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 1.308 2010/08/04 05:37:01 djm Exp $
44 .Nd OpenSSH SSH client (remote login program)
48 .Op Fl 1246AaCfgKkMNnqsTtVvXxYy
49 .Op Fl b Ar bind_address
50 .Op Fl c Ar cipher_spec
51 .Op Fl D Oo Ar bind_address : Oc Ns Ar port
52 .Op Fl e Ar escape_char
53 .Op Fl F Ar configfile
55 .Op Fl i Ar identity_file
56 .Op Fl L Oo Ar bind_address : Oc Ns Ar port : Ns Ar host : Ns Ar hostport
57 .Op Fl l Ar login_name
62 .Op Fl R Oo Ar bind_address : Oc Ns Ar port : Ns Ar host : Ns Ar hostport
64 .Op Fl W Ar host : Ns Ar port
65 .Op Fl w Ar local_tun Ns Op : Ns Ar remote_tun
66 .Oo Ar user Ns @ Oc Ns Ar hostname
71 (SSH client) is a program for logging into a remote machine and for
72 executing commands on a remote machine.
73 It is intended to replace rlogin and rsh,
74 and provide secure encrypted communications between
75 two untrusted hosts over an insecure network.
76 X11 connections and arbitrary TCP ports
77 can also be forwarded over the secure channel.
80 connects and logs into the specified
86 his/her identity to the remote machine using one of several methods
87 depending on the protocol version used (see below).
92 it is executed on the remote host instead of a login shell.
94 The options are as follows:
99 to try protocol version 1 only.
103 to try protocol version 2 only.
107 to use IPv4 addresses only.
111 to use IPv6 addresses only.
113 Enables forwarding of the authentication agent connection.
114 This can also be specified on a per-host basis in a configuration file.
116 Agent forwarding should be enabled with caution.
117 Users with the ability to bypass file permissions on the remote host
120 socket) can access the local agent through the forwarded connection.
121 An attacker cannot obtain key material from the agent,
122 however they can perform operations on the keys that enable them to
123 authenticate using the identities loaded into the agent.
125 Disables forwarding of the authentication agent connection.
126 .It Fl b Ar bind_address
129 on the local machine as the source address
131 Only useful on systems with more than one address.
133 Requests compression of all data (including stdin, stdout, stderr, and
134 data for forwarded X11 and TCP connections).
135 The compression algorithm is the same used by
139 can be controlled by the
141 option for protocol version 1.
142 Compression is desirable on modem lines and other
143 slow connections, but will only slow down things on fast networks.
144 The default value can be set on a host-by-host basis in the
145 configuration files; see the
148 .It Fl c Ar cipher_spec
149 Selects the cipher specification for encrypting the session.
151 Protocol version 1 allows specification of a single cipher.
152 The supported values are
158 (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
159 It is believed to be secure.
161 is a fast block cipher; it appears very secure and is much faster than
164 is only supported in the
166 client for interoperability with legacy protocol 1 implementations
167 that do not support the
170 Its use is strongly discouraged due to cryptographic weaknesses.
174 For protocol version 2,
176 is a comma-separated list of ciphers
177 listed in order of preference.
182 for more information.
185 .Oo Ar bind_address : Oc
191 application-level port forwarding.
192 This works by allocating a socket to listen to
194 on the local side, optionally bound to the specified
196 Whenever a connection is made to this port, the
197 connection is forwarded over the secure channel, and the application
198 protocol is then used to determine where to connect to from the
200 Currently the SOCKS4 and SOCKS5 protocols are supported, and
202 will act as a SOCKS server.
203 Only root can forward privileged ports.
204 Dynamic port forwardings can also be specified in the configuration file.
206 IPv6 addresses can be specified by enclosing the address in square brackets.
207 Only the superuser can forward privileged ports.
208 By default, the local port is bound in accordance with the
213 may be used to bind the connection to a specific address.
218 indicates that the listening port be bound for local use only, while an
221 indicates that the port should be available from all interfaces.
222 .It Fl e Ar escape_char
223 Sets the escape character for sessions with a pty (default:
225 The escape character is only recognized at the beginning of a line.
226 The escape character followed by a dot
228 closes the connection;
229 followed by control-Z suspends the connection;
230 and followed by itself sends the escape character once.
231 Setting the character to
233 disables any escapes and makes the session fully transparent.
234 .It Fl F Ar configfile
235 Specifies an alternative per-user configuration file.
236 If a configuration file is given on the command line,
237 the system-wide configuration file
238 .Pq Pa /etc/ssh/ssh_config
240 The default for the per-user configuration file is
245 to go to background just before command execution.
248 is going to ask for passwords or passphrases, but the user
249 wants it in the background.
252 The recommended way to start X11 programs at a remote site is with
254 .Ic ssh -f host xterm .
257 .Cm ExitOnForwardFailure
258 configuration option is set to
260 then a client started with
262 will wait for all remote port forwards to be successfully established
263 before placing itself in the background.
265 Allows remote hosts to connect to local forwarded ports.
267 Specify the PKCS#11 shared library
269 should use to communicate with a PKCS#11 token providing the user's
271 .It Fl i Ar identity_file
272 Selects a file from which the identity (private key) for
273 RSA or DSA authentication is read.
276 for protocol version 1, and
280 for protocol version 2.
281 Identity files may also be specified on
282 a per-host basis in the configuration file.
283 It is possible to have multiple
285 options (and multiple identities specified in
286 configuration files).
288 will also try to load certificate information from the filename obtained
291 to identity filenames.
293 Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
294 credentials to the server.
296 Disables forwarding (delegation) of GSSAPI credentials to the server.
299 .Oo Ar bind_address : Oc
300 .Ar port : host : hostport
303 Specifies that the given port on the local (client) host is to be
304 forwarded to the given host and port on the remote side.
305 This works by allocating a socket to listen to
307 on the local side, optionally bound to the specified
309 Whenever a connection is made to this port, the
310 connection is forwarded over the secure channel, and a connection is
315 from the remote machine.
316 Port forwardings can also be specified in the configuration file.
317 IPv6 addresses can be specified by enclosing the address in square brackets.
318 Only the superuser can forward privileged ports.
319 By default, the local port is bound in accordance with the
324 may be used to bind the connection to a specific address.
329 indicates that the listening port be bound for local use only, while an
332 indicates that the port should be available from all interfaces.
333 .It Fl l Ar login_name
334 Specifies the user to log in as on the remote machine.
335 This also may be specified on a per-host basis in the configuration file.
341 mode for connection sharing.
348 mode with confirmation required before slave connections are accepted.
349 Refer to the description of
355 Additionally, for protocol version 2 a comma-separated list of MAC
356 (message authentication code) algorithms can
357 be specified in order of preference.
360 keyword for more information.
362 Do not execute a remote command.
363 This is useful for just forwarding ports
364 (protocol version 2 only).
368 (actually, prevents reading from stdin).
369 This must be used when
371 is run in the background.
372 A common trick is to use this to run X11 programs on a remote machine.
374 .Ic ssh -n shadows.cs.hut.fi emacs &
375 will start an emacs on shadows.cs.hut.fi, and the X11
376 connection will be automatically forwarded over an encrypted channel.
379 program will be put in the background.
380 (This does not work if
382 needs to ask for a password or passphrase; see also the
386 Control an active connection multiplexing master process.
389 option is specified, the
391 argument is interpreted and passed to the master process.
394 (check that the master process is running),
396 (request forwardings without command execution) and
398 (request the master to exit).
400 Can be used to give options in the format used in the configuration file.
401 This is useful for specifying options for which there is no separate
403 For full details of the options listed below, and their possible values, see
406 .Bl -tag -width Ds -offset indent -compact
410 .It ChallengeResponseAuthentication
414 .It ClearAllForwardings
417 .It ConnectionAttempts
423 .It ExitOnForwardFailure
426 .It ForwardX11Trusted
428 .It GlobalKnownHostsFile
429 .It GSSAPIAuthentication
430 .It GSSAPIDelegateCredentials
433 .It HostbasedAuthentication
434 .It HostKeyAlgorithms
439 .It KbdInteractiveDevices
444 .It NoHostAuthenticationForLocalhost
445 .It NumberOfPasswordPrompts
446 .It PasswordAuthentication
447 .It PermitLocalCommand
450 .It PreferredAuthentications
453 .It PubkeyAuthentication
456 .It RhostsRSAAuthentication
457 .It RSAAuthentication
459 .It ServerAliveInterval
460 .It ServerAliveCountMax
461 .It StrictHostKeyChecking
465 .It UsePrivilegedPort
467 .It UserKnownHostsFile
474 Port to connect to on the remote host.
475 This can be specified on a
476 per-host basis in the configuration file.
479 Causes most warning and diagnostic messages to be suppressed.
482 .Oo Ar bind_address : Oc
483 .Ar port : host : hostport
486 Specifies that the given port on the remote (server) host is to be
487 forwarded to the given host and port on the local side.
488 This works by allocating a socket to listen to
490 on the remote side, and whenever a connection is made to this port, the
491 connection is forwarded over the secure channel, and a connection is
496 from the local machine.
498 Port forwardings can also be specified in the configuration file.
499 Privileged ports can be forwarded only when
500 logging in as root on the remote machine.
501 IPv6 addresses can be specified by enclosing the address in square braces.
503 By default, the listening socket on the server will be bound to the loopback
505 This may be overridden by specifying a
511 indicates that the remote socket should listen on all interfaces.
514 will only succeed if the server's
516 option is enabled (see
517 .Xr sshd_config 5 ) .
523 the listen port will be dynamically allocated on the server and reported
524 to the client at run time.
525 When used together with
527 the allocated port will be printed to the standard output.
529 Specifies the location of a control socket for connection sharing,
532 to disable connection sharing.
533 Refer to the description of
541 May be used to request invocation of a subsystem on the remote system.
542 Subsystems are a feature of the SSH2 protocol which facilitate the use
543 of SSH as a secure transport for other applications (eg.\&
545 The subsystem is specified as the remote command.
547 Disable pseudo-tty allocation.
549 Force pseudo-tty allocation.
550 This can be used to execute arbitrary
551 screen-based programs on a remote machine, which can be very useful,
552 e.g. when implementing menu services.
555 options force tty allocation, even if
559 Display the version number and exit.
564 to print debugging messages about its progress.
566 debugging connection, authentication, and configuration problems.
569 options increase the verbosity.
571 .It Fl W Ar host : Ns Ar port
572 Requests that standard input and output on the client be forwarded to
576 over the secure channel.
580 .Cm ExitOnForwardFailure
582 .Cm ClearAllForwardings
583 and works with Protocol version 2 only.
585 .Ar local_tun Ns Op : Ns Ar remote_tun
589 device forwarding with the specified
591 devices between the client
596 The devices may be specified by numerical ID or the keyword
598 which uses the next available tunnel device.
601 is not specified, it defaults to
611 directive is unset, it is set to the default tunnel mode, which is
614 Enables X11 forwarding.
615 This can also be specified on a per-host basis in a configuration file.
617 X11 forwarding should be enabled with caution.
618 Users with the ability to bypass file permissions on the remote host
619 (for the user's X authorization database)
620 can access the local X11 display through the forwarded connection.
621 An attacker may then be able to perform activities such as keystroke monitoring.
623 For this reason, X11 forwarding is subjected to X11 SECURITY extension
624 restrictions by default.
629 .Cm ForwardX11Trusted
632 for more information.
634 Disables X11 forwarding.
636 Enables trusted X11 forwarding.
637 Trusted X11 forwardings are not subjected to the X11 SECURITY extension
640 Send log information using the
643 By default this information is sent to stderr.
647 may additionally obtain configuration data from
648 a per-user configuration file and a system-wide configuration file.
649 The file format and configuration options are described in
653 exits with the exit status of the remote command or with 255
654 if an error occurred.
656 The OpenSSH SSH client supports SSH protocols 1 and 2.
657 The default is to use protocol 2 only,
658 though this can be changed via the
667 Both protocols support similar authentication methods,
668 but protocol 2 is the default since
669 it provides additional mechanisms for confidentiality
670 (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
671 and integrity (hmac-md5, hmac-sha1, umac-64, hmac-ripemd160).
672 Protocol 1 lacks a strong mechanism for ensuring the
673 integrity of the connection.
675 The methods available for authentication are:
676 GSSAPI-based authentication,
677 host-based authentication,
678 public key authentication,
679 challenge-response authentication,
680 and password authentication.
681 Authentication methods are tried in the order specified above,
682 though protocol 2 has a configuration option to change the default order:
683 .Cm PreferredAuthentications .
685 Host-based authentication works as follows:
686 If the machine the user logs in from is listed in
689 .Pa /etc/shosts.equiv
690 on the remote machine, and the user names are
691 the same on both sides, or if the files
695 exist in the user's home directory on the
696 remote machine and contain a line containing the name of the client
697 machine and the name of the user on that machine, the user is
698 considered for login.
699 Additionally, the server
701 be able to verify the client's
702 host key (see the description of
703 .Pa /etc/ssh/ssh_known_hosts
705 .Pa ~/.ssh/known_hosts ,
707 for login to be permitted.
708 This authentication method closes security holes due to IP
709 spoofing, DNS spoofing, and routing spoofing.
710 [Note to the administrator:
711 .Pa /etc/hosts.equiv ,
713 and the rlogin/rsh protocol in general, are inherently insecure and should be
714 disabled if security is desired.]
716 Public key authentication works as follows:
717 The scheme is based on public-key cryptography,
719 where encryption and decryption are done using separate keys,
720 and it is unfeasible to derive the decryption key from the encryption key.
721 The idea is that each user creates a public/private
722 key pair for authentication purposes.
723 The server knows the public key, and only the user knows the private key.
725 implements public key authentication protocol automatically,
726 using either the RSA or DSA algorithms.
727 Protocol 1 is restricted to using only RSA keys,
728 but protocol 2 may use either.
733 contains a brief discussion of the two algorithms.
736 .Pa ~/.ssh/authorized_keys
737 lists the public keys that are permitted for logging in.
738 When the user logs in, the
740 program tells the server which key pair it would like to use for
742 The client proves that it has access to the private key
743 and the server checks that the corresponding public key
744 is authorized to accept the account.
746 The user creates his/her key pair by running
748 This stores the private key in
756 and stores the public key in
757 .Pa ~/.ssh/identity.pub
759 .Pa ~/.ssh/id_dsa.pub
762 .Pa ~/.ssh/id_rsa.pub
764 in the user's home directory.
765 The user should then copy the public key
767 .Pa ~/.ssh/authorized_keys
768 in his/her home directory on the remote machine.
771 file corresponds to the conventional
773 file, and has one key
774 per line, though the lines can be very long.
775 After this, the user can log in without giving the password.
777 A variation on public key authentication
778 is available in the form of certificate authentication:
779 instead of a set of public/private keys,
780 signed certificates are used.
781 This has the advantage that a single trusted certification authority
782 can be used in place of many public/private keys.
787 for more information.
789 The most convenient way to use public key or certificate authentication
790 may be with an authentication agent.
793 for more information.
795 Challenge-response authentication works as follows:
796 The server sends an arbitrary
798 text, and prompts for a response.
799 Protocol 2 allows multiple challenges and responses;
800 protocol 1 is restricted to just one challenge/response.
801 Examples of challenge-response authentication include
802 BSD Authentication (see
804 and PAM (some non-OpenBSD systems).
806 Finally, if other authentication methods fail,
808 prompts the user for a password.
809 The password is sent to the remote
810 host for checking; however, since all communications are encrypted,
811 the password cannot be seen by someone listening on the network.
814 automatically maintains and checks a database containing
815 identification for all hosts it has ever been used with.
816 Host keys are stored in
817 .Pa ~/.ssh/known_hosts
818 in the user's home directory.
819 Additionally, the file
820 .Pa /etc/ssh/ssh_known_hosts
821 is automatically checked for known hosts.
822 Any new hosts are automatically added to the user's file.
823 If a host's identification ever changes,
825 warns about this and disables password authentication to prevent
826 server spoofing or man-in-the-middle attacks,
827 which could otherwise be used to circumvent the encryption.
829 .Cm StrictHostKeyChecking
830 option can be used to control logins to machines whose
831 host key is not known or has changed.
833 When the user's identity has been accepted by the server, the server
834 either executes the given command, or logs into the machine and gives
835 the user a normal shell on the remote machine.
836 All communication with
837 the remote command or shell will be automatically encrypted.
839 If a pseudo-terminal has been allocated (normal login session), the
840 user may use the escape characters noted below.
842 If no pseudo-tty has been allocated,
843 the session is transparent and can be used to reliably transfer binary data.
844 On most systems, setting the escape character to
846 will also make the session transparent even if a tty is used.
848 The session terminates when the command or shell on the remote
849 machine exits and all X11 and TCP connections have been closed.
850 .Sh ESCAPE CHARACTERS
851 When a pseudo-terminal has been requested,
853 supports a number of functions through the use of an escape character.
855 A single tilde character can be sent as
857 or by following the tilde by a character other than those described below.
858 The escape character must always follow a newline to be interpreted as
860 The escape character can be changed in configuration files using the
862 configuration directive or on the command line by the
866 The supported escapes (assuming the default
876 List forwarded connections.
880 at logout when waiting for forwarded connection / X11 sessions to terminate.
882 Display a list of escape characters.
884 Send a BREAK to the remote system
885 (only useful for SSH protocol version 2 and if the peer supports it).
888 Currently this allows the addition of port forwardings using the
894 It also allows the cancellation of existing remote port-forwardings
897 .Fl KR Oo Ar bind_address : Oc Ar port .
899 .Ic !\& Ns Ar command
900 allows the user to execute a local command if the
901 .Ic PermitLocalCommand
904 Basic help is available, using the
908 Request rekeying of the connection
909 (only useful for SSH protocol version 2 and if the peer supports it).
912 Forwarding of arbitrary TCP connections over the secure channel can
913 be specified either on the command line or in a configuration file.
914 One possible application of TCP forwarding is a secure connection to a
915 mail server; another is going through firewalls.
917 In the example below, we look at encrypting communication between
918 an IRC client and server, even though the IRC server does not directly
919 support encrypted communications.
920 This works as follows:
921 the user connects to the remote host using
923 specifying a port to be used to forward connections
924 to the remote server.
925 After that it is possible to start the service which is to be encrypted
926 on the client machine,
927 connecting to the same local port,
930 will encrypt and forward the connection.
932 The following example tunnels an IRC session from client machine
936 .Dq server.example.com :
937 .Bd -literal -offset 4n
938 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
939 $ irc -c '#users' -p 1234 pinky 127.0.0.1
942 This tunnels a connection to IRC server
943 .Dq server.example.com ,
949 It doesn't matter which port is used,
950 as long as it's greater than 1023
951 (remember, only root can open sockets on privileged ports)
952 and doesn't conflict with any ports already in use.
953 The connection is forwarded to port 6667 on the remote server,
954 since that's the standard port for IRC services.
960 and the remote command
962 is specified to allow an amount of time
963 (10 seconds, in the example)
964 to start the service which is to be tunnelled.
965 If no connections are made within the time specified,
973 (or see the description of the
979 and the user is using X11 (the
981 environment variable is set), the connection to the X11 display is
982 automatically forwarded to the remote side in such a way that any X11
983 programs started from the shell (or command) will go through the
984 encrypted channel, and the connection to the real X server will be made
985 from the local machine.
986 The user should not manually set
988 Forwarding of X11 connections can be
989 configured on the command line or in configuration files.
995 will point to the server machine, but with a display number greater than zero.
996 This is normal, and happens because
1000 X server on the server machine for forwarding the
1001 connections over the encrypted channel.
1004 will also automatically set up Xauthority data on the server machine.
1005 For this purpose, it will generate a random authorization cookie,
1006 store it in Xauthority on the server, and verify that any forwarded
1007 connections carry this cookie and replace it by the real cookie when
1008 the connection is opened.
1009 The real authentication cookie is never
1010 sent to the server machine (and no cookies are sent in the plain).
1016 (or see the description of the
1021 the user is using an authentication agent, the connection to the agent
1022 is automatically forwarded to the remote side.
1023 .Sh VERIFYING HOST KEYS
1024 When connecting to a server for the first time,
1025 a fingerprint of the server's public key is presented to the user
1027 .Cm StrictHostKeyChecking
1029 Fingerprints can be determined using
1032 .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1034 If the fingerprint is already known, it can be matched
1035 and the key can be accepted or rejected.
1036 Because of the difficulty of comparing host keys
1037 just by looking at hex strings,
1038 there is also support to compare host keys visually,
1045 a small ASCII graphic gets displayed on every login to a server, no matter
1046 if the session itself is interactive or not.
1047 By learning the pattern a known server produces, a user can easily
1048 find out that the host key has changed when a completely different pattern
1050 Because these patterns are not unambiguous however, a pattern that looks
1051 similar to the pattern remembered only gives a good probability that the
1052 host key is the same, not guaranteed proof.
1054 To get a listing of the fingerprints along with their random art for
1055 all known hosts, the following command line can be used:
1057 .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1059 If the fingerprint is unknown,
1060 an alternative method of verification is available:
1061 SSH fingerprints verified by DNS.
1062 An additional resource record (RR),
1064 is added to a zonefile
1065 and the connecting client is able to match the fingerprint
1066 with that of the key presented.
1068 In this example, we are connecting a client to a server,
1069 .Dq host.example.com .
1070 The SSHFP resource records should first be added to the zonefile for
1072 .Bd -literal -offset indent
1073 $ ssh-keygen -r host.example.com.
1076 The output lines will have to be added to the zonefile.
1077 To check that the zone is answering fingerprint queries:
1079 .Dl $ dig -t SSHFP host.example.com
1081 Finally the client connects:
1082 .Bd -literal -offset indent
1083 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1085 Matching host key fingerprint found in DNS.
1086 Are you sure you want to continue connecting (yes/no)?
1090 .Cm VerifyHostKeyDNS
1093 for more information.
1094 .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1096 contains support for Virtual Private Network (VPN) tunnelling
1099 network pseudo-device,
1100 allowing two networks to be joined securely.
1103 configuration option
1105 controls whether the server supports this,
1106 and at what level (layer 2 or 3 traffic).
1108 The following example would connect client network 10.0.50.0/24
1109 with remote network 10.0.99.0/24 using a point-to-point connection
1110 from 10.1.1.1 to 10.1.1.2,
1111 provided that the SSH server running on the gateway to the remote network,
1112 at 192.168.1.15, allows it.
1115 .Bd -literal -offset indent
1116 # ssh -f -w 0:1 192.168.1.15 true
1117 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1118 # route add 10.0.99.0/24 10.1.1.2
1122 .Bd -literal -offset indent
1123 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1124 # route add 10.0.50.0/24 10.1.1.1
1127 Client access may be more finely tuned via the
1128 .Pa /root/.ssh/authorized_keys
1129 file (see below) and the
1132 The following entry would permit connections on
1136 and on tun device 2 from user
1141 .Dq forced-commands-only :
1142 .Bd -literal -offset 2n
1143 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1144 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1147 Since an SSH-based setup entails a fair amount of overhead,
1148 it may be more suited to temporary setups,
1149 such as for wireless VPNs.
1150 More permanent VPNs are better provided by tools such as
1156 will normally set the following environment variables:
1157 .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1161 variable indicates the location of the X11 server.
1162 It is automatically set by
1164 to point to a value of the form
1168 indicates the host where the shell runs, and
1170 is an integer \*(Ge 1.
1172 uses this special value to forward X11 connections over the secure
1174 The user should normally not set
1177 will render the X11 connection insecure (and will require the user to
1178 manually copy any required authorization cookies).
1180 Set to the path of the user's home directory.
1184 set for compatibility with systems that use this variable.
1186 Set to the path of the user's mailbox.
1190 as specified when compiling
1195 needs a passphrase, it will read the passphrase from the current
1196 terminal if it was run from a terminal.
1199 does not have a terminal associated with it but
1203 are set, it will execute the program specified by
1205 and open an X11 window to read the passphrase.
1206 This is particularly useful when calling
1211 (Note that on some machines it
1212 may be necessary to redirect the input from
1215 .It Ev SSH_AUTH_SOCK
1216 Identifies the path of a
1218 socket used to communicate with the agent.
1219 .It Ev SSH_CONNECTION
1220 Identifies the client and server ends of the connection.
1221 The variable contains
1222 four space-separated values: client IP address, client port number,
1223 server IP address, and server port number.
1224 .It Ev SSH_ORIGINAL_COMMAND
1225 This variable contains the original command line if a forced command
1227 It can be used to extract the original arguments.
1229 This is set to the name of the tty (path to the device) associated
1230 with the current shell or command.
1231 If the current session has no tty,
1232 this variable is not set.
1234 This variable is set to indicate the present time zone if it
1235 was set when the daemon was started (i.e. the daemon passes the value
1236 on to new connections).
1238 Set to the name of the user logging in.
1244 .Pa ~/.ssh/environment ,
1245 and adds lines of the format
1247 to the environment if the file exists and users are allowed to
1248 change their environment.
1249 For more information, see the
1250 .Cm PermitUserEnvironment
1254 .Bl -tag -width Ds -compact
1256 This file is used for host-based authentication (see above).
1257 On some machines this file may need to be
1258 world-readable if the user's home directory is on an NFS partition,
1262 Additionally, this file must be owned by the user,
1263 and must not have write permissions for anyone else.
1265 permission for most machines is read/write for the user, and not
1266 accessible by others.
1269 This file is used in exactly the same way as
1271 but allows host-based authentication without permitting login with
1275 This directory is the default location for all user-specific configuration
1276 and authentication information.
1277 There is no general requirement to keep the entire contents of this directory
1278 secret, but the recommended permissions are read/write/execute for the user,
1279 and not accessible by others.
1281 .It ~/.ssh/authorized_keys
1282 Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1283 The format of this file is described in the
1286 This file is not highly sensitive, but the recommended
1287 permissions are read/write for the user, and not accessible by others.
1290 This is the per-user configuration file.
1291 The file format and configuration options are described in
1293 Because of the potential for abuse, this file must have strict permissions:
1294 read/write for the user, and not accessible by others.
1296 .It ~/.ssh/environment
1297 Contains additional definitions for environment variables; see
1304 Contains the private key for authentication.
1306 contain sensitive data and should be readable by the user but not
1307 accessible by others (read/write/execute).
1309 will simply ignore a private key file if it is accessible by others.
1310 It is possible to specify a passphrase when
1311 generating the key which will be used to encrypt the
1312 sensitive part of this file using 3DES.
1314 .It ~/.ssh/identity.pub
1315 .It ~/.ssh/id_dsa.pub
1316 .It ~/.ssh/id_rsa.pub
1317 Contains the public key for authentication.
1319 sensitive and can (but need not) be readable by anyone.
1321 .It ~/.ssh/known_hosts
1322 Contains a list of host keys for all hosts the user has logged into
1323 that are not already in the systemwide list of known host keys.
1326 for further details of the format of this file.
1329 Commands in this file are executed by
1331 when the user logs in, just before the user's shell (or command) is
1335 manual page for more information.
1337 .It /etc/hosts.equiv
1338 This file is for host-based authentication (see above).
1339 It should only be writable by root.
1341 .It /etc/shosts.equiv
1342 This file is used in exactly the same way as
1344 but allows host-based authentication without permitting login with
1347 .It Pa /etc/ssh/ssh_config
1348 Systemwide configuration file.
1349 The file format and configuration options are described in
1352 .It /etc/ssh/ssh_host_key
1353 .It /etc/ssh/ssh_host_dsa_key
1354 .It /etc/ssh/ssh_host_rsa_key
1355 These three files contain the private parts of the host keys
1356 and are used for host-based authentication.
1357 If protocol version 1 is used,
1359 must be setuid root, since the host key is readable only by root.
1360 For protocol version 2,
1364 to access the host keys,
1365 eliminating the requirement that
1367 be setuid root when host-based authentication is used.
1372 .It /etc/ssh/ssh_known_hosts
1373 Systemwide list of known host keys.
1374 This file should be prepared by the
1375 system administrator to contain the public host keys of all machines in the
1377 It should be world-readable.
1380 for further details of the format of this file.
1383 Commands in this file are executed by
1385 when the user logs in, just before the user's shell (or command) is started.
1388 manual page for more information.
1404 .%T "The Secure Shell (SSH) Protocol Assigned Numbers"
1409 .%T "The Secure Shell (SSH) Protocol Architecture"
1414 .%T "The Secure Shell (SSH) Authentication Protocol"
1419 .%T "The Secure Shell (SSH) Transport Layer Protocol"
1424 .%T "The Secure Shell (SSH) Connection Protocol"
1429 .%T "Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints"
1434 .%T "Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)"
1439 .%T "The Secure Shell (SSH) Session Channel Break Extension"
1444 .%T "The Secure Shell (SSH) Transport Layer Encryption Modes"
1449 .%T "Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol"
1454 .%T "Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol"
1459 .%T "The Secure Shell (SSH) Public Key File Format"
1463 .%T "Hash Visualization: a New Technique to improve Real-World Security"
1467 .%O "International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)"
1470 OpenSSH is a derivative of the original and free
1471 ssh 1.2.12 release by Tatu Ylonen.
1472 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1473 Theo de Raadt and Dug Song
1474 removed many bugs, re-added newer features and
1476 Markus Friedl contributed the support for SSH
1477 protocol versions 1.5 and 2.0.
projects / FreeBSD / FreeBSD.git / blob