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 1.330 2012/10/04 13:21:50 markus 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),
398 (cancel forwardings),
400 (request the master to exit), and
402 (request the master to stop accepting further multiplexing requests).
404 Can be used to give options in the format used in the configuration file.
405 This is useful for specifying options for which there is no separate
407 For full details of the options listed below, and their possible values, see
410 .Bl -tag -width Ds -offset indent -compact
414 .It ChallengeResponseAuthentication
418 .It ClearAllForwardings
421 .It ConnectionAttempts
428 .It ExitOnForwardFailure
431 .It ForwardX11Timeout
432 .It ForwardX11Trusted
434 .It GlobalKnownHostsFile
435 .It GSSAPIAuthentication
436 .It GSSAPIDelegateCredentials
439 .It HostbasedAuthentication
440 .It HostKeyAlgorithms
446 .It KbdInteractiveAuthentication
447 .It KbdInteractiveDevices
453 .It NoHostAuthenticationForLocalhost
454 .It NumberOfPasswordPrompts
455 .It PasswordAuthentication
456 .It PermitLocalCommand
459 .It PreferredAuthentications
462 .It PubkeyAuthentication
466 .It RhostsRSAAuthentication
467 .It RSAAuthentication
469 .It ServerAliveInterval
470 .It ServerAliveCountMax
471 .It StrictHostKeyChecking
475 .It UsePrivilegedPort
477 .It UserKnownHostsFile
484 Port to connect to on the remote host.
485 This can be specified on a
486 per-host basis in the configuration file.
489 Causes most warning and diagnostic messages to be suppressed.
492 .Oo Ar bind_address : Oc
493 .Ar port : host : hostport
496 Specifies that the given port on the remote (server) host is to be
497 forwarded to the given host and port on the local side.
498 This works by allocating a socket to listen to
500 on the remote side, and whenever a connection is made to this port, the
501 connection is forwarded over the secure channel, and a connection is
506 from the local machine.
508 Port forwardings can also be specified in the configuration file.
509 Privileged ports can be forwarded only when
510 logging in as root on the remote machine.
511 IPv6 addresses can be specified by enclosing the address in square brackets.
513 By default, the listening socket on the server will be bound to the loopback
515 This may be overridden by specifying a
521 indicates that the remote socket should listen on all interfaces.
524 will only succeed if the server's
526 option is enabled (see
527 .Xr sshd_config 5 ) .
533 the listen port will be dynamically allocated on the server and reported
534 to the client at run time.
535 When used together with
537 the allocated port will be printed to the standard output.
539 Specifies the location of a control socket for connection sharing,
542 to disable connection sharing.
543 Refer to the description of
551 May be used to request invocation of a subsystem on the remote system.
552 Subsystems are a feature of the SSH2 protocol which facilitate the use
553 of SSH as a secure transport for other applications (eg.\&
555 The subsystem is specified as the remote command.
557 Disable pseudo-tty allocation.
559 Force pseudo-tty allocation.
560 This can be used to execute arbitrary
561 screen-based programs on a remote machine, which can be very useful,
562 e.g. when implementing menu services.
565 options force tty allocation, even if
569 Display the version number and exit.
574 to print debugging messages about its progress.
576 debugging connection, authentication, and configuration problems.
579 options increase the verbosity.
581 .It Fl W Ar host : Ns Ar port
582 Requests that standard input and output on the client be forwarded to
586 over the secure channel.
590 .Cm ExitOnForwardFailure
592 .Cm ClearAllForwardings .
593 Works with Protocol version 2 only.
595 .Ar local_tun Ns Op : Ns Ar remote_tun
599 device forwarding with the specified
601 devices between the client
606 The devices may be specified by numerical ID or the keyword
608 which uses the next available tunnel device.
611 is not specified, it defaults to
621 directive is unset, it is set to the default tunnel mode, which is
624 Enables X11 forwarding.
625 This can also be specified on a per-host basis in a configuration file.
627 X11 forwarding should be enabled with caution.
628 Users with the ability to bypass file permissions on the remote host
629 (for the user's X authorization database)
630 can access the local X11 display through the forwarded connection.
631 An attacker may then be able to perform activities such as keystroke monitoring.
633 For this reason, X11 forwarding is subjected to X11 SECURITY extension
634 restrictions by default.
639 .Cm ForwardX11Trusted
642 for more information.
644 Disables X11 forwarding.
646 Enables trusted X11 forwarding.
647 Trusted X11 forwardings are not subjected to the X11 SECURITY extension
650 Send log information using the
653 By default this information is sent to stderr.
657 may additionally obtain configuration data from
658 a per-user configuration file and a system-wide configuration file.
659 The file format and configuration options are described in
662 The OpenSSH SSH client supports SSH protocols 1 and 2.
663 The default is to use protocol 2 only,
664 though this can be changed via the
673 Both protocols support similar authentication methods,
674 but protocol 2 is the default since
675 it provides additional mechanisms for confidentiality
676 (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
677 and integrity (hmac-md5, hmac-sha1,
678 hmac-sha2-256, hmac-sha2-512,
679 umac-64, umac-128, hmac-ripemd160).
680 Protocol 1 lacks a strong mechanism for ensuring the
681 integrity of the connection.
683 The methods available for authentication are:
684 GSSAPI-based authentication,
685 host-based authentication,
686 public key authentication,
687 challenge-response authentication,
688 and password authentication.
689 Authentication methods are tried in the order specified above,
690 though protocol 2 has a configuration option to change the default order:
691 .Cm PreferredAuthentications .
693 Host-based authentication works as follows:
694 If the machine the user logs in from is listed in
697 .Pa /etc/shosts.equiv
698 on the remote machine, and the user names are
699 the same on both sides, or if the files
703 exist in the user's home directory on the
704 remote machine and contain a line containing the name of the client
705 machine and the name of the user on that machine, the user is
706 considered for login.
707 Additionally, the server
709 be able to verify the client's
710 host key (see the description of
711 .Pa /etc/ssh/ssh_known_hosts
713 .Pa ~/.ssh/known_hosts ,
715 for login to be permitted.
716 This authentication method closes security holes due to IP
717 spoofing, DNS spoofing, and routing spoofing.
718 [Note to the administrator:
719 .Pa /etc/hosts.equiv ,
721 and the rlogin/rsh protocol in general, are inherently insecure and should be
722 disabled if security is desired.]
724 Public key authentication works as follows:
725 The scheme is based on public-key cryptography,
727 where encryption and decryption are done using separate keys,
728 and it is unfeasible to derive the decryption key from the encryption key.
729 The idea is that each user creates a public/private
730 key pair for authentication purposes.
731 The server knows the public key, and only the user knows the private key.
733 implements public key authentication protocol automatically,
734 using one of the DSA, ECDSA or RSA algorithms.
735 Protocol 1 is restricted to using only RSA keys,
736 but protocol 2 may use any.
741 contains a brief discussion of the DSA and RSA algorithms.
744 .Pa ~/.ssh/authorized_keys
745 lists the public keys that are permitted for logging in.
746 When the user logs in, the
748 program tells the server which key pair it would like to use for
750 The client proves that it has access to the private key
751 and the server checks that the corresponding public key
752 is authorized to accept the account.
754 The user creates his/her key pair by running
756 This stores the private key in
766 and stores the public key in
767 .Pa ~/.ssh/identity.pub
769 .Pa ~/.ssh/id_dsa.pub
771 .Pa ~/.ssh/id_ecdsa.pub
774 .Pa ~/.ssh/id_rsa.pub
776 in the user's home directory.
777 The user should then copy the public key
779 .Pa ~/.ssh/authorized_keys
780 in his/her home directory on the remote machine.
783 file corresponds to the conventional
785 file, and has one key
786 per line, though the lines can be very long.
787 After this, the user can log in without giving the password.
789 A variation on public key authentication
790 is available in the form of certificate authentication:
791 instead of a set of public/private keys,
792 signed certificates are used.
793 This has the advantage that a single trusted certification authority
794 can be used in place of many public/private keys.
799 for more information.
801 The most convenient way to use public key or certificate authentication
802 may be with an authentication agent.
805 for more information.
807 Challenge-response authentication works as follows:
808 The server sends an arbitrary
810 text, and prompts for a response.
811 Protocol 2 allows multiple challenges and responses;
812 protocol 1 is restricted to just one challenge/response.
813 Examples of challenge-response authentication include
814 BSD Authentication (see
816 and PAM (some non-OpenBSD systems).
818 Finally, if other authentication methods fail,
820 prompts the user for a password.
821 The password is sent to the remote
822 host for checking; however, since all communications are encrypted,
823 the password cannot be seen by someone listening on the network.
826 automatically maintains and checks a database containing
827 identification for all hosts it has ever been used with.
828 Host keys are stored in
829 .Pa ~/.ssh/known_hosts
830 in the user's home directory.
831 Additionally, the file
832 .Pa /etc/ssh/ssh_known_hosts
833 is automatically checked for known hosts.
834 Any new hosts are automatically added to the user's file.
835 If a host's identification ever changes,
837 warns about this and disables password authentication to prevent
838 server spoofing or man-in-the-middle attacks,
839 which could otherwise be used to circumvent the encryption.
841 .Cm StrictHostKeyChecking
842 option can be used to control logins to machines whose
843 host key is not known or has changed.
845 When the user's identity has been accepted by the server, the server
846 either executes the given command, or logs into the machine and gives
847 the user a normal shell on the remote machine.
848 All communication with
849 the remote command or shell will be automatically encrypted.
851 If a pseudo-terminal has been allocated (normal login session), the
852 user may use the escape characters noted below.
854 If no pseudo-tty has been allocated,
855 the session is transparent and can be used to reliably transfer binary data.
856 On most systems, setting the escape character to
858 will also make the session transparent even if a tty is used.
860 The session terminates when the command or shell on the remote
861 machine exits and all X11 and TCP connections have been closed.
862 .Sh ESCAPE CHARACTERS
863 When a pseudo-terminal has been requested,
865 supports a number of functions through the use of an escape character.
867 A single tilde character can be sent as
869 or by following the tilde by a character other than those described below.
870 The escape character must always follow a newline to be interpreted as
872 The escape character can be changed in configuration files using the
874 configuration directive or on the command line by the
878 The supported escapes (assuming the default
888 List forwarded connections.
892 at logout when waiting for forwarded connection / X11 sessions to terminate.
894 Display a list of escape characters.
896 Send a BREAK to the remote system
897 (only useful for SSH protocol version 2 and if the peer supports it).
900 Currently this allows the addition of port forwardings using the
906 It also allows the cancellation of existing port-forwardings
909 .Fl KL Oo Ar bind_address : Oc Ar port
913 .Fl KR Oo Ar bind_address : Oc Ar port
917 .Fl KD Oo Ar bind_address : Oc Ar port
919 for dynamic port-forwardings.
920 .Ic !\& Ns Ar command
921 allows the user to execute a local command if the
922 .Ic PermitLocalCommand
925 Basic help is available, using the
929 Request rekeying of the connection
930 (only useful for SSH protocol version 2 and if the peer supports it).
932 Decrease the verbosity
934 when errors are being written to stderr.
936 Increase the verbosity
938 when errors are being written to stderr.
941 Forwarding of arbitrary TCP connections over the secure channel can
942 be specified either on the command line or in a configuration file.
943 One possible application of TCP forwarding is a secure connection to a
944 mail server; another is going through firewalls.
946 In the example below, we look at encrypting communication between
947 an IRC client and server, even though the IRC server does not directly
948 support encrypted communications.
949 This works as follows:
950 the user connects to the remote host using
952 specifying a port to be used to forward connections
953 to the remote server.
954 After that it is possible to start the service which is to be encrypted
955 on the client machine,
956 connecting to the same local port,
959 will encrypt and forward the connection.
961 The following example tunnels an IRC session from client machine
965 .Dq server.example.com :
966 .Bd -literal -offset 4n
967 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
968 $ irc -c '#users' -p 1234 pinky 127.0.0.1
971 This tunnels a connection to IRC server
972 .Dq server.example.com ,
978 It doesn't matter which port is used,
979 as long as it's greater than 1023
980 (remember, only root can open sockets on privileged ports)
981 and doesn't conflict with any ports already in use.
982 The connection is forwarded to port 6667 on the remote server,
983 since that's the standard port for IRC services.
989 and the remote command
991 is specified to allow an amount of time
992 (10 seconds, in the example)
993 to start the service which is to be tunnelled.
994 If no connections are made within the time specified,
1002 (or see the description of the
1008 and the user is using X11 (the
1010 environment variable is set), the connection to the X11 display is
1011 automatically forwarded to the remote side in such a way that any X11
1012 programs started from the shell (or command) will go through the
1013 encrypted channel, and the connection to the real X server will be made
1014 from the local machine.
1015 The user should not manually set
1017 Forwarding of X11 connections can be
1018 configured on the command line or in configuration files.
1024 will point to the server machine, but with a display number greater than zero.
1025 This is normal, and happens because
1029 X server on the server machine for forwarding the
1030 connections over the encrypted channel.
1033 will also automatically set up Xauthority data on the server machine.
1034 For this purpose, it will generate a random authorization cookie,
1035 store it in Xauthority on the server, and verify that any forwarded
1036 connections carry this cookie and replace it by the real cookie when
1037 the connection is opened.
1038 The real authentication cookie is never
1039 sent to the server machine (and no cookies are sent in the plain).
1045 (or see the description of the
1050 the user is using an authentication agent, the connection to the agent
1051 is automatically forwarded to the remote side.
1052 .Sh VERIFYING HOST KEYS
1053 When connecting to a server for the first time,
1054 a fingerprint of the server's public key is presented to the user
1056 .Cm StrictHostKeyChecking
1058 Fingerprints can be determined using
1061 .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1063 If the fingerprint is already known, it can be matched
1064 and the key can be accepted or rejected.
1065 Because of the difficulty of comparing host keys
1066 just by looking at hex strings,
1067 there is also support to compare host keys visually,
1074 a small ASCII graphic gets displayed on every login to a server, no matter
1075 if the session itself is interactive or not.
1076 By learning the pattern a known server produces, a user can easily
1077 find out that the host key has changed when a completely different pattern
1079 Because these patterns are not unambiguous however, a pattern that looks
1080 similar to the pattern remembered only gives a good probability that the
1081 host key is the same, not guaranteed proof.
1083 To get a listing of the fingerprints along with their random art for
1084 all known hosts, the following command line can be used:
1086 .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1088 If the fingerprint is unknown,
1089 an alternative method of verification is available:
1090 SSH fingerprints verified by DNS.
1091 An additional resource record (RR),
1093 is added to a zonefile
1094 and the connecting client is able to match the fingerprint
1095 with that of the key presented.
1097 In this example, we are connecting a client to a server,
1098 .Dq host.example.com .
1099 The SSHFP resource records should first be added to the zonefile for
1101 .Bd -literal -offset indent
1102 $ ssh-keygen -r host.example.com.
1105 The output lines will have to be added to the zonefile.
1106 To check that the zone is answering fingerprint queries:
1108 .Dl $ dig -t SSHFP host.example.com
1110 Finally the client connects:
1111 .Bd -literal -offset indent
1112 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1114 Matching host key fingerprint found in DNS.
1115 Are you sure you want to continue connecting (yes/no)?
1119 .Cm VerifyHostKeyDNS
1122 for more information.
1123 .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1125 contains support for Virtual Private Network (VPN) tunnelling
1128 network pseudo-device,
1129 allowing two networks to be joined securely.
1132 configuration option
1134 controls whether the server supports this,
1135 and at what level (layer 2 or 3 traffic).
1137 The following example would connect client network 10.0.50.0/24
1138 with remote network 10.0.99.0/24 using a point-to-point connection
1139 from 10.1.1.1 to 10.1.1.2,
1140 provided that the SSH server running on the gateway to the remote network,
1141 at 192.168.1.15, allows it.
1144 .Bd -literal -offset indent
1145 # ssh -f -w 0:1 192.168.1.15 true
1146 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1147 # route add 10.0.99.0/24 10.1.1.2
1151 .Bd -literal -offset indent
1152 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1153 # route add 10.0.50.0/24 10.1.1.1
1156 Client access may be more finely tuned via the
1157 .Pa /root/.ssh/authorized_keys
1158 file (see below) and the
1161 The following entry would permit connections on
1165 and on tun device 2 from user
1170 .Dq forced-commands-only :
1171 .Bd -literal -offset 2n
1172 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1173 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1176 Since an SSH-based setup entails a fair amount of overhead,
1177 it may be more suited to temporary setups,
1178 such as for wireless VPNs.
1179 More permanent VPNs are better provided by tools such as
1185 will normally set the following environment variables:
1186 .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1190 variable indicates the location of the X11 server.
1191 It is automatically set by
1193 to point to a value of the form
1197 indicates the host where the shell runs, and
1199 is an integer \*(Ge 1.
1201 uses this special value to forward X11 connections over the secure
1203 The user should normally not set
1206 will render the X11 connection insecure (and will require the user to
1207 manually copy any required authorization cookies).
1209 Set to the path of the user's home directory.
1213 set for compatibility with systems that use this variable.
1215 Set to the path of the user's mailbox.
1219 as specified when compiling
1224 needs a passphrase, it will read the passphrase from the current
1225 terminal if it was run from a terminal.
1228 does not have a terminal associated with it but
1232 are set, it will execute the program specified by
1234 and open an X11 window to read the passphrase.
1235 This is particularly useful when calling
1240 (Note that on some machines it
1241 may be necessary to redirect the input from
1244 .It Ev SSH_AUTH_SOCK
1245 Identifies the path of a
1247 socket used to communicate with the agent.
1248 .It Ev SSH_CONNECTION
1249 Identifies the client and server ends of the connection.
1250 The variable contains
1251 four space-separated values: client IP address, client port number,
1252 server IP address, and server port number.
1253 .It Ev SSH_ORIGINAL_COMMAND
1254 This variable contains the original command line if a forced command
1256 It can be used to extract the original arguments.
1258 This is set to the name of the tty (path to the device) associated
1259 with the current shell or command.
1260 If the current session has no tty,
1261 this variable is not set.
1263 This variable is set to indicate the present time zone if it
1264 was set when the daemon was started (i.e. the daemon passes the value
1265 on to new connections).
1267 Set to the name of the user logging in.
1273 .Pa ~/.ssh/environment ,
1274 and adds lines of the format
1276 to the environment if the file exists and users are allowed to
1277 change their environment.
1278 For more information, see the
1279 .Cm PermitUserEnvironment
1283 .Bl -tag -width Ds -compact
1285 This file is used for host-based authentication (see above).
1286 On some machines this file may need to be
1287 world-readable if the user's home directory is on an NFS partition,
1291 Additionally, this file must be owned by the user,
1292 and must not have write permissions for anyone else.
1294 permission for most machines is read/write for the user, and not
1295 accessible by others.
1298 This file is used in exactly the same way as
1300 but allows host-based authentication without permitting login with
1304 This directory is the default location for all user-specific configuration
1305 and authentication information.
1306 There is no general requirement to keep the entire contents of this directory
1307 secret, but the recommended permissions are read/write/execute for the user,
1308 and not accessible by others.
1310 .It Pa ~/.ssh/authorized_keys
1311 Lists the public keys (DSA/ECDSA/RSA) that can be used for logging in as
1313 The format of this file is described in the
1316 This file is not highly sensitive, but the recommended
1317 permissions are read/write for the user, and not accessible by others.
1319 .It Pa ~/.ssh/config
1320 This is the per-user configuration file.
1321 The file format and configuration options are described in
1323 Because of the potential for abuse, this file must have strict permissions:
1324 read/write for the user, and not accessible by others.
1326 .It Pa ~/.ssh/environment
1327 Contains additional definitions for environment variables; see
1331 .It Pa ~/.ssh/identity
1332 .It Pa ~/.ssh/id_dsa
1333 .It Pa ~/.ssh/id_ecdsa
1334 .It Pa ~/.ssh/id_rsa
1335 Contains the private key for authentication.
1337 contain sensitive data and should be readable by the user but not
1338 accessible by others (read/write/execute).
1340 will simply ignore a private key file if it is accessible by others.
1341 It is possible to specify a passphrase when
1342 generating the key which will be used to encrypt the
1343 sensitive part of this file using 3DES.
1345 .It Pa ~/.ssh/identity.pub
1346 .It Pa ~/.ssh/id_dsa.pub
1347 .It Pa ~/.ssh/id_ecdsa.pub
1348 .It Pa ~/.ssh/id_rsa.pub
1349 Contains the public key for authentication.
1351 sensitive and can (but need not) be readable by anyone.
1353 .It Pa ~/.ssh/known_hosts
1354 Contains a list of host keys for all hosts the user has logged into
1355 that are not already in the systemwide list of known host keys.
1358 for further details of the format of this file.
1361 Commands in this file are executed by
1363 when the user logs in, just before the user's shell (or command) is
1367 manual page for more information.
1369 .It Pa /etc/hosts.equiv
1370 This file is for host-based authentication (see above).
1371 It should only be writable by root.
1373 .It Pa /etc/shosts.equiv
1374 This file is used in exactly the same way as
1376 but allows host-based authentication without permitting login with
1379 .It Pa /etc/ssh/ssh_config
1380 Systemwide configuration file.
1381 The file format and configuration options are described in
1384 .It Pa /etc/ssh/ssh_host_key
1385 .It Pa /etc/ssh/ssh_host_dsa_key
1386 .It Pa /etc/ssh/ssh_host_ecdsa_key
1387 .It Pa /etc/ssh/ssh_host_rsa_key
1388 These files contain the private parts of the host keys
1389 and are used for host-based authentication.
1390 If protocol version 1 is used,
1392 must be setuid root, since the host key is readable only by root.
1393 For protocol version 2,
1397 to access the host keys,
1398 eliminating the requirement that
1400 be setuid root when host-based authentication is used.
1405 .It Pa /etc/ssh/ssh_known_hosts
1406 Systemwide list of known host keys.
1407 This file should be prepared by the
1408 system administrator to contain the public host keys of all machines in the
1410 It should be world-readable.
1413 for further details of the format of this file.
1415 .It Pa /etc/ssh/sshrc
1416 Commands in this file are executed by
1418 when the user logs in, just before the user's shell (or command) is started.
1421 manual page for more information.
1425 exits with the exit status of the remote command or with 255
1426 if an error occurred.
1445 .%T The Secure Shell (SSH) Protocol Assigned Numbers
1453 .%T The Secure Shell (SSH) Protocol Architecture
1461 .%T The Secure Shell (SSH) Authentication Protocol
1469 .%T The Secure Shell (SSH) Transport Layer Protocol
1477 .%T The Secure Shell (SSH) Connection Protocol
1485 .%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
1493 .%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
1501 .%T The Secure Shell (SSH) Session Channel Break Extension
1510 .%T The Secure Shell (SSH) Transport Layer Encryption Modes
1517 .%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
1526 .%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
1534 .%T The Secure Shell (SSH) Public Key File Format
1542 .%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
1549 .%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
1550 .%T Hash Visualization: a New Technique to improve Real-World Security
1553 OpenSSH is a derivative of the original and free
1554 ssh 1.2.12 release by Tatu Ylonen.
1555 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1556 Theo de Raadt and Dug Song
1557 removed many bugs, re-added newer features and
1559 Markus Friedl contributed the support for SSH
1560 protocol versions 1.5 and 2.0.