1SSH(1) BSD General Commands Manual SSH(1)
2
4 ssh — OpenSSH SSH client (remote login program)
5
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]
13
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.
20 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
24 below).
25 If command is specified, it is executed on the remote host instead of a
27 login shell.
28 The options are as follows:
30 -1 Forces ssh to try protocol version 1 only.
32 -2 Forces ssh to try protocol version 2 only.
34 -4 Forces ssh to use IPv4 addresses only.
36 -6 Forces ssh to use IPv6 addresses only.
38 -A Enables forwarding of the authentication agent connection. This
40 can also be specified on a per-host basis in a configuration
41 file.
42 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
49 the agent.
50 -a Disables forwarding of the authentication agent connection.
52 -b bind_address
54 Use bind_address on the local machine as the source address of
55 the connection. Only useful on systems with more than one
56 address.
57 -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 pro‐
62 tocol 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.
66 -c cipher_spec
68 Selects the cipher specification for encrypting the session.
69 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”.
79 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:
85 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
87 arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
88 aes192-ctr,aes256-ctr
89 -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‐
100 tion file.
101 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 be
109 bound for local use only, while an empty address or ‘*’ indicates
110 that the port should be available from all interfaces.
111 -e escape_char
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
119 fully transparent.
120 -F configfile
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.
126 -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.
132 -g Allows remote hosts to connect to local forwarded ports.
134 -I smartcard_device
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).
140 -i identity_file
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‐
148 uration files).
149 -k Disables forwarding (delegation) of GSSAPI credentials to the
151 server.
152 -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 be
169 bound for local use only, while an empty address or ‘*’ indicates
170 that the port should be available from all interfaces.
171 -l login_name
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.
175 -M Places the ssh client into “master” mode for connection sharing.
177 Multiple -M options places ssh into “master” mode with confirma‐
178 tion required before slave connections are accepted. Refer to
179 the description of ControlMaster in ssh_config(5) for details.
180 -m mac_spec
182 Additionally, for protocol version 2 a comma-separated list of
183 MAC (message authentication code) algorithms can be specified in
184 order of preference. See the MACs keyword for more information.
185 -N Do not execute a remote command. This is useful for just for‐
187 warding ports (protocol version 2 only).
188 -n Redirects stdin from /dev/null (actually, prevents reading from
190 stdin). This must be used when ssh is run in the background. A
191 common trick is to use this to run X11 programs on a remote
192 machine. For example, ssh -n shadows.cs.hut.fi emacs & will
193 start an emacs on shadows.cs.hut.fi, and the X11 connection will
194 be automatically forwarded over an encrypted channel. The ssh
195 program will be put in the background. (This does not work if
196 ssh needs to ask for a password or passphrase; see also the -f
197 option.)
198 -O ctl_cmd
200 Control an active connection multiplexing master process. When
201 the -O option is specified, the ctl_cmd argument is interpreted
202 and passed to the master process. Valid commands are: “check”
203 (check that the master process is running) and “exit” (request
204 the master to exit).
205 -o option
207 Can be used to give options in the format used in the configura‐
208 tion file. This is useful for specifying options for which there
209 is no separate command-line flag. For full details of the
210 options listed below, and their possible values, see
211 ssh_config(5).
212 AddressFamily
214 BatchMode
215 BindAddress
216 ChallengeResponseAuthentication
217 CheckHostIP
218 Cipher
219 Ciphers
220 ClearAllForwardings
221 Compression
222 CompressionLevel
223 ConnectionAttempts
224 ConnectTimeout
225 ControlMaster
226 ControlPath
227 DynamicForward
228 EscapeChar
229 ExitOnForwardFailure
230 ForwardAgent
231 ForwardX11
232 ForwardX11Trusted
233 GatewayPorts
234 GlobalKnownHostsFile
235 GSSAPIAuthentication
236 GSSAPIDelegateCredentials
237 HashKnownHosts
238 Host
239 HostbasedAuthentication
240 HostKeyAlgorithms
241 HostKeyAlias
242 HostName
243 IdentityFile
244 IdentitiesOnly
245 KbdInteractiveDevices
246 LocalCommand
247 LocalForward
248 LogLevel
249 MACs
250 NoHostAuthenticationForLocalhost
251 NumberOfPasswordPrompts
252 PasswordAuthentication
253 PermitLocalCommand
254 Port
255 PreferredAuthentications
256 Protocol
257 ProxyCommand
258 PubkeyAuthentication
259 RekeyLimit
260 RemoteForward
261 RhostsRSAAuthentication
262 RSAAuthentication
263 SendEnv
264 ServerAliveInterval
265 ServerAliveCountMax
266 SmartcardDevice
267 StrictHostKeyChecking
268 TCPKeepAlive
269 Tunnel
270 TunnelDevice
271 UsePrivilegedPort
272 User
273 UserKnownHostsFile
274 VerifyHostKeyDNS
275 XAuthLocation
276 -p port
278 Port to connect to on the remote host. This can be specified on
279 a per-host basis in the configuration file.
280 -q Quiet mode. Causes all warning and diagnostic messages to be
282 suppressed.
283 -R [bind_address:]port:host:hostport
285 Specifies that the given port on the remote (server) host is to
286 be forwarded to the given host and port on the local side. This
287 works by allocating a socket to listen to port on the remote
288 side, and whenever a connection is made to this port, the connec‐
289 tion is forwarded over the secure channel, and a connection is
290 made to host port hostport from the local machine.
291 Port forwardings can also be specified in the configuration file.
293 Privileged ports can be forwarded only when logging in as root on
294 the remote machine. IPv6 addresses can be specified by enclosing
295 the address in square braces or using an alternative syntax:
296 [bind_address/]host/port/hostport.
297 By default, the listening socket on the server will be bound to
299 the loopback interface only. This may be overriden by specifying
300 a bind_address. An empty bind_address, or the address ‘*’, indi‐
301 cates that the remote socket should listen on all interfaces.
302 Specifying a remote bind_address will only succeed if the
303 server's GatewayPorts option is enabled (see sshd_config(5)).
304 -S ctl_path
306 Specifies the location of a control socket for connection shar‐
307 ing. Refer to the description of ControlPath and ControlMaster
308 in ssh_config(5) for details.
309 -s May be used to request invocation of a subsystem on the remote
311 system. Subsystems are a feature of the SSH2 protocol which
312 facilitate the use of SSH as a secure transport for other appli‐
313 cations (eg. sftp(1)). The subsystem is specified as the remote
314 command.
315 -T Disable pseudo-tty allocation.
317 -t Force pseudo-tty allocation. This can be used to execute arbi‐
319 trary screen-based programs on a remote machine, which can be
320 very useful, e.g. when implementing menu services. Multiple -t
321 options force tty allocation, even if ssh has no local tty.
322 -V Display the version number and exit.
324 -v Verbose mode. Causes ssh to print debugging messages about its
326 progress. This is helpful in debugging connection, authentica‐
327 tion, and configuration problems. Multiple -v options increase
328 the verbosity. The maximum is 3.
329 -w local_tun[:remote_tun]
331 Requests tunnel device forwarding with the specified tun(4)
332 devices between the client (local_tun) and the server
333 (remote_tun).
334 The devices may be specified by numerical ID or the keyword
336 “any”, which uses the next available tunnel device. If
337 remote_tun is not specified, it defaults to “any”. See also the
338 Tunnel and TunnelDevice directives in ssh_config(5). If the
339 Tunnel directive is unset, it is set to the default tunnel mode,
340 which is “point-to-point”.
341 -X Enables X11 forwarding. This can also be specified on a per-host
343 basis in a configuration file.
344 X11 forwarding should be enabled with caution. Users with the
346 ability to bypass file permissions on the remote host (for the
347 user's X authorization database) can access the local X11 display
348 through the forwarded connection. An attacker may then be able
349 to perform activities such as keystroke monitoring.
350 For this reason, X11 forwarding is subjected to X11 SECURITY
352 extension restrictions by default. Please refer to the ssh -Y
353 option and the ForwardX11Trusted directive in ssh_config(5) for
354 more information.
355 -x Disables X11 forwarding.
357 -Y Enables trusted X11 forwarding. Trusted X11 forwardings are not
359 subjected to the X11 SECURITY extension controls.
360 ssh may additionally obtain configuration data from a per-user configura‐
362 tion file and a system-wide configuration file. The file format and con‐
363 figuration options are described in ssh_config(5).
364 ssh exits with the exit status of the remote command or with 255 if an
366 error occurred.
367
369 The OpenSSH SSH client supports SSH protocols 1 and 2. Protocol 2 is the
370 default, with ssh falling back to protocol 1 if it detects protocol 2 is
371 unsupported. These settings may be altered using the Protocol option in
372 ssh_config(5), or enforced using the -1 and -2 options (see above). Both
373 protocols support similar authentication methods, but protocol 2 is pre‐
374 ferred since it provides additional mechanisms for confidentiality (the
375 traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour) and
376 integrity (hmac-md5, hmac-sha1, hmac-ripemd160). Protocol 1 lacks a
377 strong mechanism for ensuring the integrity of the connection.
378 The methods available for authentication are: GSSAPI-based authentica‐
380 tion, host-based authentication, public key authentication, challenge-
381 response authentication, and password authentication. Authentication
382 methods are tried in the order specified above, though protocol 2 has a
383 configuration option to change the default order:
384 PreferredAuthentications.
385 Host-based authentication works as follows: If the machine the user logs
387 in from is listed in /etc/hosts.equiv or /etc/ssh/shosts.equiv on the
388 remote machine, and the user names are the same on both sides, or if the
389 files ~/.rhosts or ~/.shosts exist in the user's home directory on the
390 remote machine and contain a line containing the name of the client
391 machine and the name of the user on that machine, the user is considered
392 for login. Additionally, the server must be able to verify the client's
393 host key (see the description of /etc/ssh/ssh_known_hosts and
394 ~/.ssh/known_hosts, below) for login to be permitted. This authentica‐
395 tion method closes security holes due to IP spoofing, DNS spoofing, and
396 routing spoofing. [Note to the administrator: /etc/hosts.equiv,
397 ~/.rhosts, and the rlogin/rsh protocol in general, are inherently inse‐
398 cure and should be disabled if security is desired.]
399 Public key authentication works as follows: The scheme is based on pub‐
401 lic-key cryptography, using cryptosystems where encryption and decryption
402 are done using separate keys, and it is unfeasible to derive the decryp‐
403 tion key from the encryption key. The idea is that each user creates a
404 public/private key pair for authentication purposes. The server knows
405 the public key, and only the user knows the private key. ssh implements
406 public key authentication protocol automatically, using either the RSA or
407 DSA algorithms. Protocol 1 is restricted to using only RSA keys, but
408 protocol 2 may use either. The HISTORY section of ssl(8) contains a
409 brief discussion of the two algorithms.
410 The file ~/.ssh/authorized_keys lists the public keys that are permitted
412 for logging in. When the user logs in, the ssh program tells the server
413 which key pair it would like to use for authentication. The client
414 proves that it has access to the private key and the server checks that
415 the corresponding public key is authorized to accept the account.
416 The user creates his/her key pair by running ssh-keygen(1). This stores
418 the private key in ~/.ssh/identity (protocol 1), ~/.ssh/id_dsa (protocol
419 2 DSA), or ~/.ssh/id_rsa (protocol 2 RSA) and stores the public key in
420 ~/.ssh/identity.pub (protocol 1), ~/.ssh/id_dsa.pub (protocol 2 DSA), or
421 ~/.ssh/id_rsa.pub (protocol 2 RSA) in the user's home directory. The
422 user should then copy the public key to ~/.ssh/authorized_keys in his/her
423 home directory on the remote machine. The authorized_keys file corre‐
424 sponds to the conventional ~/.rhosts file, and has one key per line,
425 though the lines can be very long. After this, the user can log in with‐
426 out giving the password.
427 The most convenient way to use public key authentication may be with an
429 authentication agent. See ssh-agent(1) for more information.
430 Challenge-response authentication works as follows: The server sends an
432 arbitrary "challenge" text, and prompts for a response. Protocol 2
433 allows multiple challenges and responses; protocol 1 is restricted to
434 just one challenge/response. Examples of challenge-response authentica‐
435 tion include BSD Authentication (see login.conf(5)) and PAM (some non-
436 OpenBSD systems).
437 Finally, if other authentication methods fail, ssh prompts the user for a
439 password. The password is sent to the remote host for checking; however,
440 since all communications are encrypted, the password cannot be seen by
441 someone listening on the network.
442 ssh automatically maintains and checks a database containing identifica‐
444 tion for all hosts it has ever been used with. Host keys are stored in
445 ~/.ssh/known_hosts in the user's home directory. Additionally, the file
446 /etc/ssh/ssh_known_hosts is automatically checked for known hosts. Any
447 new hosts are automatically added to the user's file. If a host's iden‐
448 tification ever changes, ssh warns about this and disables password
449 authentication to prevent server spoofing or man-in-the-middle attacks,
450 which could otherwise be used to circumvent the encryption. The
451 StrictHostKeyChecking option can be used to control logins to machines
452 whose host key is not known or has changed.
453 When the user's identity has been accepted by the server, the server
455 either executes the given command, or logs into the machine and gives the
456 user a normal shell on the remote machine. All communication with the
457 remote command or shell will be automatically encrypted.
458 If a pseudo-terminal has been allocated (normal login session), the user
460 may use the escape characters noted below.
461 If no pseudo-tty has been allocated, the session is transparent and can
463 be used to reliably transfer binary data. On most systems, setting the
464 escape character to “none” will also make the session transparent even if
465 a tty is used.
466 The session terminates when the command or shell on the remote machine
468 exits and all X11 and TCP connections have been closed.
469
471 When a pseudo-terminal has been requested, ssh supports a number of func‐
472 tions through the use of an escape character.
473 A single tilde character can be sent as ~~ or by following the tilde by a
475 character other than those described below. The escape character must
476 always follow a newline to be interpreted as special. The escape charac‐
477 ter can be changed in configuration files using the EscapeChar configura‐
478 tion directive or on the command line by the -e option.
479 The supported escapes (assuming the default ‘~’) are:
481 ~. Disconnect.
483 ~^Z Background ssh.
485 ~# List forwarded connections.
487 ~& Background ssh at logout when waiting for forwarded connection /
489 X11 sessions to terminate.
490 ~? Display a list of escape characters.
492 ~B Send a BREAK to the remote system (only useful for SSH protocol
494 version 2 and if the peer supports it).
495 ~C Open command line. Currently this allows the addition of port
497 forwardings using the -L and -R options (see above). It also
498 allows the cancellation of existing remote port-forwardings using
499 -KR[bind_address:]port. !command allows the user to execute a
500 local command if the PermitLocalCommand option is enabled in
501 ssh_config(5). Basic help is available, using the -h option.
502 ~R Request rekeying of the connection (only useful for SSH protocol
504 version 2 and if the peer supports it).
505
507 Forwarding of arbitrary TCP connections over the secure channel can be
508 specified either on the command line or in a configuration file. One
509 possible application of TCP forwarding is a secure connection to a mail
510 server; another is going through firewalls.
511 In the example below, we look at encrypting communication between an IRC
513 client and server, even though the IRC server does not directly support
514 encrypted communications. This works as follows: the user connects to
515 the remote host using ssh, specifying a port to be used to forward con‐
516 nections to the remote server. After that it is possible to start the
517 service which is to be encrypted on the client machine, connecting to the
518 same local port, and ssh will encrypt and forward the connection.
519 The following example tunnels an IRC session from client machine
521 “127.0.0.1” (localhost) to remote server “server.example.com”:
522 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
524 $ irc -c '#users' -p 1234 pinky 127.0.0.1
525 This tunnels a connection to IRC server “server.example.com”, joining
527 channel “#users”, nickname “pinky”, using port 1234. It doesn't matter
528 which port is used, as long as it's greater than 1023 (remember, only
529 root can open sockets on privileged ports) and doesn't conflict with any
530 ports already in use. The connection is forwarded to port 6667 on the
531 remote server, since that's the standard port for IRC services.
532 The -f option backgrounds ssh and the remote command “sleep 10” is speci‐
534 fied to allow an amount of time (10 seconds, in the example) to start the
535 service which is to be tunnelled. If no connections are made within the
536 time specified, ssh will exit.
537
539 If the ForwardX11 variable is set to “yes” (or see the description of the
540 -X, -x, and -Y options above) and the user is using X11 (the DISPLAY
541 environment variable is set), the connection to the X11 display is auto‐
542 matically forwarded to the remote side in such a way that any X11 pro‐
543 grams started from the shell (or command) will go through the encrypted
544 channel, and the connection to the real X server will be made from the
545 local machine. The user should not manually set DISPLAY. Forwarding of
546 X11 connections can be configured on the command line or in configuration
547 files.
548 The DISPLAY value set by ssh will point to the server machine, but with a
550 display number greater than zero. This is normal, and happens because
551 ssh creates a “proxy” X server on the server machine for forwarding the
552 connections over the encrypted channel.
553 ssh will also automatically set up Xauthority data on the server machine.
555 For this purpose, it will generate a random authorization cookie, store
556 it in Xauthority on the server, and verify that any forwarded connections
557 carry this cookie and replace it by the real cookie when the connection
558 is opened. The real authentication cookie is never sent to the server
559 machine (and no cookies are sent in the plain).
560 If the ForwardAgent variable is set to “yes” (or see the description of
562 the -A and -a options above) and the user is using an authentication
563 agent, the connection to the agent is automatically forwarded to the
564 remote side.
565
567 When connecting to a server for the first time, a fingerprint of the
568 server's public key is presented to the user (unless the option
569 StrictHostKeyChecking has been disabled). Fingerprints can be determined
570 using ssh-keygen(1):
571 $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
573 If the fingerprint is already known, it can be matched and verified, and
575 the key can be accepted. If the fingerprint is unknown, an alternative
576 method of verification is available: SSH fingerprints verified by DNS.
577 An additional resource record (RR), SSHFP, is added to a zonefile and the
578 connecting client is able to match the fingerprint with that of the key
579 presented.
580 In this example, we are connecting a client to a server,
582 “host.example.com”. The SSHFP resource records should first be added to
583 the zonefile for host.example.com:
584 $ ssh-keygen -r host.example.com.
586 The output lines will have to be added to the zonefile. To check that
588 the zone is answering fingerprint queries:
589 $ dig -t SSHFP host.example.com
591 Finally the client connects:
593 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
595 [...]
596 Matching host key fingerprint found in DNS.
597 Are you sure you want to continue connecting (yes/no)?
598 See the VerifyHostKeyDNS option in ssh_config(5) for more information.
600
602 ssh contains support for Virtual Private Network (VPN) tunnelling using
603 the tun(4) network pseudo-device, allowing two networks to be joined
604 securely. The sshd_config(5) configuration option PermitTunnel controls
605 whether the server supports this, and at what level (layer 2 or 3 traf‐
606 fic).
607 The following example would connect client network 10.0.50.0/24 with
609 remote network 10.0.99.0/24 using a point-to-point connection from
610 10.1.1.1 to 10.1.1.2, provided that the SSH server running on the gateway
611 to the remote network, at 192.168.1.15, allows it.
612 On the client:
614 # ssh -f -w 0:1 192.168.1.15 true
616 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
617 # route add 10.0.99.0/24 10.1.1.2
618 On the server:
620 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
622 # route add 10.0.50.0/24 10.1.1.1
623 Client access may be more finely tuned via the /root/.ssh/authorized_keys
625 file (see below) and the PermitRootLogin server option. The following
626 entry would permit connections on tun(4) device 1 from user “jane” and on
627 tun device 2 from user “john”, if PermitRootLogin is set to
628 “forced-commands-only”:
629 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
631 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
632 Since an SSH-based setup entails a fair amount of overhead, it may be
634 more suited to temporary setups, such as for wireless VPNs. More perma‐
635 nent VPNs are better provided by tools such as ipsecctl(8) and
636 isakmpd(8).
637
639 ssh will normally set the following environment variables:
640 DISPLAY The DISPLAY variable indicates the location of the
642 X11 server. It is automatically set by ssh to
643 point to a value of the form “hostname:n”, where
644 “hostname” indicates the host where the shell runs,
645 and ‘n’ is an integer ≥ 1. ssh uses this special
646 value to forward X11 connections over the secure
647 channel. The user should normally not set DISPLAY
648 explicitly, as that will render the X11 connection
649 insecure (and will require the user to manually
650 copy any required authorization cookies).
651 HOME Set to the path of the user's home directory.
653 LOGNAME Synonym for USER; set for compatibility with sys‐
655 tems that use this variable.
656 MAIL Set to the path of the user's mailbox.
658 PATH Set to the default PATH, as specified when compil‐
660 ing ssh.
661 SSH_ASKPASS If ssh needs a passphrase, it will read the
663 passphrase from the current terminal if it was run
664 from a terminal. If ssh does not have a terminal
665 associated with it but DISPLAY and SSH_ASKPASS are
666 set, it will execute the program specified by
667 SSH_ASKPASS and open an X11 window to read the
668 passphrase. This is particularly useful when call‐
669 ing ssh from a .xsession or related script. (Note
670 that on some machines it may be necessary to redi‐
671 rect the input from /dev/null to make this work.)
672 SSH_AUTH_SOCK Identifies the path of a UNIX-domain socket used to
674 communicate with the agent.
675 SSH_CONNECTION Identifies the client and server ends of the con‐
677 nection. The variable contains four space-sepa‐
678 rated values: client IP address, client port num‐
679 ber, server IP address, and server port number.
680 SSH_ORIGINAL_COMMAND This variable contains the original command line if
682 a forced command is executed. It can be used to
683 extract the original arguments.
684 SSH_TTY This is set to the name of the tty (path to the
686 device) associated with the current shell or com‐
687 mand. If the current session has no tty, this
688 variable is not set.
689 TZ This variable is set to indicate the present time
691 zone if it was set when the daemon was started
692 (i.e. the daemon passes the value on to new connec‐
693 tions).
694 USER Set to the name of the user logging in.
696 Additionally, ssh reads ~/.ssh/environment, and adds lines of the format
698 “VARNAME=value” to the environment if the file exists and users are
699 allowed to change their environment. For more information, see the
700 PermitUserEnvironment option in sshd_config(5).
701
703 ~/.rhosts
704 This file is used for host-based authentication (see above). On
705 some machines this file may need to be world-readable if the
706 user's home directory is on an NFS partition, because sshd(8)
707 reads it as root. Additionally, this file must be owned by the
708 user, and must not have write permissions for anyone else. The
709 recommended permission for most machines is read/write for the
710 user, and not accessible by others.
711 ~/.shosts
713 This file is used in exactly the same way as .rhosts, but allows
714 host-based authentication without permitting login with
715 rlogin/rsh.
716 ~/.ssh/authorized_keys
718 Lists the public keys (RSA/DSA) that can be used for logging in
719 as this user. The format of this file is described in the
720 sshd(8) manual page. This file is not highly sensitive, but the
721 recommended permissions are read/write for the user, and not
722 accessible by others.
723 ~/.ssh/config
725 This is the per-user configuration file. The file format and
726 configuration options are described in ssh_config(5). Because of
727 the potential for abuse, this file must have strict permissions:
728 read/write for the user, and not accessible by others.
729 ~/.ssh/environment
731 Contains additional definitions for environment variables; see
732 ENVIRONMENT, above.
733 ~/.ssh/identity
735 ~/.ssh/id_dsa
736 ~/.ssh/id_rsa
737 Contains the private key for authentication. These files contain
738 sensitive data and should be readable by the user but not acces‐
739 sible by others (read/write/execute). ssh will simply ignore a
740 private key file if it is accessible by others. It is possible
741 to specify a passphrase when generating the key which will be
742 used to encrypt the sensitive part of this file using 3DES.
743 ~/.ssh/identity.pub
745 ~/.ssh/id_dsa.pub
746 ~/.ssh/id_rsa.pub
747 Contains the public key for authentication. These files are not
748 sensitive and can (but need not) be readable by anyone.
749 ~/.ssh/known_hosts
751 Contains a list of host keys for all hosts the user has logged
752 into that are not already in the systemwide list of known host
753 keys. See sshd(8) for further details of the format of this
754 file.
755 ~/.ssh/rc
757 Commands in this file are executed by ssh when the user logs in,
758 just before the user's shell (or command) is started. See the
759 sshd(8) manual page for more information.
760 /etc/hosts.equiv
762 This file is for host-based authentication (see above). It
763 should only be writable by root.
764 /etc/ssh/shosts.equiv
766 This file is used in exactly the same way as hosts.equiv, but
767 allows host-based authentication without permitting login with
768 rlogin/rsh.
769 /etc/ssh/ssh_config
771 Systemwide configuration file. The file format and configuration
772 options are described in ssh_config(5).
773 /etc/ssh/ssh_host_key
775 /etc/ssh/ssh_host_dsa_key
776 /etc/ssh/ssh_host_rsa_key
777 These three files contain the private parts of the host keys and
778 are used for host-based authentication. If protocol version 1 is
779 used, ssh must be setuid root, since the host key is readable
780 only by root. For protocol version 2, ssh uses ssh-keysign(8) to
781 access the host keys, eliminating the requirement that ssh be
782 setuid root when host-based authentication is used. By default
783 ssh is not setuid root.
784 /etc/ssh/ssh_known_hosts
786 Systemwide list of known host keys. This file should be prepared
787 by the system administrator to contain the public host keys of
788 all machines in the organization. It should be world-readable.
789 See sshd(8) for further details of the format of this file.
790 /etc/ssh/sshrc
792 Commands in this file are executed by ssh when the user logs in,
793 just before the user's shell (or command) is started. See the
794 sshd(8) manual page for more information.
795
797 scp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), ssh-keyscan(1),
798 tun(4), hosts.equiv(5), ssh_config(5), ssh-keysign(8), sshd(8)
799 The Secure Shell (SSH) Protocol Assigned Numbers, RFC 4250, 2006.
801 The Secure Shell (SSH) Protocol Architecture, RFC 4251, 2006.
803 The Secure Shell (SSH) Authentication Protocol, RFC 4252, 2006.
805 The Secure Shell (SSH) Transport Layer Protocol, RFC 4253, 2006.
807 The Secure Shell (SSH) Connection Protocol, RFC 4254, 2006.
809 Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints, RFC
811 4255, 2006.
812 Generic Message Exchange Authentication for the Secure Shell Protocol
814 (SSH), RFC 4256, 2006.
815 The Secure Shell (SSH) Session Channel Break Extension, RFC 4335, 2006.
817 The Secure Shell (SSH) Transport Layer Encryption Modes, RFC 4344, 2006.
819 Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer
821 Protocol, RFC 4345, 2006.
822 Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer
824 Protocol, RFC 4419, 2006.
825
827 OpenSSH is a derivative of the original and free ssh 1.2.12 release by
828 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
829 de Raadt and Dug Song removed many bugs, re-added newer features and cre‐
830 ated OpenSSH. Markus Friedl contributed the support for SSH protocol
831 versions 1.5 and 2.0.
832BSD September 25, 1999 BSD