3 .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
4 .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
5 .\" All rights reserved
7 .\" As far as I am concerned, the code I have written for this software
8 .\" can be used freely for any purpose. Any derived versions of this
9 .\" software must be clearly marked as such, and if the derived work is
10 .\" incompatible with the protocol description in the RFC file, it must be
11 .\" called by a name other than "ssh" or "Secure Shell".
13 .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
14 .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
15 .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
17 .\" Redistribution and use in source and binary forms, with or without
18 .\" modification, are permitted provided that the following conditions
20 .\" 1. Redistributions of source code must retain the above copyright
21 .\" notice, this list of conditions and the following disclaimer.
22 .\" 2. Redistributions in binary form must reproduce the above copyright
23 .\" notice, this list of conditions and the following disclaimer in the
24 .\" documentation and/or other materials provided with the distribution.
26 .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
27 .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
28 .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
29 .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
30 .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
31 .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
32 .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
33 .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
34 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
35 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
37 .\" $OpenBSD: ssh.1,v 1.282 2009/02/12 03:44:25 djm Exp $
38 .Dd $Mdocdate: February 12 2009 $
43 .Nd OpenSSH SSH client (remote login program)
46 .Op Fl 1246AaCfgKkMNnqsTtVvXxYy
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 .
295 .Cm ExitOnForwardFailure
296 configuration option is set to
298 then a client started with
300 will wait for all remote port forwards to be successfully established
301 before placing itself in the background.
303 Allows remote hosts to connect to local forwarded ports.
304 .It Fl I Ar smartcard_device
307 should use to communicate with a smartcard used for storing the user's
309 This option is only available if support for smartcard devices
310 is compiled in (default is no support).
311 .It Fl i Ar identity_file
312 Selects a file from which the identity (private key) for
313 RSA or DSA authentication is read.
316 for protocol version 1, and
320 for protocol version 2.
321 Identity files may also be specified on
322 a per-host basis in the configuration file.
323 It is possible to have multiple
325 options (and multiple identities specified in
326 configuration files).
328 Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
329 credentials to the server.
331 Disables forwarding (delegation) of GSSAPI credentials to the server.
334 .Oo Ar bind_address : Oc
335 .Ar port : host : hostport
338 Specifies that the given port on the local (client) host is to be
339 forwarded to the given host and port on the remote side.
340 This works by allocating a socket to listen to
342 on the local side, optionally bound to the specified
344 Whenever a connection is made to this port, the
345 connection is forwarded over the secure channel, and a connection is
350 from the remote machine.
351 Port forwardings can also be specified in the configuration file.
352 IPv6 addresses can be specified with an alternative syntax:
355 .Op Ar bind_address No /
356 .Ar port No / Ar host No /
360 or by enclosing the address in square brackets.
361 Only the superuser can forward privileged ports.
362 By default, the local port is bound in accordance with the
367 may be used to bind the connection to a specific address.
372 indicates that the listening port be bound for local use only, while an
375 indicates that the port should be available from all interfaces.
376 .It Fl l Ar login_name
377 Specifies the user to log in as on the remote machine.
378 This also may be specified on a per-host basis in the configuration file.
384 mode for connection sharing.
391 mode with confirmation required before slave connections are accepted.
392 Refer to the description of
398 Additionally, for protocol version 2 a comma-separated list of MAC
399 (message authentication code) algorithms can
400 be specified in order of preference.
403 keyword for more information.
405 Do not execute a remote command.
406 This is useful for just forwarding ports
407 (protocol version 2 only).
411 (actually, prevents reading from stdin).
412 This must be used when
414 is run in the background.
415 A common trick is to use this to run X11 programs on a remote machine.
417 .Ic ssh -n shadows.cs.hut.fi emacs &
418 will start an emacs on shadows.cs.hut.fi, and the X11
419 connection will be automatically forwarded over an encrypted channel.
422 program will be put in the background.
423 (This does not work if
425 needs to ask for a password or passphrase; see also the
429 Control an active connection multiplexing master process.
432 option is specified, the
434 argument is interpreted and passed to the master process.
437 (check that the master process is running) and
439 (request the master to exit).
441 Can be used to give options in the format used in the configuration file.
442 This is useful for specifying options for which there is no separate
444 For full details of the options listed below, and their possible values, see
447 .Bl -tag -width Ds -offset indent -compact
451 .It ChallengeResponseAuthentication
455 .It ClearAllForwardings
458 .It ConnectionAttempts
464 .It ExitOnForwardFailure
467 .It ForwardX11Trusted
469 .It GlobalKnownHostsFile
470 .It GSSAPIAuthentication
471 .It GSSAPIDelegateCredentials
474 .It HostbasedAuthentication
475 .It HostKeyAlgorithms
480 .It KbdInteractiveDevices
485 .It NoHostAuthenticationForLocalhost
486 .It NumberOfPasswordPrompts
487 .It PasswordAuthentication
488 .It PermitLocalCommand
490 .It PreferredAuthentications
493 .It PubkeyAuthentication
496 .It RhostsRSAAuthentication
497 .It RSAAuthentication
499 .It ServerAliveInterval
500 .It ServerAliveCountMax
502 .It StrictHostKeyChecking
506 .It UsePrivilegedPort
508 .It UserKnownHostsFile
514 Port to connect to on the remote host.
515 This can be specified on a
516 per-host basis in the configuration file.
519 Causes most warning and diagnostic messages to be suppressed.
522 .Oo Ar bind_address : Oc
523 .Ar port : host : hostport
526 Specifies that the given port on the remote (server) host is to be
527 forwarded to the given host and port on the local side.
528 This works by allocating a socket to listen to
530 on the remote side, and whenever a connection is made to this port, the
531 connection is forwarded over the secure channel, and a connection is
536 from the local machine.
538 Port forwardings can also be specified in the configuration file.
539 Privileged ports can be forwarded only when
540 logging in as root on the remote machine.
541 IPv6 addresses can be specified by enclosing the address in square braces or
542 using an alternative syntax:
545 .Op Ar bind_address No /
546 .Ar host No / Ar port No /
551 By default, the listening socket on the server will be bound to the loopback
553 This may be overridden by specifying a
559 indicates that the remote socket should listen on all interfaces.
562 will only succeed if the server's
564 option is enabled (see
565 .Xr sshd_config 5 ) .
571 the listen port will be dynamically allocated on the server and reported
572 to the client at run time.
574 Specifies the location of a control socket for connection sharing.
575 Refer to the description of
583 May be used to request invocation of a subsystem on the remote system.
584 Subsystems are a feature of the SSH2 protocol which facilitate the use
585 of SSH as a secure transport for other applications (eg.\&
587 The subsystem is specified as the remote command.
589 Disable pseudo-tty allocation.
591 Force pseudo-tty allocation.
592 This can be used to execute arbitrary
593 screen-based programs on a remote machine, which can be very useful,
594 e.g. when implementing menu services.
597 options force tty allocation, even if
601 Display the version number and exit.
606 to print debugging messages about its progress.
608 debugging connection, authentication, and configuration problems.
611 options increase the verbosity.
614 .Ar local_tun Ns Op : Ns Ar remote_tun
618 device forwarding with the specified
620 devices between the client
625 The devices may be specified by numerical ID or the keyword
627 which uses the next available tunnel device.
630 is not specified, it defaults to
640 directive is unset, it is set to the default tunnel mode, which is
643 Enables X11 forwarding.
644 This can also be specified on a per-host basis in a configuration file.
646 X11 forwarding should be enabled with caution.
647 Users with the ability to bypass file permissions on the remote host
648 (for the user's X authorization database)
649 can access the local X11 display through the forwarded connection.
650 An attacker may then be able to perform activities such as keystroke monitoring.
652 For this reason, X11 forwarding is subjected to X11 SECURITY extension
653 restrictions by default.
658 .Cm ForwardX11Trusted
661 for more information.
663 Disables X11 forwarding.
665 Enables trusted X11 forwarding.
666 Trusted X11 forwardings are not subjected to the X11 SECURITY extension
669 Send log information using the
672 By default this information is sent to stderr.
676 may additionally obtain configuration data from
677 a per-user configuration file and a system-wide configuration file.
678 The file format and configuration options are described in
682 exits with the exit status of the remote command or with 255
683 if an error occurred.
685 The OpenSSH SSH client supports SSH protocols 1 and 2.
686 Protocol 2 is the default, with
688 falling back to protocol 1 if it detects protocol 2 is unsupported.
689 These settings may be altered using the
693 or enforced using the
698 Both protocols support similar authentication methods,
699 but protocol 2 is preferred since
700 it provides additional mechanisms for confidentiality
701 (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
702 and integrity (hmac-md5, hmac-sha1, umac-64, hmac-ripemd160).
703 Protocol 1 lacks a strong mechanism for ensuring the
704 integrity of the connection.
706 The methods available for authentication are:
707 GSSAPI-based authentication,
708 host-based authentication,
709 public key authentication,
710 challenge-response authentication,
711 and password authentication.
712 Authentication methods are tried in the order specified above,
713 though protocol 2 has a configuration option to change the default order:
714 .Cm PreferredAuthentications .
716 Host-based authentication works as follows:
717 If the machine the user logs in from is listed in
720 .Pa /etc/ssh/shosts.equiv
721 on the remote machine, and the user names are
722 the same on both sides, or if the files
726 exist in the user's home directory on the
727 remote machine and contain a line containing the name of the client
728 machine and the name of the user on that machine, the user is
729 considered for login.
730 Additionally, the server
732 be able to verify the client's
733 host key (see the description of
734 .Pa /etc/ssh/ssh_known_hosts
736 .Pa ~/.ssh/known_hosts ,
738 for login to be permitted.
739 This authentication method closes security holes due to IP
740 spoofing, DNS spoofing, and routing spoofing.
741 [Note to the administrator:
742 .Pa /etc/hosts.equiv ,
744 and the rlogin/rsh protocol in general, are inherently insecure and should be
745 disabled if security is desired.]
747 Public key authentication works as follows:
748 The scheme is based on public-key cryptography,
750 where encryption and decryption are done using separate keys,
751 and it is unfeasible to derive the decryption key from the encryption key.
752 The idea is that each user creates a public/private
753 key pair for authentication purposes.
754 The server knows the public key, and only the user knows the private key.
756 implements public key authentication protocol automatically,
757 using either the RSA or DSA algorithms.
758 Protocol 1 is restricted to using only RSA keys,
759 but protocol 2 may use either.
764 contains a brief discussion of the two algorithms.
767 .Pa ~/.ssh/authorized_keys
768 lists the public keys that are permitted for logging in.
769 When the user logs in, the
771 program tells the server which key pair it would like to use for
773 The client proves that it has access to the private key
774 and the server checks that the corresponding public key
775 is authorized to accept the account.
777 The user creates his/her key pair by running
779 This stores the private key in
787 and stores the public key in
788 .Pa ~/.ssh/identity.pub
790 .Pa ~/.ssh/id_dsa.pub
793 .Pa ~/.ssh/id_rsa.pub
795 in the user's home directory.
796 The user should then copy the public key
798 .Pa ~/.ssh/authorized_keys
799 in his/her home directory on the remote machine.
802 file corresponds to the conventional
804 file, and has one key
805 per line, though the lines can be very long.
806 After this, the user can log in without giving the password.
808 The most convenient way to use public key authentication may be with an
809 authentication agent.
812 for more information.
814 Challenge-response authentication works as follows:
815 The server sends an arbitrary
817 text, and prompts for a response.
818 Protocol 2 allows multiple challenges and responses;
819 protocol 1 is restricted to just one challenge/response.
820 Examples of challenge-response authentication include
821 BSD Authentication (see
823 and PAM (some non-OpenBSD systems).
825 Finally, if other authentication methods fail,
827 prompts the user for a password.
828 The password is sent to the remote
829 host for checking; however, since all communications are encrypted,
830 the password cannot be seen by someone listening on the network.
833 automatically maintains and checks a database containing
834 identification for all hosts it has ever been used with.
835 Host keys are stored in
836 .Pa ~/.ssh/known_hosts
837 in the user's home directory.
838 Additionally, the file
839 .Pa /etc/ssh/ssh_known_hosts
840 is automatically checked for known hosts.
841 Any new hosts are automatically added to the user's file.
842 If a host's identification ever changes,
844 warns about this and disables password authentication to prevent
845 server spoofing or man-in-the-middle attacks,
846 which could otherwise be used to circumvent the encryption.
848 .Cm StrictHostKeyChecking
849 option can be used to control logins to machines whose
850 host key is not known or has changed.
852 When the user's identity has been accepted by the server, the server
853 either executes the given command, or logs into the machine and gives
854 the user a normal shell on the remote machine.
855 All communication with
856 the remote command or shell will be automatically encrypted.
858 If a pseudo-terminal has been allocated (normal login session), the
859 user may use the escape characters noted below.
861 If no pseudo-tty has been allocated,
862 the session is transparent and can be used to reliably transfer binary data.
863 On most systems, setting the escape character to
865 will also make the session transparent even if a tty is used.
867 The session terminates when the command or shell on the remote
868 machine exits and all X11 and TCP connections have been closed.
869 .Sh ESCAPE CHARACTERS
870 When a pseudo-terminal has been requested,
872 supports a number of functions through the use of an escape character.
874 A single tilde character can be sent as
876 or by following the tilde by a character other than those described below.
877 The escape character must always follow a newline to be interpreted as
879 The escape character can be changed in configuration files using the
881 configuration directive or on the command line by the
885 The supported escapes (assuming the default
895 List forwarded connections.
899 at logout when waiting for forwarded connection / X11 sessions to terminate.
901 Display a list of escape characters.
903 Send a BREAK to the remote system
904 (only useful for SSH protocol version 2 and if the peer supports it).
907 Currently this allows the addition of port forwardings using the
913 It also allows the cancellation of existing remote port-forwardings
916 .Fl KR Oo Ar bind_address : Oc Ar port .
918 .Ic !\& Ns Ar command
919 allows the user to execute a local command if the
920 .Ic PermitLocalCommand
923 Basic help is available, using the
927 Request rekeying of the connection
928 (only useful for SSH protocol version 2 and if the peer supports it).
931 Forwarding of arbitrary TCP connections over the secure channel can
932 be specified either on the command line or in a configuration file.
933 One possible application of TCP forwarding is a secure connection to a
934 mail server; another is going through firewalls.
936 In the example below, we look at encrypting communication between
937 an IRC client and server, even though the IRC server does not directly
938 support encrypted communications.
939 This works as follows:
940 the user connects to the remote host using
942 specifying a port to be used to forward connections
943 to the remote server.
944 After that it is possible to start the service which is to be encrypted
945 on the client machine,
946 connecting to the same local port,
949 will encrypt and forward the connection.
951 The following example tunnels an IRC session from client machine
955 .Dq server.example.com :
956 .Bd -literal -offset 4n
957 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
958 $ irc -c '#users' -p 1234 pinky 127.0.0.1
961 This tunnels a connection to IRC server
962 .Dq server.example.com ,
968 It doesn't matter which port is used,
969 as long as it's greater than 1023
970 (remember, only root can open sockets on privileged ports)
971 and doesn't conflict with any ports already in use.
972 The connection is forwarded to port 6667 on the remote server,
973 since that's the standard port for IRC services.
979 and the remote command
981 is specified to allow an amount of time
982 (10 seconds, in the example)
983 to start the service which is to be tunnelled.
984 If no connections are made within the time specified,
992 (or see the description of the
998 and the user is using X11 (the
1000 environment variable is set), the connection to the X11 display is
1001 automatically forwarded to the remote side in such a way that any X11
1002 programs started from the shell (or command) will go through the
1003 encrypted channel, and the connection to the real X server will be made
1004 from the local machine.
1005 The user should not manually set
1007 Forwarding of X11 connections can be
1008 configured on the command line or in configuration files.
1009 Take note that X11 forwarding can represent a security hazard.
1015 will point to the server machine, but with a display number greater than zero.
1016 This is normal, and happens because
1020 X server on the server machine for forwarding the
1021 connections over the encrypted channel.
1024 will also automatically set up Xauthority data on the server machine.
1025 For this purpose, it will generate a random authorization cookie,
1026 store it in Xauthority on the server, and verify that any forwarded
1027 connections carry this cookie and replace it by the real cookie when
1028 the connection is opened.
1029 The real authentication cookie is never
1030 sent to the server machine (and no cookies are sent in the plain).
1036 (or see the description of the
1041 the user is using an authentication agent, the connection to the agent
1042 is automatically forwarded to the remote side.
1043 .Sh VERIFYING HOST KEYS
1044 When connecting to a server for the first time,
1045 a fingerprint of the server's public key is presented to the user
1047 .Cm StrictHostKeyChecking
1049 Fingerprints can be determined using
1052 .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1054 If the fingerprint is already known, it can be matched
1055 and the key can be accepted or rejected.
1056 Because of the difficulty of comparing host keys
1057 just by looking at hex strings,
1058 there is also support to compare host keys visually,
1065 a small ASCII graphic gets displayed on every login to a server, no matter
1066 if the session itself is interactive or not.
1067 By learning the pattern a known server produces, a user can easily
1068 find out that the host key has changed when a completely different pattern
1070 Because these patterns are not unambiguous however, a pattern that looks
1071 similar to the pattern remembered only gives a good probability that the
1072 host key is the same, not guaranteed proof.
1074 To get a listing of the fingerprints along with their random art for
1075 all known hosts, the following command line can be used:
1077 .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1079 If the fingerprint is unknown,
1080 an alternative method of verification is available:
1081 SSH fingerprints verified by DNS.
1082 An additional resource record (RR),
1084 is added to a zonefile
1085 and the connecting client is able to match the fingerprint
1086 with that of the key presented.
1088 In this example, we are connecting a client to a server,
1089 .Dq host.example.com .
1090 The SSHFP resource records should first be added to the zonefile for
1092 .Bd -literal -offset indent
1093 $ ssh-keygen -r host.example.com.
1096 The output lines will have to be added to the zonefile.
1097 To check that the zone is answering fingerprint queries:
1099 .Dl $ dig -t SSHFP host.example.com
1101 Finally the client connects:
1102 .Bd -literal -offset indent
1103 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1105 Matching host key fingerprint found in DNS.
1106 Are you sure you want to continue connecting (yes/no)?
1110 .Cm VerifyHostKeyDNS
1113 for more information.
1114 .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1116 contains support for Virtual Private Network (VPN) tunnelling
1119 network pseudo-device,
1120 allowing two networks to be joined securely.
1123 configuration option
1125 controls whether the server supports this,
1126 and at what level (layer 2 or 3 traffic).
1128 The following example would connect client network 10.0.50.0/24
1129 with remote network 10.0.99.0/24 using a point-to-point connection
1130 from 10.1.1.1 to 10.1.1.2,
1131 provided that the SSH server running on the gateway to the remote network,
1132 at 192.168.1.15, allows it.
1135 .Bd -literal -offset indent
1136 # ssh -f -w 0:1 192.168.1.15 true
1137 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1138 # route add 10.0.99.0/24 10.1.1.2
1142 .Bd -literal -offset indent
1143 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1144 # route add 10.0.50.0/24 10.1.1.1
1147 Client access may be more finely tuned via the
1148 .Pa /root/.ssh/authorized_keys
1149 file (see below) and the
1152 The following entry would permit connections on
1156 and on tun device 2 from user
1161 .Dq forced-commands-only :
1162 .Bd -literal -offset 2n
1163 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1164 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1167 Since an SSH-based setup entails a fair amount of overhead,
1168 it may be more suited to temporary setups,
1169 such as for wireless VPNs.
1170 More permanent VPNs are better provided by tools such as
1176 will normally set the following environment variables:
1177 .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1181 variable indicates the location of the X11 server.
1182 It is automatically set by
1184 to point to a value of the form
1188 indicates the host where the shell runs, and
1190 is an integer \*(Ge 1.
1192 uses this special value to forward X11 connections over the secure
1194 The user should normally not set
1197 will render the X11 connection insecure (and will require the user to
1198 manually copy any required authorization cookies).
1200 Set to the path of the user's home directory.
1204 set for compatibility with systems that use this variable.
1206 Set to the path of the user's mailbox.
1210 as specified when compiling
1215 needs a passphrase, it will read the passphrase from the current
1216 terminal if it was run from a terminal.
1219 does not have a terminal associated with it but
1223 are set, it will execute the program specified by
1225 and open an X11 window to read the passphrase.
1226 This is particularly useful when calling
1231 (Note that on some machines it
1232 may be necessary to redirect the input from
1235 .It Ev SSH_AUTH_SOCK
1236 Identifies the path of a
1238 socket used to communicate with the agent.
1239 .It Ev SSH_CONNECTION
1240 Identifies the client and server ends of the connection.
1241 The variable contains
1242 four space-separated values: client IP address, client port number,
1243 server IP address, and server port number.
1244 .It Ev SSH_ORIGINAL_COMMAND
1245 This variable contains the original command line if a forced command
1247 It can be used to extract the original arguments.
1249 This is set to the name of the tty (path to the device) associated
1250 with the current shell or command.
1251 If the current session has no tty,
1252 this variable is not set.
1254 This variable is set to indicate the present time zone if it
1255 was set when the daemon was started (i.e. the daemon passes the value
1256 on to new connections).
1258 Set to the name of the user logging in.
1264 .Pa ~/.ssh/environment ,
1265 and adds lines of the format
1267 to the environment if the file exists and users are allowed to
1268 change their environment.
1269 For more information, see the
1270 .Cm PermitUserEnvironment
1274 .Bl -tag -width Ds -compact
1276 This file is used for host-based authentication (see above).
1277 On some machines this file may need to be
1278 world-readable if the user's home directory is on an NFS partition,
1282 Additionally, this file must be owned by the user,
1283 and must not have write permissions for anyone else.
1285 permission for most machines is read/write for the user, and not
1286 accessible by others.
1289 This file is used in exactly the same way as
1291 but allows host-based authentication without permitting login with
1295 This directory is the default location for all user-specific configuration
1296 and authentication information.
1297 There is no general requirement to keep the entire contents of this directory
1298 secret, but the recommended permissions are read/write/execute for the user,
1299 and not accessible by others.
1301 .It ~/.ssh/authorized_keys
1302 Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1303 The format of this file is described in the
1306 This file is not highly sensitive, but the recommended
1307 permissions are read/write for the user, and not accessible by others.
1310 This is the per-user configuration file.
1311 The file format and configuration options are described in
1313 Because of the potential for abuse, this file must have strict permissions:
1314 read/write for the user, and not accessible by others.
1316 .It ~/.ssh/environment
1317 Contains additional definitions for environment variables; see
1324 Contains the private key for authentication.
1326 contain sensitive data and should be readable by the user but not
1327 accessible by others (read/write/execute).
1329 will simply ignore a private key file if it is accessible by others.
1330 It is possible to specify a passphrase when
1331 generating the key which will be used to encrypt the
1332 sensitive part of this file using 3DES.
1334 .It ~/.ssh/identity.pub
1335 .It ~/.ssh/id_dsa.pub
1336 .It ~/.ssh/id_rsa.pub
1337 Contains the public key for authentication.
1339 sensitive and can (but need not) be readable by anyone.
1341 .It ~/.ssh/known_hosts
1342 Contains a list of host keys for all hosts the user has logged into
1343 that are not already in the systemwide list of known host keys.
1346 for further details of the format of this file.
1349 Commands in this file are executed by
1351 when the user logs in, just before the user's shell (or command) is
1355 manual page for more information.
1357 .It /etc/hosts.equiv
1358 This file is for host-based authentication (see above).
1359 It should only be writable by root.
1361 .It /etc/ssh/shosts.equiv
1362 This file is used in exactly the same way as
1364 but allows host-based authentication without permitting login with
1367 .It Pa /etc/ssh/ssh_config
1368 Systemwide configuration file.
1369 The file format and configuration options are described in
1372 .It /etc/ssh/ssh_host_key
1373 .It /etc/ssh/ssh_host_dsa_key
1374 .It /etc/ssh/ssh_host_rsa_key
1375 These three files contain the private parts of the host keys
1376 and are used for host-based authentication.
1377 If protocol version 1 is used,
1379 must be setuid root, since the host key is readable only by root.
1380 For protocol version 2,
1384 to access the host keys,
1385 eliminating the requirement that
1387 be setuid root when host-based authentication is used.
1392 .It /etc/ssh/ssh_known_hosts
1393 Systemwide list of known host keys.
1394 This file should be prepared by the
1395 system administrator to contain the public host keys of all machines in the
1397 It should be world-readable.
1400 for further details of the format of this file.
1403 Commands in this file are executed by
1405 when the user logs in, just before the user's shell (or command) is started.
1408 manual page for more information.
1424 .%T "The Secure Shell (SSH) Protocol Assigned Numbers"
1429 .%T "The Secure Shell (SSH) Protocol Architecture"
1434 .%T "The Secure Shell (SSH) Authentication Protocol"
1439 .%T "The Secure Shell (SSH) Transport Layer Protocol"
1444 .%T "The Secure Shell (SSH) Connection Protocol"
1449 .%T "Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints"
1454 .%T "Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)"
1459 .%T "The Secure Shell (SSH) Session Channel Break Extension"
1464 .%T "The Secure Shell (SSH) Transport Layer Encryption Modes"
1469 .%T "Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol"
1474 .%T "Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol"
1479 .%T "The Secure Shell (SSH) Public Key File Format"
1483 .%T "Hash Visualization: a New Technique to improve Real-World Security"
1487 .%O "International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)"
1490 OpenSSH is a derivative of the original and free
1491 ssh 1.2.12 release by Tatu Ylonen.
1492 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1493 Theo de Raadt and Dug Song
1494 removed many bugs, re-added newer features and
1496 Markus Friedl contributed the support for SSH
1497 protocol versions 1.5 and 2.0.
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