3 .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
4 .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
5 .\" All rights reserved
7 .\" 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".
13 .\" 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.
17 .\" Redistribution and use in source and binary forms, with or without
18 .\" modification, are permitted provided that the following conditions
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.
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.
37 .\" $OpenBSD: ssh.1,v 1.289 2010/01/09 23:04:13 dtucker Exp $
43 .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
51 .Oo Ar bind_address : Oc
55 .Op Fl e Ar escape_char
56 .Op Fl F Ar configfile
58 .Op Fl i Ar identity_file
62 .Oo Ar bind_address : Oc
63 .Ar port : host : hostport
67 .Op Fl l Ar login_name
75 .Oo Ar bind_address : Oc
76 .Ar port : host : hostport
81 .Oo Fl w Ar local_tun Ns
82 .Op : Ns Ar remote_tun Oc
83 .Oo Ar user Ns @ Oc Ns Ar hostname
88 (SSH client) is a program for logging into a remote machine and for
89 executing commands on a remote machine.
90 It is intended to replace rlogin and rsh,
91 and provide secure encrypted communications between
92 two untrusted hosts over an insecure network.
93 X11 connections and arbitrary TCP ports
94 can also be forwarded over the secure channel.
97 connects and logs into the specified
103 his/her identity to the remote machine using one of several methods
104 depending on the protocol version used (see below).
109 it is executed on the remote host instead of a login shell.
111 The options are as follows:
116 to try protocol version 1 only.
120 to try protocol version 2 only.
124 to use IPv4 addresses only.
128 to use IPv6 addresses only.
130 Enables forwarding of the authentication agent connection.
131 This can also be specified on a per-host basis in a configuration file.
133 Agent forwarding should be enabled with caution.
134 Users with the ability to bypass file permissions on the remote host
137 socket) can access the local agent through the forwarded connection.
138 An attacker cannot obtain key material from the agent,
139 however they can perform operations on the keys that enable them to
140 authenticate using the identities loaded into the agent.
142 Disables forwarding of the authentication agent connection.
143 .It Fl b Ar bind_address
146 on the local machine as the source address
148 Only useful on systems with more than one address.
150 Requests compression of all data (including stdin, stdout, stderr, and
151 data for forwarded X11 and TCP connections).
152 The compression algorithm is the same used by
156 can be controlled by the
158 option for protocol version 1.
159 Compression is desirable on modem lines and other
160 slow connections, but will only slow down things on fast networks.
161 The default value can be set on a host-by-host basis in the
162 configuration files; see the
165 .It Fl c Ar cipher_spec
166 Selects the cipher specification for encrypting the session.
168 Protocol version 1 allows specification of a single cipher.
169 The supported values are
175 (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
176 It is believed to be secure.
178 is a fast block cipher; it appears very secure and is much faster than
181 is only supported in the
183 client for interoperability with legacy protocol 1 implementations
184 that do not support the
187 Its use is strongly discouraged due to cryptographic weaknesses.
191 For protocol version 2,
193 is a comma-separated list of ciphers
194 listed in order of preference.
197 keyword for more information.
200 .Oo Ar bind_address : Oc
206 application-level port forwarding.
207 This works by allocating a socket to listen to
209 on the local side, optionally bound to the specified
211 Whenever a connection is made to this port, the
212 connection is forwarded over the secure channel, and the application
213 protocol is then used to determine where to connect to from the
215 Currently the SOCKS4 and SOCKS5 protocols are supported, and
217 will act as a SOCKS server.
218 Only root can forward privileged ports.
219 Dynamic port forwardings can also be specified in the configuration file.
221 IPv6 addresses can be specified with an alternative syntax:
224 .Op Ar bind_address No /
228 or by enclosing the address in square brackets.
229 Only the superuser can forward privileged ports.
230 By default, the local port is bound in accordance with the
235 may be used to bind the connection to a specific address.
240 indicates that the listening port be bound for local use only, while an
243 indicates that the port should be available from all interfaces.
244 .It Fl e Ar escape_char
245 Sets the escape character for sessions with a pty (default:
247 The escape character is only recognized at the beginning of a line.
248 The escape character followed by a dot
250 closes the connection;
251 followed by control-Z suspends the connection;
252 and followed by itself sends the escape character once.
253 Setting the character to
255 disables any escapes and makes the session fully transparent.
256 .It Fl F Ar configfile
257 Specifies an alternative per-user configuration file.
258 If a configuration file is given on the command line,
259 the system-wide configuration file
260 .Pq Pa /etc/ssh/ssh_config
262 The default for the per-user configuration file is
267 to go to background just before command execution.
270 is going to ask for passwords or passphrases, but the user
271 wants it in the background.
274 The recommended way to start X11 programs at a remote site is with
276 .Ic ssh -f host xterm .
279 .Cm ExitOnForwardFailure
280 configuration option is set to
282 then a client started with
284 will wait for all remote port forwards to be successfully established
285 before placing itself in the background.
287 Allows remote hosts to connect to local forwarded ports.
288 .It Fl I Ar smartcard_device
291 should use to communicate with a smartcard used for storing the user's
293 This option is only available if support for smartcard devices
294 is compiled in (default is no support).
295 .It Fl i Ar identity_file
296 Selects a file from which the identity (private key) for
297 RSA or DSA authentication is read.
300 for protocol version 1, and
304 for protocol version 2.
305 Identity files may also be specified on
306 a per-host basis in the configuration file.
307 It is possible to have multiple
309 options (and multiple identities specified in
310 configuration files).
312 Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
313 credentials to the server.
315 Disables forwarding (delegation) of GSSAPI credentials to the server.
318 .Oo Ar bind_address : Oc
319 .Ar port : host : hostport
322 Specifies that the given port on the local (client) host is to be
323 forwarded to the given host and port on the remote side.
324 This works by allocating a socket to listen to
326 on the local side, optionally bound to the specified
328 Whenever a connection is made to this port, the
329 connection is forwarded over the secure channel, and a connection is
334 from the remote machine.
335 Port forwardings can also be specified in the configuration file.
336 IPv6 addresses can be specified with an alternative syntax:
339 .Op Ar bind_address No /
340 .Ar port No / Ar host No /
344 or by enclosing the address in square brackets.
345 Only the superuser can forward privileged ports.
346 By default, the local port is bound in accordance with the
351 may be used to bind the connection to a specific address.
356 indicates that the listening port be bound for local use only, while an
359 indicates that the port should be available from all interfaces.
360 .It Fl l Ar login_name
361 Specifies the user to log in as on the remote machine.
362 This also may be specified on a per-host basis in the configuration file.
368 mode for connection sharing.
375 mode with confirmation required before slave connections are accepted.
376 Refer to the description of
382 Additionally, for protocol version 2 a comma-separated list of MAC
383 (message authentication code) algorithms can
384 be specified in order of preference.
387 keyword for more information.
389 Do not execute a remote command.
390 This is useful for just forwarding ports
391 (protocol version 2 only).
395 (actually, prevents reading from stdin).
396 This must be used when
398 is run in the background.
399 A common trick is to use this to run X11 programs on a remote machine.
401 .Ic ssh -n shadows.cs.hut.fi emacs &
402 will start an emacs on shadows.cs.hut.fi, and the X11
403 connection will be automatically forwarded over an encrypted channel.
406 program will be put in the background.
407 (This does not work if
409 needs to ask for a password or passphrase; see also the
413 Control an active connection multiplexing master process.
416 option is specified, the
418 argument is interpreted and passed to the master process.
421 (check that the master process is running) and
423 (request the master to exit).
425 Can be used to give options in the format used in the configuration file.
426 This is useful for specifying options for which there is no separate
428 For full details of the options listed below, and their possible values, see
431 .Bl -tag -width Ds -offset indent -compact
435 .It ChallengeResponseAuthentication
439 .It ClearAllForwardings
442 .It ConnectionAttempts
448 .It ExitOnForwardFailure
451 .It ForwardX11Trusted
453 .It GlobalKnownHostsFile
454 .It GSSAPIAuthentication
455 .It GSSAPIDelegateCredentials
458 .It HostbasedAuthentication
459 .It HostKeyAlgorithms
464 .It KbdInteractiveDevices
469 .It NoHostAuthenticationForLocalhost
470 .It NumberOfPasswordPrompts
471 .It PasswordAuthentication
472 .It PermitLocalCommand
474 .It PreferredAuthentications
477 .It PubkeyAuthentication
480 .It RhostsRSAAuthentication
481 .It RSAAuthentication
483 .It ServerAliveInterval
484 .It ServerAliveCountMax
486 .It StrictHostKeyChecking
490 .It UsePrivilegedPort
492 .It UserKnownHostsFile
498 Port to connect to on the remote host.
499 This can be specified on a
500 per-host basis in the configuration file.
503 Causes most warning and diagnostic messages to be suppressed.
506 .Oo Ar bind_address : Oc
507 .Ar port : host : hostport
510 Specifies that the given port on the remote (server) host is to be
511 forwarded to the given host and port on the local side.
512 This works by allocating a socket to listen to
514 on the remote side, and whenever a connection is made to this port, the
515 connection is forwarded over the secure channel, and a connection is
520 from the local machine.
522 Port forwardings can also be specified in the configuration file.
523 Privileged ports can be forwarded only when
524 logging in as root on the remote machine.
525 IPv6 addresses can be specified by enclosing the address in square braces or
526 using an alternative syntax:
529 .Op Ar bind_address No /
530 .Ar host No / Ar port No /
535 By default, the listening socket on the server will be bound to the loopback
537 This may be overridden by specifying a
543 indicates that the remote socket should listen on all interfaces.
546 will only succeed if the server's
548 option is enabled (see
549 .Xr sshd_config 5 ) .
555 the listen port will be dynamically allocated on the server and reported
556 to the client at run time.
558 Specifies the location of a control socket for connection sharing.
559 Refer to the description of
567 May be used to request invocation of a subsystem on the remote system.
568 Subsystems are a feature of the SSH2 protocol which facilitate the use
569 of SSH as a secure transport for other applications (eg.\&
571 The subsystem is specified as the remote command.
573 Disable pseudo-tty allocation.
575 Force pseudo-tty allocation.
576 This can be used to execute arbitrary
577 screen-based programs on a remote machine, which can be very useful,
578 e.g. when implementing menu services.
581 options force tty allocation, even if
585 Display the version number and exit.
590 to print debugging messages about its progress.
592 debugging connection, authentication, and configuration problems.
595 options increase the verbosity.
598 .Ar local_tun Ns Op : Ns Ar remote_tun
602 device forwarding with the specified
604 devices between the client
609 The devices may be specified by numerical ID or the keyword
611 which uses the next available tunnel device.
614 is not specified, it defaults to
624 directive is unset, it is set to the default tunnel mode, which is
627 Enables X11 forwarding.
628 This can also be specified on a per-host basis in a configuration file.
630 X11 forwarding should be enabled with caution.
631 Users with the ability to bypass file permissions on the remote host
632 (for the user's X authorization database)
633 can access the local X11 display through the forwarded connection.
634 An attacker may then be able to perform activities such as keystroke monitoring.
636 For this reason, X11 forwarding is subjected to X11 SECURITY extension
637 restrictions by default.
642 .Cm ForwardX11Trusted
645 for more information.
647 Disables X11 forwarding.
649 Enables trusted X11 forwarding.
650 Trusted X11 forwardings are not subjected to the X11 SECURITY extension
653 Send log information using the
656 By default this information is sent to stderr.
660 may additionally obtain configuration data from
661 a per-user configuration file and a system-wide configuration file.
662 The file format and configuration options are described in
666 exits with the exit status of the remote command or with 255
667 if an error occurred.
669 The OpenSSH SSH client supports SSH protocols 1 and 2.
670 The default is to use protocol 2 only,
671 though this can be changed via the
680 Both protocols support similar authentication methods,
681 but protocol 2 is the default since
682 it provides additional mechanisms for confidentiality
683 (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
684 and integrity (hmac-md5, hmac-sha1, umac-64, hmac-ripemd160).
685 Protocol 1 lacks a strong mechanism for ensuring the
686 integrity of the connection.
688 The methods available for authentication are:
689 GSSAPI-based authentication,
690 host-based authentication,
691 public key authentication,
692 challenge-response authentication,
693 and password authentication.
694 Authentication methods are tried in the order specified above,
695 though protocol 2 has a configuration option to change the default order:
696 .Cm PreferredAuthentications .
698 Host-based authentication works as follows:
699 If the machine the user logs in from is listed in
702 .Pa /etc/shosts.equiv
703 on the remote machine, and the user names are
704 the same on both sides, or if the files
708 exist in the user's home directory on the
709 remote machine and contain a line containing the name of the client
710 machine and the name of the user on that machine, the user is
711 considered for login.
712 Additionally, the server
714 be able to verify the client's
715 host key (see the description of
716 .Pa /etc/ssh/ssh_known_hosts
718 .Pa ~/.ssh/known_hosts ,
720 for login to be permitted.
721 This authentication method closes security holes due to IP
722 spoofing, DNS spoofing, and routing spoofing.
723 [Note to the administrator:
724 .Pa /etc/hosts.equiv ,
726 and the rlogin/rsh protocol in general, are inherently insecure and should be
727 disabled if security is desired.]
729 Public key authentication works as follows:
730 The scheme is based on public-key cryptography,
732 where encryption and decryption are done using separate keys,
733 and it is unfeasible to derive the decryption key from the encryption key.
734 The idea is that each user creates a public/private
735 key pair for authentication purposes.
736 The server knows the public key, and only the user knows the private key.
738 implements public key authentication protocol automatically,
739 using either the RSA or DSA algorithms.
740 Protocol 1 is restricted to using only RSA keys,
741 but protocol 2 may use either.
746 contains a brief discussion of the two algorithms.
749 .Pa ~/.ssh/authorized_keys
750 lists the public keys that are permitted for logging in.
751 When the user logs in, the
753 program tells the server which key pair it would like to use for
755 The client proves that it has access to the private key
756 and the server checks that the corresponding public key
757 is authorized to accept the account.
759 The user creates his/her key pair by running
761 This stores the private key in
769 and stores the public key in
770 .Pa ~/.ssh/identity.pub
772 .Pa ~/.ssh/id_dsa.pub
775 .Pa ~/.ssh/id_rsa.pub
777 in the user's home directory.
778 The user should then copy the public key
780 .Pa ~/.ssh/authorized_keys
781 in his/her home directory on the remote machine.
784 file corresponds to the conventional
786 file, and has one key
787 per line, though the lines can be very long.
788 After this, the user can log in without giving the password.
790 The most convenient way to use public key authentication may be with an
791 authentication agent.
794 for more information.
796 Challenge-response authentication works as follows:
797 The server sends an arbitrary
799 text, and prompts for a response.
800 Protocol 2 allows multiple challenges and responses;
801 protocol 1 is restricted to just one challenge/response.
802 Examples of challenge-response authentication include
803 BSD Authentication (see
805 and PAM (some non-OpenBSD systems).
807 Finally, if other authentication methods fail,
809 prompts the user for a password.
810 The password is sent to the remote
811 host for checking; however, since all communications are encrypted,
812 the password cannot be seen by someone listening on the network.
815 automatically maintains and checks a database containing
816 identification for all hosts it has ever been used with.
817 Host keys are stored in
818 .Pa ~/.ssh/known_hosts
819 in the user's home directory.
820 Additionally, the file
821 .Pa /etc/ssh/ssh_known_hosts
822 is automatically checked for known hosts.
823 Any new hosts are automatically added to the user's file.
824 If a host's identification ever changes,
826 warns about this and disables password authentication to prevent
827 server spoofing or man-in-the-middle attacks,
828 which could otherwise be used to circumvent the encryption.
830 .Cm StrictHostKeyChecking
831 option can be used to control logins to machines whose
832 host key is not known or has changed.
834 When the user's identity has been accepted by the server, the server
835 either executes the given command, or logs into the machine and gives
836 the user a normal shell on the remote machine.
837 All communication with
838 the remote command or shell will be automatically encrypted.
840 If a pseudo-terminal has been allocated (normal login session), the
841 user may use the escape characters noted below.
843 If no pseudo-tty has been allocated,
844 the session is transparent and can be used to reliably transfer binary data.
845 On most systems, setting the escape character to
847 will also make the session transparent even if a tty is used.
849 The session terminates when the command or shell on the remote
850 machine exits and all X11 and TCP connections have been closed.
851 .Sh ESCAPE CHARACTERS
852 When a pseudo-terminal has been requested,
854 supports a number of functions through the use of an escape character.
856 A single tilde character can be sent as
858 or by following the tilde by a character other than those described below.
859 The escape character must always follow a newline to be interpreted as
861 The escape character can be changed in configuration files using the
863 configuration directive or on the command line by the
867 The supported escapes (assuming the default
877 List forwarded connections.
881 at logout when waiting for forwarded connection / X11 sessions to terminate.
883 Display a list of escape characters.
885 Send a BREAK to the remote system
886 (only useful for SSH protocol version 2 and if the peer supports it).
889 Currently this allows the addition of port forwardings using the
895 It also allows the cancellation of existing remote port-forwardings
898 .Fl KR Oo Ar bind_address : Oc Ar port .
900 .Ic !\& Ns Ar command
901 allows the user to execute a local command if the
902 .Ic PermitLocalCommand
905 Basic help is available, using the
909 Request rekeying of the connection
910 (only useful for SSH protocol version 2 and if the peer supports it).
913 Forwarding of arbitrary TCP connections over the secure channel can
914 be specified either on the command line or in a configuration file.
915 One possible application of TCP forwarding is a secure connection to a
916 mail server; another is going through firewalls.
918 In the example below, we look at encrypting communication between
919 an IRC client and server, even though the IRC server does not directly
920 support encrypted communications.
921 This works as follows:
922 the user connects to the remote host using
924 specifying a port to be used to forward connections
925 to the remote server.
926 After that it is possible to start the service which is to be encrypted
927 on the client machine,
928 connecting to the same local port,
931 will encrypt and forward the connection.
933 The following example tunnels an IRC session from client machine
937 .Dq server.example.com :
938 .Bd -literal -offset 4n
939 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
940 $ irc -c '#users' -p 1234 pinky 127.0.0.1
943 This tunnels a connection to IRC server
944 .Dq server.example.com ,
950 It doesn't matter which port is used,
951 as long as it's greater than 1023
952 (remember, only root can open sockets on privileged ports)
953 and doesn't conflict with any ports already in use.
954 The connection is forwarded to port 6667 on the remote server,
955 since that's the standard port for IRC services.
961 and the remote command
963 is specified to allow an amount of time
964 (10 seconds, in the example)
965 to start the service which is to be tunnelled.
966 If no connections are made within the time specified,
974 (or see the description of the
980 and the user is using X11 (the
982 environment variable is set), the connection to the X11 display is
983 automatically forwarded to the remote side in such a way that any X11
984 programs started from the shell (or command) will go through the
985 encrypted channel, and the connection to the real X server will be made
986 from the local machine.
987 The user should not manually set
989 Forwarding of X11 connections can be
990 configured on the command line or in configuration files.
996 will point to the server machine, but with a display number greater than zero.
997 This is normal, and happens because
1001 X server on the server machine for forwarding the
1002 connections over the encrypted channel.
1005 will also automatically set up Xauthority data on the server machine.
1006 For this purpose, it will generate a random authorization cookie,
1007 store it in Xauthority on the server, and verify that any forwarded
1008 connections carry this cookie and replace it by the real cookie when
1009 the connection is opened.
1010 The real authentication cookie is never
1011 sent to the server machine (and no cookies are sent in the plain).
1017 (or see the description of the
1022 the user is using an authentication agent, the connection to the agent
1023 is automatically forwarded to the remote side.
1024 .Sh VERIFYING HOST KEYS
1025 When connecting to a server for the first time,
1026 a fingerprint of the server's public key is presented to the user
1028 .Cm StrictHostKeyChecking
1030 Fingerprints can be determined using
1033 .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1035 If the fingerprint is already known, it can be matched
1036 and the key can be accepted or rejected.
1037 Because of the difficulty of comparing host keys
1038 just by looking at hex strings,
1039 there is also support to compare host keys visually,
1046 a small ASCII graphic gets displayed on every login to a server, no matter
1047 if the session itself is interactive or not.
1048 By learning the pattern a known server produces, a user can easily
1049 find out that the host key has changed when a completely different pattern
1051 Because these patterns are not unambiguous however, a pattern that looks
1052 similar to the pattern remembered only gives a good probability that the
1053 host key is the same, not guaranteed proof.
1055 To get a listing of the fingerprints along with their random art for
1056 all known hosts, the following command line can be used:
1058 .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1060 If the fingerprint is unknown,
1061 an alternative method of verification is available:
1062 SSH fingerprints verified by DNS.
1063 An additional resource record (RR),
1065 is added to a zonefile
1066 and the connecting client is able to match the fingerprint
1067 with that of the key presented.
1069 In this example, we are connecting a client to a server,
1070 .Dq host.example.com .
1071 The SSHFP resource records should first be added to the zonefile for
1073 .Bd -literal -offset indent
1074 $ ssh-keygen -r host.example.com.
1077 The output lines will have to be added to the zonefile.
1078 To check that the zone is answering fingerprint queries:
1080 .Dl $ dig -t SSHFP host.example.com
1082 Finally the client connects:
1083 .Bd -literal -offset indent
1084 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1086 Matching host key fingerprint found in DNS.
1087 Are you sure you want to continue connecting (yes/no)?
1091 .Cm VerifyHostKeyDNS
1094 for more information.
1095 .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1097 contains support for Virtual Private Network (VPN) tunnelling
1100 network pseudo-device,
1101 allowing two networks to be joined securely.
1104 configuration option
1106 controls whether the server supports this,
1107 and at what level (layer 2 or 3 traffic).
1109 The following example would connect client network 10.0.50.0/24
1110 with remote network 10.0.99.0/24 using a point-to-point connection
1111 from 10.1.1.1 to 10.1.1.2,
1112 provided that the SSH server running on the gateway to the remote network,
1113 at 192.168.1.15, allows it.
1116 .Bd -literal -offset indent
1117 # ssh -f -w 0:1 192.168.1.15 true
1118 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1119 # route add 10.0.99.0/24 10.1.1.2
1123 .Bd -literal -offset indent
1124 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1125 # route add 10.0.50.0/24 10.1.1.1
1128 Client access may be more finely tuned via the
1129 .Pa /root/.ssh/authorized_keys
1130 file (see below) and the
1133 The following entry would permit connections on
1137 and on tun device 2 from user
1142 .Dq forced-commands-only :
1143 .Bd -literal -offset 2n
1144 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1145 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1148 Since an SSH-based setup entails a fair amount of overhead,
1149 it may be more suited to temporary setups,
1150 such as for wireless VPNs.
1151 More permanent VPNs are better provided by tools such as
1157 will normally set the following environment variables:
1158 .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1162 variable indicates the location of the X11 server.
1163 It is automatically set by
1165 to point to a value of the form
1169 indicates the host where the shell runs, and
1171 is an integer \*(Ge 1.
1173 uses this special value to forward X11 connections over the secure
1175 The user should normally not set
1178 will render the X11 connection insecure (and will require the user to
1179 manually copy any required authorization cookies).
1181 Set to the path of the user's home directory.
1185 set for compatibility with systems that use this variable.
1187 Set to the path of the user's mailbox.
1191 as specified when compiling
1196 needs a passphrase, it will read the passphrase from the current
1197 terminal if it was run from a terminal.
1200 does not have a terminal associated with it but
1204 are set, it will execute the program specified by
1206 and open an X11 window to read the passphrase.
1207 This is particularly useful when calling
1212 (Note that on some machines it
1213 may be necessary to redirect the input from
1216 .It Ev SSH_AUTH_SOCK
1217 Identifies the path of a
1219 socket used to communicate with the agent.
1220 .It Ev SSH_CONNECTION
1221 Identifies the client and server ends of the connection.
1222 The variable contains
1223 four space-separated values: client IP address, client port number,
1224 server IP address, and server port number.
1225 .It Ev SSH_ORIGINAL_COMMAND
1226 This variable contains the original command line if a forced command
1228 It can be used to extract the original arguments.
1230 This is set to the name of the tty (path to the device) associated
1231 with the current shell or command.
1232 If the current session has no tty,
1233 this variable is not set.
1235 This variable is set to indicate the present time zone if it
1236 was set when the daemon was started (i.e. the daemon passes the value
1237 on to new connections).
1239 Set to the name of the user logging in.
1245 .Pa ~/.ssh/environment ,
1246 and adds lines of the format
1248 to the environment if the file exists and users are allowed to
1249 change their environment.
1250 For more information, see the
1251 .Cm PermitUserEnvironment
1255 .Bl -tag -width Ds -compact
1257 This file is used for host-based authentication (see above).
1258 On some machines this file may need to be
1259 world-readable if the user's home directory is on an NFS partition,
1263 Additionally, this file must be owned by the user,
1264 and must not have write permissions for anyone else.
1266 permission for most machines is read/write for the user, and not
1267 accessible by others.
1270 This file is used in exactly the same way as
1272 but allows host-based authentication without permitting login with
1276 This directory is the default location for all user-specific configuration
1277 and authentication information.
1278 There is no general requirement to keep the entire contents of this directory
1279 secret, but the recommended permissions are read/write/execute for the user,
1280 and not accessible by others.
1282 .It ~/.ssh/authorized_keys
1283 Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1284 The format of this file is described in the
1287 This file is not highly sensitive, but the recommended
1288 permissions are read/write for the user, and not accessible by others.
1291 This is the per-user configuration file.
1292 The file format and configuration options are described in
1294 Because of the potential for abuse, this file must have strict permissions:
1295 read/write for the user, and not accessible by others.
1297 .It ~/.ssh/environment
1298 Contains additional definitions for environment variables; see
1305 Contains the private key for authentication.
1307 contain sensitive data and should be readable by the user but not
1308 accessible by others (read/write/execute).
1310 will simply ignore a private key file if it is accessible by others.
1311 It is possible to specify a passphrase when
1312 generating the key which will be used to encrypt the
1313 sensitive part of this file using 3DES.
1315 .It ~/.ssh/identity.pub
1316 .It ~/.ssh/id_dsa.pub
1317 .It ~/.ssh/id_rsa.pub
1318 Contains the public key for authentication.
1320 sensitive and can (but need not) be readable by anyone.
1322 .It ~/.ssh/known_hosts
1323 Contains a list of host keys for all hosts the user has logged into
1324 that are not already in the systemwide list of known host keys.
1327 for further details of the format of this file.
1330 Commands in this file are executed by
1332 when the user logs in, just before the user's shell (or command) is
1336 manual page for more information.
1338 .It /etc/hosts.equiv
1339 This file is for host-based authentication (see above).
1340 It should only be writable by root.
1342 .It /etc/shosts.equiv
1343 This file is used in exactly the same way as
1345 but allows host-based authentication without permitting login with
1348 .It Pa /etc/ssh/ssh_config
1349 Systemwide configuration file.
1350 The file format and configuration options are described in
1353 .It /etc/ssh/ssh_host_key
1354 .It /etc/ssh/ssh_host_dsa_key
1355 .It /etc/ssh/ssh_host_rsa_key
1356 These three files contain the private parts of the host keys
1357 and are used for host-based authentication.
1358 If protocol version 1 is used,
1360 must be setuid root, since the host key is readable only by root.
1361 For protocol version 2,
1365 to access the host keys,
1366 eliminating the requirement that
1368 be setuid root when host-based authentication is used.
1373 .It /etc/ssh/ssh_known_hosts
1374 Systemwide list of known host keys.
1375 This file should be prepared by the
1376 system administrator to contain the public host keys of all machines in the
1378 It should be world-readable.
1381 for further details of the format of this file.
1384 Commands in this file are executed by
1386 when the user logs in, just before the user's shell (or command) is started.
1389 manual page for more information.
1405 .%T "The Secure Shell (SSH) Protocol Assigned Numbers"
1410 .%T "The Secure Shell (SSH) Protocol Architecture"
1415 .%T "The Secure Shell (SSH) Authentication Protocol"
1420 .%T "The Secure Shell (SSH) Transport Layer Protocol"
1425 .%T "The Secure Shell (SSH) Connection Protocol"
1430 .%T "Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints"
1435 .%T "Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)"
1440 .%T "The Secure Shell (SSH) Session Channel Break Extension"
1445 .%T "The Secure Shell (SSH) Transport Layer Encryption Modes"
1450 .%T "Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol"
1455 .%T "Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol"
1460 .%T "The Secure Shell (SSH) Public Key File Format"
1464 .%T "Hash Visualization: a New Technique to improve Real-World Security"
1468 .%O "International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)"
1471 OpenSSH is a derivative of the original and free
1472 ssh 1.2.12 release by Tatu Ylonen.
1473 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1474 Theo de Raadt and Dug Song
1475 removed many bugs, re-added newer features and
1477 Markus Friedl contributed the support for SSH
1478 protocol versions 1.5 and 2.0.
andersk / openssh.git / blob