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 1.303 2010/03/26 00:26:58 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
60 .Op Fl i Ar identity_file
64 .Oo Ar bind_address : Oc
65 .Ar port : host : hostport
69 .Op Fl l Ar login_name
77 .Oo Ar bind_address : Oc
78 .Ar port : host : hostport
82 .Op Fl W Ar host : Ns Ar port
83 .Oo Fl w Ar local_tun Ns
84 .Op : Ns Ar remote_tun Oc
85 .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
138 socket) can access the local agent through the forwarded connection.
139 An attacker cannot obtain key material from the agent,
140 however they can perform operations on the keys that enable them to
141 authenticate using the identities loaded into the agent.
143 Disables forwarding of the authentication agent connection.
144 .It Fl b Ar bind_address
147 on the local machine as the source address
149 Only useful on systems with more than one address.
151 Requests compression of all data (including stdin, stdout, stderr, and
152 data for forwarded X11 and TCP connections).
153 The compression algorithm is the same used by
157 can be controlled by the
159 option for protocol version 1.
160 Compression is desirable on modem lines and other
161 slow connections, but will only slow down things on fast networks.
162 The default value can be set on a host-by-host basis in the
163 configuration files; see the
166 .It Fl c Ar cipher_spec
167 Selects the cipher specification for encrypting the session.
169 Protocol version 1 allows specification of a single cipher.
170 The supported values are
176 (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
177 It is believed to be secure.
179 is a fast block cipher; it appears very secure and is much faster than
182 is only supported in the
184 client for interoperability with legacy protocol 1 implementations
185 that do not support the
188 Its use is strongly discouraged due to cryptographic weaknesses.
192 For protocol version 2,
194 is a comma-separated list of ciphers
195 listed in order of preference.
198 keyword for more information.
201 .Oo Ar bind_address : Oc
207 application-level port forwarding.
208 This works by allocating a socket to listen to
210 on the local side, optionally bound to the specified
212 Whenever a connection is made to this port, the
213 connection is forwarded over the secure channel, and the application
214 protocol is then used to determine where to connect to from the
216 Currently the SOCKS4 and SOCKS5 protocols are supported, and
218 will act as a SOCKS server.
219 Only root can forward privileged ports.
220 Dynamic port forwardings can also be specified in the configuration file.
222 IPv6 addresses can be specified with an alternative syntax:
225 .Op Ar bind_address No /
229 or by enclosing the address in square brackets.
230 Only the superuser can forward privileged ports.
231 By default, the local port is bound in accordance with the
236 may be used to bind the connection to a specific address.
241 indicates that the listening port be bound for local use only, while an
244 indicates that the port should be available from all interfaces.
245 .It Fl e Ar escape_char
246 Sets the escape character for sessions with a pty (default:
248 The escape character is only recognized at the beginning of a line.
249 The escape character followed by a dot
251 closes the connection;
252 followed by control-Z suspends the connection;
253 and followed by itself sends the escape character once.
254 Setting the character to
256 disables any escapes and makes the session fully transparent.
257 .It Fl F Ar configfile
258 Specifies an alternative per-user configuration file.
259 If a configuration file is given on the command line,
260 the system-wide configuration file
261 .Pq Pa /etc/ssh/ssh_config
263 The default for the per-user configuration file is
268 to go to background just before command execution.
271 is going to ask for passwords or passphrases, but the user
272 wants it in the background.
275 The recommended way to start X11 programs at a remote site is with
277 .Ic ssh -f host xterm .
280 .Cm ExitOnForwardFailure
281 configuration option is set to
283 then a client started with
285 will wait for all remote port forwards to be successfully established
286 before placing itself in the background.
288 Allows remote hosts to connect to local forwarded ports.
290 Specify the PKCS#11 shared library
292 should use to communicate with a PKCS#11 token providing the user's
294 .It Fl i Ar identity_file
295 Selects a file from which the identity (private key) for
296 RSA or DSA authentication is read.
299 for protocol version 1, and
303 for protocol version 2.
304 Identity files may also be specified on
305 a per-host basis in the configuration file.
306 It is possible to have multiple
308 options (and multiple identities specified in
309 configuration files).
311 will also try to load certificate information from the filename obtained
314 to identity filenames.
316 Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
317 credentials to the server.
319 Disables forwarding (delegation) of GSSAPI credentials to the server.
322 .Oo Ar bind_address : Oc
323 .Ar port : host : hostport
326 Specifies that the given port on the local (client) host is to be
327 forwarded to the given host and port on the remote side.
328 This works by allocating a socket to listen to
330 on the local side, optionally bound to the specified
332 Whenever a connection is made to this port, the
333 connection is forwarded over the secure channel, and a connection is
338 from the remote machine.
339 Port forwardings can also be specified in the configuration file.
340 IPv6 addresses can be specified with an alternative syntax:
343 .Op Ar bind_address No /
344 .Ar port No / Ar host No /
348 or by enclosing the address in square brackets.
349 Only the superuser can forward privileged ports.
350 By default, the local port is bound in accordance with the
355 may be used to bind the connection to a specific address.
360 indicates that the listening port be bound for local use only, while an
363 indicates that the port should be available from all interfaces.
364 .It Fl l Ar login_name
365 Specifies the user to log in as on the remote machine.
366 This also may be specified on a per-host basis in the configuration file.
372 mode for connection sharing.
379 mode with confirmation required before slave connections are accepted.
380 Refer to the description of
386 Additionally, for protocol version 2 a comma-separated list of MAC
387 (message authentication code) algorithms can
388 be specified in order of preference.
391 keyword for more information.
393 Do not execute a remote command.
394 This is useful for just forwarding ports
395 (protocol version 2 only).
399 (actually, prevents reading from stdin).
400 This must be used when
402 is run in the background.
403 A common trick is to use this to run X11 programs on a remote machine.
405 .Ic ssh -n shadows.cs.hut.fi emacs &
406 will start an emacs on shadows.cs.hut.fi, and the X11
407 connection will be automatically forwarded over an encrypted channel.
410 program will be put in the background.
411 (This does not work if
413 needs to ask for a password or passphrase; see also the
417 Control an active connection multiplexing master process.
420 option is specified, the
422 argument is interpreted and passed to the master process.
425 (check that the master process is running) and
427 (request the master to exit).
429 Can be used to give options in the format used in the configuration file.
430 This is useful for specifying options for which there is no separate
432 For full details of the options listed below, and their possible values, see
435 .Bl -tag -width Ds -offset indent -compact
439 .It ChallengeResponseAuthentication
443 .It ClearAllForwardings
446 .It ConnectionAttempts
452 .It ExitOnForwardFailure
455 .It ForwardX11Trusted
457 .It GlobalKnownHostsFile
458 .It GSSAPIAuthentication
459 .It GSSAPIDelegateCredentials
462 .It HostbasedAuthentication
463 .It HostKeyAlgorithms
468 .It KbdInteractiveDevices
473 .It NoHostAuthenticationForLocalhost
474 .It NumberOfPasswordPrompts
475 .It PasswordAuthentication
476 .It PermitLocalCommand
479 .It PreferredAuthentications
482 .It PubkeyAuthentication
485 .It RhostsRSAAuthentication
486 .It RSAAuthentication
488 .It ServerAliveInterval
489 .It ServerAliveCountMax
490 .It StrictHostKeyChecking
494 .It UsePrivilegedPort
496 .It UserKnownHostsFile
503 Port to connect to on the remote host.
504 This can be specified on a
505 per-host basis in the configuration file.
508 Causes most warning and diagnostic messages to be suppressed.
511 .Oo Ar bind_address : Oc
512 .Ar port : host : hostport
515 Specifies that the given port on the remote (server) host is to be
516 forwarded to the given host and port on the local side.
517 This works by allocating a socket to listen to
519 on the remote side, and whenever a connection is made to this port, the
520 connection is forwarded over the secure channel, and a connection is
525 from the local machine.
527 Port forwardings can also be specified in the configuration file.
528 Privileged ports can be forwarded only when
529 logging in as root on the remote machine.
530 IPv6 addresses can be specified by enclosing the address in square braces or
531 using an alternative syntax:
534 .Op Ar bind_address No /
535 .Ar host No / Ar port No /
540 By default, the listening socket on the server will be bound to the loopback
542 This may be overridden by specifying a
548 indicates that the remote socket should listen on all interfaces.
551 will only succeed if the server's
553 option is enabled (see
554 .Xr sshd_config 5 ) .
560 the listen port will be dynamically allocated on the server and reported
561 to the client at run time.
563 Specifies the location of a control socket for connection sharing
566 to disable connection sharing.
567 Refer to the description of
575 May be used to request invocation of a subsystem on the remote system.
576 Subsystems are a feature of the SSH2 protocol which facilitate the use
577 of SSH as a secure transport for other applications (eg.\&
579 The subsystem is specified as the remote command.
581 Disable pseudo-tty allocation.
583 Force pseudo-tty allocation.
584 This can be used to execute arbitrary
585 screen-based programs on a remote machine, which can be very useful,
586 e.g. when implementing menu services.
589 options force tty allocation, even if
593 Display the version number and exit.
598 to print debugging messages about its progress.
600 debugging connection, authentication, and configuration problems.
603 options increase the verbosity.
605 .It Fl W Ar host : Ns Ar port
606 Requests that standard input and output on the client be forwarded to
610 over the secure channel.
614 .Cm ExitOnForwardFailure
616 .Cm ClearAllForwardings
617 and works with Protocol version 2 only.
619 .Ar local_tun Ns Op : Ns Ar remote_tun
623 device forwarding with the specified
625 devices between the client
630 The devices may be specified by numerical ID or the keyword
632 which uses the next available tunnel device.
635 is not specified, it defaults to
645 directive is unset, it is set to the default tunnel mode, which is
648 Enables X11 forwarding.
649 This can also be specified on a per-host basis in a configuration file.
651 X11 forwarding should be enabled with caution.
652 Users with the ability to bypass file permissions on the remote host
653 (for the user's X authorization database)
654 can access the local X11 display through the forwarded connection.
655 An attacker may then be able to perform activities such as keystroke monitoring.
657 For this reason, X11 forwarding is subjected to X11 SECURITY extension
658 restrictions by default.
663 .Cm ForwardX11Trusted
666 for more information.
668 Disables X11 forwarding.
670 Enables trusted X11 forwarding.
671 Trusted X11 forwardings are not subjected to the X11 SECURITY extension
674 Send log information using the
677 By default this information is sent to stderr.
681 may additionally obtain configuration data from
682 a per-user configuration file and a system-wide configuration file.
683 The file format and configuration options are described in
687 exits with the exit status of the remote command or with 255
688 if an error occurred.
690 The OpenSSH SSH client supports SSH protocols 1 and 2.
691 The default is to use protocol 2 only,
692 though this can be changed via the
701 Both protocols support similar authentication methods,
702 but protocol 2 is the default since
703 it provides additional mechanisms for confidentiality
704 (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
705 and integrity (hmac-md5, hmac-sha1, umac-64, hmac-ripemd160).
706 Protocol 1 lacks a strong mechanism for ensuring the
707 integrity of the connection.
709 The methods available for authentication are:
710 GSSAPI-based authentication,
711 host-based authentication,
712 public key authentication,
713 challenge-response authentication,
714 and password authentication.
715 Authentication methods are tried in the order specified above,
716 though protocol 2 has a configuration option to change the default order:
717 .Cm PreferredAuthentications .
719 Host-based authentication works as follows:
720 If the machine the user logs in from is listed in
723 .Pa /etc/shosts.equiv
724 on the remote machine, and the user names are
725 the same on both sides, or if the files
729 exist in the user's home directory on the
730 remote machine and contain a line containing the name of the client
731 machine and the name of the user on that machine, the user is
732 considered for login.
733 Additionally, the server
735 be able to verify the client's
736 host key (see the description of
737 .Pa /etc/ssh/ssh_known_hosts
739 .Pa ~/.ssh/known_hosts ,
741 for login to be permitted.
742 This authentication method closes security holes due to IP
743 spoofing, DNS spoofing, and routing spoofing.
744 [Note to the administrator:
745 .Pa /etc/hosts.equiv ,
747 and the rlogin/rsh protocol in general, are inherently insecure and should be
748 disabled if security is desired.]
750 Public key authentication works as follows:
751 The scheme is based on public-key cryptography,
753 where encryption and decryption are done using separate keys,
754 and it is unfeasible to derive the decryption key from the encryption key.
755 The idea is that each user creates a public/private
756 key pair for authentication purposes.
757 The server knows the public key, and only the user knows the private key.
759 implements public key authentication protocol automatically,
760 using either the RSA or DSA algorithms.
761 Protocol 1 is restricted to using only RSA keys,
762 but protocol 2 may use either.
767 contains a brief discussion of the two algorithms.
770 .Pa ~/.ssh/authorized_keys
771 lists the public keys that are permitted for logging in.
772 When the user logs in, the
774 program tells the server which key pair it would like to use for
776 The client proves that it has access to the private key
777 and the server checks that the corresponding public key
778 is authorized to accept the account.
780 The user creates his/her key pair by running
782 This stores the private key in
790 and stores the public key in
791 .Pa ~/.ssh/identity.pub
793 .Pa ~/.ssh/id_dsa.pub
796 .Pa ~/.ssh/id_rsa.pub
798 in the user's home directory.
799 The user should then copy the public key
801 .Pa ~/.ssh/authorized_keys
802 in his/her home directory on the remote machine.
805 file corresponds to the conventional
807 file, and has one key
808 per line, though the lines can be very long.
809 After this, the user can log in without giving the password.
811 A variation on public key authentication
812 is available in the form of certificate authentication:
813 instead of a set of public/private keys,
814 signed certificates are used.
815 This has the advantage that a single trusted certification authority
816 can be used in place of many public/private keys.
821 for more information.
823 The most convenient way to use public key or certificate authentication
824 may be with an authentication agent.
827 for more information.
829 Challenge-response authentication works as follows:
830 The server sends an arbitrary
832 text, and prompts for a response.
833 Protocol 2 allows multiple challenges and responses;
834 protocol 1 is restricted to just one challenge/response.
835 Examples of challenge-response authentication include
836 BSD Authentication (see
838 and PAM (some non-OpenBSD systems).
840 Finally, if other authentication methods fail,
842 prompts the user for a password.
843 The password is sent to the remote
844 host for checking; however, since all communications are encrypted,
845 the password cannot be seen by someone listening on the network.
848 automatically maintains and checks a database containing
849 identification for all hosts it has ever been used with.
850 Host keys are stored in
851 .Pa ~/.ssh/known_hosts
852 in the user's home directory.
853 Additionally, the file
854 .Pa /etc/ssh/ssh_known_hosts
855 is automatically checked for known hosts.
856 Any new hosts are automatically added to the user's file.
857 If a host's identification ever changes,
859 warns about this and disables password authentication to prevent
860 server spoofing or man-in-the-middle attacks,
861 which could otherwise be used to circumvent the encryption.
863 .Cm StrictHostKeyChecking
864 option can be used to control logins to machines whose
865 host key is not known or has changed.
867 When the user's identity has been accepted by the server, the server
868 either executes the given command, or logs into the machine and gives
869 the user a normal shell on the remote machine.
870 All communication with
871 the remote command or shell will be automatically encrypted.
873 If a pseudo-terminal has been allocated (normal login session), the
874 user may use the escape characters noted below.
876 If no pseudo-tty has been allocated,
877 the session is transparent and can be used to reliably transfer binary data.
878 On most systems, setting the escape character to
880 will also make the session transparent even if a tty is used.
882 The session terminates when the command or shell on the remote
883 machine exits and all X11 and TCP connections have been closed.
884 .Sh ESCAPE CHARACTERS
885 When a pseudo-terminal has been requested,
887 supports a number of functions through the use of an escape character.
889 A single tilde character can be sent as
891 or by following the tilde by a character other than those described below.
892 The escape character must always follow a newline to be interpreted as
894 The escape character can be changed in configuration files using the
896 configuration directive or on the command line by the
900 The supported escapes (assuming the default
910 List forwarded connections.
914 at logout when waiting for forwarded connection / X11 sessions to terminate.
916 Display a list of escape characters.
918 Send a BREAK to the remote system
919 (only useful for SSH protocol version 2 and if the peer supports it).
922 Currently this allows the addition of port forwardings using the
928 It also allows the cancellation of existing remote port-forwardings
931 .Fl KR Oo Ar bind_address : Oc Ar port .
933 .Ic !\& Ns Ar command
934 allows the user to execute a local command if the
935 .Ic PermitLocalCommand
938 Basic help is available, using the
942 Request rekeying of the connection
943 (only useful for SSH protocol version 2 and if the peer supports it).
946 Forwarding of arbitrary TCP connections over the secure channel can
947 be specified either on the command line or in a configuration file.
948 One possible application of TCP forwarding is a secure connection to a
949 mail server; another is going through firewalls.
951 In the example below, we look at encrypting communication between
952 an IRC client and server, even though the IRC server does not directly
953 support encrypted communications.
954 This works as follows:
955 the user connects to the remote host using
957 specifying a port to be used to forward connections
958 to the remote server.
959 After that it is possible to start the service which is to be encrypted
960 on the client machine,
961 connecting to the same local port,
964 will encrypt and forward the connection.
966 The following example tunnels an IRC session from client machine
970 .Dq server.example.com :
971 .Bd -literal -offset 4n
972 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
973 $ irc -c '#users' -p 1234 pinky 127.0.0.1
976 This tunnels a connection to IRC server
977 .Dq server.example.com ,
983 It doesn't matter which port is used,
984 as long as it's greater than 1023
985 (remember, only root can open sockets on privileged ports)
986 and doesn't conflict with any ports already in use.
987 The connection is forwarded to port 6667 on the remote server,
988 since that's the standard port for IRC services.
994 and the remote command
996 is specified to allow an amount of time
997 (10 seconds, in the example)
998 to start the service which is to be tunnelled.
999 If no connections are made within the time specified,
1007 (or see the description of the
1013 and the user is using X11 (the
1015 environment variable is set), the connection to the X11 display is
1016 automatically forwarded to the remote side in such a way that any X11
1017 programs started from the shell (or command) will go through the
1018 encrypted channel, and the connection to the real X server will be made
1019 from the local machine.
1020 The user should not manually set
1022 Forwarding of X11 connections can be
1023 configured on the command line or in configuration files.
1029 will point to the server machine, but with a display number greater than zero.
1030 This is normal, and happens because
1034 X server on the server machine for forwarding the
1035 connections over the encrypted channel.
1038 will also automatically set up Xauthority data on the server machine.
1039 For this purpose, it will generate a random authorization cookie,
1040 store it in Xauthority on the server, and verify that any forwarded
1041 connections carry this cookie and replace it by the real cookie when
1042 the connection is opened.
1043 The real authentication cookie is never
1044 sent to the server machine (and no cookies are sent in the plain).
1050 (or see the description of the
1055 the user is using an authentication agent, the connection to the agent
1056 is automatically forwarded to the remote side.
1057 .Sh VERIFYING HOST KEYS
1058 When connecting to a server for the first time,
1059 a fingerprint of the server's public key is presented to the user
1061 .Cm StrictHostKeyChecking
1063 Fingerprints can be determined using
1066 .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1068 If the fingerprint is already known, it can be matched
1069 and the key can be accepted or rejected.
1070 Because of the difficulty of comparing host keys
1071 just by looking at hex strings,
1072 there is also support to compare host keys visually,
1079 a small ASCII graphic gets displayed on every login to a server, no matter
1080 if the session itself is interactive or not.
1081 By learning the pattern a known server produces, a user can easily
1082 find out that the host key has changed when a completely different pattern
1084 Because these patterns are not unambiguous however, a pattern that looks
1085 similar to the pattern remembered only gives a good probability that the
1086 host key is the same, not guaranteed proof.
1088 To get a listing of the fingerprints along with their random art for
1089 all known hosts, the following command line can be used:
1091 .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1093 If the fingerprint is unknown,
1094 an alternative method of verification is available:
1095 SSH fingerprints verified by DNS.
1096 An additional resource record (RR),
1098 is added to a zonefile
1099 and the connecting client is able to match the fingerprint
1100 with that of the key presented.
1102 In this example, we are connecting a client to a server,
1103 .Dq host.example.com .
1104 The SSHFP resource records should first be added to the zonefile for
1106 .Bd -literal -offset indent
1107 $ ssh-keygen -r host.example.com.
1110 The output lines will have to be added to the zonefile.
1111 To check that the zone is answering fingerprint queries:
1113 .Dl $ dig -t SSHFP host.example.com
1115 Finally the client connects:
1116 .Bd -literal -offset indent
1117 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1119 Matching host key fingerprint found in DNS.
1120 Are you sure you want to continue connecting (yes/no)?
1124 .Cm VerifyHostKeyDNS
1127 for more information.
1128 .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1130 contains support for Virtual Private Network (VPN) tunnelling
1133 network pseudo-device,
1134 allowing two networks to be joined securely.
1137 configuration option
1139 controls whether the server supports this,
1140 and at what level (layer 2 or 3 traffic).
1142 The following example would connect client network 10.0.50.0/24
1143 with remote network 10.0.99.0/24 using a point-to-point connection
1144 from 10.1.1.1 to 10.1.1.2,
1145 provided that the SSH server running on the gateway to the remote network,
1146 at 192.168.1.15, allows it.
1149 .Bd -literal -offset indent
1150 # ssh -f -w 0:1 192.168.1.15 true
1151 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1152 # route add 10.0.99.0/24 10.1.1.2
1156 .Bd -literal -offset indent
1157 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1158 # route add 10.0.50.0/24 10.1.1.1
1161 Client access may be more finely tuned via the
1162 .Pa /root/.ssh/authorized_keys
1163 file (see below) and the
1166 The following entry would permit connections on
1170 and on tun device 2 from user
1175 .Dq forced-commands-only :
1176 .Bd -literal -offset 2n
1177 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1178 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1181 Since an SSH-based setup entails a fair amount of overhead,
1182 it may be more suited to temporary setups,
1183 such as for wireless VPNs.
1184 More permanent VPNs are better provided by tools such as
1190 will normally set the following environment variables:
1191 .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1195 variable indicates the location of the X11 server.
1196 It is automatically set by
1198 to point to a value of the form
1202 indicates the host where the shell runs, and
1204 is an integer \*(Ge 1.
1206 uses this special value to forward X11 connections over the secure
1208 The user should normally not set
1211 will render the X11 connection insecure (and will require the user to
1212 manually copy any required authorization cookies).
1214 Set to the path of the user's home directory.
1218 set for compatibility with systems that use this variable.
1220 Set to the path of the user's mailbox.
1224 as specified when compiling
1229 needs a passphrase, it will read the passphrase from the current
1230 terminal if it was run from a terminal.
1233 does not have a terminal associated with it but
1237 are set, it will execute the program specified by
1239 and open an X11 window to read the passphrase.
1240 This is particularly useful when calling
1245 (Note that on some machines it
1246 may be necessary to redirect the input from
1249 .It Ev SSH_AUTH_SOCK
1250 Identifies the path of a
1252 socket used to communicate with the agent.
1253 .It Ev SSH_CONNECTION
1254 Identifies the client and server ends of the connection.
1255 The variable contains
1256 four space-separated values: client IP address, client port number,
1257 server IP address, and server port number.
1258 .It Ev SSH_ORIGINAL_COMMAND
1259 This variable contains the original command line if a forced command
1261 It can be used to extract the original arguments.
1263 This is set to the name of the tty (path to the device) associated
1264 with the current shell or command.
1265 If the current session has no tty,
1266 this variable is not set.
1268 This variable is set to indicate the present time zone if it
1269 was set when the daemon was started (i.e. the daemon passes the value
1270 on to new connections).
1272 Set to the name of the user logging in.
1278 .Pa ~/.ssh/environment ,
1279 and adds lines of the format
1281 to the environment if the file exists and users are allowed to
1282 change their environment.
1283 For more information, see the
1284 .Cm PermitUserEnvironment
1288 .Bl -tag -width Ds -compact
1290 This file is used for host-based authentication (see above).
1291 On some machines this file may need to be
1292 world-readable if the user's home directory is on an NFS partition,
1296 Additionally, this file must be owned by the user,
1297 and must not have write permissions for anyone else.
1299 permission for most machines is read/write for the user, and not
1300 accessible by others.
1303 This file is used in exactly the same way as
1305 but allows host-based authentication without permitting login with
1309 This directory is the default location for all user-specific configuration
1310 and authentication information.
1311 There is no general requirement to keep the entire contents of this directory
1312 secret, but the recommended permissions are read/write/execute for the user,
1313 and not accessible by others.
1315 .It ~/.ssh/authorized_keys
1316 Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1317 The format of this file is described in the
1320 This file is not highly sensitive, but the recommended
1321 permissions are read/write for the user, and not accessible by others.
1324 This is the per-user configuration file.
1325 The file format and configuration options are described in
1327 Because of the potential for abuse, this file must have strict permissions:
1328 read/write for the user, and not accessible by others.
1330 .It ~/.ssh/environment
1331 Contains additional definitions for environment variables; see
1338 Contains the private key for authentication.
1340 contain sensitive data and should be readable by the user but not
1341 accessible by others (read/write/execute).
1343 will simply ignore a private key file if it is accessible by others.
1344 It is possible to specify a passphrase when
1345 generating the key which will be used to encrypt the
1346 sensitive part of this file using 3DES.
1348 .It ~/.ssh/identity.pub
1349 .It ~/.ssh/id_dsa.pub
1350 .It ~/.ssh/id_rsa.pub
1351 Contains the public key for authentication.
1353 sensitive and can (but need not) be readable by anyone.
1355 .It ~/.ssh/known_hosts
1356 Contains a list of host keys for all hosts the user has logged into
1357 that are not already in the systemwide list of known host keys.
1360 for further details of the format of this file.
1363 Commands in this file are executed by
1365 when the user logs in, just before the user's shell (or command) is
1369 manual page for more information.
1371 .It /etc/hosts.equiv
1372 This file is for host-based authentication (see above).
1373 It should only be writable by root.
1375 .It /etc/shosts.equiv
1376 This file is used in exactly the same way as
1378 but allows host-based authentication without permitting login with
1381 .It Pa /etc/ssh/ssh_config
1382 Systemwide configuration file.
1383 The file format and configuration options are described in
1386 .It /etc/ssh/ssh_host_key
1387 .It /etc/ssh/ssh_host_dsa_key
1388 .It /etc/ssh/ssh_host_rsa_key
1389 These three files contain the private parts of the host keys
1390 and are used for host-based authentication.
1391 If protocol version 1 is used,
1393 must be setuid root, since the host key is readable only by root.
1394 For protocol version 2,
1398 to access the host keys,
1399 eliminating the requirement that
1401 be setuid root when host-based authentication is used.
1406 .It /etc/ssh/ssh_known_hosts
1407 Systemwide list of known host keys.
1408 This file should be prepared by the
1409 system administrator to contain the public host keys of all machines in the
1411 It should be world-readable.
1414 for further details of the format of this file.
1417 Commands in this file are executed by
1419 when the user logs in, just before the user's shell (or command) is started.
1422 manual page for more information.
1438 .%T "The Secure Shell (SSH) Protocol Assigned Numbers"
1443 .%T "The Secure Shell (SSH) Protocol Architecture"
1448 .%T "The Secure Shell (SSH) Authentication Protocol"
1453 .%T "The Secure Shell (SSH) Transport Layer Protocol"
1458 .%T "The Secure Shell (SSH) Connection Protocol"
1463 .%T "Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints"
1468 .%T "Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)"
1473 .%T "The Secure Shell (SSH) Session Channel Break Extension"
1478 .%T "The Secure Shell (SSH) Transport Layer Encryption Modes"
1483 .%T "Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol"
1488 .%T "Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol"
1493 .%T "The Secure Shell (SSH) Public Key File Format"
1497 .%T "Hash Visualization: a New Technique to improve Real-World Security"
1501 .%O "International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)"
1504 OpenSSH is a derivative of the original and free
1505 ssh 1.2.12 release by Tatu Ylonen.
1506 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1507 Theo de Raadt and Dug Song
1508 removed many bugs, re-added newer features and
1510 Markus Friedl contributed the support for SSH
1511 protocol versions 1.5 and 2.0.
projects / FreeBSD / FreeBSD.git / blob