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 1.437 2023/07/23 20:04:45 naddy Exp $
37 .Dd $Mdocdate: July 23 2023 $
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
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
69 .Op Ar command Op Ar argument ...
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 their identity to the remote machine using one of several methods
98 it will be executed on the remote host instead of a login shell.
99 A complete command line may be specified as
101 or it may have additional arguments.
102 If supplied, the arguments will be appended to the command, separated by
103 spaces, before it is sent to the server to be executed.
105 The options are as follows:
107 .Bl -tag -width Ds -compact
111 to use IPv4 addresses only.
116 to use IPv6 addresses only.
119 Enables forwarding of connections from an authentication agent such as
121 This can also be specified on a per-host basis in a configuration file.
123 Agent forwarding should be enabled with caution.
124 Users with the ability to bypass file permissions on the remote host
127 socket) can access the local agent through the forwarded connection.
128 An attacker cannot obtain key material from the agent,
129 however they can perform operations on the keys that enable them to
130 authenticate using the identities loaded into the agent.
131 A safer alternative may be to use a jump host
136 Disables forwarding of the authentication agent connection.
138 .It Fl B Ar bind_interface
139 Bind to the address of
141 before attempting to connect to the destination host.
142 This is only useful on systems with more than one address.
144 .It Fl b Ar bind_address
147 on the local machine as the source address
149 Only useful on systems with more than one address.
152 Requests compression of all data (including stdin, stdout, stderr, and
153 data for forwarded X11, TCP and
156 The compression algorithm is the same used by
158 Compression is desirable on modem lines and other
159 slow connections, but will only slow down things on fast networks.
160 The default value can be set on a host-by-host basis in the
161 configuration files; see the
166 .It Fl c Ar cipher_spec
167 Selects the cipher specification for encrypting the session.
169 is a comma-separated list of ciphers
170 listed in order of preference.
175 for more information.
179 .Oo Ar bind_address : Oc
185 application-level port forwarding.
186 This works by allocating a socket to listen to
188 on the local side, optionally bound to the specified
190 Whenever a connection is made to this port, the
191 connection is forwarded over the secure channel, and the application
192 protocol is then used to determine where to connect to from the
194 Currently the SOCKS4 and SOCKS5 protocols are supported, and
196 will act as a SOCKS server.
197 Only root can forward privileged ports.
198 Dynamic port forwardings can also be specified in the configuration file.
200 IPv6 addresses can be specified by enclosing the address in square brackets.
201 Only the superuser can forward privileged ports.
202 By default, the local port is bound in accordance with the
207 may be used to bind the connection to a specific address.
212 indicates that the listening port be bound for local use only, while an
215 indicates that the port should be available from all interfaces.
220 instead of standard error.
222 .It Fl e Ar escape_char
223 Sets the escape character for sessions with a pty (default:
225 The escape character is only recognized at the beginning of a line.
226 The escape character followed by a dot
228 closes the connection;
229 followed by control-Z suspends the connection;
230 and followed by itself sends the escape character once.
231 Setting the character to
233 disables any escapes and makes the session fully transparent.
235 .It Fl F Ar configfile
236 Specifies an alternative per-user configuration file.
237 If a configuration file is given on the command line,
238 the system-wide configuration file
239 .Pq Pa /etc/ssh/ssh_config
241 The default for the per-user configuration file is
245 no configuration files will be read.
250 to go to background just before command execution.
253 is going to ask for passwords or passphrases, but the user
254 wants it in the background.
257 The recommended way to start X11 programs at a remote site is with
259 .Ic ssh -f host xterm .
262 .Cm ExitOnForwardFailure
263 configuration option is set to
265 then a client started with
267 will wait for all remote port forwards to be successfully established
268 before placing itself in the background.
269 Refer to the description of
270 .Cm ForkAfterAuthentication
278 to print its configuration after evaluating
285 Allows remote hosts to connect to local forwarded ports.
286 If used on a multiplexed connection, then this option must be specified
287 on the master process.
290 Specify the PKCS#11 shared library
292 should use to communicate with a PKCS#11 token providing keys for user
295 .It Fl i Ar identity_file
296 Selects a file from which the identity (private key) for
297 public key authentication is read.
298 You can also specify a public key file to use the corresponding
299 private key that is loaded in
301 when the private key file is not present locally.
304 .Pa ~/.ssh/id_ecdsa ,
305 .Pa ~/.ssh/id_ecdsa_sk ,
306 .Pa ~/.ssh/id_ed25519 ,
307 .Pa ~/.ssh/id_ed25519_sk
310 Identity files may also be specified on
311 a per-host basis in the configuration file.
312 It is possible to have multiple
314 options (and multiple identities specified in
315 configuration files).
316 If no certificates have been explicitly specified by the
320 will also try to load certificate information from the filename obtained
323 to identity filenames.
325 .It Fl J Ar destination
326 Connect to the target host by first making an
328 connection to the jump host described by
330 and then establishing a TCP forwarding to the ultimate destination from
332 Multiple jump hops may be specified separated by comma characters.
333 This is a shortcut to specify a
335 configuration directive.
336 Note that configuration directives supplied on the command-line generally
337 apply to the destination host and not any specified jump hosts.
340 to specify configuration for jump hosts.
343 Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
344 credentials to the server.
347 Disables forwarding (delegation) of GSSAPI credentials to the server.
351 .Oo Ar bind_address : Oc
352 .Ar port : host : hostport
357 .Oo Ar bind_address : Oc
358 .Ar port : remote_socket
363 .Ar local_socket : host : hostport
368 .Ar local_socket : remote_socket
371 Specifies that connections to the given TCP port or Unix socket on the local
372 (client) host are to be forwarded to the given host and port, or Unix socket,
374 This works by allocating a socket to listen to either a TCP
376 on the local side, optionally bound to the specified
379 Whenever a connection is made to the local port or socket, the
380 connection is forwarded over the secure channel, and a connection is
387 from the remote machine.
389 Port forwardings can also be specified in the configuration file.
390 Only the superuser can forward privileged ports.
391 IPv6 addresses can be specified by enclosing the address in square brackets.
393 By default, the local port is bound in accordance with the
398 may be used to bind the connection to a specific address.
403 indicates that the listening port be bound for local use only, while an
406 indicates that the port should be available from all interfaces.
408 .It Fl l Ar login_name
409 Specifies the user to log in as on the remote machine.
410 This also may be specified on a per-host basis in the configuration file.
417 mode for connection sharing.
424 mode but with confirmation required using
426 before each operation that changes the multiplexing state
427 (e.g. opening a new session).
428 Refer to the description of
435 A comma-separated list of MAC (message authentication code) algorithms,
436 specified in order of preference.
441 for more information.
444 Do not execute a remote command.
445 This is useful for just forwarding ports.
446 Refer to the description of
455 (actually, prevents reading from stdin).
456 This must be used when
458 is run in the background.
459 A common trick is to use this to run X11 programs on a remote machine.
461 .Ic ssh -n shadows.cs.hut.fi emacs &
462 will start an emacs on shadows.cs.hut.fi, and the X11
463 connection will be automatically forwarded over an encrypted channel.
466 program will be put in the background.
467 (This does not work if
469 needs to ask for a password or passphrase; see also the
472 Refer to the description of
479 Control an active connection multiplexing master process.
482 option is specified, the
484 argument is interpreted and passed to the master process.
487 (check that the master process is running),
489 (request forwardings without command execution),
491 (cancel forwardings),
493 (request the master to exit), and
495 (request the master to stop accepting further multiplexing requests).
498 Can be used to give options in the format used in the configuration file.
499 This is useful for specifying options for which there is no separate
501 For full details of the options listed below, and their possible values, see
504 .Bl -tag -width Ds -offset indent -compact
510 .It CanonicalizeFallbackLocal
511 .It CanonicalizeHostname
512 .It CanonicalizeMaxDots
513 .It CanonicalizePermittedCNAMEs
514 .It CASignatureAlgorithms
518 .It ClearAllForwardings
520 .It ConnectionAttempts
526 .It EnableEscapeCommandline
528 .It ExitOnForwardFailure
530 .It ForkAfterAuthentication
533 .It ForwardX11Timeout
534 .It ForwardX11Trusted
536 .It GlobalKnownHostsFile
537 .It GSSAPIAuthentication
538 .It GSSAPIDelegateCredentials
541 .It HostbasedAcceptedAlgorithms
542 .It HostbasedAuthentication
543 .It HostKeyAlgorithms
550 .It KbdInteractiveAuthentication
551 .It KbdInteractiveDevices
553 .It KnownHostsCommand
559 .It NoHostAuthenticationForLocalhost
560 .It NumberOfPasswordPrompts
561 .It PasswordAuthentication
562 .It PermitLocalCommand
566 .It PreferredAuthentications
570 .It PubkeyAcceptedAlgorithms
571 .It PubkeyAuthentication
578 .It ServerAliveInterval
579 .It ServerAliveCountMax
583 .It StreamLocalBindMask
584 .It StreamLocalBindUnlink
585 .It StrictHostKeyChecking
591 .It UserKnownHostsFile
598 Specify a tag name that may be used to select configuration in
606 for more information.
608 Port to connect to on the remote host.
609 This can be specified on a
610 per-host basis in the configuration file.
612 .It Fl Q Ar query_option
613 Queries for the algorithms supported by one of the following features:
615 (supported symmetric ciphers),
617 (supported symmetric ciphers that support authenticated encryption),
619 (supported query terms for use with the
623 (supported message integrity codes),
625 (key exchange algorithms),
629 (valid CA signature algorithms for certificates),
631 (certificate key types),
633 (non-certificate key types),
635 (all key types and signature algorithms),
637 (supported SSH protocol versions), and
639 (supported signature algorithms).
640 Alternatively, any keyword from
644 that takes an algorithm list may be used as an alias for the corresponding
649 Causes most warning and diagnostic messages to be suppressed.
653 .Oo Ar bind_address : Oc
654 .Ar port : host : hostport
659 .Oo Ar bind_address : Oc
660 .Ar port : local_socket
665 .Ar remote_socket : host : hostport
670 .Ar remote_socket : local_socket
675 .Oo Ar bind_address : Oc
679 Specifies that connections to the given TCP port or Unix socket on the remote
680 (server) host are to be forwarded to the local side.
682 This works by allocating a socket to listen to either a TCP
684 or to a Unix socket on the remote side.
685 Whenever a connection is made to this port or Unix socket, the
686 connection is forwarded over the secure channel, and a connection
687 is made from the local machine to either an explicit destination specified by
693 or, if no explicit destination was specified,
695 will act as a SOCKS 4/5 proxy and forward connections to the destinations
696 requested by the remote SOCKS client.
698 Port forwardings can also be specified in the configuration file.
699 Privileged ports can be forwarded only when
700 logging in as root on the remote machine.
701 IPv6 addresses can be specified by enclosing the address in square brackets.
703 By default, TCP listening sockets on the server will be bound to the loopback
705 This may be overridden by specifying a
711 indicates that the remote socket should listen on all interfaces.
714 will only succeed if the server's
716 option is enabled (see
717 .Xr sshd_config 5 ) .
723 the listen port will be dynamically allocated on the server and reported
724 to the client at run time.
725 When used together with
727 the allocated port will be printed to the standard output.
730 Specifies the location of a control socket for connection sharing,
733 to disable connection sharing.
734 Refer to the description of
743 May be used to request invocation of a subsystem on the remote system.
744 Subsystems facilitate the use of SSH
745 as a secure transport for other applications (e.g.\&
747 The subsystem is specified as the remote command.
748 Refer to the description of
755 Disable pseudo-terminal allocation.
758 Force pseudo-terminal allocation.
759 This can be used to execute arbitrary
760 screen-based programs on a remote machine, which can be very useful,
761 e.g. when implementing menu services.
764 options force tty allocation, even if
769 Display the version number and exit.
775 to print debugging messages about its progress.
777 debugging connection, authentication, and configuration problems.
780 options increase the verbosity.
783 .It Fl W Ar host : Ns Ar port
784 Requests that standard input and output on the client be forwarded to
788 over the secure channel.
792 .Cm ExitOnForwardFailure
794 .Cm ClearAllForwardings ,
795 though these can be overridden in the configuration file or using
797 command line options.
800 .Ar local_tun Ns Op : Ns Ar remote_tun
804 device forwarding with the specified
806 devices between the client
811 The devices may be specified by numerical ID or the keyword
813 which uses the next available tunnel device.
816 is not specified, it defaults to
827 directive is unset, it will be set to the default tunnel mode, which is
831 forwarding mode it desired, then it should be specified before
835 Enables X11 forwarding.
836 This can also be specified on a per-host basis in a configuration file.
838 X11 forwarding should be enabled with caution.
839 Users with the ability to bypass file permissions on the remote host
840 (for the user's X authorization database)
841 can access the local X11 display through the forwarded connection.
842 An attacker may then be able to perform activities such as keystroke monitoring.
844 For this reason, X11 forwarding is subjected to X11 SECURITY extension
845 restrictions by default.
850 .Cm ForwardX11Trusted
853 for more information.
856 Disables X11 forwarding.
859 Enables trusted X11 forwarding.
860 Trusted X11 forwardings are not subjected to the X11 SECURITY extension
864 Send log information using the
867 By default this information is sent to stderr.
871 may additionally obtain configuration data from
872 a per-user configuration file and a system-wide configuration file.
873 The file format and configuration options are described in
876 The OpenSSH SSH client supports SSH protocol 2.
878 The methods available for authentication are:
879 GSSAPI-based authentication,
880 host-based authentication,
881 public key authentication,
882 keyboard-interactive authentication,
883 and password authentication.
884 Authentication methods are tried in the order specified above,
886 .Cm PreferredAuthentications
887 can be used to change the default order.
889 Host-based authentication works as follows:
890 If the machine the user logs in from is listed in
893 .Pa /etc/shosts.equiv
894 on the remote machine, the user is non-root and the user names are
895 the same on both sides, or if the files
899 exist in the user's home directory on the
900 remote machine and contain a line containing the name of the client
901 machine and the name of the user on that machine, the user is
902 considered for login.
903 Additionally, the server
905 be able to verify the client's
906 host key (see the description of
907 .Pa /etc/ssh/ssh_known_hosts
909 .Pa ~/.ssh/known_hosts ,
911 for login to be permitted.
912 This authentication method closes security holes due to IP
913 spoofing, DNS spoofing, and routing spoofing.
914 [Note to the administrator:
915 .Pa /etc/hosts.equiv ,
917 and the rlogin/rsh protocol in general, are inherently insecure and should be
918 disabled if security is desired.]
920 Public key authentication works as follows:
921 The scheme is based on public-key cryptography,
923 where encryption and decryption are done using separate keys,
924 and it is unfeasible to derive the decryption key from the encryption key.
925 The idea is that each user creates a public/private
926 key pair for authentication purposes.
927 The server knows the public key, and only the user knows the private key.
929 implements public key authentication protocol automatically,
930 using one of the DSA, ECDSA, Ed25519 or RSA algorithms.
931 The HISTORY section of
933 contains a brief discussion of the DSA and RSA algorithms.
936 .Pa ~/.ssh/authorized_keys
937 lists the public keys that are permitted for logging in.
938 When the user logs in, the
940 program tells the server which key pair it would like to use for
942 The client proves that it has access to the private key
943 and the server checks that the corresponding public key
944 is authorized to accept the account.
946 The server may inform the client of errors that prevented public key
947 authentication from succeeding after authentication completes using a
949 These may be viewed by increasing the
953 or higher (e.g. by using the
957 The user creates their key pair by running
959 This stores the private key in
964 .Pa ~/.ssh/id_ecdsa_sk
965 (authenticator-hosted ECDSA),
966 .Pa ~/.ssh/id_ed25519
968 .Pa ~/.ssh/id_ed25519_sk
969 (authenticator-hosted Ed25519),
973 and stores the public key in
974 .Pa ~/.ssh/id_dsa.pub
976 .Pa ~/.ssh/id_ecdsa.pub
978 .Pa ~/.ssh/id_ecdsa_sk.pub
979 (authenticator-hosted ECDSA),
980 .Pa ~/.ssh/id_ed25519.pub
982 .Pa ~/.ssh/id_ed25519_sk.pub
983 (authenticator-hosted Ed25519),
985 .Pa ~/.ssh/id_rsa.pub
987 in the user's home directory.
988 The user should then copy the public key
990 .Pa ~/.ssh/authorized_keys
991 in their home directory on the remote machine.
994 file corresponds to the conventional
996 file, and has one key
997 per line, though the lines can be very long.
998 After this, the user can log in without giving the password.
1000 A variation on public key authentication
1001 is available in the form of certificate authentication:
1002 instead of a set of public/private keys,
1003 signed certificates are used.
1004 This has the advantage that a single trusted certification authority
1005 can be used in place of many public/private keys.
1006 See the CERTIFICATES section of
1008 for more information.
1010 The most convenient way to use public key or certificate authentication
1011 may be with an authentication agent.
1014 and (optionally) the
1018 for more information.
1020 Keyboard-interactive authentication works as follows:
1021 The server sends an arbitrary
1023 text and prompts for a response, possibly multiple times.
1024 Examples of keyboard-interactive authentication include
1032 Finally, if other authentication methods fail,
1034 prompts the user for a password.
1035 The password is sent to the remote
1036 host for checking; however, since all communications are encrypted,
1037 the password cannot be seen by someone listening on the network.
1040 automatically maintains and checks a database containing
1041 identification for all hosts it has ever been used with.
1042 Host keys are stored in
1043 .Pa ~/.ssh/known_hosts
1044 in the user's home directory.
1045 Additionally, the file
1046 .Pa /etc/ssh/ssh_known_hosts
1047 is automatically checked for known hosts.
1048 Any new hosts are automatically added to the user's file.
1049 If a host's identification ever changes,
1051 warns about this and disables password authentication to prevent
1052 server spoofing or man-in-the-middle attacks,
1053 which could otherwise be used to circumvent the encryption.
1055 .Cm StrictHostKeyChecking
1056 option can be used to control logins to machines whose
1057 host key is not known or has changed.
1059 When the user's identity has been accepted by the server, the server
1060 either executes the given command in a non-interactive session or,
1061 if no command has been specified, logs into the machine and gives
1062 the user a normal shell as an interactive session.
1063 All communication with
1064 the remote command or shell will be automatically encrypted.
1066 If an interactive session is requested,
1068 by default will only request a pseudo-terminal (pty) for interactive
1069 sessions when the client has one.
1074 can be used to override this behaviour.
1076 If a pseudo-terminal has been allocated, the
1077 user may use the escape characters noted below.
1079 If no pseudo-terminal has been allocated,
1080 the session is transparent and can be used to reliably transfer binary data.
1081 On most systems, setting the escape character to
1083 will also make the session transparent even if a tty is used.
1085 The session terminates when the command or shell on the remote
1086 machine exits and all X11 and TCP connections have been closed.
1087 .Sh ESCAPE CHARACTERS
1088 When a pseudo-terminal has been requested,
1090 supports a number of functions through the use of an escape character.
1092 A single tilde character can be sent as
1094 or by following the tilde by a character other than those described below.
1095 The escape character must always follow a newline to be interpreted as
1097 The escape character can be changed in configuration files using the
1099 configuration directive or on the command line by the
1103 The supported escapes (assuming the default
1113 List forwarded connections.
1117 at logout when waiting for forwarded connection / X11 sessions to terminate.
1119 Display a list of escape characters.
1121 Send a BREAK to the remote system
1122 (only useful if the peer supports it).
1125 Currently this allows the addition of port forwardings using the
1130 options (see above).
1131 It also allows the cancellation of existing port-forwardings
1134 .Fl KL Oo Ar bind_address : Oc Ar port
1138 .Fl KR Oo Ar bind_address : Oc Ar port
1142 .Fl KD Oo Ar bind_address : Oc Ar port
1144 for dynamic port-forwardings.
1145 .Ic !\& Ns Ar command
1146 allows the user to execute a local command if the
1147 .Ic PermitLocalCommand
1148 option is enabled in
1150 Basic help is available, using the
1154 Request rekeying of the connection
1155 (only useful if the peer supports it).
1157 Decrease the verbosity
1159 when errors are being written to stderr.
1161 Increase the verbosity
1163 when errors are being written to stderr.
1166 Forwarding of arbitrary TCP connections over a secure channel
1167 can be specified either on the command line or in a configuration file.
1168 One possible application of TCP forwarding is a secure connection to a
1169 mail server; another is going through firewalls.
1171 In the example below, we look at encrypting communication for an IRC client,
1172 even though the IRC server it connects to does not directly
1173 support encrypted communication.
1174 This works as follows:
1175 the user connects to the remote host using
1177 specifying the ports to be used to forward the connection.
1178 After that it is possible to start the program locally,
1181 will encrypt and forward the connection to the remote server.
1183 The following example tunnels an IRC session from the client
1185 .Dq server.example.com ,
1190 using the standard IRC port, 6667:
1191 .Bd -literal -offset 4n
1192 $ ssh -f -L 6667:localhost:6667 server.example.com sleep 10
1193 $ irc -c '#users' pinky IRC/127.0.0.1
1200 and the remote command
1202 is specified to allow an amount of time
1203 (10 seconds, in the example)
1204 to start the program which is going to use the tunnel.
1205 If no connections are made within the time specified,
1213 (or see the description of the
1219 and the user is using X11 (the
1221 environment variable is set), the connection to the X11 display is
1222 automatically forwarded to the remote side in such a way that any X11
1223 programs started from the shell (or command) will go through the
1224 encrypted channel, and the connection to the real X server will be made
1225 from the local machine.
1226 The user should not manually set
1228 Forwarding of X11 connections can be
1229 configured on the command line or in configuration files.
1235 will point to the server machine, but with a display number greater than zero.
1236 This is normal, and happens because
1240 X server on the server machine for forwarding the
1241 connections over the encrypted channel.
1244 will also automatically set up Xauthority data on the server machine.
1245 For this purpose, it will generate a random authorization cookie,
1246 store it in Xauthority on the server, and verify that any forwarded
1247 connections carry this cookie and replace it by the real cookie when
1248 the connection is opened.
1249 The real authentication cookie is never
1250 sent to the server machine (and no cookies are sent in the plain).
1256 (or see the description of the
1261 the user is using an authentication agent, the connection to the agent
1262 is automatically forwarded to the remote side.
1263 .Sh VERIFYING HOST KEYS
1264 When connecting to a server for the first time,
1265 a fingerprint of the server's public key is presented to the user
1267 .Cm StrictHostKeyChecking
1269 Fingerprints can be determined using
1272 .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1274 If the fingerprint is already known, it can be matched
1275 and the key can be accepted or rejected.
1276 If only legacy (MD5) fingerprints for the server are available, the
1279 option may be used to downgrade the fingerprint algorithm to match.
1281 Because of the difficulty of comparing host keys
1282 just by looking at fingerprint strings,
1283 there is also support to compare host keys visually,
1290 a small ASCII graphic gets displayed on every login to a server, no matter
1291 if the session itself is interactive or not.
1292 By learning the pattern a known server produces, a user can easily
1293 find out that the host key has changed when a completely different pattern
1295 Because these patterns are not unambiguous however, a pattern that looks
1296 similar to the pattern remembered only gives a good probability that the
1297 host key is the same, not guaranteed proof.
1299 To get a listing of the fingerprints along with their random art for
1300 all known hosts, the following command line can be used:
1302 .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1304 If the fingerprint is unknown,
1305 an alternative method of verification is available:
1306 SSH fingerprints verified by DNS.
1307 An additional resource record (RR),
1309 is added to a zonefile
1310 and the connecting client is able to match the fingerprint
1311 with that of the key presented.
1313 In this example, we are connecting a client to a server,
1314 .Dq host.example.com .
1315 The SSHFP resource records should first be added to the zonefile for
1317 .Bd -literal -offset indent
1318 $ ssh-keygen -r host.example.com.
1321 The output lines will have to be added to the zonefile.
1322 To check that the zone is answering fingerprint queries:
1324 .Dl $ dig -t SSHFP host.example.com
1326 Finally the client connects:
1327 .Bd -literal -offset indent
1328 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1330 Matching host key fingerprint found in DNS.
1331 Are you sure you want to continue connecting (yes/no)?
1335 .Cm VerifyHostKeyDNS
1338 for more information.
1339 .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1341 contains support for Virtual Private Network (VPN) tunnelling
1344 network pseudo-device,
1345 allowing two networks to be joined securely.
1348 configuration option
1350 controls whether the server supports this,
1351 and at what level (layer 2 or 3 traffic).
1353 The following example would connect client network 10.0.50.0/24
1354 with remote network 10.0.99.0/24 using a point-to-point connection
1355 from 10.1.1.1 to 10.1.1.2,
1356 provided that the SSH server running on the gateway to the remote network,
1357 at 192.168.1.15, allows it.
1360 .Bd -literal -offset indent
1361 # ssh -f -w 0:1 192.168.1.15 true
1362 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1363 # route add 10.0.99.0/24 10.1.1.2
1367 .Bd -literal -offset indent
1368 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1369 # route add 10.0.50.0/24 10.1.1.1
1372 Client access may be more finely tuned via the
1373 .Pa /root/.ssh/authorized_keys
1374 file (see below) and the
1377 The following entry would permit connections on
1381 and on tun device 2 from user
1386 .Dq forced-commands-only :
1387 .Bd -literal -offset 2n
1388 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1389 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1392 Since an SSH-based setup entails a fair amount of overhead,
1393 it may be more suited to temporary setups,
1394 such as for wireless VPNs.
1395 More permanent VPNs are better provided by tools such as
1401 will normally set the following environment variables:
1402 .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1406 variable indicates the location of the X11 server.
1407 It is automatically set by
1409 to point to a value of the form
1413 indicates the host where the shell runs, and
1415 is an integer \*(Ge 1.
1417 uses this special value to forward X11 connections over the secure
1419 The user should normally not set
1422 will render the X11 connection insecure (and will require the user to
1423 manually copy any required authorization cookies).
1425 Set to the path of the user's home directory.
1429 set for compatibility with systems that use this variable.
1431 Set to the path of the user's mailbox.
1435 as specified when compiling
1440 needs a passphrase, it will read the passphrase from the current
1441 terminal if it was run from a terminal.
1444 does not have a terminal associated with it but
1448 are set, it will execute the program specified by
1450 and open an X11 window to read the passphrase.
1451 This is particularly useful when calling
1456 (Note that on some machines it
1457 may be necessary to redirect the input from
1460 .It Ev SSH_ASKPASS_REQUIRE
1461 Allows further control over the use of an askpass program.
1462 If this variable is set to
1466 will never attempt to use one.
1471 will prefer to use the askpass program instead of the TTY when requesting
1473 Finally, if the variable is set to
1475 then the askpass program will be used for all passphrase input regardless
1479 .It Ev SSH_AUTH_SOCK
1480 Identifies the path of a
1482 socket used to communicate with the agent.
1483 .It Ev SSH_CONNECTION
1484 Identifies the client and server ends of the connection.
1485 The variable contains
1486 four space-separated values: client IP address, client port number,
1487 server IP address, and server port number.
1488 .It Ev SSH_ORIGINAL_COMMAND
1489 This variable contains the original command line if a forced command
1491 It can be used to extract the original arguments.
1493 This is set to the name of the tty (path to the device) associated
1494 with the current shell or command.
1495 If the current session has no tty,
1496 this variable is not set.
1500 to contain the interface names assigned if tunnel forwarding was
1501 requested by the client.
1502 .It Ev SSH_USER_AUTH
1505 this variable may contain a pathname to a file that lists the authentication
1506 methods successfully used when the session was established, including any
1507 public keys that were used.
1509 This variable is set to indicate the present time zone if it
1510 was set when the daemon was started (i.e. the daemon passes the value
1511 on to new connections).
1513 Set to the name of the user logging in.
1519 .Pa ~/.ssh/environment ,
1520 and adds lines of the format
1522 to the environment if the file exists and users are allowed to
1523 change their environment.
1524 For more information, see the
1525 .Cm PermitUserEnvironment
1529 .Bl -tag -width Ds -compact
1531 This file is used for host-based authentication (see above).
1532 On some machines this file may need to be
1533 world-readable if the user's home directory is on an NFS partition,
1537 Additionally, this file must be owned by the user,
1538 and must not have write permissions for anyone else.
1540 permission for most machines is read/write for the user, and not
1541 accessible by others.
1544 This file is used in exactly the same way as
1546 but allows host-based authentication without permitting login with
1550 This directory is the default location for all user-specific configuration
1551 and authentication information.
1552 There is no general requirement to keep the entire contents of this directory
1553 secret, but the recommended permissions are read/write/execute for the user,
1554 and not accessible by others.
1556 .It Pa ~/.ssh/authorized_keys
1557 Lists the public keys (DSA, ECDSA, Ed25519, RSA)
1558 that can be used for logging in as this user.
1559 The format of this file is described in the
1562 This file is not highly sensitive, but the recommended
1563 permissions are read/write for the user, and not accessible by others.
1565 .It Pa ~/.ssh/config
1566 This is the per-user configuration file.
1567 The file format and configuration options are described in
1569 Because of the potential for abuse, this file must have strict permissions:
1570 read/write for the user, and not writable by others.
1572 .It Pa ~/.ssh/environment
1573 Contains additional definitions for environment variables; see
1577 .It Pa ~/.ssh/id_dsa
1578 .It Pa ~/.ssh/id_ecdsa
1579 .It Pa ~/.ssh/id_ecdsa_sk
1580 .It Pa ~/.ssh/id_ed25519
1581 .It Pa ~/.ssh/id_ed25519_sk
1582 .It Pa ~/.ssh/id_rsa
1583 Contains the private key for authentication.
1585 contain sensitive data and should be readable by the user but not
1586 accessible by others (read/write/execute).
1588 will simply ignore a private key file if it is accessible by others.
1589 It is possible to specify a passphrase when
1590 generating the key which will be used to encrypt the
1591 sensitive part of this file using AES-128.
1593 .It Pa ~/.ssh/id_dsa.pub
1594 .It Pa ~/.ssh/id_ecdsa.pub
1595 .It Pa ~/.ssh/id_ecdsa_sk.pub
1596 .It Pa ~/.ssh/id_ed25519.pub
1597 .It Pa ~/.ssh/id_ed25519_sk.pub
1598 .It Pa ~/.ssh/id_rsa.pub
1599 Contains the public key for authentication.
1601 sensitive and can (but need not) be readable by anyone.
1603 .It Pa ~/.ssh/known_hosts
1604 Contains a list of host keys for all hosts the user has logged into
1605 that are not already in the systemwide list of known host keys.
1608 for further details of the format of this file.
1611 Commands in this file are executed by
1613 when the user logs in, just before the user's shell (or command) is
1617 manual page for more information.
1619 .It Pa /etc/hosts.equiv
1620 This file is for host-based authentication (see above).
1621 It should only be writable by root.
1623 .It Pa /etc/shosts.equiv
1624 This file is used in exactly the same way as
1626 but allows host-based authentication without permitting login with
1629 .It Pa /etc/ssh/ssh_config
1630 Systemwide configuration file.
1631 The file format and configuration options are described in
1634 .It Pa /etc/ssh/ssh_host_key
1635 .It Pa /etc/ssh/ssh_host_dsa_key
1636 .It Pa /etc/ssh/ssh_host_ecdsa_key
1637 .It Pa /etc/ssh/ssh_host_ed25519_key
1638 .It Pa /etc/ssh/ssh_host_rsa_key
1639 These files contain the private parts of the host keys
1640 and are used for host-based authentication.
1642 .It Pa /etc/ssh/ssh_known_hosts
1643 Systemwide list of known host keys.
1644 This file should be prepared by the
1645 system administrator to contain the public host keys of all machines in the
1647 It should be world-readable.
1650 for further details of the format of this file.
1652 .It Pa /etc/ssh/sshrc
1653 Commands in this file are executed by
1655 when the user logs in, just before the user's shell (or command) is started.
1658 manual page for more information.
1662 exits with the exit status of the remote command or with 255
1663 if an error occurred.
1681 .%T The Secure Shell (SSH) Protocol Assigned Numbers
1689 .%T The Secure Shell (SSH) Protocol Architecture
1697 .%T The Secure Shell (SSH) Authentication Protocol
1705 .%T The Secure Shell (SSH) Transport Layer Protocol
1713 .%T The Secure Shell (SSH) Connection Protocol
1721 .%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
1729 .%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
1737 .%T The Secure Shell (SSH) Session Channel Break Extension
1746 .%T The Secure Shell (SSH) Transport Layer Encryption Modes
1753 .%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
1762 .%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
1770 .%T The Secure Shell (SSH) Public Key File Format
1778 .%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
1785 .%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
1786 .%T Hash Visualization: a New Technique to improve Real-World Security
1789 OpenSSH is a derivative of the original and free
1790 ssh 1.2.12 release by Tatu Ylonen.
1791 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1792 Theo de Raadt and Dug Song
1793 removed many bugs, re-added newer features and
1795 Markus Friedl contributed the support for SSH
1796 protocol versions 1.5 and 2.0.
projects / FreeBSD / FreeBSD.git / blob