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 1.334 2013/07/18 01:12:26 djm 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
52 .Op Fl e Ar escape_char
53 .Op Fl F Ar configfile
55 .Op Fl i Ar identity_file
56 .Op Fl L Oo Ar bind_address : Oc Ns Ar port : Ns Ar host : Ns Ar hostport
57 .Op Fl l Ar login_name
62 .Op Fl R Oo Ar bind_address : Oc Ns Ar port : Ns Ar host : Ns Ar hostport
64 .Op Fl W Ar host : Ns Ar port
65 .Op Fl w Ar local_tun Ns Op : Ns Ar remote_tun
66 .Oo Ar user Ns @ Oc Ns Ar hostname
70 .Fl Q Ar protocol_feature
73 (SSH client) is a program for logging into a remote machine and for
74 executing commands on a remote machine.
75 It is intended to replace rlogin and rsh,
76 and provide secure encrypted communications between
77 two untrusted hosts over an insecure network.
78 X11 connections and arbitrary TCP ports
79 can also be forwarded over the secure channel.
82 connects and logs into the specified
88 his/her identity to the remote machine using one of several methods
89 depending on the protocol version used (see below).
94 it is executed on the remote host instead of a login shell.
96 The options are as follows:
101 to try protocol version 1 only.
105 to try protocol version 2 only.
109 to use IPv4 addresses only.
113 to use IPv6 addresses only.
115 Enables forwarding of the authentication agent connection.
116 This can also be specified on a per-host basis in a configuration file.
118 Agent forwarding should be enabled with caution.
119 Users with the ability to bypass file permissions on the remote host
122 socket) can access the local agent through the forwarded connection.
123 An attacker cannot obtain key material from the agent,
124 however they can perform operations on the keys that enable them to
125 authenticate using the identities loaded into the agent.
127 Disables forwarding of the authentication agent connection.
128 .It Fl b Ar bind_address
131 on the local machine as the source address
133 Only useful on systems with more than one address.
135 Requests compression of all data (including stdin, stdout, stderr, and
136 data for forwarded X11 and TCP connections).
137 The compression algorithm is the same used by
141 can be controlled by the
143 option for protocol version 1.
144 Compression is desirable on modem lines and other
145 slow connections, but will only slow down things on fast networks.
146 The default value can be set on a host-by-host basis in the
147 configuration files; see the
150 .It Fl c Ar cipher_spec
151 Selects the cipher specification for encrypting the session.
153 Protocol version 1 allows specification of a single cipher.
154 The supported values are
160 (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
161 It is believed to be secure.
163 is a fast block cipher; it appears very secure and is much faster than
166 is only supported in the
168 client for interoperability with legacy protocol 1 implementations
169 that do not support the
172 Its use is strongly discouraged due to cryptographic weaknesses.
176 For protocol version 2,
178 is a comma-separated list of ciphers
179 listed in order of preference.
184 for more information.
187 .Oo Ar bind_address : Oc
193 application-level port forwarding.
194 This works by allocating a socket to listen to
196 on the local side, optionally bound to the specified
198 Whenever a connection is made to this port, the
199 connection is forwarded over the secure channel, and the application
200 protocol is then used to determine where to connect to from the
202 Currently the SOCKS4 and SOCKS5 protocols are supported, and
204 will act as a SOCKS server.
205 Only root can forward privileged ports.
206 Dynamic port forwardings can also be specified in the configuration file.
208 IPv6 addresses can be specified by enclosing the address in square brackets.
209 Only the superuser can forward privileged ports.
210 By default, the local port is bound in accordance with the
215 may be used to bind the connection to a specific address.
220 indicates that the listening port be bound for local use only, while an
223 indicates that the port should be available from all interfaces.
227 instead of standard error.
228 .It Fl e Ar escape_char
229 Sets the escape character for sessions with a pty (default:
231 The escape character is only recognized at the beginning of a line.
232 The escape character followed by a dot
234 closes the connection;
235 followed by control-Z suspends the connection;
236 and followed by itself sends the escape character once.
237 Setting the character to
239 disables any escapes and makes the session fully transparent.
240 .It Fl F Ar configfile
241 Specifies an alternative per-user configuration file.
242 If a configuration file is given on the command line,
243 the system-wide configuration file
244 .Pq Pa /etc/ssh/ssh_config
246 The default for the per-user configuration file is
251 to go to background just before command execution.
254 is going to ask for passwords or passphrases, but the user
255 wants it in the background.
258 The recommended way to start X11 programs at a remote site is with
260 .Ic ssh -f host xterm .
263 .Cm ExitOnForwardFailure
264 configuration option is set to
266 then a client started with
268 will wait for all remote port forwards to be successfully established
269 before placing itself in the background.
271 Allows remote hosts to connect to local forwarded ports.
273 Specify the PKCS#11 shared library
275 should use to communicate with a PKCS#11 token providing the user's
277 .It Fl i Ar identity_file
278 Selects a file from which the identity (private key) for
279 public key authentication is read.
282 for protocol version 1, and
287 for protocol version 2.
288 Identity files may also be specified on
289 a per-host basis in the configuration file.
290 It is possible to have multiple
292 options (and multiple identities specified in
293 configuration files).
295 will also try to load certificate information from the filename obtained
298 to identity filenames.
300 Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
301 credentials to the server.
303 Disables forwarding (delegation) of GSSAPI credentials to the server.
306 .Oo Ar bind_address : Oc
307 .Ar port : host : hostport
310 Specifies that the given port on the local (client) host is to be
311 forwarded to the given host and port on the remote side.
312 This works by allocating a socket to listen to
314 on the local side, optionally bound to the specified
316 Whenever a connection is made to this port, the
317 connection is forwarded over the secure channel, and a connection is
322 from the remote machine.
323 Port forwardings can also be specified in the configuration file.
324 IPv6 addresses can be specified by enclosing the address in square brackets.
325 Only the superuser can forward privileged ports.
326 By default, the local port is bound in accordance with the
331 may be used to bind the connection to a specific address.
336 indicates that the listening port be bound for local use only, while an
339 indicates that the port should be available from all interfaces.
340 .It Fl l Ar login_name
341 Specifies the user to log in as on the remote machine.
342 This also may be specified on a per-host basis in the configuration file.
348 mode for connection sharing.
355 mode with confirmation required before slave connections are accepted.
356 Refer to the description of
362 Additionally, for protocol version 2 a comma-separated list of MAC
363 (message authentication code) algorithms can
364 be specified in order of preference.
367 keyword for more information.
369 Do not execute a remote command.
370 This is useful for just forwarding ports
371 (protocol version 2 only).
375 (actually, prevents reading from stdin).
376 This must be used when
378 is run in the background.
379 A common trick is to use this to run X11 programs on a remote machine.
381 .Ic ssh -n shadows.cs.hut.fi emacs &
382 will start an emacs on shadows.cs.hut.fi, and the X11
383 connection will be automatically forwarded over an encrypted channel.
386 program will be put in the background.
387 (This does not work if
389 needs to ask for a password or passphrase; see also the
393 Control an active connection multiplexing master process.
396 option is specified, the
398 argument is interpreted and passed to the master process.
401 (check that the master process is running),
403 (request forwardings without command execution),
405 (cancel forwardings),
407 (request the master to exit), and
409 (request the master to stop accepting further multiplexing requests).
411 Can be used to give options in the format used in the configuration file.
412 This is useful for specifying options for which there is no separate
414 For full details of the options listed below, and their possible values, see
417 .Bl -tag -width Ds -offset indent -compact
421 .It ChallengeResponseAuthentication
425 .It ClearAllForwardings
428 .It ConnectionAttempts
435 .It ExitOnForwardFailure
438 .It ForwardX11Timeout
439 .It ForwardX11Trusted
441 .It GlobalKnownHostsFile
442 .It GSSAPIAuthentication
443 .It GSSAPIDelegateCredentials
446 .It HostbasedAuthentication
447 .It HostKeyAlgorithms
453 .It KbdInteractiveAuthentication
454 .It KbdInteractiveDevices
460 .It NoHostAuthenticationForLocalhost
461 .It NumberOfPasswordPrompts
462 .It PasswordAuthentication
463 .It PermitLocalCommand
466 .It PreferredAuthentications
469 .It PubkeyAuthentication
473 .It RhostsRSAAuthentication
474 .It RSAAuthentication
476 .It ServerAliveInterval
477 .It ServerAliveCountMax
478 .It StrictHostKeyChecking
482 .It UsePrivilegedPort
484 .It UserKnownHostsFile
491 Port to connect to on the remote host.
492 This can be specified on a
493 per-host basis in the configuration file.
494 .It Fl Q Ar protocol_feature
497 for the algorithms supported for the specified version 2
498 .Ar protocol_feature .
499 The queriable features are:
501 (supported symmetric ciphers),
503 (supported message integrity codes),
505 (key exchange algorithms),
508 Protocol features are treated case-insensitively.
511 Causes most warning and diagnostic messages to be suppressed.
514 .Oo Ar bind_address : Oc
515 .Ar port : host : hostport
518 Specifies that the given port on the remote (server) host is to be
519 forwarded to the given host and port on the local side.
520 This works by allocating a socket to listen to
522 on the remote side, and whenever a connection is made to this port, the
523 connection is forwarded over the secure channel, and a connection is
528 from the local machine.
530 Port forwardings can also be specified in the configuration file.
531 Privileged ports can be forwarded only when
532 logging in as root on the remote machine.
533 IPv6 addresses can be specified by enclosing the address in square brackets.
535 By default, the listening socket on the server will be bound to the loopback
537 This may be overridden by specifying a
543 indicates that the remote socket should listen on all interfaces.
546 will only succeed if the server's
548 option is enabled (see
549 .Xr sshd_config 5 ) .
555 the listen port will be dynamically allocated on the server and reported
556 to the client at run time.
557 When used together with
559 the allocated port will be printed to the standard output.
561 Specifies the location of a control socket for connection sharing,
564 to disable connection sharing.
565 Refer to the description of
573 May be used to request invocation of a subsystem on the remote system.
574 Subsystems are a feature of the SSH2 protocol which facilitate the use
575 of SSH as a secure transport for other applications (eg.\&
577 The subsystem is specified as the remote command.
579 Disable pseudo-tty allocation.
581 Force pseudo-tty allocation.
582 This can be used to execute arbitrary
583 screen-based programs on a remote machine, which can be very useful,
584 e.g. when implementing menu services.
587 options force tty allocation, even if
591 Display the version number and exit.
596 to print debugging messages about its progress.
598 debugging connection, authentication, and configuration problems.
601 options increase the verbosity.
603 .It Fl W Ar host : Ns Ar port
604 Requests that standard input and output on the client be forwarded to
608 over the secure channel.
612 .Cm ExitOnForwardFailure
614 .Cm ClearAllForwardings .
615 Works with Protocol version 2 only.
617 .Ar local_tun Ns Op : Ns Ar remote_tun
621 device forwarding with the specified
623 devices between the client
628 The devices may be specified by numerical ID or the keyword
630 which uses the next available tunnel device.
633 is not specified, it defaults to
643 directive is unset, it is set to the default tunnel mode, which is
646 Enables X11 forwarding.
647 This can also be specified on a per-host basis in a configuration file.
649 X11 forwarding should be enabled with caution.
650 Users with the ability to bypass file permissions on the remote host
651 (for the user's X authorization database)
652 can access the local X11 display through the forwarded connection.
653 An attacker may then be able to perform activities such as keystroke monitoring.
655 For this reason, X11 forwarding is subjected to X11 SECURITY extension
656 restrictions by default.
661 .Cm ForwardX11Trusted
664 for more information.
666 Disables X11 forwarding.
668 Enables trusted X11 forwarding.
669 Trusted X11 forwardings are not subjected to the X11 SECURITY extension
672 Send log information using the
675 By default this information is sent to stderr.
679 may additionally obtain configuration data from
680 a per-user configuration file and a system-wide configuration file.
681 The file format and configuration options are described in
684 The OpenSSH SSH client supports SSH protocols 1 and 2.
685 The default is to use protocol 2 only,
686 though this can be changed via the
695 Both protocols support similar authentication methods,
696 but protocol 2 is the default since
697 it provides additional mechanisms for confidentiality
698 (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
699 and integrity (hmac-md5, hmac-sha1,
700 hmac-sha2-256, hmac-sha2-512,
701 umac-64, umac-128, hmac-ripemd160).
702 Protocol 1 lacks a strong mechanism for ensuring the
703 integrity of the connection.
705 The methods available for authentication are:
706 GSSAPI-based authentication,
707 host-based authentication,
708 public key authentication,
709 challenge-response authentication,
710 and password authentication.
711 Authentication methods are tried in the order specified above,
712 though protocol 2 has a configuration option to change the default order:
713 .Cm PreferredAuthentications .
715 Host-based authentication works as follows:
716 If the machine the user logs in from is listed in
719 .Pa /etc/shosts.equiv
720 on the remote machine, and the user names are
721 the same on both sides, or if the files
725 exist in the user's home directory on the
726 remote machine and contain a line containing the name of the client
727 machine and the name of the user on that machine, the user is
728 considered for login.
729 Additionally, the server
731 be able to verify the client's
732 host key (see the description of
733 .Pa /etc/ssh/ssh_known_hosts
735 .Pa ~/.ssh/known_hosts ,
737 for login to be permitted.
738 This authentication method closes security holes due to IP
739 spoofing, DNS spoofing, and routing spoofing.
740 [Note to the administrator:
741 .Pa /etc/hosts.equiv ,
743 and the rlogin/rsh protocol in general, are inherently insecure and should be
744 disabled if security is desired.]
746 Public key authentication works as follows:
747 The scheme is based on public-key cryptography,
749 where encryption and decryption are done using separate keys,
750 and it is unfeasible to derive the decryption key from the encryption key.
751 The idea is that each user creates a public/private
752 key pair for authentication purposes.
753 The server knows the public key, and only the user knows the private key.
755 implements public key authentication protocol automatically,
756 using one of the DSA, ECDSA or RSA algorithms.
757 Protocol 1 is restricted to using only RSA keys,
758 but protocol 2 may use any.
759 The HISTORY section of
761 contains a brief discussion of the DSA and RSA algorithms.
764 .Pa ~/.ssh/authorized_keys
765 lists the public keys that are permitted for logging in.
766 When the user logs in, the
768 program tells the server which key pair it would like to use for
770 The client proves that it has access to the private key
771 and the server checks that the corresponding public key
772 is authorized to accept the account.
774 The user creates his/her key pair by running
776 This stores the private key in
786 and stores the public key in
787 .Pa ~/.ssh/identity.pub
789 .Pa ~/.ssh/id_dsa.pub
791 .Pa ~/.ssh/id_ecdsa.pub
794 .Pa ~/.ssh/id_rsa.pub
796 in the user's home directory.
797 The user should then copy the public key
799 .Pa ~/.ssh/authorized_keys
800 in his/her home directory on the remote machine.
803 file corresponds to the conventional
805 file, and has one key
806 per line, though the lines can be very long.
807 After this, the user can log in without giving the password.
809 A variation on public key authentication
810 is available in the form of certificate authentication:
811 instead of a set of public/private keys,
812 signed certificates are used.
813 This has the advantage that a single trusted certification authority
814 can be used in place of many public/private keys.
815 See the CERTIFICATES section of
817 for more information.
819 The most convenient way to use public key or certificate authentication
820 may be with an authentication agent.
823 for more information.
825 Challenge-response authentication works as follows:
826 The server sends an arbitrary
828 text, and prompts for a response.
829 Protocol 2 allows multiple challenges and responses;
830 protocol 1 is restricted to just one challenge/response.
831 Examples of challenge-response authentication include
832 BSD Authentication (see
834 and PAM (some non-OpenBSD systems).
836 Finally, if other authentication methods fail,
838 prompts the user for a password.
839 The password is sent to the remote
840 host for checking; however, since all communications are encrypted,
841 the password cannot be seen by someone listening on the network.
844 automatically maintains and checks a database containing
845 identification for all hosts it has ever been used with.
846 Host keys are stored in
847 .Pa ~/.ssh/known_hosts
848 in the user's home directory.
849 Additionally, the file
850 .Pa /etc/ssh/ssh_known_hosts
851 is automatically checked for known hosts.
852 Any new hosts are automatically added to the user's file.
853 If a host's identification ever changes,
855 warns about this and disables password authentication to prevent
856 server spoofing or man-in-the-middle attacks,
857 which could otherwise be used to circumvent the encryption.
859 .Cm StrictHostKeyChecking
860 option can be used to control logins to machines whose
861 host key is not known or has changed.
863 When the user's identity has been accepted by the server, the server
864 either executes the given command, or logs into the machine and gives
865 the user a normal shell on the remote machine.
866 All communication with
867 the remote command or shell will be automatically encrypted.
869 If a pseudo-terminal has been allocated (normal login session), the
870 user may use the escape characters noted below.
872 If no pseudo-tty has been allocated,
873 the session is transparent and can be used to reliably transfer binary data.
874 On most systems, setting the escape character to
876 will also make the session transparent even if a tty is used.
878 The session terminates when the command or shell on the remote
879 machine exits and all X11 and TCP connections have been closed.
880 .Sh ESCAPE CHARACTERS
881 When a pseudo-terminal has been requested,
883 supports a number of functions through the use of an escape character.
885 A single tilde character can be sent as
887 or by following the tilde by a character other than those described below.
888 The escape character must always follow a newline to be interpreted as
890 The escape character can be changed in configuration files using the
892 configuration directive or on the command line by the
896 The supported escapes (assuming the default
906 List forwarded connections.
910 at logout when waiting for forwarded connection / X11 sessions to terminate.
912 Display a list of escape characters.
914 Send a BREAK to the remote system
915 (only useful for SSH protocol version 2 and if the peer supports it).
918 Currently this allows the addition of port forwardings using the
924 It also allows the cancellation of existing port-forwardings
927 .Fl KL Oo Ar bind_address : Oc Ar port
931 .Fl KR Oo Ar bind_address : Oc Ar port
935 .Fl KD Oo Ar bind_address : Oc Ar port
937 for dynamic port-forwardings.
938 .Ic !\& Ns Ar command
939 allows the user to execute a local command if the
940 .Ic PermitLocalCommand
943 Basic help is available, using the
947 Request rekeying of the connection
948 (only useful for SSH protocol version 2 and if the peer supports it).
950 Decrease the verbosity
952 when errors are being written to stderr.
954 Increase the verbosity
956 when errors are being written to stderr.
959 Forwarding of arbitrary TCP connections over the secure channel can
960 be specified either on the command line or in a configuration file.
961 One possible application of TCP forwarding is a secure connection to a
962 mail server; another is going through firewalls.
964 In the example below, we look at encrypting communication between
965 an IRC client and server, even though the IRC server does not directly
966 support encrypted communications.
967 This works as follows:
968 the user connects to the remote host using
970 specifying a port to be used to forward connections
971 to the remote server.
972 After that it is possible to start the service which is to be encrypted
973 on the client machine,
974 connecting to the same local port,
977 will encrypt and forward the connection.
979 The following example tunnels an IRC session from client machine
983 .Dq server.example.com :
984 .Bd -literal -offset 4n
985 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
986 $ irc -c '#users' -p 1234 pinky 127.0.0.1
989 This tunnels a connection to IRC server
990 .Dq server.example.com ,
996 It doesn't matter which port is used,
997 as long as it's greater than 1023
998 (remember, only root can open sockets on privileged ports)
999 and doesn't conflict with any ports already in use.
1000 The connection is forwarded to port 6667 on the remote server,
1001 since that's the standard port for IRC services.
1007 and the remote command
1009 is specified to allow an amount of time
1010 (10 seconds, in the example)
1011 to start the service which is to be tunnelled.
1012 If no connections are made within the time specified,
1020 (or see the description of the
1026 and the user is using X11 (the
1028 environment variable is set), the connection to the X11 display is
1029 automatically forwarded to the remote side in such a way that any X11
1030 programs started from the shell (or command) will go through the
1031 encrypted channel, and the connection to the real X server will be made
1032 from the local machine.
1033 The user should not manually set
1035 Forwarding of X11 connections can be
1036 configured on the command line or in configuration files.
1042 will point to the server machine, but with a display number greater than zero.
1043 This is normal, and happens because
1047 X server on the server machine for forwarding the
1048 connections over the encrypted channel.
1051 will also automatically set up Xauthority data on the server machine.
1052 For this purpose, it will generate a random authorization cookie,
1053 store it in Xauthority on the server, and verify that any forwarded
1054 connections carry this cookie and replace it by the real cookie when
1055 the connection is opened.
1056 The real authentication cookie is never
1057 sent to the server machine (and no cookies are sent in the plain).
1063 (or see the description of the
1068 the user is using an authentication agent, the connection to the agent
1069 is automatically forwarded to the remote side.
1070 .Sh VERIFYING HOST KEYS
1071 When connecting to a server for the first time,
1072 a fingerprint of the server's public key is presented to the user
1074 .Cm StrictHostKeyChecking
1076 Fingerprints can be determined using
1079 .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1081 If the fingerprint is already known, it can be matched
1082 and the key can be accepted or rejected.
1083 Because of the difficulty of comparing host keys
1084 just by looking at hex strings,
1085 there is also support to compare host keys visually,
1092 a small ASCII graphic gets displayed on every login to a server, no matter
1093 if the session itself is interactive or not.
1094 By learning the pattern a known server produces, a user can easily
1095 find out that the host key has changed when a completely different pattern
1097 Because these patterns are not unambiguous however, a pattern that looks
1098 similar to the pattern remembered only gives a good probability that the
1099 host key is the same, not guaranteed proof.
1101 To get a listing of the fingerprints along with their random art for
1102 all known hosts, the following command line can be used:
1104 .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1106 If the fingerprint is unknown,
1107 an alternative method of verification is available:
1108 SSH fingerprints verified by DNS.
1109 An additional resource record (RR),
1111 is added to a zonefile
1112 and the connecting client is able to match the fingerprint
1113 with that of the key presented.
1115 In this example, we are connecting a client to a server,
1116 .Dq host.example.com .
1117 The SSHFP resource records should first be added to the zonefile for
1119 .Bd -literal -offset indent
1120 $ ssh-keygen -r host.example.com.
1123 The output lines will have to be added to the zonefile.
1124 To check that the zone is answering fingerprint queries:
1126 .Dl $ dig -t SSHFP host.example.com
1128 Finally the client connects:
1129 .Bd -literal -offset indent
1130 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1132 Matching host key fingerprint found in DNS.
1133 Are you sure you want to continue connecting (yes/no)?
1137 .Cm VerifyHostKeyDNS
1140 for more information.
1141 .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1143 contains support for Virtual Private Network (VPN) tunnelling
1146 network pseudo-device,
1147 allowing two networks to be joined securely.
1150 configuration option
1152 controls whether the server supports this,
1153 and at what level (layer 2 or 3 traffic).
1155 The following example would connect client network 10.0.50.0/24
1156 with remote network 10.0.99.0/24 using a point-to-point connection
1157 from 10.1.1.1 to 10.1.1.2,
1158 provided that the SSH server running on the gateway to the remote network,
1159 at 192.168.1.15, allows it.
1162 .Bd -literal -offset indent
1163 # ssh -f -w 0:1 192.168.1.15 true
1164 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1165 # route add 10.0.99.0/24 10.1.1.2
1169 .Bd -literal -offset indent
1170 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1171 # route add 10.0.50.0/24 10.1.1.1
1174 Client access may be more finely tuned via the
1175 .Pa /root/.ssh/authorized_keys
1176 file (see below) and the
1179 The following entry would permit connections on
1183 and on tun device 2 from user
1188 .Dq forced-commands-only :
1189 .Bd -literal -offset 2n
1190 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1191 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1194 Since an SSH-based setup entails a fair amount of overhead,
1195 it may be more suited to temporary setups,
1196 such as for wireless VPNs.
1197 More permanent VPNs are better provided by tools such as
1203 will normally set the following environment variables:
1204 .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1208 variable indicates the location of the X11 server.
1209 It is automatically set by
1211 to point to a value of the form
1215 indicates the host where the shell runs, and
1217 is an integer \*(Ge 1.
1219 uses this special value to forward X11 connections over the secure
1221 The user should normally not set
1224 will render the X11 connection insecure (and will require the user to
1225 manually copy any required authorization cookies).
1227 Set to the path of the user's home directory.
1231 set for compatibility with systems that use this variable.
1233 Set to the path of the user's mailbox.
1237 as specified when compiling
1242 needs a passphrase, it will read the passphrase from the current
1243 terminal if it was run from a terminal.
1246 does not have a terminal associated with it but
1250 are set, it will execute the program specified by
1252 and open an X11 window to read the passphrase.
1253 This is particularly useful when calling
1258 (Note that on some machines it
1259 may be necessary to redirect the input from
1262 .It Ev SSH_AUTH_SOCK
1263 Identifies the path of a
1265 socket used to communicate with the agent.
1266 .It Ev SSH_CONNECTION
1267 Identifies the client and server ends of the connection.
1268 The variable contains
1269 four space-separated values: client IP address, client port number,
1270 server IP address, and server port number.
1271 .It Ev SSH_ORIGINAL_COMMAND
1272 This variable contains the original command line if a forced command
1274 It can be used to extract the original arguments.
1276 This is set to the name of the tty (path to the device) associated
1277 with the current shell or command.
1278 If the current session has no tty,
1279 this variable is not set.
1281 This variable is set to indicate the present time zone if it
1282 was set when the daemon was started (i.e. the daemon passes the value
1283 on to new connections).
1285 Set to the name of the user logging in.
1291 .Pa ~/.ssh/environment ,
1292 and adds lines of the format
1294 to the environment if the file exists and users are allowed to
1295 change their environment.
1296 For more information, see the
1297 .Cm PermitUserEnvironment
1301 .Bl -tag -width Ds -compact
1303 This file is used for host-based authentication (see above).
1304 On some machines this file may need to be
1305 world-readable if the user's home directory is on an NFS partition,
1309 Additionally, this file must be owned by the user,
1310 and must not have write permissions for anyone else.
1312 permission for most machines is read/write for the user, and not
1313 accessible by others.
1316 This file is used in exactly the same way as
1318 but allows host-based authentication without permitting login with
1322 This directory is the default location for all user-specific configuration
1323 and authentication information.
1324 There is no general requirement to keep the entire contents of this directory
1325 secret, but the recommended permissions are read/write/execute for the user,
1326 and not accessible by others.
1328 .It Pa ~/.ssh/authorized_keys
1329 Lists the public keys (DSA/ECDSA/RSA) that can be used for logging in as
1331 The format of this file is described in the
1334 This file is not highly sensitive, but the recommended
1335 permissions are read/write for the user, and not accessible by others.
1337 .It Pa ~/.ssh/config
1338 This is the per-user configuration file.
1339 The file format and configuration options are described in
1341 Because of the potential for abuse, this file must have strict permissions:
1342 read/write for the user, and not writable by others.
1344 .It Pa ~/.ssh/environment
1345 Contains additional definitions for environment variables; see
1349 .It Pa ~/.ssh/identity
1350 .It Pa ~/.ssh/id_dsa
1351 .It Pa ~/.ssh/id_ecdsa
1352 .It Pa ~/.ssh/id_rsa
1353 Contains the private key for authentication.
1355 contain sensitive data and should be readable by the user but not
1356 accessible by others (read/write/execute).
1358 will simply ignore a private key file if it is accessible by others.
1359 It is possible to specify a passphrase when
1360 generating the key which will be used to encrypt the
1361 sensitive part of this file using 3DES.
1363 .It Pa ~/.ssh/identity.pub
1364 .It Pa ~/.ssh/id_dsa.pub
1365 .It Pa ~/.ssh/id_ecdsa.pub
1366 .It Pa ~/.ssh/id_rsa.pub
1367 Contains the public key for authentication.
1369 sensitive and can (but need not) be readable by anyone.
1371 .It Pa ~/.ssh/known_hosts
1372 Contains a list of host keys for all hosts the user has logged into
1373 that are not already in the systemwide list of known host keys.
1376 for further details of the format of this file.
1379 Commands in this file are executed by
1381 when the user logs in, just before the user's shell (or command) is
1385 manual page for more information.
1387 .It Pa /etc/hosts.equiv
1388 This file is for host-based authentication (see above).
1389 It should only be writable by root.
1391 .It Pa /etc/shosts.equiv
1392 This file is used in exactly the same way as
1394 but allows host-based authentication without permitting login with
1397 .It Pa /etc/ssh/ssh_config
1398 Systemwide configuration file.
1399 The file format and configuration options are described in
1402 .It Pa /etc/ssh/ssh_host_key
1403 .It Pa /etc/ssh/ssh_host_dsa_key
1404 .It Pa /etc/ssh/ssh_host_ecdsa_key
1405 .It Pa /etc/ssh/ssh_host_rsa_key
1406 These files contain the private parts of the host keys
1407 and are used for host-based authentication.
1408 If protocol version 1 is used,
1410 must be setuid root, since the host key is readable only by root.
1411 For protocol version 2,
1415 to access the host keys,
1416 eliminating the requirement that
1418 be setuid root when host-based authentication is used.
1423 .It Pa /etc/ssh/ssh_known_hosts
1424 Systemwide list of known host keys.
1425 This file should be prepared by the
1426 system administrator to contain the public host keys of all machines in the
1428 It should be world-readable.
1431 for further details of the format of this file.
1433 .It Pa /etc/ssh/sshrc
1434 Commands in this file are executed by
1436 when the user logs in, just before the user's shell (or command) is started.
1439 manual page for more information.
1443 exits with the exit status of the remote command or with 255
1444 if an error occurred.
1463 .%T The Secure Shell (SSH) Protocol Assigned Numbers
1471 .%T The Secure Shell (SSH) Protocol Architecture
1479 .%T The Secure Shell (SSH) Authentication Protocol
1487 .%T The Secure Shell (SSH) Transport Layer Protocol
1495 .%T The Secure Shell (SSH) Connection Protocol
1503 .%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
1511 .%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
1519 .%T The Secure Shell (SSH) Session Channel Break Extension
1528 .%T The Secure Shell (SSH) Transport Layer Encryption Modes
1535 .%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
1544 .%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
1552 .%T The Secure Shell (SSH) Public Key File Format
1560 .%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
1567 .%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
1568 .%T Hash Visualization: a New Technique to improve Real-World Security
1571 OpenSSH is a derivative of the original and free
1572 ssh 1.2.12 release by Tatu Ylonen.
1573 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1574 Theo de Raadt and Dug Song
1575 removed many bugs, re-added newer features and
1577 Markus Friedl contributed the support for SSH
1578 protocol versions 1.5 and 2.0.