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 1.308 2010/08/04 05:37:01 djm Exp $
38 .Dd $Mdocdate: August 4 2010 $
43 .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
50 .Op Fl D Oo Ar bind_address : Oc Ns Ar port
51 .Op Fl e Ar escape_char
52 .Op Fl F Ar configfile
54 .Op Fl i Ar identity_file
55 .Op Fl L Oo Ar bind_address : Oc Ns Ar port : Ns Ar host : Ns Ar hostport
56 .Op Fl l Ar login_name
61 .Op Fl R Oo Ar bind_address : Oc Ns Ar port : Ns Ar host : Ns Ar hostport
63 .Op Fl W Ar host : Ns Ar port
64 .Op Fl w Ar local_tun Ns Op : Ns Ar remote_tun
65 .Oo Ar user Ns @ Oc Ns Ar hostname
70 (SSH client) is a program for logging into a remote machine and for
71 executing commands on a remote machine.
72 It is intended to replace rlogin and rsh,
73 and provide secure encrypted communications between
74 two untrusted hosts over an insecure network.
75 X11 connections and arbitrary TCP ports
76 can also be forwarded over the secure channel.
79 connects and logs into the specified
85 his/her identity to the remote machine using one of several methods
86 depending on the protocol version used (see below).
91 it is executed on the remote host instead of a login shell.
93 The options are as follows:
98 to try protocol version 1 only.
102 to try protocol version 2 only.
106 to use IPv4 addresses only.
110 to use IPv6 addresses only.
112 Enables forwarding of the authentication agent connection.
113 This can also be specified on a per-host basis in a configuration file.
115 Agent forwarding should be enabled with caution.
116 Users with the ability to bypass file permissions on the remote host
119 socket) can access the local agent through the forwarded connection.
120 An attacker cannot obtain key material from the agent,
121 however they can perform operations on the keys that enable them to
122 authenticate using the identities loaded into the agent.
124 Disables forwarding of the authentication agent connection.
125 .It Fl b Ar bind_address
128 on the local machine as the source address
130 Only useful on systems with more than one address.
132 Requests compression of all data (including stdin, stdout, stderr, and
133 data for forwarded X11 and TCP connections).
134 The compression algorithm is the same used by
138 can be controlled by the
140 option for protocol version 1.
141 Compression is desirable on modem lines and other
142 slow connections, but will only slow down things on fast networks.
143 The default value can be set on a host-by-host basis in the
144 configuration files; see the
147 .It Fl c Ar cipher_spec
148 Selects the cipher specification for encrypting the session.
150 Protocol version 1 allows specification of a single cipher.
151 The supported values are
157 (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
158 It is believed to be secure.
160 is a fast block cipher; it appears very secure and is much faster than
163 is only supported in the
165 client for interoperability with legacy protocol 1 implementations
166 that do not support the
169 Its use is strongly discouraged due to cryptographic weaknesses.
173 For protocol version 2,
175 is a comma-separated list of ciphers
176 listed in order of preference.
181 for more information.
184 .Oo Ar bind_address : Oc
190 application-level port forwarding.
191 This works by allocating a socket to listen to
193 on the local side, optionally bound to the specified
195 Whenever a connection is made to this port, the
196 connection is forwarded over the secure channel, and the application
197 protocol is then used to determine where to connect to from the
199 Currently the SOCKS4 and SOCKS5 protocols are supported, and
201 will act as a SOCKS server.
202 Only root can forward privileged ports.
203 Dynamic port forwardings can also be specified in the configuration file.
205 IPv6 addresses can be specified by enclosing the address in square brackets.
206 Only the superuser can forward privileged ports.
207 By default, the local port is bound in accordance with the
212 may be used to bind the connection to a specific address.
217 indicates that the listening port be bound for local use only, while an
220 indicates that the port should be available from all interfaces.
221 .It Fl e Ar escape_char
222 Sets the escape character for sessions with a pty (default:
224 The escape character is only recognized at the beginning of a line.
225 The escape character followed by a dot
227 closes the connection;
228 followed by control-Z suspends the connection;
229 and followed by itself sends the escape character once.
230 Setting the character to
232 disables any escapes and makes the session fully transparent.
233 .It Fl F Ar configfile
234 Specifies an alternative per-user configuration file.
235 If a configuration file is given on the command line,
236 the system-wide configuration file
237 .Pq Pa /etc/ssh/ssh_config
239 The default for the per-user configuration file is
244 to go to background just before command execution.
247 is going to ask for passwords or passphrases, but the user
248 wants it in the background.
251 The recommended way to start X11 programs at a remote site is with
253 .Ic ssh -f host xterm .
256 .Cm ExitOnForwardFailure
257 configuration option is set to
259 then a client started with
261 will wait for all remote port forwards to be successfully established
262 before placing itself in the background.
264 Allows remote hosts to connect to local forwarded ports.
266 Specify the PKCS#11 shared library
268 should use to communicate with a PKCS#11 token providing the user's
270 .It Fl i Ar identity_file
271 Selects a file from which the identity (private key) for
272 RSA or DSA authentication is read.
275 for protocol version 1, and
279 for protocol version 2.
280 Identity files may also be specified on
281 a per-host basis in the configuration file.
282 It is possible to have multiple
284 options (and multiple identities specified in
285 configuration files).
287 will also try to load certificate information from the filename obtained
290 to identity filenames.
292 Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
293 credentials to the server.
295 Disables forwarding (delegation) of GSSAPI credentials to the server.
298 .Oo Ar bind_address : Oc
299 .Ar port : host : hostport
302 Specifies that the given port on the local (client) host is to be
303 forwarded to the given host and port on the remote side.
304 This works by allocating a socket to listen to
306 on the local side, optionally bound to the specified
308 Whenever a connection is made to this port, the
309 connection is forwarded over the secure channel, and a connection is
314 from the remote machine.
315 Port forwardings can also be specified in the configuration file.
316 IPv6 addresses can be specified by enclosing the address in square brackets.
317 Only the superuser can forward privileged ports.
318 By default, the local port is bound in accordance with the
323 may be used to bind the connection to a specific address.
328 indicates that the listening port be bound for local use only, while an
331 indicates that the port should be available from all interfaces.
332 .It Fl l Ar login_name
333 Specifies the user to log in as on the remote machine.
334 This also may be specified on a per-host basis in the configuration file.
340 mode for connection sharing.
347 mode with confirmation required before slave connections are accepted.
348 Refer to the description of
354 Additionally, for protocol version 2 a comma-separated list of MAC
355 (message authentication code) algorithms can
356 be specified in order of preference.
359 keyword for more information.
361 Do not execute a remote command.
362 This is useful for just forwarding ports
363 (protocol version 2 only).
367 (actually, prevents reading from stdin).
368 This must be used when
370 is run in the background.
371 A common trick is to use this to run X11 programs on a remote machine.
373 .Ic ssh -n shadows.cs.hut.fi emacs &
374 will start an emacs on shadows.cs.hut.fi, and the X11
375 connection will be automatically forwarded over an encrypted channel.
378 program will be put in the background.
379 (This does not work if
381 needs to ask for a password or passphrase; see also the
385 Control an active connection multiplexing master process.
388 option is specified, the
390 argument is interpreted and passed to the master process.
393 (check that the master process is running),
395 (request forwardings without command execution) and
397 (request the master to exit).
399 Can be used to give options in the format used in the configuration file.
400 This is useful for specifying options for which there is no separate
402 For full details of the options listed below, and their possible values, see
405 .Bl -tag -width Ds -offset indent -compact
409 .It ChallengeResponseAuthentication
413 .It ClearAllForwardings
416 .It ConnectionAttempts
422 .It ExitOnForwardFailure
425 .It ForwardX11Trusted
427 .It GlobalKnownHostsFile
428 .It GSSAPIAuthentication
429 .It GSSAPIDelegateCredentials
432 .It HostbasedAuthentication
433 .It HostKeyAlgorithms
438 .It KbdInteractiveDevices
443 .It NoHostAuthenticationForLocalhost
444 .It NumberOfPasswordPrompts
445 .It PasswordAuthentication
446 .It PermitLocalCommand
449 .It PreferredAuthentications
452 .It PubkeyAuthentication
455 .It RhostsRSAAuthentication
456 .It RSAAuthentication
458 .It ServerAliveInterval
459 .It ServerAliveCountMax
460 .It StrictHostKeyChecking
464 .It UsePrivilegedPort
466 .It UserKnownHostsFile
472 Port to connect to on the remote host.
473 This can be specified on a
474 per-host basis in the configuration file.
477 Causes most warning and diagnostic messages to be suppressed.
480 .Oo Ar bind_address : Oc
481 .Ar port : host : hostport
484 Specifies that the given port on the remote (server) host is to be
485 forwarded to the given host and port on the local side.
486 This works by allocating a socket to listen to
488 on the remote side, and whenever a connection is made to this port, the
489 connection is forwarded over the secure channel, and a connection is
494 from the local machine.
496 Port forwardings can also be specified in the configuration file.
497 Privileged ports can be forwarded only when
498 logging in as root on the remote machine.
499 IPv6 addresses can be specified by enclosing the address in square braces.
501 By default, the listening socket on the server will be bound to the loopback
503 This may be overridden by specifying a
509 indicates that the remote socket should listen on all interfaces.
512 will only succeed if the server's
514 option is enabled (see
515 .Xr sshd_config 5 ) .
521 the listen port will be dynamically allocated on the server and reported
522 to the client at run time.
523 When used together with
525 the allocated port will be printed to the standard output.
527 Specifies the location of a control socket for connection sharing,
530 to disable connection sharing.
531 Refer to the description of
539 May be used to request invocation of a subsystem on the remote system.
540 Subsystems are a feature of the SSH2 protocol which facilitate the use
541 of SSH as a secure transport for other applications (eg.\&
543 The subsystem is specified as the remote command.
545 Disable pseudo-tty allocation.
547 Force pseudo-tty allocation.
548 This can be used to execute arbitrary
549 screen-based programs on a remote machine, which can be very useful,
550 e.g. when implementing menu services.
553 options force tty allocation, even if
557 Display the version number and exit.
562 to print debugging messages about its progress.
564 debugging connection, authentication, and configuration problems.
567 options increase the verbosity.
569 .It Fl W Ar host : Ns Ar port
570 Requests that standard input and output on the client be forwarded to
574 over the secure channel.
578 .Cm ExitOnForwardFailure
580 .Cm ClearAllForwardings
581 and works with Protocol version 2 only.
583 .Ar local_tun Ns Op : Ns Ar remote_tun
587 device forwarding with the specified
589 devices between the client
594 The devices may be specified by numerical ID or the keyword
596 which uses the next available tunnel device.
599 is not specified, it defaults to
609 directive is unset, it is set to the default tunnel mode, which is
612 Enables X11 forwarding.
613 This can also be specified on a per-host basis in a configuration file.
615 X11 forwarding should be enabled with caution.
616 Users with the ability to bypass file permissions on the remote host
617 (for the user's X authorization database)
618 can access the local X11 display through the forwarded connection.
619 An attacker may then be able to perform activities such as keystroke monitoring.
621 For this reason, X11 forwarding is subjected to X11 SECURITY extension
622 restrictions by default.
627 .Cm ForwardX11Trusted
630 for more information.
632 Disables X11 forwarding.
634 Enables trusted X11 forwarding.
635 Trusted X11 forwardings are not subjected to the X11 SECURITY extension
638 Send log information using the
641 By default this information is sent to stderr.
645 may additionally obtain configuration data from
646 a per-user configuration file and a system-wide configuration file.
647 The file format and configuration options are described in
651 exits with the exit status of the remote command or with 255
652 if an error occurred.
654 The OpenSSH SSH client supports SSH protocols 1 and 2.
655 The default is to use protocol 2 only,
656 though this can be changed via the
665 Both protocols support similar authentication methods,
666 but protocol 2 is the default since
667 it provides additional mechanisms for confidentiality
668 (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
669 and integrity (hmac-md5, hmac-sha1, umac-64, hmac-ripemd160).
670 Protocol 1 lacks a strong mechanism for ensuring the
671 integrity of the connection.
673 The methods available for authentication are:
674 GSSAPI-based authentication,
675 host-based authentication,
676 public key authentication,
677 challenge-response authentication,
678 and password authentication.
679 Authentication methods are tried in the order specified above,
680 though protocol 2 has a configuration option to change the default order:
681 .Cm PreferredAuthentications .
683 Host-based authentication works as follows:
684 If the machine the user logs in from is listed in
687 .Pa /etc/ssh/shosts.equiv
688 on the remote machine, and the user names are
689 the same on both sides, or if the files
693 exist in the user's home directory on the
694 remote machine and contain a line containing the name of the client
695 machine and the name of the user on that machine, the user is
696 considered for login.
697 Additionally, the server
699 be able to verify the client's
700 host key (see the description of
701 .Pa /etc/ssh/ssh_known_hosts
703 .Pa ~/.ssh/known_hosts ,
705 for login to be permitted.
706 This authentication method closes security holes due to IP
707 spoofing, DNS spoofing, and routing spoofing.
708 [Note to the administrator:
709 .Pa /etc/hosts.equiv ,
711 and the rlogin/rsh protocol in general, are inherently insecure and should be
712 disabled if security is desired.]
714 Public key authentication works as follows:
715 The scheme is based on public-key cryptography,
717 where encryption and decryption are done using separate keys,
718 and it is unfeasible to derive the decryption key from the encryption key.
719 The idea is that each user creates a public/private
720 key pair for authentication purposes.
721 The server knows the public key, and only the user knows the private key.
723 implements public key authentication protocol automatically,
724 using either the RSA or DSA algorithms.
725 Protocol 1 is restricted to using only RSA keys,
726 but protocol 2 may use either.
731 contains a brief discussion of the two algorithms.
734 .Pa ~/.ssh/authorized_keys
735 lists the public keys that are permitted for logging in.
736 When the user logs in, the
738 program tells the server which key pair it would like to use for
740 The client proves that it has access to the private key
741 and the server checks that the corresponding public key
742 is authorized to accept the account.
744 The user creates his/her key pair by running
746 This stores the private key in
754 and stores the public key in
755 .Pa ~/.ssh/identity.pub
757 .Pa ~/.ssh/id_dsa.pub
760 .Pa ~/.ssh/id_rsa.pub
762 in the user's home directory.
763 The user should then copy the public key
765 .Pa ~/.ssh/authorized_keys
766 in his/her home directory on the remote machine.
769 file corresponds to the conventional
771 file, and has one key
772 per line, though the lines can be very long.
773 After this, the user can log in without giving the password.
775 A variation on public key authentication
776 is available in the form of certificate authentication:
777 instead of a set of public/private keys,
778 signed certificates are used.
779 This has the advantage that a single trusted certification authority
780 can be used in place of many public/private keys.
785 for more information.
787 The most convenient way to use public key or certificate authentication
788 may be with an 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
892 It also allows the cancellation of existing remote port-forwardings
895 .Fl KR Oo Ar bind_address : Oc Ar port .
897 .Ic !\& Ns Ar command
898 allows the user to execute a local command if the
899 .Ic PermitLocalCommand
902 Basic help is available, using the
906 Request rekeying of the connection
907 (only useful for SSH protocol version 2 and if the peer supports it).
910 Forwarding of arbitrary TCP connections over the secure channel can
911 be specified either on the command line or in a configuration file.
912 One possible application of TCP forwarding is a secure connection to a
913 mail server; another is going through firewalls.
915 In the example below, we look at encrypting communication between
916 an IRC client and server, even though the IRC server does not directly
917 support encrypted communications.
918 This works as follows:
919 the user connects to the remote host using
921 specifying a port to be used to forward connections
922 to the remote server.
923 After that it is possible to start the service which is to be encrypted
924 on the client machine,
925 connecting to the same local port,
928 will encrypt and forward the connection.
930 The following example tunnels an IRC session from client machine
934 .Dq server.example.com :
935 .Bd -literal -offset 4n
936 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
937 $ irc -c '#users' -p 1234 pinky 127.0.0.1
940 This tunnels a connection to IRC server
941 .Dq server.example.com ,
947 It doesn't matter which port is used,
948 as long as it's greater than 1023
949 (remember, only root can open sockets on privileged ports)
950 and doesn't conflict with any ports already in use.
951 The connection is forwarded to port 6667 on the remote server,
952 since that's the standard port for IRC services.
958 and the remote command
960 is specified to allow an amount of time
961 (10 seconds, in the example)
962 to start the service which is to be tunnelled.
963 If no connections are made within the time specified,
971 (or see the description of the
977 and the user is using X11 (the
979 environment variable is set), the connection to the X11 display is
980 automatically forwarded to the remote side in such a way that any X11
981 programs started from the shell (or command) will go through the
982 encrypted channel, and the connection to the real X server will be made
983 from the local machine.
984 The user should not manually set
986 Forwarding of X11 connections can be
987 configured on the command line or in configuration files.
988 Take note that X11 forwarding can represent a security hazard.
994 will point to the server machine, but with a display number greater than zero.
995 This is normal, and happens because
999 X server on the server machine for forwarding the
1000 connections over the encrypted channel.
1003 will also automatically set up Xauthority data on the server machine.
1004 For this purpose, it will generate a random authorization cookie,
1005 store it in Xauthority on the server, and verify that any forwarded
1006 connections carry this cookie and replace it by the real cookie when
1007 the connection is opened.
1008 The real authentication cookie is never
1009 sent to the server machine (and no cookies are sent in the plain).
1015 (or see the description of the
1020 the user is using an authentication agent, the connection to the agent
1021 is automatically forwarded to the remote side.
1022 .Sh VERIFYING HOST KEYS
1023 When connecting to a server for the first time,
1024 a fingerprint of the server's public key is presented to the user
1026 .Cm StrictHostKeyChecking
1028 Fingerprints can be determined using
1031 .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1033 If the fingerprint is already known, it can be matched
1034 and the key can be accepted or rejected.
1035 Because of the difficulty of comparing host keys
1036 just by looking at hex strings,
1037 there is also support to compare host keys visually,
1044 a small ASCII graphic gets displayed on every login to a server, no matter
1045 if the session itself is interactive or not.
1046 By learning the pattern a known server produces, a user can easily
1047 find out that the host key has changed when a completely different pattern
1049 Because these patterns are not unambiguous however, a pattern that looks
1050 similar to the pattern remembered only gives a good probability that the
1051 host key is the same, not guaranteed proof.
1053 To get a listing of the fingerprints along with their random art for
1054 all known hosts, the following command line can be used:
1056 .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1058 If the fingerprint is unknown,
1059 an alternative method of verification is available:
1060 SSH fingerprints verified by DNS.
1061 An additional resource record (RR),
1063 is added to a zonefile
1064 and the connecting client is able to match the fingerprint
1065 with that of the key presented.
1067 In this example, we are connecting a client to a server,
1068 .Dq host.example.com .
1069 The SSHFP resource records should first be added to the zonefile for
1071 .Bd -literal -offset indent
1072 $ ssh-keygen -r host.example.com.
1075 The output lines will have to be added to the zonefile.
1076 To check that the zone is answering fingerprint queries:
1078 .Dl $ dig -t SSHFP host.example.com
1080 Finally the client connects:
1081 .Bd -literal -offset indent
1082 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1084 Matching host key fingerprint found in DNS.
1085 Are you sure you want to continue connecting (yes/no)?
1089 .Cm VerifyHostKeyDNS
1092 for more information.
1093 .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1095 contains support for Virtual Private Network (VPN) tunnelling
1098 network pseudo-device,
1099 allowing two networks to be joined securely.
1102 configuration option
1104 controls whether the server supports this,
1105 and at what level (layer 2 or 3 traffic).
1107 The following example would connect client network 10.0.50.0/24
1108 with remote network 10.0.99.0/24 using a point-to-point connection
1109 from 10.1.1.1 to 10.1.1.2,
1110 provided that the SSH server running on the gateway to the remote network,
1111 at 192.168.1.15, allows it.
1114 .Bd -literal -offset indent
1115 # ssh -f -w 0:1 192.168.1.15 true
1116 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1117 # route add 10.0.99.0/24 10.1.1.2
1121 .Bd -literal -offset indent
1122 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1123 # route add 10.0.50.0/24 10.1.1.1
1126 Client access may be more finely tuned via the
1127 .Pa /root/.ssh/authorized_keys
1128 file (see below) and the
1131 The following entry would permit connections on
1135 and on tun device 2 from user
1140 .Dq forced-commands-only :
1141 .Bd -literal -offset 2n
1142 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1143 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1146 Since an SSH-based setup entails a fair amount of overhead,
1147 it may be more suited to temporary setups,
1148 such as for wireless VPNs.
1149 More permanent VPNs are better provided by tools such as
1155 will normally set the following environment variables:
1156 .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1160 variable indicates the location of the X11 server.
1161 It is automatically set by
1163 to point to a value of the form
1167 indicates the host where the shell runs, and
1169 is an integer \*(Ge 1.
1171 uses this special value to forward X11 connections over the secure
1173 The user should normally not set
1176 will render the X11 connection insecure (and will require the user to
1177 manually copy any required authorization cookies).
1179 Set to the path of the user's home directory.
1183 set for compatibility with systems that use this variable.
1185 Set to the path of the user's mailbox.
1189 as specified when compiling
1194 needs a passphrase, it will read the passphrase from the current
1195 terminal if it was run from a terminal.
1198 does not have a terminal associated with it but
1202 are set, it will execute the program specified by
1204 and open an X11 window to read the passphrase.
1205 This is particularly useful when calling
1210 (Note that on some machines it
1211 may be necessary to redirect the input from
1214 .It Ev SSH_AUTH_SOCK
1215 Identifies the path of a
1217 socket used to communicate with the agent.
1218 .It Ev SSH_CONNECTION
1219 Identifies the client and server ends of the connection.
1220 The variable contains
1221 four space-separated values: client IP address, client port number,
1222 server IP address, and server port number.
1223 .It Ev SSH_ORIGINAL_COMMAND
1224 This variable contains the original command line if a forced command
1226 It can be used to extract the original arguments.
1228 This is set to the name of the tty (path to the device) associated
1229 with the current shell or command.
1230 If the current session has no tty,
1231 this variable is not set.
1233 This variable is set to indicate the present time zone if it
1234 was set when the daemon was started (i.e. the daemon passes the value
1235 on to new connections).
1237 Set to the name of the user logging in.
1243 .Pa ~/.ssh/environment ,
1244 and adds lines of the format
1246 to the environment if the file exists and users are allowed to
1247 change their environment.
1248 For more information, see the
1249 .Cm PermitUserEnvironment
1253 .Bl -tag -width Ds -compact
1255 This file is used for host-based authentication (see above).
1256 On some machines this file may need to be
1257 world-readable if the user's home directory is on an NFS partition,
1261 Additionally, this file must be owned by the user,
1262 and must not have write permissions for anyone else.
1264 permission for most machines is read/write for the user, and not
1265 accessible by others.
1268 This file is used in exactly the same way as
1270 but allows host-based authentication without permitting login with
1274 This directory is the default location for all user-specific configuration
1275 and authentication information.
1276 There is no general requirement to keep the entire contents of this directory
1277 secret, but the recommended permissions are read/write/execute for the user,
1278 and not accessible by others.
1280 .It ~/.ssh/authorized_keys
1281 Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1282 The format of this file is described in the
1285 This file is not highly sensitive, but the recommended
1286 permissions are read/write for the user, and not accessible by others.
1289 This is the per-user configuration file.
1290 The file format and configuration options are described in
1292 Because of the potential for abuse, this file must have strict permissions:
1293 read/write for the user, and not accessible by others.
1295 .It ~/.ssh/environment
1296 Contains additional definitions for environment variables; see
1303 Contains the private key for authentication.
1305 contain sensitive data and should be readable by the user but not
1306 accessible by others (read/write/execute).
1308 will simply ignore a private key file if it is accessible by others.
1309 It is possible to specify a passphrase when
1310 generating the key which will be used to encrypt the
1311 sensitive part of this file using 3DES.
1313 .It ~/.ssh/identity.pub
1314 .It ~/.ssh/id_dsa.pub
1315 .It ~/.ssh/id_rsa.pub
1316 Contains the public key for authentication.
1318 sensitive and can (but need not) be readable by anyone.
1320 .It ~/.ssh/known_hosts
1321 Contains a list of host keys for all hosts the user has logged into
1322 that are not already in the systemwide list of known host keys.
1325 for further details of the format of this file.
1328 Commands in this file are executed by
1330 when the user logs in, just before the user's shell (or command) is
1334 manual page for more information.
1336 .It /etc/hosts.equiv
1337 This file is for host-based authentication (see above).
1338 It should only be writable by root.
1340 .It /etc/ssh/shosts.equiv
1341 This file is used in exactly the same way as
1343 but allows host-based authentication without permitting login with
1346 .It Pa /etc/ssh/ssh_config
1347 Systemwide configuration file.
1348 The file format and configuration options are described in
1351 .It /etc/ssh/ssh_host_key
1352 .It /etc/ssh/ssh_host_dsa_key
1353 .It /etc/ssh/ssh_host_rsa_key
1354 These three files contain the private parts of the host keys
1355 and are used for host-based authentication.
1356 If protocol version 1 is used,
1358 must be setuid root, since the host key is readable only by root.
1359 For protocol version 2,
1363 to access the host keys,
1364 eliminating the requirement that
1366 be setuid root when host-based authentication is used.
1371 .It /etc/ssh/ssh_known_hosts
1372 Systemwide list of known host keys.
1373 This file should be prepared by the
1374 system administrator to contain the public host keys of all machines in the
1376 It should be world-readable.
1379 for further details of the format of this file.
1382 Commands in this file are executed by
1384 when the user logs in, just before the user's shell (or command) is started.
1387 manual page for more information.
1403 .%T "The Secure Shell (SSH) Protocol Assigned Numbers"
1408 .%T "The Secure Shell (SSH) Protocol Architecture"
1413 .%T "The Secure Shell (SSH) Authentication Protocol"
1418 .%T "The Secure Shell (SSH) Transport Layer Protocol"
1423 .%T "The Secure Shell (SSH) Connection Protocol"
1428 .%T "Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints"
1433 .%T "Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)"
1438 .%T "The Secure Shell (SSH) Session Channel Break Extension"
1443 .%T "The Secure Shell (SSH) Transport Layer Encryption Modes"
1448 .%T "Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol"
1453 .%T "Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol"
1458 .%T "The Secure Shell (SSH) Public Key File Format"
1462 .%T "Hash Visualization: a New Technique to improve Real-World Security"
1466 .%O "International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)"
1469 OpenSSH is a derivative of the original and free
1470 ssh 1.2.12 release by Tatu Ylonen.
1471 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1472 Theo de Raadt and Dug Song
1473 removed many bugs, re-added newer features and
1475 Markus Friedl contributed the support for SSH
1476 protocol versions 1.5 and 2.0.
tuxillo's projects / dragonfly.git / blob