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 1.343 2013/12/07 11:58:46 naddy 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 Q Cm cipher | cipher-auth | mac | kex | key
63 .Op Fl R Oo Ar bind_address : Oc Ns Ar port : Ns Ar host : Ns Ar hostport
65 .Op Fl W Ar host : Ns Ar port
66 .Op Fl w Ar local_tun Ns Op : Ns Ar remote_tun
67 .Oo Ar user Ns @ Oc Ns Ar hostname
72 (SSH client) is a program for logging into a remote machine and for
73 executing commands on a remote machine.
74 It is intended to replace rlogin and rsh,
75 and provide secure encrypted communications between
76 two untrusted hosts over an insecure network.
77 X11 connections and arbitrary TCP ports
78 can also be forwarded over the secure channel.
81 connects and logs into the specified
87 his/her identity to the remote machine using one of several methods
88 depending on the protocol version used (see below).
93 it is executed on the remote host instead of a login shell.
95 The options are as follows:
100 to try protocol version 1 only.
104 to try protocol version 2 only.
108 to use IPv4 addresses only.
112 to use IPv6 addresses only.
114 Enables forwarding of the authentication agent connection.
115 This can also be specified on a per-host basis in a configuration file.
117 Agent forwarding should be enabled with caution.
118 Users with the ability to bypass file permissions on the remote host
121 socket) can access the local agent through the forwarded connection.
122 An attacker cannot obtain key material from the agent,
123 however they can perform operations on the keys that enable them to
124 authenticate using the identities loaded into the agent.
126 Disables forwarding of the authentication agent connection.
127 .It Fl b Ar bind_address
130 on the local machine as the source address
132 Only useful on systems with more than one address.
134 Requests compression of all data (including stdin, stdout, stderr, and
135 data for forwarded X11 and TCP connections).
136 The compression algorithm is the same used by
140 can be controlled by the
142 option for protocol version 1.
143 Compression is desirable on modem lines and other
144 slow connections, but will only slow down things on fast networks.
145 The default value can be set on a host-by-host basis in the
146 configuration files; see the
149 .It Fl c Ar cipher_spec
150 Selects the cipher specification for encrypting the session.
152 Protocol version 1 allows specification of a single cipher.
153 The supported values are
159 (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
160 It is believed to be secure.
162 is a fast block cipher; it appears very secure and is much faster than
165 is only supported in the
167 client for interoperability with legacy protocol 1 implementations
168 that do not support the
171 Its use is strongly discouraged due to cryptographic weaknesses.
175 For protocol version 2,
177 is a comma-separated list of ciphers
178 listed in order of preference.
183 for more information.
186 .Oo Ar bind_address : Oc
192 application-level port forwarding.
193 This works by allocating a socket to listen to
195 on the local side, optionally bound to the specified
197 Whenever a connection is made to this port, the
198 connection is forwarded over the secure channel, and the application
199 protocol is then used to determine where to connect to from the
201 Currently the SOCKS4 and SOCKS5 protocols are supported, and
203 will act as a SOCKS server.
204 Only root can forward privileged ports.
205 Dynamic port forwardings can also be specified in the configuration file.
207 IPv6 addresses can be specified by enclosing the address in square brackets.
208 Only the superuser can forward privileged ports.
209 By default, the local port is bound in accordance with the
214 may be used to bind the connection to a specific address.
219 indicates that the listening port be bound for local use only, while an
222 indicates that the port should be available from all interfaces.
226 instead of standard error.
227 .It Fl e Ar escape_char
228 Sets the escape character for sessions with a pty (default:
230 The escape character is only recognized at the beginning of a line.
231 The escape character followed by a dot
233 closes the connection;
234 followed by control-Z suspends the connection;
235 and followed by itself sends the escape character once.
236 Setting the character to
238 disables any escapes and makes the session fully transparent.
239 .It Fl F Ar configfile
240 Specifies an alternative per-user configuration file.
241 If a configuration file is given on the command line,
242 the system-wide configuration file
243 .Pq Pa /etc/ssh/ssh_config
245 The default for the per-user configuration file is
250 to go to background just before command execution.
253 is going to ask for passwords or passphrases, but the user
254 wants it in the background.
257 The recommended way to start X11 programs at a remote site is with
259 .Ic ssh -f host xterm .
262 .Cm ExitOnForwardFailure
263 configuration option is set to
265 then a client started with
267 will wait for all remote port forwards to be successfully established
268 before placing itself in the background.
270 Allows remote hosts to connect to local forwarded ports.
272 Specify the PKCS#11 shared library
274 should use to communicate with a PKCS#11 token providing the user's
276 .It Fl i Ar identity_file
277 Selects a file from which the identity (private key) for
278 public key authentication is read.
281 for protocol version 1, and
283 .Pa ~/.ssh/id_ecdsa ,
284 .Pa ~/.ssh/id_ed25519
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
422 .It CanonicalizeFallbackLocal
423 .It CanonicalizeHostname
424 .It CanonicalizeMaxDots
425 .It CanonicalizePermittedCNAMEs
426 .It ChallengeResponseAuthentication
430 .It ClearAllForwardings
433 .It ConnectionAttempts
440 .It ExitOnForwardFailure
443 .It ForwardX11Timeout
444 .It ForwardX11Trusted
446 .It GlobalKnownHostsFile
447 .It GSSAPIAuthentication
448 .It GSSAPIDelegateCredentials
451 .It HostbasedAuthentication
452 .It HostKeyAlgorithms
458 .It KbdInteractiveAuthentication
459 .It KbdInteractiveDevices
466 .It NoHostAuthenticationForLocalhost
467 .It NumberOfPasswordPrompts
468 .It PasswordAuthentication
469 .It PermitLocalCommand
472 .It PreferredAuthentications
476 .It PubkeyAuthentication
480 .It RhostsRSAAuthentication
481 .It RSAAuthentication
483 .It ServerAliveInterval
484 .It ServerAliveCountMax
485 .It StrictHostKeyChecking
489 .It UsePrivilegedPort
491 .It UserKnownHostsFile
498 Port to connect to on the remote host.
499 This can be specified on a
500 per-host basis in the configuration file.
501 .It Fl Q Cm cipher | cipher-auth | mac | kex | key
504 for the algorithms supported for the specified version 2.
505 The available features are:
507 (supported symmetric ciphers),
509 (supported symmetric ciphers that support authenticated encryption),
511 (supported message integrity codes),
513 (key exchange algorithms),
518 Causes most warning and diagnostic messages to be suppressed.
521 .Oo Ar bind_address : Oc
522 .Ar port : host : hostport
525 Specifies that the given port on the remote (server) host is to be
526 forwarded to the given host and port on the local side.
527 This works by allocating a socket to listen to
529 on the remote side, and whenever a connection is made to this port, the
530 connection is forwarded over the secure channel, and a connection is
535 from the local machine.
537 Port forwardings can also be specified in the configuration file.
538 Privileged ports can be forwarded only when
539 logging in as root on the remote machine.
540 IPv6 addresses can be specified by enclosing the address in square brackets.
542 By default, the listening socket on the server will be bound to the loopback
544 This may be overridden by specifying a
550 indicates that the remote socket should listen on all interfaces.
553 will only succeed if the server's
555 option is enabled (see
556 .Xr sshd_config 5 ) .
562 the listen port will be dynamically allocated on the server and reported
563 to the client at run time.
564 When used together with
566 the allocated port will be printed to the standard output.
568 Specifies the location of a control socket for connection sharing,
571 to disable connection sharing.
572 Refer to the description of
580 May be used to request invocation of a subsystem on the remote system.
581 Subsystems are a feature of the SSH2 protocol which facilitate the use
582 of SSH as a secure transport for other applications (eg.\&
584 The subsystem is specified as the remote command.
586 Disable pseudo-tty allocation.
588 Force pseudo-tty allocation.
589 This can be used to execute arbitrary
590 screen-based programs on a remote machine, which can be very useful,
591 e.g. when implementing menu services.
594 options force tty allocation, even if
598 Display the version number and exit.
603 to print debugging messages about its progress.
605 debugging connection, authentication, and configuration problems.
608 options increase the verbosity.
610 .It Fl W Ar host : Ns Ar port
611 Requests that standard input and output on the client be forwarded to
615 over the secure channel.
619 .Cm ExitOnForwardFailure
621 .Cm ClearAllForwardings .
622 Works with Protocol version 2 only.
624 .Ar local_tun Ns Op : Ns Ar remote_tun
628 device forwarding with the specified
630 devices between the client
635 The devices may be specified by numerical ID or the keyword
637 which uses the next available tunnel device.
640 is not specified, it defaults to
650 directive is unset, it is set to the default tunnel mode, which is
653 Enables X11 forwarding.
654 This can also be specified on a per-host basis in a configuration file.
656 X11 forwarding should be enabled with caution.
657 Users with the ability to bypass file permissions on the remote host
658 (for the user's X authorization database)
659 can access the local X11 display through the forwarded connection.
660 An attacker may then be able to perform activities such as keystroke monitoring.
662 For this reason, X11 forwarding is subjected to X11 SECURITY extension
663 restrictions by default.
668 .Cm ForwardX11Trusted
671 for more information.
673 Disables X11 forwarding.
675 Enables trusted X11 forwarding.
676 Trusted X11 forwardings are not subjected to the X11 SECURITY extension
679 Send log information using the
682 By default this information is sent to stderr.
686 may additionally obtain configuration data from
687 a per-user configuration file and a system-wide configuration file.
688 The file format and configuration options are described in
691 The OpenSSH SSH client supports SSH protocols 1 and 2.
692 The default is to use protocol 2 only,
693 though this can be changed via the
702 Both protocols support similar authentication methods,
703 but protocol 2 is the default since
704 it provides additional mechanisms for confidentiality
705 (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
706 and integrity (hmac-md5, hmac-sha1,
707 hmac-sha2-256, hmac-sha2-512,
708 umac-64, umac-128, hmac-ripemd160).
709 Protocol 1 lacks a strong mechanism for ensuring the
710 integrity of the connection.
712 The methods available for authentication are:
713 GSSAPI-based authentication,
714 host-based authentication,
715 public key authentication,
716 challenge-response authentication,
717 and password authentication.
718 Authentication methods are tried in the order specified above,
719 though protocol 2 has a configuration option to change the default order:
720 .Cm PreferredAuthentications .
722 Host-based authentication works as follows:
723 If the machine the user logs in from is listed in
726 .Pa /etc/shosts.equiv
727 on the remote machine, and the user names are
728 the same on both sides, or if the files
732 exist in the user's home directory on the
733 remote machine and contain a line containing the name of the client
734 machine and the name of the user on that machine, the user is
735 considered for login.
736 Additionally, the server
738 be able to verify the client's
739 host key (see the description of
740 .Pa /etc/ssh/ssh_known_hosts
742 .Pa ~/.ssh/known_hosts ,
744 for login to be permitted.
745 This authentication method closes security holes due to IP
746 spoofing, DNS spoofing, and routing spoofing.
747 [Note to the administrator:
748 .Pa /etc/hosts.equiv ,
750 and the rlogin/rsh protocol in general, are inherently insecure and should be
751 disabled if security is desired.]
753 Public key authentication works as follows:
754 The scheme is based on public-key cryptography,
756 where encryption and decryption are done using separate keys,
757 and it is unfeasible to derive the decryption key from the encryption key.
758 The idea is that each user creates a public/private
759 key pair for authentication purposes.
760 The server knows the public key, and only the user knows the private key.
762 implements public key authentication protocol automatically,
763 using one of the DSA, ECDSA, ED25519 or RSA algorithms.
764 Protocol 1 is restricted to using only RSA keys,
765 but protocol 2 may use any.
766 The HISTORY section of
768 contains a brief discussion of the DSA and RSA algorithms.
771 .Pa ~/.ssh/authorized_keys
772 lists the public keys that are permitted for logging in.
773 When the user logs in, the
775 program tells the server which key pair it would like to use for
777 The client proves that it has access to the private key
778 and the server checks that the corresponding public key
779 is authorized to accept the account.
781 The user creates his/her key pair by running
783 This stores the private key in
790 .Pa ~/.ssh/id_ed25519
791 (protocol 2 ED25519),
795 and stores the public key in
796 .Pa ~/.ssh/identity.pub
798 .Pa ~/.ssh/id_dsa.pub
800 .Pa ~/.ssh/id_ecdsa.pub
802 .Pa ~/.ssh/id_ed25519.pub
803 (protocol 2 ED25519),
805 .Pa ~/.ssh/id_rsa.pub
807 in the user's home directory.
808 The user should then copy the public key
810 .Pa ~/.ssh/authorized_keys
811 in his/her home directory on the remote machine.
814 file corresponds to the conventional
816 file, and has one key
817 per line, though the lines can be very long.
818 After this, the user can log in without giving the password.
820 A variation on public key authentication
821 is available in the form of certificate authentication:
822 instead of a set of public/private keys,
823 signed certificates are used.
824 This has the advantage that a single trusted certification authority
825 can be used in place of many public/private keys.
826 See the CERTIFICATES section of
828 for more information.
830 The most convenient way to use public key or certificate authentication
831 may be with an authentication agent.
834 for more information.
836 Challenge-response authentication works as follows:
837 The server sends an arbitrary
839 text, and prompts for a response.
840 Protocol 2 allows multiple challenges and responses;
841 protocol 1 is restricted to just one challenge/response.
842 Examples of challenge-response authentication include
850 Finally, if other authentication methods fail,
852 prompts the user for a password.
853 The password is sent to the remote
854 host for checking; however, since all communications are encrypted,
855 the password cannot be seen by someone listening on the network.
858 automatically maintains and checks a database containing
859 identification for all hosts it has ever been used with.
860 Host keys are stored in
861 .Pa ~/.ssh/known_hosts
862 in the user's home directory.
863 Additionally, the file
864 .Pa /etc/ssh/ssh_known_hosts
865 is automatically checked for known hosts.
866 Any new hosts are automatically added to the user's file.
867 If a host's identification ever changes,
869 warns about this and disables password authentication to prevent
870 server spoofing or man-in-the-middle attacks,
871 which could otherwise be used to circumvent the encryption.
873 .Cm StrictHostKeyChecking
874 option can be used to control logins to machines whose
875 host key is not known or has changed.
877 When the user's identity has been accepted by the server, the server
878 either executes the given command, or logs into the machine and gives
879 the user a normal shell on the remote machine.
880 All communication with
881 the remote command or shell will be automatically encrypted.
883 If a pseudo-terminal has been allocated (normal login session), the
884 user may use the escape characters noted below.
886 If no pseudo-tty has been allocated,
887 the session is transparent and can be used to reliably transfer binary data.
888 On most systems, setting the escape character to
890 will also make the session transparent even if a tty is used.
892 The session terminates when the command or shell on the remote
893 machine exits and all X11 and TCP connections have been closed.
894 .Sh ESCAPE CHARACTERS
895 When a pseudo-terminal has been requested,
897 supports a number of functions through the use of an escape character.
899 A single tilde character can be sent as
901 or by following the tilde by a character other than those described below.
902 The escape character must always follow a newline to be interpreted as
904 The escape character can be changed in configuration files using the
906 configuration directive or on the command line by the
910 The supported escapes (assuming the default
920 List forwarded connections.
924 at logout when waiting for forwarded connection / X11 sessions to terminate.
926 Display a list of escape characters.
928 Send a BREAK to the remote system
929 (only useful for SSH protocol version 2 and if the peer supports it).
932 Currently this allows the addition of port forwardings using the
938 It also allows the cancellation of existing port-forwardings
941 .Fl KL Oo Ar bind_address : Oc Ar port
945 .Fl KR Oo Ar bind_address : Oc Ar port
949 .Fl KD Oo Ar bind_address : Oc Ar port
951 for dynamic port-forwardings.
952 .Ic !\& Ns Ar command
953 allows the user to execute a local command if the
954 .Ic PermitLocalCommand
957 Basic help is available, using the
961 Request rekeying of the connection
962 (only useful for SSH protocol version 2 and if the peer supports it).
964 Decrease the verbosity
966 when errors are being written to stderr.
968 Increase the verbosity
970 when errors are being written to stderr.
973 Forwarding of arbitrary TCP connections over the secure channel can
974 be specified either on the command line or in a configuration file.
975 One possible application of TCP forwarding is a secure connection to a
976 mail server; another is going through firewalls.
978 In the example below, we look at encrypting communication between
979 an IRC client and server, even though the IRC server does not directly
980 support encrypted communications.
981 This works as follows:
982 the user connects to the remote host using
984 specifying a port to be used to forward connections
985 to the remote server.
986 After that it is possible to start the service which is to be encrypted
987 on the client machine,
988 connecting to the same local port,
991 will encrypt and forward the connection.
993 The following example tunnels an IRC session from client machine
997 .Dq server.example.com :
998 .Bd -literal -offset 4n
999 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
1000 $ irc -c '#users' -p 1234 pinky 127.0.0.1
1003 This tunnels a connection to IRC server
1004 .Dq server.example.com ,
1010 It doesn't matter which port is used,
1011 as long as it's greater than 1023
1012 (remember, only root can open sockets on privileged ports)
1013 and doesn't conflict with any ports already in use.
1014 The connection is forwarded to port 6667 on the remote server,
1015 since that's the standard port for IRC services.
1021 and the remote command
1023 is specified to allow an amount of time
1024 (10 seconds, in the example)
1025 to start the service which is to be tunnelled.
1026 If no connections are made within the time specified,
1034 (or see the description of the
1040 and the user is using X11 (the
1042 environment variable is set), the connection to the X11 display is
1043 automatically forwarded to the remote side in such a way that any X11
1044 programs started from the shell (or command) will go through the
1045 encrypted channel, and the connection to the real X server will be made
1046 from the local machine.
1047 The user should not manually set
1049 Forwarding of X11 connections can be
1050 configured on the command line or in configuration files.
1056 will point to the server machine, but with a display number greater than zero.
1057 This is normal, and happens because
1061 X server on the server machine for forwarding the
1062 connections over the encrypted channel.
1065 will also automatically set up Xauthority data on the server machine.
1066 For this purpose, it will generate a random authorization cookie,
1067 store it in Xauthority on the server, and verify that any forwarded
1068 connections carry this cookie and replace it by the real cookie when
1069 the connection is opened.
1070 The real authentication cookie is never
1071 sent to the server machine (and no cookies are sent in the plain).
1077 (or see the description of the
1082 the user is using an authentication agent, the connection to the agent
1083 is automatically forwarded to the remote side.
1084 .Sh VERIFYING HOST KEYS
1085 When connecting to a server for the first time,
1086 a fingerprint of the server's public key is presented to the user
1088 .Cm StrictHostKeyChecking
1090 Fingerprints can be determined using
1093 .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1095 If the fingerprint is already known, it can be matched
1096 and the key can be accepted or rejected.
1097 Because of the difficulty of comparing host keys
1098 just by looking at hex strings,
1099 there is also support to compare host keys visually,
1106 a small ASCII graphic gets displayed on every login to a server, no matter
1107 if the session itself is interactive or not.
1108 By learning the pattern a known server produces, a user can easily
1109 find out that the host key has changed when a completely different pattern
1111 Because these patterns are not unambiguous however, a pattern that looks
1112 similar to the pattern remembered only gives a good probability that the
1113 host key is the same, not guaranteed proof.
1115 To get a listing of the fingerprints along with their random art for
1116 all known hosts, the following command line can be used:
1118 .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1120 If the fingerprint is unknown,
1121 an alternative method of verification is available:
1122 SSH fingerprints verified by DNS.
1123 An additional resource record (RR),
1125 is added to a zonefile
1126 and the connecting client is able to match the fingerprint
1127 with that of the key presented.
1129 In this example, we are connecting a client to a server,
1130 .Dq host.example.com .
1131 The SSHFP resource records should first be added to the zonefile for
1133 .Bd -literal -offset indent
1134 $ ssh-keygen -r host.example.com.
1137 The output lines will have to be added to the zonefile.
1138 To check that the zone is answering fingerprint queries:
1140 .Dl $ dig -t SSHFP host.example.com
1142 Finally the client connects:
1143 .Bd -literal -offset indent
1144 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1146 Matching host key fingerprint found in DNS.
1147 Are you sure you want to continue connecting (yes/no)?
1151 .Cm VerifyHostKeyDNS
1154 for more information.
1155 .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1157 contains support for Virtual Private Network (VPN) tunnelling
1160 network pseudo-device,
1161 allowing two networks to be joined securely.
1164 configuration option
1166 controls whether the server supports this,
1167 and at what level (layer 2 or 3 traffic).
1169 The following example would connect client network 10.0.50.0/24
1170 with remote network 10.0.99.0/24 using a point-to-point connection
1171 from 10.1.1.1 to 10.1.1.2,
1172 provided that the SSH server running on the gateway to the remote network,
1173 at 192.168.1.15, allows it.
1176 .Bd -literal -offset indent
1177 # ssh -f -w 0:1 192.168.1.15 true
1178 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1179 # route add 10.0.99.0/24 10.1.1.2
1183 .Bd -literal -offset indent
1184 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1185 # route add 10.0.50.0/24 10.1.1.1
1188 Client access may be more finely tuned via the
1189 .Pa /root/.ssh/authorized_keys
1190 file (see below) and the
1193 The following entry would permit connections on
1197 and on tun device 2 from user
1202 .Dq forced-commands-only :
1203 .Bd -literal -offset 2n
1204 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1205 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1208 Since an SSH-based setup entails a fair amount of overhead,
1209 it may be more suited to temporary setups,
1210 such as for wireless VPNs.
1211 More permanent VPNs are better provided by tools such as
1217 will normally set the following environment variables:
1218 .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1222 variable indicates the location of the X11 server.
1223 It is automatically set by
1225 to point to a value of the form
1229 indicates the host where the shell runs, and
1231 is an integer \*(Ge 1.
1233 uses this special value to forward X11 connections over the secure
1235 The user should normally not set
1238 will render the X11 connection insecure (and will require the user to
1239 manually copy any required authorization cookies).
1241 Set to the path of the user's home directory.
1245 set for compatibility with systems that use this variable.
1247 Set to the path of the user's mailbox.
1251 as specified when compiling
1256 needs a passphrase, it will read the passphrase from the current
1257 terminal if it was run from a terminal.
1260 does not have a terminal associated with it but
1264 are set, it will execute the program specified by
1266 and open an X11 window to read the passphrase.
1267 This is particularly useful when calling
1272 (Note that on some machines it
1273 may be necessary to redirect the input from
1276 .It Ev SSH_AUTH_SOCK
1277 Identifies the path of a
1279 socket used to communicate with the agent.
1280 .It Ev SSH_CONNECTION
1281 Identifies the client and server ends of the connection.
1282 The variable contains
1283 four space-separated values: client IP address, client port number,
1284 server IP address, and server port number.
1285 .It Ev SSH_ORIGINAL_COMMAND
1286 This variable contains the original command line if a forced command
1288 It can be used to extract the original arguments.
1290 This is set to the name of the tty (path to the device) associated
1291 with the current shell or command.
1292 If the current session has no tty,
1293 this variable is not set.
1295 This variable is set to indicate the present time zone if it
1296 was set when the daemon was started (i.e. the daemon passes the value
1297 on to new connections).
1299 Set to the name of the user logging in.
1305 .Pa ~/.ssh/environment ,
1306 and adds lines of the format
1308 to the environment if the file exists and users are allowed to
1309 change their environment.
1310 For more information, see the
1311 .Cm PermitUserEnvironment
1315 .Bl -tag -width Ds -compact
1317 This file is used for host-based authentication (see above).
1318 On some machines this file may need to be
1319 world-readable if the user's home directory is on an NFS partition,
1323 Additionally, this file must be owned by the user,
1324 and must not have write permissions for anyone else.
1326 permission for most machines is read/write for the user, and not
1327 accessible by others.
1330 This file is used in exactly the same way as
1332 but allows host-based authentication without permitting login with
1336 This directory is the default location for all user-specific configuration
1337 and authentication information.
1338 There is no general requirement to keep the entire contents of this directory
1339 secret, but the recommended permissions are read/write/execute for the user,
1340 and not accessible by others.
1342 .It Pa ~/.ssh/authorized_keys
1343 Lists the public keys (DSA, ECDSA, ED25519, RSA)
1344 that can be used for logging in as this user.
1345 The format of this file is described in the
1348 This file is not highly sensitive, but the recommended
1349 permissions are read/write for the user, and not accessible by others.
1351 .It Pa ~/.ssh/config
1352 This is the per-user configuration file.
1353 The file format and configuration options are described in
1355 Because of the potential for abuse, this file must have strict permissions:
1356 read/write for the user, and not writable by others.
1358 .It Pa ~/.ssh/environment
1359 Contains additional definitions for environment variables; see
1363 .It Pa ~/.ssh/identity
1364 .It Pa ~/.ssh/id_dsa
1365 .It Pa ~/.ssh/id_ecdsa
1366 .It Pa ~/.ssh/id_ed25519
1367 .It Pa ~/.ssh/id_rsa
1368 Contains the private key for authentication.
1370 contain sensitive data and should be readable by the user but not
1371 accessible by others (read/write/execute).
1373 will simply ignore a private key file if it is accessible by others.
1374 It is possible to specify a passphrase when
1375 generating the key which will be used to encrypt the
1376 sensitive part of this file using 3DES.
1378 .It Pa ~/.ssh/identity.pub
1379 .It Pa ~/.ssh/id_dsa.pub
1380 .It Pa ~/.ssh/id_ecdsa.pub
1381 .It Pa ~/.ssh/id_ed25519.pub
1382 .It Pa ~/.ssh/id_rsa.pub
1383 Contains the public key for authentication.
1385 sensitive and can (but need not) be readable by anyone.
1387 .It Pa ~/.ssh/known_hosts
1388 Contains a list of host keys for all hosts the user has logged into
1389 that are not already in the systemwide list of known host keys.
1392 for further details of the format of this file.
1395 Commands in this file are executed by
1397 when the user logs in, just before the user's shell (or command) is
1401 manual page for more information.
1403 .It Pa /etc/hosts.equiv
1404 This file is for host-based authentication (see above).
1405 It should only be writable by root.
1407 .It Pa /etc/shosts.equiv
1408 This file is used in exactly the same way as
1410 but allows host-based authentication without permitting login with
1413 .It Pa /etc/ssh/ssh_config
1414 Systemwide configuration file.
1415 The file format and configuration options are described in
1418 .It Pa /etc/ssh/ssh_host_key
1419 .It Pa /etc/ssh/ssh_host_dsa_key
1420 .It Pa /etc/ssh/ssh_host_ecdsa_key
1421 .It Pa /etc/ssh/ssh_host_ed25519_key
1422 .It Pa /etc/ssh/ssh_host_rsa_key
1423 These files contain the private parts of the host keys
1424 and are used for host-based authentication.
1425 If protocol version 1 is used,
1427 must be setuid root, since the host key is readable only by root.
1428 For protocol version 2,
1432 to access the host keys,
1433 eliminating the requirement that
1435 be setuid root when host-based authentication is used.
1440 .It Pa /etc/ssh/ssh_known_hosts
1441 Systemwide list of known host keys.
1442 This file should be prepared by the
1443 system administrator to contain the public host keys of all machines in the
1445 It should be world-readable.
1448 for further details of the format of this file.
1450 .It Pa /etc/ssh/sshrc
1451 Commands in this file are executed by
1453 when the user logs in, just before the user's shell (or command) is started.
1456 manual page for more information.
1460 exits with the exit status of the remote command or with 255
1461 if an error occurred.
1480 .%T The Secure Shell (SSH) Protocol Assigned Numbers
1488 .%T The Secure Shell (SSH) Protocol Architecture
1496 .%T The Secure Shell (SSH) Authentication Protocol
1504 .%T The Secure Shell (SSH) Transport Layer Protocol
1512 .%T The Secure Shell (SSH) Connection Protocol
1520 .%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
1528 .%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
1536 .%T The Secure Shell (SSH) Session Channel Break Extension
1545 .%T The Secure Shell (SSH) Transport Layer Encryption Modes
1552 .%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
1561 .%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
1569 .%T The Secure Shell (SSH) Public Key File Format
1577 .%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
1584 .%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
1585 .%T Hash Visualization: a New Technique to improve Real-World Security
1588 OpenSSH is a derivative of the original and free
1589 ssh 1.2.12 release by Tatu Ylonen.
1590 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1591 Theo de Raadt and Dug Song
1592 removed many bugs, re-added newer features and
1594 Markus Friedl contributed the support for SSH
1595 protocol versions 1.5 and 2.0.