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 1.276 2008/06/26 21:11:46 jmc Exp $
43 .Nd OpenSSH SSH client (remote login program)
46 .Op Fl 1246AaCfgKkMNnqsTtVvXxY
47 .Op Fl b Ar bind_address
48 .Op Fl c Ar cipher_spec
51 .Oo Ar bind_address : Oc
55 .Op Fl e Ar escape_char
56 .Op Fl F Ar configfile
58 .Op Fl i Ar identity_file
62 .Oo Ar bind_address : Oc
63 .Ar port : host : hostport
67 .Op Fl l Ar login_name
75 .Oo Ar bind_address : Oc
76 .Ar port : host : hostport
81 .Oo Fl w Ar local_tun Ns
82 .Op : Ns Ar remote_tun Oc
83 .Oo Ar user Ns @ Oc Ns Ar hostname
88 (SSH client) is a program for logging into a remote machine and for
89 executing commands on a remote machine.
90 It is intended to replace rlogin and rsh,
91 and provide secure encrypted communications between
92 two untrusted hosts over an insecure network.
93 X11 connections and arbitrary TCP ports
94 can also be forwarded over the secure channel.
97 connects and logs into the specified
103 his/her identity to the remote machine using one of several methods
104 depending on the protocol version used (see below).
109 it is executed on the remote host instead of a login shell.
111 The options are as follows:
116 to try protocol version 1 only.
120 to try protocol version 2 only.
124 to use IPv4 addresses only.
128 to use IPv6 addresses only.
130 Enables forwarding of the authentication agent connection.
131 This can also be specified on a per-host basis in a configuration file.
133 Agent forwarding should be enabled with caution.
134 Users with the ability to bypass file permissions on the remote host
135 (for the agent's Unix-domain socket)
136 can access the local agent through the forwarded connection.
137 An attacker cannot obtain key material from the agent,
138 however they can perform operations on the keys that enable them to
139 authenticate using the identities loaded into the agent.
141 Disables forwarding of the authentication agent connection.
142 .It Fl b Ar bind_address
145 on the local machine as the source address
147 Only useful on systems with more than one address.
149 Requests compression of all data (including stdin, stdout, stderr, and
150 data for forwarded X11 and TCP connections).
151 The compression algorithm is the same used by
155 can be controlled by the
157 option for protocol version 1.
158 Compression is desirable on modem lines and other
159 slow connections, but will only slow down things on fast networks.
160 The default value can be set on a host-by-host basis in the
161 configuration files; see the
164 .It Fl c Ar cipher_spec
165 Selects the cipher specification for encrypting the session.
167 Protocol version 1 allows specification of a single cipher.
168 The supported values are
174 (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
175 It is believed to be secure.
177 is a fast block cipher; it appears very secure and is much faster than
180 is only supported in the
182 client for interoperability with legacy protocol 1 implementations
183 that do not support the
186 Its use is strongly discouraged due to cryptographic weaknesses.
190 For protocol version 2,
192 is a comma-separated list of ciphers
193 listed in order of preference.
194 The supported ciphers are:
209 .Bd -literal -offset indent
210 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
211 arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
212 aes192-ctr,aes256-ctr
216 .Oo Ar bind_address : Oc
222 application-level port forwarding.
223 This works by allocating a socket to listen to
225 on the local side, optionally bound to the specified
227 Whenever a connection is made to this port, the
228 connection is forwarded over the secure channel, and the application
229 protocol is then used to determine where to connect to from the
231 Currently the SOCKS4 and SOCKS5 protocols are supported, and
233 will act as a SOCKS server.
234 Only root can forward privileged ports.
235 Dynamic port forwardings can also be specified in the configuration file.
237 IPv6 addresses can be specified with an alternative syntax:
240 .Op Ar bind_address No /
244 or by enclosing the address in square brackets.
245 Only the superuser can forward privileged ports.
246 By default, the local port is bound in accordance with the
251 may be used to bind the connection to a specific address.
256 indicates that the listening port be bound for local use only, while an
259 indicates that the port should be available from all interfaces.
260 .It Fl e Ar escape_char
261 Sets the escape character for sessions with a pty (default:
263 The escape character is only recognized at the beginning of a line.
264 The escape character followed by a dot
266 closes the connection;
267 followed by control-Z suspends the connection;
268 and followed by itself sends the escape character once.
269 Setting the character to
271 disables any escapes and makes the session fully transparent.
272 .It Fl F Ar configfile
273 Specifies an alternative per-user configuration file.
274 If a configuration file is given on the command line,
275 the system-wide configuration file
276 .Pq Pa /etc/ssh/ssh_config
278 The default for the per-user configuration file is
283 to go to background just before command execution.
286 is going to ask for passwords or passphrases, but the user
287 wants it in the background.
290 The recommended way to start X11 programs at a remote site is with
292 .Ic ssh -f host xterm .
294 Allows remote hosts to connect to local forwarded ports.
295 .It Fl I Ar smartcard_device
298 should use to communicate with a smartcard used for storing the user's
300 This option is only available if support for smartcard devices
301 is compiled in (default is no support).
302 .It Fl i Ar identity_file
303 Selects a file from which the identity (private key) for
304 RSA or DSA authentication is read.
307 for protocol version 1, and
311 for protocol version 2.
312 Identity files may also be specified on
313 a per-host basis in the configuration file.
314 It is possible to have multiple
316 options (and multiple identities specified in
317 configuration files).
319 Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
320 credentials to the server.
322 Disables forwarding (delegation) of GSSAPI credentials to the server.
325 .Oo Ar bind_address : Oc
326 .Ar port : host : hostport
329 Specifies that the given port on the local (client) host is to be
330 forwarded to the given host and port on the remote side.
331 This works by allocating a socket to listen to
333 on the local side, optionally bound to the specified
335 Whenever a connection is made to this port, the
336 connection is forwarded over the secure channel, and a connection is
341 from the remote machine.
342 Port forwardings can also be specified in the configuration file.
343 IPv6 addresses can be specified with an alternative syntax:
346 .Op Ar bind_address No /
347 .Ar port No / Ar host No /
351 or by enclosing the address in square brackets.
352 Only the superuser can forward privileged ports.
353 By default, the local port is bound in accordance with the
358 may be used to bind the connection to a specific address.
363 indicates that the listening port be bound for local use only, while an
366 indicates that the port should be available from all interfaces.
367 .It Fl l Ar login_name
368 Specifies the user to log in as on the remote machine.
369 This also may be specified on a per-host basis in the configuration file.
375 mode for connection sharing.
382 mode with confirmation required before slave connections are accepted.
383 Refer to the description of
389 Additionally, for protocol version 2 a comma-separated list of MAC
390 (message authentication code) algorithms can
391 be specified in order of preference.
394 keyword for more information.
396 Do not execute a remote command.
397 This is useful for just forwarding ports
398 (protocol version 2 only).
402 (actually, prevents reading from stdin).
403 This must be used when
405 is run in the background.
406 A common trick is to use this to run X11 programs on a remote machine.
408 .Ic ssh -n shadows.cs.hut.fi emacs &
409 will start an emacs on shadows.cs.hut.fi, and the X11
410 connection will be automatically forwarded over an encrypted channel.
413 program will be put in the background.
414 (This does not work if
416 needs to ask for a password or passphrase; see also the
420 Control an active connection multiplexing master process.
423 option is specified, the
425 argument is interpreted and passed to the master process.
428 (check that the master process is running) and
430 (request the master to exit).
432 Can be used to give options in the format used in the configuration file.
433 This is useful for specifying options for which there is no separate
435 For full details of the options listed below, and their possible values, see
438 .Bl -tag -width Ds -offset indent -compact
442 .It ChallengeResponseAuthentication
446 .It ClearAllForwardings
449 .It ConnectionAttempts
455 .It ExitOnForwardFailure
458 .It ForwardX11Trusted
460 .It GlobalKnownHostsFile
461 .It GSSAPIAuthentication
462 .It GSSAPIDelegateCredentials
465 .It HostbasedAuthentication
466 .It HostKeyAlgorithms
471 .It KbdInteractiveDevices
476 .It NoHostAuthenticationForLocalhost
477 .It NumberOfPasswordPrompts
478 .It PasswordAuthentication
479 .It PermitLocalCommand
481 .It PreferredAuthentications
484 .It PubkeyAuthentication
487 .It RhostsRSAAuthentication
488 .It RSAAuthentication
490 .It ServerAliveInterval
491 .It ServerAliveCountMax
493 .It StrictHostKeyChecking
497 .It UsePrivilegedPort
499 .It UserKnownHostsFile
505 Port to connect to on the remote host.
506 This can be specified on a
507 per-host basis in the configuration file.
510 Causes most warning and diagnostic messages to be suppressed.
513 .Oo Ar bind_address : Oc
514 .Ar port : host : hostport
517 Specifies that the given port on the remote (server) host is to be
518 forwarded to the given host and port on the local side.
519 This works by allocating a socket to listen to
521 on the remote side, and whenever a connection is made to this port, the
522 connection is forwarded over the secure channel, and a connection is
527 from the local machine.
529 Port forwardings can also be specified in the configuration file.
530 Privileged ports can be forwarded only when
531 logging in as root on the remote machine.
532 IPv6 addresses can be specified by enclosing the address in square braces or
533 using an alternative syntax:
536 .Op Ar bind_address No /
537 .Ar host No / Ar port No /
542 By default, the listening socket on the server will be bound to the loopback
544 This may be overriden by specifying a
550 indicates that the remote socket should listen on all interfaces.
553 will only succeed if the server's
555 option is enabled (see
556 .Xr sshd_config 5 ) .
558 Specifies the location of a control socket for connection sharing.
559 Refer to the description of
567 May be used to request invocation of a subsystem on the remote system.
568 Subsystems are a feature of the SSH2 protocol which facilitate the use
569 of SSH as a secure transport for other applications (eg.\&
571 The subsystem is specified as the remote command.
573 Disable pseudo-tty allocation.
575 Force pseudo-tty allocation.
576 This can be used to execute arbitrary
577 screen-based programs on a remote machine, which can be very useful,
578 e.g. when implementing menu services.
581 options force tty allocation, even if
585 Display the version number and exit.
590 to print debugging messages about its progress.
592 debugging connection, authentication, and configuration problems.
595 options increase the verbosity.
598 .Ar local_tun Ns Op : Ns Ar remote_tun
602 device forwarding with the specified
604 devices between the client
609 The devices may be specified by numerical ID or the keyword
611 which uses the next available tunnel device.
614 is not specified, it defaults to
624 directive is unset, it is set to the default tunnel mode, which is
627 Enables X11 forwarding.
628 This can also be specified on a per-host basis in a configuration file.
630 X11 forwarding should be enabled with caution.
631 Users with the ability to bypass file permissions on the remote host
632 (for the user's X authorization database)
633 can access the local X11 display through the forwarded connection.
634 An attacker may then be able to perform activities such as keystroke monitoring.
636 For this reason, X11 forwarding is subjected to X11 SECURITY extension
637 restrictions by default.
642 .Cm ForwardX11Trusted
645 for more information.
647 Disables X11 forwarding.
649 Enables trusted X11 forwarding.
650 Trusted X11 forwardings are not subjected to the X11 SECURITY extension
655 may additionally obtain configuration data from
656 a per-user configuration file and a system-wide configuration file.
657 The file format and configuration options are described in
661 exits with the exit status of the remote command or with 255
662 if an error occurred.
664 The OpenSSH SSH client supports SSH protocols 1 and 2.
665 Protocol 2 is the default, with
667 falling back to protocol 1 if it detects protocol 2 is unsupported.
668 These settings may be altered using the
672 or enforced using the
677 Both protocols support similar authentication methods,
678 but protocol 2 is preferred since
679 it provides additional mechanisms for confidentiality
680 (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
681 and integrity (hmac-md5, hmac-sha1, umac-64, hmac-ripemd160).
682 Protocol 1 lacks a strong mechanism for ensuring the
683 integrity of the connection.
685 The methods available for authentication are:
686 GSSAPI-based authentication,
687 host-based authentication,
688 public key authentication,
689 challenge-response authentication,
690 and password authentication.
691 Authentication methods are tried in the order specified above,
692 though protocol 2 has a configuration option to change the default order:
693 .Cm PreferredAuthentications .
695 Host-based authentication works as follows:
696 If the machine the user logs in from is listed in
699 .Pa /etc/shosts.equiv
700 on the remote machine, and the user names are
701 the same on both sides, or if the files
705 exist in the user's home directory on the
706 remote machine and contain a line containing the name of the client
707 machine and the name of the user on that machine, the user is
708 considered for login.
709 Additionally, the server
711 be able to verify the client's
712 host key (see the description of
713 .Pa /etc/ssh/ssh_known_hosts
715 .Pa ~/.ssh/known_hosts ,
717 for login to be permitted.
718 This authentication method closes security holes due to IP
719 spoofing, DNS spoofing, and routing spoofing.
720 [Note to the administrator:
721 .Pa /etc/hosts.equiv ,
723 and the rlogin/rsh protocol in general, are inherently insecure and should be
724 disabled if security is desired.]
726 Public key authentication works as follows:
727 The scheme is based on public-key cryptography,
729 where encryption and decryption are done using separate keys,
730 and it is unfeasible to derive the decryption key from the encryption key.
731 The idea is that each user creates a public/private
732 key pair for authentication purposes.
733 The server knows the public key, and only the user knows the private key.
735 implements public key authentication protocol automatically,
736 using either the RSA or DSA algorithms.
737 Protocol 1 is restricted to using only RSA keys,
738 but protocol 2 may use either.
743 contains a brief discussion of the two algorithms.
746 .Pa ~/.ssh/authorized_keys
747 lists the public keys that are permitted for logging in.
748 When the user logs in, the
750 program tells the server which key pair it would like to use for
752 The client proves that it has access to the private key
753 and the server checks that the corresponding public key
754 is authorized to accept the account.
756 The user creates his/her key pair by running
758 This stores the private key in
766 and stores the public key in
767 .Pa ~/.ssh/identity.pub
769 .Pa ~/.ssh/id_dsa.pub
772 .Pa ~/.ssh/id_rsa.pub
774 in the user's home directory.
775 The user should then copy the public key
777 .Pa ~/.ssh/authorized_keys
778 in his/her home directory on the remote machine.
781 file corresponds to the conventional
783 file, and has one key
784 per line, though the lines can be very long.
785 After this, the user can log in without giving the password.
787 The most convenient way to use public key authentication may be with an
788 authentication agent.
791 for more information.
793 Challenge-response authentication works as follows:
794 The server sends an arbitrary
796 text, and prompts for a response.
797 Protocol 2 allows multiple challenges and responses;
798 protocol 1 is restricted to just one challenge/response.
799 Examples of challenge-response authentication include
800 BSD Authentication (see
802 and PAM (some non-OpenBSD systems).
804 Finally, if other authentication methods fail,
806 prompts the user for a password.
807 The password is sent to the remote
808 host for checking; however, since all communications are encrypted,
809 the password cannot be seen by someone listening on the network.
812 automatically maintains and checks a database containing
813 identification for all hosts it has ever been used with.
814 Host keys are stored in
815 .Pa ~/.ssh/known_hosts
816 in the user's home directory.
817 Additionally, the file
818 .Pa /etc/ssh/ssh_known_hosts
819 is automatically checked for known hosts.
820 Any new hosts are automatically added to the user's file.
821 If a host's identification ever changes,
823 warns about this and disables password authentication to prevent
824 server spoofing or man-in-the-middle attacks,
825 which could otherwise be used to circumvent the encryption.
827 .Cm StrictHostKeyChecking
828 option can be used to control logins to machines whose
829 host key is not known or has changed.
831 When the user's identity has been accepted by the server, the server
832 either executes the given command, or logs into the machine and gives
833 the user a normal shell on the remote machine.
834 All communication with
835 the remote command or shell will be automatically encrypted.
837 If a pseudo-terminal has been allocated (normal login session), the
838 user may use the escape characters noted below.
840 If no pseudo-tty has been allocated,
841 the session is transparent and can be used to reliably transfer binary data.
842 On most systems, setting the escape character to
844 will also make the session transparent even if a tty is used.
846 The session terminates when the command or shell on the remote
847 machine exits and all X11 and TCP connections have been closed.
848 .Sh ESCAPE CHARACTERS
849 When a pseudo-terminal has been requested,
851 supports a number of functions through the use of an escape character.
853 A single tilde character can be sent as
855 or by following the tilde by a character other than those described below.
856 The escape character must always follow a newline to be interpreted as
858 The escape character can be changed in configuration files using the
860 configuration directive or on the command line by the
864 The supported escapes (assuming the default
874 List forwarded connections.
878 at logout when waiting for forwarded connection / X11 sessions to terminate.
880 Display a list of escape characters.
882 Send a BREAK to the remote system
883 (only useful for SSH protocol version 2 and if the peer supports it).
886 Currently this allows the addition of port forwardings using the
891 It also allows the cancellation of existing remote port-forwardings
894 .Fl KR Oo Ar bind_address : Oc Ar port .
896 .Ic !\& Ns Ar command
897 allows the user to execute a local command if the
898 .Ic PermitLocalCommand
901 Basic help is available, using the
905 Request rekeying of the connection
906 (only useful for SSH protocol version 2 and if the peer supports it).
909 Forwarding of arbitrary TCP connections over the secure channel can
910 be specified either on the command line or in a configuration file.
911 One possible application of TCP forwarding is a secure connection to a
912 mail server; another is going through firewalls.
914 In the example below, we look at encrypting communication between
915 an IRC client and server, even though the IRC server does not directly
916 support encrypted communications.
917 This works as follows:
918 the user connects to the remote host using
920 specifying a port to be used to forward connections
921 to the remote server.
922 After that it is possible to start the service which is to be encrypted
923 on the client machine,
924 connecting to the same local port,
927 will encrypt and forward the connection.
929 The following example tunnels an IRC session from client machine
933 .Dq server.example.com :
934 .Bd -literal -offset 4n
935 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
936 $ irc -c '#users' -p 1234 pinky 127.0.0.1
939 This tunnels a connection to IRC server
940 .Dq server.example.com ,
946 It doesn't matter which port is used,
947 as long as it's greater than 1023
948 (remember, only root can open sockets on privileged ports)
949 and doesn't conflict with any ports already in use.
950 The connection is forwarded to port 6667 on the remote server,
951 since that's the standard port for IRC services.
957 and the remote command
959 is specified to allow an amount of time
960 (10 seconds, in the example)
961 to start the service which is to be tunnelled.
962 If no connections are made within the time specified,
970 (or see the description of the
976 and the user is using X11 (the
978 environment variable is set), the connection to the X11 display is
979 automatically forwarded to the remote side in such a way that any X11
980 programs started from the shell (or command) will go through the
981 encrypted channel, and the connection to the real X server will be made
982 from the local machine.
983 The user should not manually set
985 Forwarding of X11 connections can be
986 configured on the command line or in configuration files.
992 will point to the server machine, but with a display number greater than zero.
993 This is normal, and happens because
997 X server on the server machine for forwarding the
998 connections over the encrypted channel.
1001 will also automatically set up Xauthority data on the server machine.
1002 For this purpose, it will generate a random authorization cookie,
1003 store it in Xauthority on the server, and verify that any forwarded
1004 connections carry this cookie and replace it by the real cookie when
1005 the connection is opened.
1006 The real authentication cookie is never
1007 sent to the server machine (and no cookies are sent in the plain).
1013 (or see the description of the
1018 the user is using an authentication agent, the connection to the agent
1019 is automatically forwarded to the remote side.
1020 .Sh VERIFYING HOST KEYS
1021 When connecting to a server for the first time,
1022 a fingerprint of the server's public key is presented to the user
1024 .Cm StrictHostKeyChecking
1026 Fingerprints can be determined using
1029 .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1031 If the fingerprint is already known, it can be matched
1032 and the key can be accepted or rejected.
1033 Because of the difficulty of comparing host keys
1034 just by looking at hex strings,
1035 there is also support to compare host keys visually,
1042 a small ASCII graphic gets displayed on every login to a server, no matter
1043 if the session itself is interactive or not.
1044 By learning the pattern a known server produces, a user can easily
1045 find out that the host key has changed when a completely different pattern
1047 Because these patterns are not unambiguous however, a pattern that looks
1048 similar to the pattern remembered only gives a good probability that the
1049 host key is the same, not guaranteed proof.
1051 To get a listing of the fingerprints along with their random art for
1052 all known hosts, the following command line can be used:
1054 .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1056 If the fingerprint is unknown,
1057 an alternative method of verification is available:
1058 SSH fingerprints verified by DNS.
1059 An additional resource record (RR),
1061 is added to a zonefile
1062 and the connecting client is able to match the fingerprint
1063 with that of the key presented.
1065 In this example, we are connecting a client to a server,
1066 .Dq host.example.com .
1067 The SSHFP resource records should first be added to the zonefile for
1069 .Bd -literal -offset indent
1070 $ ssh-keygen -r host.example.com.
1073 The output lines will have to be added to the zonefile.
1074 To check that the zone is answering fingerprint queries:
1076 .Dl $ dig -t SSHFP host.example.com
1078 Finally the client connects:
1079 .Bd -literal -offset indent
1080 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1082 Matching host key fingerprint found in DNS.
1083 Are you sure you want to continue connecting (yes/no)?
1087 .Cm VerifyHostKeyDNS
1090 for more information.
1091 .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1093 contains support for Virtual Private Network (VPN) tunnelling
1096 network pseudo-device,
1097 allowing two networks to be joined securely.
1100 configuration option
1102 controls whether the server supports this,
1103 and at what level (layer 2 or 3 traffic).
1105 The following example would connect client network 10.0.50.0/24
1106 with remote network 10.0.99.0/24 using a point-to-point connection
1107 from 10.1.1.1 to 10.1.1.2,
1108 provided that the SSH server running on the gateway to the remote network,
1109 at 192.168.1.15, allows it.
1112 .Bd -literal -offset indent
1113 # ssh -f -w 0:1 192.168.1.15 true
1114 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1115 # route add 10.0.99.0/24 10.1.1.2
1119 .Bd -literal -offset indent
1120 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1121 # route add 10.0.50.0/24 10.1.1.1
1124 Client access may be more finely tuned via the
1125 .Pa /root/.ssh/authorized_keys
1126 file (see below) and the
1129 The following entry would permit connections on
1133 and on tun device 2 from user
1138 .Dq forced-commands-only :
1139 .Bd -literal -offset 2n
1140 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1141 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1144 Since an SSH-based setup entails a fair amount of overhead,
1145 it may be more suited to temporary setups,
1146 such as for wireless VPNs.
1147 More permanent VPNs are better provided by tools such as
1153 will normally set the following environment variables:
1154 .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1158 variable indicates the location of the X11 server.
1159 It is automatically set by
1161 to point to a value of the form
1165 indicates the host where the shell runs, and
1167 is an integer \*(Ge 1.
1169 uses this special value to forward X11 connections over the secure
1171 The user should normally not set
1174 will render the X11 connection insecure (and will require the user to
1175 manually copy any required authorization cookies).
1177 Set to the path of the user's home directory.
1181 set for compatibility with systems that use this variable.
1183 Set to the path of the user's mailbox.
1187 as specified when compiling
1192 needs a passphrase, it will read the passphrase from the current
1193 terminal if it was run from a terminal.
1196 does not have a terminal associated with it but
1200 are set, it will execute the program specified by
1202 and open an X11 window to read the passphrase.
1203 This is particularly useful when calling
1208 (Note that on some machines it
1209 may be necessary to redirect the input from
1212 .It Ev SSH_AUTH_SOCK
1213 Identifies the path of a
1215 socket used to communicate with the agent.
1216 .It Ev SSH_CONNECTION
1217 Identifies the client and server ends of the connection.
1218 The variable contains
1219 four space-separated values: client IP address, client port number,
1220 server IP address, and server port number.
1221 .It Ev SSH_ORIGINAL_COMMAND
1222 This variable contains the original command line if a forced command
1224 It can be used to extract the original arguments.
1226 This is set to the name of the tty (path to the device) associated
1227 with the current shell or command.
1228 If the current session has no tty,
1229 this variable is not set.
1231 This variable is set to indicate the present time zone if it
1232 was set when the daemon was started (i.e. the daemon passes the value
1233 on to new connections).
1235 Set to the name of the user logging in.
1241 .Pa ~/.ssh/environment ,
1242 and adds lines of the format
1244 to the environment if the file exists and users are allowed to
1245 change their environment.
1246 For more information, see the
1247 .Cm PermitUserEnvironment
1251 .Bl -tag -width Ds -compact
1253 This file is used for host-based authentication (see above).
1254 On some machines this file may need to be
1255 world-readable if the user's home directory is on an NFS partition,
1259 Additionally, this file must be owned by the user,
1260 and must not have write permissions for anyone else.
1262 permission for most machines is read/write for the user, and not
1263 accessible by others.
1266 This file is used in exactly the same way as
1268 but allows host-based authentication without permitting login with
1272 This directory is the default location for all user-specific configuration
1273 and authentication information.
1274 There is no general requirement to keep the entire contents of this directory
1275 secret, but the recommended permissions are read/write/execute for the user,
1276 and not accessible by others.
1278 .It ~/.ssh/authorized_keys
1279 Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1280 The format of this file is described in the
1283 This file is not highly sensitive, but the recommended
1284 permissions are read/write for the user, and not accessible by others.
1287 This is the per-user configuration file.
1288 The file format and configuration options are described in
1290 Because of the potential for abuse, this file must have strict permissions:
1291 read/write for the user, and not accessible by others.
1293 .It ~/.ssh/environment
1294 Contains additional definitions for environment variables; see
1301 Contains the private key for authentication.
1303 contain sensitive data and should be readable by the user but not
1304 accessible by others (read/write/execute).
1306 will simply ignore a private key file if it is accessible by others.
1307 It is possible to specify a passphrase when
1308 generating the key which will be used to encrypt the
1309 sensitive part of this file using 3DES.
1311 .It ~/.ssh/identity.pub
1312 .It ~/.ssh/id_dsa.pub
1313 .It ~/.ssh/id_rsa.pub
1314 Contains the public key for authentication.
1316 sensitive and can (but need not) be readable by anyone.
1318 .It ~/.ssh/known_hosts
1319 Contains a list of host keys for all hosts the user has logged into
1320 that are not already in the systemwide list of known host keys.
1323 for further details of the format of this file.
1326 Commands in this file are executed by
1328 when the user logs in, just before the user's shell (or command) is
1332 manual page for more information.
1334 .It /etc/hosts.equiv
1335 This file is for host-based authentication (see above).
1336 It should only be writable by root.
1338 .It /etc/shosts.equiv
1339 This file is used in exactly the same way as
1341 but allows host-based authentication without permitting login with
1344 .It Pa /etc/ssh/ssh_config
1345 Systemwide configuration file.
1346 The file format and configuration options are described in
1349 .It /etc/ssh/ssh_host_key
1350 .It /etc/ssh/ssh_host_dsa_key
1351 .It /etc/ssh/ssh_host_rsa_key
1352 These three files contain the private parts of the host keys
1353 and are used for host-based authentication.
1354 If protocol version 1 is used,
1356 must be setuid root, since the host key is readable only by root.
1357 For protocol version 2,
1361 to access the host keys,
1362 eliminating the requirement that
1364 be setuid root when host-based authentication is used.
1369 .It /etc/ssh/ssh_known_hosts
1370 Systemwide list of known host keys.
1371 This file should be prepared by the
1372 system administrator to contain the public host keys of all machines in the
1374 It should be world-readable.
1377 for further details of the format of this file.
1380 Commands in this file are executed by
1382 when the user logs in, just before the user's shell (or command) is started.
1385 manual page for more information.
1401 .%T "The Secure Shell (SSH) Protocol Assigned Numbers"
1406 .%T "The Secure Shell (SSH) Protocol Architecture"
1411 .%T "The Secure Shell (SSH) Authentication Protocol"
1416 .%T "The Secure Shell (SSH) Transport Layer Protocol"
1421 .%T "The Secure Shell (SSH) Connection Protocol"
1426 .%T "Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints"
1431 .%T "Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)"
1436 .%T "The Secure Shell (SSH) Session Channel Break Extension"
1441 .%T "The Secure Shell (SSH) Transport Layer Encryption Modes"
1446 .%T "Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol"
1451 .%T "Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol"
1456 .%T "The Secure Shell (SSH) Public Key File Format"
1460 .%T "Hash Visualization: a New Technique to improve Real-World Security"
1464 .%O "International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)"
1467 OpenSSH is a derivative of the original and free
1468 ssh 1.2.12 release by Tatu Ylonen.
1469 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1470 Theo de Raadt and Dug Song
1471 removed many bugs, re-added newer features and
1473 Markus Friedl contributed the support for SSH
1474 protocol versions 1.5 and 2.0.
andersk / openssh.git / blob