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 1.316 2010/11/18 15:01:00 jmc Exp $
43 .Nd OpenSSH SSH client (remote login program)
47 .Op Fl 1246AaCfgKkMNnqsTtVvXxYy
48 .Op Fl b Ar bind_address
49 .Op Fl c Ar cipher_spec
50 .Op Fl D Oo Ar bind_address : Oc Ns Ar port
51 .Op Fl e Ar escape_char
52 .Op Fl F Ar configfile
54 .Op Fl i Ar identity_file
55 .Op Fl L Oo Ar bind_address : Oc Ns Ar port : Ns Ar host : Ns Ar hostport
56 .Op Fl l Ar login_name
61 .Op Fl R Oo Ar bind_address : Oc Ns Ar port : Ns Ar host : Ns Ar hostport
63 .Op Fl W Ar host : Ns Ar port
64 .Op Fl w Ar local_tun Ns Op : Ns Ar remote_tun
65 .Oo Ar user Ns @ Oc Ns Ar hostname
70 (SSH client) is a program for logging into a remote machine and for
71 executing commands on a remote machine.
72 It is intended to replace rlogin and rsh,
73 and provide secure encrypted communications between
74 two untrusted hosts over an insecure network.
75 X11 connections and arbitrary TCP ports
76 can also be forwarded over the secure channel.
79 connects and logs into the specified
85 his/her identity to the remote machine using one of several methods
86 depending on the protocol version used (see below).
91 it is executed on the remote host instead of a login shell.
93 The options are as follows:
98 to try protocol version 1 only.
102 to try protocol version 2 only.
106 to use IPv4 addresses only.
110 to use IPv6 addresses only.
112 Enables forwarding of the authentication agent connection.
113 This can also be specified on a per-host basis in a configuration file.
115 Agent forwarding should be enabled with caution.
116 Users with the ability to bypass file permissions on the remote host
119 socket) can access the local agent through the forwarded connection.
120 An attacker cannot obtain key material from the agent,
121 however they can perform operations on the keys that enable them to
122 authenticate using the identities loaded into the agent.
124 Disables forwarding of the authentication agent connection.
125 .It Fl b Ar bind_address
128 on the local machine as the source address
130 Only useful on systems with more than one address.
132 Requests compression of all data (including stdin, stdout, stderr, and
133 data for forwarded X11 and TCP connections).
134 The compression algorithm is the same used by
138 can be controlled by the
140 option for protocol version 1.
141 Compression is desirable on modem lines and other
142 slow connections, but will only slow down things on fast networks.
143 The default value can be set on a host-by-host basis in the
144 configuration files; see the
147 .It Fl c Ar cipher_spec
148 Selects the cipher specification for encrypting the session.
150 Protocol version 1 allows specification of a single cipher.
151 The supported values are
157 (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
158 It is believed to be secure.
160 is a fast block cipher; it appears very secure and is much faster than
163 is only supported in the
165 client for interoperability with legacy protocol 1 implementations
166 that do not support the
169 Its use is strongly discouraged due to cryptographic weaknesses.
173 For protocol version 2,
175 is a comma-separated list of ciphers
176 listed in order of preference.
181 for more information.
184 .Oo Ar bind_address : Oc
190 application-level port forwarding.
191 This works by allocating a socket to listen to
193 on the local side, optionally bound to the specified
195 Whenever a connection is made to this port, the
196 connection is forwarded over the secure channel, and the application
197 protocol is then used to determine where to connect to from the
199 Currently the SOCKS4 and SOCKS5 protocols are supported, and
201 will act as a SOCKS server.
202 Only root can forward privileged ports.
203 Dynamic port forwardings can also be specified in the configuration file.
205 IPv6 addresses can be specified by enclosing the address in square brackets.
206 Only the superuser can forward privileged ports.
207 By default, the local port is bound in accordance with the
212 may be used to bind the connection to a specific address.
217 indicates that the listening port be bound for local use only, while an
220 indicates that the port should be available from all interfaces.
221 .It Fl e Ar escape_char
222 Sets the escape character for sessions with a pty (default:
224 The escape character is only recognized at the beginning of a line.
225 The escape character followed by a dot
227 closes the connection;
228 followed by control-Z suspends the connection;
229 and followed by itself sends the escape character once.
230 Setting the character to
232 disables any escapes and makes the session fully transparent.
233 .It Fl F Ar configfile
234 Specifies an alternative per-user configuration file.
235 If a configuration file is given on the command line,
236 the system-wide configuration file
237 .Pq Pa /etc/ssh/ssh_config
239 The default for the per-user configuration file is
244 to go to background just before command execution.
247 is going to ask for passwords or passphrases, but the user
248 wants it in the background.
251 The recommended way to start X11 programs at a remote site is with
253 .Ic ssh -f host xterm .
256 .Cm ExitOnForwardFailure
257 configuration option is set to
259 then a client started with
261 will wait for all remote port forwards to be successfully established
262 before placing itself in the background.
264 Allows remote hosts to connect to local forwarded ports.
266 Specify the PKCS#11 shared library
268 should use to communicate with a PKCS#11 token providing the user's
270 .It Fl i Ar identity_file
271 Selects a file from which the identity (private key) for
272 public key authentication is read.
275 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
440 .It KbdInteractiveDevices
446 .It NoHostAuthenticationForLocalhost
447 .It NumberOfPasswordPrompts
448 .It PasswordAuthentication
449 .It PermitLocalCommand
452 .It PreferredAuthentications
455 .It PubkeyAuthentication
458 .It RhostsRSAAuthentication
459 .It RSAAuthentication
461 .It ServerAliveInterval
462 .It ServerAliveCountMax
463 .It StrictHostKeyChecking
467 .It UsePrivilegedPort
469 .It UserKnownHostsFile
476 Port to connect to on the remote host.
477 This can be specified on a
478 per-host basis in the configuration file.
481 Causes most warning and diagnostic messages to be suppressed.
484 .Oo Ar bind_address : Oc
485 .Ar port : host : hostport
488 Specifies that the given port on the remote (server) host is to be
489 forwarded to the given host and port on the local side.
490 This works by allocating a socket to listen to
492 on the remote side, and whenever a connection is made to this port, the
493 connection is forwarded over the secure channel, and a connection is
498 from the local machine.
500 Port forwardings can also be specified in the configuration file.
501 Privileged ports can be forwarded only when
502 logging in as root on the remote machine.
503 IPv6 addresses can be specified by enclosing the address in square braces.
505 By default, the listening socket on the server will be bound to the loopback
507 This may be overridden by specifying a
513 indicates that the remote socket should listen on all interfaces.
516 will only succeed if the server's
518 option is enabled (see
519 .Xr sshd_config 5 ) .
525 the listen port will be dynamically allocated on the server and reported
526 to the client at run time.
527 When used together with
529 the allocated port will be printed to the standard output.
531 Specifies the location of a control socket for connection sharing,
534 to disable connection sharing.
535 Refer to the description of
543 May be used to request invocation of a subsystem on the remote system.
544 Subsystems are a feature of the SSH2 protocol which facilitate the use
545 of SSH as a secure transport for other applications (eg.\&
547 The subsystem is specified as the remote command.
549 Disable pseudo-tty allocation.
551 Force pseudo-tty allocation.
552 This can be used to execute arbitrary
553 screen-based programs on a remote machine, which can be very useful,
554 e.g. when implementing menu services.
557 options force tty allocation, even if
561 Display the version number and exit.
566 to print debugging messages about its progress.
568 debugging connection, authentication, and configuration problems.
571 options increase the verbosity.
573 .It Fl W Ar host : Ns Ar port
574 Requests that standard input and output on the client be forwarded to
578 over the secure channel.
582 .Cm ExitOnForwardFailure
584 .Cm ClearAllForwardings
585 and works with Protocol version 2 only.
587 .Ar local_tun Ns Op : Ns Ar remote_tun
591 device forwarding with the specified
593 devices between the client
598 The devices may be specified by numerical ID or the keyword
600 which uses the next available tunnel device.
603 is not specified, it defaults to
613 directive is unset, it is set to the default tunnel mode, which is
616 Enables X11 forwarding.
617 This can also be specified on a per-host basis in a configuration file.
619 X11 forwarding should be enabled with caution.
620 Users with the ability to bypass file permissions on the remote host
621 (for the user's X authorization database)
622 can access the local X11 display through the forwarded connection.
623 An attacker may then be able to perform activities such as keystroke monitoring.
625 For this reason, X11 forwarding is subjected to X11 SECURITY extension
626 restrictions by default.
631 .Cm ForwardX11Trusted
634 for more information.
636 Disables X11 forwarding.
638 Enables trusted X11 forwarding.
639 Trusted X11 forwardings are not subjected to the X11 SECURITY extension
642 Send log information using the
645 By default this information is sent to stderr.
649 may additionally obtain configuration data from
650 a per-user configuration file and a system-wide configuration file.
651 The file format and configuration options are described in
654 The OpenSSH SSH client supports SSH protocols 1 and 2.
655 The default is to use protocol 2 only,
656 though this can be changed via the
665 Both protocols support similar authentication methods,
666 but protocol 2 is the default since
667 it provides additional mechanisms for confidentiality
668 (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
669 and integrity (hmac-md5, hmac-sha1, umac-64, hmac-ripemd160).
670 Protocol 1 lacks a strong mechanism for ensuring the
671 integrity of the connection.
673 The methods available for authentication are:
674 GSSAPI-based authentication,
675 host-based authentication,
676 public key authentication,
677 challenge-response authentication,
678 and password authentication.
679 Authentication methods are tried in the order specified above,
680 though protocol 2 has a configuration option to change the default order:
681 .Cm PreferredAuthentications .
683 Host-based authentication works as follows:
684 If the machine the user logs in from is listed in
687 .Pa /etc/shosts.equiv
688 on the remote machine, and the user names are
689 the same on both sides, or if the files
693 exist in the user's home directory on the
694 remote machine and contain a line containing the name of the client
695 machine and the name of the user on that machine, the user is
696 considered for login.
697 Additionally, the server
699 be able to verify the client's
700 host key (see the description of
701 .Pa /etc/ssh/ssh_known_hosts
703 .Pa ~/.ssh/known_hosts ,
705 for login to be permitted.
706 This authentication method closes security holes due to IP
707 spoofing, DNS spoofing, and routing spoofing.
708 [Note to the administrator:
709 .Pa /etc/hosts.equiv ,
711 and the rlogin/rsh protocol in general, are inherently insecure and should be
712 disabled if security is desired.]
714 Public key authentication works as follows:
715 The scheme is based on public-key cryptography,
717 where encryption and decryption are done using separate keys,
718 and it is unfeasible to derive the decryption key from the encryption key.
719 The idea is that each user creates a public/private
720 key pair for authentication purposes.
721 The server knows the public key, and only the user knows the private key.
723 implements public key authentication protocol automatically,
724 using one of the DSA, ECDSA or RSA algorithms.
725 Protocol 1 is restricted to using only RSA keys,
726 but protocol 2 may use any.
731 contains a brief discussion of the DSA and RSA algorithms.
734 .Pa ~/.ssh/authorized_keys
735 lists the public keys that are permitted for logging in.
736 When the user logs in, the
738 program tells the server which key pair it would like to use for
740 The client proves that it has access to the private key
741 and the server checks that the corresponding public key
742 is authorized to accept the account.
744 The user creates his/her key pair by running
746 This stores the private key in
756 and stores the public key in
757 .Pa ~/.ssh/identity.pub
759 .Pa ~/.ssh/id_dsa.pub
761 .Pa ~/.ssh/id_ecdsa.pub
764 .Pa ~/.ssh/id_rsa.pub
766 in the user's home directory.
767 The user should then copy the public key
769 .Pa ~/.ssh/authorized_keys
770 in his/her home directory on the remote machine.
773 file corresponds to the conventional
775 file, and has one key
776 per line, though the lines can be very long.
777 After this, the user can log in without giving the password.
779 A variation on public key authentication
780 is available in the form of certificate authentication:
781 instead of a set of public/private keys,
782 signed certificates are used.
783 This has the advantage that a single trusted certification authority
784 can be used in place of many public/private keys.
789 for more information.
791 The most convenient way to use public key or certificate authentication
792 may be with an authentication agent.
795 for more information.
797 Challenge-response authentication works as follows:
798 The server sends an arbitrary
800 text, and prompts for a response.
801 Protocol 2 allows multiple challenges and responses;
802 protocol 1 is restricted to just one challenge/response.
803 Examples of challenge-response authentication include
804 BSD Authentication (see
806 and PAM (some non-OpenBSD systems).
808 Finally, if other authentication methods fail,
810 prompts the user for a password.
811 The password is sent to the remote
812 host for checking; however, since all communications are encrypted,
813 the password cannot be seen by someone listening on the network.
816 automatically maintains and checks a database containing
817 identification for all hosts it has ever been used with.
818 Host keys are stored in
819 .Pa ~/.ssh/known_hosts
820 in the user's home directory.
821 Additionally, the file
822 .Pa /etc/ssh/ssh_known_hosts
823 is automatically checked for known hosts.
824 Any new hosts are automatically added to the user's file.
825 If a host's identification ever changes,
827 warns about this and disables password authentication to prevent
828 server spoofing or man-in-the-middle attacks,
829 which could otherwise be used to circumvent the encryption.
831 .Cm StrictHostKeyChecking
832 option can be used to control logins to machines whose
833 host key is not known or has changed.
835 When the user's identity has been accepted by the server, the server
836 either executes the given command, or logs into the machine and gives
837 the user a normal shell on the remote machine.
838 All communication with
839 the remote command or shell will be automatically encrypted.
841 If a pseudo-terminal has been allocated (normal login session), the
842 user may use the escape characters noted below.
844 If no pseudo-tty has been allocated,
845 the session is transparent and can be used to reliably transfer binary data.
846 On most systems, setting the escape character to
848 will also make the session transparent even if a tty is used.
850 The session terminates when the command or shell on the remote
851 machine exits and all X11 and TCP connections have been closed.
852 .Sh ESCAPE CHARACTERS
853 When a pseudo-terminal has been requested,
855 supports a number of functions through the use of an escape character.
857 A single tilde character can be sent as
859 or by following the tilde by a character other than those described below.
860 The escape character must always follow a newline to be interpreted as
862 The escape character can be changed in configuration files using the
864 configuration directive or on the command line by the
868 The supported escapes (assuming the default
878 List forwarded connections.
882 at logout when waiting for forwarded connection / X11 sessions to terminate.
884 Display a list of escape characters.
886 Send a BREAK to the remote system
887 (only useful for SSH protocol version 2 and if the peer supports it).
890 Currently this allows the addition of port forwardings using the
896 It also allows the cancellation of existing remote port-forwardings
899 .Fl KR Oo Ar bind_address : Oc Ar port .
901 .Ic !\& Ns Ar command
902 allows the user to execute a local command if the
903 .Ic PermitLocalCommand
906 Basic help is available, using the
910 Request rekeying of the connection
911 (only useful for SSH protocol version 2 and if the peer supports it).
914 Forwarding of arbitrary TCP connections over the secure channel can
915 be specified either on the command line or in a configuration file.
916 One possible application of TCP forwarding is a secure connection to a
917 mail server; another is going through firewalls.
919 In the example below, we look at encrypting communication between
920 an IRC client and server, even though the IRC server does not directly
921 support encrypted communications.
922 This works as follows:
923 the user connects to the remote host using
925 specifying a port to be used to forward connections
926 to the remote server.
927 After that it is possible to start the service which is to be encrypted
928 on the client machine,
929 connecting to the same local port,
932 will encrypt and forward the connection.
934 The following example tunnels an IRC session from client machine
938 .Dq server.example.com :
939 .Bd -literal -offset 4n
940 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
941 $ irc -c '#users' -p 1234 pinky 127.0.0.1
944 This tunnels a connection to IRC server
945 .Dq server.example.com ,
951 It doesn't matter which port is used,
952 as long as it's greater than 1023
953 (remember, only root can open sockets on privileged ports)
954 and doesn't conflict with any ports already in use.
955 The connection is forwarded to port 6667 on the remote server,
956 since that's the standard port for IRC services.
962 and the remote command
964 is specified to allow an amount of time
965 (10 seconds, in the example)
966 to start the service which is to be tunnelled.
967 If no connections are made within the time specified,
975 (or see the description of the
981 and the user is using X11 (the
983 environment variable is set), the connection to the X11 display is
984 automatically forwarded to the remote side in such a way that any X11
985 programs started from the shell (or command) will go through the
986 encrypted channel, and the connection to the real X server will be made
987 from the local machine.
988 The user should not manually set
990 Forwarding of X11 connections can be
991 configured on the command line or in configuration files.
997 will point to the server machine, but with a display number greater than zero.
998 This is normal, and happens because
1002 X server on the server machine for forwarding the
1003 connections over the encrypted channel.
1006 will also automatically set up Xauthority data on the server machine.
1007 For this purpose, it will generate a random authorization cookie,
1008 store it in Xauthority on the server, and verify that any forwarded
1009 connections carry this cookie and replace it by the real cookie when
1010 the connection is opened.
1011 The real authentication cookie is never
1012 sent to the server machine (and no cookies are sent in the plain).
1018 (or see the description of the
1023 the user is using an authentication agent, the connection to the agent
1024 is automatically forwarded to the remote side.
1025 .Sh VERIFYING HOST KEYS
1026 When connecting to a server for the first time,
1027 a fingerprint of the server's public key is presented to the user
1029 .Cm StrictHostKeyChecking
1031 Fingerprints can be determined using
1034 .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1036 If the fingerprint is already known, it can be matched
1037 and the key can be accepted or rejected.
1038 Because of the difficulty of comparing host keys
1039 just by looking at hex strings,
1040 there is also support to compare host keys visually,
1047 a small ASCII graphic gets displayed on every login to a server, no matter
1048 if the session itself is interactive or not.
1049 By learning the pattern a known server produces, a user can easily
1050 find out that the host key has changed when a completely different pattern
1052 Because these patterns are not unambiguous however, a pattern that looks
1053 similar to the pattern remembered only gives a good probability that the
1054 host key is the same, not guaranteed proof.
1056 To get a listing of the fingerprints along with their random art for
1057 all known hosts, the following command line can be used:
1059 .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1061 If the fingerprint is unknown,
1062 an alternative method of verification is available:
1063 SSH fingerprints verified by DNS.
1064 An additional resource record (RR),
1066 is added to a zonefile
1067 and the connecting client is able to match the fingerprint
1068 with that of the key presented.
1070 In this example, we are connecting a client to a server,
1071 .Dq host.example.com .
1072 The SSHFP resource records should first be added to the zonefile for
1074 .Bd -literal -offset indent
1075 $ ssh-keygen -r host.example.com.
1078 The output lines will have to be added to the zonefile.
1079 To check that the zone is answering fingerprint queries:
1081 .Dl $ dig -t SSHFP host.example.com
1083 Finally the client connects:
1084 .Bd -literal -offset indent
1085 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1087 Matching host key fingerprint found in DNS.
1088 Are you sure you want to continue connecting (yes/no)?
1092 .Cm VerifyHostKeyDNS
1095 for more information.
1096 .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1098 contains support for Virtual Private Network (VPN) tunnelling
1101 network pseudo-device,
1102 allowing two networks to be joined securely.
1105 configuration option
1107 controls whether the server supports this,
1108 and at what level (layer 2 or 3 traffic).
1110 The following example would connect client network 10.0.50.0/24
1111 with remote network 10.0.99.0/24 using a point-to-point connection
1112 from 10.1.1.1 to 10.1.1.2,
1113 provided that the SSH server running on the gateway to the remote network,
1114 at 192.168.1.15, allows it.
1117 .Bd -literal -offset indent
1118 # ssh -f -w 0:1 192.168.1.15 true
1119 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1120 # route add 10.0.99.0/24 10.1.1.2
1124 .Bd -literal -offset indent
1125 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1126 # route add 10.0.50.0/24 10.1.1.1
1129 Client access may be more finely tuned via the
1130 .Pa /root/.ssh/authorized_keys
1131 file (see below) and the
1134 The following entry would permit connections on
1138 and on tun device 2 from user
1143 .Dq forced-commands-only :
1144 .Bd -literal -offset 2n
1145 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1146 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1149 Since an SSH-based setup entails a fair amount of overhead,
1150 it may be more suited to temporary setups,
1151 such as for wireless VPNs.
1152 More permanent VPNs are better provided by tools such as
1158 will normally set the following environment variables:
1159 .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1163 variable indicates the location of the X11 server.
1164 It is automatically set by
1166 to point to a value of the form
1170 indicates the host where the shell runs, and
1172 is an integer \*(Ge 1.
1174 uses this special value to forward X11 connections over the secure
1176 The user should normally not set
1179 will render the X11 connection insecure (and will require the user to
1180 manually copy any required authorization cookies).
1182 Set to the path of the user's home directory.
1186 set for compatibility with systems that use this variable.
1188 Set to the path of the user's mailbox.
1192 as specified when compiling
1197 needs a passphrase, it will read the passphrase from the current
1198 terminal if it was run from a terminal.
1201 does not have a terminal associated with it but
1205 are set, it will execute the program specified by
1207 and open an X11 window to read the passphrase.
1208 This is particularly useful when calling
1213 (Note that on some machines it
1214 may be necessary to redirect the input from
1217 .It Ev SSH_AUTH_SOCK
1218 Identifies the path of a
1220 socket used to communicate with the agent.
1221 .It Ev SSH_CONNECTION
1222 Identifies the client and server ends of the connection.
1223 The variable contains
1224 four space-separated values: client IP address, client port number,
1225 server IP address, and server port number.
1226 .It Ev SSH_ORIGINAL_COMMAND
1227 This variable contains the original command line if a forced command
1229 It can be used to extract the original arguments.
1231 This is set to the name of the tty (path to the device) associated
1232 with the current shell or command.
1233 If the current session has no tty,
1234 this variable is not set.
1236 This variable is set to indicate the present time zone if it
1237 was set when the daemon was started (i.e. the daemon passes the value
1238 on to new connections).
1240 Set to the name of the user logging in.
1246 .Pa ~/.ssh/environment ,
1247 and adds lines of the format
1249 to the environment if the file exists and users are allowed to
1250 change their environment.
1251 For more information, see the
1252 .Cm PermitUserEnvironment
1256 .Bl -tag -width Ds -compact
1258 This file is used for host-based authentication (see above).
1259 On some machines this file may need to be
1260 world-readable if the user's home directory is on an NFS partition,
1264 Additionally, this file must be owned by the user,
1265 and must not have write permissions for anyone else.
1267 permission for most machines is read/write for the user, and not
1268 accessible by others.
1271 This file is used in exactly the same way as
1273 but allows host-based authentication without permitting login with
1277 This directory is the default location for all user-specific configuration
1278 and authentication information.
1279 There is no general requirement to keep the entire contents of this directory
1280 secret, but the recommended permissions are read/write/execute for the user,
1281 and not accessible by others.
1283 .It Pa ~/.ssh/authorized_keys
1284 Lists the public keys (DSA/ECDSA/RSA) that can be used for logging in as
1286 The format of this file is described in the
1289 This file is not highly sensitive, but the recommended
1290 permissions are read/write for the user, and not accessible by others.
1292 .It Pa ~/.ssh/config
1293 This is the per-user configuration file.
1294 The file format and configuration options are described in
1296 Because of the potential for abuse, this file must have strict permissions:
1297 read/write for the user, and not accessible by others.
1299 .It Pa ~/.ssh/environment
1300 Contains additional definitions for environment variables; see
1304 .It Pa ~/.ssh/identity
1305 .It Pa ~/.ssh/id_dsa
1306 .It Pa ~/.ssh/id_ecdsa
1307 .It Pa ~/.ssh/id_rsa
1308 Contains the private key for authentication.
1310 contain sensitive data and should be readable by the user but not
1311 accessible by others (read/write/execute).
1313 will simply ignore a private key file if it is accessible by others.
1314 It is possible to specify a passphrase when
1315 generating the key which will be used to encrypt the
1316 sensitive part of this file using 3DES.
1318 .It Pa ~/.ssh/identity.pub
1319 .It Pa ~/.ssh/id_dsa.pub
1320 .It Pa ~/.ssh/id_ecdsa.pub
1321 .It Pa ~/.ssh/id_rsa.pub
1322 Contains the public key for authentication.
1324 sensitive and can (but need not) be readable by anyone.
1326 .It Pa ~/.ssh/known_hosts
1327 Contains a list of host keys for all hosts the user has logged into
1328 that are not already in the systemwide list of known host keys.
1331 for further details of the format of this file.
1334 Commands in this file are executed by
1336 when the user logs in, just before the user's shell (or command) is
1340 manual page for more information.
1342 .It Pa /etc/hosts.equiv
1343 This file is for host-based authentication (see above).
1344 It should only be writable by root.
1346 .It Pa /etc/shosts.equiv
1347 This file is used in exactly the same way as
1349 but allows host-based authentication without permitting login with
1352 .It Pa /etc/ssh/ssh_config
1353 Systemwide configuration file.
1354 The file format and configuration options are described in
1357 .It Pa /etc/ssh/ssh_host_key
1358 .It Pa /etc/ssh/ssh_host_dsa_key
1359 .It Pa /etc/ssh/ssh_host_ecdsa_key
1360 .It Pa /etc/ssh/ssh_host_rsa_key
1361 These three files contain the private parts of the host keys
1362 and are used for host-based authentication.
1363 If protocol version 1 is used,
1365 must be setuid root, since the host key is readable only by root.
1366 For protocol version 2,
1370 to access the host keys,
1371 eliminating the requirement that
1373 be setuid root when host-based authentication is used.
1378 .It Pa /etc/ssh/ssh_known_hosts
1379 Systemwide list of known host keys.
1380 This file should be prepared by the
1381 system administrator to contain the public host keys of all machines in the
1383 It should be world-readable.
1386 for further details of the format of this file.
1388 .It Pa /etc/ssh/sshrc
1389 Commands in this file are executed by
1391 when the user logs in, just before the user's shell (or command) is started.
1394 manual page for more information.
1398 exits with the exit status of the remote command or with 255
1399 if an error occurred.
1414 .%T "The Secure Shell (SSH) Protocol Assigned Numbers"
1419 .%T "The Secure Shell (SSH) Protocol Architecture"
1424 .%T "The Secure Shell (SSH) Authentication Protocol"
1429 .%T "The Secure Shell (SSH) Transport Layer Protocol"
1434 .%T "The Secure Shell (SSH) Connection Protocol"
1439 .%T "Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints"
1444 .%T "Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)"
1449 .%T "The Secure Shell (SSH) Session Channel Break Extension"
1454 .%T "The Secure Shell (SSH) Transport Layer Encryption Modes"
1459 .%T "Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol"
1464 .%T "Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol"
1469 .%T "The Secure Shell (SSH) Public Key File Format"
1474 .%T "Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer"
1478 .%T "Hash Visualization: a New Technique to improve Real-World Security"
1482 .%O "International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)"
1485 OpenSSH is a derivative of the original and free
1486 ssh 1.2.12 release by Tatu Ylonen.
1487 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1488 Theo de Raadt and Dug Song
1489 removed many bugs, re-added newer features and
1491 Markus Friedl contributed the support for SSH
1492 protocol versions 1.5 and 2.0.
projects / FreeBSD / releng / 9.0.git / blob