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 1.396 2018/07/19 10:28:47 dtucker Exp $
38 .Dd $Mdocdate: July 19 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
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
532 .It StreamLocalBindMask
533 .It StreamLocalBindUnlink
534 .It StrictHostKeyChecking
540 .It UserKnownHostsFile
548 Port to connect to on the remote host.
549 This can be specified on a
550 per-host basis in the configuration file.
552 .It Fl Q Ar query_option
555 for the algorithms supported for the specified version 2.
556 The available features are:
558 (supported symmetric ciphers),
560 (supported symmetric ciphers that support authenticated encryption),
562 (supported message integrity codes),
564 (key exchange algorithms),
568 (certificate key types),
570 (non-certificate key types), and
572 (supported SSH protocol versions).
576 Causes most warning and diagnostic messages to be suppressed.
580 .Oo Ar bind_address : Oc
581 .Ar port : host : hostport
586 .Oo Ar bind_address : Oc
587 .Ar port : local_socket
592 .Ar remote_socket : host : hostport
597 .Ar remote_socket : local_socket
602 .Oo Ar bind_address : Oc
606 Specifies that connections to the given TCP port or Unix socket on the remote
607 (server) host are to be forwarded to the local side.
609 This works by allocating a socket to listen to either a TCP
611 or to a Unix socket on the remote side.
612 Whenever a connection is made to this port or Unix socket, the
613 connection is forwarded over the secure channel, and a connection
614 is made from the local machine to either an explicit destination specified by
620 or, if no explicit destination was specified,
622 will act as a SOCKS 4/5 proxy and forward connections to the destinations
623 requested by the remote SOCKS client.
625 Port forwardings can also be specified in the configuration file.
626 Privileged ports can be forwarded only when
627 logging in as root on the remote machine.
628 IPv6 addresses can be specified by enclosing the address in square brackets.
630 By default, TCP listening sockets on the server will be bound to the loopback
632 This may be overridden by specifying a
638 indicates that the remote socket should listen on all interfaces.
641 will only succeed if the server's
643 option is enabled (see
644 .Xr sshd_config 5 ) .
650 the listen port will be dynamically allocated on the server and reported
651 to the client at run time.
652 When used together with
654 the allocated port will be printed to the standard output.
657 Specifies the location of a control socket for connection sharing,
660 to disable connection sharing.
661 Refer to the description of
670 May be used to request invocation of a subsystem on the remote system.
671 Subsystems facilitate the use of SSH
672 as a secure transport for other applications (e.g.\&
674 The subsystem is specified as the remote command.
677 Disable pseudo-terminal allocation.
680 Force pseudo-terminal allocation.
681 This can be used to execute arbitrary
682 screen-based programs on a remote machine, which can be very useful,
683 e.g. when implementing menu services.
686 options force tty allocation, even if
691 Display the version number and exit.
697 to print debugging messages about its progress.
699 debugging connection, authentication, and configuration problems.
702 options increase the verbosity.
705 .It Fl W Ar host : Ns Ar port
706 Requests that standard input and output on the client be forwarded to
710 over the secure channel.
714 .Cm ExitOnForwardFailure
716 .Cm ClearAllForwardings ,
717 though these can be overridden in the configuration file or using
719 command line options.
722 .Ar local_tun Ns Op : Ns Ar remote_tun
726 device forwarding with the specified
728 devices between the client
733 The devices may be specified by numerical ID or the keyword
735 which uses the next available tunnel device.
738 is not specified, it defaults to
749 directive is unset, it will be set to the default tunnel mode, which is
753 forwarding mode it desired, then it should be specified before
757 Enables X11 forwarding.
758 This can also be specified on a per-host basis in a configuration file.
760 X11 forwarding should be enabled with caution.
761 Users with the ability to bypass file permissions on the remote host
762 (for the user's X authorization database)
763 can access the local X11 display through the forwarded connection.
764 An attacker may then be able to perform activities such as keystroke monitoring.
766 For this reason, X11 forwarding is subjected to X11 SECURITY extension
767 restrictions by default.
772 .Cm ForwardX11Trusted
775 for more information.
778 Disables X11 forwarding.
781 Enables trusted X11 forwarding.
782 Trusted X11 forwardings are not subjected to the X11 SECURITY extension
786 Send log information using the
789 By default this information is sent to stderr.
793 may additionally obtain configuration data from
794 a per-user configuration file and a system-wide configuration file.
795 The file format and configuration options are described in
798 The OpenSSH SSH client supports SSH protocol 2.
800 The methods available for authentication are:
801 GSSAPI-based authentication,
802 host-based authentication,
803 public key authentication,
804 challenge-response authentication,
805 and password authentication.
806 Authentication methods are tried in the order specified above,
808 .Cm PreferredAuthentications
809 can be used to change the default order.
811 Host-based authentication works as follows:
812 If the machine the user logs in from is listed in
815 .Pa /etc/shosts.equiv
816 on the remote machine, and the user names are
817 the same on both sides, or if the files
821 exist in the user's home directory on the
822 remote machine and contain a line containing the name of the client
823 machine and the name of the user on that machine, the user is
824 considered for login.
825 Additionally, the server
827 be able to verify the client's
828 host key (see the description of
829 .Pa /etc/ssh/ssh_known_hosts
831 .Pa ~/.ssh/known_hosts ,
833 for login to be permitted.
834 This authentication method closes security holes due to IP
835 spoofing, DNS spoofing, and routing spoofing.
836 [Note to the administrator:
837 .Pa /etc/hosts.equiv ,
839 and the rlogin/rsh protocol in general, are inherently insecure and should be
840 disabled if security is desired.]
842 Public key authentication works as follows:
843 The scheme is based on public-key cryptography,
845 where encryption and decryption are done using separate keys,
846 and it is unfeasible to derive the decryption key from the encryption key.
847 The idea is that each user creates a public/private
848 key pair for authentication purposes.
849 The server knows the public key, and only the user knows the private key.
851 implements public key authentication protocol automatically,
852 using one of the DSA, ECDSA, Ed25519 or RSA algorithms.
853 The HISTORY section of
855 contains a brief discussion of the DSA and RSA algorithms.
858 .Pa ~/.ssh/authorized_keys
859 lists the public keys that are permitted for logging in.
860 When the user logs in, the
862 program tells the server which key pair it would like to use for
864 The client proves that it has access to the private key
865 and the server checks that the corresponding public key
866 is authorized to accept the account.
868 The server may inform the client of errors that prevented public key
869 authentication from succeeding after authentication completes using a
871 These may be viewed by increasing the
875 or higher (e.g. by using the
879 The user creates his/her key pair by running
881 This stores the private key in
886 .Pa ~/.ssh/id_ed25519
891 and stores the public key in
892 .Pa ~/.ssh/id_dsa.pub
894 .Pa ~/.ssh/id_ecdsa.pub
896 .Pa ~/.ssh/id_ed25519.pub
899 .Pa ~/.ssh/id_rsa.pub
901 in the user's home directory.
902 The user should then copy the public key
904 .Pa ~/.ssh/authorized_keys
905 in his/her home directory on the remote machine.
908 file corresponds to the conventional
910 file, and has one key
911 per line, though the lines can be very long.
912 After this, the user can log in without giving the password.
914 A variation on public key authentication
915 is available in the form of certificate authentication:
916 instead of a set of public/private keys,
917 signed certificates are used.
918 This has the advantage that a single trusted certification authority
919 can be used in place of many public/private keys.
920 See the CERTIFICATES section of
922 for more information.
924 The most convenient way to use public key or certificate authentication
925 may be with an authentication agent.
932 for more information.
934 Challenge-response authentication works as follows:
935 The server sends an arbitrary
937 text, and prompts for a response.
938 Examples of challenge-response authentication include
946 Finally, if other authentication methods fail,
948 prompts the user for a password.
949 The password is sent to the remote
950 host for checking; however, since all communications are encrypted,
951 the password cannot be seen by someone listening on the network.
954 automatically maintains and checks a database containing
955 identification for all hosts it has ever been used with.
956 Host keys are stored in
957 .Pa ~/.ssh/known_hosts
958 in the user's home directory.
959 Additionally, the file
960 .Pa /etc/ssh/ssh_known_hosts
961 is automatically checked for known hosts.
962 Any new hosts are automatically added to the user's file.
963 If a host's identification ever changes,
965 warns about this and disables password authentication to prevent
966 server spoofing or man-in-the-middle attacks,
967 which could otherwise be used to circumvent the encryption.
969 .Cm StrictHostKeyChecking
970 option can be used to control logins to machines whose
971 host key is not known or has changed.
973 When the user's identity has been accepted by the server, the server
974 either executes the given command in a non-interactive session or,
975 if no command has been specified, logs into the machine and gives
976 the user a normal shell as an interactive session.
977 All communication with
978 the remote command or shell will be automatically encrypted.
980 If an interactive session is requested
982 by default will only request a pseudo-terminal (pty) for interactive
983 sessions when the client has one.
988 can be used to override this behaviour.
990 If a pseudo-terminal has been allocated the
991 user may use the escape characters noted below.
993 If no pseudo-terminal has been allocated,
994 the session is transparent and can be used to reliably transfer binary data.
995 On most systems, setting the escape character to
997 will also make the session transparent even if a tty is used.
999 The session terminates when the command or shell on the remote
1000 machine exits and all X11 and TCP connections have been closed.
1001 .Sh ESCAPE CHARACTERS
1002 When a pseudo-terminal has been requested,
1004 supports a number of functions through the use of an escape character.
1006 A single tilde character can be sent as
1008 or by following the tilde by a character other than those described below.
1009 The escape character must always follow a newline to be interpreted as
1011 The escape character can be changed in configuration files using the
1013 configuration directive or on the command line by the
1017 The supported escapes (assuming the default
1027 List forwarded connections.
1031 at logout when waiting for forwarded connection / X11 sessions to terminate.
1033 Display a list of escape characters.
1035 Send a BREAK to the remote system
1036 (only useful if the peer supports it).
1039 Currently this allows the addition of port forwardings using the
1044 options (see above).
1045 It also allows the cancellation of existing port-forwardings
1048 .Fl KL Oo Ar bind_address : Oc Ar port
1052 .Fl KR Oo Ar bind_address : Oc Ar port
1056 .Fl KD Oo Ar bind_address : Oc Ar port
1058 for dynamic port-forwardings.
1059 .Ic !\& Ns Ar command
1060 allows the user to execute a local command if the
1061 .Ic PermitLocalCommand
1062 option is enabled in
1064 Basic help is available, using the
1068 Request rekeying of the connection
1069 (only useful if the peer supports it).
1071 Decrease the verbosity
1073 when errors are being written to stderr.
1075 Increase the verbosity
1077 when errors are being written to stderr.
1080 Forwarding of arbitrary TCP connections over the secure channel can
1081 be specified either on the command line or in a configuration file.
1082 One possible application of TCP forwarding is a secure connection to a
1083 mail server; another is going through firewalls.
1085 In the example below, we look at encrypting communication between
1086 an IRC client and server, even though the IRC server does not directly
1087 support encrypted communications.
1088 This works as follows:
1089 the user connects to the remote host using
1091 specifying a port to be used to forward connections
1092 to the remote server.
1093 After that it is possible to start the service which is to be encrypted
1094 on the client machine,
1095 connecting to the same local port,
1098 will encrypt and forward the connection.
1100 The following example tunnels an IRC session from client machine
1104 .Dq server.example.com :
1105 .Bd -literal -offset 4n
1106 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
1107 $ irc -c '#users' -p 1234 pinky 127.0.0.1
1110 This tunnels a connection to IRC server
1111 .Dq server.example.com ,
1117 It doesn't matter which port is used,
1118 as long as it's greater than 1023
1119 (remember, only root can open sockets on privileged ports)
1120 and doesn't conflict with any ports already in use.
1121 The connection is forwarded to port 6667 on the remote server,
1122 since that's the standard port for IRC services.
1128 and the remote command
1130 is specified to allow an amount of time
1131 (10 seconds, in the example)
1132 to start the service which is to be tunnelled.
1133 If no connections are made within the time specified,
1141 (or see the description of the
1147 and the user is using X11 (the
1149 environment variable is set), the connection to the X11 display is
1150 automatically forwarded to the remote side in such a way that any X11
1151 programs started from the shell (or command) will go through the
1152 encrypted channel, and the connection to the real X server will be made
1153 from the local machine.
1154 The user should not manually set
1156 Forwarding of X11 connections can be
1157 configured on the command line or in configuration files.
1163 will point to the server machine, but with a display number greater than zero.
1164 This is normal, and happens because
1168 X server on the server machine for forwarding the
1169 connections over the encrypted channel.
1172 will also automatically set up Xauthority data on the server machine.
1173 For this purpose, it will generate a random authorization cookie,
1174 store it in Xauthority on the server, and verify that any forwarded
1175 connections carry this cookie and replace it by the real cookie when
1176 the connection is opened.
1177 The real authentication cookie is never
1178 sent to the server machine (and no cookies are sent in the plain).
1184 (or see the description of the
1189 the user is using an authentication agent, the connection to the agent
1190 is automatically forwarded to the remote side.
1191 .Sh VERIFYING HOST KEYS
1192 When connecting to a server for the first time,
1193 a fingerprint of the server's public key is presented to the user
1195 .Cm StrictHostKeyChecking
1197 Fingerprints can be determined using
1200 .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1202 If the fingerprint is already known, it can be matched
1203 and the key can be accepted or rejected.
1204 If only legacy (MD5) fingerprints for the server are available, the
1207 option may be used to downgrade the fingerprint algorithm to match.
1209 Because of the difficulty of comparing host keys
1210 just by looking at fingerprint strings,
1211 there is also support to compare host keys visually,
1218 a small ASCII graphic gets displayed on every login to a server, no matter
1219 if the session itself is interactive or not.
1220 By learning the pattern a known server produces, a user can easily
1221 find out that the host key has changed when a completely different pattern
1223 Because these patterns are not unambiguous however, a pattern that looks
1224 similar to the pattern remembered only gives a good probability that the
1225 host key is the same, not guaranteed proof.
1227 To get a listing of the fingerprints along with their random art for
1228 all known hosts, the following command line can be used:
1230 .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1232 If the fingerprint is unknown,
1233 an alternative method of verification is available:
1234 SSH fingerprints verified by DNS.
1235 An additional resource record (RR),
1237 is added to a zonefile
1238 and the connecting client is able to match the fingerprint
1239 with that of the key presented.
1241 In this example, we are connecting a client to a server,
1242 .Dq host.example.com .
1243 The SSHFP resource records should first be added to the zonefile for
1245 .Bd -literal -offset indent
1246 $ ssh-keygen -r host.example.com.
1249 The output lines will have to be added to the zonefile.
1250 To check that the zone is answering fingerprint queries:
1252 .Dl $ dig -t SSHFP host.example.com
1254 Finally the client connects:
1255 .Bd -literal -offset indent
1256 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1258 Matching host key fingerprint found in DNS.
1259 Are you sure you want to continue connecting (yes/no)?
1263 .Cm VerifyHostKeyDNS
1266 for more information.
1267 .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1269 contains support for Virtual Private Network (VPN) tunnelling
1272 network pseudo-device,
1273 allowing two networks to be joined securely.
1276 configuration option
1278 controls whether the server supports this,
1279 and at what level (layer 2 or 3 traffic).
1281 The following example would connect client network 10.0.50.0/24
1282 with remote network 10.0.99.0/24 using a point-to-point connection
1283 from 10.1.1.1 to 10.1.1.2,
1284 provided that the SSH server running on the gateway to the remote network,
1285 at 192.168.1.15, allows it.
1288 .Bd -literal -offset indent
1289 # ssh -f -w 0:1 192.168.1.15 true
1290 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1291 # route add 10.0.99.0/24 10.1.1.2
1295 .Bd -literal -offset indent
1296 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1297 # route add 10.0.50.0/24 10.1.1.1
1300 Client access may be more finely tuned via the
1301 .Pa /root/.ssh/authorized_keys
1302 file (see below) and the
1305 The following entry would permit connections on
1309 and on tun device 2 from user
1314 .Dq forced-commands-only :
1315 .Bd -literal -offset 2n
1316 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1317 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1320 Since an SSH-based setup entails a fair amount of overhead,
1321 it may be more suited to temporary setups,
1322 such as for wireless VPNs.
1323 More permanent VPNs are better provided by tools such as
1329 will normally set the following environment variables:
1330 .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1334 variable indicates the location of the X11 server.
1335 It is automatically set by
1337 to point to a value of the form
1341 indicates the host where the shell runs, and
1343 is an integer \*(Ge 1.
1345 uses this special value to forward X11 connections over the secure
1347 The user should normally not set
1350 will render the X11 connection insecure (and will require the user to
1351 manually copy any required authorization cookies).
1353 Set to the path of the user's home directory.
1357 set for compatibility with systems that use this variable.
1359 Set to the path of the user's mailbox.
1363 as specified when compiling
1368 needs a passphrase, it will read the passphrase from the current
1369 terminal if it was run from a terminal.
1372 does not have a terminal associated with it but
1376 are set, it will execute the program specified by
1378 and open an X11 window to read the passphrase.
1379 This is particularly useful when calling
1384 (Note that on some machines it
1385 may be necessary to redirect the input from
1388 .It Ev SSH_AUTH_SOCK
1389 Identifies the path of a
1391 socket used to communicate with the agent.
1392 .It Ev SSH_CONNECTION
1393 Identifies the client and server ends of the connection.
1394 The variable contains
1395 four space-separated values: client IP address, client port number,
1396 server IP address, and server port number.
1397 .It Ev SSH_ORIGINAL_COMMAND
1398 This variable contains the original command line if a forced command
1400 It can be used to extract the original arguments.
1402 This is set to the name of the tty (path to the device) associated
1403 with the current shell or command.
1404 If the current session has no tty,
1405 this variable is not set.
1409 to contain the interface names assigned if tunnel forwarding was
1410 requested by the client.
1411 .It Ev SSH_USER_AUTH
1414 this variable may contain a pathname to a file that lists the authentication
1415 methods successfully used when the session was established, including any
1416 public keys that were used.
1418 This variable is set to indicate the present time zone if it
1419 was set when the daemon was started (i.e. the daemon passes the value
1420 on to new connections).
1422 Set to the name of the user logging in.
1428 .Pa ~/.ssh/environment ,
1429 and adds lines of the format
1431 to the environment if the file exists and users are allowed to
1432 change their environment.
1433 For more information, see the
1434 .Cm PermitUserEnvironment
1438 .Bl -tag -width Ds -compact
1440 This file is used for host-based authentication (see above).
1441 On some machines this file may need to be
1442 world-readable if the user's home directory is on an NFS partition,
1446 Additionally, this file must be owned by the user,
1447 and must not have write permissions for anyone else.
1449 permission for most machines is read/write for the user, and not
1450 accessible by others.
1453 This file is used in exactly the same way as
1455 but allows host-based authentication without permitting login with
1459 This directory is the default location for all user-specific configuration
1460 and authentication information.
1461 There is no general requirement to keep the entire contents of this directory
1462 secret, but the recommended permissions are read/write/execute for the user,
1463 and not accessible by others.
1465 .It Pa ~/.ssh/authorized_keys
1466 Lists the public keys (DSA, ECDSA, Ed25519, RSA)
1467 that can be used for logging in as this user.
1468 The format of this file is described in the
1471 This file is not highly sensitive, but the recommended
1472 permissions are read/write for the user, and not accessible by others.
1474 .It Pa ~/.ssh/config
1475 This is the per-user configuration file.
1476 The file format and configuration options are described in
1478 Because of the potential for abuse, this file must have strict permissions:
1479 read/write for the user, and not writable by others.
1481 .It Pa ~/.ssh/environment
1482 Contains additional definitions for environment variables; see
1486 .It Pa ~/.ssh/id_dsa
1487 .It Pa ~/.ssh/id_ecdsa
1488 .It Pa ~/.ssh/id_ed25519
1489 .It Pa ~/.ssh/id_rsa
1490 Contains the private key for authentication.
1492 contain sensitive data and should be readable by the user but not
1493 accessible by others (read/write/execute).
1495 will simply ignore a private key file if it is accessible by others.
1496 It is possible to specify a passphrase when
1497 generating the key which will be used to encrypt the
1498 sensitive part of this file using AES-128.
1500 .It Pa ~/.ssh/id_dsa.pub
1501 .It Pa ~/.ssh/id_ecdsa.pub
1502 .It Pa ~/.ssh/id_ed25519.pub
1503 .It Pa ~/.ssh/id_rsa.pub
1504 Contains the public key for authentication.
1506 sensitive and can (but need not) be readable by anyone.
1508 .It Pa ~/.ssh/known_hosts
1509 Contains a list of host keys for all hosts the user has logged into
1510 that are not already in the systemwide list of known host keys.
1513 for further details of the format of this file.
1516 Commands in this file are executed by
1518 when the user logs in, just before the user's shell (or command) is
1522 manual page for more information.
1524 .It Pa /etc/hosts.equiv
1525 This file is for host-based authentication (see above).
1526 It should only be writable by root.
1528 .It Pa /etc/shosts.equiv
1529 This file is used in exactly the same way as
1531 but allows host-based authentication without permitting login with
1534 .It Pa /etc/ssh/ssh_config
1535 Systemwide configuration file.
1536 The file format and configuration options are described in
1539 .It Pa /etc/ssh/ssh_host_key
1540 .It Pa /etc/ssh/ssh_host_dsa_key
1541 .It Pa /etc/ssh/ssh_host_ecdsa_key
1542 .It Pa /etc/ssh/ssh_host_ed25519_key
1543 .It Pa /etc/ssh/ssh_host_rsa_key
1544 These files contain the private parts of the host keys
1545 and are used for host-based authentication.
1547 .It Pa /etc/ssh/ssh_known_hosts
1548 Systemwide list of known host keys.
1549 This file should be prepared by the
1550 system administrator to contain the public host keys of all machines in the
1552 It should be world-readable.
1555 for further details of the format of this file.
1557 .It Pa /etc/ssh/sshrc
1558 Commands in this file are executed by
1560 when the user logs in, just before the user's shell (or command) is started.
1563 manual page for more information.
1567 exits with the exit status of the remote command or with 255
1568 if an error occurred.
1586 .%T The Secure Shell (SSH) Protocol Assigned Numbers
1594 .%T The Secure Shell (SSH) Protocol Architecture
1602 .%T The Secure Shell (SSH) Authentication Protocol
1610 .%T The Secure Shell (SSH) Transport Layer Protocol
1618 .%T The Secure Shell (SSH) Connection Protocol
1626 .%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
1634 .%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
1642 .%T The Secure Shell (SSH) Session Channel Break Extension
1651 .%T The Secure Shell (SSH) Transport Layer Encryption Modes
1658 .%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
1667 .%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
1675 .%T The Secure Shell (SSH) Public Key File Format
1683 .%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
1690 .%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
1691 .%T Hash Visualization: a New Technique to improve Real-World Security
1694 OpenSSH is a derivative of the original and free
1695 ssh 1.2.12 release by Tatu Ylonen.
1696 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1697 Theo de Raadt and Dug Song
1698 removed many bugs, re-added newer features and
1700 Markus Friedl contributed the support for SSH
1701 protocol versions 1.5 and 2.0.