2 .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
3 .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
4 .\" All rights reserved
6 .\" As far as I am concerned, the code I have written for this software
7 .\" can be used freely for any purpose. Any derived versions of this
8 .\" software must be clearly marked as such, and if the derived work is
9 .\" incompatible with the protocol description in the RFC file, it must be
10 .\" called by a name other than "ssh" or "Secure Shell".
12 .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
13 .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
14 .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
16 .\" Redistribution and use in source and binary forms, with or without
17 .\" modification, are permitted provided that the following conditions
19 .\" 1. Redistributions of source code must retain the above copyright
20 .\" notice, this list of conditions and the following disclaimer.
21 .\" 2. Redistributions in binary form must reproduce the above copyright
22 .\" notice, this list of conditions and the following disclaimer in the
23 .\" documentation and/or other materials provided with the distribution.
25 .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
26 .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
27 .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
28 .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
29 .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
30 .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
31 .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
32 .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
33 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
34 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
36 .\" $OpenBSD: ssh.1,v 1.343 2013/12/07 11:58:46 naddy Exp $
37 .Dd $Mdocdate: December 7 2013 $
42 .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
49 .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 Q Cm cipher | cipher-auth | mac | kex | key
62 .Op Fl R Oo Ar bind_address : Oc Ns Ar port : Ns Ar host : Ns Ar hostport
64 .Op Fl W Ar host : Ns Ar port
65 .Op Fl w Ar local_tun Ns Op : Ns Ar remote_tun
66 .Oo Ar user Ns @ Oc Ns Ar hostname
71 (SSH client) is a program for logging into a remote machine and for
72 executing commands on a remote machine.
73 It is intended to replace rlogin and rsh,
74 and provide secure encrypted communications between
75 two untrusted hosts over an insecure network.
76 X11 connections and arbitrary TCP ports
77 can also be forwarded over the secure channel.
80 connects and logs into the specified
86 his/her identity to the remote machine using one of several methods
87 depending on the protocol version used (see below).
92 it is executed on the remote host instead of a login shell.
94 The options are as follows:
99 to try protocol version 1 only.
103 to try protocol version 2 only.
107 to use IPv4 addresses only.
111 to use IPv6 addresses only.
113 Enables forwarding of the authentication agent connection.
114 This can also be specified on a per-host basis in a configuration file.
116 Agent forwarding should be enabled with caution.
117 Users with the ability to bypass file permissions on the remote host
120 socket) can access the local agent through the forwarded connection.
121 An attacker cannot obtain key material from the agent,
122 however they can perform operations on the keys that enable them to
123 authenticate using the identities loaded into the agent.
125 Disables forwarding of the authentication agent connection.
126 .It Fl b Ar bind_address
129 on the local machine as the source address
131 Only useful on systems with more than one address.
133 Requests compression of all data (including stdin, stdout, stderr, and
134 data for forwarded X11 and TCP connections).
135 The compression algorithm is the same used by
139 can be controlled by the
141 option for protocol version 1.
142 Compression is desirable on modem lines and other
143 slow connections, but will only slow down things on fast networks.
144 The default value can be set on a host-by-host basis in the
145 configuration files; see the
148 .It Fl c Ar cipher_spec
149 Selects the cipher specification for encrypting the session.
151 Protocol version 1 allows specification of a single cipher.
152 The supported values are
158 (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
159 It is believed to be secure.
161 is a fast block cipher; it appears very secure and is much faster than
164 is only supported in the
166 client for interoperability with legacy protocol 1 implementations
167 that do not support the
170 Its use is strongly discouraged due to cryptographic weaknesses.
174 For protocol version 2,
176 is a comma-separated list of ciphers
177 listed in order of preference.
182 for more information.
185 .Oo Ar bind_address : Oc
191 application-level port forwarding.
192 This works by allocating a socket to listen to
194 on the local side, optionally bound to the specified
196 Whenever a connection is made to this port, the
197 connection is forwarded over the secure channel, and the application
198 protocol is then used to determine where to connect to from the
200 Currently the SOCKS4 and SOCKS5 protocols are supported, and
202 will act as a SOCKS server.
203 Only root can forward privileged ports.
204 Dynamic port forwardings can also be specified in the configuration file.
206 IPv6 addresses can be specified by enclosing the address in square brackets.
207 Only the superuser can forward privileged ports.
208 By default, the local port is bound in accordance with the
213 may be used to bind the connection to a specific address.
218 indicates that the listening port be bound for local use only, while an
221 indicates that the port should be available from all interfaces.
225 instead of standard error.
226 .It Fl e Ar escape_char
227 Sets the escape character for sessions with a pty (default:
229 The escape character is only recognized at the beginning of a line.
230 The escape character followed by a dot
232 closes the connection;
233 followed by control-Z suspends the connection;
234 and followed by itself sends the escape character once.
235 Setting the character to
237 disables any escapes and makes the session fully transparent.
238 .It Fl F Ar configfile
239 Specifies an alternative per-user configuration file.
240 If a configuration file is given on the command line,
241 the system-wide configuration file
242 .Pq Pa /etc/ssh/ssh_config
244 The default for the per-user configuration file is
249 to go to background just before command execution.
252 is going to ask for passwords or passphrases, but the user
253 wants it in the background.
256 The recommended way to start X11 programs at a remote site is with
258 .Ic ssh -f host xterm .
261 .Cm ExitOnForwardFailure
262 configuration option is set to
264 then a client started with
266 will wait for all remote port forwards to be successfully established
267 before placing itself in the background.
269 Allows remote hosts to connect to local forwarded ports.
271 Specify the PKCS#11 shared library
273 should use to communicate with a PKCS#11 token providing the user's
275 .It Fl i Ar identity_file
276 Selects a file from which the identity (private key) for
277 public key authentication is read.
280 for protocol version 1, and
282 .Pa ~/.ssh/id_ecdsa ,
283 .Pa ~/.ssh/id_ed25519
286 for protocol version 2.
287 Identity files may also be specified on
288 a per-host basis in the configuration file.
289 It is possible to have multiple
291 options (and multiple identities specified in
292 configuration files).
294 will also try to load certificate information from the filename obtained
297 to identity filenames.
299 Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
300 credentials to the server.
302 Disables forwarding (delegation) of GSSAPI credentials to the server.
305 .Oo Ar bind_address : Oc
306 .Ar port : host : hostport
309 Specifies that the given port on the local (client) host is to be
310 forwarded to the given host and port on the remote side.
311 This works by allocating a socket to listen to
313 on the local side, optionally bound to the specified
315 Whenever a connection is made to this port, the
316 connection is forwarded over the secure channel, and a connection is
321 from the remote machine.
322 Port forwardings can also be specified in the configuration file.
323 IPv6 addresses can be specified by enclosing the address in square brackets.
324 Only the superuser can forward privileged ports.
325 By default, the local port is bound in accordance with the
330 may be used to bind the connection to a specific address.
335 indicates that the listening port be bound for local use only, while an
338 indicates that the port should be available from all interfaces.
339 .It Fl l Ar login_name
340 Specifies the user to log in as on the remote machine.
341 This also may be specified on a per-host basis in the configuration file.
347 mode for connection sharing.
354 mode with confirmation required before slave connections are accepted.
355 Refer to the description of
361 Additionally, for protocol version 2 a comma-separated list of MAC
362 (message authentication code) algorithms can
363 be specified in order of preference.
366 keyword for more information.
368 Do not execute a remote command.
369 This is useful for just forwarding ports
370 (protocol version 2 only).
374 (actually, prevents reading from stdin).
375 This must be used when
377 is run in the background.
378 A common trick is to use this to run X11 programs on a remote machine.
380 .Ic ssh -n shadows.cs.hut.fi emacs &
381 will start an emacs on shadows.cs.hut.fi, and the X11
382 connection will be automatically forwarded over an encrypted channel.
385 program will be put in the background.
386 (This does not work if
388 needs to ask for a password or passphrase; see also the
392 Control an active connection multiplexing master process.
395 option is specified, the
397 argument is interpreted and passed to the master process.
400 (check that the master process is running),
402 (request forwardings without command execution),
404 (cancel forwardings),
406 (request the master to exit), and
408 (request the master to stop accepting further multiplexing requests).
410 Can be used to give options in the format used in the configuration file.
411 This is useful for specifying options for which there is no separate
413 For full details of the options listed below, and their possible values, see
416 .Bl -tag -width Ds -offset indent -compact
421 .It CanonicalizeFallbackLocal
422 .It CanonicalizeHostname
423 .It CanonicalizeMaxDots
424 .It CanonicalizePermittedCNAMEs
425 .It ChallengeResponseAuthentication
429 .It ClearAllForwardings
432 .It ConnectionAttempts
439 .It ExitOnForwardFailure
442 .It ForwardX11Timeout
443 .It ForwardX11Trusted
445 .It GlobalKnownHostsFile
446 .It GSSAPIAuthentication
447 .It GSSAPIDelegateCredentials
450 .It HostbasedAuthentication
451 .It HostKeyAlgorithms
457 .It KbdInteractiveAuthentication
458 .It KbdInteractiveDevices
465 .It NoHostAuthenticationForLocalhost
466 .It NumberOfPasswordPrompts
467 .It PasswordAuthentication
468 .It PermitLocalCommand
471 .It PreferredAuthentications
475 .It PubkeyAuthentication
479 .It RhostsRSAAuthentication
480 .It RSAAuthentication
482 .It ServerAliveInterval
483 .It ServerAliveCountMax
484 .It StrictHostKeyChecking
488 .It UsePrivilegedPort
490 .It UserKnownHostsFile
496 Port to connect to on the remote host.
497 This can be specified on a
498 per-host basis in the configuration file.
499 .It Fl Q Cm cipher | cipher-auth | mac | kex | key
502 for the algorithms supported for the specified version 2.
503 The available features are:
505 (supported symmetric ciphers),
507 (supported symmetric ciphers that support authenticated encryption),
509 (supported message integrity codes),
511 (key exchange algorithms),
516 Causes most warning and diagnostic messages to be suppressed.
519 .Oo Ar bind_address : Oc
520 .Ar port : host : hostport
523 Specifies that the given port on the remote (server) host is to be
524 forwarded to the given host and port on the local side.
525 This works by allocating a socket to listen to
527 on the remote side, and whenever a connection is made to this port, the
528 connection is forwarded over the secure channel, and a connection is
533 from the local machine.
535 Port forwardings can also be specified in the configuration file.
536 Privileged ports can be forwarded only when
537 logging in as root on the remote machine.
538 IPv6 addresses can be specified by enclosing the address in square brackets.
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.
562 When used together with
564 the allocated port will be printed to the standard output.
566 Specifies the location of a control socket for connection sharing,
569 to disable connection sharing.
570 Refer to the description of
578 May be used to request invocation of a subsystem on the remote system.
579 Subsystems are a feature of the SSH2 protocol which facilitate the use
580 of SSH as a secure transport for other applications (eg.\&
582 The subsystem is specified as the remote command.
584 Disable pseudo-tty allocation.
586 Force pseudo-tty allocation.
587 This can be used to execute arbitrary
588 screen-based programs on a remote machine, which can be very useful,
589 e.g. when implementing menu services.
592 options force tty allocation, even if
596 Display the version number and exit.
601 to print debugging messages about its progress.
603 debugging connection, authentication, and configuration problems.
606 options increase the verbosity.
608 .It Fl W Ar host : Ns Ar port
609 Requests that standard input and output on the client be forwarded to
613 over the secure channel.
617 .Cm ExitOnForwardFailure
619 .Cm ClearAllForwardings .
620 Works with Protocol version 2 only.
622 .Ar local_tun Ns Op : Ns Ar remote_tun
626 device forwarding with the specified
628 devices between the client
633 The devices may be specified by numerical ID or the keyword
635 which uses the next available tunnel device.
638 is not specified, it defaults to
648 directive is unset, it is set to the default tunnel mode, which is
651 Enables X11 forwarding.
652 This can also be specified on a per-host basis in a configuration file.
654 X11 forwarding should be enabled with caution.
655 Users with the ability to bypass file permissions on the remote host
656 (for the user's X authorization database)
657 can access the local X11 display through the forwarded connection.
658 An attacker may then be able to perform activities such as keystroke monitoring.
660 For this reason, X11 forwarding is subjected to X11 SECURITY extension
661 restrictions by default.
666 .Cm ForwardX11Trusted
669 for more information.
671 Disables X11 forwarding.
673 Enables trusted X11 forwarding.
674 Trusted X11 forwardings are not subjected to the X11 SECURITY extension
677 Send log information using the
680 By default this information is sent to stderr.
684 may additionally obtain configuration data from
685 a per-user configuration file and a system-wide configuration file.
686 The file format and configuration options are described in
689 The OpenSSH SSH client supports SSH protocols 1 and 2.
690 The default is to use protocol 2 only,
691 though this can be changed via the
700 Both protocols support similar authentication methods,
701 but protocol 2 is the default since
702 it provides additional mechanisms for confidentiality
703 (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
704 and integrity (hmac-md5, hmac-sha1,
705 hmac-sha2-256, hmac-sha2-512,
706 umac-64, umac-128, hmac-ripemd160).
707 Protocol 1 lacks a strong mechanism for ensuring the
708 integrity of the connection.
710 The methods available for authentication are:
711 GSSAPI-based authentication,
712 host-based authentication,
713 public key authentication,
714 challenge-response authentication,
715 and password authentication.
716 Authentication methods are tried in the order specified above,
717 though protocol 2 has a configuration option to change the default order:
718 .Cm PreferredAuthentications .
720 Host-based authentication works as follows:
721 If the machine the user logs in from is listed in
724 .Pa /etc/shosts.equiv
725 on the remote machine, and the user names are
726 the same on both sides, or if the files
730 exist in the user's home directory on the
731 remote machine and contain a line containing the name of the client
732 machine and the name of the user on that machine, the user is
733 considered for login.
734 Additionally, the server
736 be able to verify the client's
737 host key (see the description of
738 .Pa /etc/ssh/ssh_known_hosts
740 .Pa ~/.ssh/known_hosts ,
742 for login to be permitted.
743 This authentication method closes security holes due to IP
744 spoofing, DNS spoofing, and routing spoofing.
745 [Note to the administrator:
746 .Pa /etc/hosts.equiv ,
748 and the rlogin/rsh protocol in general, are inherently insecure and should be
749 disabled if security is desired.]
751 Public key authentication works as follows:
752 The scheme is based on public-key cryptography,
754 where encryption and decryption are done using separate keys,
755 and it is unfeasible to derive the decryption key from the encryption key.
756 The idea is that each user creates a public/private
757 key pair for authentication purposes.
758 The server knows the public key, and only the user knows the private key.
760 implements public key authentication protocol automatically,
761 using one of the DSA, ECDSA, ED25519 or RSA algorithms.
762 Protocol 1 is restricted to using only RSA keys,
763 but protocol 2 may use any.
764 The HISTORY section of
766 contains a brief discussion of the DSA and RSA algorithms.
769 .Pa ~/.ssh/authorized_keys
770 lists the public keys that are permitted for logging in.
771 When the user logs in, the
773 program tells the server which key pair it would like to use for
775 The client proves that it has access to the private key
776 and the server checks that the corresponding public key
777 is authorized to accept the account.
779 The user creates his/her key pair by running
781 This stores the private key in
788 .Pa ~/.ssh/id_ed25519
789 (protocol 2 ED25519),
793 and stores the public key in
794 .Pa ~/.ssh/identity.pub
796 .Pa ~/.ssh/id_dsa.pub
798 .Pa ~/.ssh/id_ecdsa.pub
800 .Pa ~/.ssh/id_ed25519.pub
801 (protocol 2 ED25519),
803 .Pa ~/.ssh/id_rsa.pub
805 in the user's home directory.
806 The user should then copy the public key
808 .Pa ~/.ssh/authorized_keys
809 in his/her home directory on the remote machine.
812 file corresponds to the conventional
814 file, and has one key
815 per line, though the lines can be very long.
816 After this, the user can log in without giving the password.
818 A variation on public key authentication
819 is available in the form of certificate authentication:
820 instead of a set of public/private keys,
821 signed certificates are used.
822 This has the advantage that a single trusted certification authority
823 can be used in place of many public/private keys.
824 See the CERTIFICATES section of
826 for more information.
828 The most convenient way to use public key or certificate authentication
829 may be with an authentication agent.
832 for more information.
834 Challenge-response authentication works as follows:
835 The server sends an arbitrary
837 text, and prompts for a response.
838 Protocol 2 allows multiple challenges and responses;
839 protocol 1 is restricted to just one challenge/response.
840 Examples of challenge-response authentication include
848 Finally, if other authentication methods fail,
850 prompts the user for a password.
851 The password is sent to the remote
852 host for checking; however, since all communications are encrypted,
853 the password cannot be seen by someone listening on the network.
856 automatically maintains and checks a database containing
857 identification for all hosts it has ever been used with.
858 Host keys are stored in
859 .Pa ~/.ssh/known_hosts
860 in the user's home directory.
861 Additionally, the file
862 .Pa /etc/ssh/ssh_known_hosts
863 is automatically checked for known hosts.
864 Any new hosts are automatically added to the user's file.
865 If a host's identification ever changes,
867 warns about this and disables password authentication to prevent
868 server spoofing or man-in-the-middle attacks,
869 which could otherwise be used to circumvent the encryption.
871 .Cm StrictHostKeyChecking
872 option can be used to control logins to machines whose
873 host key is not known or has changed.
875 When the user's identity has been accepted by the server, the server
876 either executes the given command, or logs into the machine and gives
877 the user a normal shell on the remote machine.
878 All communication with
879 the remote command or shell will be automatically encrypted.
881 If a pseudo-terminal has been allocated (normal login session), the
882 user may use the escape characters noted below.
884 If no pseudo-tty has been allocated,
885 the session is transparent and can be used to reliably transfer binary data.
886 On most systems, setting the escape character to
888 will also make the session transparent even if a tty is used.
890 The session terminates when the command or shell on the remote
891 machine exits and all X11 and TCP connections have been closed.
892 .Sh ESCAPE CHARACTERS
893 When a pseudo-terminal has been requested,
895 supports a number of functions through the use of an escape character.
897 A single tilde character can be sent as
899 or by following the tilde by a character other than those described below.
900 The escape character must always follow a newline to be interpreted as
902 The escape character can be changed in configuration files using the
904 configuration directive or on the command line by the
908 The supported escapes (assuming the default
918 List forwarded connections.
922 at logout when waiting for forwarded connection / X11 sessions to terminate.
924 Display a list of escape characters.
926 Send a BREAK to the remote system
927 (only useful for SSH protocol version 2 and if the peer supports it).
930 Currently this allows the addition of port forwardings using the
936 It also allows the cancellation of existing port-forwardings
939 .Fl KL Oo Ar bind_address : Oc Ar port
943 .Fl KR Oo Ar bind_address : Oc Ar port
947 .Fl KD Oo Ar bind_address : Oc Ar port
949 for dynamic port-forwardings.
950 .Ic !\& Ns Ar command
951 allows the user to execute a local command if the
952 .Ic PermitLocalCommand
955 Basic help is available, using the
959 Request rekeying of the connection
960 (only useful for SSH protocol version 2 and if the peer supports it).
962 Decrease the verbosity
964 when errors are being written to stderr.
966 Increase the verbosity
968 when errors are being written to stderr.
971 Forwarding of arbitrary TCP connections over the secure channel can
972 be specified either on the command line or in a configuration file.
973 One possible application of TCP forwarding is a secure connection to a
974 mail server; another is going through firewalls.
976 In the example below, we look at encrypting communication between
977 an IRC client and server, even though the IRC server does not directly
978 support encrypted communications.
979 This works as follows:
980 the user connects to the remote host using
982 specifying a port to be used to forward connections
983 to the remote server.
984 After that it is possible to start the service which is to be encrypted
985 on the client machine,
986 connecting to the same local port,
989 will encrypt and forward the connection.
991 The following example tunnels an IRC session from client machine
995 .Dq server.example.com :
996 .Bd -literal -offset 4n
997 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
998 $ irc -c '#users' -p 1234 pinky 127.0.0.1
1001 This tunnels a connection to IRC server
1002 .Dq server.example.com ,
1008 It doesn't matter which port is used,
1009 as long as it's greater than 1023
1010 (remember, only root can open sockets on privileged ports)
1011 and doesn't conflict with any ports already in use.
1012 The connection is forwarded to port 6667 on the remote server,
1013 since that's the standard port for IRC services.
1019 and the remote command
1021 is specified to allow an amount of time
1022 (10 seconds, in the example)
1023 to start the service which is to be tunnelled.
1024 If no connections are made within the time specified,
1032 (or see the description of the
1038 and the user is using X11 (the
1040 environment variable is set), the connection to the X11 display is
1041 automatically forwarded to the remote side in such a way that any X11
1042 programs started from the shell (or command) will go through the
1043 encrypted channel, and the connection to the real X server will be made
1044 from the local machine.
1045 The user should not manually set
1047 Forwarding of X11 connections can be
1048 configured on the command line or in configuration files.
1054 will point to the server machine, but with a display number greater than zero.
1055 This is normal, and happens because
1059 X server on the server machine for forwarding the
1060 connections over the encrypted channel.
1063 will also automatically set up Xauthority data on the server machine.
1064 For this purpose, it will generate a random authorization cookie,
1065 store it in Xauthority on the server, and verify that any forwarded
1066 connections carry this cookie and replace it by the real cookie when
1067 the connection is opened.
1068 The real authentication cookie is never
1069 sent to the server machine (and no cookies are sent in the plain).
1075 (or see the description of the
1080 the user is using an authentication agent, the connection to the agent
1081 is automatically forwarded to the remote side.
1082 .Sh VERIFYING HOST KEYS
1083 When connecting to a server for the first time,
1084 a fingerprint of the server's public key is presented to the user
1086 .Cm StrictHostKeyChecking
1088 Fingerprints can be determined using
1091 .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1093 If the fingerprint is already known, it can be matched
1094 and the key can be accepted or rejected.
1095 Because of the difficulty of comparing host keys
1096 just by looking at hex strings,
1097 there is also support to compare host keys visually,
1104 a small ASCII graphic gets displayed on every login to a server, no matter
1105 if the session itself is interactive or not.
1106 By learning the pattern a known server produces, a user can easily
1107 find out that the host key has changed when a completely different pattern
1109 Because these patterns are not unambiguous however, a pattern that looks
1110 similar to the pattern remembered only gives a good probability that the
1111 host key is the same, not guaranteed proof.
1113 To get a listing of the fingerprints along with their random art for
1114 all known hosts, the following command line can be used:
1116 .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1118 If the fingerprint is unknown,
1119 an alternative method of verification is available:
1120 SSH fingerprints verified by DNS.
1121 An additional resource record (RR),
1123 is added to a zonefile
1124 and the connecting client is able to match the fingerprint
1125 with that of the key presented.
1127 In this example, we are connecting a client to a server,
1128 .Dq host.example.com .
1129 The SSHFP resource records should first be added to the zonefile for
1131 .Bd -literal -offset indent
1132 $ ssh-keygen -r host.example.com.
1135 The output lines will have to be added to the zonefile.
1136 To check that the zone is answering fingerprint queries:
1138 .Dl $ dig -t SSHFP host.example.com
1140 Finally the client connects:
1141 .Bd -literal -offset indent
1142 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1144 Matching host key fingerprint found in DNS.
1145 Are you sure you want to continue connecting (yes/no)?
1149 .Cm VerifyHostKeyDNS
1152 for more information.
1153 .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1155 contains support for Virtual Private Network (VPN) tunnelling
1158 network pseudo-device,
1159 allowing two networks to be joined securely.
1162 configuration option
1164 controls whether the server supports this,
1165 and at what level (layer 2 or 3 traffic).
1167 The following example would connect client network 10.0.50.0/24
1168 with remote network 10.0.99.0/24 using a point-to-point connection
1169 from 10.1.1.1 to 10.1.1.2,
1170 provided that the SSH server running on the gateway to the remote network,
1171 at 192.168.1.15, allows it.
1174 .Bd -literal -offset indent
1175 # ssh -f -w 0:1 192.168.1.15 true
1176 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1177 # route add 10.0.99.0/24 10.1.1.2
1181 .Bd -literal -offset indent
1182 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1183 # route add 10.0.50.0/24 10.1.1.1
1186 Client access may be more finely tuned via the
1187 .Pa /root/.ssh/authorized_keys
1188 file (see below) and the
1191 The following entry would permit connections on
1195 and on tun device 2 from user
1200 .Dq forced-commands-only :
1201 .Bd -literal -offset 2n
1202 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1203 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1206 Since an SSH-based setup entails a fair amount of overhead,
1207 it may be more suited to temporary setups,
1208 such as for wireless VPNs.
1209 More permanent VPNs are better provided by tools such as
1215 will normally set the following environment variables:
1216 .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1220 variable indicates the location of the X11 server.
1221 It is automatically set by
1223 to point to a value of the form
1227 indicates the host where the shell runs, and
1229 is an integer \*(Ge 1.
1231 uses this special value to forward X11 connections over the secure
1233 The user should normally not set
1236 will render the X11 connection insecure (and will require the user to
1237 manually copy any required authorization cookies).
1239 Set to the path of the user's home directory.
1243 set for compatibility with systems that use this variable.
1245 Set to the path of the user's mailbox.
1249 as specified when compiling
1254 needs a passphrase, it will read the passphrase from the current
1255 terminal if it was run from a terminal.
1258 does not have a terminal associated with it but
1262 are set, it will execute the program specified by
1264 and open an X11 window to read the passphrase.
1265 This is particularly useful when calling
1270 (Note that on some machines it
1271 may be necessary to redirect the input from
1274 .It Ev SSH_AUTH_SOCK
1275 Identifies the path of a
1277 socket used to communicate with the agent.
1278 .It Ev SSH_CONNECTION
1279 Identifies the client and server ends of the connection.
1280 The variable contains
1281 four space-separated values: client IP address, client port number,
1282 server IP address, and server port number.
1283 .It Ev SSH_ORIGINAL_COMMAND
1284 This variable contains the original command line if a forced command
1286 It can be used to extract the original arguments.
1288 This is set to the name of the tty (path to the device) associated
1289 with the current shell or command.
1290 If the current session has no tty,
1291 this variable is not set.
1293 This variable is set to indicate the present time zone if it
1294 was set when the daemon was started (i.e. the daemon passes the value
1295 on to new connections).
1297 Set to the name of the user logging in.
1303 .Pa ~/.ssh/environment ,
1304 and adds lines of the format
1306 to the environment if the file exists and users are allowed to
1307 change their environment.
1308 For more information, see the
1309 .Cm PermitUserEnvironment
1313 .Bl -tag -width Ds -compact
1315 This file is used for host-based authentication (see above).
1316 On some machines this file may need to be
1317 world-readable if the user's home directory is on an NFS partition,
1321 Additionally, this file must be owned by the user,
1322 and must not have write permissions for anyone else.
1324 permission for most machines is read/write for the user, and not
1325 accessible by others.
1328 This file is used in exactly the same way as
1330 but allows host-based authentication without permitting login with
1334 This directory is the default location for all user-specific configuration
1335 and authentication information.
1336 There is no general requirement to keep the entire contents of this directory
1337 secret, but the recommended permissions are read/write/execute for the user,
1338 and not accessible by others.
1340 .It Pa ~/.ssh/authorized_keys
1341 Lists the public keys (DSA, ECDSA, ED25519, RSA)
1342 that can be used for logging in as this user.
1343 The format of this file is described in the
1346 This file is not highly sensitive, but the recommended
1347 permissions are read/write for the user, and not accessible by others.
1349 .It Pa ~/.ssh/config
1350 This is the per-user configuration file.
1351 The file format and configuration options are described in
1353 Because of the potential for abuse, this file must have strict permissions:
1354 read/write for the user, and not writable by others.
1356 .It Pa ~/.ssh/environment
1357 Contains additional definitions for environment variables; see
1361 .It Pa ~/.ssh/identity
1362 .It Pa ~/.ssh/id_dsa
1363 .It Pa ~/.ssh/id_ecdsa
1364 .It Pa ~/.ssh/id_ed25519
1365 .It Pa ~/.ssh/id_rsa
1366 Contains the private key for authentication.
1368 contain sensitive data and should be readable by the user but not
1369 accessible by others (read/write/execute).
1371 will simply ignore a private key file if it is accessible by others.
1372 It is possible to specify a passphrase when
1373 generating the key which will be used to encrypt the
1374 sensitive part of this file using 3DES.
1376 .It Pa ~/.ssh/identity.pub
1377 .It Pa ~/.ssh/id_dsa.pub
1378 .It Pa ~/.ssh/id_ecdsa.pub
1379 .It Pa ~/.ssh/id_ed25519.pub
1380 .It Pa ~/.ssh/id_rsa.pub
1381 Contains the public key for authentication.
1383 sensitive and can (but need not) be readable by anyone.
1385 .It Pa ~/.ssh/known_hosts
1386 Contains a list of host keys for all hosts the user has logged into
1387 that are not already in the systemwide list of known host keys.
1390 for further details of the format of this file.
1393 Commands in this file are executed by
1395 when the user logs in, just before the user's shell (or command) is
1399 manual page for more information.
1401 .It Pa /etc/hosts.equiv
1402 This file is for host-based authentication (see above).
1403 It should only be writable by root.
1405 .It Pa /etc/shosts.equiv
1406 This file is used in exactly the same way as
1408 but allows host-based authentication without permitting login with
1411 .It Pa /etc/ssh/ssh_config
1412 Systemwide configuration file.
1413 The file format and configuration options are described in
1416 .It Pa /etc/ssh/ssh_host_key
1417 .It Pa /etc/ssh/ssh_host_dsa_key
1418 .It Pa /etc/ssh/ssh_host_ecdsa_key
1419 .It Pa /etc/ssh/ssh_host_ed25519_key
1420 .It Pa /etc/ssh/ssh_host_rsa_key
1421 These files contain the private parts of the host keys
1422 and are used for host-based authentication.
1423 If protocol version 1 is used,
1425 must be setuid root, since the host key is readable only by root.
1426 For protocol version 2,
1430 to access the host keys,
1431 eliminating the requirement that
1433 be setuid root when host-based authentication is used.
1438 .It Pa /etc/ssh/ssh_known_hosts
1439 Systemwide list of known host keys.
1440 This file should be prepared by the
1441 system administrator to contain the public host keys of all machines in the
1443 It should be world-readable.
1446 for further details of the format of this file.
1448 .It Pa /etc/ssh/sshrc
1449 Commands in this file are executed by
1451 when the user logs in, just before the user's shell (or command) is started.
1454 manual page for more information.
1458 exits with the exit status of the remote command or with 255
1459 if an error occurred.
1478 .%T The Secure Shell (SSH) Protocol Assigned Numbers
1486 .%T The Secure Shell (SSH) Protocol Architecture
1494 .%T The Secure Shell (SSH) Authentication Protocol
1502 .%T The Secure Shell (SSH) Transport Layer Protocol
1510 .%T The Secure Shell (SSH) Connection Protocol
1518 .%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
1526 .%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
1534 .%T The Secure Shell (SSH) Session Channel Break Extension
1543 .%T The Secure Shell (SSH) Transport Layer Encryption Modes
1550 .%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
1559 .%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
1567 .%T The Secure Shell (SSH) Public Key File Format
1575 .%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
1582 .%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
1583 .%T Hash Visualization: a New Technique to improve Real-World Security
1586 OpenSSH is a derivative of the original and free
1587 ssh 1.2.12 release by Tatu Ylonen.
1588 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1589 Theo de Raadt and Dug Song
1590 removed many bugs, re-added newer features and
1592 Markus Friedl contributed the support for SSH
1593 protocol versions 1.5 and 2.0.