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 $
37 .Dd $Mdocdate: November 18 2010 $
42 .Nd OpenSSH SSH client (remote login program)
46 .Op Fl 1246AaCfgKkMNnqsTtVvXxYy
47 .Op Fl b Ar bind_address
48 .Op Fl c Ar cipher_spec
49 .Op Fl D Oo Ar bind_address : Oc Ns Ar port
50 .Op Fl e Ar escape_char
51 .Op Fl F Ar configfile
53 .Op Fl i Ar identity_file
54 .Op Fl L Oo Ar bind_address : Oc Ns Ar port : Ns Ar host : Ns Ar hostport
55 .Op Fl l Ar login_name
60 .Op Fl R Oo Ar bind_address : Oc Ns Ar port : Ns Ar host : Ns Ar hostport
62 .Op Fl W Ar host : Ns Ar port
63 .Op Fl w Ar local_tun Ns Op : Ns Ar remote_tun
64 .Oo Ar user Ns @ Oc Ns Ar hostname
69 (SSH client) is a program for logging into a remote machine and for
70 executing commands on a remote machine.
71 It is intended to replace rlogin and rsh,
72 and provide secure encrypted communications between
73 two untrusted hosts over an insecure network.
74 X11 connections and arbitrary TCP ports
75 can also be forwarded over the secure channel.
78 connects and logs into the specified
84 his/her identity to the remote machine using one of several methods
85 depending on the protocol version used (see below).
90 it is executed on the remote host instead of a login shell.
92 The options are as follows:
97 to try protocol version 1 only.
101 to try protocol version 2 only.
105 to use IPv4 addresses only.
109 to use IPv6 addresses only.
111 Enables forwarding of the authentication agent connection.
112 This can also be specified on a per-host basis in a configuration file.
114 Agent forwarding should be enabled with caution.
115 Users with the ability to bypass file permissions on the remote host
118 socket) can access the local agent through the forwarded connection.
119 An attacker cannot obtain key material from the agent,
120 however they can perform operations on the keys that enable them to
121 authenticate using the identities loaded into the agent.
123 Disables forwarding of the authentication agent connection.
124 .It Fl b Ar bind_address
127 on the local machine as the source address
129 Only useful on systems with more than one address.
131 Requests compression of all data (including stdin, stdout, stderr, and
132 data for forwarded X11 and TCP connections).
133 The compression algorithm is the same used by
137 can be controlled by the
139 option for protocol version 1.
140 Compression is desirable on modem lines and other
141 slow connections, but will only slow down things on fast networks.
142 The default value can be set on a host-by-host basis in the
143 configuration files; see the
146 .It Fl c Ar cipher_spec
147 Selects the cipher specification for encrypting the session.
149 Protocol version 1 allows specification of a single cipher.
150 The supported values are
156 (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
157 It is believed to be secure.
159 is a fast block cipher; it appears very secure and is much faster than
162 is only supported in the
164 client for interoperability with legacy protocol 1 implementations
165 that do not support the
168 Its use is strongly discouraged due to cryptographic weaknesses.
172 For protocol version 2,
174 is a comma-separated list of ciphers
175 listed in order of preference.
180 for more information.
183 .Oo Ar bind_address : Oc
189 application-level port forwarding.
190 This works by allocating a socket to listen to
192 on the local side, optionally bound to the specified
194 Whenever a connection is made to this port, the
195 connection is forwarded over the secure channel, and the application
196 protocol is then used to determine where to connect to from the
198 Currently the SOCKS4 and SOCKS5 protocols are supported, and
200 will act as a SOCKS server.
201 Only root can forward privileged ports.
202 Dynamic port forwardings can also be specified in the configuration file.
204 IPv6 addresses can be specified by enclosing the address in square brackets.
205 Only the superuser can forward privileged ports.
206 By default, the local port is bound in accordance with the
211 may be used to bind the connection to a specific address.
216 indicates that the listening port be bound for local use only, while an
219 indicates that the port should be available from all interfaces.
220 .It Fl e Ar escape_char
221 Sets the escape character for sessions with a pty (default:
223 The escape character is only recognized at the beginning of a line.
224 The escape character followed by a dot
226 closes the connection;
227 followed by control-Z suspends the connection;
228 and followed by itself sends the escape character once.
229 Setting the character to
231 disables any escapes and makes the session fully transparent.
232 .It Fl F Ar configfile
233 Specifies an alternative per-user configuration file.
234 If a configuration file is given on the command line,
235 the system-wide configuration file
236 .Pq Pa /etc/ssh/ssh_config
238 The default for the per-user configuration file is
243 to go to background just before command execution.
246 is going to ask for passwords or passphrases, but the user
247 wants it in the background.
250 The recommended way to start X11 programs at a remote site is with
252 .Ic ssh -f host xterm .
255 .Cm ExitOnForwardFailure
256 configuration option is set to
258 then a client started with
260 will wait for all remote port forwards to be successfully established
261 before placing itself in the background.
263 Allows remote hosts to connect to local forwarded ports.
265 Specify the PKCS#11 shared library
267 should use to communicate with a PKCS#11 token providing the user's
269 .It Fl i Ar identity_file
270 Selects a file from which the identity (private key) for
271 public key authentication is read.
274 for protocol version 1, and
279 for protocol version 2.
280 Identity files may also be specified on
281 a per-host basis in the configuration file.
282 It is possible to have multiple
284 options (and multiple identities specified in
285 configuration files).
287 will also try to load certificate information from the filename obtained
290 to identity filenames.
292 Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
293 credentials to the server.
295 Disables forwarding (delegation) of GSSAPI credentials to the server.
298 .Oo Ar bind_address : Oc
299 .Ar port : host : hostport
302 Specifies that the given port on the local (client) host is to be
303 forwarded to the given host and port on the remote side.
304 This works by allocating a socket to listen to
306 on the local side, optionally bound to the specified
308 Whenever a connection is made to this port, the
309 connection is forwarded over the secure channel, and a connection is
314 from the remote machine.
315 Port forwardings can also be specified in the configuration file.
316 IPv6 addresses can be specified by enclosing the address in square brackets.
317 Only the superuser can forward privileged ports.
318 By default, the local port is bound in accordance with the
323 may be used to bind the connection to a specific address.
328 indicates that the listening port be bound for local use only, while an
331 indicates that the port should be available from all interfaces.
332 .It Fl l Ar login_name
333 Specifies the user to log in as on the remote machine.
334 This also may be specified on a per-host basis in the configuration file.
340 mode for connection sharing.
347 mode with confirmation required before slave connections are accepted.
348 Refer to the description of
354 Additionally, for protocol version 2 a comma-separated list of MAC
355 (message authentication code) algorithms can
356 be specified in order of preference.
359 keyword for more information.
361 Do not execute a remote command.
362 This is useful for just forwarding ports
363 (protocol version 2 only).
367 (actually, prevents reading from stdin).
368 This must be used when
370 is run in the background.
371 A common trick is to use this to run X11 programs on a remote machine.
373 .Ic ssh -n shadows.cs.hut.fi emacs &
374 will start an emacs on shadows.cs.hut.fi, and the X11
375 connection will be automatically forwarded over an encrypted channel.
378 program will be put in the background.
379 (This does not work if
381 needs to ask for a password or passphrase; see also the
385 Control an active connection multiplexing master process.
388 option is specified, the
390 argument is interpreted and passed to the master process.
393 (check that the master process is running),
395 (request forwardings without command execution) and
397 (request the master to exit).
399 Can be used to give options in the format used in the configuration file.
400 This is useful for specifying options for which there is no separate
402 For full details of the options listed below, and their possible values, see
405 .Bl -tag -width Ds -offset indent -compact
409 .It ChallengeResponseAuthentication
413 .It ClearAllForwardings
416 .It ConnectionAttempts
422 .It ExitOnForwardFailure
425 .It ForwardX11Trusted
427 .It GlobalKnownHostsFile
428 .It GSSAPIAuthentication
429 .It GSSAPIDelegateCredentials
432 .It HostbasedAuthentication
433 .It HostKeyAlgorithms
439 .It KbdInteractiveDevices
445 .It NoHostAuthenticationForLocalhost
446 .It NumberOfPasswordPrompts
447 .It PasswordAuthentication
448 .It PermitLocalCommand
451 .It PreferredAuthentications
454 .It PubkeyAuthentication
457 .It RhostsRSAAuthentication
458 .It RSAAuthentication
460 .It ServerAliveInterval
461 .It ServerAliveCountMax
462 .It StrictHostKeyChecking
466 .It UsePrivilegedPort
468 .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
652 The OpenSSH SSH client supports SSH protocols 1 and 2.
653 The default is to use protocol 2 only,
654 though this can be changed via the
663 Both protocols support similar authentication methods,
664 but protocol 2 is the default since
665 it provides additional mechanisms for confidentiality
666 (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
667 and integrity (hmac-md5, hmac-sha1, umac-64, hmac-ripemd160).
668 Protocol 1 lacks a strong mechanism for ensuring the
669 integrity of the connection.
671 The methods available for authentication are:
672 GSSAPI-based authentication,
673 host-based authentication,
674 public key authentication,
675 challenge-response authentication,
676 and password authentication.
677 Authentication methods are tried in the order specified above,
678 though protocol 2 has a configuration option to change the default order:
679 .Cm PreferredAuthentications .
681 Host-based authentication works as follows:
682 If the machine the user logs in from is listed in
685 .Pa /etc/ssh/shosts.equiv
686 on the remote machine, and the user names are
687 the same on both sides, or if the files
691 exist in the user's home directory on the
692 remote machine and contain a line containing the name of the client
693 machine and the name of the user on that machine, the user is
694 considered for login.
695 Additionally, the server
697 be able to verify the client's
698 host key (see the description of
699 .Pa /etc/ssh/ssh_known_hosts
701 .Pa ~/.ssh/known_hosts ,
703 for login to be permitted.
704 This authentication method closes security holes due to IP
705 spoofing, DNS spoofing, and routing spoofing.
706 [Note to the administrator:
707 .Pa /etc/hosts.equiv ,
709 and the rlogin/rsh protocol in general, are inherently insecure and should be
710 disabled if security is desired.]
712 Public key authentication works as follows:
713 The scheme is based on public-key cryptography,
715 where encryption and decryption are done using separate keys,
716 and it is unfeasible to derive the decryption key from the encryption key.
717 The idea is that each user creates a public/private
718 key pair for authentication purposes.
719 The server knows the public key, and only the user knows the private key.
721 implements public key authentication protocol automatically,
722 using one of the DSA, ECDSA or RSA algorithms.
723 Protocol 1 is restricted to using only RSA keys,
724 but protocol 2 may use any.
729 contains a brief discussion of the DSA and RSA algorithms.
732 .Pa ~/.ssh/authorized_keys
733 lists the public keys that are permitted for logging in.
734 When the user logs in, the
736 program tells the server which key pair it would like to use for
738 The client proves that it has access to the private key
739 and the server checks that the corresponding public key
740 is authorized to accept the account.
742 The user creates his/her key pair by running
744 This stores the private key in
754 and stores the public key in
755 .Pa ~/.ssh/identity.pub
757 .Pa ~/.ssh/id_dsa.pub
759 .Pa ~/.ssh/id_ecdsa.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.
990 Take note that X11 forwarding can represent a security hazard.
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 Pa ~/.ssh/authorized_keys
1283 Lists the public keys (DSA/ECDSA/RSA) that can be used for logging in as
1285 The format of this file is described in the
1288 This file is not highly sensitive, but the recommended
1289 permissions are read/write for the user, and not accessible by others.
1291 .It Pa ~/.ssh/config
1292 This is the per-user configuration file.
1293 The file format and configuration options are described in
1295 Because of the potential for abuse, this file must have strict permissions:
1296 read/write for the user, and not accessible by others.
1298 .It Pa ~/.ssh/environment
1299 Contains additional definitions for environment variables; see
1303 .It Pa ~/.ssh/identity
1304 .It Pa ~/.ssh/id_dsa
1305 .It Pa ~/.ssh/id_ecdsa
1306 .It Pa ~/.ssh/id_rsa
1307 Contains the private key for authentication.
1309 contain sensitive data and should be readable by the user but not
1310 accessible by others (read/write/execute).
1312 will simply ignore a private key file if it is accessible by others.
1313 It is possible to specify a passphrase when
1314 generating the key which will be used to encrypt the
1315 sensitive part of this file using 3DES.
1317 .It Pa ~/.ssh/identity.pub
1318 .It Pa ~/.ssh/id_dsa.pub
1319 .It Pa ~/.ssh/id_ecdsa.pub
1320 .It Pa ~/.ssh/id_rsa.pub
1321 Contains the public key for authentication.
1323 sensitive and can (but need not) be readable by anyone.
1325 .It Pa ~/.ssh/known_hosts
1326 Contains a list of host keys for all hosts the user has logged into
1327 that are not already in the systemwide list of known host keys.
1330 for further details of the format of this file.
1333 Commands in this file are executed by
1335 when the user logs in, just before the user's shell (or command) is
1339 manual page for more information.
1341 .It Pa /etc/hosts.equiv
1342 This file is for host-based authentication (see above).
1343 It should only be writable by root.
1345 .It Pa /etc/ssh/shosts.equiv
1346 This file is used in exactly the same way as
1348 but allows host-based authentication without permitting login with
1351 .It Pa /etc/ssh/ssh_config
1352 Systemwide configuration file.
1353 The file format and configuration options are described in
1356 .It Pa /etc/ssh/ssh_host_key
1357 .It Pa /etc/ssh/ssh_host_dsa_key
1358 .It Pa /etc/ssh/ssh_host_ecdsa_key
1359 .It Pa /etc/ssh/ssh_host_rsa_key
1360 These three files contain the private parts of the host keys
1361 and are used for host-based authentication.
1362 If protocol version 1 is used,
1364 must be setuid root, since the host key is readable only by root.
1365 For protocol version 2,
1369 to access the host keys,
1370 eliminating the requirement that
1372 be setuid root when host-based authentication is used.
1377 .It Pa /etc/ssh/ssh_known_hosts
1378 Systemwide list of known host keys.
1379 This file should be prepared by the
1380 system administrator to contain the public host keys of all machines in the
1382 It should be world-readable.
1385 for further details of the format of this file.
1387 .It Pa /etc/ssh/sshrc
1388 Commands in this file are executed by
1390 when the user logs in, just before the user's shell (or command) is started.
1393 manual page for more information.
1397 exits with the exit status of the remote command or with 255
1398 if an error occurred.
1413 .%T "The Secure Shell (SSH) Protocol Assigned Numbers"
1418 .%T "The Secure Shell (SSH) Protocol Architecture"
1423 .%T "The Secure Shell (SSH) Authentication Protocol"
1428 .%T "The Secure Shell (SSH) Transport Layer Protocol"
1433 .%T "The Secure Shell (SSH) Connection Protocol"
1438 .%T "Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints"
1443 .%T "Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)"
1448 .%T "The Secure Shell (SSH) Session Channel Break Extension"
1453 .%T "The Secure Shell (SSH) Transport Layer Encryption Modes"
1458 .%T "Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol"
1463 .%T "Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol"
1468 .%T "The Secure Shell (SSH) Public Key File Format"
1473 .%T "Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer"
1477 .%T "Hash Visualization: a New Technique to improve Real-World Security"
1481 .%O "International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)"
1484 OpenSSH is a derivative of the original and free
1485 ssh 1.2.12 release by Tatu Ylonen.
1486 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1487 Theo de Raadt and Dug Song
1488 removed many bugs, re-added newer features and
1490 Markus Friedl contributed the support for SSH
1491 protocol versions 1.5 and 2.0.
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