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 1.412 2020/04/17 03:34:42 djm Exp $
37 .Dd $Mdocdate: April 17 2020 $
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
71 (SSH client) is a program for logging into a remote machine and for
72 executing commands on a remote machine.
73 It is intended to provide secure encrypted communications between
74 two untrusted hosts over an insecure network.
75 X11 connections, arbitrary TCP ports and
77 sockets can also be forwarded over the secure channel.
80 connects and logs into the specified
82 which may be specified as either
84 .Oo user @ Oc hostname
88 .No ssh:// Oo user @ Oc hostname Op : port .
91 his/her identity to the remote machine using one of several methods
97 it is executed on the remote host instead of a login shell.
99 The options are as follows:
101 .Bl -tag -width Ds -compact
105 to use IPv4 addresses only.
110 to use IPv6 addresses only.
113 Enables forwarding of connections from an authentication agent such as
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.
125 A safer alternative may be to use a jump host
130 Disables forwarding of the authentication agent connection.
132 .It Fl B Ar bind_interface
133 Bind to the address of
135 before attempting to connect to the destination host.
136 This is only useful on systems with more than one address.
138 .It Fl b Ar bind_address
141 on the local machine as the source address
143 Only useful on systems with more than one address.
146 Requests compression of all data (including stdin, stdout, stderr, and
147 data for forwarded X11, TCP and
150 The compression algorithm is the same used by
152 Compression is desirable on modem lines and other
153 slow connections, but will only slow down things on fast networks.
154 The default value can be set on a host-by-host basis in the
155 configuration files; see the
159 .It Fl c Ar cipher_spec
160 Selects the cipher specification for encrypting the session.
162 is a comma-separated list of ciphers
163 listed in order of preference.
168 for more information.
172 .Oo Ar bind_address : Oc
178 application-level port forwarding.
179 This works by allocating a socket to listen to
181 on the local side, optionally bound to the specified
183 Whenever a connection is made to this port, the
184 connection is forwarded over the secure channel, and the application
185 protocol is then used to determine where to connect to from the
187 Currently the SOCKS4 and SOCKS5 protocols are supported, and
189 will act as a SOCKS server.
190 Only root can forward privileged ports.
191 Dynamic port forwardings can also be specified in the configuration file.
193 IPv6 addresses can be specified by enclosing the address in square brackets.
194 Only the superuser can forward privileged ports.
195 By default, the local port is bound in accordance with the
200 may be used to bind the connection to a specific address.
205 indicates that the listening port be bound for local use only, while an
208 indicates that the port should be available from all interfaces.
213 instead of standard error.
215 .It Fl e Ar escape_char
216 Sets the escape character for sessions with a pty (default:
218 The escape character is only recognized at the beginning of a line.
219 The escape character followed by a dot
221 closes the connection;
222 followed by control-Z suspends the connection;
223 and followed by itself sends the escape character once.
224 Setting the character to
226 disables any escapes and makes the session fully transparent.
228 .It Fl F Ar configfile
229 Specifies an alternative per-user configuration file.
230 If a configuration file is given on the command line,
231 the system-wide configuration file
232 .Pq Pa /etc/ssh/ssh_config
234 The default for the per-user configuration file is
238 no configuration files will be read.
243 to go to background just before command execution.
246 is going to ask for passwords or passphrases, but the user
247 wants it in the background.
250 The recommended way to start X11 programs at a remote site is with
252 .Ic ssh -f host xterm .
255 .Cm ExitOnForwardFailure
256 configuration option is set to
258 then a client started with
260 will wait for all remote port forwards to be successfully established
261 before placing itself in the background.
266 to print its configuration after evaluating
273 Allows remote hosts to connect to local forwarded ports.
274 If used on a multiplexed connection, then this option must be specified
275 on the master process.
278 Specify the PKCS#11 shared library
280 should use to communicate with a PKCS#11 token providing keys for user
283 .It Fl i Ar identity_file
284 Selects a file from which the identity (private key) for
285 public key authentication is read.
288 .Pa ~/.ssh/id_ecdsa ,
289 .Pa ~/.ssh/id_ecdsa_sk ,
290 .Pa ~/.ssh/id_ed25519 ,
291 .Pa ~/.ssh/id_ed25519_sk
294 Identity files may also be specified on
295 a per-host basis in the configuration file.
296 It is possible to have multiple
298 options (and multiple identities specified in
299 configuration files).
300 If no certificates have been explicitly specified by the
304 will also try to load certificate information from the filename obtained
307 to identity filenames.
309 .It Fl J Ar destination
310 Connect to the target host by first making a
312 connection to the jump host described by
314 and then establishing a TCP forwarding to the ultimate destination from
316 Multiple jump hops may be specified separated by comma characters.
317 This is a shortcut to specify a
319 configuration directive.
320 Note that configuration directives supplied on the command-line generally
321 apply to the destination host and not any specified jump hosts.
324 to specify configuration for jump hosts.
327 Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
328 credentials to the server.
331 Disables forwarding (delegation) of GSSAPI credentials to the server.
335 .Oo Ar bind_address : Oc
336 .Ar port : host : hostport
341 .Oo Ar bind_address : Oc
342 .Ar port : remote_socket
347 .Ar local_socket : host : hostport
352 .Ar local_socket : remote_socket
355 Specifies that connections to the given TCP port or Unix socket on the local
356 (client) host are to be forwarded to the given host and port, or Unix socket,
358 This works by allocating a socket to listen to either a TCP
360 on the local side, optionally bound to the specified
363 Whenever a connection is made to the local port or socket, the
364 connection is forwarded over the secure channel, and a connection is
371 from the remote machine.
373 Port forwardings can also be specified in the configuration file.
374 Only the superuser can forward privileged ports.
375 IPv6 addresses can be specified by enclosing the address in square brackets.
377 By default, the local port is bound in accordance with the
382 may be used to bind the connection to a specific address.
387 indicates that the listening port be bound for local use only, while an
390 indicates that the port should be available from all interfaces.
392 .It Fl l Ar login_name
393 Specifies the user to log in as on the remote machine.
394 This also may be specified on a per-host basis in the configuration file.
401 mode for connection sharing.
408 mode but with confirmation required using
410 before each operation that changes the multiplexing state
411 (e.g. opening a new session).
412 Refer to the description of
419 A comma-separated list of MAC (message authentication code) algorithms,
420 specified in order of preference.
423 keyword for more information.
426 Do not execute a remote command.
427 This is useful for just forwarding ports.
432 (actually, prevents reading from stdin).
433 This must be used when
435 is run in the background.
436 A common trick is to use this to run X11 programs on a remote machine.
438 .Ic ssh -n shadows.cs.hut.fi emacs &
439 will start an emacs on shadows.cs.hut.fi, and the X11
440 connection will be automatically forwarded over an encrypted channel.
443 program will be put in the background.
444 (This does not work if
446 needs to ask for a password or passphrase; see also the
451 Control an active connection multiplexing master process.
454 option is specified, the
456 argument is interpreted and passed to the master process.
459 (check that the master process is running),
461 (request forwardings without command execution),
463 (cancel forwardings),
465 (request the master to exit), and
467 (request the master to stop accepting further multiplexing requests).
470 Can be used to give options in the format used in the configuration file.
471 This is useful for specifying options for which there is no separate
473 For full details of the options listed below, and their possible values, see
476 .Bl -tag -width Ds -offset indent -compact
482 .It CanonicalizeFallbackLocal
483 .It CanonicalizeHostname
484 .It CanonicalizeMaxDots
485 .It CanonicalizePermittedCNAMEs
486 .It CASignatureAlgorithms
488 .It ChallengeResponseAuthentication
491 .It ClearAllForwardings
493 .It ConnectionAttempts
500 .It ExitOnForwardFailure
504 .It ForwardX11Timeout
505 .It ForwardX11Trusted
507 .It GlobalKnownHostsFile
508 .It GSSAPIAuthentication
509 .It GSSAPIDelegateCredentials
512 .It HostbasedAuthentication
513 .It HostbasedKeyTypes
514 .It HostKeyAlgorithms
521 .It KbdInteractiveAuthentication
522 .It KbdInteractiveDevices
529 .It NoHostAuthenticationForLocalhost
530 .It NumberOfPasswordPrompts
531 .It PasswordAuthentication
532 .It PermitLocalCommand
535 .It PreferredAuthentications
539 .It PubkeyAcceptedKeyTypes
540 .It PubkeyAuthentication
546 .It ServerAliveInterval
547 .It ServerAliveCountMax
549 .It StreamLocalBindMask
550 .It StreamLocalBindUnlink
551 .It StrictHostKeyChecking
557 .It UserKnownHostsFile
564 Port to connect to on the remote host.
565 This can be specified on a
566 per-host basis in the configuration file.
568 .It Fl Q Ar query_option
571 for the algorithms supported for the specified version 2.
572 The available features are:
574 (supported symmetric ciphers),
576 (supported symmetric ciphers that support authenticated encryption),
578 (supported query terms for use with the
582 (supported message integrity codes),
584 (key exchange algorithms),
588 (certificate key types),
590 (non-certificate key types),
592 (all key types and signature algorithms),
594 (supported SSH protocol versions), and
596 (supported signature algorithms).
597 Alternatively, any keyword from
601 that takes an algorithm list may be used as an alias for the corresponding
606 Causes most warning and diagnostic messages to be suppressed.
610 .Oo Ar bind_address : Oc
611 .Ar port : host : hostport
616 .Oo Ar bind_address : Oc
617 .Ar port : local_socket
622 .Ar remote_socket : host : hostport
627 .Ar remote_socket : local_socket
632 .Oo Ar bind_address : Oc
636 Specifies that connections to the given TCP port or Unix socket on the remote
637 (server) host are to be forwarded to the local side.
639 This works by allocating a socket to listen to either a TCP
641 or to a Unix socket on the remote side.
642 Whenever a connection is made to this port or Unix socket, the
643 connection is forwarded over the secure channel, and a connection
644 is made from the local machine to either an explicit destination specified by
650 or, if no explicit destination was specified,
652 will act as a SOCKS 4/5 proxy and forward connections to the destinations
653 requested by the remote SOCKS client.
655 Port forwardings can also be specified in the configuration file.
656 Privileged ports can be forwarded only when
657 logging in as root on the remote machine.
658 IPv6 addresses can be specified by enclosing the address in square brackets.
660 By default, TCP listening sockets on the server will be bound to the loopback
662 This may be overridden by specifying a
668 indicates that the remote socket should listen on all interfaces.
671 will only succeed if the server's
673 option is enabled (see
674 .Xr sshd_config 5 ) .
680 the listen port will be dynamically allocated on the server and reported
681 to the client at run time.
682 When used together with
684 the allocated port will be printed to the standard output.
687 Specifies the location of a control socket for connection sharing,
690 to disable connection sharing.
691 Refer to the description of
700 May be used to request invocation of a subsystem on the remote system.
701 Subsystems facilitate the use of SSH
702 as a secure transport for other applications (e.g.\&
704 The subsystem is specified as the remote command.
707 Disable pseudo-terminal allocation.
710 Force pseudo-terminal allocation.
711 This can be used to execute arbitrary
712 screen-based programs on a remote machine, which can be very useful,
713 e.g. when implementing menu services.
716 options force tty allocation, even if
721 Display the version number and exit.
727 to print debugging messages about its progress.
729 debugging connection, authentication, and configuration problems.
732 options increase the verbosity.
735 .It Fl W Ar host : Ns Ar port
736 Requests that standard input and output on the client be forwarded to
740 over the secure channel.
744 .Cm ExitOnForwardFailure
746 .Cm ClearAllForwardings ,
747 though these can be overridden in the configuration file or using
749 command line options.
752 .Ar local_tun Ns Op : Ns Ar remote_tun
756 device forwarding with the specified
758 devices between the client
763 The devices may be specified by numerical ID or the keyword
765 which uses the next available tunnel device.
768 is not specified, it defaults to
779 directive is unset, it will be set to the default tunnel mode, which is
783 forwarding mode it desired, then it should be specified before
787 Enables X11 forwarding.
788 This can also be specified on a per-host basis in a configuration file.
790 X11 forwarding should be enabled with caution.
791 Users with the ability to bypass file permissions on the remote host
792 (for the user's X authorization database)
793 can access the local X11 display through the forwarded connection.
794 An attacker may then be able to perform activities such as keystroke monitoring.
796 For this reason, X11 forwarding is subjected to X11 SECURITY extension
797 restrictions by default.
802 .Cm ForwardX11Trusted
805 for more information.
808 Disables X11 forwarding.
811 Enables trusted X11 forwarding.
812 Trusted X11 forwardings are not subjected to the X11 SECURITY extension
816 Send log information using the
819 By default this information is sent to stderr.
823 may additionally obtain configuration data from
824 a per-user configuration file and a system-wide configuration file.
825 The file format and configuration options are described in
828 The OpenSSH SSH client supports SSH protocol 2.
830 The methods available for authentication are:
831 GSSAPI-based authentication,
832 host-based authentication,
833 public key authentication,
834 challenge-response authentication,
835 and password authentication.
836 Authentication methods are tried in the order specified above,
838 .Cm PreferredAuthentications
839 can be used to change the default order.
841 Host-based authentication works as follows:
842 If the machine the user logs in from is listed in
845 .Pa /etc/shosts.equiv
846 on the remote machine, the user is non-root and the user names are
847 the same on both sides, or if the files
851 exist in the user's home directory on the
852 remote machine and contain a line containing the name of the client
853 machine and the name of the user on that machine, the user is
854 considered for login.
855 Additionally, the server
857 be able to verify the client's
858 host key (see the description of
859 .Pa /etc/ssh/ssh_known_hosts
861 .Pa ~/.ssh/known_hosts ,
863 for login to be permitted.
864 This authentication method closes security holes due to IP
865 spoofing, DNS spoofing, and routing spoofing.
866 [Note to the administrator:
867 .Pa /etc/hosts.equiv ,
869 and the rlogin/rsh protocol in general, are inherently insecure and should be
870 disabled if security is desired.]
872 Public key authentication works as follows:
873 The scheme is based on public-key cryptography,
875 where encryption and decryption are done using separate keys,
876 and it is unfeasible to derive the decryption key from the encryption key.
877 The idea is that each user creates a public/private
878 key pair for authentication purposes.
879 The server knows the public key, and only the user knows the private key.
881 implements public key authentication protocol automatically,
882 using one of the DSA, ECDSA, Ed25519 or RSA algorithms.
883 The HISTORY section of
885 contains a brief discussion of the DSA and RSA algorithms.
888 .Pa ~/.ssh/authorized_keys
889 lists the public keys that are permitted for logging in.
890 When the user logs in, the
892 program tells the server which key pair it would like to use for
894 The client proves that it has access to the private key
895 and the server checks that the corresponding public key
896 is authorized to accept the account.
898 The server may inform the client of errors that prevented public key
899 authentication from succeeding after authentication completes using a
901 These may be viewed by increasing the
905 or higher (e.g. by using the
909 The user creates his/her key pair by running
911 This stores the private key in
916 .Pa ~/.ssh/id_ecdsa_sk
917 (authenticator-hosted ECDSA),
918 .Pa ~/.ssh/id_ed25519
920 .Pa ~/.ssh/id_ed25519_sk
921 (authenticator-hosted Ed25519),
925 and stores the public key in
926 .Pa ~/.ssh/id_dsa.pub
928 .Pa ~/.ssh/id_ecdsa.pub
930 .Pa ~/.ssh/id_ecdsa_sk.pub
931 (authenticator-hosted ECDSA),
932 .Pa ~/.ssh/id_ed25519.pub
934 .Pa ~/.ssh/id_ed25519_sk.pub
935 (authenticator-hosted Ed25519),
937 .Pa ~/.ssh/id_rsa.pub
939 in the user's home directory.
940 The user should then copy the public key
942 .Pa ~/.ssh/authorized_keys
943 in his/her home directory on the remote machine.
946 file corresponds to the conventional
948 file, and has one key
949 per line, though the lines can be very long.
950 After this, the user can log in without giving the password.
952 A variation on public key authentication
953 is available in the form of certificate authentication:
954 instead of a set of public/private keys,
955 signed certificates are used.
956 This has the advantage that a single trusted certification authority
957 can be used in place of many public/private keys.
958 See the CERTIFICATES section of
960 for more information.
962 The most convenient way to use public key or certificate authentication
963 may be with an authentication agent.
970 for more information.
972 Challenge-response authentication works as follows:
973 The server sends an arbitrary
975 text, and prompts for a response.
976 Examples of challenge-response authentication include
984 Finally, if other authentication methods fail,
986 prompts the user for a password.
987 The password is sent to the remote
988 host for checking; however, since all communications are encrypted,
989 the password cannot be seen by someone listening on the network.
992 automatically maintains and checks a database containing
993 identification for all hosts it has ever been used with.
994 Host keys are stored in
995 .Pa ~/.ssh/known_hosts
996 in the user's home directory.
997 Additionally, the file
998 .Pa /etc/ssh/ssh_known_hosts
999 is automatically checked for known hosts.
1000 Any new hosts are automatically added to the user's file.
1001 If a host's identification ever changes,
1003 warns about this and disables password authentication to prevent
1004 server spoofing or man-in-the-middle attacks,
1005 which could otherwise be used to circumvent the encryption.
1007 .Cm StrictHostKeyChecking
1008 option can be used to control logins to machines whose
1009 host key is not known or has changed.
1011 When the user's identity has been accepted by the server, the server
1012 either executes the given command in a non-interactive session or,
1013 if no command has been specified, logs into the machine and gives
1014 the user a normal shell as an interactive session.
1015 All communication with
1016 the remote command or shell will be automatically encrypted.
1018 If an interactive session is requested
1020 by default will only request a pseudo-terminal (pty) for interactive
1021 sessions when the client has one.
1026 can be used to override this behaviour.
1028 If a pseudo-terminal has been allocated the
1029 user may use the escape characters noted below.
1031 If no pseudo-terminal has been allocated,
1032 the session is transparent and can be used to reliably transfer binary data.
1033 On most systems, setting the escape character to
1035 will also make the session transparent even if a tty is used.
1037 The session terminates when the command or shell on the remote
1038 machine exits and all X11 and TCP connections have been closed.
1039 .Sh ESCAPE CHARACTERS
1040 When a pseudo-terminal has been requested,
1042 supports a number of functions through the use of an escape character.
1044 A single tilde character can be sent as
1046 or by following the tilde by a character other than those described below.
1047 The escape character must always follow a newline to be interpreted as
1049 The escape character can be changed in configuration files using the
1051 configuration directive or on the command line by the
1055 The supported escapes (assuming the default
1065 List forwarded connections.
1069 at logout when waiting for forwarded connection / X11 sessions to terminate.
1071 Display a list of escape characters.
1073 Send a BREAK to the remote system
1074 (only useful if the peer supports it).
1077 Currently this allows the addition of port forwardings using the
1082 options (see above).
1083 It also allows the cancellation of existing port-forwardings
1086 .Fl KL Oo Ar bind_address : Oc Ar port
1090 .Fl KR Oo Ar bind_address : Oc Ar port
1094 .Fl KD Oo Ar bind_address : Oc Ar port
1096 for dynamic port-forwardings.
1097 .Ic !\& Ns Ar command
1098 allows the user to execute a local command if the
1099 .Ic PermitLocalCommand
1100 option is enabled in
1102 Basic help is available, using the
1106 Request rekeying of the connection
1107 (only useful if the peer supports it).
1109 Decrease the verbosity
1111 when errors are being written to stderr.
1113 Increase the verbosity
1115 when errors are being written to stderr.
1118 Forwarding of arbitrary TCP connections over a secure channel
1119 can be specified either on the command line or in a configuration file.
1120 One possible application of TCP forwarding is a secure connection to a
1121 mail server; another is going through firewalls.
1123 In the example below, we look at encrypting communication for an IRC client,
1124 even though the IRC server it connects to does not directly
1125 support encrypted communication.
1126 This works as follows:
1127 the user connects to the remote host using
1129 specifying the ports to be used to forward the connection.
1130 After that it is possible to start the program locally,
1133 will encrypt and forward the connection to the remote server.
1135 The following example tunnels an IRC session from the client
1137 .Dq server.example.com ,
1142 using the standard IRC port, 6667:
1143 .Bd -literal -offset 4n
1144 $ ssh -f -L 6667:localhost:6667 server.example.com sleep 10
1145 $ irc -c '#users' pinky IRC/127.0.0.1
1152 and the remote command
1154 is specified to allow an amount of time
1155 (10 seconds, in the example)
1156 to start the program which is going to use the tunnel.
1157 If no connections are made within the time specified,
1165 (or see the description of the
1171 and the user is using X11 (the
1173 environment variable is set), the connection to the X11 display is
1174 automatically forwarded to the remote side in such a way that any X11
1175 programs started from the shell (or command) will go through the
1176 encrypted channel, and the connection to the real X server will be made
1177 from the local machine.
1178 The user should not manually set
1180 Forwarding of X11 connections can be
1181 configured on the command line or in configuration files.
1187 will point to the server machine, but with a display number greater than zero.
1188 This is normal, and happens because
1192 X server on the server machine for forwarding the
1193 connections over the encrypted channel.
1196 will also automatically set up Xauthority data on the server machine.
1197 For this purpose, it will generate a random authorization cookie,
1198 store it in Xauthority on the server, and verify that any forwarded
1199 connections carry this cookie and replace it by the real cookie when
1200 the connection is opened.
1201 The real authentication cookie is never
1202 sent to the server machine (and no cookies are sent in the plain).
1208 (or see the description of the
1213 the user is using an authentication agent, the connection to the agent
1214 is automatically forwarded to the remote side.
1215 .Sh VERIFYING HOST KEYS
1216 When connecting to a server for the first time,
1217 a fingerprint of the server's public key is presented to the user
1219 .Cm StrictHostKeyChecking
1221 Fingerprints can be determined using
1224 .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1226 If the fingerprint is already known, it can be matched
1227 and the key can be accepted or rejected.
1228 If only legacy (MD5) fingerprints for the server are available, the
1231 option may be used to downgrade the fingerprint algorithm to match.
1233 Because of the difficulty of comparing host keys
1234 just by looking at fingerprint strings,
1235 there is also support to compare host keys visually,
1242 a small ASCII graphic gets displayed on every login to a server, no matter
1243 if the session itself is interactive or not.
1244 By learning the pattern a known server produces, a user can easily
1245 find out that the host key has changed when a completely different pattern
1247 Because these patterns are not unambiguous however, a pattern that looks
1248 similar to the pattern remembered only gives a good probability that the
1249 host key is the same, not guaranteed proof.
1251 To get a listing of the fingerprints along with their random art for
1252 all known hosts, the following command line can be used:
1254 .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1256 If the fingerprint is unknown,
1257 an alternative method of verification is available:
1258 SSH fingerprints verified by DNS.
1259 An additional resource record (RR),
1261 is added to a zonefile
1262 and the connecting client is able to match the fingerprint
1263 with that of the key presented.
1265 In this example, we are connecting a client to a server,
1266 .Dq host.example.com .
1267 The SSHFP resource records should first be added to the zonefile for
1269 .Bd -literal -offset indent
1270 $ ssh-keygen -r host.example.com.
1273 The output lines will have to be added to the zonefile.
1274 To check that the zone is answering fingerprint queries:
1276 .Dl $ dig -t SSHFP host.example.com
1278 Finally the client connects:
1279 .Bd -literal -offset indent
1280 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1282 Matching host key fingerprint found in DNS.
1283 Are you sure you want to continue connecting (yes/no)?
1287 .Cm VerifyHostKeyDNS
1290 for more information.
1291 .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1293 contains support for Virtual Private Network (VPN) tunnelling
1296 network pseudo-device,
1297 allowing two networks to be joined securely.
1300 configuration option
1302 controls whether the server supports this,
1303 and at what level (layer 2 or 3 traffic).
1305 The following example would connect client network 10.0.50.0/24
1306 with remote network 10.0.99.0/24 using a point-to-point connection
1307 from 10.1.1.1 to 10.1.1.2,
1308 provided that the SSH server running on the gateway to the remote network,
1309 at 192.168.1.15, allows it.
1312 .Bd -literal -offset indent
1313 # ssh -f -w 0:1 192.168.1.15 true
1314 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1315 # route add 10.0.99.0/24 10.1.1.2
1319 .Bd -literal -offset indent
1320 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1321 # route add 10.0.50.0/24 10.1.1.1
1324 Client access may be more finely tuned via the
1325 .Pa /root/.ssh/authorized_keys
1326 file (see below) and the
1329 The following entry would permit connections on
1333 and on tun device 2 from user
1338 .Dq forced-commands-only :
1339 .Bd -literal -offset 2n
1340 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1341 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1344 Since an SSH-based setup entails a fair amount of overhead,
1345 it may be more suited to temporary setups,
1346 such as for wireless VPNs.
1347 More permanent VPNs are better provided by tools such as
1353 will normally set the following environment variables:
1354 .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1358 variable indicates the location of the X11 server.
1359 It is automatically set by
1361 to point to a value of the form
1365 indicates the host where the shell runs, and
1367 is an integer \*(Ge 1.
1369 uses this special value to forward X11 connections over the secure
1371 The user should normally not set
1374 will render the X11 connection insecure (and will require the user to
1375 manually copy any required authorization cookies).
1377 Set to the path of the user's home directory.
1381 set for compatibility with systems that use this variable.
1383 Set to the path of the user's mailbox.
1387 as specified when compiling
1392 needs a passphrase, it will read the passphrase from the current
1393 terminal if it was run from a terminal.
1396 does not have a terminal associated with it but
1400 are set, it will execute the program specified by
1402 and open an X11 window to read the passphrase.
1403 This is particularly useful when calling
1408 (Note that on some machines it
1409 may be necessary to redirect the input from
1412 .It Ev SSH_AUTH_SOCK
1413 Identifies the path of a
1415 socket used to communicate with the agent.
1416 .It Ev SSH_CONNECTION
1417 Identifies the client and server ends of the connection.
1418 The variable contains
1419 four space-separated values: client IP address, client port number,
1420 server IP address, and server port number.
1421 .It Ev SSH_ORIGINAL_COMMAND
1422 This variable contains the original command line if a forced command
1424 It can be used to extract the original arguments.
1426 This is set to the name of the tty (path to the device) associated
1427 with the current shell or command.
1428 If the current session has no tty,
1429 this variable is not set.
1433 to contain the interface names assigned if tunnel forwarding was
1434 requested by the client.
1435 .It Ev SSH_USER_AUTH
1438 this variable may contain a pathname to a file that lists the authentication
1439 methods successfully used when the session was established, including any
1440 public keys that were used.
1442 This variable is set to indicate the present time zone if it
1443 was set when the daemon was started (i.e. the daemon passes the value
1444 on to new connections).
1446 Set to the name of the user logging in.
1452 .Pa ~/.ssh/environment ,
1453 and adds lines of the format
1455 to the environment if the file exists and users are allowed to
1456 change their environment.
1457 For more information, see the
1458 .Cm PermitUserEnvironment
1462 .Bl -tag -width Ds -compact
1464 This file is used for host-based authentication (see above).
1465 On some machines this file may need to be
1466 world-readable if the user's home directory is on an NFS partition,
1470 Additionally, this file must be owned by the user,
1471 and must not have write permissions for anyone else.
1473 permission for most machines is read/write for the user, and not
1474 accessible by others.
1477 This file is used in exactly the same way as
1479 but allows host-based authentication without permitting login with
1483 This directory is the default location for all user-specific configuration
1484 and authentication information.
1485 There is no general requirement to keep the entire contents of this directory
1486 secret, but the recommended permissions are read/write/execute for the user,
1487 and not accessible by others.
1489 .It Pa ~/.ssh/authorized_keys
1490 Lists the public keys (DSA, ECDSA, Ed25519, RSA)
1491 that can be used for logging in as this user.
1492 The format of this file is described in the
1495 This file is not highly sensitive, but the recommended
1496 permissions are read/write for the user, and not accessible by others.
1498 .It Pa ~/.ssh/config
1499 This is the per-user configuration file.
1500 The file format and configuration options are described in
1502 Because of the potential for abuse, this file must have strict permissions:
1503 read/write for the user, and not writable by others.
1505 .It Pa ~/.ssh/environment
1506 Contains additional definitions for environment variables; see
1510 .It Pa ~/.ssh/id_dsa
1511 .It Pa ~/.ssh/id_ecdsa
1512 .It Pa ~/.ssh/id_ecdsa_sk
1513 .It Pa ~/.ssh/id_ed25519
1514 .It Pa ~/.ssh/id_ed25519_sk
1515 .It Pa ~/.ssh/id_rsa
1516 Contains the private key for authentication.
1518 contain sensitive data and should be readable by the user but not
1519 accessible by others (read/write/execute).
1521 will simply ignore a private key file if it is accessible by others.
1522 It is possible to specify a passphrase when
1523 generating the key which will be used to encrypt the
1524 sensitive part of this file using AES-128.
1526 .It Pa ~/.ssh/id_dsa.pub
1527 .It Pa ~/.ssh/id_ecdsa.pub
1528 .It Pa ~/.ssh/id_ecdsa_sk.pub
1529 .It Pa ~/.ssh/id_ed25519.pub
1530 .It Pa ~/.ssh/id_ed25519_sk.pub
1531 .It Pa ~/.ssh/id_rsa.pub
1532 Contains the public key for authentication.
1534 sensitive and can (but need not) be readable by anyone.
1536 .It Pa ~/.ssh/known_hosts
1537 Contains a list of host keys for all hosts the user has logged into
1538 that are not already in the systemwide list of known host keys.
1541 for further details of the format of this file.
1544 Commands in this file are executed by
1546 when the user logs in, just before the user's shell (or command) is
1550 manual page for more information.
1552 .It Pa /etc/hosts.equiv
1553 This file is for host-based authentication (see above).
1554 It should only be writable by root.
1556 .It Pa /etc/shosts.equiv
1557 This file is used in exactly the same way as
1559 but allows host-based authentication without permitting login with
1562 .It Pa /etc/ssh/ssh_config
1563 Systemwide configuration file.
1564 The file format and configuration options are described in
1567 .It Pa /etc/ssh/ssh_host_key
1568 .It Pa /etc/ssh/ssh_host_dsa_key
1569 .It Pa /etc/ssh/ssh_host_ecdsa_key
1570 .It Pa /etc/ssh/ssh_host_ed25519_key
1571 .It Pa /etc/ssh/ssh_host_rsa_key
1572 These files contain the private parts of the host keys
1573 and are used for host-based authentication.
1575 .It Pa /etc/ssh/ssh_known_hosts
1576 Systemwide list of known host keys.
1577 This file should be prepared by the
1578 system administrator to contain the public host keys of all machines in the
1580 It should be world-readable.
1583 for further details of the format of this file.
1585 .It Pa /etc/ssh/sshrc
1586 Commands in this file are executed by
1588 when the user logs in, just before the user's shell (or command) is started.
1591 manual page for more information.
1595 exits with the exit status of the remote command or with 255
1596 if an error occurred.
1614 .%T The Secure Shell (SSH) Protocol Assigned Numbers
1622 .%T The Secure Shell (SSH) Protocol Architecture
1630 .%T The Secure Shell (SSH) Authentication Protocol
1638 .%T The Secure Shell (SSH) Transport Layer Protocol
1646 .%T The Secure Shell (SSH) Connection Protocol
1654 .%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
1662 .%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
1670 .%T The Secure Shell (SSH) Session Channel Break Extension
1679 .%T The Secure Shell (SSH) Transport Layer Encryption Modes
1686 .%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
1695 .%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
1703 .%T The Secure Shell (SSH) Public Key File Format
1711 .%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
1718 .%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
1719 .%T Hash Visualization: a New Technique to improve Real-World Security
1722 OpenSSH is a derivative of the original and free
1723 ssh 1.2.12 release by Tatu Ylonen.
1724 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1725 Theo de Raadt and Dug Song
1726 removed many bugs, re-added newer features and
1728 Markus Friedl contributed the support for SSH
1729 protocol versions 1.5 and 2.0.
projects / dragonfly.git / blob