1SSH(1) BSD General Commands Manual SSH(1)
2
4 ssh — OpenSSH SSH client (remote login program)
5
7 ssh [-46AaCfGgKkMNnqsTtVvXxYy] [-B bind_interface] [-b bind_address]
8 [-c cipher_spec] [-D [bind_address:]port] [-E log_file]
9 [-e escape_char] [-F configfile] [-I pkcs11] [-i identity_file]
10 [-J destination] [-L address] [-l login_name] [-m mac_spec]
11 [-O ctl_cmd] [-o option] [-p port] [-Q query_option] [-R address]
12 [-S ctl_path] [-W host:port] [-w local_tun[:remote_tun]] destination
13 [command]
14
16 ssh (SSH client) is a program for logging into a remote machine and for
17 executing commands on a remote machine. It is intended to provide secure
18 encrypted communications between two untrusted hosts over an insecure
19 network. X11 connections, arbitrary TCP ports and UNIX-domain sockets
20 can also be forwarded over the secure channel.
21 ssh connects and logs into the specified destination, which may be speci‐
23 fied as either [user@]hostname or a URI of the form
24 ssh://[user@]hostname[:port]. The user must prove his/her identity to
25 the remote machine using one of several methods (see below).
26 If a command is specified, it is executed on the remote host instead of a
28 login shell.
29 The options are as follows:
31 -4 Forces ssh to use IPv4 addresses only.
33 -6 Forces ssh to use IPv6 addresses only.
35 -A Enables forwarding of the authentication agent connection. This
37 can also be specified on a per-host basis in a configuration
38 file.
39 Agent forwarding should be enabled with caution. Users with the
41 ability to bypass file permissions on the remote host (for the
42 agent's UNIX-domain socket) can access the local agent through
43 the forwarded connection. An attacker cannot obtain key material
44 from the agent, however they can perform operations on the keys
45 that enable them to authenticate using the identities loaded into
46 the agent.
47 -a Disables forwarding of the authentication agent connection.
49 -B bind_interface
51 Bind to the address of bind_interface before attempting to con‐
52 nect to the destination host. This is only useful on systems
53 with more than one address.
54 -b bind_address
56 Use bind_address on the local machine as the source address of
57 the connection. Only useful on systems with more than one
58 address.
59 -C Requests compression of all data (including stdin, stdout,
61 stderr, and data for forwarded X11, TCP and UNIX-domain connec‐
62 tions). The compression algorithm is the same used by gzip(1).
63 Compression is desirable on modem lines and other slow connec‐
64 tions, but will only slow down things on fast networks. The
65 default value can be set on a host-by-host basis in the configu‐
66 ration files; see the Compression option.
67 -c cipher_spec
69 Selects the cipher specification for encrypting the session.
70 cipher_spec is a comma-separated list of ciphers listed in order
71 of preference. See the Ciphers keyword in ssh_config(5) for more
72 information.
73 -D [bind_address:]port
75 Specifies a local “dynamic” application-level port forwarding.
76 This works by allocating a socket to listen to port on the local
77 side, optionally bound to the specified bind_address. Whenever a
78 connection is made to this port, the connection is forwarded over
79 the secure channel, and the application protocol is then used to
80 determine where to connect to from the remote machine. Currently
81 the SOCKS4 and SOCKS5 protocols are supported, and ssh will act
82 as a SOCKS server. Only root can forward privileged ports.
83 Dynamic port forwardings can also be specified in the configura‐
84 tion file.
85 IPv6 addresses can be specified by enclosing the address in
87 square brackets. Only the superuser can forward privileged
88 ports. By default, the local port is bound in accordance with
89 the GatewayPorts setting. However, an explicit bind_address may
90 be used to bind the connection to a specific address. The
91 bind_address of “localhost” indicates that the listening port be
92 bound for local use only, while an empty address or ‘*’ indicates
93 that the port should be available from all interfaces.
94 -E log_file
96 Append debug logs to log_file instead of standard error.
97 -e escape_char
99 Sets the escape character for sessions with a pty (default: ‘~’).
100 The escape character is only recognized at the beginning of a
101 line. The escape character followed by a dot (‘.’) closes the
102 connection; followed by control-Z suspends the connection; and
103 followed by itself sends the escape character once. Setting the
104 character to “none” disables any escapes and makes the session
105 fully transparent.
106 -F configfile
108 Specifies an alternative per-user configuration file. If a con‐
109 figuration file is given on the command line, the system-wide
110 configuration file (/etc/ssh/ssh_config) will be ignored. The
111 default for the per-user configuration file is ~/.ssh/config.
112 -f Requests ssh to go to background just before command execution.
114 This is useful if ssh is going to ask for passwords or
115 passphrases, but the user wants it in the background. This
116 implies -n. The recommended way to start X11 programs at a
117 remote site is with something like ssh -f host xterm.
118 If the ExitOnForwardFailure configuration option is set to “yes”,
120 then a client started with -f will wait for all remote port for‐
121 wards to be successfully established before placing itself in the
122 background.
123 -G Causes ssh to print its configuration after evaluating Host and
125 Match blocks and exit.
126 -g Allows remote hosts to connect to local forwarded ports. If used
128 on a multiplexed connection, then this option must be specified
129 on the master process.
130 -I pkcs11
132 Specify the PKCS#11 shared library ssh should use to communicate
133 with a PKCS#11 token providing keys for user authentication.
134 -i identity_file
136 Selects a file from which the identity (private key) for public
137 key authentication is read. The default is ~/.ssh/id_dsa,
138 ~/.ssh/id_ecdsa, ~/.ssh/id_ed25519 and ~/.ssh/id_rsa. Identity
139 files may also be specified on a per-host basis in the configura‐
140 tion file. It is possible to have multiple -i options (and mul‐
141 tiple identities specified in configuration files). If no cer‐
142 tificates have been explicitly specified by the CertificateFile
143 directive, ssh will also try to load certificate information from
144 the filename obtained by appending -cert.pub to identity file‐
145 names.
146 -J destination
148 Connect to the target host by first making a ssh connection to
149 the jump host described by destination and then establishing a
150 TCP forwarding to the ultimate destination from there. Multiple
151 jump hops may be specified separated by comma characters. This
152 is a shortcut to specify a ProxyJump configuration directive.
153 Note that configuration directives supplied on the command-line
154 generally apply to the destination host and not any specified
155 jump hosts. Use ~/.ssh/config to specify configuration for jump
156 hosts.
157 -K Enables GSSAPI-based authentication and forwarding (delegation)
159 of GSSAPI credentials to the server.
160 -k Disables forwarding (delegation) of GSSAPI credentials to the
162 server.
163 -L [bind_address:]port:host:hostport
165 -L [bind_address:]port:remote_socket
166 -L local_socket:host:hostport
167 -L local_socket:remote_socket
168 Specifies that connections to the given TCP port or Unix socket
169 on the local (client) host are to be forwarded to the given host
170 and port, or Unix socket, on the remote side. This works by
171 allocating a socket to listen to either a TCP port on the local
172 side, optionally bound to the specified bind_address, or to a
173 Unix socket. Whenever a connection is made to the local port or
174 socket, the connection is forwarded over the secure channel, and
175 a connection is made to either host port hostport, or the Unix
176 socket remote_socket, from the remote machine.
177 Port forwardings can also be specified in the configuration file.
179 Only the superuser can forward privileged ports. IPv6 addresses
180 can be specified by enclosing the address in square brackets.
181 By default, the local port is bound in accordance with the
183 GatewayPorts setting. However, an explicit bind_address may be
184 used to bind the connection to a specific address. The
185 bind_address of “localhost” indicates that the listening port be
186 bound for local use only, while an empty address or ‘*’ indicates
187 that the port should be available from all interfaces.
188 -l login_name
190 Specifies the user to log in as on the remote machine. This also
191 may be specified on a per-host basis in the configuration file.
192 -M Places the ssh client into “master” mode for connection sharing.
194 Multiple -M options places ssh into “master” mode but with con‐
195 firmation required using ssh-askpass(1) before each operation
196 that changes the multiplexing state (e.g. opening a new session).
197 Refer to the description of ControlMaster in ssh_config(5) for
198 details.
199 -m mac_spec
201 A comma-separated list of MAC (message authentication code) algo‐
202 rithms, specified in order of preference. See the MACs keyword
203 for more information.
204 -N Do not execute a remote command. This is useful for just for‐
206 warding ports.
207 -n Redirects stdin from /dev/null (actually, prevents reading from
209 stdin). This must be used when ssh is run in the background. A
210 common trick is to use this to run X11 programs on a remote
211 machine. For example, ssh -n shadows.cs.hut.fi emacs & will
212 start an emacs on shadows.cs.hut.fi, and the X11 connection will
213 be automatically forwarded over an encrypted channel. The ssh
214 program will be put in the background. (This does not work if
215 ssh needs to ask for a password or passphrase; see also the -f
216 option.)
217 -O ctl_cmd
219 Control an active connection multiplexing master process. When
220 the -O option is specified, the ctl_cmd argument is interpreted
221 and passed to the master process. Valid commands are: “check”
222 (check that the master process is running), “forward” (request
223 forwardings without command execution), “cancel” (cancel forward‐
224 ings), “exit” (request the master to exit), and “stop” (request
225 the master to stop accepting further multiplexing requests).
226 -o option
228 Can be used to give options in the format used in the configura‐
229 tion file. This is useful for specifying options for which there
230 is no separate command-line flag. For full details of the
231 options listed below, and their possible values, see
232 ssh_config(5).
233 AddKeysToAgent
235 AddressFamily
236 BatchMode
237 BindAddress
238 CanonicalDomains
239 CanonicalizeFallbackLocal
240 CanonicalizeHostname
241 CanonicalizeMaxDots
242 CanonicalizePermittedCNAMEs
243 CASignatureAlgorithms
244 CertificateFile
245 ChallengeResponseAuthentication
246 CheckHostIP
247 Ciphers
248 ClearAllForwardings
249 Compression
250 ConnectionAttempts
251 ConnectTimeout
252 ControlMaster
253 ControlPath
254 ControlPersist
255 DynamicForward
256 EscapeChar
257 ExitOnForwardFailure
258 FingerprintHash
259 ForwardAgent
260 ForwardX11
261 ForwardX11Timeout
262 ForwardX11Trusted
263 GatewayPorts
264 GlobalKnownHostsFile
265 GSSAPIAuthentication
266 GSSAPIKeyExchange
267 GSSAPIClientIdentity
268 GSSAPIDelegateCredentials
269 GSSAPIKexAlgorithms
270 GSSAPIRenewalForcesRekey
271 GSSAPIServerIdentity
272 GSSAPITrustDns
273 HashKnownHosts
274 Host
275 HostbasedAuthentication
276 HostbasedKeyTypes
277 HostKeyAlgorithms
278 HostKeyAlias
279 Hostname
280 IdentitiesOnly
281 IdentityAgent
282 IdentityFile
283 IPQoS
284 KbdInteractiveAuthentication
285 KbdInteractiveDevices
286 KexAlgorithms
287 LocalCommand
288 LocalForward
289 LogLevel
290 MACs
291 Match
292 NoHostAuthenticationForLocalhost
293 NumberOfPasswordPrompts
294 PasswordAuthentication
295 PermitLocalCommand
296 PKCS11Provider
297 Port
298 PreferredAuthentications
299 ProxyCommand
300 ProxyJump
301 ProxyUseFdpass
302 PubkeyAcceptedKeyTypes
303 PubkeyAuthentication
304 RekeyLimit
305 RemoteCommand
306 RemoteForward
307 RequestTTY
308 SendEnv
309 ServerAliveInterval
310 ServerAliveCountMax
311 SetEnv
312 StreamLocalBindMask
313 StreamLocalBindUnlink
314 StrictHostKeyChecking
315 TCPKeepAlive
316 Tunnel
317 TunnelDevice
318 UpdateHostKeys
319 User
320 UserKnownHostsFile
321 VerifyHostKeyDNS
322 VisualHostKey
323 XAuthLocation
324 -p port
326 Port to connect to on the remote host. This can be specified on
327 a per-host basis in the configuration file.
328 -Q query_option
330 Queries ssh for the algorithms supported for the specified ver‐
331 sion 2. The available features are: cipher (supported symmetric
332 ciphers), cipher-auth (supported symmetric ciphers that support
333 authenticated encryption), help (supported query terms for use
334 with the -Q flag), mac (supported message integrity codes), kex
335 (key exchange algorithms), kex-gss (GSSAPI key exchange algo‐
336 rithms), key (key types), key-cert (certificate key types),
337 key-plain (non-certificate key types), protocol-version (sup‐
338 ported SSH protocol versions), and sig (supported signature algo‐
339 rithms).
340 -q Quiet mode. Causes most warning and diagnostic messages to be
342 suppressed.
343 -R [bind_address:]port:host:hostport
345 -R [bind_address:]port:local_socket
346 -R remote_socket:host:hostport
347 -R remote_socket:local_socket
348 -R [bind_address:]port
349 Specifies that connections to the given TCP port or Unix socket
350 on the remote (server) host are to be forwarded to the local
351 side.
352 This works by allocating a socket to listen to either a TCP port
354 or to a Unix socket on the remote side. Whenever a connection is
355 made to this port or Unix socket, the connection is forwarded
356 over the secure channel, and a connection is made from the local
357 machine to either an explicit destination specified by host port
358 hostport, or local_socket, or, if no explicit destination was
359 specified, ssh will act as a SOCKS 4/5 proxy and forward connec‐
360 tions to the destinations requested by the remote SOCKS client.
361 Port forwardings can also be specified in the configuration file.
363 Privileged ports can be forwarded only when logging in as root on
364 the remote machine. IPv6 addresses can be specified by enclosing
365 the address in square brackets.
366 By default, TCP listening sockets on the server will be bound to
368 the loopback interface only. This may be overridden by specify‐
369 ing a bind_address. An empty bind_address, or the address ‘*’,
370 indicates that the remote socket should listen on all interfaces.
371 Specifying a remote bind_address will only succeed if the
372 server's GatewayPorts option is enabled (see sshd_config(5)).
373 If the port argument is ‘0’, the listen port will be dynamically
375 allocated on the server and reported to the client at run time.
376 When used together with -O forward the allocated port will be
377 printed to the standard output.
378 -S ctl_path
380 Specifies the location of a control socket for connection shar‐
381 ing, or the string “none” to disable connection sharing. Refer
382 to the description of ControlPath and ControlMaster in
383 ssh_config(5) for details.
384 -s May be used to request invocation of a subsystem on the remote
386 system. Subsystems facilitate the use of SSH as a secure trans‐
387 port for other applications (e.g. sftp(1)). The subsystem is
388 specified as the remote command.
389 -T Disable pseudo-terminal allocation.
391 -t Force pseudo-terminal allocation. This can be used to execute
393 arbitrary screen-based programs on a remote machine, which can be
394 very useful, e.g. when implementing menu services. Multiple -t
395 options force tty allocation, even if ssh has no local tty.
396 -V Display the version number and exit.
398 -v Verbose mode. Causes ssh to print debugging messages about its
400 progress. This is helpful in debugging connection, authentica‐
401 tion, and configuration problems. Multiple -v options increase
402 the verbosity. The maximum is 3.
403 -W host:port
405 Requests that standard input and output on the client be for‐
406 warded to host on port over the secure channel. Implies -N, -T,
407 ExitOnForwardFailure and ClearAllForwardings, though these can be
408 overridden in the configuration file or using -o command line
409 options.
410 -w local_tun[:remote_tun]
412 Requests tunnel device forwarding with the specified tun(4)
413 devices between the client (local_tun) and the server
414 (remote_tun).
415 The devices may be specified by numerical ID or the keyword
417 “any”, which uses the next available tunnel device. If
418 remote_tun is not specified, it defaults to “any”. See also the
419 Tunnel and TunnelDevice directives in ssh_config(5).
420 If the Tunnel directive is unset, it will be set to the default
422 tunnel mode, which is “point-to-point”. If a different Tunnel
423 forwarding mode it desired, then it should be specified before
424 -w.
425 -X Enables X11 forwarding. This can also be specified on a per-host
427 basis in a configuration file.
428 X11 forwarding should be enabled with caution. Users with the
430 ability to bypass file permissions on the remote host (for the
431 user's X authorization database) can access the local X11 display
432 through the forwarded connection. An attacker may then be able
433 to perform activities such as keystroke monitoring.
434 For this reason, X11 forwarding is subjected to X11 SECURITY
436 extension restrictions by default. Please refer to the ssh -Y
437 option and the ForwardX11Trusted directive in ssh_config(5) for
438 more information.
439 -x Disables X11 forwarding.
441 -Y Enables trusted X11 forwarding. Trusted X11 forwardings are not
443 subjected to the X11 SECURITY extension controls.
444 -y Send log information using the syslog(3) system module. By
446 default this information is sent to stderr.
447 ssh may additionally obtain configuration data from a per-user configura‐
449 tion file and a system-wide configuration file. The file format and con‐
450 figuration options are described in ssh_config(5).
451
453 The OpenSSH SSH client supports SSH protocol 2.
454 The methods available for authentication are: GSSAPI-based authentica‐
456 tion, host-based authentication, public key authentication, challenge-
457 response authentication, and password authentication. Authentication
458 methods are tried in the order specified above, though
459 PreferredAuthentications can be used to change the default order.
460 Host-based authentication works as follows: If the machine the user logs
462 in from is listed in /etc/hosts.equiv or /etc/ssh/shosts.equiv on the
463 remote machine, and the user names are the same on both sides, or if the
464 files ~/.rhosts or ~/.shosts exist in the user's home directory on the
465 remote machine and contain a line containing the name of the client
466 machine and the name of the user on that machine, the user is considered
467 for login. Additionally, the server must be able to verify the client's
468 host key (see the description of /etc/ssh/ssh_known_hosts and
469 ~/.ssh/known_hosts, below) for login to be permitted. This authentica‐
470 tion method closes security holes due to IP spoofing, DNS spoofing, and
471 routing spoofing. [Note to the administrator: /etc/hosts.equiv,
472 ~/.rhosts, and the rlogin/rsh protocol in general, are inherently inse‐
473 cure and should be disabled if security is desired.]
474 Public key authentication works as follows: The scheme is based on pub‐
476 lic-key cryptography, using cryptosystems where encryption and decryption
477 are done using separate keys, and it is unfeasible to derive the decryp‐
478 tion key from the encryption key. The idea is that each user creates a
479 public/private key pair for authentication purposes. The server knows
480 the public key, and only the user knows the private key. ssh implements
481 public key authentication protocol automatically, using one of the DSA,
482 ECDSA, Ed25519 or RSA algorithms. The HISTORY section of ssl(8) contains
483 a brief discussion of the DSA and RSA algorithms.
484 The file ~/.ssh/authorized_keys lists the public keys that are permitted
486 for logging in. When the user logs in, the ssh program tells the server
487 which key pair it would like to use for authentication. The client
488 proves that it has access to the private key and the server checks that
489 the corresponding public key is authorized to accept the account.
490 The server may inform the client of errors that prevented public key
492 authentication from succeeding after authentication completes using a
493 different method. These may be viewed by increasing the LogLevel to
494 DEBUG or higher (e.g. by using the -v flag).
495 The user creates his/her key pair by running ssh-keygen(1). This stores
497 the private key in ~/.ssh/id_dsa (DSA), ~/.ssh/id_ecdsa (ECDSA),
498 ~/.ssh/id_ed25519 (Ed25519), or ~/.ssh/id_rsa (RSA) and stores the public
499 key in ~/.ssh/id_dsa.pub (DSA), ~/.ssh/id_ecdsa.pub (ECDSA),
500 ~/.ssh/id_ed25519.pub (Ed25519), or ~/.ssh/id_rsa.pub (RSA) in the user's
501 home directory. The user should then copy the public key to
502 ~/.ssh/authorized_keys in his/her home directory on the remote machine.
503 The authorized_keys file corresponds to the conventional ~/.rhosts file,
504 and has one key per line, though the lines can be very long. After this,
505 the user can log in without giving the password.
506 A variation on public key authentication is available in the form of cer‐
508 tificate authentication: instead of a set of public/private keys, signed
509 certificates are used. This has the advantage that a single trusted cer‐
510 tification authority can be used in place of many public/private keys.
511 See the CERTIFICATES section of ssh-keygen(1) for more information.
512 The most convenient way to use public key or certificate authentication
514 may be with an authentication agent. See ssh-agent(1) and (optionally)
515 the AddKeysToAgent directive in ssh_config(5) for more information.
516 Challenge-response authentication works as follows: The server sends an
518 arbitrary "challenge" text, and prompts for a response. Examples of
519 challenge-response authentication include BSD Authentication (see
520 login.conf(5)) and PAM (some non-OpenBSD systems).
521 Finally, if other authentication methods fail, ssh prompts the user for a
523 password. The password is sent to the remote host for checking; however,
524 since all communications are encrypted, the password cannot be seen by
525 someone listening on the network.
526 ssh automatically maintains and checks a database containing identifica‐
528 tion for all hosts it has ever been used with. Host keys are stored in
529 ~/.ssh/known_hosts in the user's home directory. Additionally, the file
530 /etc/ssh/ssh_known_hosts is automatically checked for known hosts. Any
531 new hosts are automatically added to the user's file. If a host's iden‐
532 tification ever changes, ssh warns about this and disables password
533 authentication to prevent server spoofing or man-in-the-middle attacks,
534 which could otherwise be used to circumvent the encryption. The
535 StrictHostKeyChecking option can be used to control logins to machines
536 whose host key is not known or has changed.
537 When the user's identity has been accepted by the server, the server
539 either executes the given command in a non-interactive session or, if no
540 command has been specified, logs into the machine and gives the user a
541 normal shell as an interactive session. All communication with the
542 remote command or shell will be automatically encrypted.
543 If an interactive session is requested ssh by default will only request a
545 pseudo-terminal (pty) for interactive sessions when the client has one.
546 The flags -T and -t can be used to override this behaviour.
547 If a pseudo-terminal has been allocated the user may use the escape char‐
549 acters noted below.
550 If no pseudo-terminal has been allocated, the session is transparent and
552 can be used to reliably transfer binary data. On most systems, setting
553 the escape character to “none” will also make the session transparent
554 even if a tty is used.
555 The session terminates when the command or shell on the remote machine
557 exits and all X11 and TCP connections have been closed.
558
560 When a pseudo-terminal has been requested, ssh supports a number of func‐
561 tions through the use of an escape character.
562 A single tilde character can be sent as ~~ or by following the tilde by a
564 character other than those described below. The escape character must
565 always follow a newline to be interpreted as special. The escape charac‐
566 ter can be changed in configuration files using the EscapeChar configura‐
567 tion directive or on the command line by the -e option.
568 The supported escapes (assuming the default ‘~’) are:
570 ~. Disconnect.
572 ~^Z Background ssh.
574 ~# List forwarded connections.
576 ~& Background ssh at logout when waiting for forwarded connection /
578 X11 sessions to terminate.
579 ~? Display a list of escape characters.
581 ~B Send a BREAK to the remote system (only useful if the peer sup‐
583 ports it).
584 ~C Open command line. Currently this allows the addition of port
586 forwardings using the -L, -R and -D options (see above). It also
587 allows the cancellation of existing port-forwardings with
588 -KL[bind_address:]port for local, -KR[bind_address:]port for
589 remote and -KD[bind_address:]port for dynamic port-forwardings.
590 !command allows the user to execute a local command if the
591 PermitLocalCommand option is enabled in ssh_config(5). Basic
592 help is available, using the -h option.
593 ~R Request rekeying of the connection (only useful if the peer sup‐
595 ports it).
596 ~V Decrease the verbosity (LogLevel) when errors are being written
598 to stderr.
599 ~v Increase the verbosity (LogLevel) when errors are being written
601 to stderr.
602
604 Forwarding of arbitrary TCP connections over a secure channel can be
605 specified either on the command line or in a configuration file. One
606 possible application of TCP forwarding is a secure connection to a mail
607 server; another is going through firewalls.
608 In the example below, we look at encrypting communication for an IRC
610 client, even though the IRC server it connects to does not directly sup‐
611 port encrypted communication. This works as follows: the user connects
612 to the remote host using ssh, specifying the ports to be used to forward
613 the connection. After that it is possible to start the program locally,
614 and ssh will encrypt and forward the connection to the remote server.
615 The following example tunnels an IRC session from the client to an IRC
617 server at “server.example.com”, joining channel “#users”, nickname
618 “pinky”, using the standard IRC port, 6667:
619 $ ssh -f -L 6667:localhost:6667 server.example.com sleep 10
621 $ irc -c '#users' pinky IRC/127.0.0.1
622 The -f option backgrounds ssh and the remote command “sleep 10” is speci‐
624 fied to allow an amount of time (10 seconds, in the example) to start the
625 program which is going to use the tunnel. If no connections are made
626 within the time specified, ssh will exit.
627
629 If the ForwardX11 variable is set to “yes” (or see the description of the
630 -X, -x, and -Y options above) and the user is using X11 (the DISPLAY
631 environment variable is set), the connection to the X11 display is auto‐
632 matically forwarded to the remote side in such a way that any X11 pro‐
633 grams started from the shell (or command) will go through the encrypted
634 channel, and the connection to the real X server will be made from the
635 local machine. The user should not manually set DISPLAY. Forwarding of
636 X11 connections can be configured on the command line or in configuration
637 files.
638 The DISPLAY value set by ssh will point to the server machine, but with a
640 display number greater than zero. This is normal, and happens because
641 ssh creates a “proxy” X server on the server machine for forwarding the
642 connections over the encrypted channel.
643 ssh will also automatically set up Xauthority data on the server machine.
645 For this purpose, it will generate a random authorization cookie, store
646 it in Xauthority on the server, and verify that any forwarded connections
647 carry this cookie and replace it by the real cookie when the connection
648 is opened. The real authentication cookie is never sent to the server
649 machine (and no cookies are sent in the plain).
650 If the ForwardAgent variable is set to “yes” (or see the description of
652 the -A and -a options above) and the user is using an authentication
653 agent, the connection to the agent is automatically forwarded to the
654 remote side.
655
657 When connecting to a server for the first time, a fingerprint of the
658 server's public key is presented to the user (unless the option
659 StrictHostKeyChecking has been disabled). Fingerprints can be determined
660 using ssh-keygen(1):
661 $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
663 If the fingerprint is already known, it can be matched and the key can be
665 accepted or rejected. If only legacy (MD5) fingerprints for the server
666 are available, the ssh-keygen(1) -E option may be used to downgrade the
667 fingerprint algorithm to match.
668 Because of the difficulty of comparing host keys just by looking at fin‐
670 gerprint strings, there is also support to compare host keys visually,
671 using random art. By setting the VisualHostKey option to “yes”, a small
672 ASCII graphic gets displayed on every login to a server, no matter if the
673 session itself is interactive or not. By learning the pattern a known
674 server produces, a user can easily find out that the host key has changed
675 when a completely different pattern is displayed. Because these patterns
676 are not unambiguous however, a pattern that looks similar to the pattern
677 remembered only gives a good probability that the host key is the same,
678 not guaranteed proof.
679 To get a listing of the fingerprints along with their random art for all
681 known hosts, the following command line can be used:
682 $ ssh-keygen -lv -f ~/.ssh/known_hosts
684 If the fingerprint is unknown, an alternative method of verification is
686 available: SSH fingerprints verified by DNS. An additional resource
687 record (RR), SSHFP, is added to a zonefile and the connecting client is
688 able to match the fingerprint with that of the key presented.
689 In this example, we are connecting a client to a server,
691 “host.example.com”. The SSHFP resource records should first be added to
692 the zonefile for host.example.com:
693 $ ssh-keygen -r host.example.com.
695 The output lines will have to be added to the zonefile. To check that
697 the zone is answering fingerprint queries:
698 $ dig -t SSHFP host.example.com
700 Finally the client connects:
702 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
704 [...]
705 Matching host key fingerprint found in DNS.
706 Are you sure you want to continue connecting (yes/no)?
707 See the VerifyHostKeyDNS option in ssh_config(5) for more information.
709
711 ssh contains support for Virtual Private Network (VPN) tunnelling using
712 the tun(4) network pseudo-device, allowing two networks to be joined
713 securely. The sshd_config(5) configuration option PermitTunnel controls
714 whether the server supports this, and at what level (layer 2 or 3 traf‐
715 fic).
716 The following example would connect client network 10.0.50.0/24 with
718 remote network 10.0.99.0/24 using a point-to-point connection from
719 10.1.1.1 to 10.1.1.2, provided that the SSH server running on the gateway
720 to the remote network, at 192.168.1.15, allows it.
721 On the client:
723 # ssh -f -w 0:1 192.168.1.15 true
725 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
726 # route add 10.0.99.0/24 10.1.1.2
727 On the server:
729 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
731 # route add 10.0.50.0/24 10.1.1.1
732 Client access may be more finely tuned via the /root/.ssh/authorized_keys
734 file (see below) and the PermitRootLogin server option. The following
735 entry would permit connections on tun(4) device 1 from user “jane” and on
736 tun device 2 from user “john”, if PermitRootLogin is set to
737 “forced-commands-only”:
738 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
740 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
741 Since an SSH-based setup entails a fair amount of overhead, it may be
743 more suited to temporary setups, such as for wireless VPNs. More perma‐
744 nent VPNs are better provided by tools such as ipsecctl(8) and
745 isakmpd(8).
746
748 ssh will normally set the following environment variables:
749 DISPLAY The DISPLAY variable indicates the location of the
751 X11 server. It is automatically set by ssh to
752 point to a value of the form “hostname:n”, where
753 “hostname” indicates the host where the shell runs,
754 and ‘n’ is an integer ≥ 1. ssh uses this special
755 value to forward X11 connections over the secure
756 channel. The user should normally not set DISPLAY
757 explicitly, as that will render the X11 connection
758 insecure (and will require the user to manually
759 copy any required authorization cookies).
760 HOME Set to the path of the user's home directory.
762 LOGNAME Synonym for USER; set for compatibility with sys‐
764 tems that use this variable.
765 MAIL Set to the path of the user's mailbox.
767 PATH Set to the default PATH, as specified when compil‐
769 ing ssh.
770 SSH_ASKPASS If ssh needs a passphrase, it will read the
772 passphrase from the current terminal if it was run
773 from a terminal. If ssh does not have a terminal
774 associated with it but DISPLAY and SSH_ASKPASS are
775 set, it will execute the program specified by
776 SSH_ASKPASS and open an X11 window to read the
777 passphrase. This is particularly useful when call‐
778 ing ssh from a .xsession or related script. (Note
779 that on some machines it may be necessary to redi‐
780 rect the input from /dev/null to make this work.)
781 SSH_AUTH_SOCK Identifies the path of a UNIX-domain socket used to
783 communicate with the agent.
784 SSH_CONNECTION Identifies the client and server ends of the con‐
786 nection. The variable contains four space-sepa‐
787 rated values: client IP address, client port num‐
788 ber, server IP address, and server port number.
789 SSH_ORIGINAL_COMMAND This variable contains the original command line if
791 a forced command is executed. It can be used to
792 extract the original arguments.
793 SSH_TTY This is set to the name of the tty (path to the
795 device) associated with the current shell or com‐
796 mand. If the current session has no tty, this
797 variable is not set.
798 SSH_TUNNEL Optionally set by sshd(8) to contain the interface
800 names assigned if tunnel forwarding was requested
801 by the client.
802 SSH_USER_AUTH Optionally set by sshd(8), this variable may con‐
804 tain a pathname to a file that lists the authenti‐
805 cation methods successfully used when the session
806 was established, including any public keys that
807 were used.
808 TZ This variable is set to indicate the present time
810 zone if it was set when the daemon was started
811 (i.e. the daemon passes the value on to new connec‐
812 tions).
813 USER Set to the name of the user logging in.
815 Additionally, ssh reads ~/.ssh/environment, and adds lines of the format
817 “VARNAME=value” to the environment if the file exists and users are
818 allowed to change their environment. For more information, see the
819 PermitUserEnvironment option in sshd_config(5).
820
822 ~/.rhosts
823 This file is used for host-based authentication (see above). On
824 some machines this file may need to be world-readable if the
825 user's home directory is on an NFS partition, because sshd(8)
826 reads it as root. Additionally, this file must be owned by the
827 user, and must not have write permissions for anyone else. The
828 recommended permission for most machines is read/write for the
829 user, and not accessible by others.
830 ~/.shosts
832 This file is used in exactly the same way as .rhosts, but allows
833 host-based authentication without permitting login with
834 rlogin/rsh.
835 ~/.ssh/
837 This directory is the default location for all user-specific con‐
838 figuration and authentication information. There is no general
839 requirement to keep the entire contents of this directory secret,
840 but the recommended permissions are read/write/execute for the
841 user, and not accessible by others.
842 ~/.ssh/authorized_keys
844 Lists the public keys (DSA, ECDSA, Ed25519, RSA) that can be used
845 for logging in as this user. The format of this file is
846 described in the sshd(8) manual page. This file is not highly
847 sensitive, but the recommended permissions are read/write for the
848 user, and not accessible by others.
849 ~/.ssh/config
851 This is the per-user configuration file. The file format and
852 configuration options are described in ssh_config(5). Because of
853 the potential for abuse, this file must have strict permissions:
854 read/write for the user, and not writable by others.
855 ~/.ssh/environment
857 Contains additional definitions for environment variables; see
858 ENVIRONMENT, above.
859 ~/.ssh/id_dsa
861 ~/.ssh/id_ecdsa
862 ~/.ssh/id_ed25519
863 ~/.ssh/id_rsa
864 Contains the private key for authentication. These files contain
865 sensitive data and should be readable by the user but not acces‐
866 sible by others (read/write/execute). ssh will simply ignore a
867 private key file if it is accessible by others. It is possible
868 to specify a passphrase when generating the key which will be
869 used to encrypt the sensitive part of this file using AES-128.
870 ~/.ssh/id_dsa.pub
872 ~/.ssh/id_ecdsa.pub
873 ~/.ssh/id_ed25519.pub
874 ~/.ssh/id_rsa.pub
875 Contains the public key for authentication. These files are not
876 sensitive and can (but need not) be readable by anyone.
877 ~/.ssh/known_hosts
879 Contains a list of host keys for all hosts the user has logged
880 into that are not already in the systemwide list of known host
881 keys. See sshd(8) for further details of the format of this
882 file.
883 ~/.ssh/rc
885 Commands in this file are executed by ssh when the user logs in,
886 just before the user's shell (or command) is started. See the
887 sshd(8) manual page for more information.
888 /etc/hosts.equiv
890 This file is for host-based authentication (see above). It
891 should only be writable by root.
892 /etc/ssh/shosts.equiv
894 This file is used in exactly the same way as hosts.equiv, but
895 allows host-based authentication without permitting login with
896 rlogin/rsh.
897 /etc/ssh/ssh_config
899 Systemwide configuration file. The file format and configuration
900 options are described in ssh_config(5).
901 /etc/ssh/ssh_host_key
903 /etc/ssh/ssh_host_dsa_key
904 /etc/ssh/ssh_host_ecdsa_key
905 /etc/ssh/ssh_host_ed25519_key
906 /etc/ssh/ssh_host_rsa_key
907 These files contain the private parts of the host keys and are
908 used for host-based authentication.
909 /etc/ssh/ssh_known_hosts
911 Systemwide list of known host keys. This file should be prepared
912 by the system administrator to contain the public host keys of
913 all machines in the organization. It should be world-readable.
914 See sshd(8) for further details of the format of this file.
915 /etc/ssh/sshrc
917 Commands in this file are executed by ssh when the user logs in,
918 just before the user's shell (or command) is started. See the
919 sshd(8) manual page for more information.
920
922 ssh exits with the exit status of the remote command or with 255 if an
923 error occurred.
924
926 IPv6 address can be used everywhere where IPv4 address. In all entries
927 must be the IPv6 address enclosed in square brackets. Note: The square
928 brackets are metacharacters for the shell and must be escaped in shell.
929
931 scp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), ssh-keyscan(1),
932 tun(4), ssh_config(5), ssh-keysign(8), sshd(8)
933
935 S. Lehtinen and C. Lonvick, The Secure Shell (SSH) Protocol Assigned
936 Numbers, RFC 4250, January 2006.
937 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Protocol Architecture,
939 RFC 4251, January 2006.
940 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Authentication Protocol,
942 RFC 4252, January 2006.
943 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Transport Layer
945 Protocol, RFC 4253, January 2006.
946 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Connection Protocol, RFC
948 4254, January 2006.
949 J. Schlyter and W. Griffin, Using DNS to Securely Publish Secure Shell
951 (SSH) Key Fingerprints, RFC 4255, January 2006.
952 F. Cusack and M. Forssen, Generic Message Exchange Authentication for the
954 Secure Shell Protocol (SSH), RFC 4256, January 2006.
955 J. Galbraith and P. Remaker, The Secure Shell (SSH) Session Channel Break
957 Extension, RFC 4335, January 2006.
958 M. Bellare, T. Kohno, and C. Namprempre, The Secure Shell (SSH) Transport
960 Layer Encryption Modes, RFC 4344, January 2006.
961 B. Harris, Improved Arcfour Modes for the Secure Shell (SSH) Transport
963 Layer Protocol, RFC 4345, January 2006.
964 M. Friedl, N. Provos, and W. Simpson, Diffie-Hellman Group Exchange for
966 the Secure Shell (SSH) Transport Layer Protocol, RFC 4419, March 2006.
967 J. Galbraith and R. Thayer, The Secure Shell (SSH) Public Key File
969 Format, RFC 4716, November 2006.
970 D. Stebila and J. Green, Elliptic Curve Algorithm Integration in the
972 Secure Shell Transport Layer, RFC 5656, December 2009.
973 A. Perrig and D. Song, Hash Visualization: a New Technique to improve
975 Real-World Security, 1999, International Workshop on Cryptographic
976 Techniques and E-Commerce (CrypTEC '99).
977
979 OpenSSH is a derivative of the original and free ssh 1.2.12 release by
980 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
981 de Raadt and Dug Song removed many bugs, re-added newer features and cre‐
982 ated OpenSSH. Markus Friedl contributed the support for SSH protocol
983 versions 1.5 and 2.0.
984BSD December 31, 2019 BSD