3 .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
4 .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
5 .\" All rights reserved
7 .\" As far as I am concerned, the code I have written for this software
8 .\" can be used freely for any purpose. Any derived versions of this
9 .\" software must be clearly marked as such, and if the derived work is
10 .\" incompatible with the protocol description in the RFC file, it must be
11 .\" called by a name other than "ssh" or "Secure Shell".
13 .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
14 .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
15 .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
17 .\" Redistribution and use in source and binary forms, with or without
18 .\" modification, are permitted provided that the following conditions
20 .\" 1. Redistributions of source code must retain the above copyright
21 .\" notice, this list of conditions and the following disclaimer.
22 .\" 2. Redistributions in binary form must reproduce the above copyright
23 .\" notice, this list of conditions and the following disclaimer in the
24 .\" documentation and/or other materials provided with the distribution.
26 .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
27 .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
28 .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
29 .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
30 .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
31 .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
32 .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
33 .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
34 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
35 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
37 .\" $OpenBSD: ssh.1,v 1.282 2009/02/12 03:44:25 djm Exp $
44 .Nd OpenSSH SSH client (remote login program)
47 .Op Fl 1246AaCfgKkMNnqsTtVvXxYy
48 .Op Fl b Ar bind_address
49 .Op Fl c Ar cipher_spec
52 .Oo Ar bind_address : Oc
56 .Op Fl e Ar escape_char
57 .Op Fl F Ar configfile
59 .Op Fl i Ar identity_file
63 .Oo Ar bind_address : Oc
64 .Ar port : host : hostport
68 .Op Fl l Ar login_name
76 .Oo Ar bind_address : Oc
77 .Ar port : host : hostport
82 .Oo Fl w Ar local_tun Ns
83 .Op : Ns Ar remote_tun Oc
84 .Oo Ar user Ns @ Oc Ns Ar hostname
89 (SSH client) is a program for logging into a remote machine and for
90 executing commands on a remote machine.
91 It is intended to replace rlogin and rsh,
92 and provide secure encrypted communications between
93 two untrusted hosts over an insecure network.
94 X11 connections and arbitrary TCP ports
95 can also be forwarded over the secure channel.
98 connects and logs into the specified
104 his/her identity to the remote machine using one of several methods
105 depending on the protocol version used (see below).
110 it is executed on the remote host instead of a login shell.
112 The options are as follows:
117 to try protocol version 1 only.
121 to try protocol version 2 only.
125 to use IPv4 addresses only.
129 to use IPv6 addresses only.
131 Enables forwarding of the authentication agent connection.
132 This can also be specified on a per-host basis in a configuration file.
134 Agent forwarding should be enabled with caution.
135 Users with the ability to bypass file permissions on the remote host
136 (for the agent's Unix-domain socket)
137 can access the local agent through the forwarded connection.
138 An attacker cannot obtain key material from the agent,
139 however they can perform operations on the keys that enable them to
140 authenticate using the identities loaded into the agent.
142 Disables forwarding of the authentication agent connection.
143 .It Fl b Ar bind_address
146 on the local machine as the source address
148 Only useful on systems with more than one address.
150 Requests compression of all data (including stdin, stdout, stderr, and
151 data for forwarded X11 and TCP connections).
152 The compression algorithm is the same used by
156 can be controlled by the
158 option for protocol version 1.
159 Compression is desirable on modem lines and other
160 slow connections, but will only slow down things on fast networks.
161 The default value can be set on a host-by-host basis in the
162 configuration files; see the
165 .It Fl c Ar cipher_spec
166 Selects the cipher specification for encrypting the session.
168 Protocol version 1 allows specification of a single cipher.
169 The supported values are
175 (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
176 It is believed to be secure.
178 is a fast block cipher; it appears very secure and is much faster than
181 is only supported in the
183 client for interoperability with legacy protocol 1 implementations
184 that do not support the
187 Its use is strongly discouraged due to cryptographic weaknesses.
191 For protocol version 2,
193 is a comma-separated list of ciphers
194 listed in order of preference.
195 The supported ciphers are:
210 .Bd -literal -offset indent
211 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
212 arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
213 aes192-ctr,aes256-ctr
217 .Oo Ar bind_address : Oc
223 application-level port forwarding.
224 This works by allocating a socket to listen to
226 on the local side, optionally bound to the specified
228 Whenever a connection is made to this port, the
229 connection is forwarded over the secure channel, and the application
230 protocol is then used to determine where to connect to from the
232 Currently the SOCKS4 and SOCKS5 protocols are supported, and
234 will act as a SOCKS server.
235 Only root can forward privileged ports.
236 Dynamic port forwardings can also be specified in the configuration file.
238 IPv6 addresses can be specified with an alternative syntax:
241 .Op Ar bind_address No /
245 or by enclosing the address in square brackets.
246 Only the superuser can forward privileged ports.
247 By default, the local port is bound in accordance with the
252 may be used to bind the connection to a specific address.
257 indicates that the listening port be bound for local use only, while an
260 indicates that the port should be available from all interfaces.
261 .It Fl e Ar escape_char
262 Sets the escape character for sessions with a pty (default:
264 The escape character is only recognized at the beginning of a line.
265 The escape character followed by a dot
267 closes the connection;
268 followed by control-Z suspends the connection;
269 and followed by itself sends the escape character once.
270 Setting the character to
272 disables any escapes and makes the session fully transparent.
273 .It Fl F Ar configfile
274 Specifies an alternative per-user configuration file.
275 If a configuration file is given on the command line,
276 the system-wide configuration file
277 .Pq Pa /etc/ssh/ssh_config
279 The default for the per-user configuration file is
284 to go to background just before command execution.
287 is going to ask for passwords or passphrases, but the user
288 wants it in the background.
291 The recommended way to start X11 programs at a remote site is with
293 .Ic ssh -f host xterm .
296 .Cm ExitOnForwardFailure
297 configuration option is set to
299 then a client started with
301 will wait for all remote port forwards to be successfully established
302 before placing itself in the background.
304 Allows remote hosts to connect to local forwarded ports.
305 .It Fl I Ar smartcard_device
308 should use to communicate with a smartcard used for storing the user's
310 This option is only available if support for smartcard devices
311 is compiled in (default is no support).
312 .It Fl i Ar identity_file
313 Selects a file from which the identity (private key) for
314 RSA or DSA authentication is read.
317 for protocol version 1, and
321 for protocol version 2.
322 Identity files may also be specified on
323 a per-host basis in the configuration file.
324 It is possible to have multiple
326 options (and multiple identities specified in
327 configuration files).
329 Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
330 credentials to the server.
332 Disables forwarding (delegation) of GSSAPI credentials to the server.
335 .Oo Ar bind_address : Oc
336 .Ar port : host : hostport
339 Specifies that the given port on the local (client) host is to be
340 forwarded to the given host and port on the remote side.
341 This works by allocating a socket to listen to
343 on the local side, optionally bound to the specified
345 Whenever a connection is made to this port, the
346 connection is forwarded over the secure channel, and a connection is
351 from the remote machine.
352 Port forwardings can also be specified in the configuration file.
353 IPv6 addresses can be specified with an alternative syntax:
356 .Op Ar bind_address No /
357 .Ar port No / Ar host No /
361 or by enclosing the address in square brackets.
362 Only the superuser can forward privileged ports.
363 By default, the local port is bound in accordance with the
368 may be used to bind the connection to a specific address.
373 indicates that the listening port be bound for local use only, while an
376 indicates that the port should be available from all interfaces.
377 .It Fl l Ar login_name
378 Specifies the user to log in as on the remote machine.
379 This also may be specified on a per-host basis in the configuration file.
385 mode for connection sharing.
392 mode with confirmation required before slave connections are accepted.
393 Refer to the description of
399 Additionally, for protocol version 2 a comma-separated list of MAC
400 (message authentication code) algorithms can
401 be specified in order of preference.
404 keyword for more information.
406 Do not execute a remote command.
407 This is useful for just forwarding ports
408 (protocol version 2 only).
412 (actually, prevents reading from stdin).
413 This must be used when
415 is run in the background.
416 A common trick is to use this to run X11 programs on a remote machine.
418 .Ic ssh -n shadows.cs.hut.fi emacs &
419 will start an emacs on shadows.cs.hut.fi, and the X11
420 connection will be automatically forwarded over an encrypted channel.
423 program will be put in the background.
424 (This does not work if
426 needs to ask for a password or passphrase; see also the
430 Control an active connection multiplexing master process.
433 option is specified, the
435 argument is interpreted and passed to the master process.
438 (check that the master process is running) and
440 (request the master to exit).
442 Can be used to give options in the format used in the configuration file.
443 This is useful for specifying options for which there is no separate
445 For full details of the options listed below, and their possible values, see
448 .Bl -tag -width Ds -offset indent -compact
452 .It ChallengeResponseAuthentication
456 .It ClearAllForwardings
459 .It ConnectionAttempts
465 .It ExitOnForwardFailure
468 .It ForwardX11Trusted
470 .It GlobalKnownHostsFile
471 .It GSSAPIAuthentication
472 .It GSSAPIDelegateCredentials
475 .It HostbasedAuthentication
476 .It HostKeyAlgorithms
481 .It KbdInteractiveDevices
486 .It NoHostAuthenticationForLocalhost
487 .It NumberOfPasswordPrompts
488 .It PasswordAuthentication
489 .It PermitLocalCommand
491 .It PreferredAuthentications
494 .It PubkeyAuthentication
497 .It RhostsRSAAuthentication
498 .It RSAAuthentication
500 .It ServerAliveInterval
501 .It ServerAliveCountMax
503 .It StrictHostKeyChecking
507 .It UsePrivilegedPort
509 .It UserKnownHostsFile
516 Port to connect to on the remote host.
517 This can be specified on a
518 per-host basis in the configuration file.
521 Causes most warning and diagnostic messages to be suppressed.
524 .Oo Ar bind_address : Oc
525 .Ar port : host : hostport
528 Specifies that the given port on the remote (server) host is to be
529 forwarded to the given host and port on the local side.
530 This works by allocating a socket to listen to
532 on the remote side, and whenever a connection is made to this port, the
533 connection is forwarded over the secure channel, and a connection is
538 from the local machine.
540 Port forwardings can also be specified in the configuration file.
541 Privileged ports can be forwarded only when
542 logging in as root on the remote machine.
543 IPv6 addresses can be specified by enclosing the address in square braces or
544 using an alternative syntax:
547 .Op Ar bind_address No /
548 .Ar host No / Ar port No /
553 By default, the listening socket on the server will be bound to the loopback
555 This may be overridden by specifying a
561 indicates that the remote socket should listen on all interfaces.
564 will only succeed if the server's
566 option is enabled (see
567 .Xr sshd_config 5 ) .
573 the listen port will be dynamically allocated on the server and reported
574 to the client at run time.
576 Specifies the location of a control socket for connection sharing.
577 Refer to the description of
585 May be used to request invocation of a subsystem on the remote system.
586 Subsystems are a feature of the SSH2 protocol which facilitate the use
587 of SSH as a secure transport for other applications (eg.\&
589 The subsystem is specified as the remote command.
591 Disable pseudo-tty allocation.
593 Force pseudo-tty allocation.
594 This can be used to execute arbitrary
595 screen-based programs on a remote machine, which can be very useful,
596 e.g. when implementing menu services.
599 options force tty allocation, even if
603 Display the version number and exit.
608 to print debugging messages about its progress.
610 debugging connection, authentication, and configuration problems.
613 options increase the verbosity.
616 .Ar local_tun Ns Op : Ns Ar remote_tun
620 device forwarding with the specified
622 devices between the client
627 The devices may be specified by numerical ID or the keyword
629 which uses the next available tunnel device.
632 is not specified, it defaults to
642 directive is unset, it is set to the default tunnel mode, which is
645 Enables X11 forwarding.
646 This can also be specified on a per-host basis in a configuration file.
648 X11 forwarding should be enabled with caution.
649 Users with the ability to bypass file permissions on the remote host
650 (for the user's X authorization database)
651 can access the local X11 display through the forwarded connection.
652 An attacker may then be able to perform activities such as keystroke monitoring.
654 For this reason, X11 forwarding is subjected to X11 SECURITY extension
655 restrictions by default.
660 .Cm ForwardX11Trusted
663 for more information.
665 Disables X11 forwarding.
667 Enables trusted X11 forwarding.
668 Trusted X11 forwardings are not subjected to the X11 SECURITY extension
671 Send log information using the
674 By default this information is sent to stderr.
678 may additionally obtain configuration data from
679 a per-user configuration file and a system-wide configuration file.
680 The file format and configuration options are described in
684 exits with the exit status of the remote command or with 255
685 if an error occurred.
687 The OpenSSH SSH client supports SSH protocols 1 and 2.
688 Protocol 2 is the default, with
690 falling back to protocol 1 if it detects protocol 2 is unsupported.
691 These settings may be altered using the
695 or enforced using the
700 Both protocols support similar authentication methods,
701 but protocol 2 is preferred since
702 it provides additional mechanisms for confidentiality
703 (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
704 and integrity (hmac-md5, hmac-sha1, umac-64, hmac-ripemd160).
705 Protocol 1 lacks a strong mechanism for ensuring the
706 integrity of the connection.
708 The methods available for authentication are:
709 GSSAPI-based authentication,
710 host-based authentication,
711 public key authentication,
712 challenge-response authentication,
713 and password authentication.
714 Authentication methods are tried in the order specified above,
715 though protocol 2 has a configuration option to change the default order:
716 .Cm PreferredAuthentications .
718 Host-based authentication works as follows:
719 If the machine the user logs in from is listed in
722 .Pa /etc/shosts.equiv
723 on the remote machine, and the user names are
724 the same on both sides, or if the files
728 exist in the user's home directory on the
729 remote machine and contain a line containing the name of the client
730 machine and the name of the user on that machine, the user is
731 considered for login.
732 Additionally, the server
734 be able to verify the client's
735 host key (see the description of
736 .Pa /etc/ssh/ssh_known_hosts
738 .Pa ~/.ssh/known_hosts ,
740 for login to be permitted.
741 This authentication method closes security holes due to IP
742 spoofing, DNS spoofing, and routing spoofing.
743 [Note to the administrator:
744 .Pa /etc/hosts.equiv ,
746 and the rlogin/rsh protocol in general, are inherently insecure and should be
747 disabled if security is desired.]
749 Public key authentication works as follows:
750 The scheme is based on public-key cryptography,
752 where encryption and decryption are done using separate keys,
753 and it is unfeasible to derive the decryption key from the encryption key.
754 The idea is that each user creates a public/private
755 key pair for authentication purposes.
756 The server knows the public key, and only the user knows the private key.
758 implements public key authentication protocol automatically,
759 using either the RSA or DSA algorithms.
760 Protocol 1 is restricted to using only RSA keys,
761 but protocol 2 may use either.
766 contains a brief discussion of the two algorithms.
769 .Pa ~/.ssh/authorized_keys
770 lists the public keys that are permitted for logging in.
771 When the user logs in, the
773 program tells the server which key pair it would like to use for
775 The client proves that it has access to the private key
776 and the server checks that the corresponding public key
777 is authorized to accept the account.
779 The user creates his/her key pair by running
781 This stores the private key in
789 and stores the public key in
790 .Pa ~/.ssh/identity.pub
792 .Pa ~/.ssh/id_dsa.pub
795 .Pa ~/.ssh/id_rsa.pub
797 in the user's home directory.
798 The user should then copy the public key
800 .Pa ~/.ssh/authorized_keys
801 in his/her home directory on the remote machine.
804 file corresponds to the conventional
806 file, and has one key
807 per line, though the lines can be very long.
808 After this, the user can log in without giving the password.
810 The most convenient way to use public key authentication may be with an
811 authentication agent.
814 for more information.
816 Challenge-response authentication works as follows:
817 The server sends an arbitrary
819 text, and prompts for a response.
820 Protocol 2 allows multiple challenges and responses;
821 protocol 1 is restricted to just one challenge/response.
822 Examples of challenge-response authentication include
823 BSD Authentication (see
825 and PAM (some non-OpenBSD systems).
827 Finally, if other authentication methods fail,
829 prompts the user for a password.
830 The password is sent to the remote
831 host for checking; however, since all communications are encrypted,
832 the password cannot be seen by someone listening on the network.
835 automatically maintains and checks a database containing
836 identification for all hosts it has ever been used with.
837 Host keys are stored in
838 .Pa ~/.ssh/known_hosts
839 in the user's home directory.
840 Additionally, the file
841 .Pa /etc/ssh/ssh_known_hosts
842 is automatically checked for known hosts.
843 Any new hosts are automatically added to the user's file.
844 If a host's identification ever changes,
846 warns about this and disables password authentication to prevent
847 server spoofing or man-in-the-middle attacks,
848 which could otherwise be used to circumvent the encryption.
850 .Cm StrictHostKeyChecking
851 option can be used to control logins to machines whose
852 host key is not known or has changed.
854 When the user's identity has been accepted by the server, the server
855 either executes the given command, or logs into the machine and gives
856 the user a normal shell on the remote machine.
857 All communication with
858 the remote command or shell will be automatically encrypted.
860 If a pseudo-terminal has been allocated (normal login session), the
861 user may use the escape characters noted below.
863 If no pseudo-tty has been allocated,
864 the session is transparent and can be used to reliably transfer binary data.
865 On most systems, setting the escape character to
867 will also make the session transparent even if a tty is used.
869 The session terminates when the command or shell on the remote
870 machine exits and all X11 and TCP connections have been closed.
871 .Sh ESCAPE CHARACTERS
872 When a pseudo-terminal has been requested,
874 supports a number of functions through the use of an escape character.
876 A single tilde character can be sent as
878 or by following the tilde by a character other than those described below.
879 The escape character must always follow a newline to be interpreted as
881 The escape character can be changed in configuration files using the
883 configuration directive or on the command line by the
887 The supported escapes (assuming the default
897 List forwarded connections.
901 at logout when waiting for forwarded connection / X11 sessions to terminate.
903 Display a list of escape characters.
905 Send a BREAK to the remote system
906 (only useful for SSH protocol version 2 and if the peer supports it).
909 Currently this allows the addition of port forwardings using the
915 It also allows the cancellation of existing remote port-forwardings
918 .Fl KR Oo Ar bind_address : Oc Ar port .
920 .Ic !\& Ns Ar command
921 allows the user to execute a local command if the
922 .Ic PermitLocalCommand
925 Basic help is available, using the
929 Request rekeying of the connection
930 (only useful for SSH protocol version 2 and if the peer supports it).
933 Forwarding of arbitrary TCP connections over the secure channel can
934 be specified either on the command line or in a configuration file.
935 One possible application of TCP forwarding is a secure connection to a
936 mail server; another is going through firewalls.
938 In the example below, we look at encrypting communication between
939 an IRC client and server, even though the IRC server does not directly
940 support encrypted communications.
941 This works as follows:
942 the user connects to the remote host using
944 specifying a port to be used to forward connections
945 to the remote server.
946 After that it is possible to start the service which is to be encrypted
947 on the client machine,
948 connecting to the same local port,
951 will encrypt and forward the connection.
953 The following example tunnels an IRC session from client machine
957 .Dq server.example.com :
958 .Bd -literal -offset 4n
959 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
960 $ irc -c '#users' -p 1234 pinky 127.0.0.1
963 This tunnels a connection to IRC server
964 .Dq server.example.com ,
970 It doesn't matter which port is used,
971 as long as it's greater than 1023
972 (remember, only root can open sockets on privileged ports)
973 and doesn't conflict with any ports already in use.
974 The connection is forwarded to port 6667 on the remote server,
975 since that's the standard port for IRC services.
981 and the remote command
983 is specified to allow an amount of time
984 (10 seconds, in the example)
985 to start the service which is to be tunnelled.
986 If no connections are made within the time specified,
994 (or see the description of the
1000 and the user is using X11 (the
1002 environment variable is set), the connection to the X11 display is
1003 automatically forwarded to the remote side in such a way that any X11
1004 programs started from the shell (or command) will go through the
1005 encrypted channel, and the connection to the real X server will be made
1006 from the local machine.
1007 The user should not manually set
1009 Forwarding of X11 connections can be
1010 configured on the command line or in configuration files.
1016 will point to the server machine, but with a display number greater than zero.
1017 This is normal, and happens because
1021 X server on the server machine for forwarding the
1022 connections over the encrypted channel.
1025 will also automatically set up Xauthority data on the server machine.
1026 For this purpose, it will generate a random authorization cookie,
1027 store it in Xauthority on the server, and verify that any forwarded
1028 connections carry this cookie and replace it by the real cookie when
1029 the connection is opened.
1030 The real authentication cookie is never
1031 sent to the server machine (and no cookies are sent in the plain).
1037 (or see the description of the
1042 the user is using an authentication agent, the connection to the agent
1043 is automatically forwarded to the remote side.
1044 .Sh VERIFYING HOST KEYS
1045 When connecting to a server for the first time,
1046 a fingerprint of the server's public key is presented to the user
1048 .Cm StrictHostKeyChecking
1050 Fingerprints can be determined using
1053 .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1055 If the fingerprint is already known, it can be matched
1056 and the key can be accepted or rejected.
1057 Because of the difficulty of comparing host keys
1058 just by looking at hex strings,
1059 there is also support to compare host keys visually,
1066 a small ASCII graphic gets displayed on every login to a server, no matter
1067 if the session itself is interactive or not.
1068 By learning the pattern a known server produces, a user can easily
1069 find out that the host key has changed when a completely different pattern
1071 Because these patterns are not unambiguous however, a pattern that looks
1072 similar to the pattern remembered only gives a good probability that the
1073 host key is the same, not guaranteed proof.
1075 To get a listing of the fingerprints along with their random art for
1076 all known hosts, the following command line can be used:
1078 .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1080 If the fingerprint is unknown,
1081 an alternative method of verification is available:
1082 SSH fingerprints verified by DNS.
1083 An additional resource record (RR),
1085 is added to a zonefile
1086 and the connecting client is able to match the fingerprint
1087 with that of the key presented.
1089 In this example, we are connecting a client to a server,
1090 .Dq host.example.com .
1091 The SSHFP resource records should first be added to the zonefile for
1093 .Bd -literal -offset indent
1094 $ ssh-keygen -r host.example.com.
1097 The output lines will have to be added to the zonefile.
1098 To check that the zone is answering fingerprint queries:
1100 .Dl $ dig -t SSHFP host.example.com
1102 Finally the client connects:
1103 .Bd -literal -offset indent
1104 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1106 Matching host key fingerprint found in DNS.
1107 Are you sure you want to continue connecting (yes/no)?
1111 .Cm VerifyHostKeyDNS
1114 for more information.
1115 .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1117 contains support for Virtual Private Network (VPN) tunnelling
1120 network pseudo-device,
1121 allowing two networks to be joined securely.
1124 configuration option
1126 controls whether the server supports this,
1127 and at what level (layer 2 or 3 traffic).
1129 The following example would connect client network 10.0.50.0/24
1130 with remote network 10.0.99.0/24 using a point-to-point connection
1131 from 10.1.1.1 to 10.1.1.2,
1132 provided that the SSH server running on the gateway to the remote network,
1133 at 192.168.1.15, allows it.
1136 .Bd -literal -offset indent
1137 # ssh -f -w 0:1 192.168.1.15 true
1138 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1139 # route add 10.0.99.0/24 10.1.1.2
1143 .Bd -literal -offset indent
1144 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1145 # route add 10.0.50.0/24 10.1.1.1
1148 Client access may be more finely tuned via the
1149 .Pa /root/.ssh/authorized_keys
1150 file (see below) and the
1153 The following entry would permit connections on
1157 and on tun device 2 from user
1162 .Dq forced-commands-only :
1163 .Bd -literal -offset 2n
1164 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1165 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1168 Since an SSH-based setup entails a fair amount of overhead,
1169 it may be more suited to temporary setups,
1170 such as for wireless VPNs.
1171 More permanent VPNs are better provided by tools such as
1177 will normally set the following environment variables:
1178 .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1182 variable indicates the location of the X11 server.
1183 It is automatically set by
1185 to point to a value of the form
1189 indicates the host where the shell runs, and
1191 is an integer \*(Ge 1.
1193 uses this special value to forward X11 connections over the secure
1195 The user should normally not set
1198 will render the X11 connection insecure (and will require the user to
1199 manually copy any required authorization cookies).
1201 Set to the path of the user's home directory.
1205 set for compatibility with systems that use this variable.
1207 Set to the path of the user's mailbox.
1211 as specified when compiling
1216 needs a passphrase, it will read the passphrase from the current
1217 terminal if it was run from a terminal.
1220 does not have a terminal associated with it but
1224 are set, it will execute the program specified by
1226 and open an X11 window to read the passphrase.
1227 This is particularly useful when calling
1232 (Note that on some machines it
1233 may be necessary to redirect the input from
1236 .It Ev SSH_AUTH_SOCK
1237 Identifies the path of a
1239 socket used to communicate with the agent.
1240 .It Ev SSH_CONNECTION
1241 Identifies the client and server ends of the connection.
1242 The variable contains
1243 four space-separated values: client IP address, client port number,
1244 server IP address, and server port number.
1245 .It Ev SSH_ORIGINAL_COMMAND
1246 This variable contains the original command line if a forced command
1248 It can be used to extract the original arguments.
1250 This is set to the name of the tty (path to the device) associated
1251 with the current shell or command.
1252 If the current session has no tty,
1253 this variable is not set.
1255 This variable is set to indicate the present time zone if it
1256 was set when the daemon was started (i.e. the daemon passes the value
1257 on to new connections).
1259 Set to the name of the user logging in.
1265 .Pa ~/.ssh/environment ,
1266 and adds lines of the format
1268 to the environment if the file exists and users are allowed to
1269 change their environment.
1270 For more information, see the
1271 .Cm PermitUserEnvironment
1275 .Bl -tag -width Ds -compact
1277 This file is used for host-based authentication (see above).
1278 On some machines this file may need to be
1279 world-readable if the user's home directory is on an NFS partition,
1283 Additionally, this file must be owned by the user,
1284 and must not have write permissions for anyone else.
1286 permission for most machines is read/write for the user, and not
1287 accessible by others.
1290 This file is used in exactly the same way as
1292 but allows host-based authentication without permitting login with
1296 This directory is the default location for all user-specific configuration
1297 and authentication information.
1298 There is no general requirement to keep the entire contents of this directory
1299 secret, but the recommended permissions are read/write/execute for the user,
1300 and not accessible by others.
1302 .It ~/.ssh/authorized_keys
1303 Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1304 The format of this file is described in the
1307 This file is not highly sensitive, but the recommended
1308 permissions are read/write for the user, and not accessible by others.
1311 This is the per-user configuration file.
1312 The file format and configuration options are described in
1314 Because of the potential for abuse, this file must have strict permissions:
1315 read/write for the user, and not accessible by others.
1317 .It ~/.ssh/environment
1318 Contains additional definitions for environment variables; see
1325 Contains the private key for authentication.
1327 contain sensitive data and should be readable by the user but not
1328 accessible by others (read/write/execute).
1330 will simply ignore a private key file if it is accessible by others.
1331 It is possible to specify a passphrase when
1332 generating the key which will be used to encrypt the
1333 sensitive part of this file using 3DES.
1335 .It ~/.ssh/identity.pub
1336 .It ~/.ssh/id_dsa.pub
1337 .It ~/.ssh/id_rsa.pub
1338 Contains the public key for authentication.
1340 sensitive and can (but need not) be readable by anyone.
1342 .It ~/.ssh/known_hosts
1343 Contains a list of host keys for all hosts the user has logged into
1344 that are not already in the systemwide list of known host keys.
1347 for further details of the format of this file.
1350 Commands in this file are executed by
1352 when the user logs in, just before the user's shell (or command) is
1356 manual page for more information.
1358 .It /etc/hosts.equiv
1359 This file is for host-based authentication (see above).
1360 It should only be writable by root.
1362 .It /etc/shosts.equiv
1363 This file is used in exactly the same way as
1365 but allows host-based authentication without permitting login with
1368 .It Pa /etc/ssh/ssh_config
1369 Systemwide configuration file.
1370 The file format and configuration options are described in
1373 .It /etc/ssh/ssh_host_key
1374 .It /etc/ssh/ssh_host_dsa_key
1375 .It /etc/ssh/ssh_host_rsa_key
1376 These three files contain the private parts of the host keys
1377 and are used for host-based authentication.
1378 If protocol version 1 is used,
1380 must be setuid root, since the host key is readable only by root.
1381 For protocol version 2,
1385 to access the host keys,
1386 eliminating the requirement that
1388 be setuid root when host-based authentication is used.
1393 .It /etc/ssh/ssh_known_hosts
1394 Systemwide list of known host keys.
1395 This file should be prepared by the
1396 system administrator to contain the public host keys of all machines in the
1398 It should be world-readable.
1401 for further details of the format of this file.
1404 Commands in this file are executed by
1406 when the user logs in, just before the user's shell (or command) is started.
1409 manual page for more information.
1425 .%T "The Secure Shell (SSH) Protocol Assigned Numbers"
1430 .%T "The Secure Shell (SSH) Protocol Architecture"
1435 .%T "The Secure Shell (SSH) Authentication Protocol"
1440 .%T "The Secure Shell (SSH) Transport Layer Protocol"
1445 .%T "The Secure Shell (SSH) Connection Protocol"
1450 .%T "Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints"
1455 .%T "Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)"
1460 .%T "The Secure Shell (SSH) Session Channel Break Extension"
1465 .%T "The Secure Shell (SSH) Transport Layer Encryption Modes"
1470 .%T "Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol"
1475 .%T "Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol"
1480 .%T "The Secure Shell (SSH) Public Key File Format"
1484 .%T "Hash Visualization: a New Technique to improve Real-World Security"
1488 .%O "International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)"
1491 OpenSSH is a derivative of the original and free
1492 ssh 1.2.12 release by Tatu Ylonen.
1493 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1494 Theo de Raadt and Dug Song
1495 removed many bugs, re-added newer features and
1497 Markus Friedl contributed the support for SSH
1498 protocol versions 1.5 and 2.0.
projects / FreeBSD / releng / 8.0.git / blob