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 1.391 2018/02/23 07:38:09 jmc Exp $
37 .Dd $Mdocdate: February 23 2018 $
42 .Nd OpenSSH SSH client (remote login program)
45 .Op Fl 46AaCfGgKkMNnqsTtVvXxYy
46 .Op Fl B Ar bind_interface
47 .Op Fl b Ar bind_address
48 .Op Fl c Ar cipher_spec
49 .Op Fl D Oo Ar bind_address : Oc Ns Ar port
51 .Op Fl e Ar escape_char
52 .Op Fl F Ar configfile
54 .Op Fl i Ar identity_file
55 .Op Fl J Ar destination
57 .Op Fl l Ar login_name
62 .Op Fl Q Ar query_option
65 .Op Fl W Ar host : Ns Ar port
66 .Op Fl w Ar local_tun Ns Op : Ns Ar remote_tun
71 (SSH client) is a program for logging into a remote machine and for
72 executing commands on a remote machine.
73 It is intended to provide secure encrypted communications between
74 two untrusted hosts over an insecure network.
75 X11 connections, arbitrary TCP ports and
77 sockets can also be forwarded over the secure channel.
80 connects and logs into the specified
82 which may be specified as either
84 .Oo user @ Oc hostname
88 .No ssh:// Oo user @ Oc hostname Op : port .
91 his/her identity to the remote machine using one of several methods
97 it is executed on the remote host instead of a login shell.
99 The options are as follows:
101 .Bl -tag -width Ds -compact
105 to use IPv4 addresses only.
110 to use IPv6 addresses only.
113 Enables forwarding of the authentication agent connection.
114 This can also be specified on a per-host basis in a configuration file.
116 Agent forwarding should be enabled with caution.
117 Users with the ability to bypass file permissions on the remote host
120 socket) can access the local agent through the forwarded connection.
121 An attacker cannot obtain key material from the agent,
122 however they can perform operations on the keys that enable them to
123 authenticate using the identities loaded into the agent.
126 Disables forwarding of the authentication agent connection.
128 .It Fl B Ar bind_interface
129 Bind to the address of
131 before attempting to connect to the destination host.
132 This is only useful on systems with more than one address.
134 .It Fl b Ar bind_address
137 on the local machine as the source address
139 Only useful on systems with more than one address.
142 Requests compression of all data (including stdin, stdout, stderr, and
143 data for forwarded X11, TCP and
146 The compression algorithm is the same used by
148 Compression is desirable on modem lines and other
149 slow connections, but will only slow down things on fast networks.
150 The default value can be set on a host-by-host basis in the
151 configuration files; see the
155 .It Fl c Ar cipher_spec
156 Selects the cipher specification for encrypting the session.
158 is a comma-separated list of ciphers
159 listed in order of preference.
164 for more information.
168 .Oo Ar bind_address : Oc
174 application-level port forwarding.
175 This works by allocating a socket to listen to
177 on the local side, optionally bound to the specified
179 Whenever a connection is made to this port, the
180 connection is forwarded over the secure channel, and the application
181 protocol is then used to determine where to connect to from the
183 Currently the SOCKS4 and SOCKS5 protocols are supported, and
185 will act as a SOCKS server.
186 Only root can forward privileged ports.
187 Dynamic port forwardings can also be specified in the configuration file.
189 IPv6 addresses can be specified by enclosing the address in square brackets.
190 Only the superuser can forward privileged ports.
191 By default, the local port is bound in accordance with the
196 may be used to bind the connection to a specific address.
201 indicates that the listening port be bound for local use only, while an
204 indicates that the port should be available from all interfaces.
209 instead of standard error.
211 .It Fl e Ar escape_char
212 Sets the escape character for sessions with a pty (default:
214 The escape character is only recognized at the beginning of a line.
215 The escape character followed by a dot
217 closes the connection;
218 followed by control-Z suspends the connection;
219 and followed by itself sends the escape character once.
220 Setting the character to
222 disables any escapes and makes the session fully transparent.
224 .It Fl F Ar configfile
225 Specifies an alternative per-user configuration file.
226 If a configuration file is given on the command line,
227 the system-wide configuration file
228 .Pq Pa /etc/ssh/ssh_config
230 The default for the per-user configuration file is
236 to go to background just before command execution.
239 is going to ask for passwords or passphrases, but the user
240 wants it in the background.
243 The recommended way to start X11 programs at a remote site is with
245 .Ic ssh -f host xterm .
248 .Cm ExitOnForwardFailure
249 configuration option is set to
251 then a client started with
253 will wait for all remote port forwards to be successfully established
254 before placing itself in the background.
259 to print its configuration after evaluating
266 Allows remote hosts to connect to local forwarded ports.
267 If used on a multiplexed connection, then this option must be specified
268 on the master process.
271 Specify the PKCS#11 shared library
273 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 .Pa ~/.ssh/id_ecdsa ,
282 .Pa ~/.ssh/id_ed25519
285 Identity files may also be specified on
286 a per-host basis in the configuration file.
287 It is possible to have multiple
289 options (and multiple identities specified in
290 configuration files).
291 If no certificates have been explicitly specified by the
295 will also try to load certificate information from the filename obtained
298 to identity filenames.
300 .It Fl J Ar destination
301 Connect to the target host by first making a
303 connection to the jump host described by
305 and then establishing a TCP forwarding to the ultimate destination from
307 Multiple jump hops may be specified separated by comma characters.
308 This is a shortcut to specify a
310 configuration directive.
313 Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
314 credentials to the server.
317 Disables forwarding (delegation) of GSSAPI credentials to the server.
321 .Oo Ar bind_address : Oc
322 .Ar port : host : hostport
327 .Oo Ar bind_address : Oc
328 .Ar port : remote_socket
333 .Ar local_socket : host : hostport
338 .Ar local_socket : remote_socket
341 Specifies that connections to the given TCP port or Unix socket on the local
342 (client) host are to be forwarded to the given host and port, or Unix socket,
344 This works by allocating a socket to listen to either a TCP
346 on the local side, optionally bound to the specified
349 Whenever a connection is made to the local port or socket, the
350 connection is forwarded over the secure channel, and a connection is
357 from the remote machine.
359 Port forwardings can also be specified in the configuration file.
360 Only the superuser can forward privileged ports.
361 IPv6 addresses can be specified by enclosing the address in square brackets.
363 By default, the local port is bound in accordance with the
368 may be used to bind the connection to a specific address.
373 indicates that the listening port be bound for local use only, while an
376 indicates that the port should be available from all interfaces.
378 .It Fl l Ar login_name
379 Specifies the user to log in as on the remote machine.
380 This also may be specified on a per-host basis in the configuration file.
387 mode for connection sharing.
394 mode with confirmation required before slave connections are accepted.
395 Refer to the description of
402 A comma-separated list of MAC (message authentication code) algorithms,
403 specified in order of preference.
406 keyword for more information.
409 Do not execute a remote command.
410 This is useful for just forwarding ports.
415 (actually, prevents reading from stdin).
416 This must be used when
418 is run in the background.
419 A common trick is to use this to run X11 programs on a remote machine.
421 .Ic ssh -n shadows.cs.hut.fi emacs &
422 will start an emacs on shadows.cs.hut.fi, and the X11
423 connection will be automatically forwarded over an encrypted channel.
426 program will be put in the background.
427 (This does not work if
429 needs to ask for a password or passphrase; see also the
434 Control an active connection multiplexing master process.
437 option is specified, the
439 argument is interpreted and passed to the master process.
442 (check that the master process is running),
444 (request forwardings without command execution),
446 (cancel forwardings),
448 (request the master to exit), and
450 (request the master to stop accepting further multiplexing requests).
453 Can be used to give options in the format used in the configuration file.
454 This is useful for specifying options for which there is no separate
456 For full details of the options listed below, and their possible values, see
459 .Bl -tag -width Ds -offset indent -compact
465 .It CanonicalizeFallbackLocal
466 .It CanonicalizeHostname
467 .It CanonicalizeMaxDots
468 .It CanonicalizePermittedCNAMEs
470 .It ChallengeResponseAuthentication
473 .It ClearAllForwardings
475 .It ConnectionAttempts
482 .It ExitOnForwardFailure
486 .It ForwardX11Timeout
487 .It ForwardX11Trusted
489 .It GlobalKnownHostsFile
490 .It GSSAPIAuthentication
491 .It GSSAPIDelegateCredentials
494 .It HostbasedAuthentication
495 .It HostbasedKeyTypes
496 .It HostKeyAlgorithms
504 .It KbdInteractiveAuthentication
505 .It KbdInteractiveDevices
512 .It NoHostAuthenticationForLocalhost
513 .It NumberOfPasswordPrompts
514 .It PasswordAuthentication
515 .It PermitLocalCommand
518 .It PreferredAuthentications
522 .It PubkeyAcceptedKeyTypes
523 .It PubkeyAuthentication
529 .It ServerAliveInterval
530 .It ServerAliveCountMax
531 .It StreamLocalBindMask
532 .It StreamLocalBindUnlink
533 .It StrictHostKeyChecking
538 .It UsePrivilegedPort
540 .It UserKnownHostsFile
547 Port to connect to on the remote host.
548 This can be specified on a
549 per-host basis in the configuration file.
551 .It Fl Q Ar query_option
554 for the algorithms supported for the specified version 2.
555 The available features are:
557 (supported symmetric ciphers),
559 (supported symmetric ciphers that support authenticated encryption),
561 (supported message integrity codes),
563 (key exchange algorithms),
567 (certificate key types),
569 (non-certificate key types), and
571 (supported SSH protocol versions).
575 Causes most warning and diagnostic messages to be suppressed.
579 .Oo Ar bind_address : Oc
580 .Ar port : host : hostport
585 .Oo Ar bind_address : Oc
586 .Ar port : local_socket
591 .Ar remote_socket : host : hostport
596 .Ar remote_socket : local_socket
601 .Oo Ar bind_address : Oc
605 Specifies that connections to the given TCP port or Unix socket on the remote
606 (server) host are to be forwarded to the local side.
608 This works by allocating a socket to listen to either a TCP
610 or to a Unix socket on the remote side.
611 Whenever a connection is made to this port or Unix socket, the
612 connection is forwarded over the secure channel, and a connection
613 is made from the local machine to either an explicit destination specified by
619 or, if no explicit destination was specified,
621 will act as a SOCKS 4/5 proxy and forward connections to the destinations
622 requested by the remote SOCKS client.
624 Port forwardings can also be specified in the configuration file.
625 Privileged ports can be forwarded only when
626 logging in as root on the remote machine.
627 IPv6 addresses can be specified by enclosing the address in square brackets.
629 By default, TCP listening sockets on the server will be bound to the loopback
631 This may be overridden by specifying a
637 indicates that the remote socket should listen on all interfaces.
640 will only succeed if the server's
642 option is enabled (see
643 .Xr sshd_config 5 ) .
649 the listen port will be dynamically allocated on the server and reported
650 to the client at run time.
651 When used together with
653 the allocated port will be printed to the standard output.
656 Specifies the location of a control socket for connection sharing,
659 to disable connection sharing.
660 Refer to the description of
669 May be used to request invocation of a subsystem on the remote system.
670 Subsystems facilitate the use of SSH
671 as a secure transport for other applications (e.g.\&
673 The subsystem is specified as the remote command.
676 Disable pseudo-terminal allocation.
679 Force pseudo-terminal allocation.
680 This can be used to execute arbitrary
681 screen-based programs on a remote machine, which can be very useful,
682 e.g. when implementing menu services.
685 options force tty allocation, even if
690 Display the version number and exit.
696 to print debugging messages about its progress.
698 debugging connection, authentication, and configuration problems.
701 options increase the verbosity.
704 .It Fl W Ar host : Ns Ar port
705 Requests that standard input and output on the client be forwarded to
709 over the secure channel.
713 .Cm ExitOnForwardFailure
715 .Cm ClearAllForwardings ,
716 though these can be overridden in the configuration file or using
718 command line options.
721 .Ar local_tun Ns Op : Ns Ar remote_tun
725 device forwarding with the specified
727 devices between the client
732 The devices may be specified by numerical ID or the keyword
734 which uses the next available tunnel device.
737 is not specified, it defaults to
747 directive is unset, it is set to the default tunnel mode, which is
751 Enables X11 forwarding.
752 This can also be specified on a per-host basis in a configuration file.
754 X11 forwarding should be enabled with caution.
755 Users with the ability to bypass file permissions on the remote host
756 (for the user's X authorization database)
757 can access the local X11 display through the forwarded connection.
758 An attacker may then be able to perform activities such as keystroke monitoring.
760 For this reason, X11 forwarding is subjected to X11 SECURITY extension
761 restrictions by default.
766 .Cm ForwardX11Trusted
769 for more information.
772 Disables X11 forwarding.
775 Enables trusted X11 forwarding.
776 Trusted X11 forwardings are not subjected to the X11 SECURITY extension
780 Send log information using the
783 By default this information is sent to stderr.
787 may additionally obtain configuration data from
788 a per-user configuration file and a system-wide configuration file.
789 The file format and configuration options are described in
792 The OpenSSH SSH client supports SSH protocol 2.
794 The methods available for authentication are:
795 GSSAPI-based authentication,
796 host-based authentication,
797 public key authentication,
798 challenge-response authentication,
799 and password authentication.
800 Authentication methods are tried in the order specified above,
802 .Cm PreferredAuthentications
803 can be used to change the default order.
805 Host-based authentication works as follows:
806 If the machine the user logs in from is listed in
809 .Pa /etc/shosts.equiv
810 on the remote machine, and the user names are
811 the same on both sides, or if the files
815 exist in the user's home directory on the
816 remote machine and contain a line containing the name of the client
817 machine and the name of the user on that machine, the user is
818 considered for login.
819 Additionally, the server
821 be able to verify the client's
822 host key (see the description of
823 .Pa /etc/ssh/ssh_known_hosts
825 .Pa ~/.ssh/known_hosts ,
827 for login to be permitted.
828 This authentication method closes security holes due to IP
829 spoofing, DNS spoofing, and routing spoofing.
830 [Note to the administrator:
831 .Pa /etc/hosts.equiv ,
833 and the rlogin/rsh protocol in general, are inherently insecure and should be
834 disabled if security is desired.]
836 Public key authentication works as follows:
837 The scheme is based on public-key cryptography,
839 where encryption and decryption are done using separate keys,
840 and it is unfeasible to derive the decryption key from the encryption key.
841 The idea is that each user creates a public/private
842 key pair for authentication purposes.
843 The server knows the public key, and only the user knows the private key.
845 implements public key authentication protocol automatically,
846 using one of the DSA, ECDSA, Ed25519 or RSA algorithms.
847 The HISTORY section of
849 contains a brief discussion of the DSA and RSA algorithms.
852 .Pa ~/.ssh/authorized_keys
853 lists the public keys that are permitted for logging in.
854 When the user logs in, the
856 program tells the server which key pair it would like to use for
858 The client proves that it has access to the private key
859 and the server checks that the corresponding public key
860 is authorized to accept the account.
862 The server may inform the client of errors that prevented public key
863 authentication from succeeding after authentication completes using a
865 These may be viewed by increasing the
869 or higher (e.g. by using the
873 The user creates his/her key pair by running
875 This stores the private key in
880 .Pa ~/.ssh/id_ed25519
885 and stores the public key in
886 .Pa ~/.ssh/id_dsa.pub
888 .Pa ~/.ssh/id_ecdsa.pub
890 .Pa ~/.ssh/id_ed25519.pub
893 .Pa ~/.ssh/id_rsa.pub
895 in the user's home directory.
896 The user should then copy the public key
898 .Pa ~/.ssh/authorized_keys
899 in his/her home directory on the remote machine.
902 file corresponds to the conventional
904 file, and has one key
905 per line, though the lines can be very long.
906 After this, the user can log in without giving the password.
908 A variation on public key authentication
909 is available in the form of certificate authentication:
910 instead of a set of public/private keys,
911 signed certificates are used.
912 This has the advantage that a single trusted certification authority
913 can be used in place of many public/private keys.
914 See the CERTIFICATES section of
916 for more information.
918 The most convenient way to use public key or certificate authentication
919 may be with an authentication agent.
926 for more information.
928 Challenge-response authentication works as follows:
929 The server sends an arbitrary
931 text, and prompts for a response.
932 Examples of challenge-response authentication include
940 Finally, if other authentication methods fail,
942 prompts the user for a password.
943 The password is sent to the remote
944 host for checking; however, since all communications are encrypted,
945 the password cannot be seen by someone listening on the network.
948 automatically maintains and checks a database containing
949 identification for all hosts it has ever been used with.
950 Host keys are stored in
951 .Pa ~/.ssh/known_hosts
952 in the user's home directory.
953 Additionally, the file
954 .Pa /etc/ssh/ssh_known_hosts
955 is automatically checked for known hosts.
956 Any new hosts are automatically added to the user's file.
957 If a host's identification ever changes,
959 warns about this and disables password authentication to prevent
960 server spoofing or man-in-the-middle attacks,
961 which could otherwise be used to circumvent the encryption.
963 .Cm StrictHostKeyChecking
964 option can be used to control logins to machines whose
965 host key is not known or has changed.
967 When the user's identity has been accepted by the server, the server
968 either executes the given command in a non-interactive session or,
969 if no command has been specified, logs into the machine and gives
970 the user a normal shell as an interactive session.
971 All communication with
972 the remote command or shell will be automatically encrypted.
974 If an interactive session is requested
976 by default will only request a pseudo-terminal (pty) for interactive
977 sessions when the client has one.
982 can be used to override this behaviour.
984 If a pseudo-terminal has been allocated the
985 user may use the escape characters noted below.
987 If no pseudo-terminal has been allocated,
988 the session is transparent and can be used to reliably transfer binary data.
989 On most systems, setting the escape character to
991 will also make the session transparent even if a tty is used.
993 The session terminates when the command or shell on the remote
994 machine exits and all X11 and TCP connections have been closed.
995 .Sh ESCAPE CHARACTERS
996 When a pseudo-terminal has been requested,
998 supports a number of functions through the use of an escape character.
1000 A single tilde character can be sent as
1002 or by following the tilde by a character other than those described below.
1003 The escape character must always follow a newline to be interpreted as
1005 The escape character can be changed in configuration files using the
1007 configuration directive or on the command line by the
1011 The supported escapes (assuming the default
1021 List forwarded connections.
1025 at logout when waiting for forwarded connection / X11 sessions to terminate.
1027 Display a list of escape characters.
1029 Send a BREAK to the remote system
1030 (only useful if the peer supports it).
1033 Currently this allows the addition of port forwardings using the
1038 options (see above).
1039 It also allows the cancellation of existing port-forwardings
1042 .Fl KL Oo Ar bind_address : Oc Ar port
1046 .Fl KR Oo Ar bind_address : Oc Ar port
1050 .Fl KD Oo Ar bind_address : Oc Ar port
1052 for dynamic port-forwardings.
1053 .Ic !\& Ns Ar command
1054 allows the user to execute a local command if the
1055 .Ic PermitLocalCommand
1056 option is enabled in
1058 Basic help is available, using the
1062 Request rekeying of the connection
1063 (only useful if the peer supports it).
1065 Decrease the verbosity
1067 when errors are being written to stderr.
1069 Increase the verbosity
1071 when errors are being written to stderr.
1074 Forwarding of arbitrary TCP connections over the secure channel can
1075 be specified either on the command line or in a configuration file.
1076 One possible application of TCP forwarding is a secure connection to a
1077 mail server; another is going through firewalls.
1079 In the example below, we look at encrypting communication between
1080 an IRC client and server, even though the IRC server does not directly
1081 support encrypted communications.
1082 This works as follows:
1083 the user connects to the remote host using
1085 specifying a port to be used to forward connections
1086 to the remote server.
1087 After that it is possible to start the service which is to be encrypted
1088 on the client machine,
1089 connecting to the same local port,
1092 will encrypt and forward the connection.
1094 The following example tunnels an IRC session from client machine
1098 .Dq server.example.com :
1099 .Bd -literal -offset 4n
1100 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
1101 $ irc -c '#users' -p 1234 pinky 127.0.0.1
1104 This tunnels a connection to IRC server
1105 .Dq server.example.com ,
1111 It doesn't matter which port is used,
1112 as long as it's greater than 1023
1113 (remember, only root can open sockets on privileged ports)
1114 and doesn't conflict with any ports already in use.
1115 The connection is forwarded to port 6667 on the remote server,
1116 since that's the standard port for IRC services.
1122 and the remote command
1124 is specified to allow an amount of time
1125 (10 seconds, in the example)
1126 to start the service which is to be tunnelled.
1127 If no connections are made within the time specified,
1135 (or see the description of the
1141 and the user is using X11 (the
1143 environment variable is set), the connection to the X11 display is
1144 automatically forwarded to the remote side in such a way that any X11
1145 programs started from the shell (or command) will go through the
1146 encrypted channel, and the connection to the real X server will be made
1147 from the local machine.
1148 The user should not manually set
1150 Forwarding of X11 connections can be
1151 configured on the command line or in configuration files.
1157 will point to the server machine, but with a display number greater than zero.
1158 This is normal, and happens because
1162 X server on the server machine for forwarding the
1163 connections over the encrypted channel.
1166 will also automatically set up Xauthority data on the server machine.
1167 For this purpose, it will generate a random authorization cookie,
1168 store it in Xauthority on the server, and verify that any forwarded
1169 connections carry this cookie and replace it by the real cookie when
1170 the connection is opened.
1171 The real authentication cookie is never
1172 sent to the server machine (and no cookies are sent in the plain).
1178 (or see the description of the
1183 the user is using an authentication agent, the connection to the agent
1184 is automatically forwarded to the remote side.
1185 .Sh VERIFYING HOST KEYS
1186 When connecting to a server for the first time,
1187 a fingerprint of the server's public key is presented to the user
1189 .Cm StrictHostKeyChecking
1191 Fingerprints can be determined using
1194 .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1196 If the fingerprint is already known, it can be matched
1197 and the key can be accepted or rejected.
1198 If only legacy (MD5) fingerprints for the server are available, the
1201 option may be used to downgrade the fingerprint algorithm to match.
1203 Because of the difficulty of comparing host keys
1204 just by looking at fingerprint strings,
1205 there is also support to compare host keys visually,
1212 a small ASCII graphic gets displayed on every login to a server, no matter
1213 if the session itself is interactive or not.
1214 By learning the pattern a known server produces, a user can easily
1215 find out that the host key has changed when a completely different pattern
1217 Because these patterns are not unambiguous however, a pattern that looks
1218 similar to the pattern remembered only gives a good probability that the
1219 host key is the same, not guaranteed proof.
1221 To get a listing of the fingerprints along with their random art for
1222 all known hosts, the following command line can be used:
1224 .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1226 If the fingerprint is unknown,
1227 an alternative method of verification is available:
1228 SSH fingerprints verified by DNS.
1229 An additional resource record (RR),
1231 is added to a zonefile
1232 and the connecting client is able to match the fingerprint
1233 with that of the key presented.
1235 In this example, we are connecting a client to a server,
1236 .Dq host.example.com .
1237 The SSHFP resource records should first be added to the zonefile for
1239 .Bd -literal -offset indent
1240 $ ssh-keygen -r host.example.com.
1243 The output lines will have to be added to the zonefile.
1244 To check that the zone is answering fingerprint queries:
1246 .Dl $ dig -t SSHFP host.example.com
1248 Finally the client connects:
1249 .Bd -literal -offset indent
1250 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1252 Matching host key fingerprint found in DNS.
1253 Are you sure you want to continue connecting (yes/no)?
1257 .Cm VerifyHostKeyDNS
1260 for more information.
1261 .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1263 contains support for Virtual Private Network (VPN) tunnelling
1266 network pseudo-device,
1267 allowing two networks to be joined securely.
1270 configuration option
1272 controls whether the server supports this,
1273 and at what level (layer 2 or 3 traffic).
1275 The following example would connect client network 10.0.50.0/24
1276 with remote network 10.0.99.0/24 using a point-to-point connection
1277 from 10.1.1.1 to 10.1.1.2,
1278 provided that the SSH server running on the gateway to the remote network,
1279 at 192.168.1.15, allows it.
1282 .Bd -literal -offset indent
1283 # ssh -f -w 0:1 192.168.1.15 true
1284 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1285 # route add 10.0.99.0/24 10.1.1.2
1289 .Bd -literal -offset indent
1290 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1291 # route add 10.0.50.0/24 10.1.1.1
1294 Client access may be more finely tuned via the
1295 .Pa /root/.ssh/authorized_keys
1296 file (see below) and the
1299 The following entry would permit connections on
1303 and on tun device 2 from user
1308 .Dq forced-commands-only :
1309 .Bd -literal -offset 2n
1310 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1311 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1314 Since an SSH-based setup entails a fair amount of overhead,
1315 it may be more suited to temporary setups,
1316 such as for wireless VPNs.
1317 More permanent VPNs are better provided by tools such as
1323 will normally set the following environment variables:
1324 .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1328 variable indicates the location of the X11 server.
1329 It is automatically set by
1331 to point to a value of the form
1335 indicates the host where the shell runs, and
1337 is an integer \*(Ge 1.
1339 uses this special value to forward X11 connections over the secure
1341 The user should normally not set
1344 will render the X11 connection insecure (and will require the user to
1345 manually copy any required authorization cookies).
1347 Set to the path of the user's home directory.
1351 set for compatibility with systems that use this variable.
1353 Set to the path of the user's mailbox.
1357 as specified when compiling
1362 needs a passphrase, it will read the passphrase from the current
1363 terminal if it was run from a terminal.
1366 does not have a terminal associated with it but
1370 are set, it will execute the program specified by
1372 and open an X11 window to read the passphrase.
1373 This is particularly useful when calling
1378 (Note that on some machines it
1379 may be necessary to redirect the input from
1382 .It Ev SSH_AUTH_SOCK
1383 Identifies the path of a
1385 socket used to communicate with the agent.
1386 .It Ev SSH_CONNECTION
1387 Identifies the client and server ends of the connection.
1388 The variable contains
1389 four space-separated values: client IP address, client port number,
1390 server IP address, and server port number.
1391 .It Ev SSH_ORIGINAL_COMMAND
1392 This variable contains the original command line if a forced command
1394 It can be used to extract the original arguments.
1396 This is set to the name of the tty (path to the device) associated
1397 with the current shell or command.
1398 If the current session has no tty,
1399 this variable is not set.
1403 to contain the interface names assigned if tunnel forwarding was
1404 requested by the client.
1405 .It Ev SSH_USER_AUTH
1408 this variable may contain a pathname to a file that lists the authentication
1409 methods successfully used when the session was established, including any
1410 public keys that were used.
1412 This variable is set to indicate the present time zone if it
1413 was set when the daemon was started (i.e. the daemon passes the value
1414 on to new connections).
1416 Set to the name of the user logging in.
1422 .Pa ~/.ssh/environment ,
1423 and adds lines of the format
1425 to the environment if the file exists and users are allowed to
1426 change their environment.
1427 For more information, see the
1428 .Cm PermitUserEnvironment
1432 .Bl -tag -width Ds -compact
1434 This file is used for host-based authentication (see above).
1435 On some machines this file may need to be
1436 world-readable if the user's home directory is on an NFS partition,
1440 Additionally, this file must be owned by the user,
1441 and must not have write permissions for anyone else.
1443 permission for most machines is read/write for the user, and not
1444 accessible by others.
1447 This file is used in exactly the same way as
1449 but allows host-based authentication without permitting login with
1453 This directory is the default location for all user-specific configuration
1454 and authentication information.
1455 There is no general requirement to keep the entire contents of this directory
1456 secret, but the recommended permissions are read/write/execute for the user,
1457 and not accessible by others.
1459 .It Pa ~/.ssh/authorized_keys
1460 Lists the public keys (DSA, ECDSA, Ed25519, RSA)
1461 that can be used for logging in as this user.
1462 The format of this file is described in the
1465 This file is not highly sensitive, but the recommended
1466 permissions are read/write for the user, and not accessible by others.
1468 .It Pa ~/.ssh/config
1469 This is the per-user configuration file.
1470 The file format and configuration options are described in
1472 Because of the potential for abuse, this file must have strict permissions:
1473 read/write for the user, and not writable by others.
1475 .It Pa ~/.ssh/environment
1476 Contains additional definitions for environment variables; see
1480 .It Pa ~/.ssh/id_dsa
1481 .It Pa ~/.ssh/id_ecdsa
1482 .It Pa ~/.ssh/id_ed25519
1483 .It Pa ~/.ssh/id_rsa
1484 Contains the private key for authentication.
1486 contain sensitive data and should be readable by the user but not
1487 accessible by others (read/write/execute).
1489 will simply ignore a private key file if it is accessible by others.
1490 It is possible to specify a passphrase when
1491 generating the key which will be used to encrypt the
1492 sensitive part of this file using AES-128.
1494 .It Pa ~/.ssh/id_dsa.pub
1495 .It Pa ~/.ssh/id_ecdsa.pub
1496 .It Pa ~/.ssh/id_ed25519.pub
1497 .It Pa ~/.ssh/id_rsa.pub
1498 Contains the public key for authentication.
1500 sensitive and can (but need not) be readable by anyone.
1502 .It Pa ~/.ssh/known_hosts
1503 Contains a list of host keys for all hosts the user has logged into
1504 that are not already in the systemwide list of known host keys.
1507 for further details of the format of this file.
1510 Commands in this file are executed by
1512 when the user logs in, just before the user's shell (or command) is
1516 manual page for more information.
1518 .It Pa /etc/hosts.equiv
1519 This file is for host-based authentication (see above).
1520 It should only be writable by root.
1522 .It Pa /etc/shosts.equiv
1523 This file is used in exactly the same way as
1525 but allows host-based authentication without permitting login with
1528 .It Pa /etc/ssh/ssh_config
1529 Systemwide configuration file.
1530 The file format and configuration options are described in
1533 .It Pa /etc/ssh/ssh_host_key
1534 .It Pa /etc/ssh/ssh_host_dsa_key
1535 .It Pa /etc/ssh/ssh_host_ecdsa_key
1536 .It Pa /etc/ssh/ssh_host_ed25519_key
1537 .It Pa /etc/ssh/ssh_host_rsa_key
1538 These files contain the private parts of the host keys
1539 and are used for host-based authentication.
1541 .It Pa /etc/ssh/ssh_known_hosts
1542 Systemwide list of known host keys.
1543 This file should be prepared by the
1544 system administrator to contain the public host keys of all machines in the
1546 It should be world-readable.
1549 for further details of the format of this file.
1551 .It Pa /etc/ssh/sshrc
1552 Commands in this file are executed by
1554 when the user logs in, just before the user's shell (or command) is started.
1557 manual page for more information.
1561 exits with the exit status of the remote command or with 255
1562 if an error occurred.
1580 .%T The Secure Shell (SSH) Protocol Assigned Numbers
1588 .%T The Secure Shell (SSH) Protocol Architecture
1596 .%T The Secure Shell (SSH) Authentication Protocol
1604 .%T The Secure Shell (SSH) Transport Layer Protocol
1612 .%T The Secure Shell (SSH) Connection Protocol
1620 .%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
1628 .%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
1636 .%T The Secure Shell (SSH) Session Channel Break Extension
1645 .%T The Secure Shell (SSH) Transport Layer Encryption Modes
1652 .%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
1661 .%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
1669 .%T The Secure Shell (SSH) Public Key File Format
1677 .%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
1684 .%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
1685 .%T Hash Visualization: a New Technique to improve Real-World Security
1688 OpenSSH is a derivative of the original and free
1689 ssh 1.2.12 release by Tatu Ylonen.
1690 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1691 Theo de Raadt and Dug Song
1692 removed many bugs, re-added newer features and
1694 Markus Friedl contributed the support for SSH
1695 protocol versions 1.5 and 2.0.