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 1.433 2022/11/28 01:37:36 djm Exp $
37 .Dd $Mdocdate: November 28 2022 $
42 .Nd OpenSSH remote login client
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
68 .Op Ar command Op Ar argument ...
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 their identity to the remote machine using one of several methods
97 it will be executed on the remote host instead of a login shell.
98 A complete command line may be specified as
100 or it may have additional arguments.
101 If supplied, the arguments will be appended to the command, separated by
102 spaces, before it is sent to the server to be executed.
104 The options are as follows:
106 .Bl -tag -width Ds -compact
110 to use IPv4 addresses only.
115 to use IPv6 addresses only.
118 Enables forwarding of connections from an authentication agent such as
120 This can also be specified on a per-host basis in a configuration file.
122 Agent forwarding should be enabled with caution.
123 Users with the ability to bypass file permissions on the remote host
126 socket) can access the local agent through the forwarded connection.
127 An attacker cannot obtain key material from the agent,
128 however they can perform operations on the keys that enable them to
129 authenticate using the identities loaded into the agent.
130 A safer alternative may be to use a jump host
135 Disables forwarding of the authentication agent connection.
137 .It Fl B Ar bind_interface
138 Bind to the address of
140 before attempting to connect to the destination host.
141 This is only useful on systems with more than one address.
143 .It Fl b Ar bind_address
146 on the local machine as the source address
148 Only useful on systems with more than one address.
151 Requests compression of all data (including stdin, stdout, stderr, and
152 data for forwarded X11, TCP and
155 The compression algorithm is the same used by
157 Compression is desirable on modem lines and other
158 slow connections, but will only slow down things on fast networks.
159 The default value can be set on a host-by-host basis in the
160 configuration files; see the
165 .It Fl c Ar cipher_spec
166 Selects the cipher specification for encrypting the session.
168 is a comma-separated list of ciphers
169 listed in order of preference.
174 for more information.
178 .Oo Ar bind_address : Oc
184 application-level port forwarding.
185 This works by allocating a socket to listen to
187 on the local side, optionally bound to the specified
189 Whenever a connection is made to this port, the
190 connection is forwarded over the secure channel, and the application
191 protocol is then used to determine where to connect to from the
193 Currently the SOCKS4 and SOCKS5 protocols are supported, and
195 will act as a SOCKS server.
196 Only root can forward privileged ports.
197 Dynamic port forwardings can also be specified in the configuration file.
199 IPv6 addresses can be specified by enclosing the address in square brackets.
200 Only the superuser can forward privileged ports.
201 By default, the local port is bound in accordance with the
206 may be used to bind the connection to a specific address.
211 indicates that the listening port be bound for local use only, while an
214 indicates that the port should be available from all interfaces.
219 instead of standard error.
221 .It Fl e Ar escape_char
222 Sets the escape character for sessions with a pty (default:
224 The escape character is only recognized at the beginning of a line.
225 The escape character followed by a dot
227 closes the connection;
228 followed by control-Z suspends the connection;
229 and followed by itself sends the escape character once.
230 Setting the character to
232 disables any escapes and makes the session fully transparent.
234 .It Fl F Ar configfile
235 Specifies an alternative per-user configuration file.
236 If a configuration file is given on the command line,
237 the system-wide configuration file
238 .Pq Pa /etc/ssh/ssh_config
240 The default for the per-user configuration file is
244 no configuration files will be read.
249 to go to background just before command execution.
252 is going to ask for passwords or passphrases, but the user
253 wants it in the background.
256 The recommended way to start X11 programs at a remote site is with
258 .Ic ssh -f host xterm .
261 .Cm ExitOnForwardFailure
262 configuration option is set to
264 then a client started with
266 will wait for all remote port forwards to be successfully established
267 before placing itself in the background.
268 Refer to the description of
269 .Cm ForkAfterAuthentication
277 to print its configuration after evaluating
284 Allows remote hosts to connect to local forwarded ports.
285 If used on a multiplexed connection, then this option must be specified
286 on the master process.
289 Specify the PKCS#11 shared library
291 should use to communicate with a PKCS#11 token providing keys for user
294 .It Fl i Ar identity_file
295 Selects a file from which the identity (private key) for
296 public key authentication is read.
297 You can also specify a public key file to use the corresponding
298 private key that is loaded in
300 when the private key file is not present locally.
303 .Pa ~/.ssh/id_ecdsa ,
304 .Pa ~/.ssh/id_ecdsa_sk ,
305 .Pa ~/.ssh/id_ed25519 ,
306 .Pa ~/.ssh/id_ed25519_sk
309 Identity files may also be specified on
310 a per-host basis in the configuration file.
311 It is possible to have multiple
313 options (and multiple identities specified in
314 configuration files).
315 If no certificates have been explicitly specified by the
319 will also try to load certificate information from the filename obtained
322 to identity filenames.
324 .It Fl J Ar destination
325 Connect to the target host by first making a
327 connection to the jump host described by
329 and then establishing a TCP forwarding to the ultimate destination from
331 Multiple jump hops may be specified separated by comma characters.
332 This is a shortcut to specify a
334 configuration directive.
335 Note that configuration directives supplied on the command-line generally
336 apply to the destination host and not any specified jump hosts.
339 to specify configuration for jump hosts.
342 Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
343 credentials to the server.
346 Disables forwarding (delegation) of GSSAPI credentials to the server.
350 .Oo Ar bind_address : Oc
351 .Ar port : host : hostport
356 .Oo Ar bind_address : Oc
357 .Ar port : remote_socket
362 .Ar local_socket : host : hostport
367 .Ar local_socket : remote_socket
370 Specifies that connections to the given TCP port or Unix socket on the local
371 (client) host are to be forwarded to the given host and port, or Unix socket,
373 This works by allocating a socket to listen to either a TCP
375 on the local side, optionally bound to the specified
378 Whenever a connection is made to the local port or socket, the
379 connection is forwarded over the secure channel, and a connection is
386 from the remote machine.
388 Port forwardings can also be specified in the configuration file.
389 Only the superuser can forward privileged ports.
390 IPv6 addresses can be specified by enclosing the address in square brackets.
392 By default, the local port is bound in accordance with the
397 may be used to bind the connection to a specific address.
402 indicates that the listening port be bound for local use only, while an
405 indicates that the port should be available from all interfaces.
407 .It Fl l Ar login_name
408 Specifies the user to log in as on the remote machine.
409 This also may be specified on a per-host basis in the configuration file.
416 mode for connection sharing.
423 mode but with confirmation required using
425 before each operation that changes the multiplexing state
426 (e.g. opening a new session).
427 Refer to the description of
434 A comma-separated list of MAC (message authentication code) algorithms,
435 specified in order of preference.
440 for more information.
443 Do not execute a remote command.
444 This is useful for just forwarding ports.
445 Refer to the description of
454 (actually, prevents reading from stdin).
455 This must be used when
457 is run in the background.
458 A common trick is to use this to run X11 programs on a remote machine.
460 .Ic ssh -n shadows.cs.hut.fi emacs &
461 will start an emacs on shadows.cs.hut.fi, and the X11
462 connection will be automatically forwarded over an encrypted channel.
465 program will be put in the background.
466 (This does not work if
468 needs to ask for a password or passphrase; see also the
471 Refer to the description of
478 Control an active connection multiplexing master process.
481 option is specified, the
483 argument is interpreted and passed to the master process.
486 (check that the master process is running),
488 (request forwardings without command execution),
490 (cancel forwardings),
492 (request the master to exit), and
494 (request the master to stop accepting further multiplexing requests).
497 Can be used to give options in the format used in the configuration file.
498 This is useful for specifying options for which there is no separate
500 For full details of the options listed below, and their possible values, see
503 .Bl -tag -width Ds -offset indent -compact
509 .It CanonicalizeFallbackLocal
510 .It CanonicalizeHostname
511 .It CanonicalizeMaxDots
512 .It CanonicalizePermittedCNAMEs
513 .It CASignatureAlgorithms
517 .It ClearAllForwardings
519 .It ConnectionAttempts
525 .It EnableEscapeCommandline
527 .It ExitOnForwardFailure
529 .It ForkAfterAuthentication
532 .It ForwardX11Timeout
533 .It ForwardX11Trusted
535 .It GlobalKnownHostsFile
536 .It GSSAPIAuthentication
537 .It GSSAPIDelegateCredentials
540 .It HostbasedAcceptedAlgorithms
541 .It HostbasedAuthentication
542 .It HostKeyAlgorithms
549 .It KbdInteractiveAuthentication
550 .It KbdInteractiveDevices
552 .It KnownHostsCommand
558 .It NoHostAuthenticationForLocalhost
559 .It NumberOfPasswordPrompts
560 .It PasswordAuthentication
561 .It PermitLocalCommand
565 .It PreferredAuthentications
569 .It PubkeyAcceptedAlgorithms
570 .It PubkeyAuthentication
577 .It ServerAliveInterval
578 .It ServerAliveCountMax
582 .It StreamLocalBindMask
583 .It StreamLocalBindUnlink
584 .It StrictHostKeyChecking
590 .It UserKnownHostsFile
597 Port to connect to on the remote host.
598 This can be specified on a
599 per-host basis in the configuration file.
601 .It Fl Q Ar query_option
602 Queries for the algorithms supported by one of the following features:
604 (supported symmetric ciphers),
606 (supported symmetric ciphers that support authenticated encryption),
608 (supported query terms for use with the
612 (supported message integrity codes),
614 (key exchange algorithms),
618 (certificate key types),
620 (non-certificate key types),
622 (all key types and signature algorithms),
624 (supported SSH protocol versions), and
626 (supported signature algorithms).
627 Alternatively, any keyword from
631 that takes an algorithm list may be used as an alias for the corresponding
636 Causes most warning and diagnostic messages to be suppressed.
640 .Oo Ar bind_address : Oc
641 .Ar port : host : hostport
646 .Oo Ar bind_address : Oc
647 .Ar port : local_socket
652 .Ar remote_socket : host : hostport
657 .Ar remote_socket : local_socket
662 .Oo Ar bind_address : Oc
666 Specifies that connections to the given TCP port or Unix socket on the remote
667 (server) host are to be forwarded to the local side.
669 This works by allocating a socket to listen to either a TCP
671 or to a Unix socket on the remote side.
672 Whenever a connection is made to this port or Unix socket, the
673 connection is forwarded over the secure channel, and a connection
674 is made from the local machine to either an explicit destination specified by
680 or, if no explicit destination was specified,
682 will act as a SOCKS 4/5 proxy and forward connections to the destinations
683 requested by the remote SOCKS client.
685 Port forwardings can also be specified in the configuration file.
686 Privileged ports can be forwarded only when
687 logging in as root on the remote machine.
688 IPv6 addresses can be specified by enclosing the address in square brackets.
690 By default, TCP listening sockets on the server will be bound to the loopback
692 This may be overridden by specifying a
698 indicates that the remote socket should listen on all interfaces.
701 will only succeed if the server's
703 option is enabled (see
704 .Xr sshd_config 5 ) .
710 the listen port will be dynamically allocated on the server and reported
711 to the client at run time.
712 When used together with
714 the allocated port will be printed to the standard output.
717 Specifies the location of a control socket for connection sharing,
720 to disable connection sharing.
721 Refer to the description of
730 May be used to request invocation of a subsystem on the remote system.
731 Subsystems facilitate the use of SSH
732 as a secure transport for other applications (e.g.\&
734 The subsystem is specified as the remote command.
735 Refer to the description of
742 Disable pseudo-terminal allocation.
745 Force pseudo-terminal allocation.
746 This can be used to execute arbitrary
747 screen-based programs on a remote machine, which can be very useful,
748 e.g. when implementing menu services.
751 options force tty allocation, even if
756 Display the version number and exit.
762 to print debugging messages about its progress.
764 debugging connection, authentication, and configuration problems.
767 options increase the verbosity.
770 .It Fl W Ar host : Ns Ar port
771 Requests that standard input and output on the client be forwarded to
775 over the secure channel.
779 .Cm ExitOnForwardFailure
781 .Cm ClearAllForwardings ,
782 though these can be overridden in the configuration file or using
784 command line options.
787 .Ar local_tun Ns Op : Ns Ar remote_tun
791 device forwarding with the specified
793 devices between the client
798 The devices may be specified by numerical ID or the keyword
800 which uses the next available tunnel device.
803 is not specified, it defaults to
814 directive is unset, it will be set to the default tunnel mode, which is
818 forwarding mode it desired, then it should be specified before
822 Enables X11 forwarding.
823 This can also be specified on a per-host basis in a configuration file.
825 X11 forwarding should be enabled with caution.
826 Users with the ability to bypass file permissions on the remote host
827 (for the user's X authorization database)
828 can access the local X11 display through the forwarded connection.
829 An attacker may then be able to perform activities such as keystroke monitoring.
831 For this reason, X11 forwarding is subjected to X11 SECURITY extension
832 restrictions by default.
837 .Cm ForwardX11Trusted
840 for more information.
843 Disables X11 forwarding.
846 Enables trusted X11 forwarding.
847 Trusted X11 forwardings are not subjected to the X11 SECURITY extension
851 Send log information using the
854 By default this information is sent to stderr.
858 may additionally obtain configuration data from
859 a per-user configuration file and a system-wide configuration file.
860 The file format and configuration options are described in
863 The OpenSSH SSH client supports SSH protocol 2.
865 The methods available for authentication are:
866 GSSAPI-based authentication,
867 host-based authentication,
868 public key authentication,
869 keyboard-interactive authentication,
870 and password authentication.
871 Authentication methods are tried in the order specified above,
873 .Cm PreferredAuthentications
874 can be used to change the default order.
876 Host-based authentication works as follows:
877 If the machine the user logs in from is listed in
880 .Pa /etc/shosts.equiv
881 on the remote machine, the user is non-root and the user names are
882 the same on both sides, or if the files
886 exist in the user's home directory on the
887 remote machine and contain a line containing the name of the client
888 machine and the name of the user on that machine, the user is
889 considered for login.
890 Additionally, the server
892 be able to verify the client's
893 host key (see the description of
894 .Pa /etc/ssh/ssh_known_hosts
896 .Pa ~/.ssh/known_hosts ,
898 for login to be permitted.
899 This authentication method closes security holes due to IP
900 spoofing, DNS spoofing, and routing spoofing.
901 [Note to the administrator:
902 .Pa /etc/hosts.equiv ,
904 and the rlogin/rsh protocol in general, are inherently insecure and should be
905 disabled if security is desired.]
907 Public key authentication works as follows:
908 The scheme is based on public-key cryptography,
910 where encryption and decryption are done using separate keys,
911 and it is unfeasible to derive the decryption key from the encryption key.
912 The idea is that each user creates a public/private
913 key pair for authentication purposes.
914 The server knows the public key, and only the user knows the private key.
916 implements public key authentication protocol automatically,
917 using one of the DSA, ECDSA, Ed25519 or RSA algorithms.
918 The HISTORY section of
920 contains a brief discussion of the DSA and RSA algorithms.
923 .Pa ~/.ssh/authorized_keys
924 lists the public keys that are permitted for logging in.
925 When the user logs in, the
927 program tells the server which key pair it would like to use for
929 The client proves that it has access to the private key
930 and the server checks that the corresponding public key
931 is authorized to accept the account.
933 The server may inform the client of errors that prevented public key
934 authentication from succeeding after authentication completes using a
936 These may be viewed by increasing the
940 or higher (e.g. by using the
944 The user creates their key pair by running
946 This stores the private key in
951 .Pa ~/.ssh/id_ecdsa_sk
952 (authenticator-hosted ECDSA),
953 .Pa ~/.ssh/id_ed25519
955 .Pa ~/.ssh/id_ed25519_sk
956 (authenticator-hosted Ed25519),
960 and stores the public key in
961 .Pa ~/.ssh/id_dsa.pub
963 .Pa ~/.ssh/id_ecdsa.pub
965 .Pa ~/.ssh/id_ecdsa_sk.pub
966 (authenticator-hosted ECDSA),
967 .Pa ~/.ssh/id_ed25519.pub
969 .Pa ~/.ssh/id_ed25519_sk.pub
970 (authenticator-hosted Ed25519),
972 .Pa ~/.ssh/id_rsa.pub
974 in the user's home directory.
975 The user should then copy the public key
977 .Pa ~/.ssh/authorized_keys
978 in their home directory on the remote machine.
981 file corresponds to the conventional
983 file, and has one key
984 per line, though the lines can be very long.
985 After this, the user can log in without giving the password.
987 A variation on public key authentication
988 is available in the form of certificate authentication:
989 instead of a set of public/private keys,
990 signed certificates are used.
991 This has the advantage that a single trusted certification authority
992 can be used in place of many public/private keys.
993 See the CERTIFICATES section of
995 for more information.
997 The most convenient way to use public key or certificate authentication
998 may be with an authentication agent.
1001 and (optionally) the
1005 for more information.
1007 Keyboard-interactive authentication works as follows:
1008 The server sends an arbitrary
1010 text and prompts for a response, possibly multiple times.
1011 Examples of keyboard-interactive authentication include
1019 Finally, if other authentication methods fail,
1021 prompts the user for a password.
1022 The password is sent to the remote
1023 host for checking; however, since all communications are encrypted,
1024 the password cannot be seen by someone listening on the network.
1027 automatically maintains and checks a database containing
1028 identification for all hosts it has ever been used with.
1029 Host keys are stored in
1030 .Pa ~/.ssh/known_hosts
1031 in the user's home directory.
1032 Additionally, the file
1033 .Pa /etc/ssh/ssh_known_hosts
1034 is automatically checked for known hosts.
1035 Any new hosts are automatically added to the user's file.
1036 If a host's identification ever changes,
1038 warns about this and disables password authentication to prevent
1039 server spoofing or man-in-the-middle attacks,
1040 which could otherwise be used to circumvent the encryption.
1042 .Cm StrictHostKeyChecking
1043 option can be used to control logins to machines whose
1044 host key is not known or has changed.
1046 When the user's identity has been accepted by the server, the server
1047 either executes the given command in a non-interactive session or,
1048 if no command has been specified, logs into the machine and gives
1049 the user a normal shell as an interactive session.
1050 All communication with
1051 the remote command or shell will be automatically encrypted.
1053 If an interactive session is requested,
1055 by default will only request a pseudo-terminal (pty) for interactive
1056 sessions when the client has one.
1061 can be used to override this behaviour.
1063 If a pseudo-terminal has been allocated, the
1064 user may use the escape characters noted below.
1066 If no pseudo-terminal has been allocated,
1067 the session is transparent and can be used to reliably transfer binary data.
1068 On most systems, setting the escape character to
1070 will also make the session transparent even if a tty is used.
1072 The session terminates when the command or shell on the remote
1073 machine exits and all X11 and TCP connections have been closed.
1074 .Sh ESCAPE CHARACTERS
1075 When a pseudo-terminal has been requested,
1077 supports a number of functions through the use of an escape character.
1079 A single tilde character can be sent as
1081 or by following the tilde by a character other than those described below.
1082 The escape character must always follow a newline to be interpreted as
1084 The escape character can be changed in configuration files using the
1086 configuration directive or on the command line by the
1090 The supported escapes (assuming the default
1100 List forwarded connections.
1104 at logout when waiting for forwarded connection / X11 sessions to terminate.
1106 Display a list of escape characters.
1108 Send a BREAK to the remote system
1109 (only useful if the peer supports it).
1112 Currently this allows the addition of port forwardings using the
1117 options (see above).
1118 It also allows the cancellation of existing port-forwardings
1121 .Fl KL Oo Ar bind_address : Oc Ar port
1125 .Fl KR Oo Ar bind_address : Oc Ar port
1129 .Fl KD Oo Ar bind_address : Oc Ar port
1131 for dynamic port-forwardings.
1132 .Ic !\& Ns Ar command
1133 allows the user to execute a local command if the
1134 .Ic PermitLocalCommand
1135 option is enabled in
1137 Basic help is available, using the
1141 Request rekeying of the connection
1142 (only useful if the peer supports it).
1144 Decrease the verbosity
1146 when errors are being written to stderr.
1148 Increase the verbosity
1150 when errors are being written to stderr.
1153 Forwarding of arbitrary TCP connections over a secure channel
1154 can be specified either on the command line or in a configuration file.
1155 One possible application of TCP forwarding is a secure connection to a
1156 mail server; another is going through firewalls.
1158 In the example below, we look at encrypting communication for an IRC client,
1159 even though the IRC server it connects to does not directly
1160 support encrypted communication.
1161 This works as follows:
1162 the user connects to the remote host using
1164 specifying the ports to be used to forward the connection.
1165 After that it is possible to start the program locally,
1168 will encrypt and forward the connection to the remote server.
1170 The following example tunnels an IRC session from the client
1172 .Dq server.example.com ,
1177 using the standard IRC port, 6667:
1178 .Bd -literal -offset 4n
1179 $ ssh -f -L 6667:localhost:6667 server.example.com sleep 10
1180 $ irc -c '#users' pinky IRC/127.0.0.1
1187 and the remote command
1189 is specified to allow an amount of time
1190 (10 seconds, in the example)
1191 to start the program which is going to use the tunnel.
1192 If no connections are made within the time specified,
1200 (or see the description of the
1206 and the user is using X11 (the
1208 environment variable is set), the connection to the X11 display is
1209 automatically forwarded to the remote side in such a way that any X11
1210 programs started from the shell (or command) will go through the
1211 encrypted channel, and the connection to the real X server will be made
1212 from the local machine.
1213 The user should not manually set
1215 Forwarding of X11 connections can be
1216 configured on the command line or in configuration files.
1222 will point to the server machine, but with a display number greater than zero.
1223 This is normal, and happens because
1227 X server on the server machine for forwarding the
1228 connections over the encrypted channel.
1231 will also automatically set up Xauthority data on the server machine.
1232 For this purpose, it will generate a random authorization cookie,
1233 store it in Xauthority on the server, and verify that any forwarded
1234 connections carry this cookie and replace it by the real cookie when
1235 the connection is opened.
1236 The real authentication cookie is never
1237 sent to the server machine (and no cookies are sent in the plain).
1243 (or see the description of the
1248 the user is using an authentication agent, the connection to the agent
1249 is automatically forwarded to the remote side.
1250 .Sh VERIFYING HOST KEYS
1251 When connecting to a server for the first time,
1252 a fingerprint of the server's public key is presented to the user
1254 .Cm StrictHostKeyChecking
1256 Fingerprints can be determined using
1259 .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1261 If the fingerprint is already known, it can be matched
1262 and the key can be accepted or rejected.
1263 If only legacy (MD5) fingerprints for the server are available, the
1266 option may be used to downgrade the fingerprint algorithm to match.
1268 Because of the difficulty of comparing host keys
1269 just by looking at fingerprint strings,
1270 there is also support to compare host keys visually,
1277 a small ASCII graphic gets displayed on every login to a server, no matter
1278 if the session itself is interactive or not.
1279 By learning the pattern a known server produces, a user can easily
1280 find out that the host key has changed when a completely different pattern
1282 Because these patterns are not unambiguous however, a pattern that looks
1283 similar to the pattern remembered only gives a good probability that the
1284 host key is the same, not guaranteed proof.
1286 To get a listing of the fingerprints along with their random art for
1287 all known hosts, the following command line can be used:
1289 .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1291 If the fingerprint is unknown,
1292 an alternative method of verification is available:
1293 SSH fingerprints verified by DNS.
1294 An additional resource record (RR),
1296 is added to a zonefile
1297 and the connecting client is able to match the fingerprint
1298 with that of the key presented.
1300 In this example, we are connecting a client to a server,
1301 .Dq host.example.com .
1302 The SSHFP resource records should first be added to the zonefile for
1304 .Bd -literal -offset indent
1305 $ ssh-keygen -r host.example.com.
1308 The output lines will have to be added to the zonefile.
1309 To check that the zone is answering fingerprint queries:
1311 .Dl $ dig -t SSHFP host.example.com
1313 Finally the client connects:
1314 .Bd -literal -offset indent
1315 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1317 Matching host key fingerprint found in DNS.
1318 Are you sure you want to continue connecting (yes/no)?
1322 .Cm VerifyHostKeyDNS
1325 for more information.
1326 .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1328 contains support for Virtual Private Network (VPN) tunnelling
1331 network pseudo-device,
1332 allowing two networks to be joined securely.
1335 configuration option
1337 controls whether the server supports this,
1338 and at what level (layer 2 or 3 traffic).
1340 The following example would connect client network 10.0.50.0/24
1341 with remote network 10.0.99.0/24 using a point-to-point connection
1342 from 10.1.1.1 to 10.1.1.2,
1343 provided that the SSH server running on the gateway to the remote network,
1344 at 192.168.1.15, allows it.
1347 .Bd -literal -offset indent
1348 # ssh -f -w 0:1 192.168.1.15 true
1349 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1350 # route add 10.0.99.0/24 10.1.1.2
1354 .Bd -literal -offset indent
1355 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1356 # route add 10.0.50.0/24 10.1.1.1
1359 Client access may be more finely tuned via the
1360 .Pa /root/.ssh/authorized_keys
1361 file (see below) and the
1364 The following entry would permit connections on
1368 and on tun device 2 from user
1373 .Dq forced-commands-only :
1374 .Bd -literal -offset 2n
1375 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1376 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1379 Since an SSH-based setup entails a fair amount of overhead,
1380 it may be more suited to temporary setups,
1381 such as for wireless VPNs.
1382 More permanent VPNs are better provided by tools such as
1388 will normally set the following environment variables:
1389 .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1393 variable indicates the location of the X11 server.
1394 It is automatically set by
1396 to point to a value of the form
1400 indicates the host where the shell runs, and
1402 is an integer \*(Ge 1.
1404 uses this special value to forward X11 connections over the secure
1406 The user should normally not set
1409 will render the X11 connection insecure (and will require the user to
1410 manually copy any required authorization cookies).
1412 Set to the path of the user's home directory.
1416 set for compatibility with systems that use this variable.
1418 Set to the path of the user's mailbox.
1422 as specified when compiling
1427 needs a passphrase, it will read the passphrase from the current
1428 terminal if it was run from a terminal.
1431 does not have a terminal associated with it but
1435 are set, it will execute the program specified by
1437 and open an X11 window to read the passphrase.
1438 This is particularly useful when calling
1443 (Note that on some machines it
1444 may be necessary to redirect the input from
1447 .It Ev SSH_ASKPASS_REQUIRE
1448 Allows further control over the use of an askpass program.
1449 If this variable is set to
1453 will never attempt to use one.
1458 will prefer to use the askpass program instead of the TTY when requesting
1460 Finally, if the variable is set to
1462 then the askpass program will be used for all passphrase input regardless
1466 .It Ev SSH_AUTH_SOCK
1467 Identifies the path of a
1469 socket used to communicate with the agent.
1470 .It Ev SSH_CONNECTION
1471 Identifies the client and server ends of the connection.
1472 The variable contains
1473 four space-separated values: client IP address, client port number,
1474 server IP address, and server port number.
1475 .It Ev SSH_ORIGINAL_COMMAND
1476 This variable contains the original command line if a forced command
1478 It can be used to extract the original arguments.
1480 This is set to the name of the tty (path to the device) associated
1481 with the current shell or command.
1482 If the current session has no tty,
1483 this variable is not set.
1487 to contain the interface names assigned if tunnel forwarding was
1488 requested by the client.
1489 .It Ev SSH_USER_AUTH
1492 this variable may contain a pathname to a file that lists the authentication
1493 methods successfully used when the session was established, including any
1494 public keys that were used.
1496 This variable is set to indicate the present time zone if it
1497 was set when the daemon was started (i.e. the daemon passes the value
1498 on to new connections).
1500 Set to the name of the user logging in.
1506 .Pa ~/.ssh/environment ,
1507 and adds lines of the format
1509 to the environment if the file exists and users are allowed to
1510 change their environment.
1511 For more information, see the
1512 .Cm PermitUserEnvironment
1516 .Bl -tag -width Ds -compact
1518 This file is used for host-based authentication (see above).
1519 On some machines this file may need to be
1520 world-readable if the user's home directory is on an NFS partition,
1524 Additionally, this file must be owned by the user,
1525 and must not have write permissions for anyone else.
1527 permission for most machines is read/write for the user, and not
1528 accessible by others.
1531 This file is used in exactly the same way as
1533 but allows host-based authentication without permitting login with
1537 This directory is the default location for all user-specific configuration
1538 and authentication information.
1539 There is no general requirement to keep the entire contents of this directory
1540 secret, but the recommended permissions are read/write/execute for the user,
1541 and not accessible by others.
1543 .It Pa ~/.ssh/authorized_keys
1544 Lists the public keys (DSA, ECDSA, Ed25519, RSA)
1545 that can be used for logging in as this user.
1546 The format of this file is described in the
1549 This file is not highly sensitive, but the recommended
1550 permissions are read/write for the user, and not accessible by others.
1552 .It Pa ~/.ssh/config
1553 This is the per-user configuration file.
1554 The file format and configuration options are described in
1556 Because of the potential for abuse, this file must have strict permissions:
1557 read/write for the user, and not writable by others.
1559 .It Pa ~/.ssh/environment
1560 Contains additional definitions for environment variables; see
1564 .It Pa ~/.ssh/id_dsa
1565 .It Pa ~/.ssh/id_ecdsa
1566 .It Pa ~/.ssh/id_ecdsa_sk
1567 .It Pa ~/.ssh/id_ed25519
1568 .It Pa ~/.ssh/id_ed25519_sk
1569 .It Pa ~/.ssh/id_rsa
1570 Contains the private key for authentication.
1572 contain sensitive data and should be readable by the user but not
1573 accessible by others (read/write/execute).
1575 will simply ignore a private key file if it is accessible by others.
1576 It is possible to specify a passphrase when
1577 generating the key which will be used to encrypt the
1578 sensitive part of this file using AES-128.
1580 .It Pa ~/.ssh/id_dsa.pub
1581 .It Pa ~/.ssh/id_ecdsa.pub
1582 .It Pa ~/.ssh/id_ecdsa_sk.pub
1583 .It Pa ~/.ssh/id_ed25519.pub
1584 .It Pa ~/.ssh/id_ed25519_sk.pub
1585 .It Pa ~/.ssh/id_rsa.pub
1586 Contains the public key for authentication.
1588 sensitive and can (but need not) be readable by anyone.
1590 .It Pa ~/.ssh/known_hosts
1591 Contains a list of host keys for all hosts the user has logged into
1592 that are not already in the systemwide list of known host keys.
1595 for further details of the format of this file.
1598 Commands in this file are executed by
1600 when the user logs in, just before the user's shell (or command) is
1604 manual page for more information.
1606 .It Pa /etc/hosts.equiv
1607 This file is for host-based authentication (see above).
1608 It should only be writable by root.
1610 .It Pa /etc/shosts.equiv
1611 This file is used in exactly the same way as
1613 but allows host-based authentication without permitting login with
1616 .It Pa /etc/ssh/ssh_config
1617 Systemwide configuration file.
1618 The file format and configuration options are described in
1621 .It Pa /etc/ssh/ssh_host_key
1622 .It Pa /etc/ssh/ssh_host_dsa_key
1623 .It Pa /etc/ssh/ssh_host_ecdsa_key
1624 .It Pa /etc/ssh/ssh_host_ed25519_key
1625 .It Pa /etc/ssh/ssh_host_rsa_key
1626 These files contain the private parts of the host keys
1627 and are used for host-based authentication.
1629 .It Pa /etc/ssh/ssh_known_hosts
1630 Systemwide list of known host keys.
1631 This file should be prepared by the
1632 system administrator to contain the public host keys of all machines in the
1634 It should be world-readable.
1637 for further details of the format of this file.
1639 .It Pa /etc/ssh/sshrc
1640 Commands in this file are executed by
1642 when the user logs in, just before the user's shell (or command) is started.
1645 manual page for more information.
1649 exits with the exit status of the remote command or with 255
1650 if an error occurred.
1668 .%T The Secure Shell (SSH) Protocol Assigned Numbers
1676 .%T The Secure Shell (SSH) Protocol Architecture
1684 .%T The Secure Shell (SSH) Authentication Protocol
1692 .%T The Secure Shell (SSH) Transport Layer Protocol
1700 .%T The Secure Shell (SSH) Connection Protocol
1708 .%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
1716 .%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
1724 .%T The Secure Shell (SSH) Session Channel Break Extension
1733 .%T The Secure Shell (SSH) Transport Layer Encryption Modes
1740 .%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
1749 .%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
1757 .%T The Secure Shell (SSH) Public Key File Format
1765 .%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
1772 .%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
1773 .%T Hash Visualization: a New Technique to improve Real-World Security
1776 OpenSSH is a derivative of the original and free
1777 ssh 1.2.12 release by Tatu Ylonen.
1778 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1779 Theo de Raadt and Dug Song
1780 removed many bugs, re-added newer features and
1782 Markus Friedl contributed the support for SSH
1783 protocol versions 1.5 and 2.0.
projects / FreeBSD / FreeBSD.git / blob