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 1.361 2015/07/20 18:44:12 millert Exp $
37 .Dd $Mdocdate: July 20 2015 $
42 .Nd OpenSSH SSH client (remote login program)
46 .Op Fl 1246AaCfGgKkMNnqsTtVvXxYy
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
56 .Op Fl l Ar login_name
61 .Op Fl Q Cm cipher | cipher-auth | mac | kex | key | protocol-version
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, arbitrary TCP ports and
78 sockets can also be forwarded over the secure channel.
81 connects and logs into the specified
87 his/her identity to the remote machine using one of several methods
88 depending on the protocol version used (see below).
93 it is executed on the remote host instead of a login shell.
95 The options are as follows:
97 .Bl -tag -width Ds -compact
101 to try protocol version 1 only.
106 to try protocol version 2 only.
111 to use IPv4 addresses only.
116 to use IPv6 addresses only.
119 Enables forwarding of the authentication agent connection.
120 This can also be specified on a per-host basis in a configuration file.
122 Agent forwarding should be enabled with caution.
123 Users with the ability to bypass file permissions on the remote host
126 socket) can access the local agent through the forwarded connection.
127 An attacker cannot obtain key material from the agent,
128 however they can perform operations on the keys that enable them to
129 authenticate using the identities loaded into the agent.
132 Disables forwarding of the authentication agent connection.
134 .It Fl b Ar bind_address
137 on the local machine as the source address
139 Only useful on systems with more than one address.
142 Requests compression of all data (including stdin, stdout, stderr, and
143 data for forwarded X11, TCP and
146 The compression algorithm is the same used by
150 can be controlled by the
152 option for protocol version 1.
153 Compression is desirable on modem lines and other
154 slow connections, but will only slow down things on fast networks.
155 The default value can be set on a host-by-host basis in the
156 configuration files; see the
160 .It Fl c Ar cipher_spec
161 Selects the cipher specification for encrypting the session.
163 Protocol version 1 allows specification of a single cipher.
164 The supported values are
169 For protocol version 2,
171 is a comma-separated list of ciphers
172 listed in order of preference.
177 for more information.
181 .Oo Ar bind_address : Oc
187 application-level port forwarding.
188 This works by allocating a socket to listen to
190 on the local side, optionally bound to the specified
192 Whenever a connection is made to this port, the
193 connection is forwarded over the secure channel, and the application
194 protocol is then used to determine where to connect to from the
196 Currently the SOCKS4 and SOCKS5 protocols are supported, and
198 will act as a SOCKS server.
199 Only root can forward privileged ports.
200 Dynamic port forwardings can also be specified in the configuration file.
202 IPv6 addresses can be specified by enclosing the address in square brackets.
203 Only the superuser can forward privileged ports.
204 By default, the local port is bound in accordance with the
209 may be used to bind the connection to a specific address.
214 indicates that the listening port be bound for local use only, while an
217 indicates that the port should be available from all interfaces.
222 instead of standard error.
224 .It Fl e Ar escape_char
225 Sets the escape character for sessions with a pty (default:
227 The escape character is only recognized at the beginning of a line.
228 The escape character followed by a dot
230 closes the connection;
231 followed by control-Z suspends the connection;
232 and followed by itself sends the escape character once.
233 Setting the character to
235 disables any escapes and makes the session fully transparent.
237 .It Fl F Ar configfile
238 Specifies an alternative per-user configuration file.
239 If a configuration file is given on the command line,
240 the system-wide configuration file
241 .Pq Pa /etc/ssh/ssh_config
243 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.
272 to print its configuration after evaluating
279 Allows remote hosts to connect to local forwarded ports.
280 If used on a multiplexed connection, then this option must be specified
281 on the master process.
284 Specify the PKCS#11 shared library
286 should use to communicate with a PKCS#11 token providing the user's
289 .It Fl i Ar identity_file
290 Selects a file from which the identity (private key) for
291 public key authentication is read.
294 for protocol version 1, and
296 .Pa ~/.ssh/id_ecdsa ,
297 .Pa ~/.ssh/id_ed25519
300 for protocol version 2.
301 Identity files may also be specified on
302 a per-host basis in the configuration file.
303 It is possible to have multiple
305 options (and multiple identities specified in
306 configuration files).
308 will also try to load certificate information from the filename obtained
311 to identity filenames.
314 Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
315 credentials to the server.
318 Disables forwarding (delegation) of GSSAPI credentials to the server.
322 .Oo Ar bind_address : Oc
323 .Ar port : host : hostport
328 .Oo Ar bind_address : Oc
329 .Ar port : remote_socket
334 .Ar local_socket : host : hostport
339 .Ar local_socket : remote_socket
342 Specifies that connections to the given TCP port or Unix socket on the local
343 (client) host are to be forwarded to the given host and port, or Unix socket,
345 This works by allocating a socket to listen to either a TCP
347 on the local side, optionally bound to the specified
350 Whenever a connection is made to the local port or socket, the
351 connection is forwarded over the secure channel, and a connection is
358 from the remote machine.
360 Port forwardings can also be specified in the configuration file.
361 Only the superuser can forward privileged ports.
362 IPv6 addresses can be specified by enclosing the address in square brackets.
364 By default, the local port is bound in accordance with the
369 may be used to bind the connection to a specific address.
374 indicates that the listening port be bound for local use only, while an
377 indicates that the port should be available from all interfaces.
379 .It Fl l Ar login_name
380 Specifies the user to log in as on the remote machine.
381 This also may be specified on a per-host basis in the configuration file.
388 mode for connection sharing.
395 mode with confirmation required before slave connections are accepted.
396 Refer to the description of
403 Additionally, for protocol version 2 a comma-separated list of MAC
404 (message authentication code) algorithms can
405 be specified in order of preference.
408 keyword for more information.
411 Do not execute a remote command.
412 This is useful for just forwarding ports
413 (protocol version 2 only).
418 (actually, prevents reading from stdin).
419 This must be used when
421 is run in the background.
422 A common trick is to use this to run X11 programs on a remote machine.
424 .Ic ssh -n shadows.cs.hut.fi emacs &
425 will start an emacs on shadows.cs.hut.fi, and the X11
426 connection will be automatically forwarded over an encrypted channel.
429 program will be put in the background.
430 (This does not work if
432 needs to ask for a password or passphrase; see also the
437 Control an active connection multiplexing master process.
440 option is specified, the
442 argument is interpreted and passed to the master process.
445 (check that the master process is running),
447 (request forwardings without command execution),
449 (cancel forwardings),
451 (request the master to exit), and
453 (request the master to stop accepting further multiplexing requests).
456 Can be used to give options in the format used in the configuration file.
457 This is useful for specifying options for which there is no separate
459 For full details of the options listed below, and their possible values, see
462 .Bl -tag -width Ds -offset indent -compact
467 .It CanonicalizeFallbackLocal
468 .It CanonicalizeHostname
469 .It CanonicalizeMaxDots
470 .It CanonicalizePermittedCNAMEs
471 .It ChallengeResponseAuthentication
475 .It ClearAllForwardings
478 .It ConnectionAttempts
485 .It ExitOnForwardFailure
489 .It ForwardX11Timeout
490 .It ForwardX11Trusted
492 .It GlobalKnownHostsFile
493 .It GSSAPIAuthentication
494 .It GSSAPIDelegateCredentials
497 .It HostbasedAuthentication
498 .It HostbasedKeyTypes
499 .It HostKeyAlgorithms
505 .It KbdInteractiveAuthentication
506 .It KbdInteractiveDevices
513 .It NoHostAuthenticationForLocalhost
514 .It NumberOfPasswordPrompts
515 .It PasswordAuthentication
516 .It PermitLocalCommand
519 .It PreferredAuthentications
523 .It PubkeyAcceptedKeyTypes
524 .It PubkeyAuthentication
528 .It RhostsRSAAuthentication
529 .It RSAAuthentication
531 .It ServerAliveInterval
532 .It ServerAliveCountMax
533 .It StreamLocalBindMask
534 .It StreamLocalBindUnlink
535 .It StrictHostKeyChecking
540 .It UsePrivilegedPort
542 .It UserKnownHostsFile
549 Port to connect to on the remote host.
550 This can be specified on a
551 per-host basis in the configuration file.
553 .It Fl Q Cm cipher | cipher-auth | mac | kex | key | protocol-version
556 for the algorithms supported for the specified version 2.
557 The available features are:
559 (supported symmetric ciphers),
561 (supported symmetric ciphers that support authenticated encryption),
563 (supported message integrity codes),
565 (key exchange algorithms),
569 (supported SSH protocol versions).
573 Causes most warning and diagnostic messages to be suppressed.
577 .Oo Ar bind_address : Oc
578 .Ar port : host : hostport
583 .Oo Ar bind_address : Oc
584 .Ar port : local_socket
589 .Ar remote_socket : host : hostport
594 .Ar remote_socket : local_socket
597 Specifies that connections to the given TCP port or Unix socket on the remote
598 (server) host are to be forwarded to the given host and port, or Unix socket,
600 This works by allocating a socket to listen to either a TCP
602 or to a Unix socket on the remote side.
603 Whenever a connection is made to this port or Unix socket, the
604 connection is forwarded over the secure channel, and a connection
611 from the local machine.
613 Port forwardings can also be specified in the configuration file.
614 Privileged ports can be forwarded only when
615 logging in as root on the remote machine.
616 IPv6 addresses can be specified by enclosing the address in square brackets.
618 By default, TCP listening sockets on the server will be bound to the loopback
620 This may be overridden by specifying a
626 indicates that the remote socket should listen on all interfaces.
629 will only succeed if the server's
631 option is enabled (see
632 .Xr sshd_config 5 ) .
638 the listen port will be dynamically allocated on the server and reported
639 to the client at run time.
640 When used together with
642 the allocated port will be printed to the standard output.
645 Specifies the location of a control socket for connection sharing,
648 to disable connection sharing.
649 Refer to the description of
658 May be used to request invocation of a subsystem on the remote system.
659 Subsystems are a feature of the SSH2 protocol which facilitate the use
660 of SSH as a secure transport for other applications (eg.\&
662 The subsystem is specified as the remote command.
665 Disable pseudo-terminal allocation.
668 Force pseudo-terminal allocation.
669 This can be used to execute arbitrary
670 screen-based programs on a remote machine, which can be very useful,
671 e.g. when implementing menu services.
674 options force tty allocation, even if
679 Display the version number and exit.
685 to print debugging messages about its progress.
687 debugging connection, authentication, and configuration problems.
690 options increase the verbosity.
693 .It Fl W Ar host : Ns Ar port
694 Requests that standard input and output on the client be forwarded to
698 over the secure channel.
702 .Cm ExitOnForwardFailure
704 .Cm ClearAllForwardings .
705 Works with Protocol version 2 only.
708 .Ar local_tun Ns Op : Ns Ar remote_tun
712 device forwarding with the specified
714 devices between the client
719 The devices may be specified by numerical ID or the keyword
721 which uses the next available tunnel device.
724 is not specified, it defaults to
734 directive is unset, it is set to the default tunnel mode, which is
738 Enables X11 forwarding.
739 This can also be specified on a per-host basis in a configuration file.
741 X11 forwarding should be enabled with caution.
742 Users with the ability to bypass file permissions on the remote host
743 (for the user's X authorization database)
744 can access the local X11 display through the forwarded connection.
745 An attacker may then be able to perform activities such as keystroke monitoring.
747 For this reason, X11 forwarding is subjected to X11 SECURITY extension
748 restrictions by default.
753 .Cm ForwardX11Trusted
756 for more information.
759 Disables X11 forwarding.
762 Enables trusted X11 forwarding.
763 Trusted X11 forwardings are not subjected to the X11 SECURITY extension
767 Send log information using the
770 By default this information is sent to stderr.
774 may additionally obtain configuration data from
775 a per-user configuration file and a system-wide configuration file.
776 The file format and configuration options are described in
779 The OpenSSH SSH client supports SSH protocols 1 and 2.
780 The default is to use protocol 2 only,
781 though this can be changed via the
790 Both protocols support similar authentication methods,
791 but protocol 2 is the default since
792 it provides additional mechanisms for confidentiality
793 (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
794 and integrity (hmac-md5, hmac-sha1,
795 hmac-sha2-256, hmac-sha2-512,
796 umac-64, umac-128, hmac-ripemd160).
797 Protocol 1 lacks a strong mechanism for ensuring the
798 integrity of the connection.
800 The methods available for authentication are:
801 GSSAPI-based authentication,
802 host-based authentication,
803 public key authentication,
804 challenge-response authentication,
805 and password authentication.
806 Authentication methods are tried in the order specified above,
807 though protocol 2 has a configuration option to change the default order:
808 .Cm PreferredAuthentications .
810 Host-based authentication works as follows:
811 If the machine the user logs in from is listed in
814 .Pa /etc/shosts.equiv
815 on the remote machine, and the user names are
816 the same on both sides, or if the files
820 exist in the user's home directory on the
821 remote machine and contain a line containing the name of the client
822 machine and the name of the user on that machine, the user is
823 considered for login.
824 Additionally, the server
826 be able to verify the client's
827 host key (see the description of
828 .Pa /etc/ssh/ssh_known_hosts
830 .Pa ~/.ssh/known_hosts ,
832 for login to be permitted.
833 This authentication method closes security holes due to IP
834 spoofing, DNS spoofing, and routing spoofing.
835 [Note to the administrator:
836 .Pa /etc/hosts.equiv ,
838 and the rlogin/rsh protocol in general, are inherently insecure and should be
839 disabled if security is desired.]
841 Public key authentication works as follows:
842 The scheme is based on public-key cryptography,
844 where encryption and decryption are done using separate keys,
845 and it is unfeasible to derive the decryption key from the encryption key.
846 The idea is that each user creates a public/private
847 key pair for authentication purposes.
848 The server knows the public key, and only the user knows the private key.
850 implements public key authentication protocol automatically,
851 using one of the DSA, ECDSA, Ed25519 or RSA algorithms.
852 Protocol 1 is restricted to using only RSA keys,
853 but protocol 2 may use any.
854 The HISTORY section of
856 contains a brief discussion of the DSA and RSA algorithms.
859 .Pa ~/.ssh/authorized_keys
860 lists the public keys that are permitted for logging in.
861 When the user logs in, the
863 program tells the server which key pair it would like to use for
865 The client proves that it has access to the private key
866 and the server checks that the corresponding public key
867 is authorized to accept the account.
869 The user creates his/her key pair by running
871 This stores the private key in
878 .Pa ~/.ssh/id_ed25519
879 (protocol 2 Ed25519),
883 and stores the public key in
884 .Pa ~/.ssh/identity.pub
886 .Pa ~/.ssh/id_dsa.pub
888 .Pa ~/.ssh/id_ecdsa.pub
890 .Pa ~/.ssh/id_ed25519.pub
891 (protocol 2 Ed25519),
893 .Pa ~/.ssh/id_rsa.pub
895 in the user's home directory.
896 The user should then copy the public key
898 .Pa ~/.ssh/authorized_keys
899 in his/her home directory on the remote machine.
902 file corresponds to the conventional
904 file, and has one key
905 per line, though the lines can be very long.
906 After this, the user can log in without giving the password.
908 A variation on public key authentication
909 is available in the form of certificate authentication:
910 instead of a set of public/private keys,
911 signed certificates are used.
912 This has the advantage that a single trusted certification authority
913 can be used in place of many public/private keys.
914 See the CERTIFICATES section of
916 for more information.
918 The most convenient way to use public key or certificate authentication
919 may be with an authentication agent.
922 for more information.
924 Challenge-response authentication works as follows:
925 The server sends an arbitrary
927 text, and prompts for a response.
928 Protocol 2 allows multiple challenges and responses;
929 protocol 1 is restricted to just one challenge/response.
930 Examples of challenge-response authentication include
938 Finally, if other authentication methods fail,
940 prompts the user for a password.
941 The password is sent to the remote
942 host for checking; however, since all communications are encrypted,
943 the password cannot be seen by someone listening on the network.
946 automatically maintains and checks a database containing
947 identification for all hosts it has ever been used with.
948 Host keys are stored in
949 .Pa ~/.ssh/known_hosts
950 in the user's home directory.
951 Additionally, the file
952 .Pa /etc/ssh/ssh_known_hosts
953 is automatically checked for known hosts.
954 Any new hosts are automatically added to the user's file.
955 If a host's identification ever changes,
957 warns about this and disables password authentication to prevent
958 server spoofing or man-in-the-middle attacks,
959 which could otherwise be used to circumvent the encryption.
961 .Cm StrictHostKeyChecking
962 option can be used to control logins to machines whose
963 host key is not known or has changed.
965 When the user's identity has been accepted by the server, the server
966 either executes the given command in a non-interactive session or,
967 if no command has been specified, logs into the machine and gives
968 the user a normal shell as an interactive session.
969 All communication with
970 the remote command or shell will be automatically encrypted.
972 If an interactive session is requested
974 by default will only request a pseudo-terminal (pty) for interactive
975 sessions when the client has one.
980 can be used to override this behaviour.
982 If a pseudo-terminal has been allocated the
983 user may use the escape characters noted below.
985 If no pseudo-terminal has been allocated,
986 the session is transparent and can be used to reliably transfer binary data.
987 On most systems, setting the escape character to
989 will also make the session transparent even if a tty is used.
991 The session terminates when the command or shell on the remote
992 machine exits and all X11 and TCP connections have been closed.
993 .Sh ESCAPE CHARACTERS
994 When a pseudo-terminal has been requested,
996 supports a number of functions through the use of an escape character.
998 A single tilde character can be sent as
1000 or by following the tilde by a character other than those described below.
1001 The escape character must always follow a newline to be interpreted as
1003 The escape character can be changed in configuration files using the
1005 configuration directive or on the command line by the
1009 The supported escapes (assuming the default
1019 List forwarded connections.
1023 at logout when waiting for forwarded connection / X11 sessions to terminate.
1025 Display a list of escape characters.
1027 Send a BREAK to the remote system
1028 (only useful for SSH protocol version 2 and if the peer supports it).
1031 Currently this allows the addition of port forwardings using the
1036 options (see above).
1037 It also allows the cancellation of existing port-forwardings
1040 .Fl KL Oo Ar bind_address : Oc Ar port
1044 .Fl KR Oo Ar bind_address : Oc Ar port
1048 .Fl KD Oo Ar bind_address : Oc Ar port
1050 for dynamic port-forwardings.
1051 .Ic !\& Ns Ar command
1052 allows the user to execute a local command if the
1053 .Ic PermitLocalCommand
1054 option is enabled in
1056 Basic help is available, using the
1060 Request rekeying of the connection
1061 (only useful for SSH protocol version 2 and if the peer supports it).
1063 Decrease the verbosity
1065 when errors are being written to stderr.
1067 Increase the verbosity
1069 when errors are being written to stderr.
1072 Forwarding of arbitrary TCP connections over the secure channel can
1073 be specified either on the command line or in a configuration file.
1074 One possible application of TCP forwarding is a secure connection to a
1075 mail server; another is going through firewalls.
1077 In the example below, we look at encrypting communication between
1078 an IRC client and server, even though the IRC server does not directly
1079 support encrypted communications.
1080 This works as follows:
1081 the user connects to the remote host using
1083 specifying a port to be used to forward connections
1084 to the remote server.
1085 After that it is possible to start the service which is to be encrypted
1086 on the client machine,
1087 connecting to the same local port,
1090 will encrypt and forward the connection.
1092 The following example tunnels an IRC session from client machine
1096 .Dq server.example.com :
1097 .Bd -literal -offset 4n
1098 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
1099 $ irc -c '#users' -p 1234 pinky 127.0.0.1
1102 This tunnels a connection to IRC server
1103 .Dq server.example.com ,
1109 It doesn't matter which port is used,
1110 as long as it's greater than 1023
1111 (remember, only root can open sockets on privileged ports)
1112 and doesn't conflict with any ports already in use.
1113 The connection is forwarded to port 6667 on the remote server,
1114 since that's the standard port for IRC services.
1120 and the remote command
1122 is specified to allow an amount of time
1123 (10 seconds, in the example)
1124 to start the service which is to be tunnelled.
1125 If no connections are made within the time specified,
1133 (or see the description of the
1139 and the user is using X11 (the
1141 environment variable is set), the connection to the X11 display is
1142 automatically forwarded to the remote side in such a way that any X11
1143 programs started from the shell (or command) will go through the
1144 encrypted channel, and the connection to the real X server will be made
1145 from the local machine.
1146 The user should not manually set
1148 Forwarding of X11 connections can be
1149 configured on the command line or in configuration files.
1155 will point to the server machine, but with a display number greater than zero.
1156 This is normal, and happens because
1160 X server on the server machine for forwarding the
1161 connections over the encrypted channel.
1164 will also automatically set up Xauthority data on the server machine.
1165 For this purpose, it will generate a random authorization cookie,
1166 store it in Xauthority on the server, and verify that any forwarded
1167 connections carry this cookie and replace it by the real cookie when
1168 the connection is opened.
1169 The real authentication cookie is never
1170 sent to the server machine (and no cookies are sent in the plain).
1176 (or see the description of the
1181 the user is using an authentication agent, the connection to the agent
1182 is automatically forwarded to the remote side.
1183 .Sh VERIFYING HOST KEYS
1184 When connecting to a server for the first time,
1185 a fingerprint of the server's public key is presented to the user
1187 .Cm StrictHostKeyChecking
1189 Fingerprints can be determined using
1192 .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1194 If the fingerprint is already known, it can be matched
1195 and the key can be accepted or rejected.
1196 If only legacy (MD5) fingerprints for the server are available, the
1199 option may be used to downgrade the fingerprint algorithm to match.
1201 Because of the difficulty of comparing host keys
1202 just by looking at fingerprint strings,
1203 there is also support to compare host keys visually,
1210 a small ASCII graphic gets displayed on every login to a server, no matter
1211 if the session itself is interactive or not.
1212 By learning the pattern a known server produces, a user can easily
1213 find out that the host key has changed when a completely different pattern
1215 Because these patterns are not unambiguous however, a pattern that looks
1216 similar to the pattern remembered only gives a good probability that the
1217 host key is the same, not guaranteed proof.
1219 To get a listing of the fingerprints along with their random art for
1220 all known hosts, the following command line can be used:
1222 .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1224 If the fingerprint is unknown,
1225 an alternative method of verification is available:
1226 SSH fingerprints verified by DNS.
1227 An additional resource record (RR),
1229 is added to a zonefile
1230 and the connecting client is able to match the fingerprint
1231 with that of the key presented.
1233 In this example, we are connecting a client to a server,
1234 .Dq host.example.com .
1235 The SSHFP resource records should first be added to the zonefile for
1237 .Bd -literal -offset indent
1238 $ ssh-keygen -r host.example.com.
1241 The output lines will have to be added to the zonefile.
1242 To check that the zone is answering fingerprint queries:
1244 .Dl $ dig -t SSHFP host.example.com
1246 Finally the client connects:
1247 .Bd -literal -offset indent
1248 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1250 Matching host key fingerprint found in DNS.
1251 Are you sure you want to continue connecting (yes/no)?
1255 .Cm VerifyHostKeyDNS
1258 for more information.
1259 .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1261 contains support for Virtual Private Network (VPN) tunnelling
1264 network pseudo-device,
1265 allowing two networks to be joined securely.
1268 configuration option
1270 controls whether the server supports this,
1271 and at what level (layer 2 or 3 traffic).
1273 The following example would connect client network 10.0.50.0/24
1274 with remote network 10.0.99.0/24 using a point-to-point connection
1275 from 10.1.1.1 to 10.1.1.2,
1276 provided that the SSH server running on the gateway to the remote network,
1277 at 192.168.1.15, allows it.
1280 .Bd -literal -offset indent
1281 # ssh -f -w 0:1 192.168.1.15 true
1282 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1283 # route add 10.0.99.0/24 10.1.1.2
1287 .Bd -literal -offset indent
1288 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1289 # route add 10.0.50.0/24 10.1.1.1
1292 Client access may be more finely tuned via the
1293 .Pa /root/.ssh/authorized_keys
1294 file (see below) and the
1297 The following entry would permit connections on
1301 and on tun device 2 from user
1306 .Dq forced-commands-only :
1307 .Bd -literal -offset 2n
1308 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1309 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1312 Since an SSH-based setup entails a fair amount of overhead,
1313 it may be more suited to temporary setups,
1314 such as for wireless VPNs.
1315 More permanent VPNs are better provided by tools such as
1321 will normally set the following environment variables:
1322 .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1326 variable indicates the location of the X11 server.
1327 It is automatically set by
1329 to point to a value of the form
1333 indicates the host where the shell runs, and
1335 is an integer \*(Ge 1.
1337 uses this special value to forward X11 connections over the secure
1339 The user should normally not set
1342 will render the X11 connection insecure (and will require the user to
1343 manually copy any required authorization cookies).
1345 Set to the path of the user's home directory.
1349 set for compatibility with systems that use this variable.
1351 Set to the path of the user's mailbox.
1355 as specified when compiling
1360 needs a passphrase, it will read the passphrase from the current
1361 terminal if it was run from a terminal.
1364 does not have a terminal associated with it but
1368 are set, it will execute the program specified by
1370 and open an X11 window to read the passphrase.
1371 This is particularly useful when calling
1376 (Note that on some machines it
1377 may be necessary to redirect the input from
1380 .It Ev SSH_AUTH_SOCK
1381 Identifies the path of a
1383 socket used to communicate with the agent.
1384 .It Ev SSH_CONNECTION
1385 Identifies the client and server ends of the connection.
1386 The variable contains
1387 four space-separated values: client IP address, client port number,
1388 server IP address, and server port number.
1389 .It Ev SSH_ORIGINAL_COMMAND
1390 This variable contains the original command line if a forced command
1392 It can be used to extract the original arguments.
1394 This is set to the name of the tty (path to the device) associated
1395 with the current shell or command.
1396 If the current session has no tty,
1397 this variable is not set.
1399 This variable is set to indicate the present time zone if it
1400 was set when the daemon was started (i.e. the daemon passes the value
1401 on to new connections).
1403 Set to the name of the user logging in.
1409 .Pa ~/.ssh/environment ,
1410 and adds lines of the format
1412 to the environment if the file exists and users are allowed to
1413 change their environment.
1414 For more information, see the
1415 .Cm PermitUserEnvironment
1419 .Bl -tag -width Ds -compact
1421 This file is used for host-based authentication (see above).
1422 On some machines this file may need to be
1423 world-readable if the user's home directory is on an NFS partition,
1427 Additionally, this file must be owned by the user,
1428 and must not have write permissions for anyone else.
1430 permission for most machines is read/write for the user, and not
1431 accessible by others.
1434 This file is used in exactly the same way as
1436 but allows host-based authentication without permitting login with
1440 This directory is the default location for all user-specific configuration
1441 and authentication information.
1442 There is no general requirement to keep the entire contents of this directory
1443 secret, but the recommended permissions are read/write/execute for the user,
1444 and not accessible by others.
1446 .It Pa ~/.ssh/authorized_keys
1447 Lists the public keys (DSA, ECDSA, Ed25519, RSA)
1448 that can be used for logging in as this user.
1449 The format of this file is described in the
1452 This file is not highly sensitive, but the recommended
1453 permissions are read/write for the user, and not accessible by others.
1455 .It Pa ~/.ssh/config
1456 This is the per-user configuration file.
1457 The file format and configuration options are described in
1459 Because of the potential for abuse, this file must have strict permissions:
1460 read/write for the user, and not writable by others.
1462 .It Pa ~/.ssh/environment
1463 Contains additional definitions for environment variables; see
1467 .It Pa ~/.ssh/identity
1468 .It Pa ~/.ssh/id_dsa
1469 .It Pa ~/.ssh/id_ecdsa
1470 .It Pa ~/.ssh/id_ed25519
1471 .It Pa ~/.ssh/id_rsa
1472 Contains the private key for authentication.
1474 contain sensitive data and should be readable by the user but not
1475 accessible by others (read/write/execute).
1477 will simply ignore a private key file if it is accessible by others.
1478 It is possible to specify a passphrase when
1479 generating the key which will be used to encrypt the
1480 sensitive part of this file using 3DES.
1482 .It Pa ~/.ssh/identity.pub
1483 .It Pa ~/.ssh/id_dsa.pub
1484 .It Pa ~/.ssh/id_ecdsa.pub
1485 .It Pa ~/.ssh/id_ed25519.pub
1486 .It Pa ~/.ssh/id_rsa.pub
1487 Contains the public key for authentication.
1489 sensitive and can (but need not) be readable by anyone.
1491 .It Pa ~/.ssh/known_hosts
1492 Contains a list of host keys for all hosts the user has logged into
1493 that are not already in the systemwide list of known host keys.
1496 for further details of the format of this file.
1499 Commands in this file are executed by
1501 when the user logs in, just before the user's shell (or command) is
1505 manual page for more information.
1507 .It Pa /etc/hosts.equiv
1508 This file is for host-based authentication (see above).
1509 It should only be writable by root.
1511 .It Pa /etc/shosts.equiv
1512 This file is used in exactly the same way as
1514 but allows host-based authentication without permitting login with
1517 .It Pa /etc/ssh/ssh_config
1518 Systemwide configuration file.
1519 The file format and configuration options are described in
1522 .It Pa /etc/ssh/ssh_host_key
1523 .It Pa /etc/ssh/ssh_host_dsa_key
1524 .It Pa /etc/ssh/ssh_host_ecdsa_key
1525 .It Pa /etc/ssh/ssh_host_ed25519_key
1526 .It Pa /etc/ssh/ssh_host_rsa_key
1527 These files contain the private parts of the host keys
1528 and are used for host-based authentication.
1529 If protocol version 1 is used,
1531 must be setuid root, since the host key is readable only by root.
1532 For protocol version 2,
1536 to access the host keys,
1537 eliminating the requirement that
1539 be setuid root when host-based authentication is used.
1544 .It Pa /etc/ssh/ssh_known_hosts
1545 Systemwide list of known host keys.
1546 This file should be prepared by the
1547 system administrator to contain the public host keys of all machines in the
1549 It should be world-readable.
1552 for further details of the format of this file.
1554 .It Pa /etc/ssh/sshrc
1555 Commands in this file are executed by
1557 when the user logs in, just before the user's shell (or command) is started.
1560 manual page for more information.
1564 exits with the exit status of the remote command or with 255
1565 if an error occurred.
1583 .%T The Secure Shell (SSH) Protocol Assigned Numbers
1591 .%T The Secure Shell (SSH) Protocol Architecture
1599 .%T The Secure Shell (SSH) Authentication Protocol
1607 .%T The Secure Shell (SSH) Transport Layer Protocol
1615 .%T The Secure Shell (SSH) Connection Protocol
1623 .%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
1631 .%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
1639 .%T The Secure Shell (SSH) Session Channel Break Extension
1648 .%T The Secure Shell (SSH) Transport Layer Encryption Modes
1655 .%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
1664 .%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
1672 .%T The Secure Shell (SSH) Public Key File Format
1680 .%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
1687 .%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
1688 .%T Hash Visualization: a New Technique to improve Real-World Security
1691 OpenSSH is a derivative of the original and free
1692 ssh 1.2.12 release by Tatu Ylonen.
1693 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1694 Theo de Raadt and Dug Song
1695 removed many bugs, re-added newer features and
1697 Markus Friedl contributed the support for SSH
1698 protocol versions 1.5 and 2.0.
OSDN Git Service
