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 $
38 .Dd $Mdocdate: February 23 2018 $
43 .Nd OpenSSH SSH client (remote login program)
46 .Op Fl 46AaCfGgKkMNnqsTtVvXxYy
47 .Op Fl B Ar bind_interface
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 J Ar destination
58 .Op Fl l Ar login_name
63 .Op Fl Q Ar query_option
66 .Op Fl W Ar host : Ns Ar port
67 .Op Fl w Ar local_tun Ns Op : Ns Ar remote_tun
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 provide secure encrypted communications between
75 two untrusted hosts over an insecure network.
76 X11 connections, arbitrary TCP ports and
78 sockets can also be forwarded over the secure channel.
81 connects and logs into the specified
83 which may be specified as either
85 .Oo user @ Oc hostname
89 .No ssh:// Oo user @ Oc hostname Op : port .
92 his/her identity to the remote machine using one of several methods
98 it is executed on the remote host instead of a login shell.
100 The options are as follows:
102 .Bl -tag -width Ds -compact
106 to use IPv4 addresses only.
111 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.
127 Disables forwarding of the authentication agent connection.
129 .It Fl B Ar bind_interface
130 Bind to the address of
132 before attempting to connect to the destination host.
133 This is only useful on systems with more than one address.
135 .It Fl b Ar bind_address
138 on the local machine as the source address
140 Only useful on systems with more than one address.
143 Requests compression of all data (including stdin, stdout, stderr, and
144 data for forwarded X11, TCP and
147 The compression algorithm is the same used by
149 Compression is desirable on modem lines and other
150 slow connections, but will only slow down things on fast networks.
151 The default value can be set on a host-by-host basis in the
152 configuration files; see the
156 .It Fl c Ar cipher_spec
157 Selects the cipher specification for encrypting the session.
159 is a comma-separated list of ciphers
160 listed in order of preference.
165 for more information.
169 .Oo Ar bind_address : Oc
175 application-level port forwarding.
176 This works by allocating a socket to listen to
178 on the local side, optionally bound to the specified
180 Whenever a connection is made to this port, the
181 connection is forwarded over the secure channel, and the application
182 protocol is then used to determine where to connect to from the
184 Currently the SOCKS4 and SOCKS5 protocols are supported, and
186 will act as a SOCKS server.
187 Only root can forward privileged ports.
188 Dynamic port forwardings can also be specified in the configuration file.
190 IPv6 addresses can be specified by enclosing the address in square brackets.
191 Only the superuser can forward privileged ports.
192 By default, the local port is bound in accordance with the
197 may be used to bind the connection to a specific address.
202 indicates that the listening port be bound for local use only, while an
205 indicates that the port should be available from all interfaces.
210 instead of standard error.
212 .It Fl e Ar escape_char
213 Sets the escape character for sessions with a pty (default:
215 The escape character is only recognized at the beginning of a line.
216 The escape character followed by a dot
218 closes the connection;
219 followed by control-Z suspends the connection;
220 and followed by itself sends the escape character once.
221 Setting the character to
223 disables any escapes and makes the session fully transparent.
225 .It Fl F Ar configfile
226 Specifies an alternative per-user configuration file.
227 If a configuration file is given on the command line,
228 the system-wide configuration file
229 .Pq Pa /etc/ssh/ssh_config
231 The default for the per-user configuration file is
237 to go to background just before command execution.
240 is going to ask for passwords or passphrases, but the user
241 wants it in the background.
244 The recommended way to start X11 programs at a remote site is with
246 .Ic ssh -f host xterm .
249 .Cm ExitOnForwardFailure
250 configuration option is set to
252 then a client started with
254 will wait for all remote port forwards to be successfully established
255 before placing itself in the background.
260 to print its configuration after evaluating
267 Allows remote hosts to connect to local forwarded ports.
268 If used on a multiplexed connection, then this option must be specified
269 on the master process.
272 Specify the PKCS#11 shared library
274 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 .Pa ~/.ssh/id_ecdsa ,
283 .Pa ~/.ssh/id_ed25519
286 Identity files may also be specified on
287 a per-host basis in the configuration file.
288 It is possible to have multiple
290 options (and multiple identities specified in
291 configuration files).
292 If no certificates have been explicitly specified by the
296 will also try to load certificate information from the filename obtained
299 to identity filenames.
301 .It Fl J Ar destination
302 Connect to the target host by first making a
304 connection to the jump host described by
306 and then establishing a TCP forwarding to the ultimate destination from
308 Multiple jump hops may be specified separated by comma characters.
309 This is a shortcut to specify a
311 configuration directive.
314 Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
315 credentials to the server.
318 Disables forwarding (delegation) of GSSAPI credentials to the server.
322 .Oo Ar bind_address : Oc
323 .Ar port : host : hostport
328 .Oo Ar bind_address : Oc
329 .Ar port : remote_socket
334 .Ar local_socket : host : hostport
339 .Ar local_socket : remote_socket
342 Specifies that connections to the given TCP port or Unix socket on the local
343 (client) host are to be forwarded to the given host and port, or Unix socket,
345 This works by allocating a socket to listen to either a TCP
347 on the local side, optionally bound to the specified
350 Whenever a connection is made to the local port or socket, the
351 connection is forwarded over the secure channel, and a connection is
358 from the remote machine.
360 Port forwardings can also be specified in the configuration file.
361 Only the superuser can forward privileged ports.
362 IPv6 addresses can be specified by enclosing the address in square brackets.
364 By default, the local port is bound in accordance with the
369 may be used to bind the connection to a specific address.
374 indicates that the listening port be bound for local use only, while an
377 indicates that the port should be available from all interfaces.
379 .It Fl l Ar login_name
380 Specifies the user to log in as on the remote machine.
381 This also may be specified on a per-host basis in the configuration file.
388 mode for connection sharing.
395 mode with confirmation required before slave connections are accepted.
396 Refer to the description of
403 A comma-separated list of MAC (message authentication code) algorithms,
404 specified in order of preference.
407 keyword for more information.
410 Do not execute a remote command.
411 This is useful for just forwarding ports.
416 (actually, prevents reading from stdin).
417 This must be used when
419 is run in the background.
420 A common trick is to use this to run X11 programs on a remote machine.
422 .Ic ssh -n shadows.cs.hut.fi emacs &
423 will start an emacs on shadows.cs.hut.fi, and the X11
424 connection will be automatically forwarded over an encrypted channel.
427 program will be put in the background.
428 (This does not work if
430 needs to ask for a password or passphrase; see also the
435 Control an active connection multiplexing master process.
438 option is specified, the
440 argument is interpreted and passed to the master process.
443 (check that the master process is running),
445 (request forwardings without command execution),
447 (cancel forwardings),
449 (request the master to exit), and
451 (request the master to stop accepting further multiplexing requests).
454 Can be used to give options in the format used in the configuration file.
455 This is useful for specifying options for which there is no separate
457 For full details of the options listed below, and their possible values, see
460 .Bl -tag -width Ds -offset indent -compact
466 .It CanonicalizeFallbackLocal
467 .It CanonicalizeHostname
468 .It CanonicalizeMaxDots
469 .It CanonicalizePermittedCNAMEs
471 .It ChallengeResponseAuthentication
474 .It ClearAllForwardings
476 .It ConnectionAttempts
483 .It ExitOnForwardFailure
487 .It ForwardX11Timeout
488 .It ForwardX11Trusted
490 .It GlobalKnownHostsFile
491 .It GSSAPIAuthentication
492 .It GSSAPIDelegateCredentials
495 .It HostbasedAuthentication
496 .It HostbasedKeyTypes
497 .It HostKeyAlgorithms
505 .It KbdInteractiveAuthentication
506 .It KbdInteractiveDevices
513 .It NoHostAuthenticationForLocalhost
514 .It NumberOfPasswordPrompts
515 .It PasswordAuthentication
516 .It PermitLocalCommand
519 .It PreferredAuthentications
523 .It PubkeyAcceptedKeyTypes
524 .It PubkeyAuthentication
530 .It ServerAliveInterval
531 .It ServerAliveCountMax
532 .It StreamLocalBindMask
533 .It StreamLocalBindUnlink
534 .It StrictHostKeyChecking
539 .It UsePrivilegedPort
541 .It UserKnownHostsFile
549 Port to connect to on the remote host.
550 This can be specified on a
551 per-host basis in the configuration file.
553 .It Fl Q Ar query_option
556 for the algorithms supported for the specified version 2.
557 The available features are:
559 (supported symmetric ciphers),
561 (supported symmetric ciphers that support authenticated encryption),
563 (supported message integrity codes),
565 (key exchange algorithms),
569 (certificate key types),
571 (non-certificate key types), and
573 (supported SSH protocol versions).
577 Causes most warning and diagnostic messages to be suppressed.
581 .Oo Ar bind_address : Oc
582 .Ar port : host : hostport
587 .Oo Ar bind_address : Oc
588 .Ar port : local_socket
593 .Ar remote_socket : host : hostport
598 .Ar remote_socket : local_socket
603 .Oo Ar bind_address : Oc
607 Specifies that connections to the given TCP port or Unix socket on the remote
608 (server) host are to be forwarded to the local side.
610 This works by allocating a socket to listen to either a TCP
612 or to a Unix socket on the remote side.
613 Whenever a connection is made to this port or Unix socket, the
614 connection is forwarded over the secure channel, and a connection
615 is made from the local machine to either an explicit destination specified by
621 or, if no explicit destination was specified,
623 will act as a SOCKS 4/5 proxy and forward connections to the destinations
624 requested by the remote SOCKS client.
626 Port forwardings can also be specified in the configuration file.
627 Privileged ports can be forwarded only when
628 logging in as root on the remote machine.
629 IPv6 addresses can be specified by enclosing the address in square brackets.
631 By default, TCP listening sockets on the server will be bound to the loopback
633 This may be overridden by specifying a
639 indicates that the remote socket should listen on all interfaces.
642 will only succeed if the server's
644 option is enabled (see
645 .Xr sshd_config 5 ) .
651 the listen port will be dynamically allocated on the server and reported
652 to the client at run time.
653 When used together with
655 the allocated port will be printed to the standard output.
658 Specifies the location of a control socket for connection sharing,
661 to disable connection sharing.
662 Refer to the description of
671 May be used to request invocation of a subsystem on the remote system.
672 Subsystems facilitate the use of SSH
673 as a secure transport for other applications (e.g.\&
675 The subsystem is specified as the remote command.
678 Disable pseudo-terminal allocation.
681 Force pseudo-terminal allocation.
682 This can be used to execute arbitrary
683 screen-based programs on a remote machine, which can be very useful,
684 e.g. when implementing menu services.
687 options force tty allocation, even if
692 Display the version number and exit.
698 to print debugging messages about its progress.
700 debugging connection, authentication, and configuration problems.
703 options increase the verbosity.
706 .It Fl W Ar host : Ns Ar port
707 Requests that standard input and output on the client be forwarded to
711 over the secure channel.
715 .Cm ExitOnForwardFailure
717 .Cm ClearAllForwardings ,
718 though these can be overridden in the configuration file or using
720 command line options.
723 .Ar local_tun Ns Op : Ns Ar remote_tun
727 device forwarding with the specified
729 devices between the client
734 The devices may be specified by numerical ID or the keyword
736 which uses the next available tunnel device.
739 is not specified, it defaults to
749 directive is unset, it is set to the default tunnel mode, which is
753 Enables X11 forwarding.
754 This can also be specified on a per-host basis in a configuration file.
756 X11 forwarding should be enabled with caution.
757 Users with the ability to bypass file permissions on the remote host
758 (for the user's X authorization database)
759 can access the local X11 display through the forwarded connection.
760 An attacker may then be able to perform activities such as keystroke monitoring.
762 For this reason, X11 forwarding is subjected to X11 SECURITY extension
763 restrictions by default.
768 .Cm ForwardX11Trusted
771 for more information.
774 Disables X11 forwarding.
777 Enables trusted X11 forwarding.
778 Trusted X11 forwardings are not subjected to the X11 SECURITY extension
782 Send log information using the
785 By default this information is sent to stderr.
789 may additionally obtain configuration data from
790 a per-user configuration file and a system-wide configuration file.
791 The file format and configuration options are described in
794 The OpenSSH SSH client supports SSH protocol 2.
796 The methods available for authentication are:
797 GSSAPI-based authentication,
798 host-based authentication,
799 public key authentication,
800 challenge-response authentication,
801 and password authentication.
802 Authentication methods are tried in the order specified above,
804 .Cm PreferredAuthentications
805 can be used to change the default order.
807 Host-based authentication works as follows:
808 If the machine the user logs in from is listed in
811 .Pa /etc/shosts.equiv
812 on the remote machine, and the user names are
813 the same on both sides, or if the files
817 exist in the user's home directory on the
818 remote machine and contain a line containing the name of the client
819 machine and the name of the user on that machine, the user is
820 considered for login.
821 Additionally, the server
823 be able to verify the client's
824 host key (see the description of
825 .Pa /etc/ssh/ssh_known_hosts
827 .Pa ~/.ssh/known_hosts ,
829 for login to be permitted.
830 This authentication method closes security holes due to IP
831 spoofing, DNS spoofing, and routing spoofing.
832 [Note to the administrator:
833 .Pa /etc/hosts.equiv ,
835 and the rlogin/rsh protocol in general, are inherently insecure and should be
836 disabled if security is desired.]
838 Public key authentication works as follows:
839 The scheme is based on public-key cryptography,
841 where encryption and decryption are done using separate keys,
842 and it is unfeasible to derive the decryption key from the encryption key.
843 The idea is that each user creates a public/private
844 key pair for authentication purposes.
845 The server knows the public key, and only the user knows the private key.
847 implements public key authentication protocol automatically,
848 using one of the DSA, ECDSA, Ed25519 or RSA algorithms.
849 The HISTORY section of
851 contains a brief discussion of the DSA and RSA algorithms.
854 .Pa ~/.ssh/authorized_keys
855 lists the public keys that are permitted for logging in.
856 When the user logs in, the
858 program tells the server which key pair it would like to use for
860 The client proves that it has access to the private key
861 and the server checks that the corresponding public key
862 is authorized to accept the account.
864 The server may inform the client of errors that prevented public key
865 authentication from succeeding after authentication completes using a
867 These may be viewed by increasing the
871 or higher (e.g. by using the
875 The user creates his/her key pair by running
877 This stores the private key in
882 .Pa ~/.ssh/id_ed25519
887 and stores the public key in
888 .Pa ~/.ssh/id_dsa.pub
890 .Pa ~/.ssh/id_ecdsa.pub
892 .Pa ~/.ssh/id_ed25519.pub
895 .Pa ~/.ssh/id_rsa.pub
897 in the user's home directory.
898 The user should then copy the public key
900 .Pa ~/.ssh/authorized_keys
901 in his/her home directory on the remote machine.
904 file corresponds to the conventional
906 file, and has one key
907 per line, though the lines can be very long.
908 After this, the user can log in without giving the password.
910 A variation on public key authentication
911 is available in the form of certificate authentication:
912 instead of a set of public/private keys,
913 signed certificates are used.
914 This has the advantage that a single trusted certification authority
915 can be used in place of many public/private keys.
916 See the CERTIFICATES section of
918 for more information.
920 The most convenient way to use public key or certificate authentication
921 may be with an authentication agent.
928 for more information.
930 Challenge-response authentication works as follows:
931 The server sends an arbitrary
933 text, and prompts for a response.
934 Examples of challenge-response authentication include
942 Finally, if other authentication methods fail,
944 prompts the user for a password.
945 The password is sent to the remote
946 host for checking; however, since all communications are encrypted,
947 the password cannot be seen by someone listening on the network.
950 automatically maintains and checks a database containing
951 identification for all hosts it has ever been used with.
952 Host keys are stored in
953 .Pa ~/.ssh/known_hosts
954 in the user's home directory.
955 Additionally, the file
956 .Pa /etc/ssh/ssh_known_hosts
957 is automatically checked for known hosts.
958 Any new hosts are automatically added to the user's file.
959 If a host's identification ever changes,
961 warns about this and disables password authentication to prevent
962 server spoofing or man-in-the-middle attacks,
963 which could otherwise be used to circumvent the encryption.
965 .Cm StrictHostKeyChecking
966 option can be used to control logins to machines whose
967 host key is not known or has changed.
969 When the user's identity has been accepted by the server, the server
970 either executes the given command in a non-interactive session or,
971 if no command has been specified, logs into the machine and gives
972 the user a normal shell as an interactive session.
973 All communication with
974 the remote command or shell will be automatically encrypted.
976 If an interactive session is requested
978 by default will only request a pseudo-terminal (pty) for interactive
979 sessions when the client has one.
984 can be used to override this behaviour.
986 If a pseudo-terminal has been allocated the
987 user may use the escape characters noted below.
989 If no pseudo-terminal has been allocated,
990 the session is transparent and can be used to reliably transfer binary data.
991 On most systems, setting the escape character to
993 will also make the session transparent even if a tty is used.
995 The session terminates when the command or shell on the remote
996 machine exits and all X11 and TCP connections have been closed.
997 .Sh ESCAPE CHARACTERS
998 When a pseudo-terminal has been requested,
1000 supports a number of functions through the use of an escape character.
1002 A single tilde character can be sent as
1004 or by following the tilde by a character other than those described below.
1005 The escape character must always follow a newline to be interpreted as
1007 The escape character can be changed in configuration files using the
1009 configuration directive or on the command line by the
1013 The supported escapes (assuming the default
1023 List forwarded connections.
1027 at logout when waiting for forwarded connection / X11 sessions to terminate.
1029 Display a list of escape characters.
1031 Send a BREAK to the remote system
1032 (only useful if the peer supports it).
1035 Currently this allows the addition of port forwardings using the
1040 options (see above).
1041 It also allows the cancellation of existing port-forwardings
1044 .Fl KL Oo Ar bind_address : Oc Ar port
1048 .Fl KR Oo Ar bind_address : Oc Ar port
1052 .Fl KD Oo Ar bind_address : Oc Ar port
1054 for dynamic port-forwardings.
1055 .Ic !\& Ns Ar command
1056 allows the user to execute a local command if the
1057 .Ic PermitLocalCommand
1058 option is enabled in
1060 Basic help is available, using the
1064 Request rekeying of the connection
1065 (only useful if the peer supports it).
1067 Decrease the verbosity
1069 when errors are being written to stderr.
1071 Increase the verbosity
1073 when errors are being written to stderr.
1076 Forwarding of arbitrary TCP connections over the secure channel can
1077 be specified either on the command line or in a configuration file.
1078 One possible application of TCP forwarding is a secure connection to a
1079 mail server; another is going through firewalls.
1081 In the example below, we look at encrypting communication between
1082 an IRC client and server, even though the IRC server does not directly
1083 support encrypted communications.
1084 This works as follows:
1085 the user connects to the remote host using
1087 specifying a port to be used to forward connections
1088 to the remote server.
1089 After that it is possible to start the service which is to be encrypted
1090 on the client machine,
1091 connecting to the same local port,
1094 will encrypt and forward the connection.
1096 The following example tunnels an IRC session from client machine
1100 .Dq server.example.com :
1101 .Bd -literal -offset 4n
1102 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
1103 $ irc -c '#users' -p 1234 pinky 127.0.0.1
1106 This tunnels a connection to IRC server
1107 .Dq server.example.com ,
1113 It doesn't matter which port is used,
1114 as long as it's greater than 1023
1115 (remember, only root can open sockets on privileged ports)
1116 and doesn't conflict with any ports already in use.
1117 The connection is forwarded to port 6667 on the remote server,
1118 since that's the standard port for IRC services.
1124 and the remote command
1126 is specified to allow an amount of time
1127 (10 seconds, in the example)
1128 to start the service which is to be tunnelled.
1129 If no connections are made within the time specified,
1137 (or see the description of the
1143 and the user is using X11 (the
1145 environment variable is set), the connection to the X11 display is
1146 automatically forwarded to the remote side in such a way that any X11
1147 programs started from the shell (or command) will go through the
1148 encrypted channel, and the connection to the real X server will be made
1149 from the local machine.
1150 The user should not manually set
1152 Forwarding of X11 connections can be
1153 configured on the command line or in configuration files.
1159 will point to the server machine, but with a display number greater than zero.
1160 This is normal, and happens because
1164 X server on the server machine for forwarding the
1165 connections over the encrypted channel.
1168 will also automatically set up Xauthority data on the server machine.
1169 For this purpose, it will generate a random authorization cookie,
1170 store it in Xauthority on the server, and verify that any forwarded
1171 connections carry this cookie and replace it by the real cookie when
1172 the connection is opened.
1173 The real authentication cookie is never
1174 sent to the server machine (and no cookies are sent in the plain).
1180 (or see the description of the
1185 the user is using an authentication agent, the connection to the agent
1186 is automatically forwarded to the remote side.
1187 .Sh VERIFYING HOST KEYS
1188 When connecting to a server for the first time,
1189 a fingerprint of the server's public key is presented to the user
1191 .Cm StrictHostKeyChecking
1193 Fingerprints can be determined using
1196 .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1198 If the fingerprint is already known, it can be matched
1199 and the key can be accepted or rejected.
1200 If only legacy (MD5) fingerprints for the server are available, the
1203 option may be used to downgrade the fingerprint algorithm to match.
1205 Because of the difficulty of comparing host keys
1206 just by looking at fingerprint strings,
1207 there is also support to compare host keys visually,
1214 a small ASCII graphic gets displayed on every login to a server, no matter
1215 if the session itself is interactive or not.
1216 By learning the pattern a known server produces, a user can easily
1217 find out that the host key has changed when a completely different pattern
1219 Because these patterns are not unambiguous however, a pattern that looks
1220 similar to the pattern remembered only gives a good probability that the
1221 host key is the same, not guaranteed proof.
1223 To get a listing of the fingerprints along with their random art for
1224 all known hosts, the following command line can be used:
1226 .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1228 If the fingerprint is unknown,
1229 an alternative method of verification is available:
1230 SSH fingerprints verified by DNS.
1231 An additional resource record (RR),
1233 is added to a zonefile
1234 and the connecting client is able to match the fingerprint
1235 with that of the key presented.
1237 In this example, we are connecting a client to a server,
1238 .Dq host.example.com .
1239 The SSHFP resource records should first be added to the zonefile for
1241 .Bd -literal -offset indent
1242 $ ssh-keygen -r host.example.com.
1245 The output lines will have to be added to the zonefile.
1246 To check that the zone is answering fingerprint queries:
1248 .Dl $ dig -t SSHFP host.example.com
1250 Finally the client connects:
1251 .Bd -literal -offset indent
1252 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1254 Matching host key fingerprint found in DNS.
1255 Are you sure you want to continue connecting (yes/no)?
1259 .Cm VerifyHostKeyDNS
1262 for more information.
1263 .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1265 contains support for Virtual Private Network (VPN) tunnelling
1268 network pseudo-device,
1269 allowing two networks to be joined securely.
1272 configuration option
1274 controls whether the server supports this,
1275 and at what level (layer 2 or 3 traffic).
1277 The following example would connect client network 10.0.50.0/24
1278 with remote network 10.0.99.0/24 using a point-to-point connection
1279 from 10.1.1.1 to 10.1.1.2,
1280 provided that the SSH server running on the gateway to the remote network,
1281 at 192.168.1.15, allows it.
1284 .Bd -literal -offset indent
1285 # ssh -f -w 0:1 192.168.1.15 true
1286 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1287 # route add 10.0.99.0/24 10.1.1.2
1291 .Bd -literal -offset indent
1292 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1293 # route add 10.0.50.0/24 10.1.1.1
1296 Client access may be more finely tuned via the
1297 .Pa /root/.ssh/authorized_keys
1298 file (see below) and the
1301 The following entry would permit connections on
1305 and on tun device 2 from user
1310 .Dq forced-commands-only :
1311 .Bd -literal -offset 2n
1312 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1313 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1316 Since an SSH-based setup entails a fair amount of overhead,
1317 it may be more suited to temporary setups,
1318 such as for wireless VPNs.
1319 More permanent VPNs are better provided by tools such as
1325 will normally set the following environment variables:
1326 .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1330 variable indicates the location of the X11 server.
1331 It is automatically set by
1333 to point to a value of the form
1337 indicates the host where the shell runs, and
1339 is an integer \*(Ge 1.
1341 uses this special value to forward X11 connections over the secure
1343 The user should normally not set
1346 will render the X11 connection insecure (and will require the user to
1347 manually copy any required authorization cookies).
1349 Set to the path of the user's home directory.
1353 set for compatibility with systems that use this variable.
1355 Set to the path of the user's mailbox.
1359 as specified when compiling
1364 needs a passphrase, it will read the passphrase from the current
1365 terminal if it was run from a terminal.
1368 does not have a terminal associated with it but
1372 are set, it will execute the program specified by
1374 and open an X11 window to read the passphrase.
1375 This is particularly useful when calling
1380 (Note that on some machines it
1381 may be necessary to redirect the input from
1384 .It Ev SSH_AUTH_SOCK
1385 Identifies the path of a
1387 socket used to communicate with the agent.
1388 .It Ev SSH_CONNECTION
1389 Identifies the client and server ends of the connection.
1390 The variable contains
1391 four space-separated values: client IP address, client port number,
1392 server IP address, and server port number.
1393 .It Ev SSH_ORIGINAL_COMMAND
1394 This variable contains the original command line if a forced command
1396 It can be used to extract the original arguments.
1398 This is set to the name of the tty (path to the device) associated
1399 with the current shell or command.
1400 If the current session has no tty,
1401 this variable is not set.
1405 to contain the interface names assigned if tunnel forwarding was
1406 requested by the client.
1407 .It Ev SSH_USER_AUTH
1410 this variable may contain a pathname to a file that lists the authentication
1411 methods successfully used when the session was established, including any
1412 public keys that were used.
1414 This variable is set to indicate the present time zone if it
1415 was set when the daemon was started (i.e. the daemon passes the value
1416 on to new connections).
1418 Set to the name of the user logging in.
1424 .Pa ~/.ssh/environment ,
1425 and adds lines of the format
1427 to the environment if the file exists and users are allowed to
1428 change their environment.
1429 For more information, see the
1430 .Cm PermitUserEnvironment
1434 .Bl -tag -width Ds -compact
1436 This file is used for host-based authentication (see above).
1437 On some machines this file may need to be
1438 world-readable if the user's home directory is on an NFS partition,
1442 Additionally, this file must be owned by the user,
1443 and must not have write permissions for anyone else.
1445 permission for most machines is read/write for the user, and not
1446 accessible by others.
1449 This file is used in exactly the same way as
1451 but allows host-based authentication without permitting login with
1455 This directory is the default location for all user-specific configuration
1456 and authentication information.
1457 There is no general requirement to keep the entire contents of this directory
1458 secret, but the recommended permissions are read/write/execute for the user,
1459 and not accessible by others.
1461 .It Pa ~/.ssh/authorized_keys
1462 Lists the public keys (DSA, ECDSA, Ed25519, RSA)
1463 that can be used for logging in as this user.
1464 The format of this file is described in the
1467 This file is not highly sensitive, but the recommended
1468 permissions are read/write for the user, and not accessible by others.
1470 .It Pa ~/.ssh/config
1471 This is the per-user configuration file.
1472 The file format and configuration options are described in
1474 Because of the potential for abuse, this file must have strict permissions:
1475 read/write for the user, and not writable by others.
1477 .It Pa ~/.ssh/environment
1478 Contains additional definitions for environment variables; see
1482 .It Pa ~/.ssh/id_dsa
1483 .It Pa ~/.ssh/id_ecdsa
1484 .It Pa ~/.ssh/id_ed25519
1485 .It Pa ~/.ssh/id_rsa
1486 Contains the private key for authentication.
1488 contain sensitive data and should be readable by the user but not
1489 accessible by others (read/write/execute).
1491 will simply ignore a private key file if it is accessible by others.
1492 It is possible to specify a passphrase when
1493 generating the key which will be used to encrypt the
1494 sensitive part of this file using AES-128.
1496 .It Pa ~/.ssh/id_dsa.pub
1497 .It Pa ~/.ssh/id_ecdsa.pub
1498 .It Pa ~/.ssh/id_ed25519.pub
1499 .It Pa ~/.ssh/id_rsa.pub
1500 Contains the public key for authentication.
1502 sensitive and can (but need not) be readable by anyone.
1504 .It Pa ~/.ssh/known_hosts
1505 Contains a list of host keys for all hosts the user has logged into
1506 that are not already in the systemwide list of known host keys.
1509 for further details of the format of this file.
1512 Commands in this file are executed by
1514 when the user logs in, just before the user's shell (or command) is
1518 manual page for more information.
1520 .It Pa /etc/hosts.equiv
1521 This file is for host-based authentication (see above).
1522 It should only be writable by root.
1524 .It Pa /etc/shosts.equiv
1525 This file is used in exactly the same way as
1527 but allows host-based authentication without permitting login with
1530 .It Pa /etc/ssh/ssh_config
1531 Systemwide configuration file.
1532 The file format and configuration options are described in
1535 .It Pa /etc/ssh/ssh_host_key
1536 .It Pa /etc/ssh/ssh_host_dsa_key
1537 .It Pa /etc/ssh/ssh_host_ecdsa_key
1538 .It Pa /etc/ssh/ssh_host_ed25519_key
1539 .It Pa /etc/ssh/ssh_host_rsa_key
1540 These files contain the private parts of the host keys
1541 and are used for host-based authentication.
1543 .It Pa /etc/ssh/ssh_known_hosts
1544 Systemwide list of known host keys.
1545 This file should be prepared by the
1546 system administrator to contain the public host keys of all machines in the
1548 It should be world-readable.
1551 for further details of the format of this file.
1553 .It Pa /etc/ssh/sshrc
1554 Commands in this file are executed by
1556 when the user logs in, just before the user's shell (or command) is started.
1559 manual page for more information.
1563 exits with the exit status of the remote command or with 255
1564 if an error occurred.
1582 .%T The Secure Shell (SSH) Protocol Assigned Numbers
1590 .%T The Secure Shell (SSH) Protocol Architecture
1598 .%T The Secure Shell (SSH) Authentication Protocol
1606 .%T The Secure Shell (SSH) Transport Layer Protocol
1614 .%T The Secure Shell (SSH) Connection Protocol
1622 .%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
1630 .%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
1638 .%T The Secure Shell (SSH) Session Channel Break Extension
1647 .%T The Secure Shell (SSH) Transport Layer Encryption Modes
1654 .%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
1663 .%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
1671 .%T The Secure Shell (SSH) Public Key File Format
1679 .%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
1686 .%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
1687 .%T Hash Visualization: a New Technique to improve Real-World Security
1690 OpenSSH is a derivative of the original and free
1691 ssh 1.2.12 release by Tatu Ylonen.
1692 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1693 Theo de Raadt and Dug Song
1694 removed many bugs, re-added newer features and
1696 Markus Friedl contributed the support for SSH
1697 protocol versions 1.5 and 2.0.