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] [-I pkcs11]
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] [-W host:port]
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
18 untrusted hosts over an insecure network. X11 connections and arbitrary
19 TCP 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
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
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
73 different 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
76 interoperability with legacy protocol 1 implementations that do
77 not support the 3des cipher. Its use is strongly discouraged due
78 to cryptographic 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
82 in ssh_config(5) for more information.
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.
93 Dynamic port forwardings can also be specified in the
96 IPv6 addresses can be specified by enclosing the address in
97 square brackets. Only the superuser can forward privileged
98 ports. By default, the local port is bound in accordance with
99 the GatewayPorts setting. However, an explicit bind_address may
100 be used to bind the connection to a specific address. The
101 bind_address of ``localhost'' indicates that the listening port
102 be bound for local use only, while an empty address or `*'
103 indicates that the port should be available from all interfaces.
106 Sets the escape character for sessions with a pty (default: `~').
107 The escape character is only recognized at the beginning of a
108 line. The escape character followed by a dot (`.') closes the
109 connection; followed by control-Z suspends the connection; and
110 followed by itself sends the escape character once. Setting the
111 character to ``none'' disables any escapes and makes the session
115 Specifies an alternative per-user configuration file. If a
116 configuration file is given on the command line, the system-wide
117 configuration file (/etc/ssh/ssh_config) will be ignored. The
118 default for the per-user configuration file is ~/.ssh/config.
120 -f Requests ssh to go to background just before command execution.
121 This is useful if ssh is going to ask for passwords or
122 passphrases, but the user wants it in the background. This
123 implies -n. The recommended way to start X11 programs at a
124 remote site is with something like ssh -f host xterm.
126 If the ExitOnForwardFailure configuration option is set to
127 ``yes'', then a client started with -f will wait for all remote
128 port forwards to be successfully established before placing
129 itself in the background.
131 -g Allows remote hosts to connect to local forwarded ports.
134 Specify the PKCS#11 shared library ssh should use to communicate
135 with a PKCS#11 token providing the user's private RSA key.
138 Selects a file from which the identity (private key) for public
139 key authentication is read. The default is ~/.ssh/identity for
140 protocol version 1, and ~/.ssh/id_dsa, ~/.ssh/id_ecdsa and
141 ~/.ssh/id_rsa for protocol version 2. Identity files may also be
142 specified on a per-host basis in the configuration file. It is
143 possible to have multiple -i options (and multiple identities
144 specified in configuration files). ssh will also try to load
145 certificate information from the filename obtained by appending
146 -cert.pub to identity filenames.
148 -K Enables GSSAPI-based authentication and forwarding (delegation)
149 of GSSAPI credentials to the server.
151 -k Disables forwarding (delegation) of GSSAPI credentials to the
154 -L [bind_address:]port:host:hostport
155 Specifies that the given port on the local (client) host is to be
156 forwarded to the given host and port on the remote side. This
157 works by allocating a socket to listen to port on the local side,
158 optionally bound to the specified bind_address. Whenever a
159 connection is made to this port, the connection is forwarded over
160 the secure channel, and a connection is made to host port
161 hostport from the remote machine. Port forwardings can also be
162 specified in the configuration file. IPv6 addresses can be
163 specified by enclosing the address in square brackets. Only the
164 superuser can forward privileged ports. By default, the local
165 port is bound in accordance with the GatewayPorts setting.
166 However, an explicit bind_address may be used to bind the
167 connection to a specific address. The bind_address of
168 ``localhost'' indicates that the listening port be bound for
169 local use only, while an empty address or `*' indicates that the
170 port should be available from all interfaces.
173 Specifies the user to log in as on the remote machine. This also
174 may be specified on a per-host basis in the configuration file.
176 -M Places the ssh client into ``master'' mode for connection
177 sharing. Multiple -M options places ssh into ``master'' mode
178 with confirmation required before slave connections are accepted.
179 Refer to the description of ControlMaster in ssh_config(5) for
183 Additionally, for protocol version 2 a comma-separated list of
184 MAC (message authentication code) algorithms can be specified in
185 order of preference. See the MACs keyword for more information.
187 -N Do not execute a remote command. This is useful for just
188 forwarding ports (protocol version 2 only).
190 -n Redirects stdin from /dev/null (actually, prevents reading from
191 stdin). This must be used when ssh is run in the background. A
192 common trick is to use this to run X11 programs on a remote
193 machine. For example, ssh -n shadows.cs.hut.fi emacs & will
194 start an emacs on shadows.cs.hut.fi, and the X11 connection will
195 be automatically forwarded over an encrypted channel. The ssh
196 program will be put in the background. (This does not work if
197 ssh needs to ask for a password or passphrase; see also the -f
201 Control an active connection multiplexing master process. When
202 the -O option is specified, the ctl_cmd argument is interpreted
203 and passed to the master process. Valid commands are: ``check''
204 (check that the master process is running), ``forward'' (request
205 forwardings without command execution), ``exit'' (request the
206 master to exit), and ``stop'' (request the master to stop
207 accepting further multiplexing requests).
210 Can be used to give options in the format used in the
211 configuration file. This is useful for specifying options for
212 which there is no separate command-line flag. For full details
213 of the options listed below, and their possible values, see
219 ChallengeResponseAuthentication
239 GSSAPIDelegateCredentials
242 HostbasedAuthentication
249 KbdInteractiveDevices
255 NoHostAuthenticationForLocalhost
256 NumberOfPasswordPrompts
257 PasswordAuthentication
261 PreferredAuthentications
268 RhostsRSAAuthentication
273 StrictHostKeyChecking
285 Port to connect to on the remote host. This can be specified on
286 a per-host basis in the configuration file.
288 -q Quiet mode. Causes most warning and diagnostic messages to be
291 -R [bind_address:]port:host:hostport
292 Specifies that the given port on the remote (server) host is to
293 be forwarded to the given host and port on the local side. This
294 works by allocating a socket to listen to port on the remote
295 side, and whenever a connection is made to this port, the
296 connection is forwarded over the secure channel, and a connection
297 is made to host port hostport from the local machine.
299 Port forwardings can also be specified in the configuration file.
300 Privileged ports can be forwarded only when logging in as root on
301 the remote machine. IPv6 addresses can be specified by enclosing
302 the address in square braces.
304 By default, the listening socket on the server will be bound to
305 the loopback interface only. This may be overridden by
306 specifying a bind_address. An empty bind_address, or the address
307 `*', indicates that the remote socket should listen on all
308 interfaces. Specifying a remote bind_address will only succeed
309 if the server's GatewayPorts option is enabled (see
312 If the port argument is `0', the listen port will be dynamically
313 allocated on the server and reported to the client at run time.
314 When used together with -O forward the allocated port will be
315 printed to the standard output.
318 Specifies the location of a control socket for connection
319 sharing, or the string ``none'' to disable connection sharing.
320 Refer to the description of ControlPath and ControlMaster in
321 ssh_config(5) for details.
323 -s May be used to request invocation of a subsystem on the remote
324 system. Subsystems are a feature of the SSH2 protocol which
325 facilitate the use of SSH as a secure transport for other
326 applications (eg. sftp(1)). The subsystem is specified as the
329 -T Disable pseudo-tty allocation.
331 -t Force pseudo-tty allocation. This can be used to execute
332 arbitrary screen-based programs on a remote machine, which can be
333 very useful, e.g. when implementing menu services. Multiple -t
334 options force tty allocation, even if ssh has no local tty.
336 -V Display the version number and exit.
338 -v Verbose mode. Causes ssh to print debugging messages about its
339 progress. This is helpful in debugging connection,
340 authentication, and configuration problems. Multiple -v options
341 increase the verbosity. The maximum is 3.
344 Requests that standard input and output on the client be
345 forwarded to host on port over the secure channel. Implies -N,
346 -T, ExitOnForwardFailure and ClearAllForwardings and works with
347 Protocol version 2 only.
349 -w local_tun[:remote_tun]
350 Requests tunnel device forwarding with the specified tun(4)
351 devices between the client (local_tun) and the server
354 The devices may be specified by numerical ID or the keyword
355 ``any'', which uses the next available tunnel device. If
356 remote_tun is not specified, it defaults to ``any''. See also
357 the Tunnel and TunnelDevice directives in ssh_config(5). If the
358 Tunnel directive is unset, it is set to the default tunnel mode,
359 which is ``point-to-point''.
361 -X Enables X11 forwarding. This can also be specified on a per-host
362 basis in a configuration file.
364 X11 forwarding should be enabled with caution. Users with the
365 ability to bypass file permissions on the remote host (for the
366 user's X authorization database) can access the local X11 display
367 through the forwarded connection. An attacker may then be able
368 to perform activities such as keystroke monitoring.
370 For this reason, X11 forwarding is subjected to X11 SECURITY
371 extension restrictions by default. Please refer to the ssh -Y
372 option and the ForwardX11Trusted directive in ssh_config(5) for
375 -x Disables X11 forwarding.
377 -Y Enables trusted X11 forwarding. Trusted X11 forwardings are not
378 subjected to the X11 SECURITY extension controls.
380 -y Send log information using the syslog(3) system module. By
381 default this information is sent to stderr.
383 ssh may additionally obtain configuration data from a per-user
384 configuration file and a system-wide configuration file. The file format
385 and configuration options are described in ssh_config(5).
388 The OpenSSH SSH client supports SSH protocols 1 and 2. The default is to
389 use protocol 2 only, though this can be changed via the Protocol option
390 in ssh_config(5) or the -1 and -2 options (see above). Both protocols
391 support similar authentication methods, but protocol 2 is the default
392 since it provides additional mechanisms for confidentiality (the traffic
393 is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour) and
394 integrity (hmac-md5, hmac-sha1, hmac-sha2-256, hmac-sha2-512, umac-64,
395 hmac-ripemd160). Protocol 1 lacks a strong mechanism for ensuring the
396 integrity of the connection.
398 The methods available for authentication are: GSSAPI-based
399 authentication, host-based authentication, public key authentication,
400 challenge-response authentication, and password authentication.
401 Authentication methods are tried in the order specified above, though
402 protocol 2 has a configuration option to change the default order:
403 PreferredAuthentications.
405 Host-based authentication works as follows: If the machine the user logs
406 in from is listed in /etc/hosts.equiv or /etc/shosts.equiv on the remote
407 machine, and the user names are the same on both sides, or if the files
408 ~/.rhosts or ~/.shosts exist in the user's home directory on the remote
409 machine and contain a line containing the name of the client machine and
410 the name of the user on that machine, the user is considered for login.
411 Additionally, the server must be able to verify the client's host key
412 (see the description of /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts,
413 below) for login to be permitted. This authentication method closes
414 security holes due to IP spoofing, DNS spoofing, and routing spoofing.
415 [Note to the administrator: /etc/hosts.equiv, ~/.rhosts, and the
416 rlogin/rsh protocol in general, are inherently insecure and should be
417 disabled if security is desired.]
419 Public key authentication works as follows: The scheme is based on
420 public-key cryptography, using cryptosystems where encryption and
421 decryption are done using separate keys, and it is unfeasible to derive
422 the decryption key from the encryption key. The idea is that each user
423 creates a public/private key pair for authentication purposes. The
424 server knows the public key, and only the user knows the private key.
425 ssh implements public key authentication protocol automatically, using
426 one of the DSA, ECDSA or RSA algorithms. Protocol 1 is restricted to
427 using only RSA keys, but protocol 2 may use any. The HISTORY section of
428 ssl(8) contains a brief discussion of the DSA and RSA algorithms.
430 The file ~/.ssh/authorized_keys lists the public keys that are permitted
431 for logging in. When the user logs in, the ssh program tells the server
432 which key pair it would like to use for authentication. The client
433 proves that it has access to the private key and the server checks that
434 the corresponding public key is authorized to accept the account.
436 The user creates his/her key pair by running ssh-keygen(1). This stores
437 the private key in ~/.ssh/identity (protocol 1), ~/.ssh/id_dsa (protocol
438 2 DSA), ~/.ssh/id_ecdsa (protocol 2 ECDSA), or ~/.ssh/id_rsa (protocol 2
439 RSA) and stores the public key in ~/.ssh/identity.pub (protocol 1),
440 ~/.ssh/id_dsa.pub (protocol 2 DSA), ~/.ssh/id_ecdsa.pub (protocol 2
441 ECDSA), or ~/.ssh/id_rsa.pub (protocol 2 RSA) in the user's home
442 directory. The user should then copy the public key to
443 ~/.ssh/authorized_keys in his/her home directory on the remote machine.
444 The authorized_keys file corresponds to the conventional ~/.rhosts file,
445 and has one key per line, though the lines can be very long. After this,
446 the user can log in without giving the password.
448 A variation on public key authentication is available in the form of
449 certificate authentication: instead of a set of public/private keys,
450 signed certificates are used. This has the advantage that a single
451 trusted certification authority can be used in place of many
452 public/private keys. See the CERTIFICATES section of ssh-keygen(1) for
455 The most convenient way to use public key or certificate authentication
456 may be with an authentication agent. See ssh-agent(1) for more
459 Challenge-response authentication works as follows: The server sends an
460 arbitrary "challenge" text, and prompts for a response. Protocol 2
461 allows multiple challenges and responses; protocol 1 is restricted to
462 just one challenge/response. Examples of challenge-response
463 authentication include BSD Authentication (see login.conf(5)) and PAM
464 (some non-OpenBSD systems).
466 Finally, if other authentication methods fail, ssh prompts the user for a
467 password. The password is sent to the remote host for checking; however,
468 since all communications are encrypted, the password cannot be seen by
469 someone listening on the network.
471 ssh automatically maintains and checks a database containing
472 identification for all hosts it has ever been used with. Host keys are
473 stored in ~/.ssh/known_hosts in the user's home directory. Additionally,
474 the file /etc/ssh/ssh_known_hosts is automatically checked for known
475 hosts. Any new hosts are automatically added to the user's file. If a
476 host's identification ever changes, ssh warns about this and disables
477 password authentication to prevent server spoofing or man-in-the-middle
478 attacks, which could otherwise be used to circumvent the encryption. The
479 StrictHostKeyChecking option can be used to control logins to machines
480 whose host key is not known or has changed.
482 When the user's identity has been accepted by the server, the server
483 either executes the given command, or logs into the machine and gives the
484 user a normal shell on the remote machine. All communication with the
485 remote command or shell will be automatically encrypted.
487 If a pseudo-terminal has been allocated (normal login session), the user
488 may use the escape characters noted below.
490 If no pseudo-tty has been allocated, the session is transparent and can
491 be used to reliably transfer binary data. On most systems, setting the
492 escape character to ``none'' will also make the session transparent even
495 The session terminates when the command or shell on the remote machine
496 exits and all X11 and TCP connections have been closed.
499 When a pseudo-terminal has been requested, ssh supports a number of
500 functions through the use of an escape character.
502 A single tilde character can be sent as ~~ or by following the tilde by a
503 character other than those described below. The escape character must
504 always follow a newline to be interpreted as special. The escape
505 character can be changed in configuration files using the EscapeChar
506 configuration directive or on the command line by the -e option.
508 The supported escapes (assuming the default `~') are:
514 ~# List forwarded connections.
516 ~& Background ssh at logout when waiting for forwarded connection /
517 X11 sessions to terminate.
519 ~? Display a list of escape characters.
521 ~B Send a BREAK to the remote system (only useful for SSH protocol
522 version 2 and if the peer supports it).
524 ~C Open command line. Currently this allows the addition of port
525 forwardings using the -L, -R and -D options (see above). It also
526 allows the cancellation of existing remote port-forwardings using
527 -KR[bind_address:]port. !command allows the user to execute a
528 local command if the PermitLocalCommand option is enabled in
529 ssh_config(5). Basic help is available, using the -h option.
531 ~R Request rekeying of the connection (only useful for SSH protocol
532 version 2 and if the peer supports it).
535 Forwarding of arbitrary TCP connections over the secure channel can be
536 specified either on the command line or in a configuration file. One
537 possible application of TCP forwarding is a secure connection to a mail
538 server; another is going through firewalls.
540 In the example below, we look at encrypting communication between an IRC
541 client and server, even though the IRC server does not directly support
542 encrypted communications. This works as follows: the user connects to
543 the remote host using ssh, specifying a port to be used to forward
544 connections to the remote server. After that it is possible to start the
545 service which is to be encrypted on the client machine, connecting to the
546 same local port, and ssh will encrypt and forward the connection.
548 The following example tunnels an IRC session from client machine
549 ``127.0.0.1'' (localhost) to remote server ``server.example.com'':
551 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
552 $ irc -c '#users' -p 1234 pinky 127.0.0.1
554 This tunnels a connection to IRC server ``server.example.com'', joining
555 channel ``#users'', nickname ``pinky'', using port 1234. It doesn't
556 matter which port is used, as long as it's greater than 1023 (remember,
557 only root can open sockets on privileged ports) and doesn't conflict with
558 any ports already in use. The connection is forwarded to port 6667 on
559 the remote server, since that's the standard port for IRC services.
561 The -f option backgrounds ssh and the remote command ``sleep 10'' is
562 specified to allow an amount of time (10 seconds, in the example) to
563 start the service which is to be tunnelled. If no connections are made
564 within the time specified, ssh will exit.
567 If the ForwardX11 variable is set to ``yes'' (or see the description of
568 the -X, -x, and -Y options above) and the user is using X11 (the DISPLAY
569 environment variable is set), the connection to the X11 display is
570 automatically forwarded to the remote side in such a way that any X11
571 programs started from the shell (or command) will go through the
572 encrypted channel, and the connection to the real X server will be made
573 from the local machine. The user should not manually set DISPLAY.
574 Forwarding of X11 connections can be configured on the command line or in
577 The DISPLAY value set by ssh will point to the server machine, but with a
578 display number greater than zero. This is normal, and happens because
579 ssh creates a ``proxy'' X server on the server machine for forwarding the
580 connections over the encrypted channel.
582 ssh will also automatically set up Xauthority data on the server machine.
583 For this purpose, it will generate a random authorization cookie, store
584 it in Xauthority on the server, and verify that any forwarded connections
585 carry this cookie and replace it by the real cookie when the connection
586 is opened. The real authentication cookie is never sent to the server
587 machine (and no cookies are sent in the plain).
589 If the ForwardAgent variable is set to ``yes'' (or see the description of
590 the -A and -a options above) and the user is using an authentication
591 agent, the connection to the agent is automatically forwarded to the
595 When connecting to a server for the first time, a fingerprint of the
596 server's public key is presented to the user (unless the option
597 StrictHostKeyChecking has been disabled). Fingerprints can be determined
600 $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
602 If the fingerprint is already known, it can be matched and the key can be
603 accepted or rejected. Because of the difficulty of comparing host keys
604 just by looking at hex strings, there is also support to compare host
605 keys visually, using random art. By setting the VisualHostKey option to
606 ``yes'', a small ASCII graphic gets displayed on every login to a server,
607 no matter if the session itself is interactive or not. By learning the
608 pattern a known server produces, a user can easily find out that the host
609 key has changed when a completely different pattern is displayed.
610 Because these patterns are not unambiguous however, a pattern that looks
611 similar to the pattern remembered only gives a good probability that the
612 host key is the same, not guaranteed proof.
614 To get a listing of the fingerprints along with their random art for all
615 known hosts, the following command line can be used:
617 $ ssh-keygen -lv -f ~/.ssh/known_hosts
619 If the fingerprint is unknown, an alternative method of verification is
620 available: SSH fingerprints verified by DNS. An additional resource
621 record (RR), SSHFP, is added to a zonefile and the connecting client is
622 able to match the fingerprint with that of the key presented.
624 In this example, we are connecting a client to a server,
625 ``host.example.com''. The SSHFP resource records should first be added
626 to the zonefile for host.example.com:
628 $ ssh-keygen -r host.example.com.
630 The output lines will have to be added to the zonefile. To check that
631 the zone is answering fingerprint queries:
633 $ dig -t SSHFP host.example.com
635 Finally the client connects:
637 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
639 Matching host key fingerprint found in DNS.
640 Are you sure you want to continue connecting (yes/no)?
642 See the VerifyHostKeyDNS option in ssh_config(5) for more information.
644 SSH-BASED VIRTUAL PRIVATE NETWORKS
645 ssh contains support for Virtual Private Network (VPN) tunnelling using
646 the tun(4) network pseudo-device, allowing two networks to be joined
647 securely. The sshd_config(5) configuration option PermitTunnel controls
648 whether the server supports this, and at what level (layer 2 or 3
651 The following example would connect client network 10.0.50.0/24 with
652 remote network 10.0.99.0/24 using a point-to-point connection from
653 10.1.1.1 to 10.1.1.2, provided that the SSH server running on the gateway
654 to the remote network, at 192.168.1.15, allows it.
658 # ssh -f -w 0:1 192.168.1.15 true
659 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
660 # route add 10.0.99.0/24 10.1.1.2
664 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
665 # route add 10.0.50.0/24 10.1.1.1
667 Client access may be more finely tuned via the /root/.ssh/authorized_keys
668 file (see below) and the PermitRootLogin server option. The following
669 entry would permit connections on tun(4) device 1 from user ``jane'' and
670 on tun device 2 from user ``john'', if PermitRootLogin is set to
671 ``forced-commands-only'':
673 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
674 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
676 Since an SSH-based setup entails a fair amount of overhead, it may be
677 more suited to temporary setups, such as for wireless VPNs. More
678 permanent VPNs are better provided by tools such as ipsecctl(8) and
682 ssh will normally set the following environment variables:
684 DISPLAY The DISPLAY variable indicates the location of the
685 X11 server. It is automatically set by ssh to
686 point to a value of the form ``hostname:n'', where
687 ``hostname'' indicates the host where the shell
688 runs, and `n' is an integer >= 1. ssh uses this
689 special value to forward X11 connections over the
690 secure channel. The user should normally not set
691 DISPLAY explicitly, as that will render the X11
692 connection insecure (and will require the user to
693 manually copy any required authorization cookies).
695 HOME Set to the path of the user's home directory.
697 LOGNAME Synonym for USER; set for compatibility with
698 systems that use this variable.
700 MAIL Set to the path of the user's mailbox.
702 PATH Set to the default PATH, as specified when
705 SSH_ASKPASS If ssh needs a passphrase, it will read the
706 passphrase from the current terminal if it was run
707 from a terminal. If ssh does not have a terminal
708 associated with it but DISPLAY and SSH_ASKPASS are
709 set, it will execute the program specified by
710 SSH_ASKPASS and open an X11 window to read the
711 passphrase. This is particularly useful when
712 calling ssh from a .xsession or related script.
713 (Note that on some machines it may be necessary to
714 redirect the input from /dev/null to make this
717 SSH_AUTH_SOCK Identifies the path of a UNIX-domain socket used to
718 communicate with the agent.
720 SSH_CONNECTION Identifies the client and server ends of the
721 connection. The variable contains four space-
722 separated values: client IP address, client port
723 number, server IP address, and server port number.
725 SSH_ORIGINAL_COMMAND This variable contains the original command line if
726 a forced command is executed. It can be used to
727 extract the original arguments.
729 SSH_TTY This is set to the name of the tty (path to the
730 device) associated with the current shell or
731 command. If the current session has no tty, this
734 TZ This variable is set to indicate the present time
735 zone if it was set when the daemon was started
736 (i.e. the daemon passes the value on to new
739 USER Set to the name of the user logging in.
741 Additionally, ssh reads ~/.ssh/environment, and adds lines of the format
742 ``VARNAME=value'' to the environment if the file exists and users are
743 allowed to change their environment. For more information, see the
744 PermitUserEnvironment option in sshd_config(5).
748 This file is used for host-based authentication (see above). On
749 some machines this file may need to be world-readable if the
750 user's home directory is on an NFS partition, because sshd(8)
751 reads it as root. Additionally, this file must be owned by the
752 user, and must not have write permissions for anyone else. The
753 recommended permission for most machines is read/write for the
754 user, and not accessible by others.
757 This file is used in exactly the same way as .rhosts, but allows
758 host-based authentication without permitting login with
762 This directory is the default location for all user-specific
763 configuration and authentication information. There is no
764 general requirement to keep the entire contents of this directory
765 secret, but the recommended permissions are read/write/execute
766 for the user, and not accessible by others.
768 ~/.ssh/authorized_keys
769 Lists the public keys (DSA/ECDSA/RSA) that can be used for
770 logging in as this user. The format of this file is described in
771 the sshd(8) manual page. This file is not highly sensitive, but
772 the recommended permissions are read/write for the user, and not
773 accessible by others.
776 This is the per-user configuration file. The file format and
777 configuration options are described in ssh_config(5). Because of
778 the potential for abuse, this file must have strict permissions:
779 read/write for the user, and not accessible by others.
782 Contains additional definitions for environment variables; see
789 Contains the private key for authentication. These files contain
790 sensitive data and should be readable by the user but not
791 accessible by others (read/write/execute). ssh will simply
792 ignore a private key file if it is accessible by others. It is
793 possible to specify a passphrase when generating the key which
794 will be used to encrypt the sensitive part of this file using
801 Contains the public key for authentication. These files are not
802 sensitive and can (but need not) be readable by anyone.
805 Contains a list of host keys for all hosts the user has logged
806 into that are not already in the systemwide list of known host
807 keys. See sshd(8) for further details of the format of this
811 Commands in this file are executed by ssh when the user logs in,
812 just before the user's shell (or command) is started. See the
813 sshd(8) manual page for more information.
816 This file is for host-based authentication (see above). It
817 should only be writable by root.
820 This file is used in exactly the same way as hosts.equiv, but
821 allows host-based authentication without permitting login with
825 Systemwide configuration file. The file format and configuration
826 options are described in ssh_config(5).
828 /etc/ssh/ssh_host_key
829 /etc/ssh/ssh_host_dsa_key
830 /etc/ssh/ssh_host_ecdsa_key
831 /etc/ssh/ssh_host_rsa_key
832 These three files contain the private parts of the host keys and
833 are used for host-based authentication. If protocol version 1 is
834 used, ssh must be setuid root, since the host key is readable
835 only by root. For protocol version 2, ssh uses ssh-keysign(8) to
836 access the host keys, eliminating the requirement that ssh be
837 setuid root when host-based authentication is used. By default
838 ssh is not setuid root.
840 /etc/ssh/ssh_known_hosts
841 Systemwide list of known host keys. This file should be prepared
842 by the system administrator to contain the public host keys of
843 all machines in the organization. It should be world-readable.
844 See sshd(8) for further details of the format of this file.
847 Commands in this file are executed by ssh when the user logs in,
848 just before the user's shell (or command) is started. See the
849 sshd(8) manual page for more information.
852 ssh exits with the exit status of the remote command or with 255 if an
856 scp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), ssh-keyscan(1),
857 tun(4), hosts.equiv(5), ssh_config(5), ssh-keysign(8), sshd(8)
859 The Secure Shell (SSH) Protocol Assigned Numbers, RFC 4250, 2006.
861 The Secure Shell (SSH) Protocol Architecture, RFC 4251, 2006.
863 The Secure Shell (SSH) Authentication Protocol, RFC 4252, 2006.
865 The Secure Shell (SSH) Transport Layer Protocol, RFC 4253, 2006.
867 The Secure Shell (SSH) Connection Protocol, RFC 4254, 2006.
869 Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints, RFC
872 Generic Message Exchange Authentication for the Secure Shell Protocol
873 (SSH), RFC 4256, 2006.
875 The Secure Shell (SSH) Session Channel Break Extension, RFC 4335, 2006.
877 The Secure Shell (SSH) Transport Layer Encryption Modes, RFC 4344, 2006.
879 Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer
880 Protocol, RFC 4345, 2006.
882 Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer
883 Protocol, RFC 4419, 2006.
885 The Secure Shell (SSH) Public Key File Format, RFC 4716, 2006.
887 Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer,
890 A. Perrig and D. Song, Hash Visualization: a New Technique to improve
891 Real-World Security, 1999, International Workshop on Cryptographic
892 Techniques and E-Commerce (CrypTEC '99).
895 OpenSSH is a derivative of the original and free ssh 1.2.12 release by
896 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
897 de Raadt and Dug Song removed many bugs, re-added newer features and
898 created OpenSSH. Markus Friedl contributed the support for SSH protocol
899 versions 1.5 and 2.0.
901 OpenBSD 5.0 August 2, 2011 OpenBSD 5.0
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