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. The supported ciphers
82 are: 3des-cbc, aes128-cbc, aes192-cbc, aes256-cbc, aes128-ctr,
83 aes192-ctr, aes256-ctr, arcfour128, arcfour256, arcfour, blow-
84 fish-cbc, and cast128-cbc. The default is:
86 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
87 arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
90 -D [bind_address:]port
91 Specifies a local ``dynamic'' application-level port forwarding.
92 This works by allocating a socket to listen to port on the local
93 side, optionally bound to the specified bind_address. Whenever a
94 connection is made to this port, the connection is forwarded over
95 the secure channel, and the application protocol is then used to
96 determine where to connect to from the remote machine. Currently
97 the SOCKS4 and SOCKS5 protocols are supported, and ssh will act
98 as a SOCKS server. Only root can forward privileged ports. Dy-
99 namic port forwardings can also be specified in the configuration
102 IPv6 addresses can be specified with an alternative syntax:
103 [bind_address/]port or by enclosing the address in square brack-
104 ets. Only the superuser can forward privileged ports. By de-
105 fault, the local port is bound in accordance with the
106 GatewayPorts setting. However, an explicit bind_address may be
107 used to bind the connection to a specific address. The
108 bind_address of ``localhost'' indicates that the listening port
109 be bound for local use only, while an empty address or `*' indi-
110 cates that the port should be available from all interfaces.
113 Sets the escape character for sessions with a pty (default: `~').
114 The escape character is only recognized at the beginning of a
115 line. The escape character followed by a dot (`.') closes the
116 connection; followed by control-Z suspends the connection; and
117 followed by itself sends the escape character once. Setting the
118 character to ``none'' disables any escapes and makes the session
122 Specifies an alternative per-user configuration file. If a con-
123 figuration file is given on the command line, the system-wide
124 configuration file (/etc/ssh/ssh_config) will be ignored. The
125 default for the per-user configuration file is ~/.ssh/config.
127 -f Requests ssh to go to background just before command execution.
128 This is useful if ssh is going to ask for passwords or passphras-
129 es, but the user wants it in the background. This implies -n.
130 The recommended way to start X11 programs at a remote site is
131 with something like ssh -f host xterm.
133 If the ExitOnForwardFailure configuration option is set to
134 ``yes'', then a client started with -f will wait for all remote
135 port forwards to be successfully established before placing it-
136 self in the background.
138 -g Allows remote hosts to connect to local forwarded ports.
141 Specify the device ssh should use to communicate with a smartcard
142 used for storing the user's private RSA key. This option is only
143 available if support for smartcard devices is compiled in (de-
144 fault is no support).
147 Selects a file from which the identity (private key) for RSA or
148 DSA authentication is read. The default is ~/.ssh/identity for
149 protocol version 1, and ~/.ssh/id_rsa and ~/.ssh/id_dsa for pro-
150 tocol version 2. Identity files may also be specified on a per-
151 host basis in the configuration file. It is possible to have
152 multiple -i options (and multiple identities specified in config-
155 -K Enables GSSAPI-based authentication and forwarding (delegation)
156 of GSSAPI credentials to the server.
158 -k Disables forwarding (delegation) of GSSAPI credentials to the
161 -L [bind_address:]port:host:hostport
162 Specifies that the given port on the local (client) host is to be
163 forwarded to the given host and port on the remote side. This
164 works by allocating a socket to listen to port on the local side,
165 optionally bound to the specified bind_address. Whenever a con-
166 nection is made to this port, the connection is forwarded over
167 the secure channel, and a connection is made to host port
168 hostport from the remote machine. Port forwardings can also be
169 specified in the configuration file. IPv6 addresses can be spec-
170 ified with an alternative syntax: [bind_address/]port/host/host-
171 port or by enclosing the address in square brackets. Only the
172 superuser can forward privileged ports. By default, the local
173 port is bound in accordance with the GatewayPorts setting. How-
174 ever, an explicit bind_address may be used to bind the connection
175 to a specific address. The bind_address of ``localhost'' indi-
176 cates that the listening port be bound for local use only, while
177 an empty address or `*' indicates that the port should be avail-
178 able from all interfaces.
181 Specifies the user to log in as on the remote machine. This also
182 may be specified on a per-host basis in the configuration file.
184 -M Places the ssh client into ``master'' mode for connection shar-
185 ing. Multiple -M options places ssh into ``master'' mode with
186 confirmation required before slave connections are accepted. Re-
187 fer to the description of ControlMaster in ssh_config(5) for de-
191 Additionally, for protocol version 2 a comma-separated list of
192 MAC (message authentication code) algorithms can be specified in
193 order of preference. See the MACs keyword for more information.
195 -N Do not execute a remote command. This is useful for just for-
196 warding ports (protocol version 2 only).
198 -n Redirects stdin from /dev/null (actually, prevents reading from
199 stdin). This must be used when ssh is run in the background. A
200 common trick is to use this to run X11 programs on a remote ma-
201 chine. For example, ssh -n shadows.cs.hut.fi emacs & will start
202 an emacs on shadows.cs.hut.fi, and the X11 connection will be au-
203 tomatically forwarded over an encrypted channel. The ssh program
204 will be put in the background. (This does not work if ssh needs
205 to ask for a password or passphrase; see also the -f option.)
208 Control an active connection multiplexing master process. When
209 the -O option is specified, the ctl_cmd argument is interpreted
210 and passed to the master process. Valid commands are: ``check''
211 (check that the master process is running) and ``exit'' (request
215 Can be used to give options in the format used in the configura-
216 tion file. This is useful for specifying options for which there
217 is no separate command-line flag. For full details of the op-
218 tions listed below, and their possible values, see ssh_config(5).
223 ChallengeResponseAuthentication
243 GSSAPIDelegateCredentials
246 HostbasedAuthentication
252 KbdInteractiveDevices
257 NoHostAuthenticationForLocalhost
258 NumberOfPasswordPrompts
259 PasswordAuthentication
262 PreferredAuthentications
268 RhostsRSAAuthentication
274 StrictHostKeyChecking
286 Port to connect to on the remote host. This can be specified on
287 a per-host basis in the configuration file.
289 -q Quiet mode. Causes most warning and diagnostic messages to be
292 -R [bind_address:]port:host:hostport
293 Specifies that the given port on the remote (server) host is to
294 be forwarded to the given host and port on the local side. This
295 works by allocating a socket to listen to port on the remote
296 side, and whenever a connection is made to this port, the connec-
297 tion is forwarded over the secure channel, and a connection is
298 made to host port hostport from the local machine.
300 Port forwardings can also be specified in the configuration file.
301 Privileged ports can be forwarded only when logging in as root on
302 the remote machine. IPv6 addresses can be specified by enclosing
303 the address in square braces or using an alternative syntax:
304 [bind_address/]host/port/hostport.
306 By default, the listening socket on the server will be bound to
307 the loopback interface only. This may be overridden by specify-
308 ing a bind_address. An empty bind_address, or the address `*',
309 indicates that the remote socket should listen on all interfaces.
310 Specifying a remote bind_address will only succeed if the serv-
311 er's GatewayPorts option is enabled (see sshd_config(5)).
313 If the port argument is `0', the listen port will be dynamically
314 allocated on the server and reported to the client at run time.
317 Specifies the location of a control socket for connection shar-
318 ing. Refer to the description of ControlPath and ControlMaster
319 in ssh_config(5) for details.
321 -s May be used to request invocation of a subsystem on the remote
322 system. Subsystems are a feature of the SSH2 protocol which fa-
323 cilitate the use of SSH as a secure transport for other applica-
324 tions (eg. sftp(1)). The subsystem is specified as the remote
327 -T Disable pseudo-tty allocation.
329 -t Force pseudo-tty allocation. This can be used to execute arbi-
330 trary screen-based programs on a remote machine, which can be
331 very useful, e.g. when implementing menu services. Multiple -t
332 options force tty allocation, even if ssh has no local tty.
334 -V Display the version number and exit.
336 -v Verbose mode. Causes ssh to print debugging messages about its
337 progress. This is helpful in debugging connection, authentica-
338 tion, and configuration problems. Multiple -v options increase
339 the verbosity. The maximum is 3.
341 -w local_tun[:remote_tun]
342 Requests tunnel device forwarding with the specified tun(4) de-
343 vices between the client (local_tun) and the server (remote_tun).
345 The devices may be specified by numerical ID or the keyword
346 ``any'', which uses the next available tunnel device. If
347 remote_tun is not specified, it defaults to ``any''. See also
348 the Tunnel and TunnelDevice directives in ssh_config(5). If the
349 Tunnel directive is unset, it is set to the default tunnel mode,
350 which is ``point-to-point''.
352 -X Enables X11 forwarding. This can also be specified on a per-host
353 basis in a configuration file.
355 X11 forwarding should be enabled with caution. Users with the
356 ability to bypass file permissions on the remote host (for the
357 user's X authorization database) can access the local X11 display
358 through the forwarded connection. An attacker may then be able
359 to perform activities such as keystroke monitoring.
361 For this reason, X11 forwarding is subjected to X11 SECURITY ex-
362 tension restrictions by default. Please refer to the ssh -Y op-
363 tion and the ForwardX11Trusted directive in ssh_config(5) for
366 -x Disables X11 forwarding.
368 -Y Enables trusted X11 forwarding. Trusted X11 forwardings are not
369 subjected to the X11 SECURITY extension controls.
371 -y Send log information using the syslog(3) system module. By de-
372 fault this information is sent to stderr.
374 ssh may additionally obtain configuration data from a per-user configura-
375 tion file and a system-wide configuration file. The file format and con-
376 figuration options are described in ssh_config(5).
378 ssh exits with the exit status of the remote command or with 255 if an
382 The OpenSSH SSH client supports SSH protocols 1 and 2. Protocol 2 is the
383 default, with ssh falling back to protocol 1 if it detects protocol 2 is
384 unsupported. These settings may be altered using the Protocol option in
385 ssh_config(5), or enforced using the -1 and -2 options (see above). Both
386 protocols support similar authentication methods, but protocol 2 is pre-
387 ferred since it provides additional mechanisms for confidentiality (the
388 traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour) and
389 integrity (hmac-md5, hmac-sha1, umac-64, hmac-ripemd160). Protocol 1
390 lacks a strong mechanism for ensuring the integrity of the connection.
392 The methods available for authentication are: GSSAPI-based authentica-
393 tion, host-based authentication, public key authentication, challenge-re-
394 sponse authentication, and password authentication. Authentication meth-
395 ods are tried in the order specified above, though protocol 2 has a con-
396 figuration option to change the default order: PreferredAuthentications.
398 Host-based authentication works as follows: If the machine the user logs
399 in from is listed in /etc/hosts.equiv or /etc/shosts.equiv on the remote
400 machine, and the user names are the same on both sides, or if the files
401 ~/.rhosts or ~/.shosts exist in the user's home directory on the remote
402 machine and contain a line containing the name of the client machine and
403 the name of the user on that machine, the user is considered for login.
404 Additionally, the server must be able to verify the client's host key
405 (see the description of /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts,
406 below) for login to be permitted. This authentication method closes se-
407 curity holes due to IP spoofing, DNS spoofing, and routing spoofing.
408 [Note to the administrator: /etc/hosts.equiv, ~/.rhosts, and the
409 rlogin/rsh protocol in general, are inherently insecure and should be
410 disabled if security is desired.]
412 Public key authentication works as follows: The scheme is based on pub-
413 lic-key cryptography, using cryptosystems where encryption and decryption
414 are done using separate keys, and it is unfeasible to derive the decryp-
415 tion key from the encryption key. The idea is that each user creates a
416 public/private key pair for authentication purposes. The server knows
417 the public key, and only the user knows the private key. ssh implements
418 public key authentication protocol automatically, using either the RSA or
419 DSA algorithms. Protocol 1 is restricted to using only RSA keys, but
420 protocol 2 may use either. The HISTORY section of ssl(8) contains a
421 brief discussion of the two algorithms.
423 The file ~/.ssh/authorized_keys lists the public keys that are permitted
424 for logging in. When the user logs in, the ssh program tells the server
425 which key pair it would like to use for authentication. The client
426 proves that it has access to the private key and the server checks that
427 the corresponding public key is authorized to accept the account.
429 The user creates his/her key pair by running ssh-keygen(1). This stores
430 the private key in ~/.ssh/identity (protocol 1), ~/.ssh/id_dsa (protocol
431 2 DSA), or ~/.ssh/id_rsa (protocol 2 RSA) and stores the public key in
432 ~/.ssh/identity.pub (protocol 1), ~/.ssh/id_dsa.pub (protocol 2 DSA), or
433 ~/.ssh/id_rsa.pub (protocol 2 RSA) in the user's home directory. The us-
434 er should then copy the public key to ~/.ssh/authorized_keys in his/her
435 home directory on the remote machine. The authorized_keys file corre-
436 sponds to the conventional ~/.rhosts file, and has one key per line,
437 though the lines can be very long. After this, the user can log in with-
438 out giving the password.
440 The most convenient way to use public key authentication may be with an
441 authentication agent. See ssh-agent(1) for more information.
443 Challenge-response authentication works as follows: The server sends an
444 arbitrary "challenge" text, and prompts for a response. Protocol 2 al-
445 lows multiple challenges and responses; protocol 1 is restricted to just
446 one challenge/response. Examples of challenge-response authentication
447 include BSD Authentication (see login.conf(5)) and PAM (some non-OpenBSD
450 Finally, if other authentication methods fail, ssh prompts the user for a
451 password. The password is sent to the remote host for checking; however,
452 since all communications are encrypted, the password cannot be seen by
453 someone listening on the network.
455 ssh automatically maintains and checks a database containing identifica-
456 tion for all hosts it has ever been used with. Host keys are stored in
457 ~/.ssh/known_hosts in the user's home directory. Additionally, the file
458 /etc/ssh/ssh_known_hosts is automatically checked for known hosts. Any
459 new hosts are automatically added to the user's file. If a host's iden-
460 tification ever changes, ssh warns about this and disables password au-
461 thentication to prevent server spoofing or man-in-the-middle attacks,
462 which could otherwise be used to circumvent the encryption. The
463 StrictHostKeyChecking option can be used to control logins to machines
464 whose host key is not known or has changed.
466 When the user's identity has been accepted by the server, the server ei-
467 ther executes the given command, or logs into the machine and gives the
468 user a normal shell on the remote machine. All communication with the
469 remote command or shell will be automatically encrypted.
471 If a pseudo-terminal has been allocated (normal login session), the user
472 may use the escape characters noted below.
474 If no pseudo-tty has been allocated, the session is transparent and can
475 be used to reliably transfer binary data. On most systems, setting the
476 escape character to ``none'' will also make the session transparent even
479 The session terminates when the command or shell on the remote machine
480 exits and all X11 and TCP connections have been closed.
483 When a pseudo-terminal has been requested, ssh supports a number of func-
484 tions through the use of an escape character.
486 A single tilde character can be sent as ~~ or by following the tilde by a
487 character other than those described below. The escape character must
488 always follow a newline to be interpreted as special. The escape charac-
489 ter can be changed in configuration files using the EscapeChar configura-
490 tion directive or on the command line by the -e option.
492 The supported escapes (assuming the default `~') are:
498 ~# List forwarded connections.
500 ~& Background ssh at logout when waiting for forwarded connection /
501 X11 sessions to terminate.
503 ~? Display a list of escape characters.
505 ~B Send a BREAK to the remote system (only useful for SSH protocol
506 version 2 and if the peer supports it).
508 ~C Open command line. Currently this allows the addition of port
509 forwardings using the -L, -R and -D options (see above). It also
510 allows the cancellation of existing remote port-forwardings using
511 -KR[bind_address:]port. !command allows the user to execute a
512 local command if the PermitLocalCommand option is enabled in
513 ssh_config(5). Basic help is available, using the -h option.
515 ~R Request rekeying of the connection (only useful for SSH protocol
516 version 2 and if the peer supports it).
519 Forwarding of arbitrary TCP connections over the secure channel can be
520 specified either on the command line or in a configuration file. One
521 possible application of TCP forwarding is a secure connection to a mail
522 server; another is going through firewalls.
524 In the example below, we look at encrypting communication between an IRC
525 client and server, even though the IRC server does not directly support
526 encrypted communications. This works as follows: the user connects to
527 the remote host using ssh, specifying a port to be used to forward con-
528 nections to the remote server. After that it is possible to start the
529 service which is to be encrypted on the client machine, connecting to the
530 same local port, and ssh will encrypt and forward the connection.
532 The following example tunnels an IRC session from client machine
533 ``127.0.0.1'' (localhost) to remote server ``server.example.com'':
535 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
536 $ irc -c '#users' -p 1234 pinky 127.0.0.1
538 This tunnels a connection to IRC server ``server.example.com'', joining
539 channel ``#users'', nickname ``pinky'', using port 1234. It doesn't mat-
540 ter which port is used, as long as it's greater than 1023 (remember, only
541 root can open sockets on privileged ports) and doesn't conflict with any
542 ports already in use. The connection is forwarded to port 6667 on the
543 remote server, since that's the standard port for IRC services.
545 The -f option backgrounds ssh and the remote command ``sleep 10'' is
546 specified to allow an amount of time (10 seconds, in the example) to
547 start the service which is to be tunnelled. If no connections are made
548 within the time specified, ssh will exit.
551 If the ForwardX11 variable is set to ``yes'' (or see the description of
552 the -X, -x, and -Y options above) and the user is using X11 (the DISPLAY
553 environment variable is set), the connection to the X11 display is auto-
554 matically forwarded to the remote side in such a way that any X11 pro-
555 grams started from the shell (or command) will go through the encrypted
556 channel, and the connection to the real X server will be made from the
557 local machine. The user should not manually set DISPLAY. Forwarding of
558 X11 connections can be configured on the command line or in configuration
561 The DISPLAY value set by ssh will point to the server machine, but with a
562 display number greater than zero. This is normal, and happens because
563 ssh creates a ``proxy'' X server on the server machine for forwarding the
564 connections over the encrypted channel.
566 ssh will also automatically set up Xauthority data on the server machine.
567 For this purpose, it will generate a random authorization cookie, store
568 it in Xauthority on the server, and verify that any forwarded connections
569 carry this cookie and replace it by the real cookie when the connection
570 is opened. The real authentication cookie is never sent to the server
571 machine (and no cookies are sent in the plain).
573 If the ForwardAgent variable is set to ``yes'' (or see the description of
574 the -A and -a options above) and the user is using an authentication
575 agent, the connection to the agent is automatically forwarded to the re-
579 When connecting to a server for the first time, a fingerprint of the
580 server's public key is presented to the user (unless the option
581 StrictHostKeyChecking has been disabled). Fingerprints can be determined
584 $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
586 If the fingerprint is already known, it can be matched and the key can be
587 accepted or rejected. Because of the difficulty of comparing host keys
588 just by looking at hex strings, there is also support to compare host
589 keys visually, using random art. By setting the VisualHostKey option to
590 ``yes'', a small ASCII graphic gets displayed on every login to a server,
591 no matter if the session itself is interactive or not. By learning the
592 pattern a known server produces, a user can easily find out that the host
593 key has changed when a completely different pattern is displayed. Be-
594 cause these patterns are not unambiguous however, a pattern that looks
595 similar to the pattern remembered only gives a good probability that the
596 host key is the same, not guaranteed proof.
598 To get a listing of the fingerprints along with their random art for all
599 known hosts, the following command line can be used:
601 $ ssh-keygen -lv -f ~/.ssh/known_hosts
603 If the fingerprint is unknown, an alternative method of verification is
604 available: SSH fingerprints verified by DNS. An additional resource
605 record (RR), SSHFP, is added to a zonefile and the connecting client is
606 able to match the fingerprint with that of the key presented.
608 In this example, we are connecting a client to a server,
609 ``host.example.com''. The SSHFP resource records should first be added
610 to the zonefile for host.example.com:
612 $ ssh-keygen -r host.example.com.
614 The output lines will have to be added to the zonefile. To check that
615 the zone is answering fingerprint queries:
617 $ dig -t SSHFP host.example.com
619 Finally the client connects:
621 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
623 Matching host key fingerprint found in DNS.
624 Are you sure you want to continue connecting (yes/no)?
626 See the VerifyHostKeyDNS option in ssh_config(5) for more information.
628 SSH-BASED VIRTUAL PRIVATE NETWORKS
629 ssh contains support for Virtual Private Network (VPN) tunnelling using
630 the tun(4) network pseudo-device, allowing two networks to be joined se-
631 curely. The sshd_config(5) configuration option PermitTunnel controls
632 whether the server supports this, and at what level (layer 2 or 3 traf-
635 The following example would connect client network 10.0.50.0/24 with re-
636 mote network 10.0.99.0/24 using a point-to-point connection from 10.1.1.1
637 to 10.1.1.2, provided that the SSH server running on the gateway to the
638 remote network, at 192.168.1.15, allows it.
642 # ssh -f -w 0:1 192.168.1.15 true
643 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
644 # route add 10.0.99.0/24 10.1.1.2
648 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
649 # route add 10.0.50.0/24 10.1.1.1
651 Client access may be more finely tuned via the /root/.ssh/authorized_keys
652 file (see below) and the PermitRootLogin server option. The following
653 entry would permit connections on tun(4) device 1 from user ``jane'' and
654 on tun device 2 from user ``john'', if PermitRootLogin is set to
655 ``forced-commands-only'':
657 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
658 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
660 Since an SSH-based setup entails a fair amount of overhead, it may be
661 more suited to temporary setups, such as for wireless VPNs. More perma-
662 nent VPNs are better provided by tools such as ipsecctl(8) and
666 ssh will normally set the following environment variables:
668 DISPLAY The DISPLAY variable indicates the location of the
669 X11 server. It is automatically set by ssh to
670 point to a value of the form ``hostname:n'', where
671 ``hostname'' indicates the host where the shell
672 runs, and `n' is an integer >= 1. ssh uses this
673 special value to forward X11 connections over the
674 secure channel. The user should normally not set
675 DISPLAY explicitly, as that will render the X11
676 connection insecure (and will require the user to
677 manually copy any required authorization cookies).
679 HOME Set to the path of the user's home directory.
681 LOGNAME Synonym for USER; set for compatibility with sys-
682 tems that use this variable.
684 MAIL Set to the path of the user's mailbox.
686 PATH Set to the default PATH, as specified when compil-
689 SSH_ASKPASS If ssh needs a passphrase, it will read the
690 passphrase from the current terminal if it was run
691 from a terminal. If ssh does not have a terminal
692 associated with it but DISPLAY and SSH_ASKPASS are
693 set, it will execute the program specified by
694 SSH_ASKPASS and open an X11 window to read the
695 passphrase. This is particularly useful when call-
696 ing ssh from a .xsession or related script. (Note
697 that on some machines it may be necessary to redi-
698 rect the input from /dev/null to make this work.)
700 SSH_AUTH_SOCK Identifies the path of a UNIX-domain socket used to
701 communicate with the agent.
703 SSH_CONNECTION Identifies the client and server ends of the con-
704 nection. The variable contains four space-separat-
705 ed values: client IP address, client port number,
706 server IP address, and server port number.
708 SSH_ORIGINAL_COMMAND This variable contains the original command line if
709 a forced command is executed. It can be used to
710 extract the original arguments.
712 SSH_TTY This is set to the name of the tty (path to the de-
713 vice) associated with the current shell or command.
714 If the current session has no tty, this variable is
717 TZ This variable is set to indicate the present time
718 zone if it was set when the daemon was started
719 (i.e. the daemon passes the value on to new connec-
722 USER Set to the name of the user logging in.
724 Additionally, ssh reads ~/.ssh/environment, and adds lines of the format
725 ``VARNAME=value'' to the environment if the file exists and users are al-
726 lowed to change their environment. For more information, see the
727 PermitUserEnvironment option in sshd_config(5).
731 This file is used for host-based authentication (see above). On
732 some machines this file may need to be world-readable if the us-
733 er's home directory is on an NFS partition, because sshd(8) reads
734 it as root. Additionally, this file must be owned by the user,
735 and must not have write permissions for anyone else. The recom-
736 mended permission for most machines is read/write for the user,
737 and not accessible by others.
740 This file is used in exactly the same way as .rhosts, but allows
741 host-based authentication without permitting login with
745 This directory is the default location for all user-specific con-
746 figuration and authentication information. There is no general
747 requirement to keep the entire contents of this directory secret,
748 but the recommended permissions are read/write/execute for the
749 user, and not accessible by others.
751 ~/.ssh/authorized_keys
752 Lists the public keys (RSA/DSA) that can be used for logging in
753 as this user. The format of this file is described in the
754 sshd(8) manual page. This file is not highly sensitive, but the
755 recommended permissions are read/write for the user, and not ac-
759 This is the per-user configuration file. The file format and
760 configuration options are described in ssh_config(5). Because of
761 the potential for abuse, this file must have strict permissions:
762 read/write for the user, and not accessible by others.
765 Contains additional definitions for environment variables; see
771 Contains the private key for authentication. These files contain
772 sensitive data and should be readable by the user but not acces-
773 sible by others (read/write/execute). ssh will simply ignore a
774 private key file if it is accessible by others. It is possible
775 to specify a passphrase when generating the key which will be
776 used to encrypt the sensitive part of this file using 3DES.
781 Contains the public key for authentication. These files are not
782 sensitive and can (but need not) be readable by anyone.
785 Contains a list of host keys for all hosts the user has logged
786 into that are not already in the systemwide list of known host
787 keys. See sshd(8) for further details of the format of this
791 Commands in this file are executed by ssh when the user logs in,
792 just before the user's shell (or command) is started. See the
793 sshd(8) manual page for more information.
796 This file is for host-based authentication (see above). It
797 should only be writable by root.
800 This file is used in exactly the same way as hosts.equiv, but al-
801 lows host-based authentication without permitting login with
805 Systemwide configuration file. The file format and configuration
806 options are described in ssh_config(5).
808 /etc/ssh/ssh_host_key
809 /etc/ssh/ssh_host_dsa_key
810 /etc/ssh/ssh_host_rsa_key
811 These three files contain the private parts of the host keys and
812 are used for host-based authentication. If protocol version 1 is
813 used, ssh must be setuid root, since the host key is readable on-
814 ly by root. For protocol version 2, ssh uses ssh-keysign(8) to
815 access the host keys, eliminating the requirement that ssh be se-
816 tuid root when host-based authentication is used. By default ssh
819 /etc/ssh/ssh_known_hosts
820 Systemwide list of known host keys. This file should be prepared
821 by the system administrator to contain the public host keys of
822 all machines in the organization. It should be world-readable.
823 See sshd(8) for further details of the format of this file.
826 Commands in this file are executed by ssh when the user logs in,
827 just before the user's shell (or command) is started. See the
828 sshd(8) manual page for more information.
831 scp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), ssh-keyscan(1),
832 tun(4), hosts.equiv(5), ssh_config(5), ssh-keysign(8), sshd(8)
834 The Secure Shell (SSH) Protocol Assigned Numbers, RFC 4250, 2006.
836 The Secure Shell (SSH) Protocol Architecture, RFC 4251, 2006.
838 The Secure Shell (SSH) Authentication Protocol, RFC 4252, 2006.
840 The Secure Shell (SSH) Transport Layer Protocol, RFC 4253, 2006.
842 The Secure Shell (SSH) Connection Protocol, RFC 4254, 2006.
844 Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints, RFC
847 Generic Message Exchange Authentication for the Secure Shell Protocol
848 (SSH), RFC 4256, 2006.
850 The Secure Shell (SSH) Session Channel Break Extension, RFC 4335, 2006.
852 The Secure Shell (SSH) Transport Layer Encryption Modes, RFC 4344, 2006.
854 Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer
855 Protocol, RFC 4345, 2006.
857 Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer
858 Protocol, RFC 4419, 2006.
860 The Secure Shell (SSH) Public Key File Format, RFC 4716, 2006.
862 A. Perrig and D. Song, Hash Visualization: a New Technique to improve
863 Real-World Security, 1999, International Workshop on Cryptographic
864 Techniques and E-Commerce (CrypTEC '99).
867 OpenSSH is a derivative of the original and free ssh 1.2.12 release by
868 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
869 de Raadt and Dug Song removed many bugs, re-added newer features and
870 created OpenSSH. Markus Friedl contributed the support for SSH protocol
871 versions 1.5 and 2.0.
873 OpenBSD 4.5 February 12, 2009 14
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