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 $
38 .Dd $Mdocdate: July 16 2016 $
43 .Nd OpenSSH SSH client (remote login program)
47 .Op Fl 1246AaCfGgKkMNnqsTtVvXxYy
48 .Op Fl b Ar bind_address
49 .Op Fl c Ar cipher_spec
50 .Op Fl D Oo Ar bind_address : Oc Ns Ar port
52 .Op Fl e Ar escape_char
53 .Op Fl F Ar configfile
55 .Op Fl i Ar identity_file
56 .Op Fl J Oo Ar user Ns @ Oc Ns Ar host Ns Op : Ns Ar port
58 .Op Fl l Ar login_name
63 .Op Fl Q Ar query_option
66 .Op Fl W Ar host : Ns Ar port
67 .Op Fl w Ar local_tun Ns Op : Ns Ar remote_tun
68 .Oo Ar user Ns @ Oc Ns Ar hostname
73 (SSH client) is a program for logging into a remote machine and for
74 executing commands on a remote machine.
75 It is intended to provide secure encrypted communications between
76 two untrusted hosts over an insecure network.
77 X11 connections, arbitrary TCP ports and
79 sockets can also be forwarded over the secure channel.
82 connects and logs into the specified
88 his/her identity to the remote machine using one of several methods
94 it is executed on the remote host instead of a login shell.
96 The options are as follows:
98 .Bl -tag -width Ds -compact
102 to try protocol version 1 only.
107 to try protocol version 2 only.
112 to use IPv4 addresses only.
117 to use IPv6 addresses only.
120 Enables forwarding of the authentication agent connection.
121 This can also be specified on a per-host basis in a configuration file.
123 Agent forwarding should be enabled with caution.
124 Users with the ability to bypass file permissions on the remote host
127 socket) can access the local agent through the forwarded connection.
128 An attacker cannot obtain key material from the agent,
129 however they can perform operations on the keys that enable them to
130 authenticate using the identities loaded into the agent.
133 Disables forwarding of the authentication agent connection.
135 .It Fl b Ar bind_address
138 on the local machine as the source address
140 Only useful on systems with more than one address.
143 Requests compression of all data (including stdin, stdout, stderr, and
144 data for forwarded X11, TCP and
147 The compression algorithm is the same used by
151 can be controlled by the
153 option for protocol version 1.
154 Compression is desirable on modem lines and other
155 slow connections, but will only slow down things on fast networks.
156 The default value can be set on a host-by-host basis in the
157 configuration files; see the
161 .It Fl c Ar cipher_spec
162 Selects the cipher specification for encrypting the session.
164 Protocol version 1 allows specification of a single cipher.
165 The supported values are
170 For protocol version 2,
172 is a comma-separated list of ciphers
173 listed in order of preference.
178 for more information.
182 .Oo Ar bind_address : Oc
188 application-level port forwarding.
189 This works by allocating a socket to listen to
191 on the local side, optionally bound to the specified
193 Whenever a connection is made to this port, the
194 connection is forwarded over the secure channel, and the application
195 protocol is then used to determine where to connect to from the
197 Currently the SOCKS4 and SOCKS5 protocols are supported, and
199 will act as a SOCKS server.
200 Only root can forward privileged ports.
201 Dynamic port forwardings can also be specified in the configuration file.
203 IPv6 addresses can be specified by enclosing the address in square brackets.
204 Only the superuser can forward privileged ports.
205 By default, the local port is bound in accordance with the
210 may be used to bind the connection to a specific address.
215 indicates that the listening port be bound for local use only, while an
218 indicates that the port should be available from all interfaces.
223 instead of standard error.
225 .It Fl e Ar escape_char
226 Sets the escape character for sessions with a pty (default:
228 The escape character is only recognized at the beginning of a line.
229 The escape character followed by a dot
231 closes the connection;
232 followed by control-Z suspends the connection;
233 and followed by itself sends the escape character once.
234 Setting the character to
236 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
250 to go to background just before command execution.
253 is going to ask for passwords or passphrases, but the user
254 wants it in the background.
257 The recommended way to start X11 programs at a remote site is with
259 .Ic ssh -f host xterm .
262 .Cm ExitOnForwardFailure
263 configuration option is set to
265 then a client started with
267 will wait for all remote port forwards to be successfully established
268 before placing itself in the background.
273 to print its configuration after evaluating
280 Allows remote hosts to connect to local forwarded ports.
281 If used on a multiplexed connection, then this option must be specified
282 on the master process.
285 Specify the PKCS#11 shared library
287 should use to communicate with a PKCS#11 token providing the user's
290 .It Fl i Ar identity_file
291 Selects a file from which the identity (private key) for
292 public key authentication is read.
295 for protocol version 1, and
297 .Pa ~/.ssh/id_ecdsa ,
298 .Pa ~/.ssh/id_ed25519
301 for protocol version 2.
302 Identity files may also be specified on
303 a per-host basis in the configuration file.
304 It is possible to have multiple
306 options (and multiple identities specified in
307 configuration files).
308 If no certificates have been explicitly specified by the
312 will also try to load certificate information from the filename obtained
315 to identity filenames.
324 Connect to the target host by first making a
326 connection to the jump
328 and then establishing a TCP forwarding to the ultimate destination from
330 Multiple jump hops may be specified separated by comma characters.
331 This is a shortcut to specify a
333 configuration directive.
336 Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
337 credentials to the server.
340 Disables forwarding (delegation) of GSSAPI credentials to the server.
344 .Oo Ar bind_address : Oc
345 .Ar port : host : hostport
350 .Oo Ar bind_address : Oc
351 .Ar port : remote_socket
356 .Ar local_socket : host : hostport
361 .Ar local_socket : remote_socket
364 Specifies that connections to the given TCP port or Unix socket on the local
365 (client) host are to be forwarded to the given host and port, or Unix socket,
367 This works by allocating a socket to listen to either a TCP
369 on the local side, optionally bound to the specified
372 Whenever a connection is made to the local port or socket, the
373 connection is forwarded over the secure channel, and a connection is
380 from the remote machine.
382 Port forwardings can also be specified in the configuration file.
383 Only the superuser can forward privileged ports.
384 IPv6 addresses can be specified by enclosing the address in square brackets.
386 By default, the local port is bound in accordance with the
391 may be used to bind the connection to a specific address.
396 indicates that the listening port be bound for local use only, while an
399 indicates that the port should be available from all interfaces.
401 .It Fl l Ar login_name
402 Specifies the user to log in as on the remote machine.
403 This also may be specified on a per-host basis in the configuration file.
410 mode for connection sharing.
417 mode with confirmation required before slave connections are accepted.
418 Refer to the description of
425 A comma-separated list of MAC (message authentication code) algorithms,
426 specified in order of preference.
429 keyword for more information.
432 Do not execute a remote command.
433 This is useful for just forwarding ports.
438 (actually, prevents reading from stdin).
439 This must be used when
441 is run in the background.
442 A common trick is to use this to run X11 programs on a remote machine.
444 .Ic ssh -n shadows.cs.hut.fi emacs &
445 will start an emacs on shadows.cs.hut.fi, and the X11
446 connection will be automatically forwarded over an encrypted channel.
449 program will be put in the background.
450 (This does not work if
452 needs to ask for a password or passphrase; see also the
457 Control an active connection multiplexing master process.
460 option is specified, the
462 argument is interpreted and passed to the master process.
465 (check that the master process is running),
467 (request forwardings without command execution),
469 (cancel forwardings),
471 (request the master to exit), and
473 (request the master to stop accepting further multiplexing requests).
476 Can be used to give options in the format used in the configuration file.
477 This is useful for specifying options for which there is no separate
479 For full details of the options listed below, and their possible values, see
482 .Bl -tag -width Ds -offset indent -compact
488 .It CanonicalizeFallbackLocal
489 .It CanonicalizeHostname
490 .It CanonicalizeMaxDots
491 .It CanonicalizePermittedCNAMEs
493 .It ChallengeResponseAuthentication
497 .It ClearAllForwardings
500 .It ConnectionAttempts
507 .It ExitOnForwardFailure
511 .It ForwardX11Timeout
512 .It ForwardX11Trusted
514 .It GlobalKnownHostsFile
515 .It GSSAPIAuthentication
516 .It GSSAPIDelegateCredentials
519 .It HostbasedAuthentication
520 .It HostbasedKeyTypes
521 .It HostKeyAlgorithms
529 .It KbdInteractiveAuthentication
530 .It KbdInteractiveDevices
537 .It NoHostAuthenticationForLocalhost
538 .It NumberOfPasswordPrompts
539 .It PasswordAuthentication
540 .It PermitLocalCommand
543 .It PreferredAuthentications
548 .It PubkeyAcceptedKeyTypes
549 .It PubkeyAuthentication
553 .It RhostsRSAAuthentication
554 .It RSAAuthentication
556 .It ServerAliveInterval
557 .It ServerAliveCountMax
558 .It StreamLocalBindMask
559 .It StreamLocalBindUnlink
560 .It StrictHostKeyChecking
565 .It UsePrivilegedPort
567 .It UserKnownHostsFile
575 Port to connect to on the remote host.
576 This can be specified on a
577 per-host basis in the configuration file.
579 .It Fl Q Ar query_option
582 for the algorithms supported for the specified version 2.
583 The available features are:
585 (supported symmetric ciphers),
587 (supported symmetric ciphers that support authenticated encryption),
589 (supported message integrity codes),
591 (key exchange algorithms),
595 (certificate key types),
597 (non-certificate key types), and
599 (supported SSH protocol versions).
603 Causes most warning and diagnostic messages to be suppressed.
607 .Oo Ar bind_address : Oc
608 .Ar port : host : hostport
613 .Oo Ar bind_address : Oc
614 .Ar port : local_socket
619 .Ar remote_socket : host : hostport
624 .Ar remote_socket : local_socket
627 Specifies that connections to the given TCP port or Unix socket on the remote
628 (server) host are to be forwarded to the given host and port, or Unix socket,
630 This works by allocating a socket to listen to either a TCP
632 or to a Unix socket on the remote side.
633 Whenever a connection is made to this port or Unix socket, the
634 connection is forwarded over the secure channel, and a connection
641 from the local machine.
643 Port forwardings can also be specified in the configuration file.
644 Privileged ports can be forwarded only when
645 logging in as root on the remote machine.
646 IPv6 addresses can be specified by enclosing the address in square brackets.
648 By default, TCP listening sockets on the server will be bound to the loopback
650 This may be overridden by specifying a
656 indicates that the remote socket should listen on all interfaces.
659 will only succeed if the server's
661 option is enabled (see
662 .Xr sshd_config 5 ) .
668 the listen port will be dynamically allocated on the server and reported
669 to the client at run time.
670 When used together with
672 the allocated port will be printed to the standard output.
675 Specifies the location of a control socket for connection sharing,
678 to disable connection sharing.
679 Refer to the description of
688 May be used to request invocation of a subsystem on the remote system.
689 Subsystems facilitate the use of SSH
690 as a secure transport for other applications (e.g.\&
692 The subsystem is specified as the remote command.
695 Disable pseudo-terminal allocation.
698 Force pseudo-terminal allocation.
699 This can be used to execute arbitrary
700 screen-based programs on a remote machine, which can be very useful,
701 e.g. when implementing menu services.
704 options force tty allocation, even if
709 Display the version number and exit.
715 to print debugging messages about its progress.
717 debugging connection, authentication, and configuration problems.
720 options increase the verbosity.
723 .It Fl W Ar host : Ns Ar port
724 Requests that standard input and output on the client be forwarded to
728 over the secure channel.
732 .Cm ExitOnForwardFailure
734 .Cm ClearAllForwardings ,
735 though these can be overridden in the configuration file or using
737 command line options.
740 .Ar local_tun Ns Op : Ns Ar remote_tun
744 device forwarding with the specified
746 devices between the client
751 The devices may be specified by numerical ID or the keyword
753 which uses the next available tunnel device.
756 is not specified, it defaults to
766 directive is unset, it is set to the default tunnel mode, which is
770 Enables X11 forwarding.
771 This can also be specified on a per-host basis in a configuration file.
773 X11 forwarding should be enabled with caution.
774 Users with the ability to bypass file permissions on the remote host
775 (for the user's X authorization database)
776 can access the local X11 display through the forwarded connection.
777 An attacker may then be able to perform activities such as keystroke monitoring.
779 For this reason, X11 forwarding is subjected to X11 SECURITY extension
780 restrictions by default.
785 .Cm ForwardX11Trusted
788 for more information.
791 Disables X11 forwarding.
794 Enables trusted X11 forwarding.
795 Trusted X11 forwardings are not subjected to the X11 SECURITY extension
799 Send log information using the
802 By default this information is sent to stderr.
806 may additionally obtain configuration data from
807 a per-user configuration file and a system-wide configuration file.
808 The file format and configuration options are described in
811 The OpenSSH SSH client supports SSH protocols 1 and 2.
812 The default is to use protocol 2 only,
813 though this can be changed via the
822 Protocol 1 should not be used
823 and is only offered to support legacy devices.
824 It suffers from a number of cryptographic weaknesses
825 and doesn't support many of the advanced features available for protocol 2.
827 The methods available for authentication are:
828 GSSAPI-based authentication,
829 host-based authentication,
830 public key authentication,
831 challenge-response authentication,
832 and password authentication.
833 Authentication methods are tried in the order specified above,
835 .Cm PreferredAuthentications
836 can be used to change the default order.
838 Host-based authentication works as follows:
839 If the machine the user logs in from is listed in
842 .Pa /etc/shosts.equiv
843 on the remote machine, and the user names are
844 the same on both sides, or if the files
848 exist in the user's home directory on the
849 remote machine and contain a line containing the name of the client
850 machine and the name of the user on that machine, the user is
851 considered for login.
852 Additionally, the server
854 be able to verify the client's
855 host key (see the description of
856 .Pa /etc/ssh/ssh_known_hosts
858 .Pa ~/.ssh/known_hosts ,
860 for login to be permitted.
861 This authentication method closes security holes due to IP
862 spoofing, DNS spoofing, and routing spoofing.
863 [Note to the administrator:
864 .Pa /etc/hosts.equiv ,
866 and the rlogin/rsh protocol in general, are inherently insecure and should be
867 disabled if security is desired.]
869 Public key authentication works as follows:
870 The scheme is based on public-key cryptography,
872 where encryption and decryption are done using separate keys,
873 and it is unfeasible to derive the decryption key from the encryption key.
874 The idea is that each user creates a public/private
875 key pair for authentication purposes.
876 The server knows the public key, and only the user knows the private key.
878 implements public key authentication protocol automatically,
879 using one of the DSA, ECDSA, Ed25519 or RSA algorithms.
880 The HISTORY section of
882 contains a brief discussion of the DSA and RSA algorithms.
885 .Pa ~/.ssh/authorized_keys
886 lists the public keys that are permitted for logging in.
887 When the user logs in, the
889 program tells the server which key pair it would like to use for
891 The client proves that it has access to the private key
892 and the server checks that the corresponding public key
893 is authorized to accept the account.
895 The user creates his/her key pair by running
897 This stores the private key in
904 .Pa ~/.ssh/id_ed25519
909 and stores the public key in
910 .Pa ~/.ssh/identity.pub
912 .Pa ~/.ssh/id_dsa.pub
914 .Pa ~/.ssh/id_ecdsa.pub
916 .Pa ~/.ssh/id_ed25519.pub
919 .Pa ~/.ssh/id_rsa.pub
921 in the user's home directory.
922 The user should then copy the public key
924 .Pa ~/.ssh/authorized_keys
925 in his/her home directory on the remote machine.
928 file corresponds to the conventional
930 file, and has one key
931 per line, though the lines can be very long.
932 After this, the user can log in without giving the password.
934 A variation on public key authentication
935 is available in the form of certificate authentication:
936 instead of a set of public/private keys,
937 signed certificates are used.
938 This has the advantage that a single trusted certification authority
939 can be used in place of many public/private keys.
940 See the CERTIFICATES section of
942 for more information.
944 The most convenient way to use public key or certificate authentication
945 may be with an authentication agent.
952 for more information.
954 Challenge-response authentication works as follows:
955 The server sends an arbitrary
957 text, and prompts for a response.
958 Examples of challenge-response authentication include
966 Finally, if other authentication methods fail,
968 prompts the user for a password.
969 The password is sent to the remote
970 host for checking; however, since all communications are encrypted,
971 the password cannot be seen by someone listening on the network.
974 automatically maintains and checks a database containing
975 identification for all hosts it has ever been used with.
976 Host keys are stored in
977 .Pa ~/.ssh/known_hosts
978 in the user's home directory.
979 Additionally, the file
980 .Pa /etc/ssh/ssh_known_hosts
981 is automatically checked for known hosts.
982 Any new hosts are automatically added to the user's file.
983 If a host's identification ever changes,
985 warns about this and disables password authentication to prevent
986 server spoofing or man-in-the-middle attacks,
987 which could otherwise be used to circumvent the encryption.
989 .Cm StrictHostKeyChecking
990 option can be used to control logins to machines whose
991 host key is not known or has changed.
993 When the user's identity has been accepted by the server, the server
994 either executes the given command in a non-interactive session or,
995 if no command has been specified, logs into the machine and gives
996 the user a normal shell as an interactive session.
997 All communication with
998 the remote command or shell will be automatically encrypted.
1000 If an interactive session is requested
1002 by default will only request a pseudo-terminal (pty) for interactive
1003 sessions when the client has one.
1008 can be used to override this behaviour.
1010 If a pseudo-terminal has been allocated the
1011 user may use the escape characters noted below.
1013 If no pseudo-terminal has been allocated,
1014 the session is transparent and can be used to reliably transfer binary data.
1015 On most systems, setting the escape character to
1017 will also make the session transparent even if a tty is used.
1019 The session terminates when the command or shell on the remote
1020 machine exits and all X11 and TCP connections have been closed.
1021 .Sh ESCAPE CHARACTERS
1022 When a pseudo-terminal has been requested,
1024 supports a number of functions through the use of an escape character.
1026 A single tilde character can be sent as
1028 or by following the tilde by a character other than those described below.
1029 The escape character must always follow a newline to be interpreted as
1031 The escape character can be changed in configuration files using the
1033 configuration directive or on the command line by the
1037 The supported escapes (assuming the default
1047 List forwarded connections.
1051 at logout when waiting for forwarded connection / X11 sessions to terminate.
1053 Display a list of escape characters.
1055 Send a BREAK to the remote system
1056 (only useful if the peer supports it).
1059 Currently this allows the addition of port forwardings using the
1064 options (see above).
1065 It also allows the cancellation of existing port-forwardings
1068 .Fl KL Oo Ar bind_address : Oc Ar port
1072 .Fl KR Oo Ar bind_address : Oc Ar port
1076 .Fl KD Oo Ar bind_address : Oc Ar port
1078 for dynamic port-forwardings.
1079 .Ic !\& Ns Ar command
1080 allows the user to execute a local command if the
1081 .Ic PermitLocalCommand
1082 option is enabled in
1084 Basic help is available, using the
1088 Request rekeying of the connection
1089 (only useful if the peer supports it).
1091 Decrease the verbosity
1093 when errors are being written to stderr.
1095 Increase the verbosity
1097 when errors are being written to stderr.
1100 Forwarding of arbitrary TCP connections over the secure channel can
1101 be specified either on the command line or in a configuration file.
1102 One possible application of TCP forwarding is a secure connection to a
1103 mail server; another is going through firewalls.
1105 In the example below, we look at encrypting communication between
1106 an IRC client and server, even though the IRC server does not directly
1107 support encrypted communications.
1108 This works as follows:
1109 the user connects to the remote host using
1111 specifying a port to be used to forward connections
1112 to the remote server.
1113 After that it is possible to start the service which is to be encrypted
1114 on the client machine,
1115 connecting to the same local port,
1118 will encrypt and forward the connection.
1120 The following example tunnels an IRC session from client machine
1124 .Dq server.example.com :
1125 .Bd -literal -offset 4n
1126 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
1127 $ irc -c '#users' -p 1234 pinky 127.0.0.1
1130 This tunnels a connection to IRC server
1131 .Dq server.example.com ,
1137 It doesn't matter which port is used,
1138 as long as it's greater than 1023
1139 (remember, only root can open sockets on privileged ports)
1140 and doesn't conflict with any ports already in use.
1141 The connection is forwarded to port 6667 on the remote server,
1142 since that's the standard port for IRC services.
1148 and the remote command
1150 is specified to allow an amount of time
1151 (10 seconds, in the example)
1152 to start the service which is to be tunnelled.
1153 If no connections are made within the time specified,
1161 (or see the description of the
1167 and the user is using X11 (the
1169 environment variable is set), the connection to the X11 display is
1170 automatically forwarded to the remote side in such a way that any X11
1171 programs started from the shell (or command) will go through the
1172 encrypted channel, and the connection to the real X server will be made
1173 from the local machine.
1174 The user should not manually set
1176 Forwarding of X11 connections can be
1177 configured on the command line or in configuration files.
1183 will point to the server machine, but with a display number greater than zero.
1184 This is normal, and happens because
1188 X server on the server machine for forwarding the
1189 connections over the encrypted channel.
1192 will also automatically set up Xauthority data on the server machine.
1193 For this purpose, it will generate a random authorization cookie,
1194 store it in Xauthority on the server, and verify that any forwarded
1195 connections carry this cookie and replace it by the real cookie when
1196 the connection is opened.
1197 The real authentication cookie is never
1198 sent to the server machine (and no cookies are sent in the plain).
1204 (or see the description of the
1209 the user is using an authentication agent, the connection to the agent
1210 is automatically forwarded to the remote side.
1211 .Sh VERIFYING HOST KEYS
1212 When connecting to a server for the first time,
1213 a fingerprint of the server's public key is presented to the user
1215 .Cm StrictHostKeyChecking
1217 Fingerprints can be determined using
1220 .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1222 If the fingerprint is already known, it can be matched
1223 and the key can be accepted or rejected.
1224 If only legacy (MD5) fingerprints for the server are available, the
1227 option may be used to downgrade the fingerprint algorithm to match.
1229 Because of the difficulty of comparing host keys
1230 just by looking at fingerprint strings,
1231 there is also support to compare host keys visually,
1238 a small ASCII graphic gets displayed on every login to a server, no matter
1239 if the session itself is interactive or not.
1240 By learning the pattern a known server produces, a user can easily
1241 find out that the host key has changed when a completely different pattern
1243 Because these patterns are not unambiguous however, a pattern that looks
1244 similar to the pattern remembered only gives a good probability that the
1245 host key is the same, not guaranteed proof.
1247 To get a listing of the fingerprints along with their random art for
1248 all known hosts, the following command line can be used:
1250 .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1252 If the fingerprint is unknown,
1253 an alternative method of verification is available:
1254 SSH fingerprints verified by DNS.
1255 An additional resource record (RR),
1257 is added to a zonefile
1258 and the connecting client is able to match the fingerprint
1259 with that of the key presented.
1261 In this example, we are connecting a client to a server,
1262 .Dq host.example.com .
1263 The SSHFP resource records should first be added to the zonefile for
1265 .Bd -literal -offset indent
1266 $ ssh-keygen -r host.example.com.
1269 The output lines will have to be added to the zonefile.
1270 To check that the zone is answering fingerprint queries:
1272 .Dl $ dig -t SSHFP host.example.com
1274 Finally the client connects:
1275 .Bd -literal -offset indent
1276 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1278 Matching host key fingerprint found in DNS.
1279 Are you sure you want to continue connecting (yes/no)?
1283 .Cm VerifyHostKeyDNS
1286 for more information.
1287 .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1289 contains support for Virtual Private Network (VPN) tunnelling
1292 network pseudo-device,
1293 allowing two networks to be joined securely.
1296 configuration option
1298 controls whether the server supports this,
1299 and at what level (layer 2 or 3 traffic).
1301 The following example would connect client network 10.0.50.0/24
1302 with remote network 10.0.99.0/24 using a point-to-point connection
1303 from 10.1.1.1 to 10.1.1.2,
1304 provided that the SSH server running on the gateway to the remote network,
1305 at 192.168.1.15, allows it.
1308 .Bd -literal -offset indent
1309 # ssh -f -w 0:1 192.168.1.15 true
1310 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1311 # route add 10.0.99.0/24 10.1.1.2
1315 .Bd -literal -offset indent
1316 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1317 # route add 10.0.50.0/24 10.1.1.1
1320 Client access may be more finely tuned via the
1321 .Pa /root/.ssh/authorized_keys
1322 file (see below) and the
1325 The following entry would permit connections on
1329 and on tun device 2 from user
1334 .Dq forced-commands-only :
1335 .Bd -literal -offset 2n
1336 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1337 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1340 Since an SSH-based setup entails a fair amount of overhead,
1341 it may be more suited to temporary setups,
1342 such as for wireless VPNs.
1343 More permanent VPNs are better provided by tools such as
1349 will normally set the following environment variables:
1350 .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1354 variable indicates the location of the X11 server.
1355 It is automatically set by
1357 to point to a value of the form
1361 indicates the host where the shell runs, and
1363 is an integer \*(Ge 1.
1365 uses this special value to forward X11 connections over the secure
1367 The user should normally not set
1370 will render the X11 connection insecure (and will require the user to
1371 manually copy any required authorization cookies).
1373 Set to the path of the user's home directory.
1377 set for compatibility with systems that use this variable.
1379 Set to the path of the user's mailbox.
1383 as specified when compiling
1388 needs a passphrase, it will read the passphrase from the current
1389 terminal if it was run from a terminal.
1392 does not have a terminal associated with it but
1396 are set, it will execute the program specified by
1398 and open an X11 window to read the passphrase.
1399 This is particularly useful when calling
1404 (Note that on some machines it
1405 may be necessary to redirect the input from
1408 .It Ev SSH_AUTH_SOCK
1409 Identifies the path of a
1411 socket used to communicate with the agent.
1412 .It Ev SSH_CONNECTION
1413 Identifies the client and server ends of the connection.
1414 The variable contains
1415 four space-separated values: client IP address, client port number,
1416 server IP address, and server port number.
1417 .It Ev SSH_ORIGINAL_COMMAND
1418 This variable contains the original command line if a forced command
1420 It can be used to extract the original arguments.
1422 This is set to the name of the tty (path to the device) associated
1423 with the current shell or command.
1424 If the current session has no tty,
1425 this variable is not set.
1427 This variable is set to indicate the present time zone if it
1428 was set when the daemon was started (i.e. the daemon passes the value
1429 on to new connections).
1431 Set to the name of the user logging in.
1437 .Pa ~/.ssh/environment ,
1438 and adds lines of the format
1440 to the environment if the file exists and users are allowed to
1441 change their environment.
1442 For more information, see the
1443 .Cm PermitUserEnvironment
1447 .Bl -tag -width Ds -compact
1449 This file is used for host-based authentication (see above).
1450 On some machines this file may need to be
1451 world-readable if the user's home directory is on an NFS partition,
1455 Additionally, this file must be owned by the user,
1456 and must not have write permissions for anyone else.
1458 permission for most machines is read/write for the user, and not
1459 accessible by others.
1462 This file is used in exactly the same way as
1464 but allows host-based authentication without permitting login with
1468 This directory is the default location for all user-specific configuration
1469 and authentication information.
1470 There is no general requirement to keep the entire contents of this directory
1471 secret, but the recommended permissions are read/write/execute for the user,
1472 and not accessible by others.
1474 .It Pa ~/.ssh/authorized_keys
1475 Lists the public keys (DSA, ECDSA, Ed25519, RSA)
1476 that can be used for logging in as this user.
1477 The format of this file is described in the
1480 This file is not highly sensitive, but the recommended
1481 permissions are read/write for the user, and not accessible by others.
1483 .It Pa ~/.ssh/config
1484 This is the per-user configuration file.
1485 The file format and configuration options are described in
1487 Because of the potential for abuse, this file must have strict permissions:
1488 read/write for the user, and not writable by others.
1490 .It Pa ~/.ssh/environment
1491 Contains additional definitions for environment variables; see
1495 .It Pa ~/.ssh/identity
1496 .It Pa ~/.ssh/id_dsa
1497 .It Pa ~/.ssh/id_ecdsa
1498 .It Pa ~/.ssh/id_ed25519
1499 .It Pa ~/.ssh/id_rsa
1500 Contains the private key for authentication.
1502 contain sensitive data and should be readable by the user but not
1503 accessible by others (read/write/execute).
1505 will simply ignore a private key file if it is accessible by others.
1506 It is possible to specify a passphrase when
1507 generating the key which will be used to encrypt the
1508 sensitive part of this file using 3DES.
1510 .It Pa ~/.ssh/identity.pub
1511 .It Pa ~/.ssh/id_dsa.pub
1512 .It Pa ~/.ssh/id_ecdsa.pub
1513 .It Pa ~/.ssh/id_ed25519.pub
1514 .It Pa ~/.ssh/id_rsa.pub
1515 Contains the public key for authentication.
1517 sensitive and can (but need not) be readable by anyone.
1519 .It Pa ~/.ssh/known_hosts
1520 Contains a list of host keys for all hosts the user has logged into
1521 that are not already in the systemwide list of known host keys.
1524 for further details of the format of this file.
1527 Commands in this file are executed by
1529 when the user logs in, just before the user's shell (or command) is
1533 manual page for more information.
1535 .It Pa /etc/hosts.equiv
1536 This file is for host-based authentication (see above).
1537 It should only be writable by root.
1539 .It Pa /etc/shosts.equiv
1540 This file is used in exactly the same way as
1542 but allows host-based authentication without permitting login with
1545 .It Pa /etc/ssh/ssh_config
1546 Systemwide configuration file.
1547 The file format and configuration options are described in
1550 .It Pa /etc/ssh/ssh_host_key
1551 .It Pa /etc/ssh/ssh_host_dsa_key
1552 .It Pa /etc/ssh/ssh_host_ecdsa_key
1553 .It Pa /etc/ssh/ssh_host_ed25519_key
1554 .It Pa /etc/ssh/ssh_host_rsa_key
1555 These files contain the private parts of the host keys
1556 and are used for host-based authentication.
1558 .It Pa /etc/ssh/ssh_known_hosts
1559 Systemwide list of known host keys.
1560 This file should be prepared by the
1561 system administrator to contain the public host keys of all machines in the
1563 It should be world-readable.
1566 for further details of the format of this file.
1568 .It Pa /etc/ssh/sshrc
1569 Commands in this file are executed by
1571 when the user logs in, just before the user's shell (or command) is started.
1574 manual page for more information.
1578 exits with the exit status of the remote command or with 255
1579 if an error occurred.
1597 .%T The Secure Shell (SSH) Protocol Assigned Numbers
1605 .%T The Secure Shell (SSH) Protocol Architecture
1613 .%T The Secure Shell (SSH) Authentication Protocol
1621 .%T The Secure Shell (SSH) Transport Layer Protocol
1629 .%T The Secure Shell (SSH) Connection Protocol
1637 .%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
1645 .%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
1653 .%T The Secure Shell (SSH) Session Channel Break Extension
1662 .%T The Secure Shell (SSH) Transport Layer Encryption Modes
1669 .%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
1678 .%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
1686 .%T The Secure Shell (SSH) Public Key File Format
1694 .%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
1701 .%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
1702 .%T Hash Visualization: a New Technique to improve Real-World Security
1705 OpenSSH is a derivative of the original and free
1706 ssh 1.2.12 release by Tatu Ylonen.
1707 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1708 Theo de Raadt and Dug Song
1709 removed many bugs, re-added newer features and
1711 Markus Friedl contributed the support for SSH
1712 protocol versions 1.5 and 2.0.
projects / FreeBSD / stable / 10.git / blob