2 .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
3 .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
4 .\" All rights reserved
6 .\" As far as I am concerned, the code I have written for this software
7 .\" can be used freely for any purpose. Any derived versions of this
8 .\" software must be clearly marked as such, and if the derived work is
9 .\" incompatible with the protocol description in the RFC file, it must be
10 .\" called by a name other than "ssh" or "Secure Shell".
12 .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
13 .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
14 .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
16 .\" Redistribution and use in source and binary forms, with or without
17 .\" modification, are permitted provided that the following conditions
19 .\" 1. Redistributions of source code must retain the above copyright
20 .\" notice, this list of conditions and the following disclaimer.
21 .\" 2. Redistributions in binary form must reproduce the above copyright
22 .\" notice, this list of conditions and the following disclaimer in the
23 .\" documentation and/or other materials provided with the distribution.
25 .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
26 .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
27 .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
28 .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
29 .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
30 .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
31 .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
32 .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
33 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
34 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
36 .\" $OpenBSD: ssh.1,v 1.320 2011/08/02 01:22:11 djm Exp $
37 .Dd $Mdocdate: August 2 2011 $
42 .Nd OpenSSH SSH client (remote login program)
46 .Op Fl 1246AaCfgKkMNnqsTtVvXxYy
47 .Op Fl b Ar bind_address
48 .Op Fl c Ar cipher_spec
49 .Op Fl D Oo Ar bind_address : Oc Ns Ar port
50 .Op Fl e Ar escape_char
51 .Op Fl F Ar configfile
53 .Op Fl i Ar identity_file
54 .Op Fl L Oo Ar bind_address : Oc Ns Ar port : Ns Ar host : Ns Ar hostport
55 .Op Fl l Ar login_name
60 .Op Fl R Oo Ar bind_address : Oc Ns Ar port : Ns Ar host : Ns Ar hostport
62 .Op Fl W Ar host : Ns Ar port
63 .Op Fl w Ar local_tun Ns Op : Ns Ar remote_tun
64 .Oo Ar user Ns @ Oc Ns Ar hostname
69 (SSH client) is a program for logging into a remote machine and for
70 executing commands on a remote machine.
71 It is intended to replace rlogin and rsh,
72 and provide secure encrypted communications between
73 two untrusted hosts over an insecure network.
74 X11 connections and arbitrary TCP ports
75 can also be forwarded over the secure channel.
78 connects and logs into the specified
84 his/her identity to the remote machine using one of several methods
85 depending on the protocol version used (see below).
90 it is executed on the remote host instead of a login shell.
92 The options are as follows:
97 to try protocol version 1 only.
101 to try protocol version 2 only.
105 to use IPv4 addresses only.
109 to use IPv6 addresses only.
111 Enables forwarding of the authentication agent connection.
112 This can also be specified on a per-host basis in a configuration file.
114 Agent forwarding should be enabled with caution.
115 Users with the ability to bypass file permissions on the remote host
118 socket) can access the local agent through the forwarded connection.
119 An attacker cannot obtain key material from the agent,
120 however they can perform operations on the keys that enable them to
121 authenticate using the identities loaded into the agent.
123 Disables forwarding of the authentication agent connection.
124 .It Fl b Ar bind_address
127 on the local machine as the source address
129 Only useful on systems with more than one address.
131 Requests compression of all data (including stdin, stdout, stderr, and
132 data for forwarded X11 and TCP connections).
133 The compression algorithm is the same used by
137 can be controlled by the
139 option for protocol version 1.
140 Compression is desirable on modem lines and other
141 slow connections, but will only slow down things on fast networks.
142 The default value can be set on a host-by-host basis in the
143 configuration files; see the
146 .It Fl c Ar cipher_spec
147 Selects the cipher specification for encrypting the session.
149 Protocol version 1 allows specification of a single cipher.
150 The supported values are
156 (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
157 It is believed to be secure.
159 is a fast block cipher; it appears very secure and is much faster than
162 is only supported in the
164 client for interoperability with legacy protocol 1 implementations
165 that do not support the
168 Its use is strongly discouraged due to cryptographic weaknesses.
172 For protocol version 2,
174 is a comma-separated list of ciphers
175 listed in order of preference.
180 for more information.
183 .Oo Ar bind_address : Oc
189 application-level port forwarding.
190 This works by allocating a socket to listen to
192 on the local side, optionally bound to the specified
194 Whenever a connection is made to this port, the
195 connection is forwarded over the secure channel, and the application
196 protocol is then used to determine where to connect to from the
198 Currently the SOCKS4 and SOCKS5 protocols are supported, and
200 will act as a SOCKS server.
201 Only root can forward privileged ports.
202 Dynamic port forwardings can also be specified in the configuration file.
204 IPv6 addresses can be specified by enclosing the address in square brackets.
205 Only the superuser can forward privileged ports.
206 By default, the local port is bound in accordance with the
211 may be used to bind the connection to a specific address.
216 indicates that the listening port be bound for local use only, while an
219 indicates that the port should be available from all interfaces.
220 .It Fl e Ar escape_char
221 Sets the escape character for sessions with a pty (default:
223 The escape character is only recognized at the beginning of a line.
224 The escape character followed by a dot
226 closes the connection;
227 followed by control-Z suspends the connection;
228 and followed by itself sends the escape character once.
229 Setting the character to
231 disables any escapes and makes the session fully transparent.
232 .It Fl F Ar configfile
233 Specifies an alternative per-user configuration file.
234 If a configuration file is given on the command line,
235 the system-wide configuration file
236 .Pq Pa /etc/ssh/ssh_config
238 The default for the per-user configuration file is
243 to go to background just before command execution.
246 is going to ask for passwords or passphrases, but the user
247 wants it in the background.
250 The recommended way to start X11 programs at a remote site is with
252 .Ic ssh -f host xterm .
255 .Cm ExitOnForwardFailure
256 configuration option is set to
258 then a client started with
260 will wait for all remote port forwards to be successfully established
261 before placing itself in the background.
263 Allows remote hosts to connect to local forwarded ports.
265 Specify the PKCS#11 shared library
267 should use to communicate with a PKCS#11 token providing the user's
269 .It Fl i Ar identity_file
270 Selects a file from which the identity (private key) for
271 public key authentication is read.
274 for protocol version 1, and
279 for protocol version 2.
280 Identity files may also be specified on
281 a per-host basis in the configuration file.
282 It is possible to have multiple
284 options (and multiple identities specified in
285 configuration files).
287 will also try to load certificate information from the filename obtained
290 to identity filenames.
292 Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
293 credentials to the server.
295 Disables forwarding (delegation) of GSSAPI credentials to the server.
298 .Oo Ar bind_address : Oc
299 .Ar port : host : hostport
302 Specifies that the given port on the local (client) host is to be
303 forwarded to the given host and port on the remote side.
304 This works by allocating a socket to listen to
306 on the local side, optionally bound to the specified
308 Whenever a connection is made to this port, the
309 connection is forwarded over the secure channel, and a connection is
314 from the remote machine.
315 Port forwardings can also be specified in the configuration file.
316 IPv6 addresses can be specified by enclosing the address in square brackets.
317 Only the superuser can forward privileged ports.
318 By default, the local port is bound in accordance with the
323 may be used to bind the connection to a specific address.
328 indicates that the listening port be bound for local use only, while an
331 indicates that the port should be available from all interfaces.
332 .It Fl l Ar login_name
333 Specifies the user to log in as on the remote machine.
334 This also may be specified on a per-host basis in the configuration file.
340 mode for connection sharing.
347 mode with confirmation required before slave connections are accepted.
348 Refer to the description of
354 Additionally, for protocol version 2 a comma-separated list of MAC
355 (message authentication code) algorithms can
356 be specified in order of preference.
359 keyword for more information.
361 Do not execute a remote command.
362 This is useful for just forwarding ports
363 (protocol version 2 only).
367 (actually, prevents reading from stdin).
368 This must be used when
370 is run in the background.
371 A common trick is to use this to run X11 programs on a remote machine.
373 .Ic ssh -n shadows.cs.hut.fi emacs &
374 will start an emacs on shadows.cs.hut.fi, and the X11
375 connection will be automatically forwarded over an encrypted channel.
378 program will be put in the background.
379 (This does not work if
381 needs to ask for a password or passphrase; see also the
385 Control an active connection multiplexing master process.
388 option is specified, the
390 argument is interpreted and passed to the master process.
393 (check that the master process is running),
395 (request forwardings without command execution),
397 (request the master to exit), and
399 (request the master to stop accepting further multiplexing requests).
401 Can be used to give options in the format used in the configuration file.
402 This is useful for specifying options for which there is no separate
404 For full details of the options listed below, and their possible values, see
407 .Bl -tag -width Ds -offset indent -compact
411 .It ChallengeResponseAuthentication
415 .It ClearAllForwardings
418 .It ConnectionAttempts
424 .It ExitOnForwardFailure
427 .It ForwardX11Trusted
429 .It GlobalKnownHostsFile
430 .It GSSAPIAuthentication
431 .It GSSAPIDelegateCredentials
434 .It HostbasedAuthentication
435 .It HostKeyAlgorithms
441 .It KbdInteractiveDevices
447 .It NoHostAuthenticationForLocalhost
448 .It NumberOfPasswordPrompts
449 .It PasswordAuthentication
450 .It PermitLocalCommand
453 .It PreferredAuthentications
456 .It PubkeyAuthentication
460 .It RhostsRSAAuthentication
461 .It RSAAuthentication
463 .It ServerAliveInterval
464 .It ServerAliveCountMax
465 .It StrictHostKeyChecking
469 .It UsePrivilegedPort
471 .It UserKnownHostsFile
477 Port to connect to on the remote host.
478 This can be specified on a
479 per-host basis in the configuration file.
482 Causes most warning and diagnostic messages to be suppressed.
485 .Oo Ar bind_address : Oc
486 .Ar port : host : hostport
489 Specifies that the given port on the remote (server) host is to be
490 forwarded to the given host and port on the local side.
491 This works by allocating a socket to listen to
493 on the remote side, and whenever a connection is made to this port, the
494 connection is forwarded over the secure channel, and a connection is
499 from the local machine.
501 Port forwardings can also be specified in the configuration file.
502 Privileged ports can be forwarded only when
503 logging in as root on the remote machine.
504 IPv6 addresses can be specified by enclosing the address in square braces.
506 By default, the listening socket on the server will be bound to the loopback
508 This may be overridden by specifying a
514 indicates that the remote socket should listen on all interfaces.
517 will only succeed if the server's
519 option is enabled (see
520 .Xr sshd_config 5 ) .
526 the listen port will be dynamically allocated on the server and reported
527 to the client at run time.
528 When used together with
530 the allocated port will be printed to the standard output.
532 Specifies the location of a control socket for connection sharing,
535 to disable connection sharing.
536 Refer to the description of
544 May be used to request invocation of a subsystem on the remote system.
545 Subsystems are a feature of the SSH2 protocol which facilitate the use
546 of SSH as a secure transport for other applications (eg.\&
548 The subsystem is specified as the remote command.
550 Disable pseudo-tty allocation.
552 Force pseudo-tty allocation.
553 This can be used to execute arbitrary
554 screen-based programs on a remote machine, which can be very useful,
555 e.g. when implementing menu services.
558 options force tty allocation, even if
562 Display the version number and exit.
567 to print debugging messages about its progress.
569 debugging connection, authentication, and configuration problems.
572 options increase the verbosity.
574 .It Fl W Ar host : Ns Ar port
575 Requests that standard input and output on the client be forwarded to
579 over the secure channel.
583 .Cm ExitOnForwardFailure
585 .Cm ClearAllForwardings
586 and works with Protocol version 2 only.
588 .Ar local_tun Ns Op : Ns Ar remote_tun
592 device forwarding with the specified
594 devices between the client
599 The devices may be specified by numerical ID or the keyword
601 which uses the next available tunnel device.
604 is not specified, it defaults to
614 directive is unset, it is set to the default tunnel mode, which is
617 Enables X11 forwarding.
618 This can also be specified on a per-host basis in a configuration file.
620 X11 forwarding should be enabled with caution.
621 Users with the ability to bypass file permissions on the remote host
622 (for the user's X authorization database)
623 can access the local X11 display through the forwarded connection.
624 An attacker may then be able to perform activities such as keystroke monitoring.
626 For this reason, X11 forwarding is subjected to X11 SECURITY extension
627 restrictions by default.
632 .Cm ForwardX11Trusted
635 for more information.
637 Disables X11 forwarding.
639 Enables trusted X11 forwarding.
640 Trusted X11 forwardings are not subjected to the X11 SECURITY extension
643 Send log information using the
646 By default this information is sent to stderr.
650 may additionally obtain configuration data from
651 a per-user configuration file and a system-wide configuration file.
652 The file format and configuration options are described in
655 The OpenSSH SSH client supports SSH protocols 1 and 2.
656 The default is to use protocol 2 only,
657 though this can be changed via the
666 Both protocols support similar authentication methods,
667 but protocol 2 is the default since
668 it provides additional mechanisms for confidentiality
669 (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
670 and integrity (hmac-md5, hmac-sha1,
671 hmac-sha2-256, hmac-sha2-512,
672 umac-64, hmac-ripemd160).
673 Protocol 1 lacks a strong mechanism for ensuring the
674 integrity of the connection.
676 The methods available for authentication are:
677 GSSAPI-based authentication,
678 host-based authentication,
679 public key authentication,
680 challenge-response authentication,
681 and password authentication.
682 Authentication methods are tried in the order specified above,
683 though protocol 2 has a configuration option to change the default order:
684 .Cm PreferredAuthentications .
686 Host-based authentication works as follows:
687 If the machine the user logs in from is listed in
690 .Pa /etc/ssh/shosts.equiv
691 on the remote machine, and the user names are
692 the same on both sides, or if the files
696 exist in the user's home directory on the
697 remote machine and contain a line containing the name of the client
698 machine and the name of the user on that machine, the user is
699 considered for login.
700 Additionally, the server
702 be able to verify the client's
703 host key (see the description of
704 .Pa /etc/ssh/ssh_known_hosts
706 .Pa ~/.ssh/known_hosts ,
708 for login to be permitted.
709 This authentication method closes security holes due to IP
710 spoofing, DNS spoofing, and routing spoofing.
711 [Note to the administrator:
712 .Pa /etc/hosts.equiv ,
714 and the rlogin/rsh protocol in general, are inherently insecure and should be
715 disabled if security is desired.]
717 Public key authentication works as follows:
718 The scheme is based on public-key cryptography,
720 where encryption and decryption are done using separate keys,
721 and it is unfeasible to derive the decryption key from the encryption key.
722 The idea is that each user creates a public/private
723 key pair for authentication purposes.
724 The server knows the public key, and only the user knows the private key.
726 implements public key authentication protocol automatically,
727 using one of the DSA, ECDSA or RSA algorithms.
728 Protocol 1 is restricted to using only RSA keys,
729 but protocol 2 may use any.
734 contains a brief discussion of the DSA and RSA algorithms.
737 .Pa ~/.ssh/authorized_keys
738 lists the public keys that are permitted for logging in.
739 When the user logs in, the
741 program tells the server which key pair it would like to use for
743 The client proves that it has access to the private key
744 and the server checks that the corresponding public key
745 is authorized to accept the account.
747 The user creates his/her key pair by running
749 This stores the private key in
759 and stores the public key in
760 .Pa ~/.ssh/identity.pub
762 .Pa ~/.ssh/id_dsa.pub
764 .Pa ~/.ssh/id_ecdsa.pub
767 .Pa ~/.ssh/id_rsa.pub
769 in the user's home directory.
770 The user should then copy the public key
772 .Pa ~/.ssh/authorized_keys
773 in his/her home directory on the remote machine.
776 file corresponds to the conventional
778 file, and has one key
779 per line, though the lines can be very long.
780 After this, the user can log in without giving the password.
782 A variation on public key authentication
783 is available in the form of certificate authentication:
784 instead of a set of public/private keys,
785 signed certificates are used.
786 This has the advantage that a single trusted certification authority
787 can be used in place of many public/private keys.
792 for more information.
794 The most convenient way to use public key or certificate authentication
795 may be with an authentication agent.
798 for more information.
800 Challenge-response authentication works as follows:
801 The server sends an arbitrary
803 text, and prompts for a response.
804 Protocol 2 allows multiple challenges and responses;
805 protocol 1 is restricted to just one challenge/response.
806 Examples of challenge-response authentication include
807 BSD Authentication (see
809 and PAM (some non-OpenBSD systems).
811 Finally, if other authentication methods fail,
813 prompts the user for a password.
814 The password is sent to the remote
815 host for checking; however, since all communications are encrypted,
816 the password cannot be seen by someone listening on the network.
819 automatically maintains and checks a database containing
820 identification for all hosts it has ever been used with.
821 Host keys are stored in
822 .Pa ~/.ssh/known_hosts
823 in the user's home directory.
824 Additionally, the file
825 .Pa /etc/ssh/ssh_known_hosts
826 is automatically checked for known hosts.
827 Any new hosts are automatically added to the user's file.
828 If a host's identification ever changes,
830 warns about this and disables password authentication to prevent
831 server spoofing or man-in-the-middle attacks,
832 which could otherwise be used to circumvent the encryption.
834 .Cm StrictHostKeyChecking
835 option can be used to control logins to machines whose
836 host key is not known or has changed.
838 When the user's identity has been accepted by the server, the server
839 either executes the given command, or logs into the machine and gives
840 the user a normal shell on the remote machine.
841 All communication with
842 the remote command or shell will be automatically encrypted.
844 If a pseudo-terminal has been allocated (normal login session), the
845 user may use the escape characters noted below.
847 If no pseudo-tty has been allocated,
848 the session is transparent and can be used to reliably transfer binary data.
849 On most systems, setting the escape character to
851 will also make the session transparent even if a tty is used.
853 The session terminates when the command or shell on the remote
854 machine exits and all X11 and TCP connections have been closed.
855 .Sh ESCAPE CHARACTERS
856 When a pseudo-terminal has been requested,
858 supports a number of functions through the use of an escape character.
860 A single tilde character can be sent as
862 or by following the tilde by a character other than those described below.
863 The escape character must always follow a newline to be interpreted as
865 The escape character can be changed in configuration files using the
867 configuration directive or on the command line by the
871 The supported escapes (assuming the default
881 List forwarded connections.
885 at logout when waiting for forwarded connection / X11 sessions to terminate.
887 Display a list of escape characters.
889 Send a BREAK to the remote system
890 (only useful for SSH protocol version 2 and if the peer supports it).
893 Currently this allows the addition of port forwardings using the
899 It also allows the cancellation of existing remote port-forwardings
902 .Fl KR Oo Ar bind_address : Oc Ar port .
904 .Ic !\& Ns Ar command
905 allows the user to execute a local command if the
906 .Ic PermitLocalCommand
909 Basic help is available, using the
913 Request rekeying of the connection
914 (only useful for SSH protocol version 2 and if the peer supports it).
917 Forwarding of arbitrary TCP connections over the secure channel can
918 be specified either on the command line or in a configuration file.
919 One possible application of TCP forwarding is a secure connection to a
920 mail server; another is going through firewalls.
922 In the example below, we look at encrypting communication between
923 an IRC client and server, even though the IRC server does not directly
924 support encrypted communications.
925 This works as follows:
926 the user connects to the remote host using
928 specifying a port to be used to forward connections
929 to the remote server.
930 After that it is possible to start the service which is to be encrypted
931 on the client machine,
932 connecting to the same local port,
935 will encrypt and forward the connection.
937 The following example tunnels an IRC session from client machine
941 .Dq server.example.com :
942 .Bd -literal -offset 4n
943 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
944 $ irc -c '#users' -p 1234 pinky 127.0.0.1
947 This tunnels a connection to IRC server
948 .Dq server.example.com ,
954 It doesn't matter which port is used,
955 as long as it's greater than 1023
956 (remember, only root can open sockets on privileged ports)
957 and doesn't conflict with any ports already in use.
958 The connection is forwarded to port 6667 on the remote server,
959 since that's the standard port for IRC services.
965 and the remote command
967 is specified to allow an amount of time
968 (10 seconds, in the example)
969 to start the service which is to be tunnelled.
970 If no connections are made within the time specified,
978 (or see the description of the
984 and the user is using X11 (the
986 environment variable is set), the connection to the X11 display is
987 automatically forwarded to the remote side in such a way that any X11
988 programs started from the shell (or command) will go through the
989 encrypted channel, and the connection to the real X server will be made
990 from the local machine.
991 The user should not manually set
993 Forwarding of X11 connections can be
994 configured on the command line or in configuration files.
995 Take note that X11 forwarding can represent a security hazard.
1001 will point to the server machine, but with a display number greater than zero.
1002 This is normal, and happens because
1006 X server on the server machine for forwarding the
1007 connections over the encrypted channel.
1010 will also automatically set up Xauthority data on the server machine.
1011 For this purpose, it will generate a random authorization cookie,
1012 store it in Xauthority on the server, and verify that any forwarded
1013 connections carry this cookie and replace it by the real cookie when
1014 the connection is opened.
1015 The real authentication cookie is never
1016 sent to the server machine (and no cookies are sent in the plain).
1022 (or see the description of the
1027 the user is using an authentication agent, the connection to the agent
1028 is automatically forwarded to the remote side.
1029 .Sh VERIFYING HOST KEYS
1030 When connecting to a server for the first time,
1031 a fingerprint of the server's public key is presented to the user
1033 .Cm StrictHostKeyChecking
1035 Fingerprints can be determined using
1038 .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1040 If the fingerprint is already known, it can be matched
1041 and the key can be accepted or rejected.
1042 Because of the difficulty of comparing host keys
1043 just by looking at hex strings,
1044 there is also support to compare host keys visually,
1051 a small ASCII graphic gets displayed on every login to a server, no matter
1052 if the session itself is interactive or not.
1053 By learning the pattern a known server produces, a user can easily
1054 find out that the host key has changed when a completely different pattern
1056 Because these patterns are not unambiguous however, a pattern that looks
1057 similar to the pattern remembered only gives a good probability that the
1058 host key is the same, not guaranteed proof.
1060 To get a listing of the fingerprints along with their random art for
1061 all known hosts, the following command line can be used:
1063 .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1065 If the fingerprint is unknown,
1066 an alternative method of verification is available:
1067 SSH fingerprints verified by DNS.
1068 An additional resource record (RR),
1070 is added to a zonefile
1071 and the connecting client is able to match the fingerprint
1072 with that of the key presented.
1074 In this example, we are connecting a client to a server,
1075 .Dq host.example.com .
1076 The SSHFP resource records should first be added to the zonefile for
1078 .Bd -literal -offset indent
1079 $ ssh-keygen -r host.example.com.
1082 The output lines will have to be added to the zonefile.
1083 To check that the zone is answering fingerprint queries:
1085 .Dl $ dig -t SSHFP host.example.com
1087 Finally the client connects:
1088 .Bd -literal -offset indent
1089 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1091 Matching host key fingerprint found in DNS.
1092 Are you sure you want to continue connecting (yes/no)?
1096 .Cm VerifyHostKeyDNS
1099 for more information.
1100 .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1102 contains support for Virtual Private Network (VPN) tunnelling
1105 network pseudo-device,
1106 allowing two networks to be joined securely.
1109 configuration option
1111 controls whether the server supports this,
1112 and at what level (layer 2 or 3 traffic).
1114 The following example would connect client network 10.0.50.0/24
1115 with remote network 10.0.99.0/24 using a point-to-point connection
1116 from 10.1.1.1 to 10.1.1.2,
1117 provided that the SSH server running on the gateway to the remote network,
1118 at 192.168.1.15, allows it.
1121 .Bd -literal -offset indent
1122 # ssh -f -w 0:1 192.168.1.15 true
1123 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1124 # route add 10.0.99.0/24 10.1.1.2
1128 .Bd -literal -offset indent
1129 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1130 # route add 10.0.50.0/24 10.1.1.1
1133 Client access may be more finely tuned via the
1134 .Pa /root/.ssh/authorized_keys
1135 file (see below) and the
1138 The following entry would permit connections on
1142 and on tun device 2 from user
1147 .Dq forced-commands-only :
1148 .Bd -literal -offset 2n
1149 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1150 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1153 Since an SSH-based setup entails a fair amount of overhead,
1154 it may be more suited to temporary setups,
1155 such as for wireless VPNs.
1156 More permanent VPNs are better provided by tools such as
1162 will normally set the following environment variables:
1163 .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1167 variable indicates the location of the X11 server.
1168 It is automatically set by
1170 to point to a value of the form
1174 indicates the host where the shell runs, and
1176 is an integer \*(Ge 1.
1178 uses this special value to forward X11 connections over the secure
1180 The user should normally not set
1183 will render the X11 connection insecure (and will require the user to
1184 manually copy any required authorization cookies).
1186 Set to the path of the user's home directory.
1190 set for compatibility with systems that use this variable.
1192 Set to the path of the user's mailbox.
1196 as specified when compiling
1201 needs a passphrase, it will read the passphrase from the current
1202 terminal if it was run from a terminal.
1205 does not have a terminal associated with it but
1209 are set, it will execute the program specified by
1211 and open an X11 window to read the passphrase.
1212 This is particularly useful when calling
1217 (Note that on some machines it
1218 may be necessary to redirect the input from
1221 .It Ev SSH_AUTH_SOCK
1222 Identifies the path of a
1224 socket used to communicate with the agent.
1225 .It Ev SSH_CONNECTION
1226 Identifies the client and server ends of the connection.
1227 The variable contains
1228 four space-separated values: client IP address, client port number,
1229 server IP address, and server port number.
1230 .It Ev SSH_ORIGINAL_COMMAND
1231 This variable contains the original command line if a forced command
1233 It can be used to extract the original arguments.
1235 This is set to the name of the tty (path to the device) associated
1236 with the current shell or command.
1237 If the current session has no tty,
1238 this variable is not set.
1240 This variable is set to indicate the present time zone if it
1241 was set when the daemon was started (i.e. the daemon passes the value
1242 on to new connections).
1244 Set to the name of the user logging in.
1250 .Pa ~/.ssh/environment ,
1251 and adds lines of the format
1253 to the environment if the file exists and users are allowed to
1254 change their environment.
1255 For more information, see the
1256 .Cm PermitUserEnvironment
1260 .Bl -tag -width Ds -compact
1262 This file is used for host-based authentication (see above).
1263 On some machines this file may need to be
1264 world-readable if the user's home directory is on an NFS partition,
1268 Additionally, this file must be owned by the user,
1269 and must not have write permissions for anyone else.
1271 permission for most machines is read/write for the user, and not
1272 accessible by others.
1275 This file is used in exactly the same way as
1277 but allows host-based authentication without permitting login with
1281 This directory is the default location for all user-specific configuration
1282 and authentication information.
1283 There is no general requirement to keep the entire contents of this directory
1284 secret, but the recommended permissions are read/write/execute for the user,
1285 and not accessible by others.
1287 .It Pa ~/.ssh/authorized_keys
1288 Lists the public keys (DSA/ECDSA/RSA) that can be used for logging in as
1290 The format of this file is described in the
1293 This file is not highly sensitive, but the recommended
1294 permissions are read/write for the user, and not accessible by others.
1296 .It Pa ~/.ssh/config
1297 This is the per-user configuration file.
1298 The file format and configuration options are described in
1300 Because of the potential for abuse, this file must have strict permissions:
1301 read/write for the user, and not accessible by others.
1303 .It Pa ~/.ssh/environment
1304 Contains additional definitions for environment variables; see
1308 .It Pa ~/.ssh/identity
1309 .It Pa ~/.ssh/id_dsa
1310 .It Pa ~/.ssh/id_ecdsa
1311 .It Pa ~/.ssh/id_rsa
1312 Contains the private key for authentication.
1314 contain sensitive data and should be readable by the user but not
1315 accessible by others (read/write/execute).
1317 will simply ignore a private key file if it is accessible by others.
1318 It is possible to specify a passphrase when
1319 generating the key which will be used to encrypt the
1320 sensitive part of this file using 3DES.
1322 .It Pa ~/.ssh/identity.pub
1323 .It Pa ~/.ssh/id_dsa.pub
1324 .It Pa ~/.ssh/id_ecdsa.pub
1325 .It Pa ~/.ssh/id_rsa.pub
1326 Contains the public key for authentication.
1328 sensitive and can (but need not) be readable by anyone.
1330 .It Pa ~/.ssh/known_hosts
1331 Contains a list of host keys for all hosts the user has logged into
1332 that are not already in the systemwide list of known host keys.
1335 for further details of the format of this file.
1338 Commands in this file are executed by
1340 when the user logs in, just before the user's shell (or command) is
1344 manual page for more information.
1346 .It Pa /etc/hosts.equiv
1347 This file is for host-based authentication (see above).
1348 It should only be writable by root.
1350 .It Pa /etc/ssh/shosts.equiv
1351 This file is used in exactly the same way as
1353 but allows host-based authentication without permitting login with
1356 .It Pa /etc/ssh/ssh_config
1357 Systemwide configuration file.
1358 The file format and configuration options are described in
1361 .It Pa /etc/ssh/ssh_host_key
1362 .It Pa /etc/ssh/ssh_host_dsa_key
1363 .It Pa /etc/ssh/ssh_host_ecdsa_key
1364 .It Pa /etc/ssh/ssh_host_rsa_key
1365 These three files contain the private parts of the host keys
1366 and are used for host-based authentication.
1367 If protocol version 1 is used,
1369 must be setuid root, since the host key is readable only by root.
1370 For protocol version 2,
1374 to access the host keys,
1375 eliminating the requirement that
1377 be setuid root when host-based authentication is used.
1382 .It Pa /etc/ssh/ssh_known_hosts
1383 Systemwide list of known host keys.
1384 This file should be prepared by the
1385 system administrator to contain the public host keys of all machines in the
1387 It should be world-readable.
1390 for further details of the format of this file.
1392 .It Pa /etc/ssh/sshrc
1393 Commands in this file are executed by
1395 when the user logs in, just before the user's shell (or command) is started.
1398 manual page for more information.
1402 exits with the exit status of the remote command or with 255
1403 if an error occurred.
1418 .%T "The Secure Shell (SSH) Protocol Assigned Numbers"
1423 .%T "The Secure Shell (SSH) Protocol Architecture"
1428 .%T "The Secure Shell (SSH) Authentication Protocol"
1433 .%T "The Secure Shell (SSH) Transport Layer Protocol"
1438 .%T "The Secure Shell (SSH) Connection Protocol"
1443 .%T "Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints"
1448 .%T "Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)"
1453 .%T "The Secure Shell (SSH) Session Channel Break Extension"
1458 .%T "The Secure Shell (SSH) Transport Layer Encryption Modes"
1463 .%T "Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol"
1468 .%T "Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol"
1473 .%T "The Secure Shell (SSH) Public Key File Format"
1478 .%T "Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer"
1482 .%T "Hash Visualization: a New Technique to improve Real-World Security"
1486 .%O "International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)"
1489 OpenSSH is a derivative of the original and free
1490 ssh 1.2.12 release by Tatu Ylonen.
1491 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1492 Theo de Raadt and Dug Song
1493 removed many bugs, re-added newer features and
1495 Markus Friedl contributed the support for SSH
1496 protocol versions 1.5 and 2.0.
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