1SSH(1) BSD General Commands Manual SSH(1)
2
4 ssh — OpenSSH remote login client
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 connections from an authentication agent
37 such as ssh-agent(1). This can also be specified on a per-host
38 basis in a configuration 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. A safer alternative may be to use a jump host (see
47 -J).
48 -a Disables forwarding of the authentication agent connection.
50 -B bind_interface
52 Bind to the address of bind_interface before attempting to con‐
53 nect to the destination host. This is only useful on systems
54 with more than one address.
55 -b bind_address
57 Use bind_address on the local machine as the source address of
58 the connection. Only useful on systems with more than one ad‐
59 dress.
60 -C Requests compression of all data (including stdin, stdout,
62 stderr, and data for forwarded X11, TCP and UNIX-domain connec‐
63 tions). The compression algorithm is the same used by gzip(1).
64 Compression is desirable on modem lines and other slow connec‐
65 tions, but will only slow down things on fast networks. The de‐
66 fault value can be set on a host-by-host basis in the configura‐
67 tion files; see the Compression option.
68 -c cipher_spec
70 Selects the cipher specification for encrypting the session.
71 cipher_spec is a comma-separated list of ciphers listed in order
72 of preference. See the Ciphers keyword in ssh_config(5) for more
73 information.
74 -D [bind_address:]port
76 Specifies a local “dynamic” application-level port forwarding.
77 This works by allocating a socket to listen to port on the local
78 side, optionally bound to the specified bind_address. Whenever a
79 connection is made to this port, the connection is forwarded over
80 the secure channel, and the application protocol is then used to
81 determine where to connect to from the remote machine. Currently
82 the SOCKS4 and SOCKS5 protocols are supported, and ssh will act
83 as a SOCKS server. Only root can forward privileged ports. Dy‐
84 namic port forwardings can also be specified in the configuration
85 file.
86 IPv6 addresses can be specified by enclosing the address in
88 square brackets. Only the superuser can forward privileged
89 ports. By default, the local port is bound in accordance with
90 the GatewayPorts setting. However, an explicit bind_address may
91 be used to bind the connection to a specific address. The
92 bind_address of “localhost” indicates that the listening port be
93 bound for local use only, while an empty address or ‘*’ indicates
94 that the port should be available from all interfaces.
95 -E log_file
97 Append debug logs to log_file instead of standard error.
98 -e escape_char
100 Sets the escape character for sessions with a pty (default: ‘~’).
101 The escape character is only recognized at the beginning of a
102 line. The escape character followed by a dot (‘.’) closes the
103 connection; followed by control-Z suspends the connection; and
104 followed by itself sends the escape character once. Setting the
105 character to “none” disables any escapes and makes the session
106 fully transparent.
107 -F configfile
109 Specifies an alternative per-user configuration file. If a con‐
110 figuration file is given on the command line, the system-wide
111 configuration file (/etc/ssh/ssh_config) will be ignored. The
112 default for the per-user configuration file is ~/.ssh/config. If
113 set to “none”, no configuration files will be read.
114 -f Requests ssh to go to background just before command execution.
116 This is useful if ssh is going to ask for passwords or
117 passphrases, but the user wants it in the background. This im‐
118 plies -n. The recommended way to start X11 programs at a remote
119 site is with something like ssh -f host xterm.
120 If the ExitOnForwardFailure configuration option is set to “yes”,
122 then a client started with -f will wait for all remote port for‐
123 wards to be successfully established before placing itself in the
124 background.
125 -G Causes ssh to print its configuration after evaluating Host and
127 Match blocks and exit.
128 -g Allows remote hosts to connect to local forwarded ports. If used
130 on a multiplexed connection, then this option must be specified
131 on the master process.
132 -I pkcs11
134 Specify the PKCS#11 shared library ssh should use to communicate
135 with a PKCS#11 token providing keys for user authentication.
136 -i identity_file
138 Selects a file from which the identity (private key) for public
139 key authentication is read. The default is ~/.ssh/id_dsa,
140 ~/.ssh/id_ecdsa, ~/.ssh/id_ecdsa_sk, ~/.ssh/id_ed25519,
141 ~/.ssh/id_ed25519_sk and ~/.ssh/id_rsa. Identity files may also
142 be specified on a per-host basis in the configuration file. It
143 is possible to have multiple -i options (and multiple identities
144 specified in configuration files). If no certificates have been
145 explicitly specified by the CertificateFile directive, ssh will
146 also try to load certificate information from the filename ob‐
147 tained by appending -cert.pub to identity filenames.
148 -J destination
150 Connect to the target host by first making a ssh connection to
151 the jump host described by destination and then establishing a
152 TCP forwarding to the ultimate destination from there. Multiple
153 jump hops may be specified separated by comma characters. This
154 is a shortcut to specify a ProxyJump configuration directive.
155 Note that configuration directives supplied on the command-line
156 generally apply to the destination host and not any specified
157 jump hosts. Use ~/.ssh/config to specify configuration for jump
158 hosts.
159 -K Enables GSSAPI-based authentication and forwarding (delegation)
161 of GSSAPI credentials to the server.
162 -k Disables forwarding (delegation) of GSSAPI credentials to the
164 server.
165 -L [bind_address:]port:host:hostport
167 -L [bind_address:]port:remote_socket
168 -L local_socket:host:hostport
169 -L local_socket:remote_socket
170 Specifies that connections to the given TCP port or Unix socket
171 on the local (client) host are to be forwarded to the given host
172 and port, or Unix socket, on the remote side. This works by al‐
173 locating a socket to listen to either a TCP port on the local
174 side, optionally bound to the specified bind_address, or to a
175 Unix socket. Whenever a connection is made to the local port or
176 socket, the connection is forwarded over the secure channel, and
177 a connection is made to either host port hostport, or the Unix
178 socket remote_socket, from the remote machine.
179 Port forwardings can also be specified in the configuration file.
181 Only the superuser can forward privileged ports. IPv6 addresses
182 can be specified by enclosing the address in square brackets.
183 By default, the local port is bound in accordance with the
185 GatewayPorts setting. However, an explicit bind_address may be
186 used to bind the connection to a specific address. The
187 bind_address of “localhost” indicates that the listening port be
188 bound for local use only, while an empty address or ‘*’ indicates
189 that the port should be available from all interfaces.
190 -l login_name
192 Specifies the user to log in as on the remote machine. This also
193 may be specified on a per-host basis in the configuration file.
194 -M Places the ssh client into “master” mode for connection sharing.
196 Multiple -M options places ssh into “master” mode but with con‐
197 firmation required using ssh-askpass(1) before each operation
198 that changes the multiplexing state (e.g. opening a new session).
199 Refer to the description of ControlMaster in ssh_config(5) for
200 details.
201 -m mac_spec
203 A comma-separated list of MAC (message authentication code) algo‐
204 rithms, specified in order of preference. See the MACs keyword
205 for more information.
206 -N Do not execute a remote command. This is useful for just for‐
208 warding ports.
209 -n Redirects stdin from /dev/null (actually, prevents reading from
211 stdin). This must be used when ssh is run in the background. A
212 common trick is to use this to run X11 programs on a remote ma‐
213 chine. For example, ssh -n shadows.cs.hut.fi emacs & will start
214 an emacs on shadows.cs.hut.fi, and the X11 connection will be au‐
215 tomatically forwarded over an encrypted channel. The ssh program
216 will be put in the background. (This does not work if ssh needs
217 to ask for a password or passphrase; see also the -f option.)
218 -O ctl_cmd
220 Control an active connection multiplexing master process. When
221 the -O option is specified, the ctl_cmd argument is interpreted
222 and passed to the master process. Valid commands are: “check”
223 (check that the master process is running), “forward” (request
224 forwardings without command execution), “cancel” (cancel forward‐
225 ings), “exit” (request the master to exit), and “stop” (request
226 the master to stop accepting further multiplexing requests).
227 -o option
229 Can be used to give options in the format used in the configura‐
230 tion file. This is useful for specifying options for which there
231 is no separate command-line flag. For full details of the op‐
232 tions listed below, and their possible values, see 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 HostbasedAcceptedAlgorithms
276 HostbasedAuthentication
277 HostKeyAlgorithms
278 HostKeyAlias
279 Hostname
280 IdentitiesOnly
281 IdentityAgent
282 IdentityFile
283 IPQoS
284 KbdInteractiveAuthentication
285 KbdInteractiveDevices
286 KexAlgorithms
287 KnownHostsCommand
288 LocalCommand
289 LocalForward
290 LogLevel
291 MACs
292 Match
293 NoHostAuthenticationForLocalhost
294 NumberOfPasswordPrompts
295 PasswordAuthentication
296 PermitLocalCommand
297 PermitRemoteOpen
298 PKCS11Provider
299 Port
300 PreferredAuthentications
301 ProxyCommand
302 ProxyJump
303 ProxyUseFdpass
304 PubkeyAcceptedAlgorithms
305 PubkeyAuthentication
306 RekeyLimit
307 RemoteCommand
308 RemoteForward
309 RequestTTY
310 SendEnv
311 ServerAliveInterval
312 ServerAliveCountMax
313 SetEnv
314 StreamLocalBindMask
315 StreamLocalBindUnlink
316 StrictHostKeyChecking
317 TCPKeepAlive
318 Tunnel
319 TunnelDevice
320 UpdateHostKeys
321 User
322 UserKnownHostsFile
323 VerifyHostKeyDNS
324 VisualHostKey
325 XAuthLocation
326 -p port
328 Port to connect to on the remote host. This can be specified on
329 a per-host basis in the configuration file.
330 -Q query_option
332 Queries ssh for the algorithms supported for the specified ver‐
333 sion 2. The available features are: cipher (supported symmetric
334 ciphers), cipher-auth (supported symmetric ciphers that support
335 authenticated encryption), help (supported query terms for use
336 with the -Q flag), mac (supported message integrity codes), kex
337 (key exchange algorithms), kex-gss (GSSAPI key exchange algo‐
338 rithms), key (key types), key-cert (certificate key types),
339 key-plain (non-certificate key types), key-sig (all key types and
340 signature algorithms), protocol-version (supported SSH protocol
341 versions), and sig (supported signature algorithms). Alterna‐
342 tively, any keyword from ssh_config(5) or sshd_config(5) that
343 takes an algorithm list may be used as an alias for the corre‐
344 sponding query_option.
345 -q Quiet mode. Causes most warning and diagnostic messages to be
347 suppressed.
348 -R [bind_address:]port:host:hostport
350 -R [bind_address:]port:local_socket
351 -R remote_socket:host:hostport
352 -R remote_socket:local_socket
353 -R [bind_address:]port
354 Specifies that connections to the given TCP port or Unix socket
355 on the remote (server) host are to be forwarded to the local
356 side.
357 This works by allocating a socket to listen to either a TCP port
359 or to a Unix socket on the remote side. Whenever a connection is
360 made to this port or Unix socket, the connection is forwarded
361 over the secure channel, and a connection is made from the local
362 machine to either an explicit destination specified by host port
363 hostport, or local_socket, or, if no explicit destination was
364 specified, ssh will act as a SOCKS 4/5 proxy and forward connec‐
365 tions to the destinations requested by the remote SOCKS client.
366 Port forwardings can also be specified in the configuration file.
368 Privileged ports can be forwarded only when logging in as root on
369 the remote machine. IPv6 addresses can be specified by enclosing
370 the address in square brackets.
371 By default, TCP listening sockets on the server will be bound to
373 the loopback interface only. This may be overridden by specify‐
374 ing a bind_address. An empty bind_address, or the address ‘*’,
375 indicates that the remote socket should listen on all interfaces.
376 Specifying a remote bind_address will only succeed if the
377 server's GatewayPorts option is enabled (see sshd_config(5)).
378 If the port argument is ‘0’, the listen port will be dynamically
380 allocated on the server and reported to the client at run time.
381 When used together with -O forward the allocated port will be
382 printed to the standard output.
383 -S ctl_path
385 Specifies the location of a control socket for connection shar‐
386 ing, or the string “none” to disable connection sharing. Refer
387 to the description of ControlPath and ControlMaster in
388 ssh_config(5) for details.
389 -s May be used to request invocation of a subsystem on the remote
391 system. Subsystems facilitate the use of SSH as a secure trans‐
392 port for other applications (e.g. sftp(1)). The subsystem is
393 specified as the remote command.
394 -T Disable pseudo-terminal allocation.
396 -t Force pseudo-terminal allocation. This can be used to execute
398 arbitrary screen-based programs on a remote machine, which can be
399 very useful, e.g. when implementing menu services. Multiple -t
400 options force tty allocation, even if ssh has no local tty.
401 -V Display the version number and exit.
403 -v Verbose mode. Causes ssh to print debugging messages about its
405 progress. This is helpful in debugging connection, authentica‐
406 tion, and configuration problems. Multiple -v options increase
407 the verbosity. The maximum is 3.
408 -W host:port
410 Requests that standard input and output on the client be for‐
411 warded to host on port over the secure channel. Implies -N, -T,
412 ExitOnForwardFailure and ClearAllForwardings, though these can be
413 overridden in the configuration file or using -o command line op‐
414 tions.
415 -w local_tun[:remote_tun]
417 Requests tunnel device forwarding with the specified tun(4) de‐
418 vices between the client (local_tun) and the server (remote_tun).
419 The devices may be specified by numerical ID or the keyword
421 “any”, which uses the next available tunnel device. If
422 remote_tun is not specified, it defaults to “any”. See also the
423 Tunnel and TunnelDevice directives in ssh_config(5).
424 If the Tunnel directive is unset, it will be set to the default
426 tunnel mode, which is “point-to-point”. If a different Tunnel
427 forwarding mode it desired, then it should be specified before
428 -w.
429 -X Enables X11 forwarding. This can also be specified on a per-host
431 basis in a configuration file.
432 X11 forwarding should be enabled with caution. Users with the
434 ability to bypass file permissions on the remote host (for the
435 user's X authorization database) can access the local X11 display
436 through the forwarded connection. An attacker may then be able
437 to perform activities such as keystroke monitoring.
438 For this reason, X11 forwarding is subjected to X11 SECURITY ex‐
440 tension restrictions by default. Please refer to the ssh -Y op‐
441 tion and the ForwardX11Trusted directive in ssh_config(5) for
442 more information.
443 -x Disables X11 forwarding.
445 -Y Enables trusted X11 forwarding. Trusted X11 forwardings are not
447 subjected to the X11 SECURITY extension controls.
448 -y Send log information using the syslog(3) system module. By de‐
450 fault this information is sent to stderr.
451 ssh may additionally obtain configuration data from a per-user configura‐
453 tion file and a system-wide configuration file. The file format and con‐
454 figuration options are described in ssh_config(5).
455
457 The OpenSSH SSH client supports SSH protocol 2.
458 The methods available for authentication are: GSSAPI-based authentica‐
460 tion, host-based authentication, public key authentication, challenge-re‐
461 sponse authentication, and password authentication. Authentication meth‐
462 ods are tried in the order specified above, though
463 PreferredAuthentications can be used to change the default order.
464 Host-based authentication works as follows: If the machine the user logs
466 in from is listed in /etc/hosts.equiv or /etc/ssh/shosts.equiv on the re‐
467 mote machine, the user is non-root and the user names are the same on
468 both sides, or if the files ~/.rhosts or ~/.shosts exist in the user's
469 home directory on the remote machine and contain a line containing the
470 name of the client machine and the name of the user on that machine, the
471 user is considered for login. Additionally, the server must be able to
472 verify the client's host key (see the description of
473 /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts, below) for login to be
474 permitted. This authentication method closes security holes due to IP
475 spoofing, DNS spoofing, and routing spoofing. [Note to the administra‐
476 tor: /etc/hosts.equiv, ~/.rhosts, and the rlogin/rsh protocol in general,
477 are inherently insecure and should be disabled if security is desired.]
478 Public key authentication works as follows: The scheme is based on pub‐
480 lic-key cryptography, using cryptosystems where encryption and decryption
481 are done using separate keys, and it is unfeasible to derive the decryp‐
482 tion key from the encryption key. The idea is that each user creates a
483 public/private key pair for authentication purposes. The server knows
484 the public key, and only the user knows the private key. ssh implements
485 public key authentication protocol automatically, using one of the DSA,
486 ECDSA, Ed25519 or RSA algorithms. The HISTORY section of ssl(8) contains
487 a brief discussion of the DSA and RSA algorithms.
488 The file ~/.ssh/authorized_keys lists the public keys that are permitted
490 for logging in. When the user logs in, the ssh program tells the server
491 which key pair it would like to use for authentication. The client
492 proves that it has access to the private key and the server checks that
493 the corresponding public key is authorized to accept the account.
494 The server may inform the client of errors that prevented public key au‐
496 thentication from succeeding after authentication completes using a dif‐
497 ferent method. These may be viewed by increasing the LogLevel to DEBUG
498 or higher (e.g. by using the -v flag).
499 The user creates his/her key pair by running ssh-keygen(1). This stores
501 the private key in ~/.ssh/id_dsa (DSA), ~/.ssh/id_ecdsa (ECDSA),
502 ~/.ssh/id_ecdsa_sk (authenticator-hosted ECDSA), ~/.ssh/id_ed25519
503 (Ed25519), ~/.ssh/id_ed25519_sk (authenticator-hosted Ed25519), or
504 ~/.ssh/id_rsa (RSA) and stores the public key in ~/.ssh/id_dsa.pub (DSA),
505 ~/.ssh/id_ecdsa.pub (ECDSA), ~/.ssh/id_ecdsa_sk.pub (authenticator-hosted
506 ECDSA), ~/.ssh/id_ed25519.pub (Ed25519), ~/.ssh/id_ed25519_sk.pub (au‐
507 thenticator-hosted Ed25519), or ~/.ssh/id_rsa.pub (RSA) in the user's
508 home directory. The user should then copy the public key to
509 ~/.ssh/authorized_keys in his/her home directory on the remote machine.
510 The authorized_keys file corresponds to the conventional ~/.rhosts file,
511 and has one key per line, though the lines can be very long. After this,
512 the user can log in without giving the password.
513 A variation on public key authentication is available in the form of cer‐
515 tificate authentication: instead of a set of public/private keys, signed
516 certificates are used. This has the advantage that a single trusted cer‐
517 tification authority can be used in place of many public/private keys.
518 See the CERTIFICATES section of ssh-keygen(1) for more information.
519 The most convenient way to use public key or certificate authentication
521 may be with an authentication agent. See ssh-agent(1) and (optionally)
522 the AddKeysToAgent directive in ssh_config(5) for more information.
523 Challenge-response authentication works as follows: The server sends an
525 arbitrary "challenge" text, and prompts for a response. Examples of
526 challenge-response authentication include BSD Authentication (see
527 login.conf(5)) and PAM (some non-OpenBSD systems).
528 Finally, if other authentication methods fail, ssh prompts the user for a
530 password. The password is sent to the remote host for checking; however,
531 since all communications are encrypted, the password cannot be seen by
532 someone listening on the network.
533 ssh automatically maintains and checks a database containing identifica‐
535 tion for all hosts it has ever been used with. Host keys are stored in
536 ~/.ssh/known_hosts in the user's home directory. Additionally, the file
537 /etc/ssh/ssh_known_hosts is automatically checked for known hosts. Any
538 new hosts are automatically added to the user's file. If a host's iden‐
539 tification ever changes, ssh warns about this and disables password au‐
540 thentication to prevent server spoofing or man-in-the-middle attacks,
541 which could otherwise be used to circumvent the encryption. The
542 StrictHostKeyChecking option can be used to control logins to machines
543 whose host key is not known or has changed.
544 When the user's identity has been accepted by the server, the server ei‐
546 ther executes the given command in a non-interactive session or, if no
547 command has been specified, logs into the machine and gives the user a
548 normal shell as an interactive session. All communication with the re‐
549 mote command or shell will be automatically encrypted.
550 If an interactive session is requested ssh by default will only request a
552 pseudo-terminal (pty) for interactive sessions when the client has one.
553 The flags -T and -t can be used to override this behaviour.
554 If a pseudo-terminal has been allocated the user may use the escape char‐
556 acters noted below.
557 If no pseudo-terminal has been allocated, the session is transparent and
559 can be used to reliably transfer binary data. On most systems, setting
560 the escape character to “none” will also make the session transparent
561 even if a tty is used.
562 The session terminates when the command or shell on the remote machine
564 exits and all X11 and TCP connections have been closed.
565
567 When a pseudo-terminal has been requested, ssh supports a number of func‐
568 tions through the use of an escape character.
569 A single tilde character can be sent as ~~ or by following the tilde by a
571 character other than those described below. The escape character must
572 always follow a newline to be interpreted as special. The escape charac‐
573 ter can be changed in configuration files using the EscapeChar configura‐
574 tion directive or on the command line by the -e option.
575 The supported escapes (assuming the default ‘~’) are:
577 ~. Disconnect.
579 ~^Z Background ssh.
581 ~# List forwarded connections.
583 ~& Background ssh at logout when waiting for forwarded connection /
585 X11 sessions to terminate.
586 ~? Display a list of escape characters.
588 ~B Send a BREAK to the remote system (only useful if the peer sup‐
590 ports it).
591 ~C Open command line. Currently this allows the addition of port
593 forwardings using the -L, -R and -D options (see above). It also
594 allows the cancellation of existing port-forwardings with
595 -KL[bind_address:]port for local, -KR[bind_address:]port for re‐
596 mote and -KD[bind_address:]port for dynamic port-forwardings.
597 !command allows the user to execute a local command if the
598 PermitLocalCommand option is enabled in ssh_config(5). Basic
599 help is available, using the -h option.
600 ~R Request rekeying of the connection (only useful if the peer sup‐
602 ports it).
603 ~V Decrease the verbosity (LogLevel) when errors are being written
605 to stderr.
606 ~v Increase the verbosity (LogLevel) when errors are being written
608 to stderr.
609
611 Forwarding of arbitrary TCP connections over a secure channel can be
612 specified either on the command line or in a configuration file. One
613 possible application of TCP forwarding is a secure connection to a mail
614 server; another is going through firewalls.
615 In the example below, we look at encrypting communication for an IRC
617 client, even though the IRC server it connects to does not directly sup‐
618 port encrypted communication. This works as follows: the user connects
619 to the remote host using ssh, specifying the ports to be used to forward
620 the connection. After that it is possible to start the program locally,
621 and ssh will encrypt and forward the connection to the remote server.
622 The following example tunnels an IRC session from the client to an IRC
624 server at “server.example.com”, joining channel “#users”, nickname
625 “pinky”, using the standard IRC port, 6667:
626 $ ssh -f -L 6667:localhost:6667 server.example.com sleep 10
628 $ irc -c '#users' pinky IRC/127.0.0.1
629 The -f option backgrounds ssh and the remote command “sleep 10” is speci‐
631 fied to allow an amount of time (10 seconds, in the example) to start the
632 program which is going to use the tunnel. If no connections are made
633 within the time specified, ssh will exit.
634
636 If the ForwardX11 variable is set to “yes” (or see the description of the
637 -X, -x, and -Y options above) and the user is using X11 (the DISPLAY en‐
638 vironment variable is set), the connection to the X11 display is automat‐
639 ically forwarded to the remote side in such a way that any X11 programs
640 started from the shell (or command) will go through the encrypted chan‐
641 nel, and the connection to the real X server will be made from the local
642 machine. The user should not manually set DISPLAY. Forwarding of X11
643 connections can be configured on the command line or in configuration
644 files.
645 The DISPLAY value set by ssh will point to the server machine, but with a
647 display number greater than zero. This is normal, and happens because
648 ssh creates a “proxy” X server on the server machine for forwarding the
649 connections over the encrypted channel.
650 ssh will also automatically set up Xauthority data on the server machine.
652 For this purpose, it will generate a random authorization cookie, store
653 it in Xauthority on the server, and verify that any forwarded connections
654 carry this cookie and replace it by the real cookie when the connection
655 is opened. The real authentication cookie is never sent to the server
656 machine (and no cookies are sent in the plain).
657 If the ForwardAgent variable is set to “yes” (or see the description of
659 the -A and -a options above) and the user is using an authentication
660 agent, the connection to the agent is automatically forwarded to the re‐
661 mote side.
662
664 When connecting to a server for the first time, a fingerprint of the
665 server's public key is presented to the user (unless the option
666 StrictHostKeyChecking has been disabled). Fingerprints can be determined
667 using ssh-keygen(1):
668 $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
670 If the fingerprint is already known, it can be matched and the key can be
672 accepted or rejected. If only legacy (MD5) fingerprints for the server
673 are available, the ssh-keygen(1) -E option may be used to downgrade the
674 fingerprint algorithm to match.
675 Because of the difficulty of comparing host keys just by looking at fin‐
677 gerprint strings, there is also support to compare host keys visually,
678 using random art. By setting the VisualHostKey option to “yes”, a small
679 ASCII graphic gets displayed on every login to a server, no matter if the
680 session itself is interactive or not. By learning the pattern a known
681 server produces, a user can easily find out that the host key has changed
682 when a completely different pattern is displayed. Because these patterns
683 are not unambiguous however, a pattern that looks similar to the pattern
684 remembered only gives a good probability that the host key is the same,
685 not guaranteed proof.
686 To get a listing of the fingerprints along with their random art for all
688 known hosts, the following command line can be used:
689 $ ssh-keygen -lv -f ~/.ssh/known_hosts
691 If the fingerprint is unknown, an alternative method of verification is
693 available: SSH fingerprints verified by DNS. An additional resource
694 record (RR), SSHFP, is added to a zonefile and the connecting client is
695 able to match the fingerprint with that of the key presented.
696 In this example, we are connecting a client to a server,
698 “host.example.com”. The SSHFP resource records should first be added to
699 the zonefile for host.example.com:
700 $ ssh-keygen -r host.example.com.
702 The output lines will have to be added to the zonefile. To check that
704 the zone is answering fingerprint queries:
705 $ dig -t SSHFP host.example.com
707 Finally the client connects:
709 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
711 [...]
712 Matching host key fingerprint found in DNS.
713 Are you sure you want to continue connecting (yes/no)?
714 See the VerifyHostKeyDNS option in ssh_config(5) for more information.
716
718 ssh contains support for Virtual Private Network (VPN) tunnelling using
719 the tun(4) network pseudo-device, allowing two networks to be joined se‐
720 curely. The sshd_config(5) configuration option PermitTunnel controls
721 whether the server supports this, and at what level (layer 2 or 3 traf‐
722 fic).
723 The following example would connect client network 10.0.50.0/24 with re‐
725 mote network 10.0.99.0/24 using a point-to-point connection from 10.1.1.1
726 to 10.1.1.2, provided that the SSH server running on the gateway to the
727 remote network, at 192.168.1.15, allows it.
728 On the client:
730 # ssh -f -w 0:1 192.168.1.15 true
732 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
733 # route add 10.0.99.0/24 10.1.1.2
734 On the server:
736 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
738 # route add 10.0.50.0/24 10.1.1.1
739 Client access may be more finely tuned via the /root/.ssh/authorized_keys
741 file (see below) and the PermitRootLogin server option. The following
742 entry would permit connections on tun(4) device 1 from user “jane” and on
743 tun device 2 from user “john”, if PermitRootLogin is set to
744 “forced-commands-only”:
745 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
747 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
748 Since an SSH-based setup entails a fair amount of overhead, it may be
750 more suited to temporary setups, such as for wireless VPNs. More perma‐
751 nent VPNs are better provided by tools such as ipsecctl(8) and
752 isakmpd(8).
753
755 ssh will normally set the following environment variables:
756 DISPLAY The DISPLAY variable indicates the location of the
758 X11 server. It is automatically set by ssh to
759 point to a value of the form “hostname:n”, where
760 “hostname” indicates the host where the shell runs,
761 and ‘n’ is an integer ≥ 1. ssh uses this special
762 value to forward X11 connections over the secure
763 channel. The user should normally not set DISPLAY
764 explicitly, as that will render the X11 connection
765 insecure (and will require the user to manually
766 copy any required authorization cookies).
767 HOME Set to the path of the user's home directory.
769 LOGNAME Synonym for USER; set for compatibility with sys‐
771 tems that use this variable.
772 MAIL Set to the path of the user's mailbox.
774 PATH Set to the default PATH, as specified when compil‐
776 ing ssh.
777 SSH_ASKPASS If ssh needs a passphrase, it will read the
779 passphrase from the current terminal if it was run
780 from a terminal. If ssh does not have a terminal
781 associated with it but DISPLAY and SSH_ASKPASS are
782 set, it will execute the program specified by
783 SSH_ASKPASS and open an X11 window to read the
784 passphrase. This is particularly useful when call‐
785 ing ssh from a .xsession or related script. (Note
786 that on some machines it may be necessary to redi‐
787 rect the input from /dev/null to make this work.)
788 SSH_ASKPASS_REQUIRE Allows further control over the use of an askpass
790 program. If this variable is set to “never” then
791 ssh will never attempt to use one. If it is set to
792 “prefer”, then ssh will prefer to use the askpass
793 program instead of the TTY when requesting pass‐
794 words. Finally, if the variable is set to “force”,
795 then the askpass program will be used for all
796 passphrase input regardless of whether DISPLAY is
797 set.
798 SSH_AUTH_SOCK Identifies the path of a UNIX-domain socket used to
800 communicate with the agent.
801 SSH_CONNECTION Identifies the client and server ends of the con‐
803 nection. The variable contains four space-sepa‐
804 rated values: client IP address, client port num‐
805 ber, server IP address, and server port number.
806 SSH_ORIGINAL_COMMAND This variable contains the original command line if
808 a forced command is executed. It can be used to
809 extract the original arguments.
810 SSH_TTY This is set to the name of the tty (path to the de‐
812 vice) associated with the current shell or command.
813 If the current session has no tty, this variable is
814 not set.
815 SSH_TUNNEL Optionally set by sshd(8) to contain the interface
817 names assigned if tunnel forwarding was requested
818 by the client.
819 SSH_USER_AUTH Optionally set by sshd(8), this variable may con‐
821 tain a pathname to a file that lists the authenti‐
822 cation methods successfully used when the session
823 was established, including any public keys that
824 were used.
825 TZ This variable is set to indicate the present time
827 zone if it was set when the daemon was started
828 (i.e. the daemon passes the value on to new connec‐
829 tions).
830 USER Set to the name of the user logging in.
832 Additionally, ssh reads ~/.ssh/environment, and adds lines of the format
834 “VARNAME=value” to the environment if the file exists and users are al‐
835 lowed to change their environment. For more information, see the
836 PermitUserEnvironment option in sshd_config(5).
837
839 ~/.rhosts
840 This file is used for host-based authentication (see above). On
841 some machines this file may need to be world-readable if the
842 user's home directory is on an NFS partition, because sshd(8)
843 reads it as root. Additionally, this file must be owned by the
844 user, and must not have write permissions for anyone else. The
845 recommended permission for most machines is read/write for the
846 user, and not accessible by others.
847 ~/.shosts
849 This file is used in exactly the same way as .rhosts, but allows
850 host-based authentication without permitting login with
851 rlogin/rsh.
852 ~/.ssh/
854 This directory is the default location for all user-specific con‐
855 figuration and authentication information. There is no general
856 requirement to keep the entire contents of this directory secret,
857 but the recommended permissions are read/write/execute for the
858 user, and not accessible by others.
859 ~/.ssh/authorized_keys
861 Lists the public keys (DSA, ECDSA, Ed25519, RSA) that can be used
862 for logging in as this user. The format of this file is de‐
863 scribed in the sshd(8) manual page. This file is not highly sen‐
864 sitive, but the recommended permissions are read/write for the
865 user, and not accessible by others.
866 ~/.ssh/config
868 This is the per-user configuration file. The file format and
869 configuration options are described in ssh_config(5). Because of
870 the potential for abuse, this file must have strict permissions:
871 read/write for the user, and not writable by others.
872 ~/.ssh/environment
874 Contains additional definitions for environment variables; see
875 ENVIRONMENT, above.
876 ~/.ssh/id_dsa
878 ~/.ssh/id_ecdsa
879 ~/.ssh/id_ecdsa_sk
880 ~/.ssh/id_ed25519
881 ~/.ssh/id_ed25519_sk
882 ~/.ssh/id_rsa
883 Contains the private key for authentication. These files contain
884 sensitive data and should be readable by the user but not acces‐
885 sible by others (read/write/execute). ssh will simply ignore a
886 private key file if it is accessible by others. It is possible
887 to specify a passphrase when generating the key which will be
888 used to encrypt the sensitive part of this file using AES-128.
889 ~/.ssh/id_dsa.pub
891 ~/.ssh/id_ecdsa.pub
892 ~/.ssh/id_ecdsa_sk.pub
893 ~/.ssh/id_ed25519.pub
894 ~/.ssh/id_ed25519_sk.pub
895 ~/.ssh/id_rsa.pub
896 Contains the public key for authentication. These files are not
897 sensitive and can (but need not) be readable by anyone.
898 ~/.ssh/known_hosts
900 Contains a list of host keys for all hosts the user has logged
901 into that are not already in the systemwide list of known host
902 keys. See sshd(8) for further details of the format of this
903 file.
904 ~/.ssh/rc
906 Commands in this file are executed by ssh when the user logs in,
907 just before the user's shell (or command) is started. See the
908 sshd(8) manual page for more information.
909 /etc/hosts.equiv
911 This file is for host-based authentication (see above). It
912 should only be writable by root.
913 /etc/ssh/shosts.equiv
915 This file is used in exactly the same way as hosts.equiv, but al‐
916 lows host-based authentication without permitting login with
917 rlogin/rsh.
918 /etc/ssh/ssh_config
920 Systemwide configuration file. The file format and configuration
921 options are described in ssh_config(5).
922 /etc/ssh/ssh_host_key
924 /etc/ssh/ssh_host_dsa_key
925 /etc/ssh/ssh_host_ecdsa_key
926 /etc/ssh/ssh_host_ed25519_key
927 /etc/ssh/ssh_host_rsa_key
928 These files contain the private parts of the host keys and are
929 used for host-based authentication.
930 /etc/ssh/ssh_known_hosts
932 Systemwide list of known host keys. This file should be prepared
933 by the system administrator to contain the public host keys of
934 all machines in the organization. It should be world-readable.
935 See sshd(8) for further details of the format of this file.
936 /etc/ssh/sshrc
938 Commands in this file are executed by ssh when the user logs in,
939 just before the user's shell (or command) is started. See the
940 sshd(8) manual page for more information.
941
943 ssh exits with the exit status of the remote command or with 255 if an
944 error occurred.
945
947 IPv6 address can be used everywhere where IPv4 address. In all entries
948 must be the IPv6 address enclosed in square brackets. Note: The square
949 brackets are metacharacters for the shell and must be escaped in shell.
950
952 scp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), ssh-keyscan(1),
953 tun(4), ssh_config(5), ssh-keysign(8), sshd(8)
954
956 S. Lehtinen and C. Lonvick, The Secure Shell (SSH) Protocol Assigned
957 Numbers, RFC 4250, January 2006.
958 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Protocol Architecture,
960 RFC 4251, January 2006.
961 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Authentication Protocol,
963 RFC 4252, January 2006.
964 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Transport Layer
966 Protocol, RFC 4253, January 2006.
967 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Connection Protocol, RFC
969 4254, January 2006.
970 J. Schlyter and W. Griffin, Using DNS to Securely Publish Secure Shell
972 (SSH) Key Fingerprints, RFC 4255, January 2006.
973 F. Cusack and M. Forssen, Generic Message Exchange Authentication for the
975 Secure Shell Protocol (SSH), RFC 4256, January 2006.
976 J. Galbraith and P. Remaker, The Secure Shell (SSH) Session Channel Break
978 Extension, RFC 4335, January 2006.
979 M. Bellare, T. Kohno, and C. Namprempre, The Secure Shell (SSH) Transport
981 Layer Encryption Modes, RFC 4344, January 2006.
982 B. Harris, Improved Arcfour Modes for the Secure Shell (SSH) Transport
984 Layer Protocol, RFC 4345, January 2006.
985 M. Friedl, N. Provos, and W. Simpson, Diffie-Hellman Group Exchange for
987 the Secure Shell (SSH) Transport Layer Protocol, RFC 4419, March 2006.
988 J. Galbraith and R. Thayer, The Secure Shell (SSH) Public Key File
990 Format, RFC 4716, November 2006.
991 D. Stebila and J. Green, Elliptic Curve Algorithm Integration in the
993 Secure Shell Transport Layer, RFC 5656, December 2009.
994 A. Perrig and D. Song, Hash Visualization: a New Technique to improve
996 Real-World Security, 1999, International Workshop on Cryptographic
997 Techniques and E-Commerce (CrypTEC '99).
998
1000 OpenSSH is a derivative of the original and free ssh 1.2.12 release by
1001 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
1002 de Raadt and Dug Song removed many bugs, re-added newer features and cre‐
1003 ated OpenSSH. Markus Friedl contributed the support for SSH protocol
1004 versions 1.5 and 2.0.
1005BSD February 15, 2021 BSD