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 1.402 2019/03/16 19:14:21 jmc Exp $
37 .Dd $Mdocdate: March 16 2019 $
42 .Nd OpenSSH SSH client (remote login program)
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 the authentication agent connection.
114 This can also be specified on a per-host basis in a configuration file.
116 Agent forwarding should be enabled with caution.
117 Users with the ability to bypass file permissions on the remote host
120 socket) can access the local agent through the forwarded connection.
121 An attacker cannot obtain key material from the agent,
122 however they can perform operations on the keys that enable them to
123 authenticate using the identities loaded into the agent.
126 Disables forwarding of the authentication agent connection.
128 .It Fl B Ar bind_interface
129 Bind to the address of
131 before attempting to connect to the destination host.
132 This is only useful on systems with more than one address.
134 .It Fl b Ar bind_address
137 on the local machine as the source address
139 Only useful on systems with more than one address.
142 Requests compression of all data (including stdin, stdout, stderr, and
143 data for forwarded X11, TCP and
146 The compression algorithm is the same used by
148 Compression is desirable on modem lines and other
149 slow connections, but will only slow down things on fast networks.
150 The default value can be set on a host-by-host basis in the
151 configuration files; see the
155 .It Fl c Ar cipher_spec
156 Selects the cipher specification for encrypting the session.
158 is a comma-separated list of ciphers
159 listed in order of preference.
164 for more information.
168 .Oo Ar bind_address : Oc
174 application-level port forwarding.
175 This works by allocating a socket to listen to
177 on the local side, optionally bound to the specified
179 Whenever a connection is made to this port, the
180 connection is forwarded over the secure channel, and the application
181 protocol is then used to determine where to connect to from the
183 Currently the SOCKS4 and SOCKS5 protocols are supported, and
185 will act as a SOCKS server.
186 Only root can forward privileged ports.
187 Dynamic port forwardings can also be specified in the configuration file.
189 IPv6 addresses can be specified by enclosing the address in square brackets.
190 Only the superuser can forward privileged ports.
191 By default, the local port is bound in accordance with the
196 may be used to bind the connection to a specific address.
201 indicates that the listening port be bound for local use only, while an
204 indicates that the port should be available from all interfaces.
209 instead of standard error.
211 .It Fl e Ar escape_char
212 Sets the escape character for sessions with a pty (default:
214 The escape character is only recognized at the beginning of a line.
215 The escape character followed by a dot
217 closes the connection;
218 followed by control-Z suspends the connection;
219 and followed by itself sends the escape character once.
220 Setting the character to
222 disables any escapes and makes the session fully transparent.
224 .It Fl F Ar configfile
225 Specifies an alternative per-user configuration file.
226 If a configuration file is given on the command line,
227 the system-wide configuration file
228 .Pq Pa /etc/ssh/ssh_config
230 The default for the per-user configuration file is
236 to go to background just before command execution.
239 is going to ask for passwords or passphrases, but the user
240 wants it in the background.
243 The recommended way to start X11 programs at a remote site is with
245 .Ic ssh -f host xterm .
248 .Cm ExitOnForwardFailure
249 configuration option is set to
251 then a client started with
253 will wait for all remote port forwards to be successfully established
254 before placing itself in the background.
259 to print its configuration after evaluating
266 Allows remote hosts to connect to local forwarded ports.
267 If used on a multiplexed connection, then this option must be specified
268 on the master process.
271 Specify the PKCS#11 shared library
273 should use to communicate with a PKCS#11 token providing keys for user
276 .It Fl i Ar identity_file
277 Selects a file from which the identity (private key) for
278 public key authentication is read.
281 .Pa ~/.ssh/id_ecdsa ,
282 .Pa ~/.ssh/id_ed25519
285 Identity files may also be specified on
286 a per-host basis in the configuration file.
287 It is possible to have multiple
289 options (and multiple identities specified in
290 configuration files).
291 If no certificates have been explicitly specified by the
295 will also try to load certificate information from the filename obtained
298 to identity filenames.
300 .It Fl J Ar destination
301 Connect to the target host by first making a
303 connection to the jump host described by
305 and then establishing a TCP forwarding to the ultimate destination from
307 Multiple jump hops may be specified separated by comma characters.
308 This is a shortcut to specify a
310 configuration directive.
311 Note that configuration directives supplied on the command-line generally
312 apply to the destination host and not any specified jump hosts.
315 to specify configuration for jump hosts.
318 Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
319 credentials to the server.
322 Disables forwarding (delegation) of GSSAPI credentials to the server.
326 .Oo Ar bind_address : Oc
327 .Ar port : host : hostport
332 .Oo Ar bind_address : Oc
333 .Ar port : remote_socket
338 .Ar local_socket : host : hostport
343 .Ar local_socket : remote_socket
346 Specifies that connections to the given TCP port or Unix socket on the local
347 (client) host are to be forwarded to the given host and port, or Unix socket,
349 This works by allocating a socket to listen to either a TCP
351 on the local side, optionally bound to the specified
354 Whenever a connection is made to the local port or socket, the
355 connection is forwarded over the secure channel, and a connection is
362 from the remote machine.
364 Port forwardings can also be specified in the configuration file.
365 Only the superuser can forward privileged ports.
366 IPv6 addresses can be specified by enclosing the address in square brackets.
368 By default, the local port is bound in accordance with the
373 may be used to bind the connection to a specific address.
378 indicates that the listening port be bound for local use only, while an
381 indicates that the port should be available from all interfaces.
383 .It Fl l Ar login_name
384 Specifies the user to log in as on the remote machine.
385 This also may be specified on a per-host basis in the configuration file.
392 mode for connection sharing.
399 mode but with confirmation required using
401 before each operation that changes the multiplexing state
402 (e.g. opening a new session).
403 Refer to the description of
410 A comma-separated list of MAC (message authentication code) algorithms,
411 specified in order of preference.
414 keyword for more information.
417 Do not execute a remote command.
418 This is useful for just forwarding ports.
423 (actually, prevents reading from stdin).
424 This must be used when
426 is run in the background.
427 A common trick is to use this to run X11 programs on a remote machine.
429 .Ic ssh -n shadows.cs.hut.fi emacs &
430 will start an emacs on shadows.cs.hut.fi, and the X11
431 connection will be automatically forwarded over an encrypted channel.
434 program will be put in the background.
435 (This does not work if
437 needs to ask for a password or passphrase; see also the
442 Control an active connection multiplexing master process.
445 option is specified, the
447 argument is interpreted and passed to the master process.
450 (check that the master process is running),
452 (request forwardings without command execution),
454 (cancel forwardings),
456 (request the master to exit), and
458 (request the master to stop accepting further multiplexing requests).
461 Can be used to give options in the format used in the configuration file.
462 This is useful for specifying options for which there is no separate
464 For full details of the options listed below, and their possible values, see
467 .Bl -tag -width Ds -offset indent -compact
473 .It CanonicalizeFallbackLocal
474 .It CanonicalizeHostname
475 .It CanonicalizeMaxDots
476 .It CanonicalizePermittedCNAMEs
477 .It CASignatureAlgorithms
479 .It ChallengeResponseAuthentication
482 .It ClearAllForwardings
484 .It ConnectionAttempts
491 .It ExitOnForwardFailure
495 .It ForwardX11Timeout
496 .It ForwardX11Trusted
498 .It GlobalKnownHostsFile
499 .It GSSAPIAuthentication
500 .It GSSAPIDelegateCredentials
503 .It HostbasedAuthentication
504 .It HostbasedKeyTypes
505 .It HostKeyAlgorithms
512 .It KbdInteractiveAuthentication
513 .It KbdInteractiveDevices
520 .It NoHostAuthenticationForLocalhost
521 .It NumberOfPasswordPrompts
522 .It PasswordAuthentication
523 .It PermitLocalCommand
526 .It PreferredAuthentications
530 .It PubkeyAcceptedKeyTypes
531 .It PubkeyAuthentication
537 .It ServerAliveInterval
538 .It ServerAliveCountMax
540 .It StreamLocalBindMask
541 .It StreamLocalBindUnlink
542 .It StrictHostKeyChecking
548 .It UserKnownHostsFile
555 Port to connect to on the remote host.
556 This can be specified on a
557 per-host basis in the configuration file.
559 .It Fl Q Ar query_option
562 for the algorithms supported for the specified version 2.
563 The available features are:
565 (supported symmetric ciphers),
567 (supported symmetric ciphers that support authenticated encryption),
569 (supported query terms for use with the
573 (supported message integrity codes),
575 (key exchange algorithms),
579 (certificate key types),
581 (non-certificate key types),
583 (supported SSH protocol versions), and
585 (supported signature algorithms).
589 Causes most warning and diagnostic messages to be suppressed.
593 .Oo Ar bind_address : Oc
594 .Ar port : host : hostport
599 .Oo Ar bind_address : Oc
600 .Ar port : local_socket
605 .Ar remote_socket : host : hostport
610 .Ar remote_socket : local_socket
615 .Oo Ar bind_address : Oc
619 Specifies that connections to the given TCP port or Unix socket on the remote
620 (server) host are to be forwarded to the local side.
622 This works by allocating a socket to listen to either a TCP
624 or to a Unix socket on the remote side.
625 Whenever a connection is made to this port or Unix socket, the
626 connection is forwarded over the secure channel, and a connection
627 is made from the local machine to either an explicit destination specified by
633 or, if no explicit destination was specified,
635 will act as a SOCKS 4/5 proxy and forward connections to the destinations
636 requested by the remote SOCKS client.
638 Port forwardings can also be specified in the configuration file.
639 Privileged ports can be forwarded only when
640 logging in as root on the remote machine.
641 IPv6 addresses can be specified by enclosing the address in square brackets.
643 By default, TCP listening sockets on the server will be bound to the loopback
645 This may be overridden by specifying a
651 indicates that the remote socket should listen on all interfaces.
654 will only succeed if the server's
656 option is enabled (see
657 .Xr sshd_config 5 ) .
663 the listen port will be dynamically allocated on the server and reported
664 to the client at run time.
665 When used together with
667 the allocated port will be printed to the standard output.
670 Specifies the location of a control socket for connection sharing,
673 to disable connection sharing.
674 Refer to the description of
683 May be used to request invocation of a subsystem on the remote system.
684 Subsystems facilitate the use of SSH
685 as a secure transport for other applications (e.g.\&
687 The subsystem is specified as the remote command.
690 Disable pseudo-terminal allocation.
693 Force pseudo-terminal allocation.
694 This can be used to execute arbitrary
695 screen-based programs on a remote machine, which can be very useful,
696 e.g. when implementing menu services.
699 options force tty allocation, even if
704 Display the version number and exit.
710 to print debugging messages about its progress.
712 debugging connection, authentication, and configuration problems.
715 options increase the verbosity.
718 .It Fl W Ar host : Ns Ar port
719 Requests that standard input and output on the client be forwarded to
723 over the secure channel.
727 .Cm ExitOnForwardFailure
729 .Cm ClearAllForwardings ,
730 though these can be overridden in the configuration file or using
732 command line options.
735 .Ar local_tun Ns Op : Ns Ar remote_tun
739 device forwarding with the specified
741 devices between the client
746 The devices may be specified by numerical ID or the keyword
748 which uses the next available tunnel device.
751 is not specified, it defaults to
762 directive is unset, it will be set to the default tunnel mode, which is
766 forwarding mode it desired, then it should be specified before
770 Enables X11 forwarding.
771 This can also be specified on a per-host basis in a configuration file.
773 X11 forwarding should be enabled with caution.
774 Users with the ability to bypass file permissions on the remote host
775 (for the user's X authorization database)
776 can access the local X11 display through the forwarded connection.
777 An attacker may then be able to perform activities such as keystroke monitoring.
779 For this reason, X11 forwarding is subjected to X11 SECURITY extension
780 restrictions by default.
785 .Cm ForwardX11Trusted
788 for more information.
791 Disables X11 forwarding.
794 Enables trusted X11 forwarding.
795 Trusted X11 forwardings are not subjected to the X11 SECURITY extension
799 Send log information using the
802 By default this information is sent to stderr.
806 may additionally obtain configuration data from
807 a per-user configuration file and a system-wide configuration file.
808 The file format and configuration options are described in
811 The OpenSSH SSH client supports SSH protocol 2.
813 The methods available for authentication are:
814 GSSAPI-based authentication,
815 host-based authentication,
816 public key authentication,
817 challenge-response authentication,
818 and password authentication.
819 Authentication methods are tried in the order specified above,
821 .Cm PreferredAuthentications
822 can be used to change the default order.
824 Host-based authentication works as follows:
825 If the machine the user logs in from is listed in
828 .Pa /etc/shosts.equiv
829 on the remote machine, and the user names are
830 the same on both sides, or if the files
834 exist in the user's home directory on the
835 remote machine and contain a line containing the name of the client
836 machine and the name of the user on that machine, the user is
837 considered for login.
838 Additionally, the server
840 be able to verify the client's
841 host key (see the description of
842 .Pa /etc/ssh/ssh_known_hosts
844 .Pa ~/.ssh/known_hosts ,
846 for login to be permitted.
847 This authentication method closes security holes due to IP
848 spoofing, DNS spoofing, and routing spoofing.
849 [Note to the administrator:
850 .Pa /etc/hosts.equiv ,
852 and the rlogin/rsh protocol in general, are inherently insecure and should be
853 disabled if security is desired.]
855 Public key authentication works as follows:
856 The scheme is based on public-key cryptography,
858 where encryption and decryption are done using separate keys,
859 and it is unfeasible to derive the decryption key from the encryption key.
860 The idea is that each user creates a public/private
861 key pair for authentication purposes.
862 The server knows the public key, and only the user knows the private key.
864 implements public key authentication protocol automatically,
865 using one of the DSA, ECDSA, Ed25519 or RSA algorithms.
866 The HISTORY section of
868 contains a brief discussion of the DSA and RSA algorithms.
871 .Pa ~/.ssh/authorized_keys
872 lists the public keys that are permitted for logging in.
873 When the user logs in, the
875 program tells the server which key pair it would like to use for
877 The client proves that it has access to the private key
878 and the server checks that the corresponding public key
879 is authorized to accept the account.
881 The server may inform the client of errors that prevented public key
882 authentication from succeeding after authentication completes using a
884 These may be viewed by increasing the
888 or higher (e.g. by using the
892 The user creates his/her key pair by running
894 This stores the private key in
899 .Pa ~/.ssh/id_ed25519
904 and stores the public key in
905 .Pa ~/.ssh/id_dsa.pub
907 .Pa ~/.ssh/id_ecdsa.pub
909 .Pa ~/.ssh/id_ed25519.pub
912 .Pa ~/.ssh/id_rsa.pub
914 in the user's home directory.
915 The user should then copy the public key
917 .Pa ~/.ssh/authorized_keys
918 in his/her home directory on the remote machine.
921 file corresponds to the conventional
923 file, and has one key
924 per line, though the lines can be very long.
925 After this, the user can log in without giving the password.
927 A variation on public key authentication
928 is available in the form of certificate authentication:
929 instead of a set of public/private keys,
930 signed certificates are used.
931 This has the advantage that a single trusted certification authority
932 can be used in place of many public/private keys.
933 See the CERTIFICATES section of
935 for more information.
937 The most convenient way to use public key or certificate authentication
938 may be with an authentication agent.
945 for more information.
947 Challenge-response authentication works as follows:
948 The server sends an arbitrary
950 text, and prompts for a response.
951 Examples of challenge-response authentication include
959 Finally, if other authentication methods fail,
961 prompts the user for a password.
962 The password is sent to the remote
963 host for checking; however, since all communications are encrypted,
964 the password cannot be seen by someone listening on the network.
967 automatically maintains and checks a database containing
968 identification for all hosts it has ever been used with.
969 Host keys are stored in
970 .Pa ~/.ssh/known_hosts
971 in the user's home directory.
972 Additionally, the file
973 .Pa /etc/ssh/ssh_known_hosts
974 is automatically checked for known hosts.
975 Any new hosts are automatically added to the user's file.
976 If a host's identification ever changes,
978 warns about this and disables password authentication to prevent
979 server spoofing or man-in-the-middle attacks,
980 which could otherwise be used to circumvent the encryption.
982 .Cm StrictHostKeyChecking
983 option can be used to control logins to machines whose
984 host key is not known or has changed.
986 When the user's identity has been accepted by the server, the server
987 either executes the given command in a non-interactive session or,
988 if no command has been specified, logs into the machine and gives
989 the user a normal shell as an interactive session.
990 All communication with
991 the remote command or shell will be automatically encrypted.
993 If an interactive session is requested
995 by default will only request a pseudo-terminal (pty) for interactive
996 sessions when the client has one.
1001 can be used to override this behaviour.
1003 If a pseudo-terminal has been allocated the
1004 user may use the escape characters noted below.
1006 If no pseudo-terminal has been allocated,
1007 the session is transparent and can be used to reliably transfer binary data.
1008 On most systems, setting the escape character to
1010 will also make the session transparent even if a tty is used.
1012 The session terminates when the command or shell on the remote
1013 machine exits and all X11 and TCP connections have been closed.
1014 .Sh ESCAPE CHARACTERS
1015 When a pseudo-terminal has been requested,
1017 supports a number of functions through the use of an escape character.
1019 A single tilde character can be sent as
1021 or by following the tilde by a character other than those described below.
1022 The escape character must always follow a newline to be interpreted as
1024 The escape character can be changed in configuration files using the
1026 configuration directive or on the command line by the
1030 The supported escapes (assuming the default
1040 List forwarded connections.
1044 at logout when waiting for forwarded connection / X11 sessions to terminate.
1046 Display a list of escape characters.
1048 Send a BREAK to the remote system
1049 (only useful if the peer supports it).
1052 Currently this allows the addition of port forwardings using the
1057 options (see above).
1058 It also allows the cancellation of existing port-forwardings
1061 .Fl KL Oo Ar bind_address : Oc Ar port
1065 .Fl KR Oo Ar bind_address : Oc Ar port
1069 .Fl KD Oo Ar bind_address : Oc Ar port
1071 for dynamic port-forwardings.
1072 .Ic !\& Ns Ar command
1073 allows the user to execute a local command if the
1074 .Ic PermitLocalCommand
1075 option is enabled in
1077 Basic help is available, using the
1081 Request rekeying of the connection
1082 (only useful if the peer supports it).
1084 Decrease the verbosity
1086 when errors are being written to stderr.
1088 Increase the verbosity
1090 when errors are being written to stderr.
1093 Forwarding of arbitrary TCP connections over a secure channel
1094 can be specified either on the command line or in a configuration file.
1095 One possible application of TCP forwarding is a secure connection to a
1096 mail server; another is going through firewalls.
1098 In the example below, we look at encrypting communication for an IRC client,
1099 even though the IRC server it connects to does not directly
1100 support encrypted communication.
1101 This works as follows:
1102 the user connects to the remote host using
1104 specifying the ports to be used to forward the connection.
1105 After that it is possible to start the program locally,
1108 will encrypt and forward the connection to the remote server.
1110 The following example tunnels an IRC session from the client
1112 .Dq server.example.com ,
1117 using the standard IRC port, 6667:
1118 .Bd -literal -offset 4n
1119 $ ssh -f -L 6667:localhost:6667 server.example.com sleep 10
1120 $ irc -c '#users' pinky IRC/127.0.0.1
1127 and the remote command
1129 is specified to allow an amount of time
1130 (10 seconds, in the example)
1131 to start the program which is going to use the tunnel.
1132 If no connections are made within the time specified,
1140 (or see the description of the
1146 and the user is using X11 (the
1148 environment variable is set), the connection to the X11 display is
1149 automatically forwarded to the remote side in such a way that any X11
1150 programs started from the shell (or command) will go through the
1151 encrypted channel, and the connection to the real X server will be made
1152 from the local machine.
1153 The user should not manually set
1155 Forwarding of X11 connections can be
1156 configured on the command line or in configuration files.
1162 will point to the server machine, but with a display number greater than zero.
1163 This is normal, and happens because
1167 X server on the server machine for forwarding the
1168 connections over the encrypted channel.
1171 will also automatically set up Xauthority data on the server machine.
1172 For this purpose, it will generate a random authorization cookie,
1173 store it in Xauthority on the server, and verify that any forwarded
1174 connections carry this cookie and replace it by the real cookie when
1175 the connection is opened.
1176 The real authentication cookie is never
1177 sent to the server machine (and no cookies are sent in the plain).
1183 (or see the description of the
1188 the user is using an authentication agent, the connection to the agent
1189 is automatically forwarded to the remote side.
1190 .Sh VERIFYING HOST KEYS
1191 When connecting to a server for the first time,
1192 a fingerprint of the server's public key is presented to the user
1194 .Cm StrictHostKeyChecking
1196 Fingerprints can be determined using
1199 .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1201 If the fingerprint is already known, it can be matched
1202 and the key can be accepted or rejected.
1203 If only legacy (MD5) fingerprints for the server are available, the
1206 option may be used to downgrade the fingerprint algorithm to match.
1208 Because of the difficulty of comparing host keys
1209 just by looking at fingerprint strings,
1210 there is also support to compare host keys visually,
1217 a small ASCII graphic gets displayed on every login to a server, no matter
1218 if the session itself is interactive or not.
1219 By learning the pattern a known server produces, a user can easily
1220 find out that the host key has changed when a completely different pattern
1222 Because these patterns are not unambiguous however, a pattern that looks
1223 similar to the pattern remembered only gives a good probability that the
1224 host key is the same, not guaranteed proof.
1226 To get a listing of the fingerprints along with their random art for
1227 all known hosts, the following command line can be used:
1229 .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1231 If the fingerprint is unknown,
1232 an alternative method of verification is available:
1233 SSH fingerprints verified by DNS.
1234 An additional resource record (RR),
1236 is added to a zonefile
1237 and the connecting client is able to match the fingerprint
1238 with that of the key presented.
1240 In this example, we are connecting a client to a server,
1241 .Dq host.example.com .
1242 The SSHFP resource records should first be added to the zonefile for
1244 .Bd -literal -offset indent
1245 $ ssh-keygen -r host.example.com.
1248 The output lines will have to be added to the zonefile.
1249 To check that the zone is answering fingerprint queries:
1251 .Dl $ dig -t SSHFP host.example.com
1253 Finally the client connects:
1254 .Bd -literal -offset indent
1255 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1257 Matching host key fingerprint found in DNS.
1258 Are you sure you want to continue connecting (yes/no)?
1262 .Cm VerifyHostKeyDNS
1265 for more information.
1266 .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1268 contains support for Virtual Private Network (VPN) tunnelling
1271 network pseudo-device,
1272 allowing two networks to be joined securely.
1275 configuration option
1277 controls whether the server supports this,
1278 and at what level (layer 2 or 3 traffic).
1280 The following example would connect client network 10.0.50.0/24
1281 with remote network 10.0.99.0/24 using a point-to-point connection
1282 from 10.1.1.1 to 10.1.1.2,
1283 provided that the SSH server running on the gateway to the remote network,
1284 at 192.168.1.15, allows it.
1287 .Bd -literal -offset indent
1288 # ssh -f -w 0:1 192.168.1.15 true
1289 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1290 # route add 10.0.99.0/24 10.1.1.2
1294 .Bd -literal -offset indent
1295 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1296 # route add 10.0.50.0/24 10.1.1.1
1299 Client access may be more finely tuned via the
1300 .Pa /root/.ssh/authorized_keys
1301 file (see below) and the
1304 The following entry would permit connections on
1308 and on tun device 2 from user
1313 .Dq forced-commands-only :
1314 .Bd -literal -offset 2n
1315 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1316 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1319 Since an SSH-based setup entails a fair amount of overhead,
1320 it may be more suited to temporary setups,
1321 such as for wireless VPNs.
1322 More permanent VPNs are better provided by tools such as
1328 will normally set the following environment variables:
1329 .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1333 variable indicates the location of the X11 server.
1334 It is automatically set by
1336 to point to a value of the form
1340 indicates the host where the shell runs, and
1342 is an integer \*(Ge 1.
1344 uses this special value to forward X11 connections over the secure
1346 The user should normally not set
1349 will render the X11 connection insecure (and will require the user to
1350 manually copy any required authorization cookies).
1352 Set to the path of the user's home directory.
1356 set for compatibility with systems that use this variable.
1358 Set to the path of the user's mailbox.
1362 as specified when compiling
1367 needs a passphrase, it will read the passphrase from the current
1368 terminal if it was run from a terminal.
1371 does not have a terminal associated with it but
1375 are set, it will execute the program specified by
1377 and open an X11 window to read the passphrase.
1378 This is particularly useful when calling
1383 (Note that on some machines it
1384 may be necessary to redirect the input from
1387 .It Ev SSH_AUTH_SOCK
1388 Identifies the path of a
1390 socket used to communicate with the agent.
1391 .It Ev SSH_CONNECTION
1392 Identifies the client and server ends of the connection.
1393 The variable contains
1394 four space-separated values: client IP address, client port number,
1395 server IP address, and server port number.
1396 .It Ev SSH_ORIGINAL_COMMAND
1397 This variable contains the original command line if a forced command
1399 It can be used to extract the original arguments.
1401 This is set to the name of the tty (path to the device) associated
1402 with the current shell or command.
1403 If the current session has no tty,
1404 this variable is not set.
1408 to contain the interface names assigned if tunnel forwarding was
1409 requested by the client.
1410 .It Ev SSH_USER_AUTH
1413 this variable may contain a pathname to a file that lists the authentication
1414 methods successfully used when the session was established, including any
1415 public keys that were used.
1417 This variable is set to indicate the present time zone if it
1418 was set when the daemon was started (i.e. the daemon passes the value
1419 on to new connections).
1421 Set to the name of the user logging in.
1427 .Pa ~/.ssh/environment ,
1428 and adds lines of the format
1430 to the environment if the file exists and users are allowed to
1431 change their environment.
1432 For more information, see the
1433 .Cm PermitUserEnvironment
1437 .Bl -tag -width Ds -compact
1439 This file is used for host-based authentication (see above).
1440 On some machines this file may need to be
1441 world-readable if the user's home directory is on an NFS partition,
1445 Additionally, this file must be owned by the user,
1446 and must not have write permissions for anyone else.
1448 permission for most machines is read/write for the user, and not
1449 accessible by others.
1452 This file is used in exactly the same way as
1454 but allows host-based authentication without permitting login with
1458 This directory is the default location for all user-specific configuration
1459 and authentication information.
1460 There is no general requirement to keep the entire contents of this directory
1461 secret, but the recommended permissions are read/write/execute for the user,
1462 and not accessible by others.
1464 .It Pa ~/.ssh/authorized_keys
1465 Lists the public keys (DSA, ECDSA, Ed25519, RSA)
1466 that can be used for logging in as this user.
1467 The format of this file is described in the
1470 This file is not highly sensitive, but the recommended
1471 permissions are read/write for the user, and not accessible by others.
1473 .It Pa ~/.ssh/config
1474 This is the per-user configuration file.
1475 The file format and configuration options are described in
1477 Because of the potential for abuse, this file must have strict permissions:
1478 read/write for the user, and not writable by others.
1480 .It Pa ~/.ssh/environment
1481 Contains additional definitions for environment variables; see
1485 .It Pa ~/.ssh/id_dsa
1486 .It Pa ~/.ssh/id_ecdsa
1487 .It Pa ~/.ssh/id_ed25519
1488 .It Pa ~/.ssh/id_rsa
1489 Contains the private key for authentication.
1491 contain sensitive data and should be readable by the user but not
1492 accessible by others (read/write/execute).
1494 will simply ignore a private key file if it is accessible by others.
1495 It is possible to specify a passphrase when
1496 generating the key which will be used to encrypt the
1497 sensitive part of this file using AES-128.
1499 .It Pa ~/.ssh/id_dsa.pub
1500 .It Pa ~/.ssh/id_ecdsa.pub
1501 .It Pa ~/.ssh/id_ed25519.pub
1502 .It Pa ~/.ssh/id_rsa.pub
1503 Contains the public key for authentication.
1505 sensitive and can (but need not) be readable by anyone.
1507 .It Pa ~/.ssh/known_hosts
1508 Contains a list of host keys for all hosts the user has logged into
1509 that are not already in the systemwide list of known host keys.
1512 for further details of the format of this file.
1515 Commands in this file are executed by
1517 when the user logs in, just before the user's shell (or command) is
1521 manual page for more information.
1523 .It Pa /etc/hosts.equiv
1524 This file is for host-based authentication (see above).
1525 It should only be writable by root.
1527 .It Pa /etc/shosts.equiv
1528 This file is used in exactly the same way as
1530 but allows host-based authentication without permitting login with
1533 .It Pa /etc/ssh/ssh_config
1534 Systemwide configuration file.
1535 The file format and configuration options are described in
1538 .It Pa /etc/ssh/ssh_host_key
1539 .It Pa /etc/ssh/ssh_host_dsa_key
1540 .It Pa /etc/ssh/ssh_host_ecdsa_key
1541 .It Pa /etc/ssh/ssh_host_ed25519_key
1542 .It Pa /etc/ssh/ssh_host_rsa_key
1543 These files contain the private parts of the host keys
1544 and are used for host-based authentication.
1546 .It Pa /etc/ssh/ssh_known_hosts
1547 Systemwide list of known host keys.
1548 This file should be prepared by the
1549 system administrator to contain the public host keys of all machines in the
1551 It should be world-readable.
1554 for further details of the format of this file.
1556 .It Pa /etc/ssh/sshrc
1557 Commands in this file are executed by
1559 when the user logs in, just before the user's shell (or command) is started.
1562 manual page for more information.
1566 exits with the exit status of the remote command or with 255
1567 if an error occurred.
1585 .%T The Secure Shell (SSH) Protocol Assigned Numbers
1593 .%T The Secure Shell (SSH) Protocol Architecture
1601 .%T The Secure Shell (SSH) Authentication Protocol
1609 .%T The Secure Shell (SSH) Transport Layer Protocol
1617 .%T The Secure Shell (SSH) Connection Protocol
1625 .%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
1633 .%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
1641 .%T The Secure Shell (SSH) Session Channel Break Extension
1650 .%T The Secure Shell (SSH) Transport Layer Encryption Modes
1657 .%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
1666 .%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
1674 .%T The Secure Shell (SSH) Public Key File Format
1682 .%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
1689 .%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
1690 .%T Hash Visualization: a New Technique to improve Real-World Security
1693 OpenSSH is a derivative of the original and free
1694 ssh 1.2.12 release by Tatu Ylonen.
1695 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1696 Theo de Raadt and Dug Song
1697 removed many bugs, re-added newer features and
1699 Markus Friedl contributed the support for SSH
1700 protocol versions 1.5 and 2.0.
projects / dragonfly.git / blob