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 1.376 2016/07/16 06:57:55 jmc Exp $
37 .Dd $Mdocdate: July 16 2016 $
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
55 .Op Fl J Oo Ar user Ns @ Oc Ns Ar host Ns Op : Ns Ar port
57 .Op Fl l Ar login_name
62 .Op Fl Q Ar query_option
65 .Op Fl W Ar host : Ns Ar port
66 .Op Fl w Ar local_tun Ns Op : Ns Ar remote_tun
67 .Oo Ar user Ns @ Oc Ns Ar hostname
72 (SSH client) is a program for logging into a remote machine and for
73 executing commands on a remote machine.
74 It is intended to 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
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).
307 If no certificates have been explicitly specified by the
311 will also try to load certificate information from the filename obtained
314 to identity filenames.
323 Connect to the target host by first making a
325 connection to the jump
327 and then establishing a TCP forwarding to the ultimate destination from
329 Multiple jump hops may be specified separated by comma characters.
330 This is a shortcut to specify a
332 configuration directive.
335 Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
336 credentials to the server.
339 Disables forwarding (delegation) of GSSAPI credentials to the server.
343 .Oo Ar bind_address : Oc
344 .Ar port : host : hostport
349 .Oo Ar bind_address : Oc
350 .Ar port : remote_socket
355 .Ar local_socket : host : hostport
360 .Ar local_socket : remote_socket
363 Specifies that connections to the given TCP port or Unix socket on the local
364 (client) host are to be forwarded to the given host and port, or Unix socket,
366 This works by allocating a socket to listen to either a TCP
368 on the local side, optionally bound to the specified
371 Whenever a connection is made to the local port or socket, the
372 connection is forwarded over the secure channel, and a connection is
379 from the remote machine.
381 Port forwardings can also be specified in the configuration file.
382 Only the superuser can forward privileged ports.
383 IPv6 addresses can be specified by enclosing the address in square brackets.
385 By default, the local port is bound in accordance with the
390 may be used to bind the connection to a specific address.
395 indicates that the listening port be bound for local use only, while an
398 indicates that the port should be available from all interfaces.
400 .It Fl l Ar login_name
401 Specifies the user to log in as on the remote machine.
402 This also may be specified on a per-host basis in the configuration file.
409 mode for connection sharing.
416 mode with confirmation required before slave connections are accepted.
417 Refer to the description of
424 A comma-separated list of MAC (message authentication code) algorithms,
425 specified in order of preference.
428 keyword for more information.
431 Do not execute a remote command.
432 This is useful for just forwarding ports.
437 (actually, prevents reading from stdin).
438 This must be used when
440 is run in the background.
441 A common trick is to use this to run X11 programs on a remote machine.
443 .Ic ssh -n shadows.cs.hut.fi emacs &
444 will start an emacs on shadows.cs.hut.fi, and the X11
445 connection will be automatically forwarded over an encrypted channel.
448 program will be put in the background.
449 (This does not work if
451 needs to ask for a password or passphrase; see also the
456 Control an active connection multiplexing master process.
459 option is specified, the
461 argument is interpreted and passed to the master process.
464 (check that the master process is running),
466 (request forwardings without command execution),
468 (cancel forwardings),
470 (request the master to exit), and
472 (request the master to stop accepting further multiplexing requests).
475 Can be used to give options in the format used in the configuration file.
476 This is useful for specifying options for which there is no separate
478 For full details of the options listed below, and their possible values, see
481 .Bl -tag -width Ds -offset indent -compact
487 .It CanonicalizeFallbackLocal
488 .It CanonicalizeHostname
489 .It CanonicalizeMaxDots
490 .It CanonicalizePermittedCNAMEs
492 .It ChallengeResponseAuthentication
496 .It ClearAllForwardings
499 .It ConnectionAttempts
506 .It ExitOnForwardFailure
510 .It ForwardX11Timeout
511 .It ForwardX11Trusted
513 .It GlobalKnownHostsFile
514 .It GSSAPIAuthentication
515 .It GSSAPIDelegateCredentials
518 .It HostbasedAuthentication
519 .It HostbasedKeyTypes
520 .It HostKeyAlgorithms
528 .It KbdInteractiveAuthentication
529 .It KbdInteractiveDevices
536 .It NoHostAuthenticationForLocalhost
537 .It NumberOfPasswordPrompts
538 .It PasswordAuthentication
539 .It PermitLocalCommand
542 .It PreferredAuthentications
547 .It PubkeyAcceptedKeyTypes
548 .It PubkeyAuthentication
552 .It RhostsRSAAuthentication
553 .It RSAAuthentication
555 .It ServerAliveInterval
556 .It ServerAliveCountMax
557 .It StreamLocalBindMask
558 .It StreamLocalBindUnlink
559 .It StrictHostKeyChecking
564 .It UsePrivilegedPort
566 .It UserKnownHostsFile
573 Port to connect to on the remote host.
574 This can be specified on a
575 per-host basis in the configuration file.
577 .It Fl Q Ar query_option
580 for the algorithms supported for the specified version 2.
581 The available features are:
583 (supported symmetric ciphers),
585 (supported symmetric ciphers that support authenticated encryption),
587 (supported message integrity codes),
589 (key exchange algorithms),
593 (certificate key types),
595 (non-certificate key types), and
597 (supported SSH protocol versions).
601 Causes most warning and diagnostic messages to be suppressed.
605 .Oo Ar bind_address : Oc
606 .Ar port : host : hostport
611 .Oo Ar bind_address : Oc
612 .Ar port : local_socket
617 .Ar remote_socket : host : hostport
622 .Ar remote_socket : local_socket
625 Specifies that connections to the given TCP port or Unix socket on the remote
626 (server) host are to be forwarded to the given host and port, or Unix socket,
628 This works by allocating a socket to listen to either a TCP
630 or to a Unix socket on the remote side.
631 Whenever a connection is made to this port or Unix socket, the
632 connection is forwarded over the secure channel, and a connection
639 from the local machine.
641 Port forwardings can also be specified in the configuration file.
642 Privileged ports can be forwarded only when
643 logging in as root on the remote machine.
644 IPv6 addresses can be specified by enclosing the address in square brackets.
646 By default, TCP listening sockets on the server will be bound to the loopback
648 This may be overridden by specifying a
654 indicates that the remote socket should listen on all interfaces.
657 will only succeed if the server's
659 option is enabled (see
660 .Xr sshd_config 5 ) .
666 the listen port will be dynamically allocated on the server and reported
667 to the client at run time.
668 When used together with
670 the allocated port will be printed to the standard output.
673 Specifies the location of a control socket for connection sharing,
676 to disable connection sharing.
677 Refer to the description of
686 May be used to request invocation of a subsystem on the remote system.
687 Subsystems facilitate the use of SSH
688 as a secure transport for other applications (e.g.\&
690 The subsystem is specified as the remote command.
693 Disable pseudo-terminal allocation.
696 Force pseudo-terminal allocation.
697 This can be used to execute arbitrary
698 screen-based programs on a remote machine, which can be very useful,
699 e.g. when implementing menu services.
702 options force tty allocation, even if
707 Display the version number and exit.
713 to print debugging messages about its progress.
715 debugging connection, authentication, and configuration problems.
718 options increase the verbosity.
721 .It Fl W Ar host : Ns Ar port
722 Requests that standard input and output on the client be forwarded to
726 over the secure channel.
730 .Cm ExitOnForwardFailure
732 .Cm ClearAllForwardings ,
733 though these can be overridden in the configuration file or using
735 command line options.
738 .Ar local_tun Ns Op : Ns Ar remote_tun
742 device forwarding with the specified
744 devices between the client
749 The devices may be specified by numerical ID or the keyword
751 which uses the next available tunnel device.
754 is not specified, it defaults to
764 directive is unset, it is set to the default tunnel mode, which is
768 Enables X11 forwarding.
769 This can also be specified on a per-host basis in a configuration file.
771 X11 forwarding should be enabled with caution.
772 Users with the ability to bypass file permissions on the remote host
773 (for the user's X authorization database)
774 can access the local X11 display through the forwarded connection.
775 An attacker may then be able to perform activities such as keystroke monitoring.
777 For this reason, X11 forwarding is subjected to X11 SECURITY extension
778 restrictions by default.
783 .Cm ForwardX11Trusted
786 for more information.
789 Disables X11 forwarding.
792 Enables trusted X11 forwarding.
793 Trusted X11 forwardings are not subjected to the X11 SECURITY extension
797 Send log information using the
800 By default this information is sent to stderr.
804 may additionally obtain configuration data from
805 a per-user configuration file and a system-wide configuration file.
806 The file format and configuration options are described in
809 The OpenSSH SSH client supports SSH protocols 1 and 2.
810 The default is to use protocol 2 only,
811 though this can be changed via the
820 Protocol 1 should not be used
821 and is only offered to support legacy devices.
822 It suffers from a number of cryptographic weaknesses
823 and doesn't support many of the advanced features available for protocol 2.
825 The methods available for authentication are:
826 GSSAPI-based authentication,
827 host-based authentication,
828 public key authentication,
829 challenge-response authentication,
830 and password authentication.
831 Authentication methods are tried in the order specified above,
833 .Cm PreferredAuthentications
834 can be used to change the default order.
836 Host-based authentication works as follows:
837 If the machine the user logs in from is listed in
840 .Pa /etc/ssh/shosts.equiv
841 on the remote machine, and the user names are
842 the same on both sides, or if the files
846 exist in the user's home directory on the
847 remote machine and contain a line containing the name of the client
848 machine and the name of the user on that machine, the user is
849 considered for login.
850 Additionally, the server
852 be able to verify the client's
853 host key (see the description of
854 .Pa /etc/ssh/ssh_known_hosts
856 .Pa ~/.ssh/known_hosts ,
858 for login to be permitted.
859 This authentication method closes security holes due to IP
860 spoofing, DNS spoofing, and routing spoofing.
861 [Note to the administrator:
862 .Pa /etc/hosts.equiv ,
864 and the rlogin/rsh protocol in general, are inherently insecure and should be
865 disabled if security is desired.]
867 Public key authentication works as follows:
868 The scheme is based on public-key cryptography,
870 where encryption and decryption are done using separate keys,
871 and it is unfeasible to derive the decryption key from the encryption key.
872 The idea is that each user creates a public/private
873 key pair for authentication purposes.
874 The server knows the public key, and only the user knows the private key.
876 implements public key authentication protocol automatically,
877 using one of the DSA, ECDSA, Ed25519 or RSA algorithms.
878 The HISTORY section of
880 contains a brief discussion of the DSA and RSA algorithms.
883 .Pa ~/.ssh/authorized_keys
884 lists the public keys that are permitted for logging in.
885 When the user logs in, the
887 program tells the server which key pair it would like to use for
889 The client proves that it has access to the private key
890 and the server checks that the corresponding public key
891 is authorized to accept the account.
893 The user creates his/her key pair by running
895 This stores the private key in
902 .Pa ~/.ssh/id_ed25519
907 and stores the public key in
908 .Pa ~/.ssh/identity.pub
910 .Pa ~/.ssh/id_dsa.pub
912 .Pa ~/.ssh/id_ecdsa.pub
914 .Pa ~/.ssh/id_ed25519.pub
917 .Pa ~/.ssh/id_rsa.pub
919 in the user's home directory.
920 The user should then copy the public key
922 .Pa ~/.ssh/authorized_keys
923 in his/her home directory on the remote machine.
926 file corresponds to the conventional
928 file, and has one key
929 per line, though the lines can be very long.
930 After this, the user can log in without giving the password.
932 A variation on public key authentication
933 is available in the form of certificate authentication:
934 instead of a set of public/private keys,
935 signed certificates are used.
936 This has the advantage that a single trusted certification authority
937 can be used in place of many public/private keys.
938 See the CERTIFICATES section of
940 for more information.
942 The most convenient way to use public key or certificate authentication
943 may be with an authentication agent.
950 for more information.
952 Challenge-response authentication works as follows:
953 The server sends an arbitrary
955 text, and prompts for a response.
956 Examples of challenge-response authentication include
964 Finally, if other authentication methods fail,
966 prompts the user for a password.
967 The password is sent to the remote
968 host for checking; however, since all communications are encrypted,
969 the password cannot be seen by someone listening on the network.
972 automatically maintains and checks a database containing
973 identification for all hosts it has ever been used with.
974 Host keys are stored in
975 .Pa ~/.ssh/known_hosts
976 in the user's home directory.
977 Additionally, the file
978 .Pa /etc/ssh/ssh_known_hosts
979 is automatically checked for known hosts.
980 Any new hosts are automatically added to the user's file.
981 If a host's identification ever changes,
983 warns about this and disables password authentication to prevent
984 server spoofing or man-in-the-middle attacks,
985 which could otherwise be used to circumvent the encryption.
987 .Cm StrictHostKeyChecking
988 option can be used to control logins to machines whose
989 host key is not known or has changed.
991 When the user's identity has been accepted by the server, the server
992 either executes the given command in a non-interactive session or,
993 if no command has been specified, logs into the machine and gives
994 the user a normal shell as an interactive session.
995 All communication with
996 the remote command or shell will be automatically encrypted.
998 If an interactive session is requested
1000 by default will only request a pseudo-terminal (pty) for interactive
1001 sessions when the client has one.
1006 can be used to override this behaviour.
1008 If a pseudo-terminal has been allocated the
1009 user may use the escape characters noted below.
1011 If no pseudo-terminal has been allocated,
1012 the session is transparent and can be used to reliably transfer binary data.
1013 On most systems, setting the escape character to
1015 will also make the session transparent even if a tty is used.
1017 The session terminates when the command or shell on the remote
1018 machine exits and all X11 and TCP connections have been closed.
1019 .Sh ESCAPE CHARACTERS
1020 When a pseudo-terminal has been requested,
1022 supports a number of functions through the use of an escape character.
1024 A single tilde character can be sent as
1026 or by following the tilde by a character other than those described below.
1027 The escape character must always follow a newline to be interpreted as
1029 The escape character can be changed in configuration files using the
1031 configuration directive or on the command line by the
1035 The supported escapes (assuming the default
1045 List forwarded connections.
1049 at logout when waiting for forwarded connection / X11 sessions to terminate.
1051 Display a list of escape characters.
1053 Send a BREAK to the remote system
1054 (only useful if the peer supports it).
1057 Currently this allows the addition of port forwardings using the
1062 options (see above).
1063 It also allows the cancellation of existing port-forwardings
1066 .Fl KL Oo Ar bind_address : Oc Ar port
1070 .Fl KR Oo Ar bind_address : Oc Ar port
1074 .Fl KD Oo Ar bind_address : Oc Ar port
1076 for dynamic port-forwardings.
1077 .Ic !\& Ns Ar command
1078 allows the user to execute a local command if the
1079 .Ic PermitLocalCommand
1080 option is enabled in
1082 Basic help is available, using the
1086 Request rekeying of the connection
1087 (only useful if the peer supports it).
1089 Decrease the verbosity
1091 when errors are being written to stderr.
1093 Increase the verbosity
1095 when errors are being written to stderr.
1098 Forwarding of arbitrary TCP connections over the secure channel can
1099 be specified either on the command line or in a configuration file.
1100 One possible application of TCP forwarding is a secure connection to a
1101 mail server; another is going through firewalls.
1103 In the example below, we look at encrypting communication between
1104 an IRC client and server, even though the IRC server does not directly
1105 support encrypted communications.
1106 This works as follows:
1107 the user connects to the remote host using
1109 specifying a port to be used to forward connections
1110 to the remote server.
1111 After that it is possible to start the service which is to be encrypted
1112 on the client machine,
1113 connecting to the same local port,
1116 will encrypt and forward the connection.
1118 The following example tunnels an IRC session from client machine
1122 .Dq server.example.com :
1123 .Bd -literal -offset 4n
1124 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
1125 $ irc -c '#users' -p 1234 pinky 127.0.0.1
1128 This tunnels a connection to IRC server
1129 .Dq server.example.com ,
1135 It doesn't matter which port is used,
1136 as long as it's greater than 1023
1137 (remember, only root can open sockets on privileged ports)
1138 and doesn't conflict with any ports already in use.
1139 The connection is forwarded to port 6667 on the remote server,
1140 since that's the standard port for IRC services.
1146 and the remote command
1148 is specified to allow an amount of time
1149 (10 seconds, in the example)
1150 to start the service which is to be tunnelled.
1151 If no connections are made within the time specified,
1159 (or see the description of the
1165 and the user is using X11 (the
1167 environment variable is set), the connection to the X11 display is
1168 automatically forwarded to the remote side in such a way that any X11
1169 programs started from the shell (or command) will go through the
1170 encrypted channel, and the connection to the real X server will be made
1171 from the local machine.
1172 The user should not manually set
1174 Forwarding of X11 connections can be
1175 configured on the command line or in configuration files.
1176 Take note that X11 forwarding can represent a security hazard.
1182 will point to the server machine, but with a display number greater than zero.
1183 This is normal, and happens because
1187 X server on the server machine for forwarding the
1188 connections over the encrypted channel.
1191 will also automatically set up Xauthority data on the server machine.
1192 For this purpose, it will generate a random authorization cookie,
1193 store it in Xauthority on the server, and verify that any forwarded
1194 connections carry this cookie and replace it by the real cookie when
1195 the connection is opened.
1196 The real authentication cookie is never
1197 sent to the server machine (and no cookies are sent in the plain).
1203 (or see the description of the
1208 the user is using an authentication agent, the connection to the agent
1209 is automatically forwarded to the remote side.
1210 .Sh VERIFYING HOST KEYS
1211 When connecting to a server for the first time,
1212 a fingerprint of the server's public key is presented to the user
1214 .Cm StrictHostKeyChecking
1216 Fingerprints can be determined using
1219 .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1221 If the fingerprint is already known, it can be matched
1222 and the key can be accepted or rejected.
1223 If only legacy (MD5) fingerprints for the server are available, the
1226 option may be used to downgrade the fingerprint algorithm to match.
1228 Because of the difficulty of comparing host keys
1229 just by looking at fingerprint strings,
1230 there is also support to compare host keys visually,
1237 a small ASCII graphic gets displayed on every login to a server, no matter
1238 if the session itself is interactive or not.
1239 By learning the pattern a known server produces, a user can easily
1240 find out that the host key has changed when a completely different pattern
1242 Because these patterns are not unambiguous however, a pattern that looks
1243 similar to the pattern remembered only gives a good probability that the
1244 host key is the same, not guaranteed proof.
1246 To get a listing of the fingerprints along with their random art for
1247 all known hosts, the following command line can be used:
1249 .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1251 If the fingerprint is unknown,
1252 an alternative method of verification is available:
1253 SSH fingerprints verified by DNS.
1254 An additional resource record (RR),
1256 is added to a zonefile
1257 and the connecting client is able to match the fingerprint
1258 with that of the key presented.
1260 In this example, we are connecting a client to a server,
1261 .Dq host.example.com .
1262 The SSHFP resource records should first be added to the zonefile for
1264 .Bd -literal -offset indent
1265 $ ssh-keygen -r host.example.com.
1268 The output lines will have to be added to the zonefile.
1269 To check that the zone is answering fingerprint queries:
1271 .Dl $ dig -t SSHFP host.example.com
1273 Finally the client connects:
1274 .Bd -literal -offset indent
1275 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1277 Matching host key fingerprint found in DNS.
1278 Are you sure you want to continue connecting (yes/no)?
1282 .Cm VerifyHostKeyDNS
1285 for more information.
1286 .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1288 contains support for Virtual Private Network (VPN) tunnelling
1291 network pseudo-device,
1292 allowing two networks to be joined securely.
1295 configuration option
1297 controls whether the server supports this,
1298 and at what level (layer 2 or 3 traffic).
1300 The following example would connect client network 10.0.50.0/24
1301 with remote network 10.0.99.0/24 using a point-to-point connection
1302 from 10.1.1.1 to 10.1.1.2,
1303 provided that the SSH server running on the gateway to the remote network,
1304 at 192.168.1.15, allows it.
1307 .Bd -literal -offset indent
1308 # ssh -f -w 0:1 192.168.1.15 true
1309 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1310 # route add 10.0.99.0/24 10.1.1.2
1314 .Bd -literal -offset indent
1315 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1316 # route add 10.0.50.0/24 10.1.1.1
1319 Client access may be more finely tuned via the
1320 .Pa /root/.ssh/authorized_keys
1321 file (see below) and the
1324 The following entry would permit connections on
1328 and on tun device 2 from user
1333 .Dq forced-commands-only :
1334 .Bd -literal -offset 2n
1335 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1336 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1339 Since an SSH-based setup entails a fair amount of overhead,
1340 it may be more suited to temporary setups,
1341 such as for wireless VPNs.
1342 More permanent VPNs are better provided by tools such as
1348 will normally set the following environment variables:
1349 .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1353 variable indicates the location of the X11 server.
1354 It is automatically set by
1356 to point to a value of the form
1360 indicates the host where the shell runs, and
1362 is an integer \*(Ge 1.
1364 uses this special value to forward X11 connections over the secure
1366 The user should normally not set
1369 will render the X11 connection insecure (and will require the user to
1370 manually copy any required authorization cookies).
1372 Set to the path of the user's home directory.
1376 set for compatibility with systems that use this variable.
1378 Set to the path of the user's mailbox.
1382 as specified when compiling
1387 needs a passphrase, it will read the passphrase from the current
1388 terminal if it was run from a terminal.
1391 does not have a terminal associated with it but
1395 are set, it will execute the program specified by
1397 and open an X11 window to read the passphrase.
1398 This is particularly useful when calling
1403 (Note that on some machines it
1404 may be necessary to redirect the input from
1407 .It Ev SSH_AUTH_SOCK
1408 Identifies the path of a
1410 socket used to communicate with the agent.
1411 .It Ev SSH_CONNECTION
1412 Identifies the client and server ends of the connection.
1413 The variable contains
1414 four space-separated values: client IP address, client port number,
1415 server IP address, and server port number.
1416 .It Ev SSH_ORIGINAL_COMMAND
1417 This variable contains the original command line if a forced command
1419 It can be used to extract the original arguments.
1421 This is set to the name of the tty (path to the device) associated
1422 with the current shell or command.
1423 If the current session has no tty,
1424 this variable is not set.
1426 This variable is set to indicate the present time zone if it
1427 was set when the daemon was started (i.e. the daemon passes the value
1428 on to new connections).
1430 Set to the name of the user logging in.
1436 .Pa ~/.ssh/environment ,
1437 and adds lines of the format
1439 to the environment if the file exists and users are allowed to
1440 change their environment.
1441 For more information, see the
1442 .Cm PermitUserEnvironment
1446 .Bl -tag -width Ds -compact
1448 This file is used for host-based authentication (see above).
1449 On some machines this file may need to be
1450 world-readable if the user's home directory is on an NFS partition,
1454 Additionally, this file must be owned by the user,
1455 and must not have write permissions for anyone else.
1457 permission for most machines is read/write for the user, and not
1458 accessible by others.
1461 This file is used in exactly the same way as
1463 but allows host-based authentication without permitting login with
1467 This directory is the default location for all user-specific configuration
1468 and authentication information.
1469 There is no general requirement to keep the entire contents of this directory
1470 secret, but the recommended permissions are read/write/execute for the user,
1471 and not accessible by others.
1473 .It Pa ~/.ssh/authorized_keys
1474 Lists the public keys (DSA, ECDSA, Ed25519, RSA)
1475 that can be used for logging in as this user.
1476 The format of this file is described in the
1479 This file is not highly sensitive, but the recommended
1480 permissions are read/write for the user, and not accessible by others.
1482 .It Pa ~/.ssh/config
1483 This is the per-user configuration file.
1484 The file format and configuration options are described in
1486 Because of the potential for abuse, this file must have strict permissions:
1487 read/write for the user, and not writable by others.
1489 .It Pa ~/.ssh/environment
1490 Contains additional definitions for environment variables; see
1494 .It Pa ~/.ssh/identity
1495 .It Pa ~/.ssh/id_dsa
1496 .It Pa ~/.ssh/id_ecdsa
1497 .It Pa ~/.ssh/id_ed25519
1498 .It Pa ~/.ssh/id_rsa
1499 Contains the private key for authentication.
1501 contain sensitive data and should be readable by the user but not
1502 accessible by others (read/write/execute).
1504 will simply ignore a private key file if it is accessible by others.
1505 It is possible to specify a passphrase when
1506 generating the key which will be used to encrypt the
1507 sensitive part of this file using 3DES.
1509 .It Pa ~/.ssh/identity.pub
1510 .It Pa ~/.ssh/id_dsa.pub
1511 .It Pa ~/.ssh/id_ecdsa.pub
1512 .It Pa ~/.ssh/id_ed25519.pub
1513 .It Pa ~/.ssh/id_rsa.pub
1514 Contains the public key for authentication.
1516 sensitive and can (but need not) be readable by anyone.
1518 .It Pa ~/.ssh/known_hosts
1519 Contains a list of host keys for all hosts the user has logged into
1520 that are not already in the systemwide list of known host keys.
1523 for further details of the format of this file.
1526 Commands in this file are executed by
1528 when the user logs in, just before the user's shell (or command) is
1532 manual page for more information.
1534 .It Pa /etc/hosts.equiv
1535 This file is for host-based authentication (see above).
1536 It should only be writable by root.
1538 .It Pa /etc/ssh/shosts.equiv
1539 This file is used in exactly the same way as
1541 but allows host-based authentication without permitting login with
1544 .It Pa /etc/ssh/ssh_config
1545 Systemwide configuration file.
1546 The file format and configuration options are described in
1549 .It Pa /etc/ssh/ssh_host_key
1550 .It Pa /etc/ssh/ssh_host_dsa_key
1551 .It Pa /etc/ssh/ssh_host_ecdsa_key
1552 .It Pa /etc/ssh/ssh_host_ed25519_key
1553 .It Pa /etc/ssh/ssh_host_rsa_key
1554 These files contain the private parts of the host keys
1555 and are used for host-based authentication.
1557 .It Pa /etc/ssh/ssh_known_hosts
1558 Systemwide list of known host keys.
1559 This file should be prepared by the
1560 system administrator to contain the public host keys of all machines in the
1562 It should be world-readable.
1565 for further details of the format of this file.
1567 .It Pa /etc/ssh/sshrc
1568 Commands in this file are executed by
1570 when the user logs in, just before the user's shell (or command) is started.
1573 manual page for more information.
1577 exits with the exit status of the remote command or with 255
1578 if an error occurred.
1596 .%T The Secure Shell (SSH) Protocol Assigned Numbers
1604 .%T The Secure Shell (SSH) Protocol Architecture
1612 .%T The Secure Shell (SSH) Authentication Protocol
1620 .%T The Secure Shell (SSH) Transport Layer Protocol
1628 .%T The Secure Shell (SSH) Connection Protocol
1636 .%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
1644 .%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
1652 .%T The Secure Shell (SSH) Session Channel Break Extension
1661 .%T The Secure Shell (SSH) Transport Layer Encryption Modes
1668 .%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
1677 .%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
1685 .%T The Secure Shell (SSH) Public Key File Format
1693 .%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
1700 .%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
1701 .%T Hash Visualization: a New Technique to improve Real-World Security
1704 OpenSSH is a derivative of the original and free
1705 ssh 1.2.12 release by Tatu Ylonen.
1706 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1707 Theo de Raadt and Dug Song
1708 removed many bugs, re-added newer features and
1710 Markus Friedl contributed the support for SSH
1711 protocol versions 1.5 and 2.0.
projects / dragonfly.git / blob