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 1.369 2016/02/17 07:38:19 jmc Exp $
38 .Dd $Mdocdate: February 17 2016 $
43 .Nd OpenSSH SSH client (remote login program)
47 .Op Fl 1246AaCfGgKkMNnqsTtVvXxYy
48 .Op Fl b Ar bind_address
49 .Op Fl c Ar cipher_spec
50 .Op Fl D Oo Ar bind_address : Oc Ns Ar port
52 .Op Fl e Ar escape_char
53 .Op Fl F Ar configfile
55 .Op Fl i Ar identity_file
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
67 .Oo Ar user Ns @ Oc Ns Ar hostname
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
87 his/her identity to the remote machine using one of several methods
93 it is executed on the remote host instead of a login shell.
95 The options are as follows:
97 .Bl -tag -width Ds -compact
101 to try protocol version 1 only.
106 to try protocol version 2 only.
111 to use IPv4 addresses only.
116 to use IPv6 addresses only.
119 Enables forwarding of the authentication agent connection.
120 This can also be specified on a per-host basis in a configuration file.
122 Agent forwarding should be enabled with caution.
123 Users with the ability to bypass file permissions on the remote host
126 socket) can access the local agent through the forwarded connection.
127 An attacker cannot obtain key material from the agent,
128 however they can perform operations on the keys that enable them to
129 authenticate using the identities loaded into the agent.
132 Disables forwarding of the authentication agent connection.
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
150 can be controlled by the
152 option for protocol version 1.
153 Compression is desirable on modem lines and other
154 slow connections, but will only slow down things on fast networks.
155 The default value can be set on a host-by-host basis in the
156 configuration files; see the
160 .It Fl c Ar cipher_spec
161 Selects the cipher specification for encrypting the session.
163 Protocol version 1 allows specification of a single cipher.
164 The supported values are
169 For protocol version 2,
171 is a comma-separated list of ciphers
172 listed in order of preference.
177 for more information.
181 .Oo Ar bind_address : Oc
187 application-level port forwarding.
188 This works by allocating a socket to listen to
190 on the local side, optionally bound to the specified
192 Whenever a connection is made to this port, the
193 connection is forwarded over the secure channel, and the application
194 protocol is then used to determine where to connect to from the
196 Currently the SOCKS4 and SOCKS5 protocols are supported, and
198 will act as a SOCKS server.
199 Only root can forward privileged ports.
200 Dynamic port forwardings can also be specified in the configuration file.
202 IPv6 addresses can be specified by enclosing the address in square brackets.
203 Only the superuser can forward privileged ports.
204 By default, the local port is bound in accordance with the
209 may be used to bind the connection to a specific address.
214 indicates that the listening port be bound for local use only, while an
217 indicates that the port should be available from all interfaces.
222 instead of standard error.
224 .It Fl e Ar escape_char
225 Sets the escape character for sessions with a pty (default:
227 The escape character is only recognized at the beginning of a line.
228 The escape character followed by a dot
230 closes the connection;
231 followed by control-Z suspends the connection;
232 and followed by itself sends the escape character once.
233 Setting the character to
235 disables any escapes and makes the session fully transparent.
237 .It Fl F Ar configfile
238 Specifies an alternative per-user configuration file.
239 If a configuration file is given on the command line,
240 the system-wide configuration file
241 .Pq Pa /etc/ssh/ssh_config
243 The default for the per-user configuration file is
249 to go to background just before command execution.
252 is going to ask for passwords or passphrases, but the user
253 wants it in the background.
256 The recommended way to start X11 programs at a remote site is with
258 .Ic ssh -f host xterm .
261 .Cm ExitOnForwardFailure
262 configuration option is set to
264 then a client started with
266 will wait for all remote port forwards to be successfully established
267 before placing itself in the background.
272 to print its configuration after evaluating
279 Allows remote hosts to connect to local forwarded ports.
280 If used on a multiplexed connection, then this option must be specified
281 on the master process.
284 Specify the PKCS#11 shared library
286 should use to communicate with a PKCS#11 token providing the user's
289 .It Fl i Ar identity_file
290 Selects a file from which the identity (private key) for
291 public key authentication is read.
294 for protocol version 1, and
296 .Pa ~/.ssh/id_ecdsa ,
297 .Pa ~/.ssh/id_ed25519
300 for protocol version 2.
301 Identity files may also be specified on
302 a per-host basis in the configuration file.
303 It is possible to have multiple
305 options (and multiple identities specified in
306 configuration files).
307 If no certificates have been explicitly specified by the
311 will also try to load certificate information from the filename obtained
314 to identity filenames.
317 Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
318 credentials to the server.
321 Disables forwarding (delegation) of GSSAPI credentials to the server.
325 .Oo Ar bind_address : Oc
326 .Ar port : host : hostport
331 .Oo Ar bind_address : Oc
332 .Ar port : remote_socket
337 .Ar local_socket : host : hostport
342 .Ar local_socket : remote_socket
345 Specifies that connections to the given TCP port or Unix socket on the local
346 (client) host are to be forwarded to the given host and port, or Unix socket,
348 This works by allocating a socket to listen to either a TCP
350 on the local side, optionally bound to the specified
353 Whenever a connection is made to the local port or socket, the
354 connection is forwarded over the secure channel, and a connection is
361 from the remote machine.
363 Port forwardings can also be specified in the configuration file.
364 Only the superuser can forward privileged ports.
365 IPv6 addresses can be specified by enclosing the address in square brackets.
367 By default, the local port is bound in accordance with the
372 may be used to bind the connection to a specific address.
377 indicates that the listening port be bound for local use only, while an
380 indicates that the port should be available from all interfaces.
382 .It Fl l Ar login_name
383 Specifies the user to log in as on the remote machine.
384 This also may be specified on a per-host basis in the configuration file.
391 mode for connection sharing.
398 mode with confirmation required before slave connections are accepted.
399 Refer to the description of
406 A comma-separated list of MAC (message authentication code) algorithms,
407 specified in order of preference.
410 keyword for more information.
413 Do not execute a remote command.
414 This is useful for just forwarding ports.
419 (actually, prevents reading from stdin).
420 This must be used when
422 is run in the background.
423 A common trick is to use this to run X11 programs on a remote machine.
425 .Ic ssh -n shadows.cs.hut.fi emacs &
426 will start an emacs on shadows.cs.hut.fi, and the X11
427 connection will be automatically forwarded over an encrypted channel.
430 program will be put in the background.
431 (This does not work if
433 needs to ask for a password or passphrase; see also the
438 Control an active connection multiplexing master process.
441 option is specified, the
443 argument is interpreted and passed to the master process.
446 (check that the master process is running),
448 (request forwardings without command execution),
450 (cancel forwardings),
452 (request the master to exit), and
454 (request the master to stop accepting further multiplexing requests).
457 Can be used to give options in the format used in the configuration file.
458 This is useful for specifying options for which there is no separate
460 For full details of the options listed below, and their possible values, see
463 .Bl -tag -width Ds -offset indent -compact
469 .It CanonicalizeFallbackLocal
470 .It CanonicalizeHostname
471 .It CanonicalizeMaxDots
472 .It CanonicalizePermittedCNAMEs
474 .It ChallengeResponseAuthentication
478 .It ClearAllForwardings
481 .It ConnectionAttempts
488 .It ExitOnForwardFailure
492 .It ForwardX11Timeout
493 .It ForwardX11Trusted
495 .It GlobalKnownHostsFile
496 .It GSSAPIAuthentication
497 .It GSSAPIDelegateCredentials
500 .It HostbasedAuthentication
501 .It HostbasedKeyTypes
502 .It HostKeyAlgorithms
508 .It KbdInteractiveAuthentication
509 .It KbdInteractiveDevices
516 .It NoHostAuthenticationForLocalhost
517 .It NumberOfPasswordPrompts
518 .It PasswordAuthentication
519 .It PermitLocalCommand
522 .It PreferredAuthentications
526 .It PubkeyAcceptedKeyTypes
527 .It PubkeyAuthentication
531 .It RhostsRSAAuthentication
532 .It RSAAuthentication
534 .It ServerAliveInterval
535 .It ServerAliveCountMax
536 .It StreamLocalBindMask
537 .It StreamLocalBindUnlink
538 .It StrictHostKeyChecking
543 .It UsePrivilegedPort
545 .It UserKnownHostsFile
553 Port to connect to on the remote host.
554 This can be specified on a
555 per-host basis in the configuration file.
557 .It Fl Q Ar query_option
560 for the algorithms supported for the specified version 2.
561 The available features are:
563 (supported symmetric ciphers),
565 (supported symmetric ciphers that support authenticated encryption),
567 (supported message integrity codes),
569 (key exchange algorithms),
573 (certificate key types),
575 (non-certificate key types), and
577 (supported SSH protocol versions).
581 Causes most warning and diagnostic messages to be suppressed.
585 .Oo Ar bind_address : Oc
586 .Ar port : host : hostport
591 .Oo Ar bind_address : Oc
592 .Ar port : local_socket
597 .Ar remote_socket : host : hostport
602 .Ar remote_socket : local_socket
605 Specifies that connections to the given TCP port or Unix socket on the remote
606 (server) host are to be forwarded to the given host and port, or Unix socket,
608 This works by allocating a socket to listen to either a TCP
610 or to a Unix socket on the remote side.
611 Whenever a connection is made to this port or Unix socket, the
612 connection is forwarded over the secure channel, and a connection
619 from the local machine.
621 Port forwardings can also be specified in the configuration file.
622 Privileged ports can be forwarded only when
623 logging in as root on the remote machine.
624 IPv6 addresses can be specified by enclosing the address in square brackets.
626 By default, TCP listening sockets on the server will be bound to the loopback
628 This may be overridden by specifying a
634 indicates that the remote socket should listen on all interfaces.
637 will only succeed if the server's
639 option is enabled (see
640 .Xr sshd_config 5 ) .
646 the listen port will be dynamically allocated on the server and reported
647 to the client at run time.
648 When used together with
650 the allocated port will be printed to the standard output.
653 Specifies the location of a control socket for connection sharing,
656 to disable connection sharing.
657 Refer to the description of
666 May be used to request invocation of a subsystem on the remote system.
667 Subsystems facilitate the use of SSH
668 as a secure transport for other applications (e.g.\&
670 The subsystem is specified as the remote command.
673 Disable pseudo-terminal allocation.
676 Force pseudo-terminal allocation.
677 This can be used to execute arbitrary
678 screen-based programs on a remote machine, which can be very useful,
679 e.g. when implementing menu services.
682 options force tty allocation, even if
687 Display the version number and exit.
693 to print debugging messages about its progress.
695 debugging connection, authentication, and configuration problems.
698 options increase the verbosity.
701 .It Fl W Ar host : Ns Ar port
702 Requests that standard input and output on the client be forwarded to
706 over the secure channel.
710 .Cm ExitOnForwardFailure
712 .Cm ClearAllForwardings .
715 .Ar local_tun Ns Op : Ns Ar remote_tun
719 device forwarding with the specified
721 devices between the client
726 The devices may be specified by numerical ID or the keyword
728 which uses the next available tunnel device.
731 is not specified, it defaults to
741 directive is unset, it is set to the default tunnel mode, which is
745 Enables X11 forwarding.
746 This can also be specified on a per-host basis in a configuration file.
748 X11 forwarding should be enabled with caution.
749 Users with the ability to bypass file permissions on the remote host
750 (for the user's X authorization database)
751 can access the local X11 display through the forwarded connection.
752 An attacker may then be able to perform activities such as keystroke monitoring.
754 For this reason, X11 forwarding is subjected to X11 SECURITY extension
755 restrictions by default.
760 .Cm ForwardX11Trusted
763 for more information.
766 Disables X11 forwarding.
769 Enables trusted X11 forwarding.
770 Trusted X11 forwardings are not subjected to the X11 SECURITY extension
774 Send log information using the
777 By default this information is sent to stderr.
781 may additionally obtain configuration data from
782 a per-user configuration file and a system-wide configuration file.
783 The file format and configuration options are described in
786 The OpenSSH SSH client supports SSH protocols 1 and 2.
787 The default is to use protocol 2 only,
788 though this can be changed via the
797 Protocol 1 should not be used
798 and is only offered to support legacy devices.
799 It suffers from a number of cryptographic weaknesses
800 and doesn't support many of the advanced features available for protocol 2.
802 The methods available for authentication are:
803 GSSAPI-based authentication,
804 host-based authentication,
805 public key authentication,
806 challenge-response authentication,
807 and password authentication.
808 Authentication methods are tried in the order specified above,
810 .Cm PreferredAuthentications
811 can be used to change the default order.
813 Host-based authentication works as follows:
814 If the machine the user logs in from is listed in
817 .Pa /etc/shosts.equiv
818 on the remote machine, and the user names are
819 the same on both sides, or if the files
823 exist in the user's home directory on the
824 remote machine and contain a line containing the name of the client
825 machine and the name of the user on that machine, the user is
826 considered for login.
827 Additionally, the server
829 be able to verify the client's
830 host key (see the description of
831 .Pa /etc/ssh/ssh_known_hosts
833 .Pa ~/.ssh/known_hosts ,
835 for login to be permitted.
836 This authentication method closes security holes due to IP
837 spoofing, DNS spoofing, and routing spoofing.
838 [Note to the administrator:
839 .Pa /etc/hosts.equiv ,
841 and the rlogin/rsh protocol in general, are inherently insecure and should be
842 disabled if security is desired.]
844 Public key authentication works as follows:
845 The scheme is based on public-key cryptography,
847 where encryption and decryption are done using separate keys,
848 and it is unfeasible to derive the decryption key from the encryption key.
849 The idea is that each user creates a public/private
850 key pair for authentication purposes.
851 The server knows the public key, and only the user knows the private key.
853 implements public key authentication protocol automatically,
854 using one of the DSA, ECDSA, Ed25519 or RSA algorithms.
855 The HISTORY section of
857 contains a brief discussion of the DSA and RSA algorithms.
860 .Pa ~/.ssh/authorized_keys
861 lists the public keys that are permitted for logging in.
862 When the user logs in, the
864 program tells the server which key pair it would like to use for
866 The client proves that it has access to the private key
867 and the server checks that the corresponding public key
868 is authorized to accept the account.
870 The user creates his/her key pair by running
872 This stores the private key in
879 .Pa ~/.ssh/id_ed25519
884 and stores the public key in
885 .Pa ~/.ssh/identity.pub
887 .Pa ~/.ssh/id_dsa.pub
889 .Pa ~/.ssh/id_ecdsa.pub
891 .Pa ~/.ssh/id_ed25519.pub
894 .Pa ~/.ssh/id_rsa.pub
896 in the user's home directory.
897 The user should then copy the public key
899 .Pa ~/.ssh/authorized_keys
900 in his/her home directory on the remote machine.
903 file corresponds to the conventional
905 file, and has one key
906 per line, though the lines can be very long.
907 After this, the user can log in without giving the password.
909 A variation on public key authentication
910 is available in the form of certificate authentication:
911 instead of a set of public/private keys,
912 signed certificates are used.
913 This has the advantage that a single trusted certification authority
914 can be used in place of many public/private keys.
915 See the CERTIFICATES section of
917 for more information.
919 The most convenient way to use public key or certificate authentication
920 may be with an authentication agent.
927 for more information.
929 Challenge-response authentication works as follows:
930 The server sends an arbitrary
932 text, and prompts for a response.
933 Examples of challenge-response authentication include
941 Finally, if other authentication methods fail,
943 prompts the user for a password.
944 The password is sent to the remote
945 host for checking; however, since all communications are encrypted,
946 the password cannot be seen by someone listening on the network.
949 automatically maintains and checks a database containing
950 identification for all hosts it has ever been used with.
951 Host keys are stored in
952 .Pa ~/.ssh/known_hosts
953 in the user's home directory.
954 Additionally, the file
955 .Pa /etc/ssh/ssh_known_hosts
956 is automatically checked for known hosts.
957 Any new hosts are automatically added to the user's file.
958 If a host's identification ever changes,
960 warns about this and disables password authentication to prevent
961 server spoofing or man-in-the-middle attacks,
962 which could otherwise be used to circumvent the encryption.
964 .Cm StrictHostKeyChecking
965 option can be used to control logins to machines whose
966 host key is not known or has changed.
968 When the user's identity has been accepted by the server, the server
969 either executes the given command in a non-interactive session or,
970 if no command has been specified, logs into the machine and gives
971 the user a normal shell as an interactive session.
972 All communication with
973 the remote command or shell will be automatically encrypted.
975 If an interactive session is requested
977 by default will only request a pseudo-terminal (pty) for interactive
978 sessions when the client has one.
983 can be used to override this behaviour.
985 If a pseudo-terminal has been allocated the
986 user may use the escape characters noted below.
988 If no pseudo-terminal has been allocated,
989 the session is transparent and can be used to reliably transfer binary data.
990 On most systems, setting the escape character to
992 will also make the session transparent even if a tty is used.
994 The session terminates when the command or shell on the remote
995 machine exits and all X11 and TCP connections have been closed.
996 .Sh ESCAPE CHARACTERS
997 When a pseudo-terminal has been requested,
999 supports a number of functions through the use of an escape character.
1001 A single tilde character can be sent as
1003 or by following the tilde by a character other than those described below.
1004 The escape character must always follow a newline to be interpreted as
1006 The escape character can be changed in configuration files using the
1008 configuration directive or on the command line by the
1012 The supported escapes (assuming the default
1022 List forwarded connections.
1026 at logout when waiting for forwarded connection / X11 sessions to terminate.
1028 Display a list of escape characters.
1030 Send a BREAK to the remote system
1031 (only useful if the peer supports it).
1034 Currently this allows the addition of port forwardings using the
1039 options (see above).
1040 It also allows the cancellation of existing port-forwardings
1043 .Fl KL Oo Ar bind_address : Oc Ar port
1047 .Fl KR Oo Ar bind_address : Oc Ar port
1051 .Fl KD Oo Ar bind_address : Oc Ar port
1053 for dynamic port-forwardings.
1054 .Ic !\& Ns Ar command
1055 allows the user to execute a local command if the
1056 .Ic PermitLocalCommand
1057 option is enabled in
1059 Basic help is available, using the
1063 Request rekeying of the connection
1064 (only useful if the peer supports it).
1066 Decrease the verbosity
1068 when errors are being written to stderr.
1070 Increase the verbosity
1072 when errors are being written to stderr.
1075 Forwarding of arbitrary TCP connections over the secure channel can
1076 be specified either on the command line or in a configuration file.
1077 One possible application of TCP forwarding is a secure connection to a
1078 mail server; another is going through firewalls.
1080 In the example below, we look at encrypting communication between
1081 an IRC client and server, even though the IRC server does not directly
1082 support encrypted communications.
1083 This works as follows:
1084 the user connects to the remote host using
1086 specifying a port to be used to forward connections
1087 to the remote server.
1088 After that it is possible to start the service which is to be encrypted
1089 on the client machine,
1090 connecting to the same local port,
1093 will encrypt and forward the connection.
1095 The following example tunnels an IRC session from client machine
1099 .Dq server.example.com :
1100 .Bd -literal -offset 4n
1101 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
1102 $ irc -c '#users' -p 1234 pinky 127.0.0.1
1105 This tunnels a connection to IRC server
1106 .Dq server.example.com ,
1112 It doesn't matter which port is used,
1113 as long as it's greater than 1023
1114 (remember, only root can open sockets on privileged ports)
1115 and doesn't conflict with any ports already in use.
1116 The connection is forwarded to port 6667 on the remote server,
1117 since that's the standard port for IRC services.
1123 and the remote command
1125 is specified to allow an amount of time
1126 (10 seconds, in the example)
1127 to start the service which is to be tunnelled.
1128 If no connections are made within the time specified,
1136 (or see the description of the
1142 and the user is using X11 (the
1144 environment variable is set), the connection to the X11 display is
1145 automatically forwarded to the remote side in such a way that any X11
1146 programs started from the shell (or command) will go through the
1147 encrypted channel, and the connection to the real X server will be made
1148 from the local machine.
1149 The user should not manually set
1151 Forwarding of X11 connections can be
1152 configured on the command line or in configuration files.
1158 will point to the server machine, but with a display number greater than zero.
1159 This is normal, and happens because
1163 X server on the server machine for forwarding the
1164 connections over the encrypted channel.
1167 will also automatically set up Xauthority data on the server machine.
1168 For this purpose, it will generate a random authorization cookie,
1169 store it in Xauthority on the server, and verify that any forwarded
1170 connections carry this cookie and replace it by the real cookie when
1171 the connection is opened.
1172 The real authentication cookie is never
1173 sent to the server machine (and no cookies are sent in the plain).
1179 (or see the description of the
1184 the user is using an authentication agent, the connection to the agent
1185 is automatically forwarded to the remote side.
1186 .Sh VERIFYING HOST KEYS
1187 When connecting to a server for the first time,
1188 a fingerprint of the server's public key is presented to the user
1190 .Cm StrictHostKeyChecking
1192 Fingerprints can be determined using
1195 .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1197 If the fingerprint is already known, it can be matched
1198 and the key can be accepted or rejected.
1199 If only legacy (MD5) fingerprints for the server are available, the
1202 option may be used to downgrade the fingerprint algorithm to match.
1204 Because of the difficulty of comparing host keys
1205 just by looking at fingerprint strings,
1206 there is also support to compare host keys visually,
1213 a small ASCII graphic gets displayed on every login to a server, no matter
1214 if the session itself is interactive or not.
1215 By learning the pattern a known server produces, a user can easily
1216 find out that the host key has changed when a completely different pattern
1218 Because these patterns are not unambiguous however, a pattern that looks
1219 similar to the pattern remembered only gives a good probability that the
1220 host key is the same, not guaranteed proof.
1222 To get a listing of the fingerprints along with their random art for
1223 all known hosts, the following command line can be used:
1225 .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1227 If the fingerprint is unknown,
1228 an alternative method of verification is available:
1229 SSH fingerprints verified by DNS.
1230 An additional resource record (RR),
1232 is added to a zonefile
1233 and the connecting client is able to match the fingerprint
1234 with that of the key presented.
1236 In this example, we are connecting a client to a server,
1237 .Dq host.example.com .
1238 The SSHFP resource records should first be added to the zonefile for
1240 .Bd -literal -offset indent
1241 $ ssh-keygen -r host.example.com.
1244 The output lines will have to be added to the zonefile.
1245 To check that the zone is answering fingerprint queries:
1247 .Dl $ dig -t SSHFP host.example.com
1249 Finally the client connects:
1250 .Bd -literal -offset indent
1251 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1253 Matching host key fingerprint found in DNS.
1254 Are you sure you want to continue connecting (yes/no)?
1258 .Cm VerifyHostKeyDNS
1261 for more information.
1262 .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1264 contains support for Virtual Private Network (VPN) tunnelling
1267 network pseudo-device,
1268 allowing two networks to be joined securely.
1271 configuration option
1273 controls whether the server supports this,
1274 and at what level (layer 2 or 3 traffic).
1276 The following example would connect client network 10.0.50.0/24
1277 with remote network 10.0.99.0/24 using a point-to-point connection
1278 from 10.1.1.1 to 10.1.1.2,
1279 provided that the SSH server running on the gateway to the remote network,
1280 at 192.168.1.15, allows it.
1283 .Bd -literal -offset indent
1284 # ssh -f -w 0:1 192.168.1.15 true
1285 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1286 # route add 10.0.99.0/24 10.1.1.2
1290 .Bd -literal -offset indent
1291 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1292 # route add 10.0.50.0/24 10.1.1.1
1295 Client access may be more finely tuned via the
1296 .Pa /root/.ssh/authorized_keys
1297 file (see below) and the
1300 The following entry would permit connections on
1304 and on tun device 2 from user
1309 .Dq forced-commands-only :
1310 .Bd -literal -offset 2n
1311 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1312 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1315 Since an SSH-based setup entails a fair amount of overhead,
1316 it may be more suited to temporary setups,
1317 such as for wireless VPNs.
1318 More permanent VPNs are better provided by tools such as
1324 will normally set the following environment variables:
1325 .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1329 variable indicates the location of the X11 server.
1330 It is automatically set by
1332 to point to a value of the form
1336 indicates the host where the shell runs, and
1338 is an integer \*(Ge 1.
1340 uses this special value to forward X11 connections over the secure
1342 The user should normally not set
1345 will render the X11 connection insecure (and will require the user to
1346 manually copy any required authorization cookies).
1348 Set to the path of the user's home directory.
1352 set for compatibility with systems that use this variable.
1354 Set to the path of the user's mailbox.
1358 as specified when compiling
1363 needs a passphrase, it will read the passphrase from the current
1364 terminal if it was run from a terminal.
1367 does not have a terminal associated with it but
1371 are set, it will execute the program specified by
1373 and open an X11 window to read the passphrase.
1374 This is particularly useful when calling
1379 (Note that on some machines it
1380 may be necessary to redirect the input from
1383 .It Ev SSH_AUTH_SOCK
1384 Identifies the path of a
1386 socket used to communicate with the agent.
1387 .It Ev SSH_CONNECTION
1388 Identifies the client and server ends of the connection.
1389 The variable contains
1390 four space-separated values: client IP address, client port number,
1391 server IP address, and server port number.
1392 .It Ev SSH_ORIGINAL_COMMAND
1393 This variable contains the original command line if a forced command
1395 It can be used to extract the original arguments.
1397 This is set to the name of the tty (path to the device) associated
1398 with the current shell or command.
1399 If the current session has no tty,
1400 this variable is not set.
1402 This variable is set to indicate the present time zone if it
1403 was set when the daemon was started (i.e. the daemon passes the value
1404 on to new connections).
1406 Set to the name of the user logging in.
1412 .Pa ~/.ssh/environment ,
1413 and adds lines of the format
1415 to the environment if the file exists and users are allowed to
1416 change their environment.
1417 For more information, see the
1418 .Cm PermitUserEnvironment
1422 .Bl -tag -width Ds -compact
1424 This file is used for host-based authentication (see above).
1425 On some machines this file may need to be
1426 world-readable if the user's home directory is on an NFS partition,
1430 Additionally, this file must be owned by the user,
1431 and must not have write permissions for anyone else.
1433 permission for most machines is read/write for the user, and not
1434 accessible by others.
1437 This file is used in exactly the same way as
1439 but allows host-based authentication without permitting login with
1443 This directory is the default location for all user-specific configuration
1444 and authentication information.
1445 There is no general requirement to keep the entire contents of this directory
1446 secret, but the recommended permissions are read/write/execute for the user,
1447 and not accessible by others.
1449 .It Pa ~/.ssh/authorized_keys
1450 Lists the public keys (DSA, ECDSA, Ed25519, RSA)
1451 that can be used for logging in as this user.
1452 The format of this file is described in the
1455 This file is not highly sensitive, but the recommended
1456 permissions are read/write for the user, and not accessible by others.
1458 .It Pa ~/.ssh/config
1459 This is the per-user configuration file.
1460 The file format and configuration options are described in
1462 Because of the potential for abuse, this file must have strict permissions:
1463 read/write for the user, and not writable by others.
1465 .It Pa ~/.ssh/environment
1466 Contains additional definitions for environment variables; see
1470 .It Pa ~/.ssh/identity
1471 .It Pa ~/.ssh/id_dsa
1472 .It Pa ~/.ssh/id_ecdsa
1473 .It Pa ~/.ssh/id_ed25519
1474 .It Pa ~/.ssh/id_rsa
1475 Contains the private key for authentication.
1477 contain sensitive data and should be readable by the user but not
1478 accessible by others (read/write/execute).
1480 will simply ignore a private key file if it is accessible by others.
1481 It is possible to specify a passphrase when
1482 generating the key which will be used to encrypt the
1483 sensitive part of this file using 3DES.
1485 .It Pa ~/.ssh/identity.pub
1486 .It Pa ~/.ssh/id_dsa.pub
1487 .It Pa ~/.ssh/id_ecdsa.pub
1488 .It Pa ~/.ssh/id_ed25519.pub
1489 .It Pa ~/.ssh/id_rsa.pub
1490 Contains the public key for authentication.
1492 sensitive and can (but need not) be readable by anyone.
1494 .It Pa ~/.ssh/known_hosts
1495 Contains a list of host keys for all hosts the user has logged into
1496 that are not already in the systemwide list of known host keys.
1499 for further details of the format of this file.
1502 Commands in this file are executed by
1504 when the user logs in, just before the user's shell (or command) is
1508 manual page for more information.
1510 .It Pa /etc/hosts.equiv
1511 This file is for host-based authentication (see above).
1512 It should only be writable by root.
1514 .It Pa /etc/shosts.equiv
1515 This file is used in exactly the same way as
1517 but allows host-based authentication without permitting login with
1520 .It Pa /etc/ssh/ssh_config
1521 Systemwide configuration file.
1522 The file format and configuration options are described in
1525 .It Pa /etc/ssh/ssh_host_key
1526 .It Pa /etc/ssh/ssh_host_dsa_key
1527 .It Pa /etc/ssh/ssh_host_ecdsa_key
1528 .It Pa /etc/ssh/ssh_host_ed25519_key
1529 .It Pa /etc/ssh/ssh_host_rsa_key
1530 These files contain the private parts of the host keys
1531 and are used for host-based authentication.
1533 .It Pa /etc/ssh/ssh_known_hosts
1534 Systemwide list of known host keys.
1535 This file should be prepared by the
1536 system administrator to contain the public host keys of all machines in the
1538 It should be world-readable.
1541 for further details of the format of this file.
1543 .It Pa /etc/ssh/sshrc
1544 Commands in this file are executed by
1546 when the user logs in, just before the user's shell (or command) is started.
1549 manual page for more information.
1553 exits with the exit status of the remote command or with 255
1554 if an error occurred.
1572 .%T The Secure Shell (SSH) Protocol Assigned Numbers
1580 .%T The Secure Shell (SSH) Protocol Architecture
1588 .%T The Secure Shell (SSH) Authentication Protocol
1596 .%T The Secure Shell (SSH) Transport Layer Protocol
1604 .%T The Secure Shell (SSH) Connection Protocol
1612 .%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
1620 .%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
1628 .%T The Secure Shell (SSH) Session Channel Break Extension
1637 .%T The Secure Shell (SSH) Transport Layer Encryption Modes
1644 .%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
1653 .%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
1661 .%T The Secure Shell (SSH) Public Key File Format
1669 .%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
1676 .%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
1677 .%T Hash Visualization: a New Technique to improve Real-World Security
1680 OpenSSH is a derivative of the original and free
1681 ssh 1.2.12 release by Tatu Ylonen.
1682 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1683 Theo de Raadt and Dug Song
1684 removed many bugs, re-added newer features and
1686 Markus Friedl contributed the support for SSH
1687 protocol versions 1.5 and 2.0.