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 their identity to the
25 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. Refer to the description of ForkAfterAuthentication
125 in ssh_config(5) for details.
126 -G Causes ssh to print its configuration after evaluating Host and
128 Match blocks and exit.
129 -g Allows remote hosts to connect to local forwarded ports. If used
131 on a multiplexed connection, then this option must be specified
132 on the master process.
133 -I pkcs11
135 Specify the PKCS#11 shared library ssh should use to communicate
136 with a PKCS#11 token providing keys for user authentication.
137 -i identity_file
139 Selects a file from which the identity (private key) for public
140 key authentication is read. The default is ~/.ssh/id_dsa,
141 ~/.ssh/id_ecdsa, ~/.ssh/id_ecdsa_sk, ~/.ssh/id_ed25519,
142 ~/.ssh/id_ed25519_sk and ~/.ssh/id_rsa. Identity files may also
143 be specified on a per-host basis in the configuration file. It
144 is possible to have multiple -i options (and multiple identities
145 specified in configuration files). If no certificates have been
146 explicitly specified by the CertificateFile directive, ssh will
147 also try to load certificate information from the filename ob‐
148 tained by appending -cert.pub to identity filenames.
149 -J destination
151 Connect to the target host by first making a ssh connection to
152 the jump host described by destination and then establishing a
153 TCP forwarding to the ultimate destination from there. Multiple
154 jump hops may be specified separated by comma characters. This
155 is a shortcut to specify a ProxyJump configuration directive.
156 Note that configuration directives supplied on the command-line
157 generally apply to the destination host and not any specified
158 jump hosts. Use ~/.ssh/config to specify configuration for jump
159 hosts.
160 -K Enables GSSAPI-based authentication and forwarding (delegation)
162 of GSSAPI credentials to the server.
163 -k Disables forwarding (delegation) of GSSAPI credentials to the
165 server.
166 -L [bind_address:]port:host:hostport
168 -L [bind_address:]port:remote_socket
169 -L local_socket:host:hostport
170 -L local_socket:remote_socket
171 Specifies that connections to the given TCP port or Unix socket
172 on the local (client) host are to be forwarded to the given host
173 and port, or Unix socket, on the remote side. This works by al‐
174 locating a socket to listen to either a TCP port on the local
175 side, optionally bound to the specified bind_address, or to a
176 Unix socket. Whenever a connection is made to the local port or
177 socket, the connection is forwarded over the secure channel, and
178 a connection is made to either host port hostport, or the Unix
179 socket remote_socket, from the remote machine.
180 Port forwardings can also be specified in the configuration file.
182 Only the superuser can forward privileged ports. IPv6 addresses
183 can be specified by enclosing the address in square brackets.
184 By default, the local port is bound in accordance with the
186 GatewayPorts setting. However, an explicit bind_address may be
187 used to bind the connection to a specific address. The
188 bind_address of “localhost” indicates that the listening port be
189 bound for local use only, while an empty address or ‘*’ indicates
190 that the port should be available from all interfaces.
191 -l login_name
193 Specifies the user to log in as on the remote machine. This also
194 may be specified on a per-host basis in the configuration file.
195 -M Places the ssh client into “master” mode for connection sharing.
197 Multiple -M options places ssh into “master” mode but with con‐
198 firmation required using ssh-askpass(1) before each operation
199 that changes the multiplexing state (e.g. opening a new session).
200 Refer to the description of ControlMaster in ssh_config(5) for
201 details.
202 -m mac_spec
204 A comma-separated list of MAC (message authentication code) algo‐
205 rithms, specified in order of preference. See the MACs keyword
206 for more information.
207 -N Do not execute a remote command. This is useful for just for‐
209 warding ports. Refer to the description of SessionType in
210 ssh_config(5) for details.
211 -n Redirects stdin from /dev/null (actually, prevents reading from
213 stdin). This must be used when ssh is run in the background. A
214 common trick is to use this to run X11 programs on a remote ma‐
215 chine. For example, ssh -n shadows.cs.hut.fi emacs & will start
216 an emacs on shadows.cs.hut.fi, and the X11 connection will be au‐
217 tomatically forwarded over an encrypted channel. The ssh program
218 will be put in the background. (This does not work if ssh needs
219 to ask for a password or passphrase; see also the -f option.)
220 Refer to the description of StdinNull in ssh_config(5) for de‐
221 tails.
222 -O ctl_cmd
224 Control an active connection multiplexing master process. When
225 the -O option is specified, the ctl_cmd argument is interpreted
226 and passed to the master process. Valid commands are: “check”
227 (check that the master process is running), “forward” (request
228 forwardings without command execution), “cancel” (cancel forward‐
229 ings), “exit” (request the master to exit), and “stop” (request
230 the master to stop accepting further multiplexing requests).
231 -o option
233 Can be used to give options in the format used in the configura‐
234 tion file. This is useful for specifying options for which there
235 is no separate command-line flag. For full details of the op‐
236 tions listed below, and their possible values, see ssh_config(5).
237 AddKeysToAgent
239 AddressFamily
240 BatchMode
241 BindAddress
242 CanonicalDomains
243 CanonicalizeFallbackLocal
244 CanonicalizeHostname
245 CanonicalizeMaxDots
246 CanonicalizePermittedCNAMEs
247 CASignatureAlgorithms
248 CertificateFile
249 CheckHostIP
250 Ciphers
251 ClearAllForwardings
252 Compression
253 ConnectionAttempts
254 ConnectTimeout
255 ControlMaster
256 ControlPath
257 ControlPersist
258 DynamicForward
259 EscapeChar
260 ExitOnForwardFailure
261 FingerprintHash
262 ForkAfterAuthentication
263 ForwardAgent
264 ForwardX11
265 ForwardX11Timeout
266 ForwardX11Trusted
267 GatewayPorts
268 GlobalKnownHostsFile
269 GSSAPIAuthentication
270 GSSAPIKeyExchange
271 GSSAPIClientIdentity
272 GSSAPIDelegateCredentials
273 GSSAPIKexAlgorithms
274 GSSAPIRenewalForcesRekey
275 GSSAPIServerIdentity
276 GSSAPITrustDns
277 HashKnownHosts
278 Host
279 HostbasedAcceptedAlgorithms
280 HostbasedAuthentication
281 HostKeyAlgorithms
282 HostKeyAlias
283 Hostname
284 IdentitiesOnly
285 IdentityAgent
286 IdentityFile
287 IPQoS
288 KbdInteractiveAuthentication
289 KbdInteractiveDevices
290 KexAlgorithms
291 KnownHostsCommand
292 LocalCommand
293 LocalForward
294 LogLevel
295 MACs
296 Match
297 NoHostAuthenticationForLocalhost
298 NumberOfPasswordPrompts
299 PasswordAuthentication
300 PermitLocalCommand
301 PermitRemoteOpen
302 PKCS11Provider
303 Port
304 PreferredAuthentications
305 ProxyCommand
306 ProxyJump
307 ProxyUseFdpass
308 PubkeyAcceptedAlgorithms
309 PubkeyAuthentication
310 RekeyLimit
311 RemoteCommand
312 RemoteForward
313 RequestTTY
314 SendEnv
315 ServerAliveInterval
316 ServerAliveCountMax
317 SessionType
318 SetEnv
319 StdinNull
320 StreamLocalBindMask
321 StreamLocalBindUnlink
322 StrictHostKeyChecking
323 TCPKeepAlive
324 Tunnel
325 TunnelDevice
326 UpdateHostKeys
327 User
328 UserKnownHostsFile
329 VerifyHostKeyDNS
330 VisualHostKey
331 XAuthLocation
332 -p port
334 Port to connect to on the remote host. This can be specified on
335 a per-host basis in the configuration file.
336 -Q query_option
338 Queries for the algorithms supported by one of the following fea‐
339 tures: cipher (supported symmetric ciphers), cipher-auth (sup‐
340 ported symmetric ciphers that support authenticated encryption),
341 help (supported query terms for use with the -Q flag), mac (sup‐
342 ported message integrity codes), kex (key exchange algorithms),
343 kex-gss (GSSAPI key exchange algorithms), key (key types),
344 key-cert (certificate key types), key-plain (non-certificate key
345 types), key-sig (all key types and signature algorithms),
346 protocol-version (supported SSH protocol versions), and sig (sup‐
347 ported signature algorithms). Alternatively, any keyword from
348 ssh_config(5) or sshd_config(5) that takes an algorithm list may
349 be used as an alias for the corresponding query_option.
350 -q Quiet mode. Causes most warning and diagnostic messages to be
352 suppressed.
353 -R [bind_address:]port:host:hostport
355 -R [bind_address:]port:local_socket
356 -R remote_socket:host:hostport
357 -R remote_socket:local_socket
358 -R [bind_address:]port
359 Specifies that connections to the given TCP port or Unix socket
360 on the remote (server) host are to be forwarded to the local
361 side.
362 This works by allocating a socket to listen to either a TCP port
364 or to a Unix socket on the remote side. Whenever a connection is
365 made to this port or Unix socket, the connection is forwarded
366 over the secure channel, and a connection is made from the local
367 machine to either an explicit destination specified by host port
368 hostport, or local_socket, or, if no explicit destination was
369 specified, ssh will act as a SOCKS 4/5 proxy and forward connec‐
370 tions to the destinations requested by the remote SOCKS client.
371 Port forwardings can also be specified in the configuration file.
373 Privileged ports can be forwarded only when logging in as root on
374 the remote machine. IPv6 addresses can be specified by enclosing
375 the address in square brackets.
376 By default, TCP listening sockets on the server will be bound to
378 the loopback interface only. This may be overridden by specify‐
379 ing a bind_address. An empty bind_address, or the address ‘*’,
380 indicates that the remote socket should listen on all interfaces.
381 Specifying a remote bind_address will only succeed if the
382 server's GatewayPorts option is enabled (see sshd_config(5)).
383 If the port argument is ‘0’, the listen port will be dynamically
385 allocated on the server and reported to the client at run time.
386 When used together with -O forward the allocated port will be
387 printed to the standard output.
388 -S ctl_path
390 Specifies the location of a control socket for connection shar‐
391 ing, or the string “none” to disable connection sharing. Refer
392 to the description of ControlPath and ControlMaster in
393 ssh_config(5) for details.
394 -s May be used to request invocation of a subsystem on the remote
396 system. Subsystems facilitate the use of SSH as a secure trans‐
397 port for other applications (e.g. sftp(1)). The subsystem is
398 specified as the remote command. Refer to the description of
399 SessionType in ssh_config(5) for details.
400 -T Disable pseudo-terminal allocation.
402 -t Force pseudo-terminal allocation. This can be used to execute
404 arbitrary screen-based programs on a remote machine, which can be
405 very useful, e.g. when implementing menu services. Multiple -t
406 options force tty allocation, even if ssh has no local tty.
407 -V Display the version number and exit.
409 -v Verbose mode. Causes ssh to print debugging messages about its
411 progress. This is helpful in debugging connection, authentica‐
412 tion, and configuration problems. Multiple -v options increase
413 the verbosity. The maximum is 3.
414 -W host:port
416 Requests that standard input and output on the client be for‐
417 warded to host on port over the secure channel. Implies -N, -T,
418 ExitOnForwardFailure and ClearAllForwardings, though these can be
419 overridden in the configuration file or using -o command line op‐
420 tions.
421 -w local_tun[:remote_tun]
423 Requests tunnel device forwarding with the specified tun(4) de‐
424 vices between the client (local_tun) and the server (remote_tun).
425 The devices may be specified by numerical ID or the keyword
427 “any”, which uses the next available tunnel device. If
428 remote_tun is not specified, it defaults to “any”. See also the
429 Tunnel and TunnelDevice directives in ssh_config(5).
430 If the Tunnel directive is unset, it will be set to the default
432 tunnel mode, which is “point-to-point”. If a different Tunnel
433 forwarding mode it desired, then it should be specified before
434 -w.
435 -X Enables X11 forwarding. This can also be specified on a per-host
437 basis in a configuration file.
438 X11 forwarding should be enabled with caution. Users with the
440 ability to bypass file permissions on the remote host (for the
441 user's X authorization database) can access the local X11 display
442 through the forwarded connection. An attacker may then be able
443 to perform activities such as keystroke monitoring.
444 For this reason, X11 forwarding is subjected to X11 SECURITY ex‐
446 tension restrictions by default. Please refer to the ssh -Y op‐
447 tion and the ForwardX11Trusted directive in ssh_config(5) for
448 more information.
449 -x Disables X11 forwarding.
451 -Y Enables trusted X11 forwarding. Trusted X11 forwardings are not
453 subjected to the X11 SECURITY extension controls.
454 -y Send log information using the syslog(3) system module. By de‐
456 fault this information is sent to stderr.
457 ssh may additionally obtain configuration data from a per-user configura‐
459 tion file and a system-wide configuration file. The file format and con‐
460 figuration options are described in ssh_config(5).
461
463 The OpenSSH SSH client supports SSH protocol 2.
464 The methods available for authentication are: GSSAPI-based authentica‐
466 tion, host-based authentication, public key authentication, keyboard-in‐
467 teractive authentication, and password authentication. Authentication
468 methods are tried in the order specified above, though
469 PreferredAuthentications can be used to change the default order.
470 Host-based authentication works as follows: If the machine the user logs
472 in from is listed in /etc/hosts.equiv or /etc/ssh/shosts.equiv on the re‐
473 mote machine, the user is non-root and the user names are the same on
474 both sides, or if the files ~/.rhosts or ~/.shosts exist in the user's
475 home directory on the remote machine and contain a line containing the
476 name of the client machine and the name of the user on that machine, the
477 user is considered for login. Additionally, the server must be able to
478 verify the client's host key (see the description of
479 /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts, below) for login to be
480 permitted. This authentication method closes security holes due to IP
481 spoofing, DNS spoofing, and routing spoofing. [Note to the administra‐
482 tor: /etc/hosts.equiv, ~/.rhosts, and the rlogin/rsh protocol in general,
483 are inherently insecure and should be disabled if security is desired.]
484 Public key authentication works as follows: The scheme is based on pub‐
486 lic-key cryptography, using cryptosystems where encryption and decryption
487 are done using separate keys, and it is unfeasible to derive the decryp‐
488 tion key from the encryption key. The idea is that each user creates a
489 public/private key pair for authentication purposes. The server knows
490 the public key, and only the user knows the private key. ssh implements
491 public key authentication protocol automatically, using one of the DSA,
492 ECDSA, Ed25519 or RSA algorithms. The HISTORY section of ssl(8) contains
493 a brief discussion of the DSA and RSA algorithms.
494 The file ~/.ssh/authorized_keys lists the public keys that are permitted
496 for logging in. When the user logs in, the ssh program tells the server
497 which key pair it would like to use for authentication. The client
498 proves that it has access to the private key and the server checks that
499 the corresponding public key is authorized to accept the account.
500 The server may inform the client of errors that prevented public key au‐
502 thentication from succeeding after authentication completes using a dif‐
503 ferent method. These may be viewed by increasing the LogLevel to DEBUG
504 or higher (e.g. by using the -v flag).
505 The user creates their key pair by running ssh-keygen(1). This stores
507 the private key in ~/.ssh/id_dsa (DSA), ~/.ssh/id_ecdsa (ECDSA),
508 ~/.ssh/id_ecdsa_sk (authenticator-hosted ECDSA), ~/.ssh/id_ed25519
509 (Ed25519), ~/.ssh/id_ed25519_sk (authenticator-hosted Ed25519), or
510 ~/.ssh/id_rsa (RSA) and stores the public key in ~/.ssh/id_dsa.pub (DSA),
511 ~/.ssh/id_ecdsa.pub (ECDSA), ~/.ssh/id_ecdsa_sk.pub (authenticator-hosted
512 ECDSA), ~/.ssh/id_ed25519.pub (Ed25519), ~/.ssh/id_ed25519_sk.pub (au‐
513 thenticator-hosted Ed25519), or ~/.ssh/id_rsa.pub (RSA) in the user's
514 home directory. The user should then copy the public key to
515 ~/.ssh/authorized_keys in their home directory on the remote machine.
516 The authorized_keys file corresponds to the conventional ~/.rhosts file,
517 and has one key per line, though the lines can be very long. After this,
518 the user can log in without giving the password.
519 A variation on public key authentication is available in the form of cer‐
521 tificate authentication: instead of a set of public/private keys, signed
522 certificates are used. This has the advantage that a single trusted cer‐
523 tification authority can be used in place of many public/private keys.
524 See the CERTIFICATES section of ssh-keygen(1) for more information.
525 The most convenient way to use public key or certificate authentication
527 may be with an authentication agent. See ssh-agent(1) and (optionally)
528 the AddKeysToAgent directive in ssh_config(5) for more information.
529 Keyboard-interactive authentication works as follows: The server sends an
531 arbitrary "challenge" text and prompts for a response, possibly multiple
532 times. Examples of keyboard-interactive authentication include BSD Au‐
533 thentication (see login.conf(5)) and PAM (some non-OpenBSD systems).
534 Finally, if other authentication methods fail, ssh prompts the user for a
536 password. The password is sent to the remote host for checking; however,
537 since all communications are encrypted, the password cannot be seen by
538 someone listening on the network.
539 ssh automatically maintains and checks a database containing identifica‐
541 tion for all hosts it has ever been used with. Host keys are stored in
542 ~/.ssh/known_hosts in the user's home directory. Additionally, the file
543 /etc/ssh/ssh_known_hosts is automatically checked for known hosts. Any
544 new hosts are automatically added to the user's file. If a host's iden‐
545 tification ever changes, ssh warns about this and disables password au‐
546 thentication to prevent server spoofing or man-in-the-middle attacks,
547 which could otherwise be used to circumvent the encryption. The
548 StrictHostKeyChecking option can be used to control logins to machines
549 whose host key is not known or has changed.
550 When the user's identity has been accepted by the server, the server ei‐
552 ther executes the given command in a non-interactive session or, if no
553 command has been specified, logs into the machine and gives the user a
554 normal shell as an interactive session. All communication with the re‐
555 mote command or shell will be automatically encrypted.
556 If an interactive session is requested ssh by default will only request a
558 pseudo-terminal (pty) for interactive sessions when the client has one.
559 The flags -T and -t can be used to override this behaviour.
560 If a pseudo-terminal has been allocated the user may use the escape char‐
562 acters noted below.
563 If no pseudo-terminal has been allocated, the session is transparent and
565 can be used to reliably transfer binary data. On most systems, setting
566 the escape character to “none” will also make the session transparent
567 even if a tty is used.
568 The session terminates when the command or shell on the remote machine
570 exits and all X11 and TCP connections have been closed.
571
573 When a pseudo-terminal has been requested, ssh supports a number of func‐
574 tions through the use of an escape character.
575 A single tilde character can be sent as ~~ or by following the tilde by a
577 character other than those described below. The escape character must
578 always follow a newline to be interpreted as special. The escape charac‐
579 ter can be changed in configuration files using the EscapeChar configura‐
580 tion directive or on the command line by the -e option.
581 The supported escapes (assuming the default ‘~’) are:
583 ~. Disconnect.
585 ~^Z Background ssh.
587 ~# List forwarded connections.
589 ~& Background ssh at logout when waiting for forwarded connection /
591 X11 sessions to terminate.
592 ~? Display a list of escape characters.
594 ~B Send a BREAK to the remote system (only useful if the peer sup‐
596 ports it).
597 ~C Open command line. Currently this allows the addition of port
599 forwardings using the -L, -R and -D options (see above). It also
600 allows the cancellation of existing port-forwardings with
601 -KL[bind_address:]port for local, -KR[bind_address:]port for re‐
602 mote and -KD[bind_address:]port for dynamic port-forwardings.
603 !command allows the user to execute a local command if the
604 PermitLocalCommand option is enabled in ssh_config(5). Basic
605 help is available, using the -h option.
606 ~R Request rekeying of the connection (only useful if the peer sup‐
608 ports it).
609 ~V Decrease the verbosity (LogLevel) when errors are being written
611 to stderr.
612 ~v Increase the verbosity (LogLevel) when errors are being written
614 to stderr.
615
617 Forwarding of arbitrary TCP connections over a secure channel can be
618 specified either on the command line or in a configuration file. One
619 possible application of TCP forwarding is a secure connection to a mail
620 server; another is going through firewalls.
621 In the example below, we look at encrypting communication for an IRC
623 client, even though the IRC server it connects to does not directly sup‐
624 port encrypted communication. This works as follows: the user connects
625 to the remote host using ssh, specifying the ports to be used to forward
626 the connection. After that it is possible to start the program locally,
627 and ssh will encrypt and forward the connection to the remote server.
628 The following example tunnels an IRC session from the client to an IRC
630 server at “server.example.com”, joining channel “#users”, nickname
631 “pinky”, using the standard IRC port, 6667:
632 $ ssh -f -L 6667:localhost:6667 server.example.com sleep 10
634 $ irc -c '#users' pinky IRC/127.0.0.1
635 The -f option backgrounds ssh and the remote command “sleep 10” is speci‐
637 fied to allow an amount of time (10 seconds, in the example) to start the
638 program which is going to use the tunnel. If no connections are made
639 within the time specified, ssh will exit.
640
642 If the ForwardX11 variable is set to “yes” (or see the description of the
643 -X, -x, and -Y options above) and the user is using X11 (the DISPLAY en‐
644 vironment variable is set), the connection to the X11 display is automat‐
645 ically forwarded to the remote side in such a way that any X11 programs
646 started from the shell (or command) will go through the encrypted chan‐
647 nel, and the connection to the real X server will be made from the local
648 machine. The user should not manually set DISPLAY. Forwarding of X11
649 connections can be configured on the command line or in configuration
650 files.
651 The DISPLAY value set by ssh will point to the server machine, but with a
653 display number greater than zero. This is normal, and happens because
654 ssh creates a “proxy” X server on the server machine for forwarding the
655 connections over the encrypted channel.
656 ssh will also automatically set up Xauthority data on the server machine.
658 For this purpose, it will generate a random authorization cookie, store
659 it in Xauthority on the server, and verify that any forwarded connections
660 carry this cookie and replace it by the real cookie when the connection
661 is opened. The real authentication cookie is never sent to the server
662 machine (and no cookies are sent in the plain).
663 If the ForwardAgent variable is set to “yes” (or see the description of
665 the -A and -a options above) and the user is using an authentication
666 agent, the connection to the agent is automatically forwarded to the re‐
667 mote side.
668
670 When connecting to a server for the first time, a fingerprint of the
671 server's public key is presented to the user (unless the option
672 StrictHostKeyChecking has been disabled). Fingerprints can be determined
673 using ssh-keygen(1):
674 $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
676 If the fingerprint is already known, it can be matched and the key can be
678 accepted or rejected. If only legacy (MD5) fingerprints for the server
679 are available, the ssh-keygen(1) -E option may be used to downgrade the
680 fingerprint algorithm to match.
681 Because of the difficulty of comparing host keys just by looking at fin‐
683 gerprint strings, there is also support to compare host keys visually,
684 using random art. By setting the VisualHostKey option to “yes”, a small
685 ASCII graphic gets displayed on every login to a server, no matter if the
686 session itself is interactive or not. By learning the pattern a known
687 server produces, a user can easily find out that the host key has changed
688 when a completely different pattern is displayed. Because these patterns
689 are not unambiguous however, a pattern that looks similar to the pattern
690 remembered only gives a good probability that the host key is the same,
691 not guaranteed proof.
692 To get a listing of the fingerprints along with their random art for all
694 known hosts, the following command line can be used:
695 $ ssh-keygen -lv -f ~/.ssh/known_hosts
697 If the fingerprint is unknown, an alternative method of verification is
699 available: SSH fingerprints verified by DNS. An additional resource
700 record (RR), SSHFP, is added to a zonefile and the connecting client is
701 able to match the fingerprint with that of the key presented.
702 In this example, we are connecting a client to a server,
704 “host.example.com”. The SSHFP resource records should first be added to
705 the zonefile for host.example.com:
706 $ ssh-keygen -r host.example.com.
708 The output lines will have to be added to the zonefile. To check that
710 the zone is answering fingerprint queries:
711 $ dig -t SSHFP host.example.com
713 Finally the client connects:
715 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
717 [...]
718 Matching host key fingerprint found in DNS.
719 Are you sure you want to continue connecting (yes/no)?
720 See the VerifyHostKeyDNS option in ssh_config(5) for more information.
722
724 ssh contains support for Virtual Private Network (VPN) tunnelling using
725 the tun(4) network pseudo-device, allowing two networks to be joined se‐
726 curely. The sshd_config(5) configuration option PermitTunnel controls
727 whether the server supports this, and at what level (layer 2 or 3 traf‐
728 fic).
729 The following example would connect client network 10.0.50.0/24 with re‐
731 mote network 10.0.99.0/24 using a point-to-point connection from 10.1.1.1
732 to 10.1.1.2, provided that the SSH server running on the gateway to the
733 remote network, at 192.168.1.15, allows it.
734 On the client:
736 # ssh -f -w 0:1 192.168.1.15 true
738 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
739 # route add 10.0.99.0/24 10.1.1.2
740 On the server:
742 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
744 # route add 10.0.50.0/24 10.1.1.1
745 Client access may be more finely tuned via the /root/.ssh/authorized_keys
747 file (see below) and the PermitRootLogin server option. The following
748 entry would permit connections on tun(4) device 1 from user “jane” and on
749 tun device 2 from user “john”, if PermitRootLogin is set to
750 “forced-commands-only”:
751 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
753 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
754 Since an SSH-based setup entails a fair amount of overhead, it may be
756 more suited to temporary setups, such as for wireless VPNs. More perma‐
757 nent VPNs are better provided by tools such as ipsecctl(8) and
758 isakmpd(8).
759
761 ssh will normally set the following environment variables:
762 DISPLAY The DISPLAY variable indicates the location of the
764 X11 server. It is automatically set by ssh to
765 point to a value of the form “hostname:n”, where
766 “hostname” indicates the host where the shell runs,
767 and ‘n’ is an integer ≥ 1. ssh uses this special
768 value to forward X11 connections over the secure
769 channel. The user should normally not set DISPLAY
770 explicitly, as that will render the X11 connection
771 insecure (and will require the user to manually
772 copy any required authorization cookies).
773 HOME Set to the path of the user's home directory.
775 LOGNAME Synonym for USER; set for compatibility with sys‐
777 tems that use this variable.
778 MAIL Set to the path of the user's mailbox.
780 PATH Set to the default PATH, as specified when compil‐
782 ing ssh.
783 SSH_ASKPASS If ssh needs a passphrase, it will read the
785 passphrase from the current terminal if it was run
786 from a terminal. If ssh does not have a terminal
787 associated with it but DISPLAY and SSH_ASKPASS are
788 set, it will execute the program specified by
789 SSH_ASKPASS and open an X11 window to read the
790 passphrase. This is particularly useful when call‐
791 ing ssh from a .xsession or related script. (Note
792 that on some machines it may be necessary to redi‐
793 rect the input from /dev/null to make this work.)
794 SSH_ASKPASS_REQUIRE Allows further control over the use of an askpass
796 program. If this variable is set to “never” then
797 ssh will never attempt to use one. If it is set to
798 “prefer”, then ssh will prefer to use the askpass
799 program instead of the TTY when requesting pass‐
800 words. Finally, if the variable is set to “force”,
801 then the askpass program will be used for all
802 passphrase input regardless of whether DISPLAY is
803 set.
804 SSH_AUTH_SOCK Identifies the path of a UNIX-domain socket used to
806 communicate with the agent.
807 SSH_CONNECTION Identifies the client and server ends of the con‐
809 nection. The variable contains four space-sepa‐
810 rated values: client IP address, client port num‐
811 ber, server IP address, and server port number.
812 SSH_ORIGINAL_COMMAND This variable contains the original command line if
814 a forced command is executed. It can be used to
815 extract the original arguments.
816 SSH_TTY This is set to the name of the tty (path to the de‐
818 vice) associated with the current shell or command.
819 If the current session has no tty, this variable is
820 not set.
821 SSH_TUNNEL Optionally set by sshd(8) to contain the interface
823 names assigned if tunnel forwarding was requested
824 by the client.
825 SSH_USER_AUTH Optionally set by sshd(8), this variable may con‐
827 tain a pathname to a file that lists the authenti‐
828 cation methods successfully used when the session
829 was established, including any public keys that
830 were used.
831 TZ This variable is set to indicate the present time
833 zone if it was set when the daemon was started
834 (i.e. the daemon passes the value on to new connec‐
835 tions).
836 USER Set to the name of the user logging in.
838 Additionally, ssh reads ~/.ssh/environment, and adds lines of the format
840 “VARNAME=value” to the environment if the file exists and users are al‐
841 lowed to change their environment. For more information, see the
842 PermitUserEnvironment option in sshd_config(5).
843
845 ~/.rhosts
846 This file is used for host-based authentication (see above). On
847 some machines this file may need to be world-readable if the
848 user's home directory is on an NFS partition, because sshd(8)
849 reads it as root. Additionally, this file must be owned by the
850 user, and must not have write permissions for anyone else. The
851 recommended permission for most machines is read/write for the
852 user, and not accessible by others.
853 ~/.shosts
855 This file is used in exactly the same way as .rhosts, but allows
856 host-based authentication without permitting login with
857 rlogin/rsh.
858 ~/.ssh/
860 This directory is the default location for all user-specific con‐
861 figuration and authentication information. There is no general
862 requirement to keep the entire contents of this directory secret,
863 but the recommended permissions are read/write/execute for the
864 user, and not accessible by others.
865 ~/.ssh/authorized_keys
867 Lists the public keys (DSA, ECDSA, Ed25519, RSA) that can be used
868 for logging in as this user. The format of this file is de‐
869 scribed in the sshd(8) manual page. This file is not highly sen‐
870 sitive, but the recommended permissions are read/write for the
871 user, and not accessible by others.
872 ~/.ssh/config
874 This is the per-user configuration file. The file format and
875 configuration options are described in ssh_config(5). Because of
876 the potential for abuse, this file must have strict permissions:
877 read/write for the user, and not writable by others.
878 ~/.ssh/environment
880 Contains additional definitions for environment variables; see
881 ENVIRONMENT, above.
882 ~/.ssh/id_dsa
884 ~/.ssh/id_ecdsa
885 ~/.ssh/id_ecdsa_sk
886 ~/.ssh/id_ed25519
887 ~/.ssh/id_ed25519_sk
888 ~/.ssh/id_rsa
889 Contains the private key for authentication. These files contain
890 sensitive data and should be readable by the user but not acces‐
891 sible by others (read/write/execute). ssh will simply ignore a
892 private key file if it is accessible by others. It is possible
893 to specify a passphrase when generating the key which will be
894 used to encrypt the sensitive part of this file using AES-128.
895 ~/.ssh/id_dsa.pub
897 ~/.ssh/id_ecdsa.pub
898 ~/.ssh/id_ecdsa_sk.pub
899 ~/.ssh/id_ed25519.pub
900 ~/.ssh/id_ed25519_sk.pub
901 ~/.ssh/id_rsa.pub
902 Contains the public key for authentication. These files are not
903 sensitive and can (but need not) be readable by anyone.
904 ~/.ssh/known_hosts
906 Contains a list of host keys for all hosts the user has logged
907 into that are not already in the systemwide list of known host
908 keys. See sshd(8) for further details of the format of this
909 file.
910 ~/.ssh/rc
912 Commands in this file are executed by ssh when the user logs in,
913 just before the user's shell (or command) is started. See the
914 sshd(8) manual page for more information.
915 /etc/hosts.equiv
917 This file is for host-based authentication (see above). It
918 should only be writable by root.
919 /etc/ssh/shosts.equiv
921 This file is used in exactly the same way as hosts.equiv, but al‐
922 lows host-based authentication without permitting login with
923 rlogin/rsh.
924 /etc/ssh/ssh_config
926 Systemwide configuration file. The file format and configuration
927 options are described in ssh_config(5).
928 /etc/ssh/ssh_host_key
930 /etc/ssh/ssh_host_dsa_key
931 /etc/ssh/ssh_host_ecdsa_key
932 /etc/ssh/ssh_host_ed25519_key
933 /etc/ssh/ssh_host_rsa_key
934 These files contain the private parts of the host keys and are
935 used for host-based authentication.
936 /etc/ssh/ssh_known_hosts
938 Systemwide list of known host keys. This file should be prepared
939 by the system administrator to contain the public host keys of
940 all machines in the organization. It should be world-readable.
941 See sshd(8) for further details of the format of this file.
942 /etc/ssh/sshrc
944 Commands in this file are executed by ssh when the user logs in,
945 just before the user's shell (or command) is started. See the
946 sshd(8) manual page for more information.
947
949 ssh exits with the exit status of the remote command or with 255 if an
950 error occurred.
951
953 IPv6 address can be used everywhere where IPv4 address. In all entries
954 must be the IPv6 address enclosed in square brackets. Note: The square
955 brackets are metacharacters for the shell and must be escaped in shell.
956
958 scp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), ssh-keyscan(1),
959 tun(4), ssh_config(5), ssh-keysign(8), sshd(8)
960
962 S. Lehtinen and C. Lonvick, The Secure Shell (SSH) Protocol Assigned
963 Numbers, RFC 4250, January 2006.
964 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Protocol Architecture,
966 RFC 4251, January 2006.
967 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Authentication Protocol,
969 RFC 4252, January 2006.
970 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Transport Layer
972 Protocol, RFC 4253, January 2006.
973 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Connection Protocol, RFC
975 4254, January 2006.
976 J. Schlyter and W. Griffin, Using DNS to Securely Publish Secure Shell
978 (SSH) Key Fingerprints, RFC 4255, January 2006.
979 F. Cusack and M. Forssen, Generic Message Exchange Authentication for the
981 Secure Shell Protocol (SSH), RFC 4256, January 2006.
982 J. Galbraith and P. Remaker, The Secure Shell (SSH) Session Channel Break
984 Extension, RFC 4335, January 2006.
985 M. Bellare, T. Kohno, and C. Namprempre, The Secure Shell (SSH) Transport
987 Layer Encryption Modes, RFC 4344, January 2006.
988 B. Harris, Improved Arcfour Modes for the Secure Shell (SSH) Transport
990 Layer Protocol, RFC 4345, January 2006.
991 M. Friedl, N. Provos, and W. Simpson, Diffie-Hellman Group Exchange for
993 the Secure Shell (SSH) Transport Layer Protocol, RFC 4419, March 2006.
994 J. Galbraith and R. Thayer, The Secure Shell (SSH) Public Key File
996 Format, RFC 4716, November 2006.
997 D. Stebila and J. Green, Elliptic Curve Algorithm Integration in the
999 Secure Shell Transport Layer, RFC 5656, December 2009.
1000 A. Perrig and D. Song, Hash Visualization: a New Technique to improve
1002 Real-World Security, 1999, International Workshop on Cryptographic
1003 Techniques and E-Commerce (CrypTEC '99).
1004
1006 OpenSSH is a derivative of the original and free ssh 1.2.12 release by
1007 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
1008 de Raadt and Dug Song removed many bugs, re-added newer features and cre‐
1009 ated OpenSSH. Markus Friedl contributed the support for SSH protocol
1010 versions 1.5 and 2.0.
1011BSD July 28, 2021 BSD