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 1.326 2012/06/18 12:17:18 dtucker Exp $
37 .Dd $Mdocdate: June 18 2012 $
42 .Nd OpenSSH SSH client (remote login program)
46 .Op Fl 1246AaCfgKkMNnqsTtVvXxYy
47 .Op Fl b Ar bind_address
48 .Op Fl c Ar cipher_spec
49 .Op Fl D Oo Ar bind_address : Oc Ns Ar port
50 .Op Fl e Ar escape_char
51 .Op Fl F Ar configfile
53 .Op Fl i Ar identity_file
54 .Op Fl L Oo Ar bind_address : Oc Ns Ar port : Ns Ar host : Ns Ar hostport
55 .Op Fl l Ar login_name
60 .Op Fl R Oo Ar bind_address : Oc Ns Ar port : Ns Ar host : Ns Ar hostport
62 .Op Fl W Ar host : Ns Ar port
63 .Op Fl w Ar local_tun Ns Op : Ns Ar remote_tun
64 .Oo Ar user Ns @ Oc Ns Ar hostname
69 (SSH client) is a program for logging into a remote machine and for
70 executing commands on a remote machine.
71 It is intended to replace rlogin and rsh,
72 and provide secure encrypted communications between
73 two untrusted hosts over an insecure network.
74 X11 connections and arbitrary TCP ports
75 can also be forwarded over the secure channel.
78 connects and logs into the specified
84 his/her identity to the remote machine using one of several methods
85 depending on the protocol version used (see below).
90 it is executed on the remote host instead of a login shell.
92 The options are as follows:
97 to try protocol version 1 only.
101 to try protocol version 2 only.
105 to use IPv4 addresses only.
109 to use IPv6 addresses only.
111 Enables forwarding of the authentication agent connection.
112 This can also be specified on a per-host basis in a configuration file.
114 Agent forwarding should be enabled with caution.
115 Users with the ability to bypass file permissions on the remote host
118 socket) can access the local agent through the forwarded connection.
119 An attacker cannot obtain key material from the agent,
120 however they can perform operations on the keys that enable them to
121 authenticate using the identities loaded into the agent.
123 Disables forwarding of the authentication agent connection.
124 .It Fl b Ar bind_address
127 on the local machine as the source address
129 Only useful on systems with more than one address.
131 Requests compression of all data (including stdin, stdout, stderr, and
132 data for forwarded X11 and TCP connections).
133 The compression algorithm is the same used by
137 can be controlled by the
139 option for protocol version 1.
140 Compression is desirable on modem lines and other
141 slow connections, but will only slow down things on fast networks.
142 The default value can be set on a host-by-host basis in the
143 configuration files; see the
146 .It Fl c Ar cipher_spec
147 Selects the cipher specification for encrypting the session.
149 Protocol version 1 allows specification of a single cipher.
150 The supported values are
156 (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
157 It is believed to be secure.
159 is a fast block cipher; it appears very secure and is much faster than
162 is only supported in the
164 client for interoperability with legacy protocol 1 implementations
165 that do not support the
168 Its use is strongly discouraged due to cryptographic weaknesses.
172 For protocol version 2,
174 is a comma-separated list of ciphers
175 listed in order of preference.
180 for more information.
183 .Oo Ar bind_address : Oc
189 application-level port forwarding.
190 This works by allocating a socket to listen to
192 on the local side, optionally bound to the specified
194 Whenever a connection is made to this port, the
195 connection is forwarded over the secure channel, and the application
196 protocol is then used to determine where to connect to from the
198 Currently the SOCKS4 and SOCKS5 protocols are supported, and
200 will act as a SOCKS server.
201 Only root can forward privileged ports.
202 Dynamic port forwardings can also be specified in the configuration file.
204 IPv6 addresses can be specified by enclosing the address in square brackets.
205 Only the superuser can forward privileged ports.
206 By default, the local port is bound in accordance with the
211 may be used to bind the connection to a specific address.
216 indicates that the listening port be bound for local use only, while an
219 indicates that the port should be available from all interfaces.
220 .It Fl e Ar escape_char
221 Sets the escape character for sessions with a pty (default:
223 The escape character is only recognized at the beginning of a line.
224 The escape character followed by a dot
226 closes the connection;
227 followed by control-Z suspends the connection;
228 and followed by itself sends the escape character once.
229 Setting the character to
231 disables any escapes and makes the session fully transparent.
232 .It Fl F Ar configfile
233 Specifies an alternative per-user configuration file.
234 If a configuration file is given on the command line,
235 the system-wide configuration file
236 .Pq Pa /etc/ssh/ssh_config
238 The default for the per-user configuration file is
243 to go to background just before command execution.
246 is going to ask for passwords or passphrases, but the user
247 wants it in the background.
250 The recommended way to start X11 programs at a remote site is with
252 .Ic ssh -f host xterm .
255 .Cm ExitOnForwardFailure
256 configuration option is set to
258 then a client started with
260 will wait for all remote port forwards to be successfully established
261 before placing itself in the background.
263 Allows remote hosts to connect to local forwarded ports.
265 Specify the PKCS#11 shared library
267 should use to communicate with a PKCS#11 token providing the user's
269 .It Fl i Ar identity_file
270 Selects a file from which the identity (private key) for
271 public key authentication is read.
274 for protocol version 1, and
279 for protocol version 2.
280 Identity files may also be specified on
281 a per-host basis in the configuration file.
282 It is possible to have multiple
284 options (and multiple identities specified in
285 configuration files).
287 will also try to load certificate information from the filename obtained
290 to identity filenames.
292 Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
293 credentials to the server.
295 Disables forwarding (delegation) of GSSAPI credentials to the server.
298 .Oo Ar bind_address : Oc
299 .Ar port : host : hostport
302 Specifies that the given port on the local (client) host is to be
303 forwarded to the given host and port on the remote side.
304 This works by allocating a socket to listen to
306 on the local side, optionally bound to the specified
308 Whenever a connection is made to this port, the
309 connection is forwarded over the secure channel, and a connection is
314 from the remote machine.
315 Port forwardings can also be specified in the configuration file.
316 IPv6 addresses can be specified by enclosing the address in square brackets.
317 Only the superuser can forward privileged ports.
318 By default, the local port is bound in accordance with the
323 may be used to bind the connection to a specific address.
328 indicates that the listening port be bound for local use only, while an
331 indicates that the port should be available from all interfaces.
332 .It Fl l Ar login_name
333 Specifies the user to log in as on the remote machine.
334 This also may be specified on a per-host basis in the configuration file.
340 mode for connection sharing.
347 mode with confirmation required before slave connections are accepted.
348 Refer to the description of
354 Additionally, for protocol version 2 a comma-separated list of MAC
355 (message authentication code) algorithms can
356 be specified in order of preference.
359 keyword for more information.
361 Do not execute a remote command.
362 This is useful for just forwarding ports
363 (protocol version 2 only).
367 (actually, prevents reading from stdin).
368 This must be used when
370 is run in the background.
371 A common trick is to use this to run X11 programs on a remote machine.
373 .Ic ssh -n shadows.cs.hut.fi emacs &
374 will start an emacs on shadows.cs.hut.fi, and the X11
375 connection will be automatically forwarded over an encrypted channel.
378 program will be put in the background.
379 (This does not work if
381 needs to ask for a password or passphrase; see also the
385 Control an active connection multiplexing master process.
388 option is specified, the
390 argument is interpreted and passed to the master process.
393 (check that the master process is running),
395 (request forwardings without command execution),
397 (cancel forwardings),
399 (request the master to exit), and
401 (request the master to stop accepting further multiplexing requests).
403 Can be used to give options in the format used in the configuration file.
404 This is useful for specifying options for which there is no separate
406 For full details of the options listed below, and their possible values, see
409 .Bl -tag -width Ds -offset indent -compact
413 .It ChallengeResponseAuthentication
417 .It ClearAllForwardings
420 .It ConnectionAttempts
427 .It ExitOnForwardFailure
430 .It ForwardX11Timeout
431 .It ForwardX11Trusted
433 .It GlobalKnownHostsFile
434 .It GSSAPIAuthentication
435 .It GSSAPIDelegateCredentials
438 .It HostbasedAuthentication
439 .It HostKeyAlgorithms
445 .It KbdInteractiveAuthentication
446 .It KbdInteractiveDevices
452 .It NoHostAuthenticationForLocalhost
453 .It NumberOfPasswordPrompts
454 .It PasswordAuthentication
455 .It PermitLocalCommand
458 .It PreferredAuthentications
461 .It PubkeyAuthentication
465 .It RhostsRSAAuthentication
466 .It RSAAuthentication
468 .It ServerAliveInterval
469 .It ServerAliveCountMax
470 .It StrictHostKeyChecking
474 .It UsePrivilegedPort
476 .It UserKnownHostsFile
482 Port to connect to on the remote host.
483 This can be specified on a
484 per-host basis in the configuration file.
487 Causes most warning and diagnostic messages to be suppressed.
490 .Oo Ar bind_address : Oc
491 .Ar port : host : hostport
494 Specifies that the given port on the remote (server) host is to be
495 forwarded to the given host and port on the local side.
496 This works by allocating a socket to listen to
498 on the remote side, and whenever a connection is made to this port, the
499 connection is forwarded over the secure channel, and a connection is
504 from the local machine.
506 Port forwardings can also be specified in the configuration file.
507 Privileged ports can be forwarded only when
508 logging in as root on the remote machine.
509 IPv6 addresses can be specified by enclosing the address in square brackets.
511 By default, the listening socket on the server will be bound to the loopback
513 This may be overridden by specifying a
519 indicates that the remote socket should listen on all interfaces.
522 will only succeed if the server's
524 option is enabled (see
525 .Xr sshd_config 5 ) .
531 the listen port will be dynamically allocated on the server and reported
532 to the client at run time.
533 When used together with
535 the allocated port will be printed to the standard output.
537 Specifies the location of a control socket for connection sharing,
540 to disable connection sharing.
541 Refer to the description of
549 May be used to request invocation of a subsystem on the remote system.
550 Subsystems are a feature of the SSH2 protocol which facilitate the use
551 of SSH as a secure transport for other applications (eg.\&
553 The subsystem is specified as the remote command.
555 Disable pseudo-tty allocation.
557 Force pseudo-tty allocation.
558 This can be used to execute arbitrary
559 screen-based programs on a remote machine, which can be very useful,
560 e.g. when implementing menu services.
563 options force tty allocation, even if
567 Display the version number and exit.
572 to print debugging messages about its progress.
574 debugging connection, authentication, and configuration problems.
577 options increase the verbosity.
579 .It Fl W Ar host : Ns Ar port
580 Requests that standard input and output on the client be forwarded to
584 over the secure channel.
588 .Cm ExitOnForwardFailure
590 .Cm ClearAllForwardings .
591 Works with Protocol version 2 only.
593 .Ar local_tun Ns Op : Ns Ar remote_tun
597 device forwarding with the specified
599 devices between the client
604 The devices may be specified by numerical ID or the keyword
606 which uses the next available tunnel device.
609 is not specified, it defaults to
619 directive is unset, it is set to the default tunnel mode, which is
622 Enables X11 forwarding.
623 This can also be specified on a per-host basis in a configuration file.
625 X11 forwarding should be enabled with caution.
626 Users with the ability to bypass file permissions on the remote host
627 (for the user's X authorization database)
628 can access the local X11 display through the forwarded connection.
629 An attacker may then be able to perform activities such as keystroke monitoring.
631 For this reason, X11 forwarding is subjected to X11 SECURITY extension
632 restrictions by default.
637 .Cm ForwardX11Trusted
640 for more information.
642 Disables X11 forwarding.
644 Enables trusted X11 forwarding.
645 Trusted X11 forwardings are not subjected to the X11 SECURITY extension
648 Send log information using the
651 By default this information is sent to stderr.
655 may additionally obtain configuration data from
656 a per-user configuration file and a system-wide configuration file.
657 The file format and configuration options are described in
660 The OpenSSH SSH client supports SSH protocols 1 and 2.
661 The default is to use protocol 2 only,
662 though this can be changed via the
671 Both protocols support similar authentication methods,
672 but protocol 2 is the default since
673 it provides additional mechanisms for confidentiality
674 (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
675 and integrity (hmac-md5, hmac-sha1,
676 hmac-sha2-256, hmac-sha2-512,
677 umac-64, hmac-ripemd160).
678 Protocol 1 lacks a strong mechanism for ensuring the
679 integrity of the connection.
681 The methods available for authentication are:
682 GSSAPI-based authentication,
683 host-based authentication,
684 public key authentication,
685 challenge-response authentication,
686 and password authentication.
687 Authentication methods are tried in the order specified above,
688 though protocol 2 has a configuration option to change the default order:
689 .Cm PreferredAuthentications .
691 Host-based authentication works as follows:
692 If the machine the user logs in from is listed in
695 .Pa /etc/ssh/shosts.equiv
696 on the remote machine, and the user names are
697 the same on both sides, or if the files
701 exist in the user's home directory on the
702 remote machine and contain a line containing the name of the client
703 machine and the name of the user on that machine, the user is
704 considered for login.
705 Additionally, the server
707 be able to verify the client's
708 host key (see the description of
709 .Pa /etc/ssh/ssh_known_hosts
711 .Pa ~/.ssh/known_hosts ,
713 for login to be permitted.
714 This authentication method closes security holes due to IP
715 spoofing, DNS spoofing, and routing spoofing.
716 [Note to the administrator:
717 .Pa /etc/hosts.equiv ,
719 and the rlogin/rsh protocol in general, are inherently insecure and should be
720 disabled if security is desired.]
722 Public key authentication works as follows:
723 The scheme is based on public-key cryptography,
725 where encryption and decryption are done using separate keys,
726 and it is unfeasible to derive the decryption key from the encryption key.
727 The idea is that each user creates a public/private
728 key pair for authentication purposes.
729 The server knows the public key, and only the user knows the private key.
731 implements public key authentication protocol automatically,
732 using one of the DSA, ECDSA or RSA algorithms.
733 Protocol 1 is restricted to using only RSA keys,
734 but protocol 2 may use any.
739 contains a brief discussion of the DSA and RSA algorithms.
742 .Pa ~/.ssh/authorized_keys
743 lists the public keys that are permitted for logging in.
744 When the user logs in, the
746 program tells the server which key pair it would like to use for
748 The client proves that it has access to the private key
749 and the server checks that the corresponding public key
750 is authorized to accept the account.
752 The user creates his/her key pair by running
754 This stores the private key in
764 and stores the public key in
765 .Pa ~/.ssh/identity.pub
767 .Pa ~/.ssh/id_dsa.pub
769 .Pa ~/.ssh/id_ecdsa.pub
772 .Pa ~/.ssh/id_rsa.pub
774 in the user's home directory.
775 The user should then copy the public key
777 .Pa ~/.ssh/authorized_keys
778 in his/her home directory on the remote machine.
781 file corresponds to the conventional
783 file, and has one key
784 per line, though the lines can be very long.
785 After this, the user can log in without giving the password.
787 A variation on public key authentication
788 is available in the form of certificate authentication:
789 instead of a set of public/private keys,
790 signed certificates are used.
791 This has the advantage that a single trusted certification authority
792 can be used in place of many public/private keys.
797 for more information.
799 The most convenient way to use public key or certificate authentication
800 may be with an authentication agent.
803 for more information.
805 Challenge-response authentication works as follows:
806 The server sends an arbitrary
808 text, and prompts for a response.
809 Protocol 2 allows multiple challenges and responses;
810 protocol 1 is restricted to just one challenge/response.
811 Examples of challenge-response authentication include
812 BSD Authentication (see
814 and PAM (some non-OpenBSD systems).
816 Finally, if other authentication methods fail,
818 prompts the user for a password.
819 The password is sent to the remote
820 host for checking; however, since all communications are encrypted,
821 the password cannot be seen by someone listening on the network.
824 automatically maintains and checks a database containing
825 identification for all hosts it has ever been used with.
826 Host keys are stored in
827 .Pa ~/.ssh/known_hosts
828 in the user's home directory.
829 Additionally, the file
830 .Pa /etc/ssh/ssh_known_hosts
831 is automatically checked for known hosts.
832 Any new hosts are automatically added to the user's file.
833 If a host's identification ever changes,
835 warns about this and disables password authentication to prevent
836 server spoofing or man-in-the-middle attacks,
837 which could otherwise be used to circumvent the encryption.
839 .Cm StrictHostKeyChecking
840 option can be used to control logins to machines whose
841 host key is not known or has changed.
843 When the user's identity has been accepted by the server, the server
844 either executes the given command, or logs into the machine and gives
845 the user a normal shell on the remote machine.
846 All communication with
847 the remote command or shell will be automatically encrypted.
849 If a pseudo-terminal has been allocated (normal login session), the
850 user may use the escape characters noted below.
852 If no pseudo-tty has been allocated,
853 the session is transparent and can be used to reliably transfer binary data.
854 On most systems, setting the escape character to
856 will also make the session transparent even if a tty is used.
858 The session terminates when the command or shell on the remote
859 machine exits and all X11 and TCP connections have been closed.
860 .Sh ESCAPE CHARACTERS
861 When a pseudo-terminal has been requested,
863 supports a number of functions through the use of an escape character.
865 A single tilde character can be sent as
867 or by following the tilde by a character other than those described below.
868 The escape character must always follow a newline to be interpreted as
870 The escape character can be changed in configuration files using the
872 configuration directive or on the command line by the
876 The supported escapes (assuming the default
886 List forwarded connections.
890 at logout when waiting for forwarded connection / X11 sessions to terminate.
892 Display a list of escape characters.
894 Send a BREAK to the remote system
895 (only useful for SSH protocol version 2 and if the peer supports it).
898 Currently this allows the addition of port forwardings using the
904 It also allows the cancellation of existing port-forwardings
907 .Fl KL Oo Ar bind_address : Oc Ar port
911 .Fl KR Oo Ar bind_address : Oc Ar port
915 .Fl KD Oo Ar bind_address : Oc Ar port
917 for dynamic port-forwardings.
918 .Ic !\& Ns Ar command
919 allows the user to execute a local command if the
920 .Ic PermitLocalCommand
923 Basic help is available, using the
927 Request rekeying of the connection
928 (only useful for SSH protocol version 2 and if the peer supports it).
931 Forwarding of arbitrary TCP connections over the secure channel can
932 be specified either on the command line or in a configuration file.
933 One possible application of TCP forwarding is a secure connection to a
934 mail server; another is going through firewalls.
936 In the example below, we look at encrypting communication between
937 an IRC client and server, even though the IRC server does not directly
938 support encrypted communications.
939 This works as follows:
940 the user connects to the remote host using
942 specifying a port to be used to forward connections
943 to the remote server.
944 After that it is possible to start the service which is to be encrypted
945 on the client machine,
946 connecting to the same local port,
949 will encrypt and forward the connection.
951 The following example tunnels an IRC session from client machine
955 .Dq server.example.com :
956 .Bd -literal -offset 4n
957 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
958 $ irc -c '#users' -p 1234 pinky 127.0.0.1
961 This tunnels a connection to IRC server
962 .Dq server.example.com ,
968 It doesn't matter which port is used,
969 as long as it's greater than 1023
970 (remember, only root can open sockets on privileged ports)
971 and doesn't conflict with any ports already in use.
972 The connection is forwarded to port 6667 on the remote server,
973 since that's the standard port for IRC services.
979 and the remote command
981 is specified to allow an amount of time
982 (10 seconds, in the example)
983 to start the service which is to be tunnelled.
984 If no connections are made within the time specified,
992 (or see the description of the
998 and the user is using X11 (the
1000 environment variable is set), the connection to the X11 display is
1001 automatically forwarded to the remote side in such a way that any X11
1002 programs started from the shell (or command) will go through the
1003 encrypted channel, and the connection to the real X server will be made
1004 from the local machine.
1005 The user should not manually set
1007 Forwarding of X11 connections can be
1008 configured on the command line or in configuration files.
1009 Take note that X11 forwarding can represent a security hazard.
1015 will point to the server machine, but with a display number greater than zero.
1016 This is normal, and happens because
1020 X server on the server machine for forwarding the
1021 connections over the encrypted channel.
1024 will also automatically set up Xauthority data on the server machine.
1025 For this purpose, it will generate a random authorization cookie,
1026 store it in Xauthority on the server, and verify that any forwarded
1027 connections carry this cookie and replace it by the real cookie when
1028 the connection is opened.
1029 The real authentication cookie is never
1030 sent to the server machine (and no cookies are sent in the plain).
1036 (or see the description of the
1041 the user is using an authentication agent, the connection to the agent
1042 is automatically forwarded to the remote side.
1043 .Sh VERIFYING HOST KEYS
1044 When connecting to a server for the first time,
1045 a fingerprint of the server's public key is presented to the user
1047 .Cm StrictHostKeyChecking
1049 Fingerprints can be determined using
1052 .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1054 If the fingerprint is already known, it can be matched
1055 and the key can be accepted or rejected.
1056 Because of the difficulty of comparing host keys
1057 just by looking at hex strings,
1058 there is also support to compare host keys visually,
1065 a small ASCII graphic gets displayed on every login to a server, no matter
1066 if the session itself is interactive or not.
1067 By learning the pattern a known server produces, a user can easily
1068 find out that the host key has changed when a completely different pattern
1070 Because these patterns are not unambiguous however, a pattern that looks
1071 similar to the pattern remembered only gives a good probability that the
1072 host key is the same, not guaranteed proof.
1074 To get a listing of the fingerprints along with their random art for
1075 all known hosts, the following command line can be used:
1077 .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1079 If the fingerprint is unknown,
1080 an alternative method of verification is available:
1081 SSH fingerprints verified by DNS.
1082 An additional resource record (RR),
1084 is added to a zonefile
1085 and the connecting client is able to match the fingerprint
1086 with that of the key presented.
1088 In this example, we are connecting a client to a server,
1089 .Dq host.example.com .
1090 The SSHFP resource records should first be added to the zonefile for
1092 .Bd -literal -offset indent
1093 $ ssh-keygen -r host.example.com.
1096 The output lines will have to be added to the zonefile.
1097 To check that the zone is answering fingerprint queries:
1099 .Dl $ dig -t SSHFP host.example.com
1101 Finally the client connects:
1102 .Bd -literal -offset indent
1103 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1105 Matching host key fingerprint found in DNS.
1106 Are you sure you want to continue connecting (yes/no)?
1110 .Cm VerifyHostKeyDNS
1113 for more information.
1114 .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1116 contains support for Virtual Private Network (VPN) tunnelling
1119 network pseudo-device,
1120 allowing two networks to be joined securely.
1123 configuration option
1125 controls whether the server supports this,
1126 and at what level (layer 2 or 3 traffic).
1128 The following example would connect client network 10.0.50.0/24
1129 with remote network 10.0.99.0/24 using a point-to-point connection
1130 from 10.1.1.1 to 10.1.1.2,
1131 provided that the SSH server running on the gateway to the remote network,
1132 at 192.168.1.15, allows it.
1135 .Bd -literal -offset indent
1136 # ssh -f -w 0:1 192.168.1.15 true
1137 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1138 # route add 10.0.99.0/24 10.1.1.2
1142 .Bd -literal -offset indent
1143 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1144 # route add 10.0.50.0/24 10.1.1.1
1147 Client access may be more finely tuned via the
1148 .Pa /root/.ssh/authorized_keys
1149 file (see below) and the
1152 The following entry would permit connections on
1156 and on tun device 2 from user
1161 .Dq forced-commands-only :
1162 .Bd -literal -offset 2n
1163 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1164 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1167 Since an SSH-based setup entails a fair amount of overhead,
1168 it may be more suited to temporary setups,
1169 such as for wireless VPNs.
1170 More permanent VPNs are better provided by tools such as
1176 will normally set the following environment variables:
1177 .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1181 variable indicates the location of the X11 server.
1182 It is automatically set by
1184 to point to a value of the form
1188 indicates the host where the shell runs, and
1190 is an integer \*(Ge 1.
1192 uses this special value to forward X11 connections over the secure
1194 The user should normally not set
1197 will render the X11 connection insecure (and will require the user to
1198 manually copy any required authorization cookies).
1200 Set to the path of the user's home directory.
1204 set for compatibility with systems that use this variable.
1206 Set to the path of the user's mailbox.
1210 as specified when compiling
1215 needs a passphrase, it will read the passphrase from the current
1216 terminal if it was run from a terminal.
1219 does not have a terminal associated with it but
1223 are set, it will execute the program specified by
1225 and open an X11 window to read the passphrase.
1226 This is particularly useful when calling
1231 (Note that on some machines it
1232 may be necessary to redirect the input from
1235 .It Ev SSH_AUTH_SOCK
1236 Identifies the path of a
1238 socket used to communicate with the agent.
1239 .It Ev SSH_CONNECTION
1240 Identifies the client and server ends of the connection.
1241 The variable contains
1242 four space-separated values: client IP address, client port number,
1243 server IP address, and server port number.
1244 .It Ev SSH_ORIGINAL_COMMAND
1245 This variable contains the original command line if a forced command
1247 It can be used to extract the original arguments.
1249 This is set to the name of the tty (path to the device) associated
1250 with the current shell or command.
1251 If the current session has no tty,
1252 this variable is not set.
1254 This variable is set to indicate the present time zone if it
1255 was set when the daemon was started (i.e. the daemon passes the value
1256 on to new connections).
1258 Set to the name of the user logging in.
1264 .Pa ~/.ssh/environment ,
1265 and adds lines of the format
1267 to the environment if the file exists and users are allowed to
1268 change their environment.
1269 For more information, see the
1270 .Cm PermitUserEnvironment
1274 .Bl -tag -width Ds -compact
1276 This file is used for host-based authentication (see above).
1277 On some machines this file may need to be
1278 world-readable if the user's home directory is on an NFS partition,
1282 Additionally, this file must be owned by the user,
1283 and must not have write permissions for anyone else.
1285 permission for most machines is read/write for the user, and not
1286 accessible by others.
1289 This file is used in exactly the same way as
1291 but allows host-based authentication without permitting login with
1295 This directory is the default location for all user-specific configuration
1296 and authentication information.
1297 There is no general requirement to keep the entire contents of this directory
1298 secret, but the recommended permissions are read/write/execute for the user,
1299 and not accessible by others.
1301 .It Pa ~/.ssh/authorized_keys
1302 Lists the public keys (DSA/ECDSA/RSA) that can be used for logging in as
1304 The format of this file is described in the
1307 This file is not highly sensitive, but the recommended
1308 permissions are read/write for the user, and not accessible by others.
1310 .It Pa ~/.ssh/config
1311 This is the per-user configuration file.
1312 The file format and configuration options are described in
1314 Because of the potential for abuse, this file must have strict permissions:
1315 read/write for the user, and not accessible by others.
1317 .It Pa ~/.ssh/environment
1318 Contains additional definitions for environment variables; see
1322 .It Pa ~/.ssh/identity
1323 .It Pa ~/.ssh/id_dsa
1324 .It Pa ~/.ssh/id_ecdsa
1325 .It Pa ~/.ssh/id_rsa
1326 Contains the private key for authentication.
1328 contain sensitive data and should be readable by the user but not
1329 accessible by others (read/write/execute).
1331 will simply ignore a private key file if it is accessible by others.
1332 It is possible to specify a passphrase when
1333 generating the key which will be used to encrypt the
1334 sensitive part of this file using 3DES.
1336 .It Pa ~/.ssh/identity.pub
1337 .It Pa ~/.ssh/id_dsa.pub
1338 .It Pa ~/.ssh/id_ecdsa.pub
1339 .It Pa ~/.ssh/id_rsa.pub
1340 Contains the public key for authentication.
1342 sensitive and can (but need not) be readable by anyone.
1344 .It Pa ~/.ssh/known_hosts
1345 Contains a list of host keys for all hosts the user has logged into
1346 that are not already in the systemwide list of known host keys.
1349 for further details of the format of this file.
1352 Commands in this file are executed by
1354 when the user logs in, just before the user's shell (or command) is
1358 manual page for more information.
1360 .It Pa /etc/hosts.equiv
1361 This file is for host-based authentication (see above).
1362 It should only be writable by root.
1364 .It Pa /etc/ssh/shosts.equiv
1365 This file is used in exactly the same way as
1367 but allows host-based authentication without permitting login with
1370 .It Pa /etc/ssh/ssh_config
1371 Systemwide configuration file.
1372 The file format and configuration options are described in
1375 .It Pa /etc/ssh/ssh_host_key
1376 .It Pa /etc/ssh/ssh_host_dsa_key
1377 .It Pa /etc/ssh/ssh_host_ecdsa_key
1378 .It Pa /etc/ssh/ssh_host_rsa_key
1379 These files contain the private parts of the host keys
1380 and are used for host-based authentication.
1381 If protocol version 1 is used,
1383 must be setuid root, since the host key is readable only by root.
1384 For protocol version 2,
1388 to access the host keys,
1389 eliminating the requirement that
1391 be setuid root when host-based authentication is used.
1396 .It Pa /etc/ssh/ssh_known_hosts
1397 Systemwide list of known host keys.
1398 This file should be prepared by the
1399 system administrator to contain the public host keys of all machines in the
1401 It should be world-readable.
1404 for further details of the format of this file.
1406 .It Pa /etc/ssh/sshrc
1407 Commands in this file are executed by
1409 when the user logs in, just before the user's shell (or command) is started.
1412 manual page for more information.
1416 exits with the exit status of the remote command or with 255
1417 if an error occurred.
1432 .%T "The Secure Shell (SSH) Protocol Assigned Numbers"
1437 .%T "The Secure Shell (SSH) Protocol Architecture"
1442 .%T "The Secure Shell (SSH) Authentication Protocol"
1447 .%T "The Secure Shell (SSH) Transport Layer Protocol"
1452 .%T "The Secure Shell (SSH) Connection Protocol"
1457 .%T "Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints"
1462 .%T "Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)"
1467 .%T "The Secure Shell (SSH) Session Channel Break Extension"
1472 .%T "The Secure Shell (SSH) Transport Layer Encryption Modes"
1477 .%T "Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol"
1482 .%T "Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol"
1487 .%T "The Secure Shell (SSH) Public Key File Format"
1492 .%T "Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer"
1496 .%T "Hash Visualization: a New Technique to improve Real-World Security"
1500 .%O "International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)"
1503 OpenSSH is a derivative of the original and free
1504 ssh 1.2.12 release by Tatu Ylonen.
1505 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1506 Theo de Raadt and Dug Song
1507 removed many bugs, re-added newer features and
1509 Markus Friedl contributed the support for SSH
1510 protocol versions 1.5 and 2.0.