1 SSH(1) OpenBSD Reference Manual SSH(1)
4 ssh - OpenSSH SSH client (remote login program)
7 ssh [-1246AaCfgKkMNnqsTtVvXxYy] [-b bind_address] [-c cipher_spec]
8 [-D [bind_address:]port] [-e escape_char] [-F configfile]
9 [-i identity_file] [-L [bind_address:]port:host:hostport]
10 [-l login_name] [-m mac_spec] [-O ctl_cmd] [-o option] [-p port]
11 [-R [bind_address:]port:host:hostport] [-S ctl_path]
12 [-w local_tun[:remote_tun]] [user@]hostname [command]
15 ssh (SSH client) is a program for logging into a remote machine and for
16 executing commands on a remote machine. It is intended to replace rlogin
17 and rsh, and provide secure encrypted communications between two untrust-
18 ed hosts over an insecure network. X11 connections and arbitrary TCP
19 ports can also be forwarded over the secure channel.
21 ssh connects and logs into the specified hostname (with optional user
22 name). The user must prove his/her identity to the remote machine using
23 one of several methods depending on the protocol version used (see be-
26 If command is specified, it is executed on the remote host instead of a
29 The options are as follows:
31 -1 Forces ssh to try protocol version 1 only.
33 -2 Forces ssh to try protocol version 2 only.
35 -4 Forces ssh to use IPv4 addresses only.
37 -6 Forces ssh to use IPv6 addresses only.
39 -A Enables forwarding of the authentication agent connection. This
40 can also be specified on a per-host basis in a configuration
43 Agent forwarding should be enabled with caution. Users with the
44 ability to bypass file permissions on the remote host (for the
45 agent's Unix-domain socket) can access the local agent through
46 the forwarded connection. An attacker cannot obtain key material
47 from the agent, however they can perform operations on the keys
48 that enable them to authenticate using the identities loaded into
51 -a Disables forwarding of the authentication agent connection.
54 Use bind_address on the local machine as the source address of
55 the connection. Only useful on systems with more than one ad-
58 -C Requests compression of all data (including stdin, stdout,
59 stderr, and data for forwarded X11 and TCP connections). The
60 compression algorithm is the same used by gzip(1), and the
61 ``level'' can be controlled by the CompressionLevel option for
62 protocol version 1. Compression is desirable on modem lines and
63 other slow connections, but will only slow down things on fast
64 networks. The default value can be set on a host-by-host basis
65 in the configuration files; see the Compression option.
68 Selects the cipher specification for encrypting the session.
70 Protocol version 1 allows specification of a single cipher. The
71 supported values are ``3des'', ``blowfish'', and ``des''. 3des
72 (triple-des) is an encrypt-decrypt-encrypt triple with three dif-
73 ferent keys. It is believed to be secure. blowfish is a fast
74 block cipher; it appears very secure and is much faster than
75 3des. des is only supported in the ssh client for interoperabil-
76 ity with legacy protocol 1 implementations that do not support
77 the 3des cipher. Its use is strongly discouraged due to crypto-
78 graphic weaknesses. The default is ``3des''.
80 For protocol version 2, cipher_spec is a comma-separated list of
81 ciphers listed in order of preference. See the Ciphers keyword
84 -D [bind_address:]port
85 Specifies a local ``dynamic'' application-level port forwarding.
86 This works by allocating a socket to listen to port on the local
87 side, optionally bound to the specified bind_address. Whenever a
88 connection is made to this port, the connection is forwarded over
89 the secure channel, and the application protocol is then used to
90 determine where to connect to from the remote machine. Currently
91 the SOCKS4 and SOCKS5 protocols are supported, and ssh will act
92 as a SOCKS server. Only root can forward privileged ports. Dy-
93 namic port forwardings can also be specified in the configuration
96 IPv6 addresses can be specified with an alternative syntax:
97 [bind_address/]port or by enclosing the address in square brack-
98 ets. Only the superuser can forward privileged ports. By de-
99 fault, the local port is bound in accordance with the
100 GatewayPorts setting. However, an explicit bind_address may be
101 used to bind the connection to a specific address. The
102 bind_address of ``localhost'' indicates that the listening port
103 be bound for local use only, while an empty address or `*' indi-
104 cates that the port should be available from all interfaces.
107 Sets the escape character for sessions with a pty (default: `~').
108 The escape character is only recognized at the beginning of a
109 line. The escape character followed by a dot (`.') closes the
110 connection; followed by control-Z suspends the connection; and
111 followed by itself sends the escape character once. Setting the
112 character to ``none'' disables any escapes and makes the session
116 Specifies an alternative per-user configuration file. If a con-
117 figuration file is given on the command line, the system-wide
118 configuration file (/etc/ssh/ssh_config) will be ignored. The
119 default for the per-user configuration file is ~/.ssh/config.
121 -f Requests ssh to go to background just before command execution.
122 This is useful if ssh is going to ask for passwords or passphras-
123 es, but the user wants it in the background. This implies -n.
124 The recommended way to start X11 programs at a remote site is
125 with something like ssh -f host xterm.
127 If the ExitOnForwardFailure configuration option is set to
128 ``yes'', then a client started with -f will wait for all remote
129 port forwards to be successfully established before placing it-
130 self in the background.
132 -g Allows remote hosts to connect to local forwarded ports.
135 Specify the device ssh should use to communicate with a smartcard
136 used for storing the user's private RSA key. This option is only
137 available if support for smartcard devices is compiled in (de-
138 fault is no support).
141 Selects a file from which the identity (private key) for RSA or
142 DSA authentication is read. The default is ~/.ssh/identity for
143 protocol version 1, and ~/.ssh/id_rsa and ~/.ssh/id_dsa for pro-
144 tocol version 2. Identity files may also be specified on a per-
145 host basis in the configuration file. It is possible to have
146 multiple -i options (and multiple identities specified in config-
149 -K Enables GSSAPI-based authentication and forwarding (delegation)
150 of GSSAPI credentials to the server.
152 -k Disables forwarding (delegation) of GSSAPI credentials to the
155 -L [bind_address:]port:host:hostport
156 Specifies that the given port on the local (client) host is to be
157 forwarded to the given host and port on the remote side. This
158 works by allocating a socket to listen to port on the local side,
159 optionally bound to the specified bind_address. Whenever a con-
160 nection is made to this port, the connection is forwarded over
161 the secure channel, and a connection is made to host port
162 hostport from the remote machine. Port forwardings can also be
163 specified in the configuration file. IPv6 addresses can be spec-
164 ified with an alternative syntax: [bind_address/]port/host/host-
165 port or by enclosing the address in square brackets. Only the
166 superuser can forward privileged ports. By default, the local
167 port is bound in accordance with the GatewayPorts setting. How-
168 ever, an explicit bind_address may be used to bind the connection
169 to a specific address. The bind_address of ``localhost'' indi-
170 cates that the listening port be bound for local use only, while
171 an empty address or `*' indicates that the port should be avail-
172 able from all interfaces.
175 Specifies the user to log in as on the remote machine. This also
176 may be specified on a per-host basis in the configuration file.
178 -M Places the ssh client into ``master'' mode for connection shar-
179 ing. Multiple -M options places ssh into ``master'' mode with
180 confirmation required before slave connections are accepted. Re-
181 fer to the description of ControlMaster in ssh_config(5) for de-
185 Additionally, for protocol version 2 a comma-separated list of
186 MAC (message authentication code) algorithms can be specified in
187 order of preference. See the MACs keyword for more information.
189 -N Do not execute a remote command. This is useful for just for-
190 warding ports (protocol version 2 only).
192 -n Redirects stdin from /dev/null (actually, prevents reading from
193 stdin). This must be used when ssh is run in the background. A
194 common trick is to use this to run X11 programs on a remote ma-
195 chine. For example, ssh -n shadows.cs.hut.fi emacs & will start
196 an emacs on shadows.cs.hut.fi, and the X11 connection will be au-
197 tomatically forwarded over an encrypted channel. The ssh program
198 will be put in the background. (This does not work if ssh needs
199 to ask for a password or passphrase; see also the -f option.)
202 Control an active connection multiplexing master process. When
203 the -O option is specified, the ctl_cmd argument is interpreted
204 and passed to the master process. Valid commands are: ``check''
205 (check that the master process is running) and ``exit'' (request
209 Can be used to give options in the format used in the configura-
210 tion file. This is useful for specifying options for which there
211 is no separate command-line flag. For full details of the op-
212 tions listed below, and their possible values, see ssh_config(5).
217 ChallengeResponseAuthentication
237 GSSAPIDelegateCredentials
240 HostbasedAuthentication
246 KbdInteractiveDevices
251 NoHostAuthenticationForLocalhost
252 NumberOfPasswordPrompts
253 PasswordAuthentication
256 PreferredAuthentications
262 RhostsRSAAuthentication
268 StrictHostKeyChecking
280 Port to connect to on the remote host. This can be specified on
281 a per-host basis in the configuration file.
283 -q Quiet mode. Causes most warning and diagnostic messages to be
286 -R [bind_address:]port:host:hostport
287 Specifies that the given port on the remote (server) host is to
288 be forwarded to the given host and port on the local side. This
289 works by allocating a socket to listen to port on the remote
290 side, and whenever a connection is made to this port, the connec-
291 tion is forwarded over the secure channel, and a connection is
292 made to host port hostport from the local machine.
294 Port forwardings can also be specified in the configuration file.
295 Privileged ports can be forwarded only when logging in as root on
296 the remote machine. IPv6 addresses can be specified by enclosing
297 the address in square braces or using an alternative syntax:
298 [bind_address/]host/port/hostport.
300 By default, the listening socket on the server will be bound to
301 the loopback interface only. This may be overridden by specify-
302 ing a bind_address. An empty bind_address, or the address `*',
303 indicates that the remote socket should listen on all interfaces.
304 Specifying a remote bind_address will only succeed if the serv-
305 er's GatewayPorts option is enabled (see sshd_config(5)).
307 If the port argument is `0', the listen port will be dynamically
308 allocated on the server and reported to the client at run time.
311 Specifies the location of a control socket for connection shar-
312 ing. Refer to the description of ControlPath and ControlMaster
313 in ssh_config(5) for details.
315 -s May be used to request invocation of a subsystem on the remote
316 system. Subsystems are a feature of the SSH2 protocol which fa-
317 cilitate the use of SSH as a secure transport for other applica-
318 tions (eg. sftp(1)). The subsystem is specified as the remote
321 -T Disable pseudo-tty allocation.
323 -t Force pseudo-tty allocation. This can be used to execute arbi-
324 trary screen-based programs on a remote machine, which can be
325 very useful, e.g. when implementing menu services. Multiple -t
326 options force tty allocation, even if ssh has no local tty.
328 -V Display the version number and exit.
330 -v Verbose mode. Causes ssh to print debugging messages about its
331 progress. This is helpful in debugging connection, authentica-
332 tion, and configuration problems. Multiple -v options increase
333 the verbosity. The maximum is 3.
335 -w local_tun[:remote_tun]
336 Requests tunnel device forwarding with the specified tun(4) de-
337 vices between the client (local_tun) and the server (remote_tun).
339 The devices may be specified by numerical ID or the keyword
340 ``any'', which uses the next available tunnel device. If
341 remote_tun is not specified, it defaults to ``any''. See also
342 the Tunnel and TunnelDevice directives in ssh_config(5). If the
343 Tunnel directive is unset, it is set to the default tunnel mode,
344 which is ``point-to-point''.
346 -X Enables X11 forwarding. This can also be specified on a per-host
347 basis in a configuration file.
349 X11 forwarding should be enabled with caution. Users with the
350 ability to bypass file permissions on the remote host (for the
351 user's X authorization database) can access the local X11 display
352 through the forwarded connection. An attacker may then be able
353 to perform activities such as keystroke monitoring.
355 For this reason, X11 forwarding is subjected to X11 SECURITY ex-
356 tension restrictions by default. Please refer to the ssh -Y op-
357 tion and the ForwardX11Trusted directive in ssh_config(5) for
360 -x Disables X11 forwarding.
362 -Y Enables trusted X11 forwarding. Trusted X11 forwardings are not
363 subjected to the X11 SECURITY extension controls.
365 -y Send log information using the syslog(3) system module. By de-
366 fault this information is sent to stderr.
368 ssh may additionally obtain configuration data from a per-user configura-
369 tion file and a system-wide configuration file. The file format and con-
370 figuration options are described in ssh_config(5).
372 ssh exits with the exit status of the remote command or with 255 if an
376 The OpenSSH SSH client supports SSH protocols 1 and 2. Protocol 2 is the
377 default, with ssh falling back to protocol 1 if it detects protocol 2 is
378 unsupported. These settings may be altered using the Protocol option in
379 ssh_config(5), or enforced using the -1 and -2 options (see above). Both
380 protocols support similar authentication methods, but protocol 2 is pre-
381 ferred since it provides additional mechanisms for confidentiality (the
382 traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour) and
383 integrity (hmac-md5, hmac-sha1, umac-64, hmac-ripemd160). Protocol 1
384 lacks a strong mechanism for ensuring the integrity of the connection.
386 The methods available for authentication are: GSSAPI-based authentica-
387 tion, host-based authentication, public key authentication, challenge-re-
388 sponse authentication, and password authentication. Authentication meth-
389 ods are tried in the order specified above, though protocol 2 has a con-
390 figuration option to change the default order: PreferredAuthentications.
392 Host-based authentication works as follows: If the machine the user logs
393 in from is listed in /etc/hosts.equiv or /etc/shosts.equiv on the remote
394 machine, and the user names are the same on both sides, or if the files
395 ~/.rhosts or ~/.shosts exist in the user's home directory on the remote
396 machine and contain a line containing the name of the client machine and
397 the name of the user on that machine, the user is considered for login.
398 Additionally, the server must be able to verify the client's host key
399 (see the description of /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts,
400 below) for login to be permitted. This authentication method closes se-
401 curity holes due to IP spoofing, DNS spoofing, and routing spoofing.
402 [Note to the administrator: /etc/hosts.equiv, ~/.rhosts, and the
403 rlogin/rsh protocol in general, are inherently insecure and should be
404 disabled if security is desired.]
406 Public key authentication works as follows: The scheme is based on pub-
407 lic-key cryptography, using cryptosystems where encryption and decryption
408 are done using separate keys, and it is unfeasible to derive the decryp-
409 tion key from the encryption key. The idea is that each user creates a
410 public/private key pair for authentication purposes. The server knows
411 the public key, and only the user knows the private key. ssh implements
412 public key authentication protocol automatically, using either the RSA or
413 DSA algorithms. Protocol 1 is restricted to using only RSA keys, but
414 protocol 2 may use either. The HISTORY section of ssl(8) contains a
415 brief discussion of the two algorithms.
417 The file ~/.ssh/authorized_keys lists the public keys that are permitted
418 for logging in. When the user logs in, the ssh program tells the server
419 which key pair it would like to use for authentication. The client
420 proves that it has access to the private key and the server checks that
421 the corresponding public key is authorized to accept the account.
423 The user creates his/her key pair by running ssh-keygen(1). This stores
424 the private key in ~/.ssh/identity (protocol 1), ~/.ssh/id_dsa (protocol
425 2 DSA), or ~/.ssh/id_rsa (protocol 2 RSA) and stores the public key in
426 ~/.ssh/identity.pub (protocol 1), ~/.ssh/id_dsa.pub (protocol 2 DSA), or
427 ~/.ssh/id_rsa.pub (protocol 2 RSA) in the user's home directory. The us-
428 er should then copy the public key to ~/.ssh/authorized_keys in his/her
429 home directory on the remote machine. The authorized_keys file corre-
430 sponds to the conventional ~/.rhosts file, and has one key per line,
431 though the lines can be very long. After this, the user can log in with-
432 out giving the password.
434 The most convenient way to use public key authentication may be with an
435 authentication agent. See ssh-agent(1) for more information.
437 Challenge-response authentication works as follows: The server sends an
438 arbitrary "challenge" text, and prompts for a response. Protocol 2 al-
439 lows multiple challenges and responses; protocol 1 is restricted to just
440 one challenge/response. Examples of challenge-response authentication
441 include BSD Authentication (see login.conf(5)) and PAM (some non-OpenBSD
444 Finally, if other authentication methods fail, ssh prompts the user for a
445 password. The password is sent to the remote host for checking; however,
446 since all communications are encrypted, the password cannot be seen by
447 someone listening on the network.
449 ssh automatically maintains and checks a database containing identifica-
450 tion for all hosts it has ever been used with. Host keys are stored in
451 ~/.ssh/known_hosts in the user's home directory. Additionally, the file
452 /etc/ssh/ssh_known_hosts is automatically checked for known hosts. Any
453 new hosts are automatically added to the user's file. If a host's iden-
454 tification ever changes, ssh warns about this and disables password au-
455 thentication to prevent server spoofing or man-in-the-middle attacks,
456 which could otherwise be used to circumvent the encryption. The
457 StrictHostKeyChecking option can be used to control logins to machines
458 whose host key is not known or has changed.
460 When the user's identity has been accepted by the server, the server ei-
461 ther executes the given command, or logs into the machine and gives the
462 user a normal shell on the remote machine. All communication with the
463 remote command or shell will be automatically encrypted.
465 If a pseudo-terminal has been allocated (normal login session), the user
466 may use the escape characters noted below.
468 If no pseudo-tty has been allocated, the session is transparent and can
469 be used to reliably transfer binary data. On most systems, setting the
470 escape character to ``none'' will also make the session transparent even
473 The session terminates when the command or shell on the remote machine
474 exits and all X11 and TCP connections have been closed.
477 When a pseudo-terminal has been requested, ssh supports a number of func-
478 tions through the use of an escape character.
480 A single tilde character can be sent as ~~ or by following the tilde by a
481 character other than those described below. The escape character must
482 always follow a newline to be interpreted as special. The escape charac-
483 ter can be changed in configuration files using the EscapeChar configura-
484 tion directive or on the command line by the -e option.
486 The supported escapes (assuming the default `~') are:
492 ~# List forwarded connections.
494 ~& Background ssh at logout when waiting for forwarded connection /
495 X11 sessions to terminate.
497 ~? Display a list of escape characters.
499 ~B Send a BREAK to the remote system (only useful for SSH protocol
500 version 2 and if the peer supports it).
502 ~C Open command line. Currently this allows the addition of port
503 forwardings using the -L, -R and -D options (see above). It also
504 allows the cancellation of existing remote port-forwardings using
505 -KR[bind_address:]port. !command allows the user to execute a
506 local command if the PermitLocalCommand option is enabled in
507 ssh_config(5). Basic help is available, using the -h option.
509 ~R Request rekeying of the connection (only useful for SSH protocol
510 version 2 and if the peer supports it).
513 Forwarding of arbitrary TCP connections over the secure channel can be
514 specified either on the command line or in a configuration file. One
515 possible application of TCP forwarding is a secure connection to a mail
516 server; another is going through firewalls.
518 In the example below, we look at encrypting communication between an IRC
519 client and server, even though the IRC server does not directly support
520 encrypted communications. This works as follows: the user connects to
521 the remote host using ssh, specifying a port to be used to forward con-
522 nections to the remote server. After that it is possible to start the
523 service which is to be encrypted on the client machine, connecting to the
524 same local port, and ssh will encrypt and forward the connection.
526 The following example tunnels an IRC session from client machine
527 ``127.0.0.1'' (localhost) to remote server ``server.example.com'':
529 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
530 $ irc -c '#users' -p 1234 pinky 127.0.0.1
532 This tunnels a connection to IRC server ``server.example.com'', joining
533 channel ``#users'', nickname ``pinky'', using port 1234. It doesn't mat-
534 ter which port is used, as long as it's greater than 1023 (remember, only
535 root can open sockets on privileged ports) and doesn't conflict with any
536 ports already in use. The connection is forwarded to port 6667 on the
537 remote server, since that's the standard port for IRC services.
539 The -f option backgrounds ssh and the remote command ``sleep 10'' is
540 specified to allow an amount of time (10 seconds, in the example) to
541 start the service which is to be tunnelled. If no connections are made
542 within the time specified, ssh will exit.
545 If the ForwardX11 variable is set to ``yes'' (or see the description of
546 the -X, -x, and -Y options above) and the user is using X11 (the DISPLAY
547 environment variable is set), the connection to the X11 display is auto-
548 matically forwarded to the remote side in such a way that any X11 pro-
549 grams started from the shell (or command) will go through the encrypted
550 channel, and the connection to the real X server will be made from the
551 local machine. The user should not manually set DISPLAY. Forwarding of
552 X11 connections can be configured on the command line or in configuration
555 The DISPLAY value set by ssh will point to the server machine, but with a
556 display number greater than zero. This is normal, and happens because
557 ssh creates a ``proxy'' X server on the server machine for forwarding the
558 connections over the encrypted channel.
560 ssh will also automatically set up Xauthority data on the server machine.
561 For this purpose, it will generate a random authorization cookie, store
562 it in Xauthority on the server, and verify that any forwarded connections
563 carry this cookie and replace it by the real cookie when the connection
564 is opened. The real authentication cookie is never sent to the server
565 machine (and no cookies are sent in the plain).
567 If the ForwardAgent variable is set to ``yes'' (or see the description of
568 the -A and -a options above) and the user is using an authentication
569 agent, the connection to the agent is automatically forwarded to the re-
573 When connecting to a server for the first time, a fingerprint of the
574 server's public key is presented to the user (unless the option
575 StrictHostKeyChecking has been disabled). Fingerprints can be determined
578 $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
580 If the fingerprint is already known, it can be matched and the key can be
581 accepted or rejected. Because of the difficulty of comparing host keys
582 just by looking at hex strings, there is also support to compare host
583 keys visually, using random art. By setting the VisualHostKey option to
584 ``yes'', a small ASCII graphic gets displayed on every login to a server,
585 no matter if the session itself is interactive or not. By learning the
586 pattern a known server produces, a user can easily find out that the host
587 key has changed when a completely different pattern is displayed. Be-
588 cause these patterns are not unambiguous however, a pattern that looks
589 similar to the pattern remembered only gives a good probability that the
590 host key is the same, not guaranteed proof.
592 To get a listing of the fingerprints along with their random art for all
593 known hosts, the following command line can be used:
595 $ ssh-keygen -lv -f ~/.ssh/known_hosts
597 If the fingerprint is unknown, an alternative method of verification is
598 available: SSH fingerprints verified by DNS. An additional resource
599 record (RR), SSHFP, is added to a zonefile and the connecting client is
600 able to match the fingerprint with that of the key presented.
602 In this example, we are connecting a client to a server,
603 ``host.example.com''. The SSHFP resource records should first be added
604 to the zonefile for host.example.com:
606 $ ssh-keygen -r host.example.com.
608 The output lines will have to be added to the zonefile. To check that
609 the zone is answering fingerprint queries:
611 $ dig -t SSHFP host.example.com
613 Finally the client connects:
615 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
617 Matching host key fingerprint found in DNS.
618 Are you sure you want to continue connecting (yes/no)?
620 See the VerifyHostKeyDNS option in ssh_config(5) for more information.
622 SSH-BASED VIRTUAL PRIVATE NETWORKS
623 ssh contains support for Virtual Private Network (VPN) tunnelling using
624 the tun(4) network pseudo-device, allowing two networks to be joined se-
625 curely. The sshd_config(5) configuration option PermitTunnel controls
626 whether the server supports this, and at what level (layer 2 or 3 traf-
629 The following example would connect client network 10.0.50.0/24 with re-
630 mote network 10.0.99.0/24 using a point-to-point connection from 10.1.1.1
631 to 10.1.1.2, provided that the SSH server running on the gateway to the
632 remote network, at 192.168.1.15, allows it.
636 # ssh -f -w 0:1 192.168.1.15 true
637 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
638 # route add 10.0.99.0/24 10.1.1.2
642 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
643 # route add 10.0.50.0/24 10.1.1.1
645 Client access may be more finely tuned via the /root/.ssh/authorized_keys
646 file (see below) and the PermitRootLogin server option. The following
647 entry would permit connections on tun(4) device 1 from user ``jane'' and
648 on tun device 2 from user ``john'', if PermitRootLogin is set to
649 ``forced-commands-only'':
651 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
652 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
654 Since an SSH-based setup entails a fair amount of overhead, it may be
655 more suited to temporary setups, such as for wireless VPNs. More perma-
656 nent VPNs are better provided by tools such as ipsecctl(8) and
660 ssh will normally set the following environment variables:
662 DISPLAY The DISPLAY variable indicates the location of the
663 X11 server. It is automatically set by ssh to
664 point to a value of the form ``hostname:n'', where
665 ``hostname'' indicates the host where the shell
666 runs, and `n' is an integer >= 1. ssh uses this
667 special value to forward X11 connections over the
668 secure channel. The user should normally not set
669 DISPLAY explicitly, as that will render the X11
670 connection insecure (and will require the user to
671 manually copy any required authorization cookies).
673 HOME Set to the path of the user's home directory.
675 LOGNAME Synonym for USER; set for compatibility with sys-
676 tems that use this variable.
678 MAIL Set to the path of the user's mailbox.
680 PATH Set to the default PATH, as specified when compil-
683 SSH_ASKPASS If ssh needs a passphrase, it will read the
684 passphrase from the current terminal if it was run
685 from a terminal. If ssh does not have a terminal
686 associated with it but DISPLAY and SSH_ASKPASS are
687 set, it will execute the program specified by
688 SSH_ASKPASS and open an X11 window to read the
689 passphrase. This is particularly useful when call-
690 ing ssh from a .xsession or related script. (Note
691 that on some machines it may be necessary to redi-
692 rect the input from /dev/null to make this work.)
694 SSH_AUTH_SOCK Identifies the path of a UNIX-domain socket used to
695 communicate with the agent.
697 SSH_CONNECTION Identifies the client and server ends of the con-
698 nection. The variable contains four space-separat-
699 ed values: client IP address, client port number,
700 server IP address, and server port number.
702 SSH_ORIGINAL_COMMAND This variable contains the original command line if
703 a forced command is executed. It can be used to
704 extract the original arguments.
706 SSH_TTY This is set to the name of the tty (path to the de-
707 vice) associated with the current shell or command.
708 If the current session has no tty, this variable is
711 TZ This variable is set to indicate the present time
712 zone if it was set when the daemon was started
713 (i.e. the daemon passes the value on to new connec-
716 USER Set to the name of the user logging in.
718 Additionally, ssh reads ~/.ssh/environment, and adds lines of the format
719 ``VARNAME=value'' to the environment if the file exists and users are al-
720 lowed to change their environment. For more information, see the
721 PermitUserEnvironment option in sshd_config(5).
725 This file is used for host-based authentication (see above). On
726 some machines this file may need to be world-readable if the us-
727 er's home directory is on an NFS partition, because sshd(8) reads
728 it as root. Additionally, this file must be owned by the user,
729 and must not have write permissions for anyone else. The recom-
730 mended permission for most machines is read/write for the user,
731 and not accessible by others.
734 This file is used in exactly the same way as .rhosts, but allows
735 host-based authentication without permitting login with
739 This directory is the default location for all user-specific con-
740 figuration and authentication information. There is no general
741 requirement to keep the entire contents of this directory secret,
742 but the recommended permissions are read/write/execute for the
743 user, and not accessible by others.
745 ~/.ssh/authorized_keys
746 Lists the public keys (RSA/DSA) that can be used for logging in
747 as this user. The format of this file is described in the
748 sshd(8) manual page. This file is not highly sensitive, but the
749 recommended permissions are read/write for the user, and not ac-
753 This is the per-user configuration file. The file format and
754 configuration options are described in ssh_config(5). Because of
755 the potential for abuse, this file must have strict permissions:
756 read/write for the user, and not accessible by others.
759 Contains additional definitions for environment variables; see
765 Contains the private key for authentication. These files contain
766 sensitive data and should be readable by the user but not acces-
767 sible by others (read/write/execute). ssh will simply ignore a
768 private key file if it is accessible by others. It is possible
769 to specify a passphrase when generating the key which will be
770 used to encrypt the sensitive part of this file using 3DES.
775 Contains the public key for authentication. These files are not
776 sensitive and can (but need not) be readable by anyone.
779 Contains a list of host keys for all hosts the user has logged
780 into that are not already in the systemwide list of known host
781 keys. See sshd(8) for further details of the format of this
785 Commands in this file are executed by ssh when the user logs in,
786 just before the user's shell (or command) is started. See the
787 sshd(8) manual page for more information.
790 This file is for host-based authentication (see above). It
791 should only be writable by root.
794 This file is used in exactly the same way as hosts.equiv, but al-
795 lows host-based authentication without permitting login with
799 Systemwide configuration file. The file format and configuration
800 options are described in ssh_config(5).
802 /etc/ssh/ssh_host_key
803 /etc/ssh/ssh_host_dsa_key
804 /etc/ssh/ssh_host_rsa_key
805 These three files contain the private parts of the host keys and
806 are used for host-based authentication. If protocol version 1 is
807 used, ssh must be setuid root, since the host key is readable on-
808 ly by root. For protocol version 2, ssh uses ssh-keysign(8) to
809 access the host keys, eliminating the requirement that ssh be se-
810 tuid root when host-based authentication is used. By default ssh
813 /etc/ssh/ssh_known_hosts
814 Systemwide list of known host keys. This file should be prepared
815 by the system administrator to contain the public host keys of
816 all machines in the organization. It should be world-readable.
817 See sshd(8) for further details of the format of this file.
820 Commands in this file are executed by ssh when the user logs in,
821 just before the user's shell (or command) is started. See the
822 sshd(8) manual page for more information.
825 scp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), ssh-keyscan(1),
826 tun(4), hosts.equiv(5), ssh_config(5), ssh-keysign(8), sshd(8)
828 The Secure Shell (SSH) Protocol Assigned Numbers, RFC 4250, 2006.
830 The Secure Shell (SSH) Protocol Architecture, RFC 4251, 2006.
832 The Secure Shell (SSH) Authentication Protocol, RFC 4252, 2006.
834 The Secure Shell (SSH) Transport Layer Protocol, RFC 4253, 2006.
836 The Secure Shell (SSH) Connection Protocol, RFC 4254, 2006.
838 Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints, RFC
841 Generic Message Exchange Authentication for the Secure Shell Protocol
842 (SSH), RFC 4256, 2006.
844 The Secure Shell (SSH) Session Channel Break Extension, RFC 4335, 2006.
846 The Secure Shell (SSH) Transport Layer Encryption Modes, RFC 4344, 2006.
848 Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer
849 Protocol, RFC 4345, 2006.
851 Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer
852 Protocol, RFC 4419, 2006.
854 The Secure Shell (SSH) Public Key File Format, RFC 4716, 2006.
856 A. Perrig and D. Song, Hash Visualization: a New Technique to improve
857 Real-World Security, 1999, International Workshop on Cryptographic
858 Techniques and E-Commerce (CrypTEC '99).
861 OpenSSH is a derivative of the original and free ssh 1.2.12 release by
862 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
863 de Raadt and Dug Song removed many bugs, re-added newer features and
864 created OpenSSH. Markus Friedl contributed the support for SSH protocol
865 versions 1.5 and 2.0.
867 OpenBSD 4.6 March 19, 2009 14
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