1SSH(1) BSD General Commands Manual SSH(1)
2
4 ssh — OpenSSH SSH client (remote login program)
5
7 ssh [-1246AaCfGgKkMNnqsTtVvXxYy] [-b bind_address] [-c cipher_spec]
8 [-D [bind_address:]port] [-E log_file] [-e escape_char]
9 [-F configfile] [-I pkcs11] [-i identity_file]
10 [-J [user@]host[:port]] [-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]]
13 [user@]hostname [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 hostname (with optional user
23 name). The user must prove his/her identity to the remote machine using
24 one of several methods (see below).
25 If command is specified, it is executed on the remote host instead of a
27 login shell.
28 The options are as follows:
30 -1 Forces ssh to try protocol version 1 only.
32 -2 Forces ssh to try protocol version 2 only.
34 -4 Forces ssh to use IPv4 addresses only.
36 -6 Forces ssh to use IPv6 addresses only.
38 -A Enables forwarding of the authentication agent connection. This
40 can also be specified on a per-host basis in a configuration
41 file.
42 Agent forwarding should be enabled with caution. Users with the
44 ability to bypass file permissions on the remote host (for the
45 agent's UNIX-domain socket) can access the local agent through
46 the forwarded connection. An attacker cannot obtain key material
47 from the agent, however they can perform operations on the keys
48 that enable them to authenticate using the identities loaded into
49 the agent.
50 -a Disables forwarding of the authentication agent connection.
52 -b bind_address
54 Use bind_address on the local machine as the source address of
55 the connection. Only useful on systems with more than one
56 address.
57 -C Requests compression of all data (including stdin, stdout,
59 stderr, and data for forwarded X11, TCP and UNIX-domain connec‐
60 tions). The compression algorithm is the same used by gzip(1),
61 and the “level” can be controlled by the CompressionLevel option
62 for protocol version 1. Compression is desirable on modem lines
63 and other slow connections, but will only slow down things on
64 fast networks. The default value can be set on a host-by-host
65 basis in the configuration files; see the Compression option.
66 -c cipher_spec
68 Selects the cipher specification for encrypting the session.
69 Protocol version 1 allows specification of a single cipher. The
71 supported values are “3des”, “blowfish”, and “des”. For protocol
72 version 2, cipher_spec is a comma-separated list of ciphers
73 listed in order of preference. See the Ciphers keyword in
74 ssh_config(5) for more information.
75 -D [bind_address:]port
77 Specifies a local “dynamic” application-level port forwarding.
78 This works by allocating a socket to listen to port on the local
79 side, optionally bound to the specified bind_address. Whenever a
80 connection is made to this port, the connection is forwarded over
81 the secure channel, and the application protocol is then used to
82 determine where to connect to from the remote machine. Currently
83 the SOCKS4 and SOCKS5 protocols are supported, and ssh will act
84 as a SOCKS server. Only root can forward privileged ports.
85 Dynamic port forwardings can also be specified in the configura‐
86 tion file.
87 IPv6 addresses can be specified by enclosing the address in
89 square brackets. Only the superuser can forward privileged
90 ports. By default, the local port is bound in accordance with
91 the GatewayPorts setting. However, an explicit bind_address may
92 be used to bind the connection to a specific address. The
93 bind_address of “localhost” indicates that the listening port be
94 bound for local use only, while an empty address or ‘*’ indicates
95 that the port should be available from all interfaces.
96 -E log_file
98 Append debug logs to log_file instead of standard error.
99 -e escape_char
101 Sets the escape character for sessions with a pty (default: ‘~’).
102 The escape character is only recognized at the beginning of a
103 line. The escape character followed by a dot (‘.’) closes the
104 connection; followed by control-Z suspends the connection; and
105 followed by itself sends the escape character once. Setting the
106 character to “none” disables any escapes and makes the session
107 fully transparent.
108 -F configfile
110 Specifies an alternative per-user configuration file. If a con‐
111 figuration file is given on the command line, the system-wide
112 configuration file (/etc/ssh/ssh_config) will be ignored. The
113 default for the per-user configuration file is ~/.ssh/config.
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
118 implies -n. The recommended way to start X11 programs at a
119 remote 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 the user's private RSA key.
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/identity for
140 protocol version 1, and ~/.ssh/id_dsa, ~/.ssh/id_ecdsa,
141 ~/.ssh/id_ed25519 and ~/.ssh/id_rsa for protocol version 2.
142 Identity files may also be specified on a per-host basis in the
143 configuration file. It is possible to have multiple -i options
144 (and multiple identities specified in configuration files). If
145 no certificates have been explicitly specified by the
146 CertificateFile directive, ssh will also try to load certificate
147 information from the filename obtained by appending -cert.pub to
148 identity filenames.
149 -J [user@]host[:port]
151 Connect to the target host by first making a ssh connection to
152 the jump host and then establishing a TCP forwarding to the ulti‐
153 mate destination from there. Multiple jump hops may be specified
154 separated by comma characters. This is a shortcut to specify a
155 ProxyJump configuration directive.
156 -K Enables GSSAPI-based authentication and forwarding (delegation)
158 of GSSAPI credentials to the server.
159 -k Disables forwarding (delegation) of GSSAPI credentials to the
161 server.
162 -L [bind_address:]port:host:hostport
164 -L [bind_address:]port:remote_socket
165 -L local_socket:host:hostport
166 -L local_socket:remote_socket
167 Specifies that connections to the given TCP port or Unix socket
168 on the local (client) host are to be forwarded to the given host
169 and port, or Unix socket, on the remote side. This works by
170 allocating a socket to listen to either a TCP port on the local
171 side, optionally bound to the specified bind_address, or to a
172 Unix socket. Whenever a connection is made to the local port or
173 socket, the connection is forwarded over the secure channel, and
174 a connection is made to either host port hostport, or the Unix
175 socket remote_socket, from the remote machine.
176 Port forwardings can also be specified in the configuration file.
178 Only the superuser can forward privileged ports. IPv6 addresses
179 can be specified by enclosing the address in square brackets.
180 By default, the local port is bound in accordance with the
182 GatewayPorts setting. However, an explicit bind_address may be
183 used to bind the connection to a specific address. The
184 bind_address of “localhost” indicates that the listening port be
185 bound for local use only, while an empty address or ‘*’ indicates
186 that the port should be available from all interfaces.
187 -l login_name
189 Specifies the user to log in as on the remote machine. This also
190 may be specified on a per-host basis in the configuration file.
191 -M Places the ssh client into “master” mode for connection sharing.
193 Multiple -M options places ssh into “master” mode with confirma‐
194 tion required before slave connections are accepted. Refer to
195 the description of ControlMaster in ssh_config(5) for details.
196 -m mac_spec
198 A comma-separated list of MAC (message authentication code) algo‐
199 rithms, specified in order of preference. See the MACs keyword
200 for more information.
201 -N Do not execute a remote command. This is useful for just for‐
203 warding ports.
204 -n Redirects stdin from /dev/null (actually, prevents reading from
206 stdin). This must be used when ssh is run in the background. A
207 common trick is to use this to run X11 programs on a remote
208 machine. For example, ssh -n shadows.cs.hut.fi emacs & will
209 start an emacs on shadows.cs.hut.fi, and the X11 connection will
210 be automatically forwarded over an encrypted channel. The ssh
211 program will be put in the background. (This does not work if
212 ssh needs to ask for a password or passphrase; see also the -f
213 option.)
214 -O ctl_cmd
216 Control an active connection multiplexing master process. When
217 the -O option is specified, the ctl_cmd argument is interpreted
218 and passed to the master process. Valid commands are: “check”
219 (check that the master process is running), “forward” (request
220 forwardings without command execution), “cancel” (cancel forward‐
221 ings), “exit” (request the master to exit), and “stop” (request
222 the master to stop accepting further multiplexing requests).
223 -o option
225 Can be used to give options in the format used in the configura‐
226 tion file. This is useful for specifying options for which there
227 is no separate command-line flag. For full details of the
228 options listed below, and their possible values, see
229 ssh_config(5).
230 AddKeysToAgent
232 AddressFamily
233 BatchMode
234 BindAddress
235 CanonicalDomains
236 CanonicalizeFallbackLocal
237 CanonicalizeHostname
238 CanonicalizeMaxDots
239 CanonicalizePermittedCNAMEs
240 CertificateFile
241 ChallengeResponseAuthentication
242 CheckHostIP
243 Cipher
244 Ciphers
245 ClearAllForwardings
246 Compression
247 CompressionLevel
248 ConnectionAttempts
249 ConnectTimeout
250 ControlMaster
251 ControlPath
252 ControlPersist
253 DynamicForward
254 EscapeChar
255 ExitOnForwardFailure
256 FingerprintHash
257 ForwardAgent
258 ForwardX11
259 ForwardX11Timeout
260 ForwardX11Trusted
261 GatewayPorts
262 GlobalKnownHostsFile
263 GSSAPIAuthentication
264 GSSAPIKeyExchange
265 GSSAPIClientIdentity
266 GSSAPIDelegateCredentials
267 GSSAPIRenewalForcesRekey
268 GSSAPITrustDns
269 GSSAPIKexAlgorithms
270 HashKnownHosts
271 Host
272 HostbasedAuthentication
273 HostbasedKeyTypes
274 HostKeyAlgorithms
275 HostKeyAlias
276 HostName
277 IdentitiesOnly
278 IdentityAgent
279 IdentityFile
280 Include
281 IPQoS
282 KbdInteractiveAuthentication
283 KbdInteractiveDevices
284 KexAlgorithms
285 LocalCommand
286 LocalForward
287 LogLevel
288 MACs
289 Match
290 NoHostAuthenticationForLocalhost
291 NumberOfPasswordPrompts
292 PasswordAuthentication
293 PermitLocalCommand
294 PKCS11Provider
295 Port
296 PreferredAuthentications
297 Protocol
298 ProxyCommand
299 ProxyJump
300 ProxyUseFdpass
301 PubkeyAcceptedKeyTypes
302 PubkeyAuthentication
303 RekeyLimit
304 RemoteForward
305 RequestTTY
306 RhostsRSAAuthentication
307 RSAAuthentication
308 SendEnv
309 ServerAliveInterval
310 ServerAliveCountMax
311 StreamLocalBindMask
312 StreamLocalBindUnlink
313 StrictHostKeyChecking
314 TCPKeepAlive
315 Tunnel
316 TunnelDevice
317 UpdateHostKeys
318 UsePrivilegedPort
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), mac (supported message integrity
334 codes), kex (key exchange algorithms), key (key types), key-cert
335 (certificate key types), key-plain (non-certificate key types),
336 and protocol-version (supported SSH protocol versions).
337 -q Quiet mode. Causes most warning and diagnostic messages to be
339 suppressed.
340 -R [bind_address:]port:host:hostport
342 -R [bind_address:]port:local_socket
343 -R remote_socket:host:hostport
344 -R remote_socket:local_socket
345 Specifies that connections to the given TCP port or Unix socket
346 on the remote (server) host are to be forwarded to the given host
347 and port, or Unix socket, on the local side. This works by allo‐
348 cating a socket to listen to either a TCP port or to a Unix
349 socket on the remote side. Whenever a connection is made to this
350 port or Unix socket, the connection is forwarded over the secure
351 channel, and a connection is made to either host port hostport,
352 or local_socket, from the local machine.
353 Port forwardings can also be specified in the configuration file.
355 Privileged ports can be forwarded only when logging in as root on
356 the remote machine. IPv6 addresses can be specified by enclosing
357 the address in square brackets.
358 By default, TCP listening sockets on the server will be bound to
360 the loopback interface only. This may be overridden by specify‐
361 ing a bind_address. An empty bind_address, or the address ‘*’,
362 indicates that the remote socket should listen on all interfaces.
363 Specifying a remote bind_address will only succeed if the
364 server's GatewayPorts option is enabled (see sshd_config(5)).
365 If the port argument is ‘0’, the listen port will be dynamically
367 allocated on the server and reported to the client at run time.
368 When used together with -O forward the allocated port will be
369 printed to the standard output.
370 -S ctl_path
372 Specifies the location of a control socket for connection shar‐
373 ing, or the string “none” to disable connection sharing. Refer
374 to the description of ControlPath and ControlMaster in
375 ssh_config(5) for details.
376 -s May be used to request invocation of a subsystem on the remote
378 system. Subsystems facilitate the use of SSH as a secure trans‐
379 port for other applications (e.g. sftp(1)). The subsystem is
380 specified as the remote command.
381 -T Disable pseudo-terminal allocation.
383 -t Force pseudo-terminal allocation. This can be used to execute
385 arbitrary screen-based programs on a remote machine, which can be
386 very useful, e.g. when implementing menu services. Multiple -t
387 options force tty allocation, even if ssh has no local tty.
388 -V Display the version number and exit.
390 -v Verbose mode. Causes ssh to print debugging messages about its
392 progress. This is helpful in debugging connection, authentica‐
393 tion, and configuration problems. Multiple -v options increase
394 the verbosity. The maximum is 3.
395 -W host:port
397 Requests that standard input and output on the client be for‐
398 warded to host on port over the secure channel. Implies -N, -T,
399 ExitOnForwardFailure and ClearAllForwardings, though these can be
400 overridden in the configuration file or using -o command line
401 options.
402 -w local_tun[:remote_tun]
404 Requests tunnel device forwarding with the specified tun(4)
405 devices between the client (local_tun) and the server
406 (remote_tun).
407 The devices may be specified by numerical ID or the keyword
409 “any”, which uses the next available tunnel device. If
410 remote_tun is not specified, it defaults to “any”. See also the
411 Tunnel and TunnelDevice directives in ssh_config(5). If the
412 Tunnel directive is unset, it is set to the default tunnel mode,
413 which is “point-to-point”.
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 protocols 1 and 2. The default is to
443 use protocol 2 only, though this can be changed via the Protocol option
444 in ssh_config(5) or the -1 and -2 options (see above). Protocol 1 should
445 not be used and is only offered to support legacy devices. It suffers
446 from a number of cryptographic weaknesses and doesn't support many of the
447 advanced features available for protocol 2.
448 The methods available for authentication are: GSSAPI-based authentica‐
450 tion, host-based authentication, public key authentication, challenge-
451 response authentication, and password authentication. Authentication
452 methods are tried in the order specified above, though
453 PreferredAuthentications can be used to change the default order.
454 Host-based authentication works as follows: If the machine the user logs
456 in from is listed in /etc/hosts.equiv or /etc/ssh/shosts.equiv on the
457 remote machine, and the user names are the same on both sides, or if the
458 files ~/.rhosts or ~/.shosts exist in the user's home directory on the
459 remote machine and contain a line containing the name of the client
460 machine and the name of the user on that machine, the user is considered
461 for login. Additionally, the server must be able to verify the client's
462 host key (see the description of /etc/ssh/ssh_known_hosts and
463 ~/.ssh/known_hosts, below) for login to be permitted. This authentica‐
464 tion method closes security holes due to IP spoofing, DNS spoofing, and
465 routing spoofing. [Note to the administrator: /etc/hosts.equiv,
466 ~/.rhosts, and the rlogin/rsh protocol in general, are inherently inse‐
467 cure and should be disabled if security is desired.]
468 Public key authentication works as follows: The scheme is based on pub‐
470 lic-key cryptography, using cryptosystems where encryption and decryption
471 are done using separate keys, and it is unfeasible to derive the decryp‐
472 tion key from the encryption key. The idea is that each user creates a
473 public/private key pair for authentication purposes. The server knows
474 the public key, and only the user knows the private key. ssh implements
475 public key authentication protocol automatically, using one of the DSA,
476 ECDSA, Ed25519 or RSA algorithms. The HISTORY section of ssl(8) contains
477 a brief discussion of the DSA and RSA algorithms.
478 The file ~/.ssh/authorized_keys lists the public keys that are permitted
480 for logging in. When the user logs in, the ssh program tells the server
481 which key pair it would like to use for authentication. The client
482 proves that it has access to the private key and the server checks that
483 the corresponding public key is authorized to accept the account.
484 The user creates his/her key pair by running ssh-keygen(1). This stores
486 the private key in ~/.ssh/identity (protocol 1), ~/.ssh/id_dsa (DSA),
487 ~/.ssh/id_ecdsa (ECDSA), ~/.ssh/id_ed25519 (Ed25519), or ~/.ssh/id_rsa
488 (RSA) and stores the public key in ~/.ssh/identity.pub (protocol 1),
489 ~/.ssh/id_dsa.pub (DSA), ~/.ssh/id_ecdsa.pub (ECDSA),
490 ~/.ssh/id_ed25519.pub (Ed25519), or ~/.ssh/id_rsa.pub (RSA) in the user's
491 home directory. The user should then copy the public key to
492 ~/.ssh/authorized_keys in his/her home directory on the remote machine.
493 The authorized_keys file corresponds to the conventional ~/.rhosts file,
494 and has one key per line, though the lines can be very long. After this,
495 the user can log in without giving the password.
496 A variation on public key authentication is available in the form of cer‐
498 tificate authentication: instead of a set of public/private keys, signed
499 certificates are used. This has the advantage that a single trusted cer‐
500 tification authority can be used in place of many public/private keys.
501 See the CERTIFICATES section of ssh-keygen(1) for more information.
502 The most convenient way to use public key or certificate authentication
504 may be with an authentication agent. See ssh-agent(1) and (optionally)
505 the AddKeysToAgent directive in ssh_config(5) for more information.
506 Challenge-response authentication works as follows: The server sends an
508 arbitrary "challenge" text, and prompts for a response. Examples of
509 challenge-response authentication include BSD Authentication (see
510 login.conf(5)) and PAM (some non-OpenBSD systems).
511 Finally, if other authentication methods fail, ssh prompts the user for a
513 password. The password is sent to the remote host for checking; however,
514 since all communications are encrypted, the password cannot be seen by
515 someone listening on the network.
516 ssh automatically maintains and checks a database containing identifica‐
518 tion for all hosts it has ever been used with. Host keys are stored in
519 ~/.ssh/known_hosts in the user's home directory. Additionally, the file
520 /etc/ssh/ssh_known_hosts is automatically checked for known hosts. Any
521 new hosts are automatically added to the user's file. If a host's iden‐
522 tification ever changes, ssh warns about this and disables password
523 authentication to prevent server spoofing or man-in-the-middle attacks,
524 which could otherwise be used to circumvent the encryption. The
525 StrictHostKeyChecking option can be used to control logins to machines
526 whose host key is not known or has changed.
527 When the user's identity has been accepted by the server, the server
529 either executes the given command in a non-interactive session or, if no
530 command has been specified, logs into the machine and gives the user a
531 normal shell as an interactive session. All communication with the
532 remote command or shell will be automatically encrypted.
533 If an interactive session is requested ssh by default will only request a
535 pseudo-terminal (pty) for interactive sessions when the client has one.
536 The flags -T and -t can be used to override this behaviour.
537 If a pseudo-terminal has been allocated the user may use the escape char‐
539 acters noted below.
540 If no pseudo-terminal has been allocated, the session is transparent and
542 can be used to reliably transfer binary data. On most systems, setting
543 the escape character to “none” will also make the session transparent
544 even if a tty is used.
545 The session terminates when the command or shell on the remote machine
547 exits and all X11 and TCP connections have been closed.
548
550 When a pseudo-terminal has been requested, ssh supports a number of func‐
551 tions through the use of an escape character.
552 A single tilde character can be sent as ~~ or by following the tilde by a
554 character other than those described below. The escape character must
555 always follow a newline to be interpreted as special. The escape charac‐
556 ter can be changed in configuration files using the EscapeChar configura‐
557 tion directive or on the command line by the -e option.
558 The supported escapes (assuming the default ‘~’) are:
560 ~. Disconnect.
562 ~^Z Background ssh.
564 ~# List forwarded connections.
566 ~& Background ssh at logout when waiting for forwarded connection /
568 X11 sessions to terminate.
569 ~? Display a list of escape characters.
571 ~B Send a BREAK to the remote system (only useful if the peer sup‐
573 ports it).
574 ~C Open command line. Currently this allows the addition of port
576 forwardings using the -L, -R and -D options (see above). It also
577 allows the cancellation of existing port-forwardings with
578 -KL[bind_address:]port for local, -KR[bind_address:]port for
579 remote and -KD[bind_address:]port for dynamic port-forwardings.
580 !command allows the user to execute a local command if the
581 PermitLocalCommand option is enabled in ssh_config(5). Basic
582 help is available, using the -h option.
583 ~R Request rekeying of the connection (only useful if the peer sup‐
585 ports it).
586 ~V Decrease the verbosity (LogLevel) when errors are being written
588 to stderr.
589 ~v Increase the verbosity (LogLevel) when errors are being written
591 to stderr.
592
594 Forwarding of arbitrary TCP connections over the secure channel can be
595 specified either on the command line or in a configuration file. One
596 possible application of TCP forwarding is a secure connection to a mail
597 server; another is going through firewalls.
598 In the example below, we look at encrypting communication between an IRC
600 client and server, even though the IRC server does not directly support
601 encrypted communications. This works as follows: the user connects to
602 the remote host using ssh, specifying a port to be used to forward con‐
603 nections to the remote server. After that it is possible to start the
604 service which is to be encrypted on the client machine, connecting to the
605 same local port, and ssh will encrypt and forward the connection.
606 The following example tunnels an IRC session from client machine
608 “127.0.0.1” (localhost) to remote server “server.example.com”:
609 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
611 $ irc -c '#users' -p 1234 pinky 127.0.0.1
612 This tunnels a connection to IRC server “server.example.com”, joining
614 channel “#users”, nickname “pinky”, using port 1234. It doesn't matter
615 which port is used, as long as it's greater than 1023 (remember, only
616 root can open sockets on privileged ports) and doesn't conflict with any
617 ports already in use. The connection is forwarded to port 6667 on the
618 remote server, since that's the standard port for IRC services.
619 The -f option backgrounds ssh and the remote command “sleep 10” is speci‐
621 fied to allow an amount of time (10 seconds, in the example) to start the
622 service which is to be tunnelled. If no connections are made within the
623 time specified, ssh will exit.
624
626 If the ForwardX11 variable is set to “yes” (or see the description of the
627 -X, -x, and -Y options above) and the user is using X11 (the DISPLAY
628 environment variable is set), the connection to the X11 display is auto‐
629 matically forwarded to the remote side in such a way that any X11 pro‐
630 grams started from the shell (or command) will go through the encrypted
631 channel, and the connection to the real X server will be made from the
632 local machine. The user should not manually set DISPLAY. Forwarding of
633 X11 connections can be configured on the command line or in configuration
634 files.
635 The DISPLAY value set by ssh will point to the server machine, but with a
637 display number greater than zero. This is normal, and happens because
638 ssh creates a “proxy” X server on the server machine for forwarding the
639 connections over the encrypted channel.
640 ssh will also automatically set up Xauthority data on the server machine.
642 For this purpose, it will generate a random authorization cookie, store
643 it in Xauthority on the server, and verify that any forwarded connections
644 carry this cookie and replace it by the real cookie when the connection
645 is opened. The real authentication cookie is never sent to the server
646 machine (and no cookies are sent in the plain).
647 If the ForwardAgent variable is set to “yes” (or see the description of
649 the -A and -a options above) and the user is using an authentication
650 agent, the connection to the agent is automatically forwarded to the
651 remote side.
652
654 When connecting to a server for the first time, a fingerprint of the
655 server's public key is presented to the user (unless the option
656 StrictHostKeyChecking has been disabled). Fingerprints can be determined
657 using ssh-keygen(1):
658 $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
660 If the fingerprint is already known, it can be matched and the key can be
662 accepted or rejected. If only legacy (MD5) fingerprints for the server
663 are available, the ssh-keygen(1) -E option may be used to downgrade the
664 fingerprint algorithm to match.
665 Because of the difficulty of comparing host keys just by looking at fin‐
667 gerprint strings, there is also support to compare host keys visually,
668 using random art. By setting the VisualHostKey option to “yes”, a small
669 ASCII graphic gets displayed on every login to a server, no matter if the
670 session itself is interactive or not. By learning the pattern a known
671 server produces, a user can easily find out that the host key has changed
672 when a completely different pattern is displayed. Because these patterns
673 are not unambiguous however, a pattern that looks similar to the pattern
674 remembered only gives a good probability that the host key is the same,
675 not guaranteed proof.
676 To get a listing of the fingerprints along with their random art for all
678 known hosts, the following command line can be used:
679 $ ssh-keygen -lv -f ~/.ssh/known_hosts
681 If the fingerprint is unknown, an alternative method of verification is
683 available: SSH fingerprints verified by DNS. An additional resource
684 record (RR), SSHFP, is added to a zonefile and the connecting client is
685 able to match the fingerprint with that of the key presented.
686 In this example, we are connecting a client to a server,
688 “host.example.com”. The SSHFP resource records should first be added to
689 the zonefile for host.example.com:
690 $ ssh-keygen -r host.example.com.
692 The output lines will have to be added to the zonefile. To check that
694 the zone is answering fingerprint queries:
695 $ dig -t SSHFP host.example.com
697 Finally the client connects:
699 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
701 [...]
702 Matching host key fingerprint found in DNS.
703 Are you sure you want to continue connecting (yes/no)?
704 See the VerifyHostKeyDNS option in ssh_config(5) for more information.
706
708 ssh contains support for Virtual Private Network (VPN) tunnelling using
709 the tun(4) network pseudo-device, allowing two networks to be joined
710 securely. The sshd_config(5) configuration option PermitTunnel controls
711 whether the server supports this, and at what level (layer 2 or 3 traf‐
712 fic).
713 The following example would connect client network 10.0.50.0/24 with
715 remote network 10.0.99.0/24 using a point-to-point connection from
716 10.1.1.1 to 10.1.1.2, provided that the SSH server running on the gateway
717 to the remote network, at 192.168.1.15, allows it.
718 On the client:
720 # ssh -f -w 0:1 192.168.1.15 true
722 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
723 # route add 10.0.99.0/24 10.1.1.2
724 On the server:
726 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
728 # route add 10.0.50.0/24 10.1.1.1
729 Client access may be more finely tuned via the /root/.ssh/authorized_keys
731 file (see below) and the PermitRootLogin server option. The following
732 entry would permit connections on tun(4) device 1 from user “jane” and on
733 tun device 2 from user “john”, if PermitRootLogin is set to
734 “forced-commands-only”:
735 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
737 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
738 Since an SSH-based setup entails a fair amount of overhead, it may be
740 more suited to temporary setups, such as for wireless VPNs. More perma‐
741 nent VPNs are better provided by tools such as ipsecctl(8) and
742 isakmpd(8).
743
745 ssh will normally set the following environment variables:
746 DISPLAY The DISPLAY variable indicates the location of the
748 X11 server. It is automatically set by ssh to
749 point to a value of the form “hostname:n”, where
750 “hostname” indicates the host where the shell runs,
751 and ‘n’ is an integer ≥ 1. ssh uses this special
752 value to forward X11 connections over the secure
753 channel. The user should normally not set DISPLAY
754 explicitly, as that will render the X11 connection
755 insecure (and will require the user to manually
756 copy any required authorization cookies).
757 HOME Set to the path of the user's home directory.
759 LOGNAME Synonym for USER; set for compatibility with sys‐
761 tems that use this variable.
762 MAIL Set to the path of the user's mailbox.
764 PATH Set to the default PATH, as specified when compil‐
766 ing ssh.
767 SSH_ASKPASS If ssh needs a passphrase, it will read the
769 passphrase from the current terminal if it was run
770 from a terminal. If ssh does not have a terminal
771 associated with it but DISPLAY and SSH_ASKPASS are
772 set, it will execute the program specified by
773 SSH_ASKPASS and open an X11 window to read the
774 passphrase. This is particularly useful when call‐
775 ing ssh from a .xsession or related script. (Note
776 that on some machines it may be necessary to redi‐
777 rect the input from /dev/null to make this work.)
778 SSH_AUTH_SOCK Identifies the path of a UNIX-domain socket used to
780 communicate with the agent.
781 SSH_CONNECTION Identifies the client and server ends of the con‐
783 nection. The variable contains four space-sepa‐
784 rated values: client IP address, client port num‐
785 ber, server IP address, and server port number.
786 SSH_ORIGINAL_COMMAND This variable contains the original command line if
788 a forced command is executed. It can be used to
789 extract the original arguments.
790 SSH_USER_AUTH This variable contains, for SSH2 only, a comma-sep‐
792 arated list of authentication methods that were
793 successfuly used to authenticate. When possible,
794 these methods are extended with detailed informa‐
795 tion on the credential used.
796 SSH_TTY This is set to the name of the tty (path to the
798 device) associated with the current shell or com‐
799 mand. If the current session has no tty, this
800 variable is not set.
801 TZ This variable is set to indicate the present time
803 zone if it was set when the daemon was started
804 (i.e. the daemon passes the value on to new connec‐
805 tions).
806 USER Set to the name of the user logging in.
808 Additionally, ssh reads ~/.ssh/environment, and adds lines of the format
810 “VARNAME=value” to the environment if the file exists and users are
811 allowed to change their environment. For more information, see the
812 PermitUserEnvironment option in sshd_config(5).
813
815 SSH_USE_STRONG_RNG
816 The reseeding of the OpenSSL random generator is usually done
817 from /dev/urandom. If the SSH_USE_STRONG_RNG environment vari‐
818 able is set to value other than 0 the OpenSSL random generator is
819 reseeded from /dev/random. The number of bytes read is defined
820 by the SSH_USE_STRONG_RNG value. Minimum is 14 bytes. This set‐
821 ting is not recommended on the computers without the hardware
822 random generator because insufficient entropy causes the connec‐
823 tion to be blocked until enough entropy is available.
824
826 ~/.rhosts
827 This file is used for host-based authentication (see above). On
828 some machines this file may need to be world-readable if the
829 user's home directory is on an NFS partition, because sshd(8)
830 reads it as root. Additionally, this file must be owned by the
831 user, and must not have write permissions for anyone else. The
832 recommended permission for most machines is read/write for the
833 user, and not accessible by others.
834 ~/.shosts
836 This file is used in exactly the same way as .rhosts, but allows
837 host-based authentication without permitting login with
838 rlogin/rsh.
839 ~/.ssh/
841 This directory is the default location for all user-specific con‐
842 figuration and authentication information. There is no general
843 requirement to keep the entire contents of this directory secret,
844 but the recommended permissions are read/write/execute for the
845 user, and not accessible by others.
846 ~/.ssh/authorized_keys
848 Lists the public keys (DSA, ECDSA, Ed25519, RSA) that can be used
849 for logging in as this user. The format of this file is
850 described in the sshd(8) manual page. This file is not highly
851 sensitive, but the recommended permissions are read/write for the
852 user, and not accessible by others.
853 ~/.ssh/config
855 This is the per-user configuration file. The file format and
856 configuration options are described in ssh_config(5). Because of
857 the potential for abuse, this file must have strict permissions:
858 read/write for the user, and not writable by others.
859 ~/.ssh/environment
861 Contains additional definitions for environment variables; see
862 ENVIRONMENT, above.
863 ~/.ssh/identity
865 ~/.ssh/id_dsa
866 ~/.ssh/id_ecdsa
867 ~/.ssh/id_ed25519
868 ~/.ssh/id_rsa
869 Contains the private key for authentication. These files contain
870 sensitive data and should be readable by the user but not acces‐
871 sible by others (read/write/execute). ssh will simply ignore a
872 private key file if it is accessible by others. It is possible
873 to specify a passphrase when generating the key which will be
874 used to encrypt the sensitive part of this file using 3DES.
875 ~/.ssh/identity.pub
877 ~/.ssh/id_dsa.pub
878 ~/.ssh/id_ecdsa.pub
879 ~/.ssh/id_ed25519.pub
880 ~/.ssh/id_rsa.pub
881 Contains the public key for authentication. These files are not
882 sensitive and can (but need not) be readable by anyone.
883 ~/.ssh/known_hosts
885 Contains a list of host keys for all hosts the user has logged
886 into that are not already in the systemwide list of known host
887 keys. See sshd(8) for further details of the format of this
888 file.
889 ~/.ssh/rc
891 Commands in this file are executed by ssh when the user logs in,
892 just before the user's shell (or command) is started. See the
893 sshd(8) manual page for more information.
894 /etc/hosts.equiv
896 This file is for host-based authentication (see above). It
897 should only be writable by root.
898 /etc/ssh/shosts.equiv
900 This file is used in exactly the same way as hosts.equiv, but
901 allows host-based authentication without permitting login with
902 rlogin/rsh.
903 /etc/ssh/ssh_config
905 Systemwide configuration file. The file format and configuration
906 options are described in ssh_config(5).
907 /etc/ssh/ssh_host_key
909 /etc/ssh/ssh_host_dsa_key
910 /etc/ssh/ssh_host_ecdsa_key
911 /etc/ssh/ssh_host_ed25519_key
912 /etc/ssh/ssh_host_rsa_key
913 These files contain the private parts of the host keys and are
914 used for host-based authentication.
915 /etc/ssh/ssh_known_hosts
917 Systemwide list of known host keys. This file should be prepared
918 by the system administrator to contain the public host keys of
919 all machines in the organization. It should be world-readable.
920 See sshd(8) for further details of the format of this file.
921 /etc/ssh/sshrc
923 Commands in this file are executed by ssh when the user logs in,
924 just before the user's shell (or command) is started. See the
925 sshd(8) manual page for more information.
926
928 ssh exits with the exit status of the remote command or with 255 if an
929 error occurred.
930
932 IPv6 address can be used everywhere where IPv4 address. In all entries
933 must be the IPv6 address enclosed in square brackets. Note: The square
934 brackets are metacharacters for the shell and must be escaped in shell.
935
937 scp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), ssh-keyscan(1),
938 tun(4), ssh_config(5), ssh-keysign(8), sshd(8)
939
941 S. Lehtinen and C. Lonvick, The Secure Shell (SSH) Protocol Assigned
942 Numbers, RFC 4250, January 2006.
943 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Protocol Architecture,
945 RFC 4251, January 2006.
946 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Authentication Protocol,
948 RFC 4252, January 2006.
949 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Transport Layer
951 Protocol, RFC 4253, January 2006.
952 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Connection Protocol, RFC
954 4254, January 2006.
955 J. Schlyter and W. Griffin, Using DNS to Securely Publish Secure Shell
957 (SSH) Key Fingerprints, RFC 4255, January 2006.
958 F. Cusack and M. Forssen, Generic Message Exchange Authentication for the
960 Secure Shell Protocol (SSH), RFC 4256, January 2006.
961 J. Galbraith and P. Remaker, The Secure Shell (SSH) Session Channel Break
963 Extension, RFC 4335, January 2006.
964 M. Bellare, T. Kohno, and C. Namprempre, The Secure Shell (SSH) Transport
966 Layer Encryption Modes, RFC 4344, January 2006.
967 B. Harris, Improved Arcfour Modes for the Secure Shell (SSH) Transport
969 Layer Protocol, RFC 4345, January 2006.
970 M. Friedl, N. Provos, and W. Simpson, Diffie-Hellman Group Exchange for
972 the Secure Shell (SSH) Transport Layer Protocol, RFC 4419, March 2006.
973 J. Galbraith and R. Thayer, The Secure Shell (SSH) Public Key File
975 Format, RFC 4716, November 2006.
976 D. Stebila and J. Green, Elliptic Curve Algorithm Integration in the
978 Secure Shell Transport Layer, RFC 5656, December 2009.
979 A. Perrig and D. Song, Hash Visualization: a New Technique to improve
981 Real-World Security, 1999, International Workshop on Cryptographic
982 Techniques and E-Commerce (CrypTEC '99).
983
985 OpenSSH is a derivative of the original and free ssh 1.2.12 release by
986 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
987 de Raadt and Dug Song removed many bugs, re-added newer features and cre‐
988 ated OpenSSH. Markus Friedl contributed the support for SSH protocol
989 versions 1.5 and 2.0.
990BSD June 21, 2019 BSD