1SSH(1) BSD General Commands Manual SSH(1)
2
4 ssh — OpenSSH SSH client (remote login program)
5
7 ssh [-1246AaCfgKkMNnqsTtVvXxYy] [-b bind_address] [-c cipher_spec] [-D
8 [bind_address:]port] [-e escape_char] [-F configfile] [-I pkcs11]
9 [-i identity_file] [-L [bind_address:]port:host:hostport]
10 [-l login_name] [-m mac_spec] [-O ctl_cmd] [-o option] [-p port] [-R
11 [bind_address:]port:host:hostport] [-S ctl_path] [-W host:port] [-w
12 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. See the Ciphers keyword
82 for more information.
83 -D [bind_address:]port
85 Specifies a local “dynamic” application-level port forwarding.
86 This works by allocating a socket to listen to port on the local
87 side, optionally bound to the specified bind_address. Whenever a
88 connection is made to this port, the connection is forwarded over
89 the secure channel, and the application protocol is then used to
90 determine where to connect to from the remote machine. Currently
91 the SOCKS4 and SOCKS5 protocols are supported, and ssh will act
92 as a SOCKS server. Only root can forward privileged ports.
93 Dynamic port forwardings can also be specified in the configura‐
94 tion file.
95 IPv6 addresses can be specified with an alternative syntax:
97 [bind_address/]port or by enclosing the address in square brack‐
98 ets. Only the superuser can forward privileged ports. By
99 default, the local port is bound in accordance with the
100 GatewayPorts setting. However, an explicit bind_address may be
101 used to bind the connection to a specific address. The
102 bind_address of “localhost” indicates that the listening port be
103 bound for local use only, while an empty address or ‘*’ indicates
104 that the port should be available from all interfaces.
105 -e escape_char
107 Sets the escape character for sessions with a pty (default: ‘~’).
108 The escape character is only recognized at the beginning of a
109 line. The escape character followed by a dot (‘.’) closes the
110 connection; followed by control-Z suspends the connection; and
111 followed by itself sends the escape character once. Setting the
112 character to “none” disables any escapes and makes the session
113 fully transparent.
114 -F configfile
116 Specifies an alternative per-user configuration file. If a con‐
117 figuration file is given on the command line, the system-wide
118 configuration file (/etc/ssh/ssh_config) will be ignored. The
119 default for the per-user configuration file is ~/.ssh/config.
120 -f Requests ssh to go to background just before command execution.
122 This is useful if ssh is going to ask for passwords or
123 passphrases, but the user wants it in the background. This
124 implies -n. The recommended way to start X11 programs at a
125 remote site is with something like ssh -f host xterm.
126 If the ExitOnForwardFailure configuration option is set to “yes”,
128 then a client started with -f will wait for all remote port for‐
129 wards to be successfully established before placing itself in the
130 background.
131 -g Allows remote hosts to connect to local forwarded ports.
133 -I pkcs11
135 Specify the PKCS#11 shared library ssh should use to communicate
136 with a PKCS#11 token providing the user's private RSA key.
137 -i identity_file
139 Selects a file from which the identity (private key) for RSA or
140 DSA authentication is read. The default is ~/.ssh/identity for
141 protocol version 1, and ~/.ssh/id_rsa and ~/.ssh/id_dsa for pro‐
142 tocol version 2. Identity files may also be specified on a per-
143 host basis in the configuration file. It is possible to have
144 multiple -i options (and multiple identities specified in config‐
145 uration files). ssh will also try to load certificate informa‐
146 tion from the filename obtained by appending -cert.pub to iden‐
147 tity filenames.
148 -K Enables GSSAPI-based authentication and forwarding (delegation)
150 of GSSAPI credentials to the server.
151 -k Disables forwarding (delegation) of GSSAPI credentials to the
153 server.
154 -L [bind_address:]port:host:hostport
156 Specifies that the given port on the local (client) host is to be
157 forwarded to the given host and port on the remote side. This
158 works by allocating a socket to listen to port on the local side,
159 optionally bound to the specified bind_address. Whenever a con‐
160 nection is made to this port, the connection is forwarded over
161 the secure channel, and a connection is made to host port
162 hostport from the remote machine. Port forwardings can also be
163 specified in the configuration file. IPv6 addresses can be spec‐
164 ified with an alternative syntax:
165 [bind_address/]port/host/hostport or by enclosing the address in
166 square brackets. Only the superuser can forward privileged
167 ports. By default, the local port is bound in accordance with
168 the GatewayPorts setting. However, an explicit bind_address may
169 be used to bind the connection to a specific address. The
170 bind_address of “localhost” indicates that the listening port be
171 bound for local use only, while an empty address or ‘*’ indicates
172 that the port should be available from all interfaces.
173 -l login_name
175 Specifies the user to log in as on the remote machine. This also
176 may be specified on a per-host basis in the configuration file.
177 -M Places the ssh client into “master” mode for connection sharing.
179 Multiple -M options places ssh into “master” mode with confirma‐
180 tion required before slave connections are accepted. Refer to
181 the description of ControlMaster in ssh_config(5) for details.
182 -m mac_spec
184 Additionally, for protocol version 2 a comma-separated list of
185 MAC (message authentication code) algorithms can be specified in
186 order of preference. See the MACs keyword for more information.
187 -N Do not execute a remote command. This is useful for just for‐
189 warding ports (protocol version 2 only).
190 -n Redirects stdin from /dev/null (actually, prevents reading from
192 stdin). This must be used when ssh is run in the background. A
193 common trick is to use this to run X11 programs on a remote
194 machine. For example, ssh -n shadows.cs.hut.fi emacs & will
195 start an emacs on shadows.cs.hut.fi, and the X11 connection will
196 be automatically forwarded over an encrypted channel. The ssh
197 program will be put in the background. (This does not work if
198 ssh needs to ask for a password or passphrase; see also the -f
199 option.)
200 -O ctl_cmd
202 Control an active connection multiplexing master process. When
203 the -O option is specified, the ctl_cmd argument is interpreted
204 and passed to the master process. Valid commands are: “check”
205 (check that the master process is running) and “exit” (request
206 the master to exit).
207 -o option
209 Can be used to give options in the format used in the configura‐
210 tion file. This is useful for specifying options for which there
211 is no separate command-line flag. For full details of the
212 options listed below, and their possible values, see
213 ssh_config(5).
214 AddressFamily
216 BatchMode
217 BindAddress
218 ChallengeResponseAuthentication
219 CheckHostIP
220 Cipher
221 Ciphers
222 ClearAllForwardings
223 Compression
224 CompressionLevel
225 ConnectionAttempts
226 ConnectTimeout
227 ControlMaster
228 ControlPath
229 DynamicForward
230 EscapeChar
231 ExitOnForwardFailure
232 ForwardAgent
233 ForwardX11
234 ForwardX11Trusted
235 GatewayPorts
236 GlobalKnownHostsFile
237 GSSAPIAuthentication
238 GSSAPIDelegateCredentials
239 HashKnownHosts
240 Host
241 HostbasedAuthentication
242 HostKeyAlgorithms
243 HostKeyAlias
244 HostName
245 IdentityFile
246 IdentitiesOnly
247 KbdInteractiveDevices
248 LocalCommand
249 LocalForward
250 LogLevel
251 MACs
252 NoHostAuthenticationForLocalhost
253 NumberOfPasswordPrompts
254 PasswordAuthentication
255 PermitLocalCommand
256 PKCS11Provider
257 Port
258 PreferredAuthentications
259 Protocol
260 ProxyCommand
261 PubkeyAuthentication
262 RekeyLimit
263 RemoteForward
264 RhostsRSAAuthentication
265 RSAAuthentication
266 SendEnv
267 ServerAliveInterval
268 ServerAliveCountMax
269 StrictHostKeyChecking
270 TCPKeepAlive
271 Tunnel
272 TunnelDevice
273 UsePrivilegedPort
274 User
275 UserKnownHostsFile
276 VerifyHostKeyDNS
277 VisualHostKey
278 XAuthLocation
279 -p port
281 Port to connect to on the remote host. This can be specified on
282 a per-host basis in the configuration file.
283 -q Quiet mode. Causes most warning and diagnostic messages to be
285 suppressed.
286 -R [bind_address:]port:host:hostport
288 Specifies that the given port on the remote (server) host is to
289 be forwarded to the given host and port on the local side. This
290 works by allocating a socket to listen to port on the remote
291 side, and whenever a connection is made to this port, the connec‐
292 tion is forwarded over the secure channel, and a connection is
293 made to host port hostport from the local machine.
294 Port forwardings can also be specified in the configuration file.
296 Privileged ports can be forwarded only when logging in as root on
297 the remote machine. IPv6 addresses can be specified by enclosing
298 the address in square braces or using an alternative syntax:
299 [bind_address/]host/port/hostport.
300 By default, the listening socket on the server will be bound to
302 the loopback interface only. This may be overridden by specify‐
303 ing a bind_address. An empty bind_address, or the address ‘*’,
304 indicates that the remote socket should listen on all interfaces.
305 Specifying a remote bind_address will only succeed if the
306 server's GatewayPorts option is enabled (see sshd_config(5)).
307 If the port argument is ‘0’, the listen port will be dynamically
309 allocated on the server and reported to the client at run time.
310 -S ctl_path
312 Specifies the location of a control socket for connection sharing
313 or the string “none” to disable connection sharing. Refer to the
314 description of ControlPath and ControlMaster in ssh_config(5) for
315 details.
316 -s May be used to request invocation of a subsystem on the remote
318 system. Subsystems are a feature of the SSH2 protocol which
319 facilitate the use of SSH as a secure transport for other appli‐
320 cations (eg. sftp(1)). The subsystem is specified as the remote
321 command.
322 -T Disable pseudo-tty allocation.
324 -t Force pseudo-tty allocation. This can be used to execute arbi‐
326 trary screen-based programs on a remote machine, which can be
327 very useful, e.g. when implementing menu services. Multiple -t
328 options force tty allocation, even if ssh has no local tty.
329 -V Display the version number and exit.
331 -v Verbose mode. Causes ssh to print debugging messages about its
333 progress. This is helpful in debugging connection, authentica‐
334 tion, and configuration problems. Multiple -v options increase
335 the verbosity. The maximum is 3.
336 -W host:port
338 Requests that standard input and output on the client be for‐
339 warded to host on port over the secure channel. Implies -N, -T,
340 ExitOnForwardFailure and ClearAllForwardings and works with Pro‐
341 tocol version 2 only.
342 -w local_tun[:remote_tun]
344 Requests tunnel device forwarding with the specified tun(4)
345 devices between the client (local_tun) and the server
346 (remote_tun).
347 The devices may be specified by numerical ID or the keyword
349 “any”, which uses the next available tunnel device. If
350 remote_tun is not specified, it defaults to “any”. See also the
351 Tunnel and TunnelDevice directives in ssh_config(5). If the
352 Tunnel directive is unset, it is set to the default tunnel mode,
353 which is “point-to-point”.
354 -X Enables X11 forwarding. This can also be specified on a per-host
356 basis in a configuration file.
357 X11 forwarding should be enabled with caution. Users with the
359 ability to bypass file permissions on the remote host (for the
360 user's X authorization database) can access the local X11 display
361 through the forwarded connection. An attacker may then be able
362 to perform activities such as keystroke monitoring.
363 For this reason, X11 forwarding is subjected to X11 SECURITY
365 extension restrictions by default. Please refer to the ssh -Y
366 option and the ForwardX11Trusted directive in ssh_config(5) for
367 more information.
368 -x Disables X11 forwarding.
370 -Y Enables trusted X11 forwarding. Trusted X11 forwardings are not
372 subjected to the X11 SECURITY extension controls.
373 -y Send log information using the syslog(3) system module. By
375 default this information is sent to stderr.
376 ssh may additionally obtain configuration data from a per-user configura‐
378 tion file and a system-wide configuration file. The file format and con‐
379 figuration options are described in ssh_config(5).
380 ssh exits with the exit status of the remote command or with 255 if an
382 error occurred.
383
385 The OpenSSH SSH client supports SSH protocols 1 and 2. The default is to
386 use protocol 2 only, though this can be changed via the Protocol option
387 in ssh_config(5) or the -1 and -2 options (see above). Both protocols
388 support similar authentication methods, but protocol 2 is the default
389 since it provides additional mechanisms for confidentiality (the traffic
390 is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour) and
391 integrity (hmac-md5, hmac-sha1, umac-64, hmac-ripemd160). Protocol 1
392 lacks a strong mechanism for ensuring the integrity of the connection.
393 The methods available for authentication are: GSSAPI-based authentica‐
395 tion, host-based authentication, public key authentication, challenge-
396 response authentication, and password authentication. Authentication
397 methods are tried in the order specified above, though protocol 2 has a
398 configuration option to change the default order:
399 PreferredAuthentications.
400 Host-based authentication works as follows: If the machine the user logs
402 in from is listed in /etc/hosts.equiv or /etc/ssh/shosts.equiv on the
403 remote machine, and the user names are the same on both sides, or if the
404 files ~/.rhosts or ~/.shosts exist in the user's home directory on the
405 remote machine and contain a line containing the name of the client
406 machine and the name of the user on that machine, the user is considered
407 for login. Additionally, the server must be able to verify the client's
408 host key (see the description of /etc/ssh/ssh_known_hosts and
409 ~/.ssh/known_hosts, below) for login to be permitted. This authentica‐
410 tion method closes security holes due to IP spoofing, DNS spoofing, and
411 routing spoofing. [Note to the administrator: /etc/hosts.equiv,
412 ~/.rhosts, and the rlogin/rsh protocol in general, are inherently inse‐
413 cure and should be disabled if security is desired.]
414 Public key authentication works as follows: The scheme is based on pub‐
416 lic-key cryptography, using cryptosystems where encryption and decryption
417 are done using separate keys, and it is unfeasible to derive the decryp‐
418 tion key from the encryption key. The idea is that each user creates a
419 public/private key pair for authentication purposes. The server knows
420 the public key, and only the user knows the private key. ssh implements
421 public key authentication protocol automatically, using either the RSA or
422 DSA algorithms. Protocol 1 is restricted to using only RSA keys, but
423 protocol 2 may use either. The HISTORY section of ssl(8) contains a
424 brief discussion of the two algorithms.
425 The file ~/.ssh/authorized_keys lists the public keys that are permitted
427 for logging in. When the user logs in, the ssh program tells the server
428 which key pair it would like to use for authentication. The client
429 proves that it has access to the private key and the server checks that
430 the corresponding public key is authorized to accept the account.
431 The user creates his/her key pair by running ssh-keygen(1). This stores
433 the private key in ~/.ssh/identity (protocol 1), ~/.ssh/id_dsa (protocol
434 2 DSA), or ~/.ssh/id_rsa (protocol 2 RSA) and stores the public key in
435 ~/.ssh/identity.pub (protocol 1), ~/.ssh/id_dsa.pub (protocol 2 DSA), or
436 ~/.ssh/id_rsa.pub (protocol 2 RSA) in the user's home directory. The
437 user should then copy the public key to ~/.ssh/authorized_keys in his/her
438 home directory on the remote machine. The authorized_keys file corre‐
439 sponds to the conventional ~/.rhosts file, and has one key per line,
440 though the lines can be very long. After this, the user can log in with‐
441 out giving the password.
442 A variation on public key authentication is available in the form of cer‐
444 tificate authentication: instead of a set of public/private keys, signed
445 certificates are used. This has the advantage that a single trusted cer‐
446 tification authority can be used in place of many public/private keys.
447 See the CERTIFICATES section of ssh-keygen(1) for more information.
448 The most convenient way to use public key or certificate authentication
450 may be with an authentication agent. See ssh-agent(1) for more informa‐
451 tion.
452 Challenge-response authentication works as follows: The server sends an
454 arbitrary "challenge" text, and prompts for a response. Protocol 2
455 allows multiple challenges and responses; protocol 1 is restricted to
456 just one challenge/response. Examples of challenge-response authentica‐
457 tion include BSD Authentication (see login.conf(5)) and PAM (some non-
458 OpenBSD systems).
459 Finally, if other authentication methods fail, ssh prompts the user for a
461 password. The password is sent to the remote host for checking; however,
462 since all communications are encrypted, the password cannot be seen by
463 someone listening on the network.
464 ssh automatically maintains and checks a database containing identifica‐
466 tion for all hosts it has ever been used with. Host keys are stored in
467 ~/.ssh/known_hosts in the user's home directory. Additionally, the file
468 /etc/ssh/ssh_known_hosts is automatically checked for known hosts. Any
469 new hosts are automatically added to the user's file. If a host's iden‐
470 tification ever changes, ssh warns about this and disables password
471 authentication to prevent server spoofing or man-in-the-middle attacks,
472 which could otherwise be used to circumvent the encryption. The
473 StrictHostKeyChecking option can be used to control logins to machines
474 whose host key is not known or has changed.
475 When the user's identity has been accepted by the server, the server
477 either executes the given command, or logs into the machine and gives the
478 user a normal shell on the remote machine. All communication with the
479 remote command or shell will be automatically encrypted.
480 If a pseudo-terminal has been allocated (normal login session), the user
482 may use the escape characters noted below.
483 If no pseudo-tty has been allocated, the session is transparent and can
485 be used to reliably transfer binary data. On most systems, setting the
486 escape character to “none” will also make the session transparent even if
487 a tty is used.
488 The session terminates when the command or shell on the remote machine
490 exits and all X11 and TCP connections have been closed.
491
493 When a pseudo-terminal has been requested, ssh supports a number of func‐
494 tions through the use of an escape character.
495 A single tilde character can be sent as ~~ or by following the tilde by a
497 character other than those described below. The escape character must
498 always follow a newline to be interpreted as special. The escape charac‐
499 ter can be changed in configuration files using the EscapeChar configura‐
500 tion directive or on the command line by the -e option.
501 The supported escapes (assuming the default ‘~’) are:
503 ~. Disconnect.
505 ~^Z Background ssh.
507 ~# List forwarded connections.
509 ~& Background ssh at logout when waiting for forwarded connection /
511 X11 sessions to terminate.
512 ~? Display a list of escape characters.
514 ~B Send a BREAK to the remote system (only useful for SSH protocol
516 version 2 and if the peer supports it).
517 ~C Open command line. Currently this allows the addition of port
519 forwardings using the -L, -R and -D options (see above). It also
520 allows the cancellation of existing remote port-forwardings using
521 -KR[bind_address:]port. !command allows the user to execute a
522 local command if the PermitLocalCommand option is enabled in
523 ssh_config(5). Basic help is available, using the -h option.
524 ~R Request rekeying of the connection (only useful for SSH protocol
526 version 2 and if the peer supports it).
527
529 Forwarding of arbitrary TCP connections over the secure channel can be
530 specified either on the command line or in a configuration file. One
531 possible application of TCP forwarding is a secure connection to a mail
532 server; another is going through firewalls.
533 In the example below, we look at encrypting communication between an IRC
535 client and server, even though the IRC server does not directly support
536 encrypted communications. This works as follows: the user connects to
537 the remote host using ssh, specifying a port to be used to forward con‐
538 nections to the remote server. After that it is possible to start the
539 service which is to be encrypted on the client machine, connecting to the
540 same local port, and ssh will encrypt and forward the connection.
541 The following example tunnels an IRC session from client machine
543 “127.0.0.1” (localhost) to remote server “server.example.com”:
544 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
546 $ irc -c '#users' -p 1234 pinky 127.0.0.1
547 This tunnels a connection to IRC server “server.example.com”, joining
549 channel “#users”, nickname “pinky”, using port 1234. It doesn't matter
550 which port is used, as long as it's greater than 1023 (remember, only
551 root can open sockets on privileged ports) and doesn't conflict with any
552 ports already in use. The connection is forwarded to port 6667 on the
553 remote server, since that's the standard port for IRC services.
554 The -f option backgrounds ssh and the remote command “sleep 10” is speci‐
556 fied to allow an amount of time (10 seconds, in the example) to start the
557 service which is to be tunnelled. If no connections are made within the
558 time specified, ssh will exit.
559
561 If the ForwardX11 variable is set to “yes” (or see the description of the
562 -X, -x, and -Y options above) and the user is using X11 (the DISPLAY
563 environment variable is set), the connection to the X11 display is auto‐
564 matically forwarded to the remote side in such a way that any X11 pro‐
565 grams started from the shell (or command) will go through the encrypted
566 channel, and the connection to the real X server will be made from the
567 local machine. The user should not manually set DISPLAY. Forwarding of
568 X11 connections can be configured on the command line or in configuration
569 files.
570 The DISPLAY value set by ssh will point to the server machine, but with a
572 display number greater than zero. This is normal, and happens because
573 ssh creates a “proxy” X server on the server machine for forwarding the
574 connections over the encrypted channel.
575 ssh will also automatically set up Xauthority data on the server machine.
577 For this purpose, it will generate a random authorization cookie, store
578 it in Xauthority on the server, and verify that any forwarded connections
579 carry this cookie and replace it by the real cookie when the connection
580 is opened. The real authentication cookie is never sent to the server
581 machine (and no cookies are sent in the plain).
582 If the ForwardAgent variable is set to “yes” (or see the description of
584 the -A and -a options above) and the user is using an authentication
585 agent, the connection to the agent is automatically forwarded to the
586 remote side.
587
589 When connecting to a server for the first time, a fingerprint of the
590 server's public key is presented to the user (unless the option
591 StrictHostKeyChecking has been disabled). Fingerprints can be determined
592 using ssh-keygen(1):
593 $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
595 If the fingerprint is already known, it can be matched and the key can be
597 accepted or rejected. Because of the difficulty of comparing host keys
598 just by looking at hex strings, there is also support to compare host
599 keys visually, using random art. By setting the VisualHostKey option to
600 “yes”, a small ASCII graphic gets displayed on every login to a server,
601 no matter if the session itself is interactive or not. By learning the
602 pattern a known server produces, a user can easily find out that the host
603 key has changed when a completely different pattern is displayed.
604 Because these patterns are not unambiguous however, a pattern that looks
605 similar to the pattern remembered only gives a good probability that the
606 host key is the same, not guaranteed proof.
607 To get a listing of the fingerprints along with their random art for all
609 known hosts, the following command line can be used:
610 $ ssh-keygen -lv -f ~/.ssh/known_hosts
612 If the fingerprint is unknown, an alternative method of verification is
614 available: SSH fingerprints verified by DNS. An additional resource
615 record (RR), SSHFP, is added to a zonefile and the connecting client is
616 able to match the fingerprint with that of the key presented.
617 In this example, we are connecting a client to a server,
619 “host.example.com”. The SSHFP resource records should first be added to
620 the zonefile for host.example.com:
621 $ ssh-keygen -r host.example.com.
623 The output lines will have to be added to the zonefile. To check that
625 the zone is answering fingerprint queries:
626 $ dig -t SSHFP host.example.com
628 Finally the client connects:
630 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
632 [...]
633 Matching host key fingerprint found in DNS.
634 Are you sure you want to continue connecting (yes/no)?
635 See the VerifyHostKeyDNS option in ssh_config(5) for more information.
637
639 ssh contains support for Virtual Private Network (VPN) tunnelling using
640 the tun(4) network pseudo-device, allowing two networks to be joined
641 securely. The sshd_config(5) configuration option PermitTunnel controls
642 whether the server supports this, and at what level (layer 2 or 3 traf‐
643 fic).
644 The following example would connect client network 10.0.50.0/24 with
646 remote network 10.0.99.0/24 using a point-to-point connection from
647 10.1.1.1 to 10.1.1.2, provided that the SSH server running on the gateway
648 to the remote network, at 192.168.1.15, allows it.
649 On the client:
651 # ssh -f -w 0:1 192.168.1.15 true
653 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
654 # route add 10.0.99.0/24 10.1.1.2
655 On the server:
657 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
659 # route add 10.0.50.0/24 10.1.1.1
660 Client access may be more finely tuned via the /root/.ssh/authorized_keys
662 file (see below) and the PermitRootLogin server option. The following
663 entry would permit connections on tun(4) device 1 from user “jane” and on
664 tun device 2 from user “john”, if PermitRootLogin is set to
665 “forced-commands-only”:
666 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
668 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
669 Since an SSH-based setup entails a fair amount of overhead, it may be
671 more suited to temporary setups, such as for wireless VPNs. More perma‐
672 nent VPNs are better provided by tools such as ipsecctl(8) and
673 isakmpd(8).
674
676 ssh will normally set the following environment variables:
677 DISPLAY The DISPLAY variable indicates the location of the
679 X11 server. It is automatically set by ssh to
680 point to a value of the form “hostname:n”, where
681 “hostname” indicates the host where the shell runs,
682 and ‘n’ is an integer ≥ 1. ssh uses this special
683 value to forward X11 connections over the secure
684 channel. The user should normally not set DISPLAY
685 explicitly, as that will render the X11 connection
686 insecure (and will require the user to manually
687 copy any required authorization cookies).
688 HOME Set to the path of the user's home directory.
690 LOGNAME Synonym for USER; set for compatibility with sys‐
692 tems that use this variable.
693 MAIL Set to the path of the user's mailbox.
695 PATH Set to the default PATH, as specified when compil‐
697 ing ssh.
698 SSH_ASKPASS If ssh needs a passphrase, it will read the
700 passphrase from the current terminal if it was run
701 from a terminal. If ssh does not have a terminal
702 associated with it but DISPLAY and SSH_ASKPASS are
703 set, it will execute the program specified by
704 SSH_ASKPASS and open an X11 window to read the
705 passphrase. This is particularly useful when call‐
706 ing ssh from a .xsession or related script. (Note
707 that on some machines it may be necessary to redi‐
708 rect the input from /dev/null to make this work.)
709 SSH_AUTH_SOCK Identifies the path of a UNIX-domain socket used to
711 communicate with the agent.
712 SSH_CONNECTION Identifies the client and server ends of the con‐
714 nection. The variable contains four space-sepa‐
715 rated values: client IP address, client port num‐
716 ber, server IP address, and server port number.
717 SSH_ORIGINAL_COMMAND This variable contains the original command line if
719 a forced command is executed. It can be used to
720 extract the original arguments.
721 SSH_TTY This is set to the name of the tty (path to the
723 device) associated with the current shell or com‐
724 mand. If the current session has no tty, this
725 variable is not set.
726 TZ This variable is set to indicate the present time
728 zone if it was set when the daemon was started
729 (i.e. the daemon passes the value on to new connec‐
730 tions).
731 USER Set to the name of the user logging in.
733 Additionally, ssh reads ~/.ssh/environment, and adds lines of the format
735 “VARNAME=value” to the environment if the file exists and users are
736 allowed to change their environment. For more information, see the
737 PermitUserEnvironment option in sshd_config(5).
738
740 ~/.rhosts
741 This file is used for host-based authentication (see above). On
742 some machines this file may need to be world-readable if the
743 user's home directory is on an NFS partition, because sshd(8)
744 reads it as root. Additionally, this file must be owned by the
745 user, and must not have write permissions for anyone else. The
746 recommended permission for most machines is read/write for the
747 user, and not accessible by others.
748 ~/.shosts
750 This file is used in exactly the same way as .rhosts, but allows
751 host-based authentication without permitting login with
752 rlogin/rsh.
753 ~/.ssh/
755 This directory is the default location for all user-specific con‐
756 figuration and authentication information. There is no general
757 requirement to keep the entire contents of this directory secret,
758 but the recommended permissions are read/write/execute for the
759 user, and not accessible by others.
760 ~/.ssh/authorized_keys
762 Lists the public keys (RSA/DSA) that can be used for logging in
763 as this user. The format of this file is described in the
764 sshd(8) manual page. This file is not highly sensitive, but the
765 recommended permissions are read/write for the user, and not
766 accessible by others.
767 ~/.ssh/config
769 This is the per-user configuration file. The file format and
770 configuration options are described in ssh_config(5). Because of
771 the potential for abuse, this file must have strict permissions:
772 read/write for the user, and not accessible by others.
773 ~/.ssh/environment
775 Contains additional definitions for environment variables; see
776 ENVIRONMENT, above.
777 ~/.ssh/identity
779 ~/.ssh/id_dsa
780 ~/.ssh/id_rsa
781 Contains the private key for authentication. These files contain
782 sensitive data and should be readable by the user but not acces‐
783 sible by others (read/write/execute). ssh will simply ignore a
784 private key file if it is accessible by others. It is possible
785 to specify a passphrase when generating the key which will be
786 used to encrypt the sensitive part of this file using 3DES.
787 ~/.ssh/identity.pub
789 ~/.ssh/id_dsa.pub
790 ~/.ssh/id_rsa.pub
791 Contains the public key for authentication. These files are not
792 sensitive and can (but need not) be readable by anyone.
793 ~/.ssh/known_hosts
795 Contains a list of host keys for all hosts the user has logged
796 into that are not already in the systemwide list of known host
797 keys. See sshd(8) for further details of the format of this
798 file.
799 ~/.ssh/rc
801 Commands in this file are executed by ssh when the user logs in,
802 just before the user's shell (or command) is started. See the
803 sshd(8) manual page for more information.
804 /etc/hosts.equiv
806 This file is for host-based authentication (see above). It
807 should only be writable by root.
808 /etc/ssh/shosts.equiv
810 This file is used in exactly the same way as hosts.equiv, but
811 allows host-based authentication without permitting login with
812 rlogin/rsh.
813 /etc/ssh/ssh_config
815 Systemwide configuration file. The file format and configuration
816 options are described in ssh_config(5).
817 /etc/ssh/ssh_host_key
819 /etc/ssh/ssh_host_dsa_key
820 /etc/ssh/ssh_host_rsa_key
821 These three files contain the private parts of the host keys and
822 are used for host-based authentication. If protocol version 1 is
823 used, ssh must be setuid root, since the host key is readable
824 only by root. For protocol version 2, ssh uses ssh-keysign(8) to
825 access the host keys, eliminating the requirement that ssh be
826 setuid root when host-based authentication is used. By default
827 ssh is not setuid root.
828 /etc/ssh/ssh_known_hosts
830 Systemwide list of known host keys. This file should be prepared
831 by the system administrator to contain the public host keys of
832 all machines in the organization. It should be world-readable.
833 See sshd(8) for further details of the format of this file.
834 /etc/ssh/sshrc
836 Commands in this file are executed by ssh when the user logs in,
837 just before the user's shell (or command) is started. See the
838 sshd(8) manual page for more information.
839
841 scp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), ssh-keyscan(1),
842 tun(4), hosts.equiv(5), ssh_config(5), ssh-keysign(8), sshd(8)
843 The Secure Shell (SSH) Protocol Assigned Numbers, RFC 4250, 2006.
845 The Secure Shell (SSH) Protocol Architecture, RFC 4251, 2006.
847 The Secure Shell (SSH) Authentication Protocol, RFC 4252, 2006.
849 The Secure Shell (SSH) Transport Layer Protocol, RFC 4253, 2006.
851 The Secure Shell (SSH) Connection Protocol, RFC 4254, 2006.
853 Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints, RFC
855 4255, 2006.
856 Generic Message Exchange Authentication for the Secure Shell Protocol
858 (SSH), RFC 4256, 2006.
859 The Secure Shell (SSH) Session Channel Break Extension, RFC 4335, 2006.
861 The Secure Shell (SSH) Transport Layer Encryption Modes, RFC 4344, 2006.
863 Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer
865 Protocol, RFC 4345, 2006.
866 Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer
868 Protocol, RFC 4419, 2006.
869 The Secure Shell (SSH) Public Key File Format, RFC 4716, 2006.
871 A. Perrig and D. Song, Hash Visualization: a New Technique to improve
873 Real-World Security, 1999, International Workshop on Cryptographic
874 Techniques and E-Commerce (CrypTEC '99).
875
877 OpenSSH is a derivative of the original and free ssh 1.2.12 release by
878 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
879 de Raadt and Dug Song removed many bugs, re-added newer features and cre‐
880 ated OpenSSH. Markus Friedl contributed the support for SSH protocol
881 versions 1.5 and 2.0.
882BSD June 22, 2019 BSD