the user's password.
374Another purpose of
Damien Miller32aa1441999-10-29 09:15:49 +1000375this mechanism is to prevent man-in-the-middle attacks which could
Damien Miller7684ee12000-03-17 23:40:15 +1100376otherwise be used to circumvent the encryption.
377The
Damien Miller32aa1441999-10-29 09:15:49 +1000378.Cm StrictHostKeyChecking
379option (see below) can be used to prevent logins to machines whose
380host key is not known or has changed.
Damien Miller0bc1bd82000-11-13 22:57:25 +1100381.Pp
382The options are as follows:
Damien Miller32aa1441999-10-29 09:15:49 +1000383.Bl -tag -width Ds
384.It Fl a
Damien Miller450a7a12000-03-26 13:04:51 +1000385Disables forwarding of the authentication agent connection.
Damien Millerb1715dc2000-05-30 13:44:51 +1000386.It Fl A
387Enables forwarding of the authentication agent connection.
388This can also be specified on a per-host basis in a configuration file.
Ben Lindstrome0f88042001-04-30 13:06:24 +0000389.It Fl b Ar bind_address
390Specify the interface to transmit from on machines with multiple
391interfaces or aliased addresses.
Damien Miller32aa1441999-10-29 09:15:49 +1000392.It Fl c Ar blowfish|3des
Damien Miller22c77262000-04-13 12:26:34 +1000393Selects the cipher to use for encrypting the session.
Damien Miller32aa1441999-10-29 09:15:49 +1000394.Ar 3des
Damien Miller7684ee12000-03-17 23:40:15 +1100395is used by default.
Damien Miller22c77262000-04-13 12:26:34 +1000396It is believed to be secure.
Damien Miller32aa1441999-10-29 09:15:49 +1000397.Ar 3des
398(triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
399It is presumably more secure than the
400.Ar des
Damien Miller69b69aa2000-10-28 14:19:58 +1100401cipher which is no longer fully supported in
Damien Miller30c3d422000-05-09 11:02:59 +1000402.Nm ssh .
Damien Miller32aa1441999-10-29 09:15:49 +1000403.Ar blowfish
404is a fast block cipher, it appears very secure and is much faster than
Damien Miller7684ee12000-03-17 23:40:15 +1100405.Ar 3des .
Ben Lindstrom5ab6ae12001-02-10 22:08:03 +0000406.It Fl c Ar cipher_spec
Damien Miller30c3d422000-05-09 11:02:59 +1000407Additionally, for protocol version 2 a comma-separated list of ciphers can
Damien Miller874d77b2000-10-14 16:23:11 +1100408be specified in order of preference.
Ben Lindstrom5ab6ae12001-02-10 22:08:03 +0000409See
410.Cm Ciphers
411for more information.
Damien Miller32aa1441999-10-29 09:15:49 +1000412.It Fl e Ar ch|^ch|none
413Sets the escape character for sessions with a pty (default:
414.Ql ~ ) .
Damien Miller7684ee12000-03-17 23:40:15 +1100415The escape character is only recognized at the beginning of a line.
416The escape character followed by a dot
Damien Miller32aa1441999-10-29 09:15:49 +1000417.Pq Ql \&.
418closes the connection, followed
419by control-Z suspends the connection, and followed by itself sends the
Damien Miller7684ee12000-03-17 23:40:15 +1100420escape character once.
421Setting the character to
Damien Miller32aa1441999-10-29 09:15:49 +1000422.Dq none
423disables any escapes and makes the session fully transparent.
424.It Fl f
425Requests
426.Nm
Damien Miller7684ee12000-03-17 23:40:15 +1100427to go to background just before command execution.
428This is useful if
Damien Miller32aa1441999-10-29 09:15:49 +1000429.Nm
430is going to ask for passwords or passphrases, but the user
Damien Miller7684ee12000-03-17 23:40:15 +1100431wants it in the background.
Damien Miller22c77262000-04-13 12:26:34 +1000432This implies
Damien Miller32aa1441999-10-29 09:15:49 +1000433.Fl n .
434The recommended way to start X11 programs at a remote site is with
435something like
436.Ic ssh -f host xterm .
Damien Miller396691a2000-01-20 22:44:08 +1100437.It Fl g
438Allows remote hosts to connect to local forwarded ports.
Damien Miller32aa1441999-10-29 09:15:49 +1000439.It Fl i Ar identity_file
Damien Miller22c77262000-04-13 12:26:34 +1000440Selects the file from which the identity (private key) for
Damien Miller0bc1bd82000-11-13 22:57:25 +1100441RSA or DSA authentication is read.
Damien Miller22c77262000-04-13 12:26:34 +1000442Default is
Damien Millere247cc42000-05-07 12:03:14 +1000443.Pa $HOME/.ssh/identity
Damien Miller7684ee12000-03-17 23:40:15 +1100444in the user's home directory.
445Identity files may also be specified on
446a per-host basis in the configuration file.
447It is possible to have multiple
Damien Miller32aa1441999-10-29 09:15:49 +1000448.Fl i
449options (and multiple identities specified in
450configuration files).
Ben Lindstromae996bf2001-08-06 21:27:53 +0000451.It Fl I Ar smartcard_device
452Specifies which smartcard device to use. The argument is
453the device
454.Nm
455should use to communicate with a smartcard used for storing the user's
456private RSA key.
Damien Miller32aa1441999-10-29 09:15:49 +1000457.It Fl k
Damien Miller450a7a12000-03-26 13:04:51 +1000458Disables forwarding of Kerberos tickets and AFS tokens.
459This may also be specified on a per-host basis in the configuration file.
Damien Miller32aa1441999-10-29 09:15:49 +1000460.It Fl l Ar login_name
Damien Miller7684ee12000-03-17 23:40:15 +1100461Specifies the user to log in as on the remote machine.
462This also may be specified on a per-host basis in the configuration file.
Ben Lindstrom06b33aa2001-02-15 03:01:59 +0000463.It Fl m Ar mac_spec
464Additionally, for protocol version 2 a comma-separated list of MAC
465(message authentication code) algorithms can
466be specified in order of preference.
467See the
468.Cm MACs
469keyword for more information.
Damien Miller32aa1441999-10-29 09:15:49 +1000470.It Fl n
471Redirects stdin from
472.Pa /dev/null
473(actually, prevents reading from stdin).
474This must be used when
475.Nm
Damien Miller7684ee12000-03-17 23:40:15 +1100476is run in the background.
477A common trick is to use this to run X11 programs on a remote machine.
478For example,
Damien Miller32aa1441999-10-29 09:15:49 +1000479.Ic ssh -n shadows.cs.hut.fi emacs &
480will start an emacs on shadows.cs.hut.fi, and the X11
481connection will be automatically forwarded over an encrypted channel.
482The
483.Nm
484program will be put in the background.
485(This does not work if
486.Nm
487needs to ask for a password or passphrase; see also the
488.Fl f
489option.)
Damien Millerdcb6ecd2000-05-17 22:34:22 +1000490.It Fl N
491Do not execute a remote command.
Ben Lindstromc72745a2000-12-02 19:03:54 +0000492This is useful if you just want to forward ports
Damien Millerdcb6ecd2000-05-17 22:34:22 +1000493(protocol version 2 only).
Damien Miller32aa1441999-10-29 09:15:49 +1000494.It Fl o Ar option
495Can be used to give options in the format used in the config file.
496This is useful for specifying options for which there is no separate
Damien Miller7684ee12000-03-17 23:40:15 +1100497command-line flag.
498The option has the same format as a line in the configuration file.
Damien Miller32aa1441999-10-29 09:15:49 +1000499.It Fl p Ar port
Damien Miller7684ee12000-03-17 23:40:15 +1100500Port to connect to on the remote host.
501This can be specified on a
Damien Miller32aa1441999-10-29 09:15:49 +1000502per-host basis in the configuration file.
503.It Fl P
504Use a non-privileged port for outgoing connections.
505This can be used if your firewall does
506not permit connections from privileged ports.
Damien Millera34a28b1999-12-14 10:47:15 +1100507Note that this option turns off
Damien Miller32aa1441999-10-29 09:15:49 +1000508.Cm RhostsAuthentication
509and
Kevin Stevesfcec7f82000-12-15 19:55:48 +0000510.Cm RhostsRSAAuthentication
511for older servers.
Damien Miller32aa1441999-10-29 09:15:49 +1000512.It Fl q
Damien Miller7684ee12000-03-17 23:40:15 +1100513Quiet mode.
514Causes all warning and diagnostic messages to be suppressed.
515Only fatal errors are displayed.
Damien Miller832562e2001-01-30 09:30:01 +1100516.It Fl s
Ben Lindstrom24643222001-06-25 05:08:11 +0000517May be used to request invocation of a subsystem on the remote system. Subsystems are a feature of the SSH2 protocol which facilitate the use
518of SSH as a secure transport for other applications (eg. sftp). The
Damien Miller832562e2001-01-30 09:30:01 +1100519subsystem is specified as the remote command.
Damien Miller32aa1441999-10-29 09:15:49 +1000520.It Fl t
Damien Miller7684ee12000-03-17 23:40:15 +1100521Force pseudo-tty allocation.
Damien Miller450a7a12000-03-26 13:04:51 +1000522This can be used to execute arbitrary
Damien Miller7684ee12000-03-17 23:40:15 +1100523screen-based programs on a remote machine, which can be very useful,
524e.g., when implementing menu services.
Ben Lindstrom4dccfa52000-12-28 16:40:05 +0000525Multiple
526.Fl t
527options force tty allocation, even if
528.Nm
529has no local tty.
Damien Millerdcb6ecd2000-05-17 22:34:22 +1000530.It Fl T
Ben Lindstromc72745a2000-12-02 19:03:54 +0000531Disable pseudo-tty allocation.
Damien Miller32aa1441999-10-29 09:15:49 +1000532.It Fl v
Damien Miller7684ee12000-03-17 23:40:15 +1100533Verbose mode.
534Causes
Damien Miller32aa1441999-10-29 09:15:49 +1000535.Nm
Damien Miller7684ee12000-03-17 23:40:15 +1100536to print debugging messages about its progress.
537This is helpful in
Damien Miller32aa1441999-10-29 09:15:49 +1000538debugging connection, authentication, and configuration problems.
Ben Lindstrom4dccfa52000-12-28 16:40:05 +0000539Multiple
540.Fl v
541options increases the verbosity.
Damien Miller874d77b2000-10-14 16:23:11 +1100542Maximum is 3.
Damien Miller32aa1441999-10-29 09:15:49 +1000543.It Fl x
Damien Miller7684ee12000-03-17 23:40:15 +1100544Disables X11 forwarding.
Damien Miller32aa1441999-10-29 09:15:49 +1000545.It Fl X
546Enables X11 forwarding.
Damien Millerb1715dc2000-05-30 13:44:51 +1000547This can also be specified on a per-host basis in a configuration file.
Damien Miller32aa1441999-10-29 09:15:49 +1000548.It Fl C
549Requests compression of all data (including stdin, stdout, stderr, and
Damien Miller7684ee12000-03-17 23:40:15 +1100550data for forwarded X11 and TCP/IP connections).
551The compression algorithm is the same used by
Damien Miller396691a2000-01-20 22:44:08 +1100552.Xr gzip 1 ,
553and the
Damien Miller32aa1441999-10-29 09:15:49 +1000554.Dq level
555can be controlled by the
556.Cm CompressionLevel
Damien Miller7684ee12000-03-17 23:40:15 +1100557option (see below).
558Compression is desirable on modem lines and other
Damien Miller32aa1441999-10-29 09:15:49 +1000559slow connections, but will only slow down things on fast networks.
560The default value can be set on a host-by-host basis in the
561configuration files; see the
Ben Lindstrome2b9b062001-08-06 20:50:55 +0000562.Cm Compression
Damien Miller32aa1441999-10-29 09:15:49 +1000563option below.
564.It Fl L Ar port:host:hostport
565Specifies that the given port on the local (client) host is to be
Damien Miller7684ee12000-03-17 23:40:15 +1100566forwarded to the given host and port on the remote side.
567This works by allocating a socket to listen to
Damien Miller32aa1441999-10-29 09:15:49 +1000568.Ar port
569on the local side, and whenever a connection is made to this port, the
570connection is forwarded over the secure channel, and a connection is
571made to
Damien Miller34132e52000-01-14 15:45:46 +1100572.Ar host
573port
574.Ar hostport
Damien Miller7684ee12000-03-17 23:40:15 +1100575from the remote machine.
576Port forwardings can also be specified in the configuration file.
577Only root can forward privileged ports.
Damien Miller34132e52000-01-14 15:45:46 +1100578IPv6 addresses can be specified with an alternative syntax:
579.Ar port/host/hostport
Damien Miller32aa1441999-10-29 09:15:49 +1000580.It Fl R Ar port:host:hostport
581Specifies that the given port on the remote (server) host is to be
Damien Miller7684ee12000-03-17 23:40:15 +1100582forwarded to the given host and port on the local side.
583This works by allocating a socket to listen to
Damien Miller32aa1441999-10-29 09:15:49 +1000584.Ar port
585on the remote side, and whenever a connection is made to this port, the
586connection is forwarded over the secure channel, and a connection is
587made to
Damien Miller34132e52000-01-14 15:45:46 +1100588.Ar host
589port
590.Ar hostport
Damien Miller7684ee12000-03-17 23:40:15 +1100591from the local machine.
592Port forwardings can also be specified in the configuration file.
593Privileged ports can be forwarded only when
Damien Miller32aa1441999-10-29 09:15:49 +1000594logging in as root on the remote machine.
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000595IPv6 addresses can be specified with an alternative syntax:
596.Ar port/host/hostport
Ben Lindstrom5ab6ae12001-02-10 22:08:03 +0000597.It Fl 1
598Forces
599.Nm
600to try protocol version 1 only.
Damien Miller4af51302000-04-16 11:18:38 +1000601.It Fl 2
602Forces
603.Nm
Damien Millere247cc42000-05-07 12:03:14 +1000604to try protocol version 2 only.
Damien Miller34132e52000-01-14 15:45:46 +1100605.It Fl 4
606Forces
607.Nm
608to use IPv4 addresses only.
609.It Fl 6
610Forces
611.Nm
612to use IPv6 addresses only.
Damien Miller32aa1441999-10-29 09:15:49 +1000613.El
614.Sh CONFIGURATION FILES
615.Nm
616obtains configuration data from the following sources (in this order):
617command line options, user's configuration file
618.Pq Pa $HOME/.ssh/config ,
619and system-wide configuration file
Damien Miller886c63a2000-01-20 23:13:36 +1100620.Pq Pa /etc/ssh_config .
Damien Miller32aa1441999-10-29 09:15:49 +1000621For each parameter, the first obtained value
Damien Miller7684ee12000-03-17 23:40:15 +1100622will be used.
623The configuration files contain sections bracketed by
624.Dq Host
625specifications, and that section is only applied for hosts that
626match one of the patterns given in the specification.
627The matched host name is the one given on the command line.
Damien Miller32aa1441999-10-29 09:15:49 +1000628.Pp
629Since the first obtained value for each parameter is used, more
630host-specific declarations should be given near the beginning of the
631file, and general defaults at the end.
632.Pp
633The configuration file has the following format:
634.Pp
635Empty lines and lines starting with
636.Ql #
637are comments.
638.Pp
639Otherwise a line is of the format
640.Dq keyword arguments .
641The possible
642keywords and their meanings are as follows (note that the
643configuration files are case-sensitive):
644.Bl -tag -width Ds
645.It Cm Host
646Restricts the following declarations (up to the next
647.Cm Host
648keyword) to be only for those hosts that match one of the patterns
649given after the keyword.
650.Ql \&*
651and
652.Ql ?
653can be used as wildcards in the
Damien Miller7684ee12000-03-17 23:40:15 +1100654patterns.
655A single
Damien Miller32aa1441999-10-29 09:15:49 +1000656.Ql \&*
657as a pattern can be used to provide global
Damien Miller7684ee12000-03-17 23:40:15 +1100658defaults for all hosts.
659The host is the
Damien Miller32aa1441999-10-29 09:15:49 +1000660.Ar hostname
661argument given on the command line (i.e., the name is not converted to
662a canonicalized host name before matching).
663.It Cm AFSTokenPassing
Damien Miller450a7a12000-03-26 13:04:51 +1000664Specifies whether to pass AFS tokens to remote host.
665The argument to this keyword must be
Damien Miller32aa1441999-10-29 09:15:49 +1000666.Dq yes
667or
668.Dq no .
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000669This option applies to protocol version 1 only.
Damien Miller32aa1441999-10-29 09:15:49 +1000670.It Cm BatchMode
671If set to
672.Dq yes ,
Damien Miller7684ee12000-03-17 23:40:15 +1100673passphrase/password querying will be disabled.
674This option is useful in scripts and other batch jobs where you have no
675user to supply the password.
676The argument must be
Damien Miller32aa1441999-10-29 09:15:49 +1000677.Dq yes
678or
679.Dq no .
Ben Lindstrom89b0bd62001-03-29 00:27:11 +0000680The default is
681.Dq no .
Ben Lindstrome0f88042001-04-30 13:06:24 +0000682.It Cm BindAddress
683Specify the interface to transmit from on machines with multiple
684interfaces or aliased addresses.
685Note that this option does not work if
686.Cm UsePrivilegedPort
687is set to
688.Dq yes .
Damien Miller396691a2000-01-20 22:44:08 +1100689.It Cm CheckHostIP
690If this flag is set to
691.Dq yes ,
Ben Lindstrom89b0bd62001-03-29 00:27:11 +0000692ssh will additionally check the host IP address in the
Damien Miller396691a2000-01-20 22:44:08 +1100693.Pa known_hosts
Damien Miller450a7a12000-03-26 13:04:51 +1000694file.
695This allows ssh to detect if a host key changed due to DNS spoofing.
Damien Miller396691a2000-01-20 22:44:08 +1100696If the option is set to
697.Dq no ,
698the check will not be executed.
Ben Lindstrom89b0bd62001-03-29 00:27:11 +0000699The default is
700.Dq yes .
Damien Miller32aa1441999-10-29 09:15:49 +1000701.It Cm Cipher
Damien Miller50a41ed2000-10-16 12:14:42 +1100702Specifies the cipher to use for encrypting the session
Damien Miller69b69aa2000-10-28 14:19:58 +1100703in protocol version 1.
Damien Miller7684ee12000-03-17 23:40:15 +1100704Currently,
Ben Lindstrom24643222001-06-25 05:08:11 +0000705.Dq blowfish
Damien Miller32aa1441999-10-29 09:15:49 +1000706and
707.Dq 3des
Damien Miller7684ee12000-03-17 23:40:15 +1100708are supported.
709The default is
Damien Miller32aa1441999-10-29 09:15:49 +1000710.Dq 3des .
Damien Miller22c77262000-04-13 12:26:34 +1000711.It Cm Ciphers
712Specifies the ciphers allowed for protocol version 2
713in order of preference.
714Multiple ciphers must be comma-separated.
715The default is
Ben Lindstrom5ab6ae12001-02-10 22:08:03 +0000716.Pp
717.Bd -literal
Ben Lindstromec26fb12001-03-06 01:05:23 +0000718 ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000719 aes192-cbc,aes256-cbc''
Ben Lindstrom5ab6ae12001-02-10 22:08:03 +0000720.Ed
Damien Miller32aa1441999-10-29 09:15:49 +1000721.It Cm Compression
Damien Miller7684ee12000-03-17 23:40:15 +1100722Specifies whether to use compression.
723The argument must be
Damien Miller32aa1441999-10-29 09:15:49 +1000724.Dq yes
725or
726.Dq no .
Ben Lindstrom89b0bd62001-03-29 00:27:11 +0000727The default is
728.Dq no .
Damien Miller32aa1441999-10-29 09:15:49 +1000729.It Cm CompressionLevel
Ben Lindstrom89b0bd62001-03-29 00:27:11 +0000730Specifies the compression level to use if compression is enabled.
Damien Miller7684ee12000-03-17 23:40:15 +1100731The argument must be an integer from 1 (fast) to 9 (slow, best).
732The default level is 6, which is good for most applications.
733The meaning of the values is the same as in
Damien Miller396691a2000-01-20 22:44:08 +1100734.Xr gzip 1 .
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000735Note that this option applies to protocol version 1 only.
Damien Miller32aa1441999-10-29 09:15:49 +1000736.It Cm ConnectionAttempts
737Specifies the number of tries (one per second) to make before falling
Damien Miller7684ee12000-03-17 23:40:15 +1100738back to rsh or exiting.
739The argument must be an integer.
740This may be useful in scripts if the connection sometimes fails.
Ben Lindstromf9cedb92001-08-06 21:07:11 +0000741The default is 1.
Damien Miller32aa1441999-10-29 09:15:49 +1000742.It Cm EscapeChar
743Sets the escape character (default:
744.Ql ~ ) .
745The escape character can also
Damien Miller7684ee12000-03-17 23:40:15 +1100746be set on the command line.
747The argument should be a single character,
Damien Miller32aa1441999-10-29 09:15:49 +1000748.Ql ^
749followed by a letter, or
750.Dq none
751to disable the escape
752character entirely (making the connection transparent for binary
753data).
Damien Miller22c77262000-04-13 12:26:34 +1000754.It Cm FallBackToRsh
Damien Miller32aa1441999-10-29 09:15:49 +1000755Specifies that if connecting via
756.Nm
757fails due to a connection refused error (there is no
758.Xr sshd 8
Damien Miller22c77262000-04-13 12:26:34 +1000759listening on the remote host),
Damien Miller32aa1441999-10-29 09:15:49 +1000760.Xr rsh 1
761should automatically be used instead (after a suitable warning about
Damien Miller7684ee12000-03-17 23:40:15 +1100762the session being unencrypted).
763The argument must be
Damien Miller32aa1441999-10-29 09:15:49 +1000764.Dq yes
765or
766.Dq no .
Ben Lindstrom89b0bd62001-03-29 00:27:11 +0000767The default is
768.Dq no .
Damien Miller32aa1441999-10-29 09:15:49 +1000769.It Cm ForwardAgent
770Specifies whether the connection to the authentication agent (if any)
Damien Miller7684ee12000-03-17 23:40:15 +1100771will be forwarded to the remote machine.
772The argument must be
Damien Miller32aa1441999-10-29 09:15:49 +1000773.Dq yes
774or
775.Dq no .
Damien Millerb1715dc2000-05-30 13:44:51 +1000776The default is
777.Dq no .
Damien Miller32aa1441999-10-29 09:15:49 +1000778.It Cm ForwardX11
779Specifies whether X11 connections will be automatically redirected
Damien Miller22c77262000-04-13 12:26:34 +1000780over the secure channel and
Damien Miller32aa1441999-10-29 09:15:49 +1000781.Ev DISPLAY
Damien Miller7684ee12000-03-17 23:40:15 +1100782set.
Damien Miller22c77262000-04-13 12:26:34 +1000783The argument must be
Damien Miller32aa1441999-10-29 09:15:49 +1000784.Dq yes
785or
786.Dq no .
Damien Miller98c7ad62000-03-09 21:27:49 +1100787The default is
788.Dq no .
Damien Miller32aa1441999-10-29 09:15:49 +1000789.It Cm GatewayPorts
790Specifies whether remote hosts are allowed to connect to local
791forwarded ports.
792The argument must be
793.Dq yes
794or
795.Dq no .
796The default is
797.Dq no .
798.It Cm GlobalKnownHostsFile
Ben Lindstromd6481ea2001-06-25 04:37:41 +0000799Specifies a file to use for the global
Ben Lindstrom1e621742001-03-06 01:10:53 +0000800host key database instead of
Damien Miller886c63a2000-01-20 23:13:36 +1100801.Pa /etc/ssh_known_hosts .
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000802.It Cm HostbasedAuthentication
803Specifies whether to try rhosts based authentication with public key
804authentication.
805The argument must be
806.Dq yes
807or
808.Dq no .
809The default is
Ben Lindstrom1bfe2912001-06-05 19:37:25 +0000810.Dq no .
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000811This option applies to protocol version 2 only and
812is similar to
813.Cm RhostsRSAAuthentication .
Ben Lindstrom982dbbc2001-04-17 18:11:36 +0000814.It Cm HostKeyAlgorithms
Ben Lindstromf0609f82001-05-04 22:38:43 +0000815Specifies the protocol version 2 host key algorithms
Ben Lindstrom982dbbc2001-04-17 18:11:36 +0000816that the client wants to use in order of preference.
817The default for this option is:
818.Dq ssh-rsa,ssh-dss
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000819.It Cm HostKeyAlias
820Specifies an alias that should be used instead of the
821real host name when looking up or saving the host key
822in the host key database files.
823This option is useful for tunneling ssh connections
824or if you have multiple servers running on a single host.
Damien Miller32aa1441999-10-29 09:15:49 +1000825.It Cm HostName
Damien Miller7684ee12000-03-17 23:40:15 +1100826Specifies the real host name to log into.
827This can be used to specify nicknames or abbreviations for hosts.
828Default is the name given on the command line.
829Numeric IP addresses are also permitted (both on the command line and in
Damien Miller32aa1441999-10-29 09:15:49 +1000830.Cm HostName
831specifications).
832.It Cm IdentityFile
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000833Specifies the file from which the user's RSA or DSA authentication identity
Damien Miller32aa1441999-10-29 09:15:49 +1000834is read (default
Damien Millere247cc42000-05-07 12:03:14 +1000835.Pa $HOME/.ssh/identity
Damien Miller32aa1441999-10-29 09:15:49 +1000836in the user's home directory).
837Additionally, any identities represented by the authentication agent
Damien Miller7684ee12000-03-17 23:40:15 +1100838will be used for authentication.
839The file name may use the tilde
840syntax to refer to a user's home directory.
841It is possible to have
Damien Miller32aa1441999-10-29 09:15:49 +1000842multiple identity files specified in configuration files; all these
843identities will be tried in sequence.
844.It Cm KeepAlive
845Specifies whether the system should send keepalive messages to the
Damien Miller7684ee12000-03-17 23:40:15 +1100846other side.
847If they are sent, death of the connection or crash of one
848of the machines will be properly noticed.
849However, this means that
Damien Miller32aa1441999-10-29 09:15:49 +1000850connections will die if the route is down temporarily, and some people
Damien Miller450a7a12000-03-26 13:04:51 +1000851find it annoying.
Damien Miller32aa1441999-10-29 09:15:49 +1000852.Pp
853The default is
854.Dq yes
855(to send keepalives), and the client will notice
Damien Miller7684ee12000-03-17 23:40:15 +1100856if the network goes down or the remote host dies.
857This is important in scripts, and many users want it too.
Damien Miller32aa1441999-10-29 09:15:49 +1000858.Pp
859To disable keepalives, the value should be set to
860.Dq no
861in both the server and the client configuration files.
862.It Cm KerberosAuthentication
Damien Miller450a7a12000-03-26 13:04:51 +1000863Specifies whether Kerberos authentication will be used.
864The argument to this keyword must be
Damien Miller32aa1441999-10-29 09:15:49 +1000865.Dq yes
866or
867.Dq no .
868.It Cm KerberosTgtPassing
Damien Miller450a7a12000-03-26 13:04:51 +1000869Specifies whether a Kerberos TGT will be forwarded to the server.
870This will only work if the Kerberos server is actually an AFS kaserver.
871The argument to this keyword must be
Damien Miller32aa1441999-10-29 09:15:49 +1000872.Dq yes
873or
874.Dq no .
875.It Cm LocalForward
876Specifies that a TCP/IP port on the local machine be forwarded over
Damien Miller7684ee12000-03-17 23:40:15 +1100877the secure channel to given host:port from the remote machine.
878The first argument must be a port number, and the second must be
879host:port.
880Multiple forwardings may be specified, and additional
881forwardings can be given on the command line.
882Only the superuser can forward privileged ports.
Damien Miller5ce662a1999-11-11 17:57:39 +1100883.It Cm LogLevel
884Gives the verbosity level that is used when logging messages from
885.Nm ssh .
886The possible values are:
Ben Lindstromdb65e8f2001-01-19 04:26:52 +0000887QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
888The default is INFO.
Ben Lindstrom06b33aa2001-02-15 03:01:59 +0000889.It Cm MACs
Ben Lindstrom24643222001-06-25 05:08:11 +0000890Specifies the MAC (message authentication code) algorithms
Ben Lindstrom06b33aa2001-02-15 03:01:59 +0000891in order of preference.
892The MAC algorithm is used in protocol version 2
893for data integrity protection.
894Multiple algorithms must be comma-separated.
895The default is
Ben Lindstrombd0e2de2001-06-05 19:52:52 +0000896.Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
Damien Miller32aa1441999-10-29 09:15:49 +1000897.It Cm NumberOfPasswordPrompts
Damien Miller450a7a12000-03-26 13:04:51 +1000898Specifies the number of password prompts before giving up.
899The argument to this keyword must be an integer.
900Default is 3.
Damien Miller396691a2000-01-20 22:44:08 +1100901.It Cm PasswordAuthentication
Damien Miller7684ee12000-03-17 23:40:15 +1100902Specifies whether to use password authentication.
903The argument to this keyword must be
Damien Miller396691a2000-01-20 22:44:08 +1100904.Dq yes
905or
906.Dq no .
Ben Lindstrom89b0bd62001-03-29 00:27:11 +0000907The default is
908.Dq yes .
Damien Miller32aa1441999-10-29 09:15:49 +1000909.It Cm Port
Damien Miller7684ee12000-03-17 23:40:15 +1100910Specifies the port number to connect on the remote host.
911Default is 22.
Damien Miller1e42f302001-03-19 23:59:11 +1100912.It Cm PreferredAuthentications
Ben Lindstrom24643222001-06-25 05:08:11 +0000913Specifies the order in which the client should try protocol 2
914authentication methods. This allows a client to prefer one method (e.g.
Damien Miller1e42f302001-03-19 23:59:11 +1100915.Cm keyboard-interactive )
916over another method (e.g.
917.Cm password )
918The default for this option is:
Ben Lindstroma9086a12001-08-06 20:58:51 +0000919.Dq hostbased,publickey,keyboard-interactive,password
Damien Miller22c77262000-04-13 12:26:34 +1000920.It Cm Protocol
921Specifies the protocol versions
922.Nm
923should support in order of preference.
924The possible values are
925.Dq 1
926and
927.Dq 2 .
928Multiple versions must be comma-separated.
929The default is
Ben Lindstromf3272352001-04-05 23:29:59 +0000930.Dq 2,1 .
Damien Millere247cc42000-05-07 12:03:14 +1000931This means that
932.Nm
Ben Lindstromf3272352001-04-05 23:29:59 +0000933tries version 2 and falls back to version 1
934if version 2 is not available.
Damien Miller32aa1441999-10-29 09:15:49 +1000935.It Cm ProxyCommand
Damien Miller7684ee12000-03-17 23:40:15 +1100936Specifies the command to use to connect to the server.
937The command
938string extends to the end of the line, and is executed with
939.Pa /bin/sh .
940In the command string,
941.Ql %h
942will be substituted by the host name to
943connect and
944.Ql %p
945by the port.
946The command can be basically anything,
947and should read from its standard input and write to its standard output.
948It should eventually connect an
Damien Miller32aa1441999-10-29 09:15:49 +1000949.Xr sshd 8
950server running on some machine, or execute
951.Ic sshd -i
Damien Miller7684ee12000-03-17 23:40:15 +1100952somewhere.
953Host key management will be done using the
Damien Miller32aa1441999-10-29 09:15:49 +1000954HostName of the host being connected (defaulting to the name typed by
955the user).
Damien Milleraae6c611999-12-06 11:47:28 +1100956Note that
957.Cm CheckHostIP
958is not available for connects with a proxy command.
Damien Miller32aa1441999-10-29 09:15:49 +1000959.Pp
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000960.It Cm PubkeyAuthentication
961Specifies whether to try public key authentication.
962The argument to this keyword must be
963.Dq yes
964or
965.Dq no .
966The default is
967.Dq yes .
968This option applies to protocol version 2 only.
Damien Miller32aa1441999-10-29 09:15:49 +1000969.It Cm RemoteForward
970Specifies that a TCP/IP port on the remote machine be forwarded over
Damien Miller7684ee12000-03-17 23:40:15 +1100971the secure channel to given host:port from the local machine.
972The first argument must be a port number, and the second must be
973host:port.
974Multiple forwardings may be specified, and additional
975forwardings can be given on the command line.
976Only the superuser can forward privileged ports.
Damien Miller32aa1441999-10-29 09:15:49 +1000977.It Cm RhostsAuthentication
Damien Miller7684ee12000-03-17 23:40:15 +1100978Specifies whether to try rhosts based authentication.
979Note that this
Damien Miller32aa1441999-10-29 09:15:49 +1000980declaration only affects the client side and has no effect whatsoever
Damien Miller7684ee12000-03-17 23:40:15 +1100981on security.
982Disabling rhosts authentication may reduce
Damien Miller32aa1441999-10-29 09:15:49 +1000983authentication time on slow connections when rhosts authentication is
Damien Miller7684ee12000-03-17 23:40:15 +1100984not used.
985Most servers do not permit RhostsAuthentication because it
Ben Lindstrom24643222001-06-25 05:08:11 +0000986is not secure (see
Ben Lindstromd2bf0d62001-06-25 04:10:54 +0000987.Cm RhostsRSAAuthentication ) .
Damien Miller7684ee12000-03-17 23:40:15 +1100988The argument to this keyword must be
Damien Miller32aa1441999-10-29 09:15:49 +1000989.Dq yes
990or
991.Dq no .
Ben Lindstrom89b0bd62001-03-29 00:27:11 +0000992The default is
993.Dq yes .
Ben Lindstromc65e6a02001-04-23 13:02:16 +0000994This option applies to protocol version 1 only.
Damien Miller32aa1441999-10-29 09:15:49 +1000995.It Cm RhostsRSAAuthentication
996Specifies whether to try rhosts based authentication with RSA host
Damien Miller7684ee12000-03-17 23:40:15 +1100997authentication.
Damien Miller7684ee12000-03-17 23:40:15 +1100998The argument must be
Damien Miller32aa1441999-10-29 09:15:49 +1000999.Dq yes
1000or
1001.Dq no .
Ben Lindstrom89b0bd62001-03-29 00:27:11 +00001002The default is
1003.Dq yes .
Ben Lindstromc65e6a02001-04-23 13:02:16 +00001004This option applies to protocol version 1 only.
Damien Miller32aa1441999-10-29 09:15:49 +10001005.It Cm RSAAuthentication
Damien Miller7684ee12000-03-17 23:40:15 +11001006Specifies whether to try RSA authentication.
1007The argument to this keyword must be
Damien Miller32aa1441999-10-29 09:15:49 +10001008.Dq yes
1009or
1010.Dq no .
1011RSA authentication will only be
1012attempted if the identity file exists, or an authentication agent is
1013running.
Ben Lindstrom89b0bd62001-03-29 00:27:11 +00001014The default is
1015.Dq yes .
Damien Millere247cc42000-05-07 12:03:14 +10001016Note that this option applies to protocol version 1 only.
Damien Miller33804262001-02-04 23:20:18 +11001017.It Cm ChallengeResponseAuthentication
1018Specifies whether to use challenge response authentication.
Damien Miller7684ee12000-03-17 23:40:15 +11001019The argument to this keyword must be
Damien Miller95def091999-11-25 00:26:21 +11001020.Dq yes
1021or
1022.Dq no .
1023The default is
Ben Lindstrom0076d752001-08-06 20:53:26 +00001024.Dq yes .
Ben Lindstromae996bf2001-08-06 21:27:53 +00001025.It Cm SmartcardDevice
1026Specifies which smartcard device to use. The argument to this keyword is
1027the device
1028.Nm
1029should use to communicate with a smartcard used for storing the user's
1030private RSA key. By default, no device is specified and smartcard support
1031is not activated.
Damien Miller32aa1441999-10-29 09:15:49 +10001032.It Cm StrictHostKeyChecking
1033If this flag is set to
Damien Miller22c77262000-04-13 12:26:34 +10001034.Dq yes ,
Damien Miller32aa1441999-10-29 09:15:49 +10001035.Nm
Ben Lindstromeb930d42001-01-29 08:37:08 +00001036will never automatically add host keys to the
Damien Miller32aa1441999-10-29 09:15:49 +10001037.Pa $HOME/.ssh/known_hosts
Ben Lindstromd6481ea2001-06-25 04:37:41 +00001038file, and refuses to connect to hosts whose host key has changed.
Damien Miller7684ee12000-03-17 23:40:15 +11001039This provides maximum protection against trojan horse attacks.
1040However, it can be somewhat annoying if you don't have good
Damien Miller886c63a2000-01-20 23:13:36 +11001041.Pa /etc/ssh_known_hosts
Damien Miller32aa1441999-10-29 09:15:49 +10001042files installed and frequently
Ben Lindstromeb930d42001-01-29 08:37:08 +00001043connect to new hosts.
1044This option forces the user to manually
1045add all new hosts.
1046If this flag is set to
1047.Dq no ,
1048.Nm
1049will automatically add new host keys to the
1050user known hosts files.
1051If this flag is set to
1052.Dq ask ,
1053new host keys
1054will be added to the user known host files only after the user
1055has confirmed that is what they really want to do, and
1056.Nm
1057will refuse to connect to hosts whose host key has changed.
Damien Miller7684ee12000-03-17 23:40:15 +11001058The host keys of
Ben Lindstromeb930d42001-01-29 08:37:08 +00001059known hosts will be verified automatically in all cases.
Damien Miller7684ee12000-03-17 23:40:15 +11001060The argument must be
Ben Lindstromeb930d42001-01-29 08:37:08 +00001061.Dq yes ,
1062.Dq no
Damien Miller32aa1441999-10-29 09:15:49 +10001063or
Ben Lindstromeb930d42001-01-29 08:37:08 +00001064.Dq ask .
1065The default is
1066.Dq ask .
Damien Miller32aa1441999-10-29 09:15:49 +10001067.It Cm UsePrivilegedPort
1068Specifies whether to use a privileged port for outgoing connections.
1069The argument must be
1070.Dq yes
1071or
1072.Dq no .
1073The default is
Ben Lindstromcebc8582001-03-08 03:39:10 +00001074.Dq no .
Ben Lindstromc65e6a02001-04-23 13:02:16 +00001075Note that you need to set this option to
1076.Dq yes
1077if you want to use
Damien Miller32aa1441999-10-29 09:15:49 +10001078.Cm RhostsAuthentication
1079and
Kevin Stevesfcec7f82000-12-15 19:55:48 +00001080.Cm RhostsRSAAuthentication
Ben Lindstromc65e6a02001-04-23 13:02:16 +00001081with older servers.
Damien Miller396691a2000-01-20 22:44:08 +11001082.It Cm User
Damien Miller7684ee12000-03-17 23:40:15 +11001083Specifies the user to log in as.
1084This can be useful if you have a different user name on different machines.
1085This saves the trouble of
Damien Miller396691a2000-01-20 22:44:08 +11001086having to remember to give the user name on the command line.
1087.It Cm UserKnownHostsFile
Ben Lindstromd6481ea2001-06-25 04:37:41 +00001088Specifies a file to use for the user
Ben Lindstrom1e621742001-03-06 01:10:53 +00001089host key database instead of
Damien Miller396691a2000-01-20 22:44:08 +11001090.Pa $HOME/.ssh/known_hosts .
Damien Miller32aa1441999-10-29 09:15:49 +10001091.It Cm UseRsh
Damien Miller7684ee12000-03-17 23:40:15 +11001092Specifies that rlogin/rsh should be used for this host.
1093It is possible that the host does not at all support the
Damien Miller32aa1441999-10-29 09:15:49 +10001094.Nm
Damien Miller7684ee12000-03-17 23:40:15 +11001095protocol.
1096This causes
Damien Miller32aa1441999-10-29 09:15:49 +10001097.Nm
Damien Miller7684ee12000-03-17 23:40:15 +11001098to immediately execute
Damien Miller32aa1441999-10-29 09:15:49 +10001099.Xr rsh 1 .
1100All other options (except
1101.Cm HostName )
Damien Miller7684ee12000-03-17 23:40:15 +11001102are ignored if this has been specified.
1103The argument must be
Damien Miller32aa1441999-10-29 09:15:49 +10001104.Dq yes
1105or
1106.Dq no .
Damien Millerd3a18572000-06-07 19:55:44 +10001107.It Cm XAuthLocation
1108Specifies the location of the
1109.Xr xauth 1
1110program.
1111The default is
1112.Pa /usr/X11R6/bin/xauth .
Damien Miller7b28dc52000-09-05 13:34:53 +11001113.El
Damien Miller32aa1441999-10-29 09:15:49 +10001114.Sh ENVIRONMENT
1115.Nm
1116will normally set the following environment variables:
1117.Bl -tag -width Ds
1118.It Ev DISPLAY
1119The
1120.Ev DISPLAY
Damien Miller7684ee12000-03-17 23:40:15 +11001121variable indicates the location of the X11 server.
Damien Miller22c77262000-04-13 12:26:34 +10001122It is automatically set by
Damien Miller32aa1441999-10-29 09:15:49 +10001123.Nm
1124to point to a value of the form
1125.Dq hostname:n
1126where hostname indicates
Damien Miller7684ee12000-03-17 23:40:15 +11001127the host where the shell runs, and n is an integer >= 1.
1128.Nm
1129uses this special value to forward X11 connections over the secure
1130channel.
Ben Lindstromc65e6a02001-04-23 13:02:16 +00001131The user should normally not set
1132.Ev DISPLAY
1133explicitly, as that
Damien Miller32aa1441999-10-29 09:15:49 +10001134will render the X11 connection insecure (and will require the user to
1135manually copy any required authorization cookies).
1136.It Ev HOME
1137Set to the path of the user's home directory.
1138.It Ev LOGNAME
1139Synonym for
1140.Ev USER ;
1141set for compatibility with systems that use this variable.
1142.It Ev MAIL
1143Set to point the user's mailbox.
Damien Miller7684ee12000-03-17 23:40:15 +11001144.It Ev PATH
Damien Miller32aa1441999-10-29 09:15:49 +10001145Set to the default
1146.Ev PATH ,
1147as specified when compiling
1148.Nm ssh .
Ben Lindstrom5bf5d672001-07-04 04:31:38 +00001149.It Ev SSH_ASKPASS
1150If
1151.Nm
1152needs a passphrase, it will read the passphrase from the current
1153terminal if it was run from a terminal.
1154If
1155.Nm
1156does not have a terminal associated with it but
1157.Ev DISPLAY
1158and
1159.Ev SSH_ASKPASS
1160are set, it will execute the program specified by
1161.Ev SSH_ASKPASS
1162and open an X11 window to read the passphrase.
1163This is particularly useful when calling
1164.Nm
1165from a
1166.Pa .Xsession
1167or related script.
1168(Note that on some machines it
1169may be necessary to redirect the input from
1170.Pa /dev/null
1171to make this work.)
Damien Miller32aa1441999-10-29 09:15:49 +10001172.It Ev SSH_AUTH_SOCK
1173indicates the path of a unix-domain socket used to communicate with the
1174agent.
1175.It Ev SSH_CLIENT
Damien Miller7684ee12000-03-17 23:40:15 +11001176Identifies the client end of the connection.
1177The variable contains
Damien Miller32aa1441999-10-29 09:15:49 +10001178three space-separated values: client ip-address, client port number,
1179and server port number.
Ben Lindstrom4dccfa52000-12-28 16:40:05 +00001180.It Ev SSH_ORIGINAL_COMMAND
1181The variable contains the original command line if a forced command
1182is executed.
1183It can be used to extract the original arguments.
Damien Miller32aa1441999-10-29 09:15:49 +10001184.It Ev SSH_TTY
1185This is set to the name of the tty (path to the device) associated
Damien Miller7684ee12000-03-17 23:40:15 +11001186with the current shell or command.
1187If the current session has no tty,
Damien Miller32aa1441999-10-29 09:15:49 +10001188this variable is not set.
1189.It Ev TZ
1190The timezone variable is set to indicate the present timezone if it
Damien Miller942da032000-08-18 13:59:06 +10001191was set when the daemon was started (i.e., the daemon passes the value
Damien Miller32aa1441999-10-29 09:15:49 +10001192on to new connections).
1193.It Ev USER
1194Set to the name of the user logging in.
1195.El
1196.Pp
Damien Miller22c77262000-04-13 12:26:34 +10001197Additionally,
Damien Miller32aa1441999-10-29 09:15:49 +10001198.Nm
Damien Miller22c77262000-04-13 12:26:34 +10001199reads
1200.Pa $HOME/.ssh/environment ,
Damien Miller32aa1441999-10-29 09:15:49 +10001201and adds lines of the format
1202.Dq VARNAME=value
1203to the environment.
1204.Sh FILES
Damien Miller98c7ad62000-03-09 21:27:49 +11001205.Bl -tag -width Ds
Ben Lindstromd6481ea2001-06-25 04:37:41 +00001206.It Pa $HOME/.ssh/known_hosts
Damien Miller32aa1441999-10-29 09:15:49 +10001207Records host keys for all hosts the user has logged into (that are not
1208in
Ben Lindstromd6481ea2001-06-25 04:37:41 +00001209.Pa /etc/ssh_known_hosts .
Damien Miller32aa1441999-10-29 09:15:49 +10001210See
1211.Xr sshd 8 .
Ben Lindstrom18a82ac2001-04-11 15:59:35 +00001212.It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa
1213Contains the authentication identity of the user.
1214They are for protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively.
Damien Millere247cc42000-05-07 12:03:14 +10001215These files
1216contain sensitive data and should be readable by the user but not
Damien Miller32aa1441999-10-29 09:15:49 +10001217accessible by others (read/write/execute).
1218Note that
1219.Nm
Damien Millere247cc42000-05-07 12:03:14 +10001220ignores a private key file if it is accessible by others.
Damien Miller32aa1441999-10-29 09:15:49 +10001221It is possible to specify a passphrase when
1222generating the key; the passphrase will be used to encrypt the
1223sensitive part of this file using 3DES.
Ben Lindstrom18a82ac2001-04-11 15:59:35 +00001224.It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub
Damien Miller32aa1441999-10-29 09:15:49 +10001225Contains the public key for authentication (public part of the
Damien Miller7684ee12000-03-17 23:40:15 +11001226identity file in human-readable form).
Damien Millere247cc42000-05-07 12:03:14 +10001227The contents of the
1228.Pa $HOME/.ssh/identity.pub
1229file should be added to
Damien Miller32aa1441999-10-29 09:15:49 +10001230.Pa $HOME/.ssh/authorized_keys
1231on all machines
Ben Lindstrom18a82ac2001-04-11 15:59:35 +00001232where you wish to log in using protocol version 1 RSA authentication.
Damien Millere247cc42000-05-07 12:03:14 +10001233The contents of the
1234.Pa $HOME/.ssh/id_dsa.pub
Ben Lindstrom18a82ac2001-04-11 15:59:35 +00001235and
1236.Pa $HOME/.ssh/id_rsa.pub
Damien Millere247cc42000-05-07 12:03:14 +10001237file should be added to
Ben Lindstromf96704d2001-06-25 04:17:12 +00001238.Pa $HOME/.ssh/authorized_keys
Damien Millere247cc42000-05-07 12:03:14 +10001239on all machines
Ben Lindstrom18a82ac2001-04-11 15:59:35 +00001240where you wish to log in using protocol version 2 DSA/RSA authentication.
Damien Millere247cc42000-05-07 12:03:14 +10001241These files are not
Damien Miller7684ee12000-03-17 23:40:15 +11001242sensitive and can (but need not) be readable by anyone.
Damien Millere247cc42000-05-07 12:03:14 +10001243These files are
Kevin Steves3c034ae2001-02-05 13:47:11 +00001244never used automatically and are not necessary; they are only provided for
Damien Miller32aa1441999-10-29 09:15:49 +10001245the convenience of the user.
1246.It Pa $HOME/.ssh/config
Damien Miller7684ee12000-03-17 23:40:15 +11001247This is the per-user configuration file.
1248The format of this file is described above.
1249This file is used by the
Damien Miller32aa1441999-10-29 09:15:49 +10001250.Nm
Damien Miller7684ee12000-03-17 23:40:15 +11001251client.
1252This file does not usually contain any sensitive information,
Damien Miller32aa1441999-10-29 09:15:49 +10001253but the recommended permissions are read/write for the user, and not
1254accessible by others.
1255.It Pa $HOME/.ssh/authorized_keys
Ben Lindstromf96704d2001-06-25 04:17:12 +00001256Lists the public keys (RSA/DSA) that can be used for logging in as this user.
Damien Miller7684ee12000-03-17 23:40:15 +11001257The format of this file is described in the
Damien Miller32aa1441999-10-29 09:15:49 +10001258.Xr sshd 8
Damien Miller7684ee12000-03-17 23:40:15 +11001259manual page.
Ben Lindstromd6481ea2001-06-25 04:37:41 +00001260In the simplest form the format is the same as the .pub
1261identity files.
Damien Millere247cc42000-05-07 12:03:14 +10001262This file is not highly sensitive, but the recommended
1263permissions are read/write for the user, and not accessible by others.
Ben Lindstromd6481ea2001-06-25 04:37:41 +00001264.It Pa /etc/ssh_known_hosts
Damien Miller7684ee12000-03-17 23:40:15 +11001265Systemwide list of known host keys.
Ben Lindstromd6481ea2001-06-25 04:37:41 +00001266This file should be prepared by the
Damien Miller32aa1441999-10-29 09:15:49 +10001267system administrator to contain the public host keys of all machines in the
Damien Miller7684ee12000-03-17 23:40:15 +11001268organization.
1269This file should be world-readable.
1270This file contains
Damien Miller32aa1441999-10-29 09:15:49 +10001271public keys, one per line, in the following format (fields separated
Ben Lindstromd6481ea2001-06-25 04:37:41 +00001272by spaces): system name, public key and optional comment field.
Damien Miller7684ee12000-03-17 23:40:15 +11001273When different names are used
Damien Miller32aa1441999-10-29 09:15:49 +10001274for the same machine, all such names should be listed, separated by
Damien Miller7684ee12000-03-17 23:40:15 +11001275commas.
1276The format is described on the
Damien Miller32aa1441999-10-29 09:15:49 +10001277.Xr sshd 8
1278manual page.
1279.Pp
1280The canonical system name (as returned by name servers) is used by
1281.Xr sshd 8
1282to verify the client host when logging in; other names are needed because
1283.Nm
1284does not convert the user-supplied name to a canonical name before
1285checking the key, because someone with access to the name servers
1286would then be able to fool host authentication.
Damien Miller886c63a2000-01-20 23:13:36 +11001287.It Pa /etc/ssh_config
Damien Miller7684ee12000-03-17 23:40:15 +11001288Systemwide configuration file.
1289This file provides defaults for those
Damien Miller32aa1441999-10-29 09:15:49 +10001290values that are not specified in the user's configuration file, and
Damien Miller7684ee12000-03-17 23:40:15 +11001291for those users who do not have a configuration file.
1292This file must be world-readable.
Damien Miller32aa1441999-10-29 09:15:49 +10001293.It Pa $HOME/.rhosts
1294This file is used in
1295.Pa \&.rhosts
1296authentication to list the
Damien Miller7684ee12000-03-17 23:40:15 +11001297host/user pairs that are permitted to log in.
1298(Note that this file is
Damien Miller32aa1441999-10-29 09:15:49 +10001299also used by rlogin and rsh, which makes using this file insecure.)
1300Each line of the file contains a host name (in the canonical form
1301returned by name servers), and then a user name on that host,
Damien Miller7684ee12000-03-17 23:40:15 +11001302separated by a space.
Ben Lindstromebd888d2001-03-05 05:49:29 +00001303On some machines this file may need to be
Damien Miller32aa1441999-10-29 09:15:49 +10001304world-readable if the user's home directory is on a NFS partition,
1305because
1306.Xr sshd 8
Damien Miller7684ee12000-03-17 23:40:15 +11001307reads it as root.
1308Additionally, this file must be owned by the user,
1309and must not have write permissions for anyone else.
1310The recommended
Damien Miller32aa1441999-10-29 09:15:49 +10001311permission for most machines is read/write for the user, and not
1312accessible by others.
1313.Pp
1314Note that by default
1315.Xr sshd 8
1316will be installed so that it requires successful RSA host
Damien Miller7684ee12000-03-17 23:40:15 +11001317authentication before permitting \s+2.\s0rhosts authentication.
1318If your server machine does not have the client's host key in
Damien Miller886c63a2000-01-20 23:13:36 +11001319.Pa /etc/ssh_known_hosts ,
Damien Miller32aa1441999-10-29 09:15:49 +10001320you can store it in
1321.Pa $HOME/.ssh/known_hosts .
1322The easiest way to do this is to
1323connect back to the client from the server machine using ssh; this
Damien Millere247cc42000-05-07 12:03:14 +10001324will automatically add the host key to
Damien Miller32aa1441999-10-29 09:15:49 +10001325.Pa $HOME/.ssh/known_hosts .
1326.It Pa $HOME/.shosts
1327This file is used exactly the same way as
1328.Pa \&.rhosts .
1329The purpose for
1330having this file is to be able to use rhosts authentication with
1331.Nm
1332without permitting login with
1333.Xr rlogin 1
1334or
1335.Xr rsh 1 .
1336.It Pa /etc/hosts.equiv
1337This file is used during
Damien Miller7684ee12000-03-17 23:40:15 +11001338.Pa \&.rhosts authentication.
1339It contains
Damien Miller32aa1441999-10-29 09:15:49 +10001340canonical hosts names, one per line (the full format is described on
1341the
1342.Xr sshd 8
Damien Miller7684ee12000-03-17 23:40:15 +11001343manual page).
1344If the client host is found in this file, login is
Damien Miller32aa1441999-10-29 09:15:49 +10001345automatically permitted provided client and server user names are the
Damien Miller7684ee12000-03-17 23:40:15 +11001346same.
1347Additionally, successful RSA host authentication is normally
1348required.
1349This file should only be writable by root.
Damien Miller886c63a2000-01-20 23:13:36 +11001350.It Pa /etc/shosts.equiv
Damien Miller22c77262000-04-13 12:26:34 +10001351This file is processed exactly as
Damien Miller32aa1441999-10-29 09:15:49 +10001352.Pa /etc/hosts.equiv .
1353This file may be useful to permit logins using
1354.Nm
1355but not using rsh/rlogin.
Damien Miller886c63a2000-01-20 23:13:36 +11001356.It Pa /etc/sshrc
Damien Miller32aa1441999-10-29 09:15:49 +10001357Commands in this file are executed by
1358.Nm
1359when the user logs in just before the user's shell (or command) is started.
1360See the
1361.Xr sshd 8
1362manual page for more information.
1363.It Pa $HOME/.ssh/rc
1364Commands in this file are executed by
1365.Nm
1366when the user logs in just before the user's shell (or command) is
1367started.
Damien Miller22c77262000-04-13 12:26:34 +10001368See the
Damien Miller32aa1441999-10-29 09:15:49 +10001369.Xr sshd 8
1370manual page for more information.
Damien Miller4f0fa561999-12-26 14:24:41 +11001371.It Pa $HOME/.ssh/environment
1372Contains additional definitions for environment variables, see section
1373.Sx ENVIRONMENT
1374above.
Damien Miller7b28dc52000-09-05 13:34:53 +11001375.El
Damien Miller0bc1bd82000-11-13 22:57:25 +11001376.Sh AUTHORS
Ben Lindstrom035782e2001-01-29 08:34:16 +00001377OpenSSH is a derivative of the original and free
1378ssh 1.2.12 release by Tatu Ylonen.
1379Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1380Theo de Raadt and Dug Song
1381removed many bugs, re-added newer features and
1382created OpenSSH.
1383Markus Friedl contributed the support for SSH
1384protocol versions 1.5 and 2.0.
Damien Miller32aa1441999-10-29 09:15:49 +10001385.Sh SEE ALSO
1386.Xr rlogin 1 ,
1387.Xr rsh 1 ,
1388.Xr scp 1 ,
Damien Miller33804262001-02-04 23:20:18 +11001389.Xr sftp 1 ,
Damien Miller32aa1441999-10-29 09:15:49 +10001390.Xr ssh-add 1 ,
1391.Xr ssh-agent 1 ,
1392.Xr ssh-keygen 1 ,
1393.Xr telnet 1 ,
Ben Lindstrom5ab6ae12001-02-10 22:08:03 +00001394.Xr sshd 8
Ben Lindstrom160ec622001-04-22 17:17:46 +00001395.Rs
1396.%A T. Ylonen
1397.%A T. Kivinen
1398.%A M. Saarinen
1399.%A T. Rinne
1400.%A S. Lehtinen
1401.%T "SSH Protocol Architecture"
Ben Lindstrom020a8692001-08-06 21:38:10 +00001402.%N draft-ietf-secsh-architecture-09.txt
1403.%D July 2001
Ben Lindstrom160ec622001-04-22 17:17:46 +00001404.%O work in progress material
1405.Re