1 SSH(1) BSD General Commands Manual SSH(1)
4 ssh - OpenSSH SSH client (remote login program)
7 ssh [-1246AaCfgkMNnqsTtVvXxY] [-b bind_address] [-c cipher_spec] [-D
8 [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] [-R
11 [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
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 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.
99 Dynamic port forwardings can also be specified in the configura-
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
105 default, 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
129 passphrases, but the user wants it in the background. This
130 implies -n. The recommended way to start X11 programs at a
131 remote site is with something like ssh -f host xterm.
133 -g Allows remote hosts to connect to local forwarded ports.
136 Specify the device ssh should use to communicate with a smartcard
137 used for storing the user's private RSA key. This option is only
138 available if support for smartcard devices is compiled in
139 (default is no support).
142 Selects a file from which the identity (private key) for RSA or
143 DSA authentication is read. The default is ~/.ssh/identity for
144 protocol version 1, and ~/.ssh/id_rsa and ~/.ssh/id_dsa for pro-
145 tocol version 2. Identity files may also be specified on a per-
146 host basis in the configuration file. It is possible to have
147 multiple -i options (and multiple identities specified in config-
150 -k Disables forwarding (delegation) of GSSAPI credentials to the
153 -L [bind_address:]port:host:hostport
154 Specifies that the given port on the local (client) host is to be
155 forwarded to the given host and port on the remote side. This
156 works by allocating a socket to listen to port on the local side,
157 optionally bound to the specified bind_address. Whenever a con-
158 nection is made to this port, the connection is forwarded over
159 the secure channel, and a connection is made to host port
160 hostport from the remote machine. Port forwardings can also be
161 specified in the configuration file. IPv6 addresses can be spec-
162 ified with an alternative syntax:
163 [bind_address/]port/host/hostport or by enclosing the address in
164 square brackets. Only the superuser can forward privileged
165 ports. By default, the local port is bound in accordance with
166 the GatewayPorts setting. However, an explicit bind_address may
167 be used to bind the connection to a specific address. The
168 bind_address of ``localhost'' indicates that the listening port
169 be bound for local use only, while an empty address or '*' indi-
170 cates that the 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 shar-
177 ing. Multiple -M options places ssh into ``master'' mode with
178 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 for-
188 warding 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) and ``exit'' (request
208 Can be used to give options in the format used in the configura-
209 tion file. This is useful for specifying options for which there
210 is no separate command-line flag. For full details of the
211 options listed below, and their possible values, see
217 ChallengeResponseAuthentication
237 GSSAPIDelegateCredentials
240 HostbasedAuthentication
246 KbdInteractiveDevices
251 NoHostAuthenticationForLocalhost
252 NumberOfPasswordPrompts
253 PasswordAuthentication
256 PreferredAuthentications
262 RhostsRSAAuthentication
268 StrictHostKeyChecking
279 Port to connect to on the remote host. This can be specified on
280 a per-host basis in the configuration file.
282 -q Quiet mode. Causes all warning and diagnostic messages to be
285 -R [bind_address:]port:host:hostport
286 Specifies that the given port on the remote (server) host is to
287 be forwarded to the given host and port on the local side. This
288 works by allocating a socket to listen to port on the remote
289 side, and whenever a connection is made to this port, the connec-
290 tion is forwarded over the secure channel, and a connection is
291 made to host port hostport from the local machine.
293 Port forwardings can also be specified in the configuration file.
294 Privileged ports can be forwarded only when logging in as root on
295 the remote machine. IPv6 addresses can be specified by enclosing
296 the address in square braces or using an alternative syntax:
297 [bind_address/]host/port/hostport.
299 By default, the listening socket on the server will be bound to
300 the loopback interface only. This may be overriden by specifying
301 a bind_address. An empty bind_address, or the address '*', indi-
302 cates that the remote socket should listen on all interfaces.
303 Specifying a remote bind_address will only succeed if the
304 server's GatewayPorts option is enabled (see sshd_config(5)).
307 Specifies the location of a control socket for connection shar-
308 ing. Refer to the description of ControlPath and ControlMaster
309 in ssh_config(5) for details.
311 -s May be used to request invocation of a subsystem on the remote
312 system. Subsystems are a feature of the SSH2 protocol which
313 facilitate the use of SSH as a secure transport for other appli-
314 cations (eg. sftp(1)). The subsystem is specified as the remote
317 -T Disable pseudo-tty allocation.
319 -t Force pseudo-tty allocation. This can be used to execute arbi-
320 trary screen-based programs on a remote machine, which can be
321 very useful, e.g. when implementing menu services. Multiple -t
322 options force tty allocation, even if ssh has no local tty.
324 -V Display the version number and exit.
326 -v Verbose mode. Causes ssh to print debugging messages about its
327 progress. This is helpful in debugging connection, authentica-
328 tion, and configuration problems. Multiple -v options increase
329 the verbosity. The maximum is 3.
331 -w local_tun[:remote_tun]
332 Requests tunnel device forwarding with the specified tun(4)
333 devices between the client (local_tun) and the server
336 The devices may be specified by numerical ID or the keyword
337 ``any'', which uses the next available tunnel device. If
338 remote_tun is not specified, it defaults to ``any''. See also
339 the Tunnel and TunnelDevice directives in ssh_config(5). If the
340 Tunnel directive is unset, it is set to the default tunnel mode,
341 which is ``point-to-point''.
343 -X Enables X11 forwarding. This can also be specified on a per-host
344 basis in a configuration file.
346 X11 forwarding should be enabled with caution. Users with the
347 ability to bypass file permissions on the remote host (for the
348 user's X authorization database) can access the local X11 display
349 through the forwarded connection. An attacker may then be able
350 to perform activities such as keystroke monitoring.
352 For this reason, X11 forwarding is subjected to X11 SECURITY
353 extension restrictions by default. Please refer to the ssh -Y
354 option and the ForwardX11Trusted directive in ssh_config(5) for
357 -x Disables X11 forwarding.
359 -Y Enables trusted X11 forwarding. Trusted X11 forwardings are not
360 subjected to the X11 SECURITY extension controls.
362 ssh may additionally obtain configuration data from a per-user configura-
363 tion file and a system-wide configuration file. The file format and con-
364 figuration options are described in ssh_config(5).
366 ssh exits with the exit status of the remote command or with 255 if an
370 The OpenSSH SSH client supports SSH protocols 1 and 2. Protocol 2 is the
371 default, with ssh falling back to protocol 1 if it detects protocol 2 is
372 unsupported. These settings may be altered using the Protocol option in
373 ssh_config(5), or enforced using the -1 and -2 options (see above). Both
374 protocols support similar authentication methods, but protocol 2 is pre-
375 ferred since it provides additional mechanisms for confidentiality (the
376 traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour) and
377 integrity (hmac-md5, hmac-sha1, hmac-ripemd160). Protocol 1 lacks a
378 strong mechanism for ensuring the integrity of the connection.
380 The methods available for authentication are: GSSAPI-based authentica-
381 tion, host-based authentication, public key authentication, challenge-
382 response authentication, and password authentication. Authentication
383 methods are tried in the order specified above, though protocol 2 has a
384 configuration option to change the default order:
385 PreferredAuthentications.
387 Host-based authentication works as follows: If the machine the user logs
388 in from is listed in /etc/hosts.equiv or /etc/shosts.equiv on the remote
389 machine, and the user names are the same on both sides, or if the files
390 ~/.rhosts or ~/.shosts exist in the user's home directory on the remote
391 machine and contain a line containing the name of the client machine and
392 the name of the user on that machine, the user is considered for login.
393 Additionally, the server must be able to verify the client's host key
394 (see the description of /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts,
395 below) for login to be permitted. This authentication method closes
396 security holes due to IP spoofing, DNS spoofing, and routing spoofing.
397 [Note to the administrator: /etc/hosts.equiv, ~/.rhosts, and the
398 rlogin/rsh protocol in general, are inherently insecure and should be
399 disabled if security is desired.]
401 Public key authentication works as follows: The scheme is based on pub-
402 lic-key cryptography, using cryptosystems where encryption and decryption
403 are done using separate keys, and it is unfeasible to derive the decryp-
404 tion key from the encryption key. The idea is that each user creates a
405 public/private key pair for authentication purposes. The server knows
406 the public key, and only the user knows the private key. ssh implements
407 public key authentication protocol automatically, using either the RSA or
408 DSA algorithms. Protocol 1 is restricted to using only RSA keys, but
409 protocol 2 may use either. The HISTORY section of ssl(8) contains a
410 brief discussion of the two algorithms.
412 The file ~/.ssh/authorized_keys lists the public keys that are permitted
413 for logging in. When the user logs in, the ssh program tells the server
414 which key pair it would like to use for authentication. The client
415 proves that it has access to the private key and the server checks that
416 the corresponding public key is authorized to accept the account.
418 The user creates his/her key pair by running ssh-keygen(1). This stores
419 the private key in ~/.ssh/identity (protocol 1), ~/.ssh/id_dsa (protocol
420 2 DSA), or ~/.ssh/id_rsa (protocol 2 RSA) and stores the public key in
421 ~/.ssh/identity.pub (protocol 1), ~/.ssh/id_dsa.pub (protocol 2 DSA), or
422 ~/.ssh/id_rsa.pub (protocol 2 RSA) in the user's home directory. The
423 user should then copy the public key to ~/.ssh/authorized_keys in his/her
424 home directory on the remote machine. The authorized_keys file corre-
425 sponds to the conventional ~/.rhosts file, and has one key per line,
426 though the lines can be very long. After this, the user can log in with-
427 out giving the password.
429 The most convenient way to use public key authentication may be with an
430 authentication agent. See ssh-agent(1) for more information.
432 Challenge-response authentication works as follows: The server sends an
433 arbitrary "challenge" text, and prompts for a response. Protocol 2
434 allows multiple challenges and responses; protocol 1 is restricted to
435 just one challenge/response. Examples of challenge-response authentica-
436 tion include BSD Authentication (see login.conf(5)) and PAM (some non-
439 Finally, if other authentication methods fail, ssh prompts the user for a
440 password. The password is sent to the remote host for checking; however,
441 since all communications are encrypted, the password cannot be seen by
442 someone listening on the network.
444 ssh automatically maintains and checks a database containing identifica-
445 tion for all hosts it has ever been used with. Host keys are stored in
446 ~/.ssh/known_hosts in the user's home directory. Additionally, the file
447 /etc/ssh/ssh_known_hosts is automatically checked for known hosts. Any
448 new hosts are automatically added to the user's file. If a host's iden-
449 tification ever changes, ssh warns about this and disables password
450 authentication to prevent server spoofing or man-in-the-middle attacks,
451 which could otherwise be used to circumvent the encryption. The
452 StrictHostKeyChecking option can be used to control logins to machines
453 whose host key is not known or has changed.
455 When the user's identity has been accepted by the server, the server
456 either executes the given command, or logs into the machine and gives the
457 user a normal shell on the remote machine. All communication with the
458 remote command or shell will be automatically encrypted.
460 If a pseudo-terminal has been allocated (normal login session), the user
461 may use the escape characters noted below.
463 If no pseudo-tty has been allocated, the session is transparent and can
464 be used to reliably transfer binary data. On most systems, setting the
465 escape character to ``none'' will also make the session transparent even
468 The session terminates when the command or shell on the remote machine
469 exits and all X11 and TCP connections have been closed.
472 When a pseudo-terminal has been requested, ssh supports a number of func-
473 tions through the use of an escape character.
475 A single tilde character can be sent as ~~ or by following the tilde by a
476 character other than those described below. The escape character must
477 always follow a newline to be interpreted as special. The escape charac-
478 ter can be changed in configuration files using the EscapeChar configura-
479 tion directive or on the command line by the -e option.
481 The supported escapes (assuming the default '~') are:
487 ~# List forwarded connections.
489 ~& Background ssh at logout when waiting for forwarded connection /
490 X11 sessions to terminate.
492 ~? Display a list of escape characters.
494 ~B Send a BREAK to the remote system (only useful for SSH protocol
495 version 2 and if the peer supports it).
497 ~C Open command line. Currently this allows the addition of port
498 forwardings using the -L and -R options (see above). It also
499 allows the cancellation of existing remote port-forwardings using
500 -KR[bind_address:]port. !command allows the user to execute a
501 local command if the PermitLocalCommand option is enabled in
502 ssh_config(5). Basic help is available, using the -h option.
504 ~R Request rekeying of the connection (only useful for SSH protocol
505 version 2 and if the peer supports it).
508 Forwarding of arbitrary TCP connections over the secure channel can be
509 specified either on the command line or in a configuration file. One
510 possible application of TCP forwarding is a secure connection to a mail
511 server; another is going through firewalls.
513 In the example below, we look at encrypting communication between an IRC
514 client and server, even though the IRC server does not directly support
515 encrypted communications. This works as follows: the user connects to
516 the remote host using ssh, specifying a port to be used to forward con-
517 nections to the remote server. After that it is possible to start the
518 service which is to be encrypted on the client machine, connecting to the
519 same local port, and ssh will encrypt and forward the connection.
521 The following example tunnels an IRC session from client machine
522 ``127.0.0.1'' (localhost) to remote server ``server.example.com'':
524 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
525 $ irc -c '#users' -p 1234 pinky 127.0.0.1
527 This tunnels a connection to IRC server ``server.example.com'', joining
528 channel ``#users'', nickname ``pinky'', using port 1234. It doesn't mat-
529 ter which port is used, as long as it's greater than 1023 (remember, only
530 root can open sockets on privileged ports) and doesn't conflict with any
531 ports already in use. The connection is forwarded to port 6667 on the
532 remote server, since that's the standard port for IRC services.
534 The -f option backgrounds ssh and the remote command ``sleep 10'' is
535 specified to allow an amount of time (10 seconds, in the example) to
536 start the service which is to be tunnelled. If no connections are made
537 within the time specified, ssh will exit.
540 If the ForwardX11 variable is set to ``yes'' (or see the description of
541 the -X, -x, and -Y options above) and the user is using X11 (the DISPLAY
542 environment variable is set), the connection to the X11 display is auto-
543 matically forwarded to the remote side in such a way that any X11 pro-
544 grams started from the shell (or command) will go through the encrypted
545 channel, and the connection to the real X server will be made from the
546 local machine. The user should not manually set DISPLAY. Forwarding of
547 X11 connections can be configured on the command line or in configuration
550 The DISPLAY value set by ssh will point to the server machine, but with a
551 display number greater than zero. This is normal, and happens because
552 ssh creates a ``proxy'' X server on the server machine for forwarding the
553 connections over the encrypted channel.
555 ssh will also automatically set up Xauthority data on the server machine.
556 For this purpose, it will generate a random authorization cookie, store
557 it in Xauthority on the server, and verify that any forwarded connections
558 carry this cookie and replace it by the real cookie when the connection
559 is opened. The real authentication cookie is never sent to the server
560 machine (and no cookies are sent in the plain).
562 If the ForwardAgent variable is set to ``yes'' (or see the description of
563 the -A and -a options above) and the user is using an authentication
564 agent, the connection to the agent is automatically forwarded to the
568 When connecting to a server for the first time, a fingerprint of the
569 server's public key is presented to the user (unless the option
570 StrictHostKeyChecking has been disabled). Fingerprints can be determined
573 $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
575 If the fingerprint is already known, it can be matched and verified, and
576 the key can be accepted. If the fingerprint is unknown, an alternative
577 method of verification is available: SSH fingerprints verified by DNS.
578 An additional resource record (RR), SSHFP, is added to a zonefile and the
579 connecting client is able to match the fingerprint with that of the key
582 In this example, we are connecting a client to a server,
583 ``host.example.com''. The SSHFP resource records should first be added
584 to the zonefile for host.example.com:
586 $ ssh-keygen -r host.example.com.
588 The output lines will have to be added to the zonefile. To check that
589 the zone is answering fingerprint queries:
591 $ dig -t SSHFP host.example.com
593 Finally the client connects:
595 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
597 Matching host key fingerprint found in DNS.
598 Are you sure you want to continue connecting (yes/no)?
600 See the VerifyHostKeyDNS option in ssh_config(5) for more information.
602 SSH-BASED VIRTUAL PRIVATE NETWORKS
603 ssh contains support for Virtual Private Network (VPN) tunnelling using
604 the tun(4) network pseudo-device, allowing two networks to be joined
605 securely. The sshd_config(5) configuration option PermitTunnel controls
606 whether the server supports this, and at what level (layer 2 or 3 traf-
609 The following example would connect client network 10.0.50.0/24 with
610 remote network 10.0.99.0/24, provided that the SSH server running on the
611 gateway to the remote network, at 192.168.1.15, allows it:
613 # ssh -f -w 0:1 192.168.1.15 true
614 # ifconfig tun0 10.0.50.1 10.0.99.1 netmask 255.255.255.252
616 Client access may be more finely tuned via the /root/.ssh/authorized_keys
617 file (see below) and the PermitRootLogin server option. The following
618 entry would permit connections on tun(4) device 1 from user ``jane'' and
619 on tun device 2 from user ``john'', if PermitRootLogin is set to
620 ``forced-commands-only'':
622 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
623 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
625 Since a SSH-based setup entails a fair amount of overhead, it may be more
626 suited to temporary setups, such as for wireless VPNs. More permanent
627 VPNs are better provided by tools such as ipsecctl(8) and isakmpd(8).
630 ssh will normally set the following environment variables:
632 DISPLAY The DISPLAY variable indicates the location of the
633 X11 server. It is automatically set by ssh to
634 point to a value of the form ``hostname:n'', where
635 ``hostname'' indicates the host where the shell
636 runs, and 'n' is an integer >= 1. ssh uses this
637 special value to forward X11 connections over the
638 secure channel. The user should normally not set
639 DISPLAY explicitly, as that will render the X11
640 connection insecure (and will require the user to
641 manually copy any required authorization cookies).
643 HOME Set to the path of the user's home directory.
645 LOGNAME Synonym for USER; set for compatibility with sys-
646 tems that use this variable.
648 MAIL Set to the path of the user's mailbox.
650 PATH Set to the default PATH, as specified when compil-
653 SSH_ASKPASS If ssh needs a passphrase, it will read the
654 passphrase from the current terminal if it was run
655 from a terminal. If ssh does not have a terminal
656 associated with it but DISPLAY and SSH_ASKPASS are
657 set, it will execute the program specified by
658 SSH_ASKPASS and open an X11 window to read the
659 passphrase. This is particularly useful when call-
660 ing ssh from a .xsession or related script. (Note
661 that on some machines it may be necessary to redi-
662 rect the input from /dev/null to make this work.)
664 SSH_AUTH_SOCK Identifies the path of a UNIX-domain socket used to
665 communicate with the agent.
667 SSH_CONNECTION Identifies the client and server ends of the con-
668 nection. The variable contains four space-sepa-
669 rated values: client IP address, client port num-
670 ber, server IP address, and server port number.
672 SSH_ORIGINAL_COMMAND This variable contains the original command line if
673 a forced command is executed. It can be used to
674 extract the original arguments.
676 SSH_TTY This is set to the name of the tty (path to the
677 device) associated with the current shell or com-
678 mand. If the current session has no tty, this
681 TZ This variable is set to indicate the present time
682 zone if it was set when the daemon was started
683 (i.e. the daemon passes the value on to new connec-
686 USER Set to the name of the user logging in.
688 Additionally, ssh reads ~/.ssh/environment, and adds lines of the format
689 ``VARNAME=value'' to the environment if the file exists and users are
690 allowed to change their environment. For more information, see the
691 PermitUserEnvironment option in sshd_config(5).
695 This file is used for host-based authentication (see above). On
696 some machines this file may need to be world-readable if the
697 user's home directory is on an NFS partition, because sshd(8)
698 reads it as root. Additionally, this file must be owned by the
699 user, and must not have write permissions for anyone else. The
700 recommended permission for most machines is read/write for the
701 user, and not accessible by others.
704 This file is used in exactly the same way as .rhosts, but allows
705 host-based authentication without permitting login with
708 ~/.ssh/authorized_keys
709 Lists the public keys (RSA/DSA) that can be used for logging in
710 as this user. The format of this file is described in the
711 sshd(8) manual page. This file is not highly sensitive, but the
712 recommended permissions are read/write for the user, and not
713 accessible by others.
716 This is the per-user configuration file. The file format and
717 configuration options are described in ssh_config(5). Because of
718 the potential for abuse, this file must have strict permissions:
719 read/write for the user, and not accessible by others.
722 Contains additional definitions for environment variables; see
728 Contains the private key for authentication. These files contain
729 sensitive data and should be readable by the user but not acces-
730 sible by others (read/write/execute). ssh will simply ignore a
731 private key file if it is accessible by others. It is possible
732 to specify a passphrase when generating the key which will be
733 used to encrypt the sensitive part of this file using 3DES.
738 Contains the public key for authentication. These files are not
739 sensitive and can (but need not) be readable by anyone.
742 Contains a list of host keys for all hosts the user has logged
743 into that are not already in the systemwide list of known host
744 keys. See sshd(8) for further details of the format of this
748 Commands in this file are executed by ssh when the user logs in,
749 just before the user's shell (or command) is started. See the
750 sshd(8) manual page for more information.
753 This file is for host-based authentication (see above). It
754 should only be writable by root.
757 This file is used in exactly the same way as hosts.equiv, but
758 allows host-based authentication without permitting login with
762 Systemwide configuration file. The file format and configuration
763 options are described in ssh_config(5).
765 /etc/ssh/ssh_host_key
766 /etc/ssh/ssh_host_dsa_key
767 /etc/ssh/ssh_host_rsa_key
768 These three files contain the private parts of the host keys and
769 are used for host-based authentication. If protocol version 1 is
770 used, ssh must be setuid root, since the host key is readable
771 only by root. For protocol version 2, ssh uses ssh-keysign(8) to
772 access the host keys, eliminating the requirement that ssh be
773 setuid root when host-based authentication is used. By default
774 ssh is not setuid root.
776 /etc/ssh/ssh_known_hosts
777 Systemwide list of known host keys. This file should be prepared
778 by the system administrator to contain the public host keys of
779 all machines in the organization. It should be world-readable.
780 See sshd(8) for further details of the format of this file.
783 Commands in this file are executed by ssh when the user logs in,
784 just before the user's shell (or command) is started. See the
785 sshd(8) manual page for more information.
788 scp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), ssh-keyscan(1),
789 tun(4), hosts.equiv(5), ssh_config(5), ssh-keysign(8), sshd(8)
791 The Secure Shell (SSH) Protocol Assigned Numbers, RFC 4250, 2006.
793 The Secure Shell (SSH) Protocol Architecture, RFC 4251, 2006.
795 The Secure Shell (SSH) Authentication Protocol, RFC 4252, 2006.
797 The Secure Shell (SSH) Transport Layer Protocol, RFC 4253, 2006.
799 The Secure Shell (SSH) Connection Protocol, RFC 4254, 2006.
801 Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints, RFC
804 Generic Message Exchange Authentication for the Secure Shell Protocol
805 (SSH), RFC 4256, 2006.
807 The Secure Shell (SSH) Session Channel Break Extension, RFC 4335, 2006.
809 The Secure Shell (SSH) Transport Layer Encryption Modes, RFC 4344, 2006.
811 Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer
812 Protocol, RFC 4345, 2006.
814 Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer
815 Protocol, RFC 4419, 2006.
818 OpenSSH is a derivative of the original and free ssh 1.2.12 release by
819 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
820 de Raadt and Dug Song removed many bugs, re-added newer features and cre-
821 ated OpenSSH. Markus Friedl contributed the support for SSH protocol
822 versions 1.5 and 2.0.
824 BSD September 25, 1999 BSD
andersk / gssapi-openssh.git / blob