3 .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
4 .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
5 .\" All rights reserved
7 .\" As far as I am concerned, the code I have written for this software
8 .\" can be used freely for any purpose. Any derived versions of this
9 .\" software must be clearly marked as such, and if the derived work is
10 .\" incompatible with the protocol description in the RFC file, it must be
11 .\" called by a name other than "ssh" or "Secure Shell".
13 .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
14 .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
15 .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
17 .\" Redistribution and use in source and binary forms, with or without
18 .\" modification, are permitted provided that the following conditions
20 .\" 1. Redistributions of source code must retain the above copyright
21 .\" notice, this list of conditions and the following disclaimer.
22 .\" 2. Redistributions in binary form must reproduce the above copyright
23 .\" notice, this list of conditions and the following disclaimer in the
24 .\" documentation and/or other materials provided with the distribution.
26 .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
27 .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
28 .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
29 .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
30 .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
31 .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
32 .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
33 .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
34 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
35 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
37 .\" $OpenBSD: ssh.1,v 1.262 2006/07/02 23:01:55 stevesk Exp $
38 .Dd September 25, 1999
43 .Nd OpenSSH SSH client (remote login program)
46 .Op Fl 1246AaCfgkMNnqsTtVvXxY
47 .Op Fl b Ar bind_address
48 .Op Fl c Ar cipher_spec
51 .Oo Ar bind_address : Oc
55 .Op Fl e Ar escape_char
56 .Op Fl F Ar configfile
58 .Op Fl i Ar identity_file
62 .Oo Ar bind_address : Oc
63 .Ar port : host : hostport
67 .Op Fl l Ar login_name
75 .Oo Ar bind_address : Oc
76 .Ar port : host : hostport
81 .Oo Fl w Ar local_tun Ns
82 .Op : Ns Ar remote_tun Oc
83 .Oo Ar user Ns @ Oc Ns Ar hostname
88 (SSH client) is a program for logging into a remote machine and for
89 executing commands on a remote machine.
90 It is intended to replace rlogin and rsh,
91 and provide secure encrypted communications between
92 two untrusted hosts over an insecure network.
93 X11 connections and arbitrary TCP ports
94 can also be forwarded over the secure channel.
97 connects and logs into the specified
103 his/her identity to the remote machine using one of several methods
104 depending on the protocol version used (see below).
109 it is executed on the remote host instead of a login shell.
111 The options are as follows:
116 to try protocol version 1 only.
120 to try protocol version 2 only.
124 to use IPv4 addresses only.
128 to use IPv6 addresses only.
130 Enables forwarding of the authentication agent connection.
131 This can also be specified on a per-host basis in a configuration file.
133 Agent forwarding should be enabled with caution.
134 Users with the ability to bypass file permissions on the remote host
135 (for the agent's Unix-domain socket)
136 can access the local agent through the forwarded connection.
137 An attacker cannot obtain key material from the agent,
138 however they can perform operations on the keys that enable them to
139 authenticate using the identities loaded into the agent.
141 Disables forwarding of the authentication agent connection.
142 .It Fl b Ar bind_address
145 on the local machine as the source address
147 Only useful on systems with more than one address.
149 Requests compression of all data (including stdin, stdout, stderr, and
150 data for forwarded X11 and TCP connections).
151 The compression algorithm is the same used by
155 can be controlled by the
157 option for protocol version 1.
158 Compression is desirable on modem lines and other
159 slow connections, but will only slow down things on fast networks.
160 The default value can be set on a host-by-host basis in the
161 configuration files; see the
164 .It Fl c Ar cipher_spec
165 Selects the cipher specification for encrypting the session.
167 Protocol version 1 allows specification of a single cipher.
168 The supported values are
174 (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
175 It is believed to be secure.
177 is a fast block cipher; it appears very secure and is much faster than
180 is only supported in the
182 client for interoperability with legacy protocol 1 implementations
183 that do not support the
186 Its use is strongly discouraged due to cryptographic weaknesses.
190 For protocol version 2,
192 is a comma-separated list of ciphers
193 listed in order of preference.
194 The supported ciphers are:
209 .Bd -literal -offset indent
210 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
211 arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
212 aes192-ctr,aes256-ctr
216 .Oo Ar bind_address : Oc
222 application-level port forwarding.
223 This works by allocating a socket to listen to
225 on the local side, optionally bound to the specified
227 Whenever a connection is made to this port, the
228 connection is forwarded over the secure channel, and the application
229 protocol is then used to determine where to connect to from the
231 Currently the SOCKS4 and SOCKS5 protocols are supported, and
233 will act as a SOCKS server.
234 Only root can forward privileged ports.
235 Dynamic port forwardings can also be specified in the configuration file.
237 IPv6 addresses can be specified with an alternative syntax:
240 .Op Ar bind_address No /
244 or by enclosing the address in square brackets.
245 Only the superuser can forward privileged ports.
246 By default, the local port is bound in accordance with the
251 may be used to bind the connection to a specific address.
256 indicates that the listening port be bound for local use only, while an
259 indicates that the port should be available from all interfaces.
260 .It Fl e Ar escape_char
261 Sets the escape character for sessions with a pty (default:
263 The escape character is only recognized at the beginning of a line.
264 The escape character followed by a dot
266 closes the connection;
267 followed by control-Z suspends the connection;
268 and followed by itself sends the escape character once.
269 Setting the character to
271 disables any escapes and makes the session fully transparent.
272 .It Fl F Ar configfile
273 Specifies an alternative per-user configuration file.
274 If a configuration file is given on the command line,
275 the system-wide configuration file
276 .Pq Pa /etc/ssh/ssh_config
278 The default for the per-user configuration file is
283 to go to background just before command execution.
286 is going to ask for passwords or passphrases, but the user
287 wants it in the background.
290 The recommended way to start X11 programs at a remote site is with
292 .Ic ssh -f host xterm .
294 Allows remote hosts to connect to local forwarded ports.
295 .It Fl I Ar smartcard_device
298 should use to communicate with a smartcard used for storing the user's
300 This option is only available if support for smartcard devices
301 is compiled in (default is no support).
302 .It Fl i Ar identity_file
303 Selects a file from which the identity (private key) for
304 RSA or DSA authentication is read.
307 for protocol version 1, and
311 for protocol version 2.
312 Identity files may also be specified on
313 a per-host basis in the configuration file.
314 It is possible to have multiple
316 options (and multiple identities specified in
317 configuration files).
319 Disables forwarding (delegation) of GSSAPI credentials to the server.
322 .Oo Ar bind_address : Oc
323 .Ar port : host : hostport
326 Specifies that the given port on the local (client) host is to be
327 forwarded to the given host and port on the remote side.
328 This works by allocating a socket to listen to
330 on the local side, optionally bound to the specified
332 Whenever a connection is made to this port, the
333 connection is forwarded over the secure channel, and a connection is
338 from the remote machine.
339 Port forwardings can also be specified in the configuration file.
340 IPv6 addresses can be specified with an alternative syntax:
343 .Op Ar bind_address No /
344 .Ar port No / Ar host No /
348 or by enclosing the address in square brackets.
349 Only the superuser can forward privileged ports.
350 By default, the local port is bound in accordance with the
355 may be used to bind the connection to a specific address.
360 indicates that the listening port be bound for local use only, while an
363 indicates that the port should be available from all interfaces.
364 .It Fl l Ar login_name
365 Specifies the user to log in as on the remote machine.
366 This also may be specified on a per-host basis in the configuration file.
372 mode for connection sharing.
379 mode with confirmation required before slave connections are accepted.
380 Refer to the description of
386 Additionally, for protocol version 2 a comma-separated list of MAC
387 (message authentication code) algorithms can
388 be specified in order of preference.
391 keyword for more information.
393 Do not execute a remote command.
394 This is useful for just forwarding ports
395 (protocol version 2 only).
399 (actually, prevents reading from stdin).
400 This must be used when
402 is run in the background.
403 A common trick is to use this to run X11 programs on a remote machine.
405 .Ic ssh -n shadows.cs.hut.fi emacs &
406 will start an emacs on shadows.cs.hut.fi, and the X11
407 connection will be automatically forwarded over an encrypted channel.
410 program will be put in the background.
411 (This does not work if
413 needs to ask for a password or passphrase; see also the
417 Control an active connection multiplexing master process.
420 option is specified, the
422 argument is interpreted and passed to the master process.
425 (check that the master process is running) and
427 (request the master to exit).
429 Can be used to give options in the format used in the configuration file.
430 This is useful for specifying options for which there is no separate
432 For full details of the options listed below, and their possible values, see
435 .Bl -tag -width Ds -offset indent -compact
439 .It ChallengeResponseAuthentication
443 .It ClearAllForwardings
446 .It ConnectionAttempts
454 .It ForwardX11Trusted
456 .It GlobalKnownHostsFile
457 .It GSSAPIAuthentication
458 .It GSSAPIDelegateCredentials
461 .It HostbasedAuthentication
462 .It HostKeyAlgorithms
467 .It KbdInteractiveDevices
472 .It NoHostAuthenticationForLocalhost
473 .It NumberOfPasswordPrompts
474 .It PasswordAuthentication
475 .It PermitLocalCommand
477 .It PreferredAuthentications
480 .It PubkeyAuthentication
483 .It RhostsRSAAuthentication
484 .It RSAAuthentication
486 .It ServerAliveInterval
487 .It ServerAliveCountMax
489 .It StrictHostKeyChecking
493 .It UsePrivilegedPort
495 .It UserKnownHostsFile
500 Port to connect to on the remote host.
501 This can be specified on a
502 per-host basis in the configuration file.
505 Causes all warning and diagnostic messages to be suppressed.
508 .Oo Ar bind_address : Oc
509 .Ar port : host : hostport
512 Specifies that the given port on the remote (server) host is to be
513 forwarded to the given host and port on the local side.
514 This works by allocating a socket to listen to
516 on the remote side, and whenever a connection is made to this port, the
517 connection is forwarded over the secure channel, and a connection is
522 from the local machine.
524 Port forwardings can also be specified in the configuration file.
525 Privileged ports can be forwarded only when
526 logging in as root on the remote machine.
527 IPv6 addresses can be specified by enclosing the address in square braces or
528 using an alternative syntax:
531 .Op Ar bind_address No /
532 .Ar host No / Ar port No /
537 By default, the listening socket on the server will be bound to the loopback
539 This may be overriden by specifying a
545 indicates that the remote socket should listen on all interfaces.
548 will only succeed if the server's
550 option is enabled (see
551 .Xr sshd_config 5 ) .
553 Specifies the location of a control socket for connection sharing.
554 Refer to the description of
562 May be used to request invocation of a subsystem on the remote system.
563 Subsystems are a feature of the SSH2 protocol which facilitate the use
564 of SSH as a secure transport for other applications (eg.\&
566 The subsystem is specified as the remote command.
568 Disable pseudo-tty allocation.
570 Force pseudo-tty allocation.
571 This can be used to execute arbitrary
572 screen-based programs on a remote machine, which can be very useful,
573 e.g. when implementing menu services.
576 options force tty allocation, even if
580 Display the version number and exit.
585 to print debugging messages about its progress.
587 debugging connection, authentication, and configuration problems.
590 options increase the verbosity.
593 .Ar local_tun Ns Op : Ns Ar remote_tun
597 device forwarding with the specified
599 devices between the client
604 The devices may be specified by numerical ID or the keyword
606 which uses the next available tunnel device.
609 is not specified, it defaults to
619 directive is unset, it is set to the default tunnel mode, which is
622 Enables X11 forwarding.
623 This can also be specified on a per-host basis in a configuration file.
625 X11 forwarding should be enabled with caution.
626 Users with the ability to bypass file permissions on the remote host
627 (for the user's X authorization database)
628 can access the local X11 display through the forwarded connection.
629 An attacker may then be able to perform activities such as keystroke monitoring.
631 For this reason, X11 forwarding is subjected to X11 SECURITY extension
632 restrictions by default.
637 .Cm ForwardX11Trusted
640 for more information.
642 Disables X11 forwarding.
644 Enables trusted X11 forwarding.
645 Trusted X11 forwardings are not subjected to the X11 SECURITY extension
650 may additionally obtain configuration data from
651 a per-user configuration file and a system-wide configuration file.
652 The file format and configuration options are described in
656 exits with the exit status of the remote command or with 255
657 if an error occurred.
659 The OpenSSH SSH client supports SSH protocols 1 and 2.
660 Protocol 2 is the default, with
662 falling back to protocol 1 if it detects protocol 2 is unsupported.
663 These settings may be altered using the
667 or enforced using the
672 Both protocols support similar authentication methods,
673 but protocol 2 is preferred since
674 it provides additional mechanisms for confidentiality
675 (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
676 and integrity (hmac-md5, hmac-sha1, hmac-ripemd160).
677 Protocol 1 lacks a strong mechanism for ensuring the
678 integrity of the connection.
680 The methods available for authentication are:
681 GSSAPI-based authentication,
682 host-based authentication,
683 public key authentication,
684 challenge-response authentication,
685 and password authentication.
686 Authentication methods are tried in the order specified above,
687 though protocol 2 has a configuration option to change the default order:
688 .Cm PreferredAuthentications .
690 Host-based authentication works as follows:
691 If the machine the user logs in from is listed in
694 .Pa /etc/shosts.equiv
695 on the remote machine, and the user names are
696 the same on both sides, or if the files
700 exist in the user's home directory on the
701 remote machine and contain a line containing the name of the client
702 machine and the name of the user on that machine, the user is
703 considered for login.
704 Additionally, the server
706 be able to verify the client's
707 host key (see the description of
708 .Pa /etc/ssh/ssh_known_hosts
710 .Pa ~/.ssh/known_hosts ,
712 for login to be permitted.
713 This authentication method closes security holes due to IP
714 spoofing, DNS spoofing, and routing spoofing.
715 [Note to the administrator:
716 .Pa /etc/hosts.equiv ,
718 and the rlogin/rsh protocol in general, are inherently insecure and should be
719 disabled if security is desired.]
721 Public key authentication works as follows:
722 The scheme is based on public-key cryptography,
724 where encryption and decryption are done using separate keys,
725 and it is unfeasible to derive the decryption key from the encryption key.
726 The idea is that each user creates a public/private
727 key pair for authentication purposes.
728 The server knows the public key, and only the user knows the private key.
730 implements public key authentication protocol automatically,
731 using either the RSA or DSA algorithms.
732 Protocol 1 is restricted to using only RSA keys,
733 but protocol 2 may use either.
738 contains a brief discussion of the two algorithms.
741 .Pa ~/.ssh/authorized_keys
742 lists the public keys that are permitted for logging in.
743 When the user logs in, the
745 program tells the server which key pair it would like to use for
747 The client proves that it has access to the private key
748 and the server checks that the corresponding public key
749 is authorized to accept the account.
751 The user creates his/her key pair by running
753 This stores the private key in
761 and stores the public key in
762 .Pa ~/.ssh/identity.pub
764 .Pa ~/.ssh/id_dsa.pub
767 .Pa ~/.ssh/id_rsa.pub
769 in the user's home directory.
770 The user should then copy the public key
772 .Pa ~/.ssh/authorized_keys
773 in his/her home directory on the remote machine.
776 file corresponds to the conventional
778 file, and has one key
779 per line, though the lines can be very long.
780 After this, the user can log in without giving the password.
782 The most convenient way to use public key authentication may be with an
783 authentication agent.
786 for more information.
788 Challenge-response authentication works as follows:
789 The server sends an arbitrary
791 text, and prompts for a response.
792 Protocol 2 allows multiple challenges and responses;
793 protocol 1 is restricted to just one challenge/response.
794 Examples of challenge-response authentication include
795 BSD Authentication (see
797 and PAM (some non-OpenBSD systems).
799 Finally, if other authentication methods fail,
801 prompts the user for a password.
802 The password is sent to the remote
803 host for checking; however, since all communications are encrypted,
804 the password cannot be seen by someone listening on the network.
807 automatically maintains and checks a database containing
808 identification for all hosts it has ever been used with.
809 Host keys are stored in
810 .Pa ~/.ssh/known_hosts
811 in the user's home directory.
812 Additionally, the file
813 .Pa /etc/ssh/ssh_known_hosts
814 is automatically checked for known hosts.
815 Any new hosts are automatically added to the user's file.
816 If a host's identification ever changes,
818 warns about this and disables password authentication to prevent
819 server spoofing or man-in-the-middle attacks,
820 which could otherwise be used to circumvent the encryption.
822 .Cm StrictHostKeyChecking
823 option can be used to control logins to machines whose
824 host key is not known or has changed.
826 When the user's identity has been accepted by the server, the server
827 either executes the given command, or logs into the machine and gives
828 the user a normal shell on the remote machine.
829 All communication with
830 the remote command or shell will be automatically encrypted.
832 If a pseudo-terminal has been allocated (normal login session), the
833 user may use the escape characters noted below.
835 If no pseudo-tty has been allocated,
836 the session is transparent and can be used to reliably transfer binary data.
837 On most systems, setting the escape character to
839 will also make the session transparent even if a tty is used.
841 The session terminates when the command or shell on the remote
842 machine exits and all X11 and TCP connections have been closed.
843 .Sh ESCAPE CHARACTERS
844 When a pseudo-terminal has been requested,
846 supports a number of functions through the use of an escape character.
848 A single tilde character can be sent as
850 or by following the tilde by a character other than those described below.
851 The escape character must always follow a newline to be interpreted as
853 The escape character can be changed in configuration files using the
855 configuration directive or on the command line by the
859 The supported escapes (assuming the default
869 List forwarded connections.
873 at logout when waiting for forwarded connection / X11 sessions to terminate.
875 Display a list of escape characters.
877 Send a BREAK to the remote system
878 (only useful for SSH protocol version 2 and if the peer supports it).
881 Currently this allows the addition of port forwardings using the
886 It also allows the cancellation of existing remote port-forwardings
889 .Fl KR Oo Ar bind_address : Oc Ar port .
891 .Ic !\& Ns Ar command
892 allows the user to execute a local command if the
893 .Ic PermitLocalCommand
896 Basic help is available, using the
900 Request rekeying of the connection
901 (only useful for SSH protocol version 2 and if the peer supports it).
904 Forwarding of arbitrary TCP connections over the secure channel can
905 be specified either on the command line or in a configuration file.
906 One possible application of TCP forwarding is a secure connection to a
907 mail server; another is going through firewalls.
909 In the example below, we look at encrypting communication between
910 an IRC client and server, even though the IRC server does not directly
911 support encrypted communications.
912 This works as follows:
913 the user connects to the remote host using
915 specifying a port to be used to forward connections
916 to the remote server.
917 After that it is possible to start the service which is to be encrypted
918 on the client machine,
919 connecting to the same local port,
922 will encrypt and forward the connection.
924 The following example tunnels an IRC session from client machine
928 .Dq server.example.com :
929 .Bd -literal -offset 4n
930 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
931 $ irc -c '#users' -p 1234 pinky 127.0.0.1
934 This tunnels a connection to IRC server
935 .Dq server.example.com ,
941 It doesn't matter which port is used,
942 as long as it's greater than 1023
943 (remember, only root can open sockets on privileged ports)
944 and doesn't conflict with any ports already in use.
945 The connection is forwarded to port 6667 on the remote server,
946 since that's the standard port for IRC services.
952 and the remote command
954 is specified to allow an amount of time
955 (10 seconds, in the example)
956 to start the service which is to be tunnelled.
957 If no connections are made within the time specified,
965 (or see the description of the
971 and the user is using X11 (the
973 environment variable is set), the connection to the X11 display is
974 automatically forwarded to the remote side in such a way that any X11
975 programs started from the shell (or command) will go through the
976 encrypted channel, and the connection to the real X server will be made
977 from the local machine.
978 The user should not manually set
980 Forwarding of X11 connections can be
981 configured on the command line or in configuration files.
987 will point to the server machine, but with a display number greater than zero.
988 This is normal, and happens because
992 X server on the server machine for forwarding the
993 connections over the encrypted channel.
996 will also automatically set up Xauthority data on the server machine.
997 For this purpose, it will generate a random authorization cookie,
998 store it in Xauthority on the server, and verify that any forwarded
999 connections carry this cookie and replace it by the real cookie when
1000 the connection is opened.
1001 The real authentication cookie is never
1002 sent to the server machine (and no cookies are sent in the plain).
1008 (or see the description of the
1013 the user is using an authentication agent, the connection to the agent
1014 is automatically forwarded to the remote side.
1015 .Sh VERIFYING HOST KEYS
1016 When connecting to a server for the first time,
1017 a fingerprint of the server's public key is presented to the user
1019 .Cm StrictHostKeyChecking
1021 Fingerprints can be determined using
1024 .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1026 If the fingerprint is already known,
1027 it can be matched and verified,
1028 and the key can be accepted.
1029 If the fingerprint is unknown,
1030 an alternative method of verification is available:
1031 SSH fingerprints verified by DNS.
1032 An additional resource record (RR),
1034 is added to a zonefile
1035 and the connecting client is able to match the fingerprint
1036 with that of the key presented.
1038 In this example, we are connecting a client to a server,
1039 .Dq host.example.com .
1040 The SSHFP resource records should first be added to the zonefile for
1042 .Bd -literal -offset indent
1043 $ ssh-keygen -r host.example.com.
1046 The output lines will have to be added to the zonefile.
1047 To check that the zone is answering fingerprint queries:
1049 .Dl $ dig -t SSHFP host.example.com
1051 Finally the client connects:
1052 .Bd -literal -offset indent
1053 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1055 Matching host key fingerprint found in DNS.
1056 Are you sure you want to continue connecting (yes/no)?
1060 .Cm VerifyHostKeyDNS
1063 for more information.
1064 .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1066 contains support for Virtual Private Network (VPN) tunnelling
1069 network pseudo-device,
1070 allowing two networks to be joined securely.
1073 configuration option
1075 controls whether the server supports this,
1076 and at what level (layer 2 or 3 traffic).
1078 The following example would connect client network 10.0.50.0/24
1079 with remote network 10.0.99.0/24, provided that the SSH server
1080 running on the gateway to the remote network,
1081 at 192.168.1.15, allows it:
1082 .Bd -literal -offset indent
1083 # ssh -f -w 0:1 192.168.1.15 true
1084 # ifconfig tun0 10.0.50.1 10.0.99.1 netmask 255.255.255.252
1087 Client access may be more finely tuned via the
1088 .Pa /root/.ssh/authorized_keys
1089 file (see below) and the
1092 The following entry would permit connections on
1096 and on tun device 2 from user
1101 .Dq forced-commands-only :
1102 .Bd -literal -offset 2n
1103 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1104 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1107 Since a SSH-based setup entails a fair amount of overhead,
1108 it may be more suited to temporary setups,
1109 such as for wireless VPNs.
1110 More permanent VPNs are better provided by tools such as
1116 will normally set the following environment variables:
1117 .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1121 variable indicates the location of the X11 server.
1122 It is automatically set by
1124 to point to a value of the form
1128 indicates the host where the shell runs, and
1130 is an integer \*(Ge 1.
1132 uses this special value to forward X11 connections over the secure
1134 The user should normally not set
1137 will render the X11 connection insecure (and will require the user to
1138 manually copy any required authorization cookies).
1140 Set to the path of the user's home directory.
1144 set for compatibility with systems that use this variable.
1146 Set to the path of the user's mailbox.
1150 as specified when compiling
1155 needs a passphrase, it will read the passphrase from the current
1156 terminal if it was run from a terminal.
1159 does not have a terminal associated with it but
1163 are set, it will execute the program specified by
1165 and open an X11 window to read the passphrase.
1166 This is particularly useful when calling
1171 (Note that on some machines it
1172 may be necessary to redirect the input from
1175 .It Ev SSH_AUTH_SOCK
1176 Identifies the path of a
1178 socket used to communicate with the agent.
1179 .It Ev SSH_CONNECTION
1180 Identifies the client and server ends of the connection.
1181 The variable contains
1182 four space-separated values: client IP address, client port number,
1183 server IP address, and server port number.
1184 .It Ev SSH_ORIGINAL_COMMAND
1185 This variable contains the original command line if a forced command
1187 It can be used to extract the original arguments.
1189 This is set to the name of the tty (path to the device) associated
1190 with the current shell or command.
1191 If the current session has no tty,
1192 this variable is not set.
1194 This variable is set to indicate the present time zone if it
1195 was set when the daemon was started (i.e. the daemon passes the value
1196 on to new connections).
1198 Set to the name of the user logging in.
1204 .Pa ~/.ssh/environment ,
1205 and adds lines of the format
1207 to the environment if the file exists and users are allowed to
1208 change their environment.
1209 For more information, see the
1210 .Cm PermitUserEnvironment
1214 .Bl -tag -width Ds -compact
1216 This file is used for host-based authentication (see above).
1217 On some machines this file may need to be
1218 world-readable if the user's home directory is on an NFS partition,
1222 Additionally, this file must be owned by the user,
1223 and must not have write permissions for anyone else.
1225 permission for most machines is read/write for the user, and not
1226 accessible by others.
1229 This file is used in exactly the same way as
1231 but allows host-based authentication without permitting login with
1234 .It ~/.ssh/authorized_keys
1235 Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1236 The format of this file is described in the
1239 This file is not highly sensitive, but the recommended
1240 permissions are read/write for the user, and not accessible by others.
1243 This is the per-user configuration file.
1244 The file format and configuration options are described in
1246 Because of the potential for abuse, this file must have strict permissions:
1247 read/write for the user, and not accessible by others.
1249 .It ~/.ssh/environment
1250 Contains additional definitions for environment variables; see
1257 Contains the private key for authentication.
1259 contain sensitive data and should be readable by the user but not
1260 accessible by others (read/write/execute).
1262 will simply ignore a private key file if it is accessible by others.
1263 It is possible to specify a passphrase when
1264 generating the key which will be used to encrypt the
1265 sensitive part of this file using 3DES.
1267 .It ~/.ssh/identity.pub
1268 .It ~/.ssh/id_dsa.pub
1269 .It ~/.ssh/id_rsa.pub
1270 Contains the public key for authentication.
1272 sensitive and can (but need not) be readable by anyone.
1274 .It ~/.ssh/known_hosts
1275 Contains a list of host keys for all hosts the user has logged into
1276 that are not already in the systemwide list of known host keys.
1279 for further details of the format of this file.
1282 Commands in this file are executed by
1284 when the user logs in, just before the user's shell (or command) is
1288 manual page for more information.
1290 .It /etc/hosts.equiv
1291 This file is for host-based authentication (see above).
1292 It should only be writable by root.
1294 .It /etc/shosts.equiv
1295 This file is used in exactly the same way as
1297 but allows host-based authentication without permitting login with
1300 .It Pa /etc/ssh/ssh_config
1301 Systemwide configuration file.
1302 The file format and configuration options are described in
1305 .It /etc/ssh/ssh_host_key
1306 .It /etc/ssh/ssh_host_dsa_key
1307 .It /etc/ssh/ssh_host_rsa_key
1308 These three files contain the private parts of the host keys
1309 and are used for host-based authentication.
1310 If protocol version 1 is used,
1312 must be setuid root, since the host key is readable only by root.
1313 For protocol version 2,
1317 to access the host keys,
1318 eliminating the requirement that
1320 be setuid root when host-based authentication is used.
1325 .It /etc/ssh/ssh_known_hosts
1326 Systemwide list of known host keys.
1327 This file should be prepared by the
1328 system administrator to contain the public host keys of all machines in the
1330 It should be world-readable.
1333 for further details of the format of this file.
1336 Commands in this file are executed by
1338 when the user logs in, just before the user's shell (or command) is started.
1341 manual page for more information.
1357 .%T "The Secure Shell (SSH) Protocol Assigned Numbers"
1362 .%T "The Secure Shell (SSH) Protocol Architecture"
1367 .%T "The Secure Shell (SSH) Authentication Protocol"
1372 .%T "The Secure Shell (SSH) Transport Layer Protocol"
1377 .%T "The Secure Shell (SSH) Connection Protocol"
1382 .%T "Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints"
1387 .%T "Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)"
1392 .%T "The Secure Shell (SSH) Session Channel Break Extension"
1397 .%T "The Secure Shell (SSH) Transport Layer Encryption Modes"
1402 .%T "Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol"
1407 .%T "Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol"
1411 OpenSSH is a derivative of the original and free
1412 ssh 1.2.12 release by Tatu Ylonen.
1413 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1414 Theo de Raadt and Dug Song
1415 removed many bugs, re-added newer features and
1417 Markus Friedl contributed the support for SSH
1418 protocol versions 1.5 and 2.0.