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 the user's private RSA key.
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 -K Enables GSSAPI-based authentication and forwarding (delegation)
155 of GSSAPI credentials to the server.
156 -k Disables forwarding (delegation) of GSSAPI credentials to the
158 server.
159 -L [bind_address:]port:host:hostport
161 -L [bind_address:]port:remote_socket
162 -L local_socket:host:hostport
163 -L local_socket:remote_socket
164 Specifies that connections to the given TCP port or Unix socket
165 on the local (client) host are to be forwarded to the given host
166 and port, or Unix socket, on the remote side. This works by
167 allocating a socket to listen to either a TCP port on the local
168 side, optionally bound to the specified bind_address, or to a
169 Unix socket. Whenever a connection is made to the local port or
170 socket, the connection is forwarded over the secure channel, and
171 a connection is made to either host port hostport, or the Unix
172 socket remote_socket, from the remote machine.
173 Port forwardings can also be specified in the configuration file.
175 Only the superuser can forward privileged ports. IPv6 addresses
176 can be specified by enclosing the address in square brackets.
177 By default, the local port is bound in accordance with the
179 GatewayPorts setting. However, an explicit bind_address may be
180 used to bind the connection to a specific address. The
181 bind_address of “localhost” indicates that the listening port be
182 bound for local use only, while an empty address or ‘*’ indicates
183 that the port should be available from all interfaces.
184 -l login_name
186 Specifies the user to log in as on the remote machine. This also
187 may be specified on a per-host basis in the configuration file.
188 -M Places the ssh client into “master” mode for connection sharing.
190 Multiple -M options places ssh into “master” mode with confirma‐
191 tion required before slave connections are accepted. Refer to
192 the description of ControlMaster in ssh_config(5) for details.
193 -m mac_spec
195 A comma-separated list of MAC (message authentication code) algo‐
196 rithms, specified in order of preference. See the MACs keyword
197 for more information.
198 -N Do not execute a remote command. This is useful for just for‐
200 warding ports.
201 -n Redirects stdin from /dev/null (actually, prevents reading from
203 stdin). This must be used when ssh is run in the background. A
204 common trick is to use this to run X11 programs on a remote
205 machine. For example, ssh -n shadows.cs.hut.fi emacs & will
206 start an emacs on shadows.cs.hut.fi, and the X11 connection will
207 be automatically forwarded over an encrypted channel. The ssh
208 program will be put in the background. (This does not work if
209 ssh needs to ask for a password or passphrase; see also the -f
210 option.)
211 -O ctl_cmd
213 Control an active connection multiplexing master process. When
214 the -O option is specified, the ctl_cmd argument is interpreted
215 and passed to the master process. Valid commands are: “check”
216 (check that the master process is running), “forward” (request
217 forwardings without command execution), “cancel” (cancel forward‐
218 ings), “exit” (request the master to exit), and “stop” (request
219 the master to stop accepting further multiplexing requests).
220 -o option
222 Can be used to give options in the format used in the configura‐
223 tion file. This is useful for specifying options for which there
224 is no separate command-line flag. For full details of the
225 options listed below, and their possible values, see
226 ssh_config(5).
227 AddKeysToAgent
229 AddressFamily
230 BatchMode
231 BindAddress
232 CanonicalDomains
233 CanonicalizeFallbackLocal
234 CanonicalizeHostname
235 CanonicalizeMaxDots
236 CanonicalizePermittedCNAMEs
237 CertificateFile
238 ChallengeResponseAuthentication
239 CheckHostIP
240 Ciphers
241 ClearAllForwardings
242 Compression
243 ConnectionAttempts
244 ConnectTimeout
245 ControlMaster
246 ControlPath
247 ControlPersist
248 DynamicForward
249 EscapeChar
250 ExitOnForwardFailure
251 FingerprintHash
252 ForwardAgent
253 ForwardX11
254 ForwardX11Timeout
255 ForwardX11Trusted
256 GatewayPorts
257 GlobalKnownHostsFile
258 GSSAPIAuthentication
259 GSSAPIKeyExchange
260 GSSAPIClientIdentity
261 GSSAPIDelegateCredentials
262 GSSAPIRenewalForcesRekey
263 GSSAPITrustDNS
264 GSSAPIKexAlgorithms
265 HashKnownHosts
266 Host
267 HostbasedAuthentication
268 HostbasedKeyTypes
269 HostKeyAlgorithms
270 HostKeyAlias
271 HostName
272 IdentitiesOnly
273 IdentityAgent
274 IdentityFile
275 IPQoS
276 KbdInteractiveAuthentication
277 KbdInteractiveDevices
278 KexAlgorithms
279 LocalCommand
280 LocalForward
281 LogLevel
282 MACs
283 Match
284 NoHostAuthenticationForLocalhost
285 NumberOfPasswordPrompts
286 PasswordAuthentication
287 PermitLocalCommand
288 PKCS11Provider
289 Port
290 PreferredAuthentications
291 ProxyCommand
292 ProxyJump
293 ProxyUseFdpass
294 PubkeyAcceptedKeyTypes
295 PubkeyAuthentication
296 RekeyLimit
297 RemoteCommand
298 RemoteForward
299 RequestTTY
300 SendEnv
301 ServerAliveInterval
302 ServerAliveCountMax
303 SetEnv
304 StreamLocalBindMask
305 StreamLocalBindUnlink
306 StrictHostKeyChecking
307 TCPKeepAlive
308 Tunnel
309 TunnelDevice
310 UpdateHostKeys
311 User
312 UserKnownHostsFile
313 VerifyHostKeyDNS
314 VisualHostKey
315 XAuthLocation
316 -p port
318 Port to connect to on the remote host. This can be specified on
319 a per-host basis in the configuration file.
320 -Q query_option
322 Queries ssh for the algorithms supported for the specified ver‐
323 sion 2. The available features are: cipher (supported symmetric
324 ciphers), cipher-auth (supported symmetric ciphers that support
325 authenticated encryption), mac (supported message integrity
326 codes), kex (key exchange algorithms), key (key types), key-cert
327 (certificate key types), key-plain (non-certificate key types),
328 and protocol-version (supported SSH protocol versions).
329 -q Quiet mode. Causes most warning and diagnostic messages to be
331 suppressed.
332 -R [bind_address:]port:host:hostport
334 -R [bind_address:]port:local_socket
335 -R remote_socket:host:hostport
336 -R remote_socket:local_socket
337 -R [bind_address:]port
338 Specifies that connections to the given TCP port or Unix socket
339 on the remote (server) host are to be forwarded to the local
340 side.
341 This works by allocating a socket to listen to either a TCP port
343 or to a Unix socket on the remote side. Whenever a connection is
344 made to this port or Unix socket, the connection is forwarded
345 over the secure channel, and a connection is made from the local
346 machine to either an explicit destination specified by host port
347 hostport, or local_socket, or, if no explicit destination was
348 specified, ssh will act as a SOCKS 4/5 proxy and forward connec‐
349 tions to the destinations requested by the remote SOCKS client.
350 Port forwardings can also be specified in the configuration file.
352 Privileged ports can be forwarded only when logging in as root on
353 the remote machine. IPv6 addresses can be specified by enclosing
354 the address in square brackets.
355 By default, TCP listening sockets on the server will be bound to
357 the loopback interface only. This may be overridden by specify‐
358 ing a bind_address. An empty bind_address, or the address ‘*’,
359 indicates that the remote socket should listen on all interfaces.
360 Specifying a remote bind_address will only succeed if the
361 server's GatewayPorts option is enabled (see sshd_config(5)).
362 If the port argument is ‘0’, the listen port will be dynamically
364 allocated on the server and reported to the client at run time.
365 When used together with -O forward the allocated port will be
366 printed to the standard output.
367 -S ctl_path
369 Specifies the location of a control socket for connection shar‐
370 ing, or the string “none” to disable connection sharing. Refer
371 to the description of ControlPath and ControlMaster in
372 ssh_config(5) for details.
373 -s May be used to request invocation of a subsystem on the remote
375 system. Subsystems facilitate the use of SSH as a secure trans‐
376 port for other applications (e.g. sftp(1)). The subsystem is
377 specified as the remote command.
378 -T Disable pseudo-terminal allocation.
380 -t Force pseudo-terminal allocation. This can be used to execute
382 arbitrary screen-based programs on a remote machine, which can be
383 very useful, e.g. when implementing menu services. Multiple -t
384 options force tty allocation, even if ssh has no local tty.
385 -V Display the version number and exit.
387 -v Verbose mode. Causes ssh to print debugging messages about its
389 progress. This is helpful in debugging connection, authentica‐
390 tion, and configuration problems. Multiple -v options increase
391 the verbosity. The maximum is 3.
392 -W host:port
394 Requests that standard input and output on the client be for‐
395 warded to host on port over the secure channel. Implies -N, -T,
396 ExitOnForwardFailure and ClearAllForwardings, though these can be
397 overridden in the configuration file or using -o command line
398 options.
399 -w local_tun[:remote_tun]
401 Requests tunnel device forwarding with the specified tun(4)
402 devices between the client (local_tun) and the server
403 (remote_tun).
404 The devices may be specified by numerical ID or the keyword
406 “any”, which uses the next available tunnel device. If
407 remote_tun is not specified, it defaults to “any”. See also the
408 Tunnel and TunnelDevice directives in ssh_config(5).
409 If the Tunnel directive is unset, it will be set to the default
411 tunnel mode, which is “point-to-point”. If a different Tunnel
412 forwarding mode it desired, then it should be specified before
413 -w.
414 -X Enables X11 forwarding. This can also be specified on a per-host
416 basis in a configuration file.
417 X11 forwarding should be enabled with caution. Users with the
419 ability to bypass file permissions on the remote host (for the
420 user's X authorization database) can access the local X11 display
421 through the forwarded connection. An attacker may then be able
422 to perform activities such as keystroke monitoring.
423 For this reason, X11 forwarding is subjected to X11 SECURITY
425 extension restrictions by default. Please refer to the ssh -Y
426 option and the ForwardX11Trusted directive in ssh_config(5) for
427 more information.
428 -x Disables X11 forwarding.
430 -Y Enables trusted X11 forwarding. Trusted X11 forwardings are not
432 subjected to the X11 SECURITY extension controls.
433 -y Send log information using the syslog(3) system module. By
435 default this information is sent to stderr.
436 ssh may additionally obtain configuration data from a per-user configura‐
438 tion file and a system-wide configuration file. The file format and con‐
439 figuration options are described in ssh_config(5).
440
442 The OpenSSH SSH client supports SSH protocol 2.
443 The methods available for authentication are: GSSAPI-based authentica‐
445 tion, host-based authentication, public key authentication, challenge-
446 response authentication, and password authentication. Authentication
447 methods are tried in the order specified above, though
448 PreferredAuthentications can be used to change the default order.
449 Host-based authentication works as follows: If the machine the user logs
451 in from is listed in /etc/hosts.equiv or /etc/ssh/shosts.equiv on the
452 remote machine, and the user names are the same on both sides, or if the
453 files ~/.rhosts or ~/.shosts exist in the user's home directory on the
454 remote machine and contain a line containing the name of the client
455 machine and the name of the user on that machine, the user is considered
456 for login. Additionally, the server must be able to verify the client's
457 host key (see the description of /etc/ssh/ssh_known_hosts and
458 ~/.ssh/known_hosts, below) for login to be permitted. This authentica‐
459 tion method closes security holes due to IP spoofing, DNS spoofing, and
460 routing spoofing. [Note to the administrator: /etc/hosts.equiv,
461 ~/.rhosts, and the rlogin/rsh protocol in general, are inherently inse‐
462 cure and should be disabled if security is desired.]
463 Public key authentication works as follows: The scheme is based on pub‐
465 lic-key cryptography, using cryptosystems where encryption and decryption
466 are done using separate keys, and it is unfeasible to derive the decryp‐
467 tion key from the encryption key. The idea is that each user creates a
468 public/private key pair for authentication purposes. The server knows
469 the public key, and only the user knows the private key. ssh implements
470 public key authentication protocol automatically, using one of the DSA,
471 ECDSA, Ed25519 or RSA algorithms. The HISTORY section of ssl(8) contains
472 a brief discussion of the DSA and RSA algorithms.
473 The file ~/.ssh/authorized_keys lists the public keys that are permitted
475 for logging in. When the user logs in, the ssh program tells the server
476 which key pair it would like to use for authentication. The client
477 proves that it has access to the private key and the server checks that
478 the corresponding public key is authorized to accept the account.
479 The server may inform the client of errors that prevented public key
481 authentication from succeeding after authentication completes using a
482 different method. These may be viewed by increasing the LogLevel to
483 DEBUG or higher (e.g. by using the -v flag).
484 The user creates his/her key pair by running ssh-keygen(1). This stores
486 the private key in ~/.ssh/id_dsa (DSA), ~/.ssh/id_ecdsa (ECDSA),
487 ~/.ssh/id_ed25519 (Ed25519), or ~/.ssh/id_rsa (RSA) and stores the public
488 key in ~/.ssh/id_dsa.pub (DSA), ~/.ssh/id_ecdsa.pub (ECDSA),
489 ~/.ssh/id_ed25519.pub (Ed25519), or ~/.ssh/id_rsa.pub (RSA) in the user's
490 home directory. The user should then copy the public key to
491 ~/.ssh/authorized_keys in his/her home directory on the remote machine.
492 The authorized_keys file corresponds to the conventional ~/.rhosts file,
493 and has one key per line, though the lines can be very long. After this,
494 the user can log in without giving the password.
495 A variation on public key authentication is available in the form of cer‐
497 tificate authentication: instead of a set of public/private keys, signed
498 certificates are used. This has the advantage that a single trusted cer‐
499 tification authority can be used in place of many public/private keys.
500 See the CERTIFICATES section of ssh-keygen(1) for more information.
501 The most convenient way to use public key or certificate authentication
503 may be with an authentication agent. See ssh-agent(1) and (optionally)
504 the AddKeysToAgent directive in ssh_config(5) for more information.
505 Challenge-response authentication works as follows: The server sends an
507 arbitrary "challenge" text, and prompts for a response. Examples of
508 challenge-response authentication include BSD Authentication (see
509 login.conf(5)) and PAM (some non-OpenBSD systems).
510 Finally, if other authentication methods fail, ssh prompts the user for a
512 password. The password is sent to the remote host for checking; however,
513 since all communications are encrypted, the password cannot be seen by
514 someone listening on the network.
515 ssh automatically maintains and checks a database containing identifica‐
517 tion for all hosts it has ever been used with. Host keys are stored in
518 ~/.ssh/known_hosts in the user's home directory. Additionally, the file
519 /etc/ssh/ssh_known_hosts is automatically checked for known hosts. Any
520 new hosts are automatically added to the user's file. If a host's iden‐
521 tification ever changes, ssh warns about this and disables password
522 authentication to prevent server spoofing or man-in-the-middle attacks,
523 which could otherwise be used to circumvent the encryption. The
524 StrictHostKeyChecking option can be used to control logins to machines
525 whose host key is not known or has changed.
526 When the user's identity has been accepted by the server, the server
528 either executes the given command in a non-interactive session or, if no
529 command has been specified, logs into the machine and gives the user a
530 normal shell as an interactive session. All communication with the
531 remote command or shell will be automatically encrypted.
532 If an interactive session is requested ssh by default will only request a
534 pseudo-terminal (pty) for interactive sessions when the client has one.
535 The flags -T and -t can be used to override this behaviour.
536 If a pseudo-terminal has been allocated the user may use the escape char‐
538 acters noted below.
539 If no pseudo-terminal has been allocated, the session is transparent and
541 can be used to reliably transfer binary data. On most systems, setting
542 the escape character to “none” will also make the session transparent
543 even if a tty is used.
544 The session terminates when the command or shell on the remote machine
546 exits and all X11 and TCP connections have been closed.
547
549 When a pseudo-terminal has been requested, ssh supports a number of func‐
550 tions through the use of an escape character.
551 A single tilde character can be sent as ~~ or by following the tilde by a
553 character other than those described below. The escape character must
554 always follow a newline to be interpreted as special. The escape charac‐
555 ter can be changed in configuration files using the EscapeChar configura‐
556 tion directive or on the command line by the -e option.
557 The supported escapes (assuming the default ‘~’) are:
559 ~. Disconnect.
561 ~^Z Background ssh.
563 ~# List forwarded connections.
565 ~& Background ssh at logout when waiting for forwarded connection /
567 X11 sessions to terminate.
568 ~? Display a list of escape characters.
570 ~B Send a BREAK to the remote system (only useful if the peer sup‐
572 ports it).
573 ~C Open command line. Currently this allows the addition of port
575 forwardings using the -L, -R and -D options (see above). It also
576 allows the cancellation of existing port-forwardings with
577 -KL[bind_address:]port for local, -KR[bind_address:]port for
578 remote and -KD[bind_address:]port for dynamic port-forwardings.
579 !command allows the user to execute a local command if the
580 PermitLocalCommand option is enabled in ssh_config(5). Basic
581 help is available, using the -h option.
582 ~R Request rekeying of the connection (only useful if the peer sup‐
584 ports it).
585 ~V Decrease the verbosity (LogLevel) when errors are being written
587 to stderr.
588 ~v Increase the verbosity (LogLevel) when errors are being written
590 to stderr.
591
593 Forwarding of arbitrary TCP connections over the secure channel can be
594 specified either on the command line or in a configuration file. One
595 possible application of TCP forwarding is a secure connection to a mail
596 server; another is going through firewalls.
597 In the example below, we look at encrypting communication between an IRC
599 client and server, even though the IRC server does not directly support
600 encrypted communications. This works as follows: the user connects to
601 the remote host using ssh, specifying a port to be used to forward con‐
602 nections to the remote server. After that it is possible to start the
603 service which is to be encrypted on the client machine, connecting to the
604 same local port, and ssh will encrypt and forward the connection.
605 The following example tunnels an IRC session from client machine
607 “127.0.0.1” (localhost) to remote server “server.example.com”:
608 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
610 $ irc -c '#users' -p 1234 pinky 127.0.0.1
611 This tunnels a connection to IRC server “server.example.com”, joining
613 channel “#users”, nickname “pinky”, using port 1234. It doesn't matter
614 which port is used, as long as it's greater than 1023 (remember, only
615 root can open sockets on privileged ports) and doesn't conflict with any
616 ports already in use. The connection is forwarded to port 6667 on the
617 remote server, since that's the standard port for IRC services.
618 The -f option backgrounds ssh and the remote command “sleep 10” is speci‐
620 fied to allow an amount of time (10 seconds, in the example) to start the
621 service which is to be tunnelled. If no connections are made within the
622 time specified, ssh will exit.
623
625 If the ForwardX11 variable is set to “yes” (or see the description of the
626 -X, -x, and -Y options above) and the user is using X11 (the DISPLAY
627 environment variable is set), the connection to the X11 display is auto‐
628 matically forwarded to the remote side in such a way that any X11 pro‐
629 grams started from the shell (or command) will go through the encrypted
630 channel, and the connection to the real X server will be made from the
631 local machine. The user should not manually set DISPLAY. Forwarding of
632 X11 connections can be configured on the command line or in configuration
633 files.
634 The DISPLAY value set by ssh will point to the server machine, but with a
636 display number greater than zero. This is normal, and happens because
637 ssh creates a “proxy” X server on the server machine for forwarding the
638 connections over the encrypted channel.
639 ssh will also automatically set up Xauthority data on the server machine.
641 For this purpose, it will generate a random authorization cookie, store
642 it in Xauthority on the server, and verify that any forwarded connections
643 carry this cookie and replace it by the real cookie when the connection
644 is opened. The real authentication cookie is never sent to the server
645 machine (and no cookies are sent in the plain).
646 If the ForwardAgent variable is set to “yes” (or see the description of
648 the -A and -a options above) and the user is using an authentication
649 agent, the connection to the agent is automatically forwarded to the
650 remote side.
651
653 When connecting to a server for the first time, a fingerprint of the
654 server's public key is presented to the user (unless the option
655 StrictHostKeyChecking has been disabled). Fingerprints can be determined
656 using ssh-keygen(1):
657 $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
659 If the fingerprint is already known, it can be matched and the key can be
661 accepted or rejected. If only legacy (MD5) fingerprints for the server
662 are available, the ssh-keygen(1) -E option may be used to downgrade the
663 fingerprint algorithm to match.
664 Because of the difficulty of comparing host keys just by looking at fin‐
666 gerprint strings, there is also support to compare host keys visually,
667 using random art. By setting the VisualHostKey option to “yes”, a small
668 ASCII graphic gets displayed on every login to a server, no matter if the
669 session itself is interactive or not. By learning the pattern a known
670 server produces, a user can easily find out that the host key has changed
671 when a completely different pattern is displayed. Because these patterns
672 are not unambiguous however, a pattern that looks similar to the pattern
673 remembered only gives a good probability that the host key is the same,
674 not guaranteed proof.
675 To get a listing of the fingerprints along with their random art for all
677 known hosts, the following command line can be used:
678 $ ssh-keygen -lv -f ~/.ssh/known_hosts
680 If the fingerprint is unknown, an alternative method of verification is
682 available: SSH fingerprints verified by DNS. An additional resource
683 record (RR), SSHFP, is added to a zonefile and the connecting client is
684 able to match the fingerprint with that of the key presented.
685 In this example, we are connecting a client to a server,
687 “host.example.com”. The SSHFP resource records should first be added to
688 the zonefile for host.example.com:
689 $ ssh-keygen -r host.example.com.
691 The output lines will have to be added to the zonefile. To check that
693 the zone is answering fingerprint queries:
694 $ dig -t SSHFP host.example.com
696 Finally the client connects:
698 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
700 [...]
701 Matching host key fingerprint found in DNS.
702 Are you sure you want to continue connecting (yes/no)?
703 See the VerifyHostKeyDNS option in ssh_config(5) for more information.
705
707 ssh contains support for Virtual Private Network (VPN) tunnelling using
708 the tun(4) network pseudo-device, allowing two networks to be joined
709 securely. The sshd_config(5) configuration option PermitTunnel controls
710 whether the server supports this, and at what level (layer 2 or 3 traf‐
711 fic).
712 The following example would connect client network 10.0.50.0/24 with
714 remote network 10.0.99.0/24 using a point-to-point connection from
715 10.1.1.1 to 10.1.1.2, provided that the SSH server running on the gateway
716 to the remote network, at 192.168.1.15, allows it.
717 On the client:
719 # ssh -f -w 0:1 192.168.1.15 true
721 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
722 # route add 10.0.99.0/24 10.1.1.2
723 On the server:
725 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
727 # route add 10.0.50.0/24 10.1.1.1
728 Client access may be more finely tuned via the /root/.ssh/authorized_keys
730 file (see below) and the PermitRootLogin server option. The following
731 entry would permit connections on tun(4) device 1 from user “jane” and on
732 tun device 2 from user “john”, if PermitRootLogin is set to
733 “forced-commands-only”:
734 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
736 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
737 Since an SSH-based setup entails a fair amount of overhead, it may be
739 more suited to temporary setups, such as for wireless VPNs. More perma‐
740 nent VPNs are better provided by tools such as ipsecctl(8) and
741 isakmpd(8).
742
744 ssh will normally set the following environment variables:
745 DISPLAY The DISPLAY variable indicates the location of the
747 X11 server. It is automatically set by ssh to
748 point to a value of the form “hostname:n”, where
749 “hostname” indicates the host where the shell runs,
750 and ‘n’ is an integer ≥ 1. ssh uses this special
751 value to forward X11 connections over the secure
752 channel. The user should normally not set DISPLAY
753 explicitly, as that will render the X11 connection
754 insecure (and will require the user to manually
755 copy any required authorization cookies).
756 HOME Set to the path of the user's home directory.
758 LOGNAME Synonym for USER; set for compatibility with sys‐
760 tems that use this variable.
761 MAIL Set to the path of the user's mailbox.
763 PATH Set to the default PATH, as specified when compil‐
765 ing ssh.
766 SSH_ASKPASS If ssh needs a passphrase, it will read the
768 passphrase from the current terminal if it was run
769 from a terminal. If ssh does not have a terminal
770 associated with it but DISPLAY and SSH_ASKPASS are
771 set, it will execute the program specified by
772 SSH_ASKPASS and open an X11 window to read the
773 passphrase. This is particularly useful when call‐
774 ing ssh from a .xsession or related script. (Note
775 that on some machines it may be necessary to redi‐
776 rect the input from /dev/null to make this work.)
777 SSH_AUTH_SOCK Identifies the path of a UNIX-domain socket used to
779 communicate with the agent.
780 SSH_CONNECTION Identifies the client and server ends of the con‐
782 nection. The variable contains four space-sepa‐
783 rated values: client IP address, client port num‐
784 ber, server IP address, and server port number.
785 SSH_ORIGINAL_COMMAND This variable contains the original command line if
787 a forced command is executed. It can be used to
788 extract the original arguments.
789 SSH_TTY This is set to the name of the tty (path to the
791 device) associated with the current shell or com‐
792 mand. If the current session has no tty, this
793 variable is not set.
794 SSH_TUNNEL Optionally set by sshd(8) to contain the interface
796 names assigned if tunnel forwarding was requested
797 by the client.
798 SSH_USER_AUTH Optionally set by sshd(8), this variable may con‐
800 tain a pathname to a file that lists the authenti‐
801 cation methods successfully used when the session
802 was established, including any public keys that
803 were used.
804 TZ This variable is set to indicate the present time
806 zone if it was set when the daemon was started
807 (i.e. the daemon passes the value on to new connec‐
808 tions).
809 USER Set to the name of the user logging in.
811 Additionally, ssh reads ~/.ssh/environment, and adds lines of the format
813 “VARNAME=value” to the environment if the file exists and users are
814 allowed to change their environment. For more information, see the
815 PermitUserEnvironment option in sshd_config(5).
816
818 ~/.rhosts
819 This file is used for host-based authentication (see above). On
820 some machines this file may need to be world-readable if the
821 user's home directory is on an NFS partition, because sshd(8)
822 reads it as root. Additionally, this file must be owned by the
823 user, and must not have write permissions for anyone else. The
824 recommended permission for most machines is read/write for the
825 user, and not accessible by others.
826 ~/.shosts
828 This file is used in exactly the same way as .rhosts, but allows
829 host-based authentication without permitting login with
830 rlogin/rsh.
831 ~/.ssh/
833 This directory is the default location for all user-specific con‐
834 figuration and authentication information. There is no general
835 requirement to keep the entire contents of this directory secret,
836 but the recommended permissions are read/write/execute for the
837 user, and not accessible by others.
838 ~/.ssh/authorized_keys
840 Lists the public keys (DSA, ECDSA, Ed25519, RSA) that can be used
841 for logging in as this user. The format of this file is
842 described in the sshd(8) manual page. This file is not highly
843 sensitive, but the recommended permissions are read/write for the
844 user, and not accessible by others.
845 ~/.ssh/config
847 This is the per-user configuration file. The file format and
848 configuration options are described in ssh_config(5). Because of
849 the potential for abuse, this file must have strict permissions:
850 read/write for the user, and not writable by others.
851 ~/.ssh/environment
853 Contains additional definitions for environment variables; see
854 ENVIRONMENT, above.
855 ~/.ssh/id_dsa
857 ~/.ssh/id_ecdsa
858 ~/.ssh/id_ed25519
859 ~/.ssh/id_rsa
860 Contains the private key for authentication. These files contain
861 sensitive data and should be readable by the user but not acces‐
862 sible by others (read/write/execute). ssh will simply ignore a
863 private key file if it is accessible by others. It is possible
864 to specify a passphrase when generating the key which will be
865 used to encrypt the sensitive part of this file using AES-128.
866 ~/.ssh/id_dsa.pub
868 ~/.ssh/id_ecdsa.pub
869 ~/.ssh/id_ed25519.pub
870 ~/.ssh/id_rsa.pub
871 Contains the public key for authentication. These files are not
872 sensitive and can (but need not) be readable by anyone.
873 ~/.ssh/known_hosts
875 Contains a list of host keys for all hosts the user has logged
876 into that are not already in the systemwide list of known host
877 keys. See sshd(8) for further details of the format of this
878 file.
879 ~/.ssh/rc
881 Commands in this file are executed by ssh when the user logs in,
882 just before the user's shell (or command) is started. See the
883 sshd(8) manual page for more information.
884 /etc/hosts.equiv
886 This file is for host-based authentication (see above). It
887 should only be writable by root.
888 /etc/ssh/shosts.equiv
890 This file is used in exactly the same way as hosts.equiv, but
891 allows host-based authentication without permitting login with
892 rlogin/rsh.
893 /etc/ssh/ssh_config
895 Systemwide configuration file. The file format and configuration
896 options are described in ssh_config(5).
897 /etc/ssh/ssh_host_key
899 /etc/ssh/ssh_host_dsa_key
900 /etc/ssh/ssh_host_ecdsa_key
901 /etc/ssh/ssh_host_ed25519_key
902 /etc/ssh/ssh_host_rsa_key
903 These files contain the private parts of the host keys and are
904 used for host-based authentication.
905 /etc/ssh/ssh_known_hosts
907 Systemwide list of known host keys. This file should be prepared
908 by the system administrator to contain the public host keys of
909 all machines in the organization. It should be world-readable.
910 See sshd(8) for further details of the format of this file.
911 /etc/ssh/sshrc
913 Commands in this file are executed by ssh when the user logs in,
914 just before the user's shell (or command) is started. See the
915 sshd(8) manual page for more information.
916
918 ssh exits with the exit status of the remote command or with 255 if an
919 error occurred.
920
922 IPv6 address can be used everywhere where IPv4 address. In all entries
923 must be the IPv6 address enclosed in square brackets. Note: The square
924 brackets are metacharacters for the shell and must be escaped in shell.
925
927 scp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), ssh-keyscan(1),
928 tun(4), ssh_config(5), ssh-keysign(8), sshd(8)
929
931 S. Lehtinen and C. Lonvick, The Secure Shell (SSH) Protocol Assigned
932 Numbers, RFC 4250, January 2006.
933 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Protocol Architecture,
935 RFC 4251, January 2006.
936 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Authentication Protocol,
938 RFC 4252, January 2006.
939 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Transport Layer
941 Protocol, RFC 4253, January 2006.
942 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Connection Protocol, RFC
944 4254, January 2006.
945 J. Schlyter and W. Griffin, Using DNS to Securely Publish Secure Shell
947 (SSH) Key Fingerprints, RFC 4255, January 2006.
948 F. Cusack and M. Forssen, Generic Message Exchange Authentication for the
950 Secure Shell Protocol (SSH), RFC 4256, January 2006.
951 J. Galbraith and P. Remaker, The Secure Shell (SSH) Session Channel Break
953 Extension, RFC 4335, January 2006.
954 M. Bellare, T. Kohno, and C. Namprempre, The Secure Shell (SSH) Transport
956 Layer Encryption Modes, RFC 4344, January 2006.
957 B. Harris, Improved Arcfour Modes for the Secure Shell (SSH) Transport
959 Layer Protocol, RFC 4345, January 2006.
960 M. Friedl, N. Provos, and W. Simpson, Diffie-Hellman Group Exchange for
962 the Secure Shell (SSH) Transport Layer Protocol, RFC 4419, March 2006.
963 J. Galbraith and R. Thayer, The Secure Shell (SSH) Public Key File
965 Format, RFC 4716, November 2006.
966 D. Stebila and J. Green, Elliptic Curve Algorithm Integration in the
968 Secure Shell Transport Layer, RFC 5656, December 2009.
969 A. Perrig and D. Song, Hash Visualization: a New Technique to improve
971 Real-World Security, 1999, International Workshop on Cryptographic
972 Techniques and E-Commerce (CrypTEC '99).
973
975 OpenSSH is a derivative of the original and free ssh 1.2.12 release by
976 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
977 de Raadt and Dug Song removed many bugs, re-added newer features and cre‐
978 ated OpenSSH. Markus Friedl contributed the support for SSH protocol
979 versions 1.5 and 2.0.
980BSD October 26, 2019 BSD