1SSH(1)                    BSD General Commands Manual                   SSH(1)
2

NAME

4     ssh — OpenSSH SSH client (remote login program)
5

SYNOPSIS

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

DESCRIPTION

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
22     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
27     If a command is specified, it is executed on the remote host instead of a
28     login shell.
29
30     The options are as follows:
31
32     -4      Forces ssh to use IPv4 addresses only.
33
34     -6      Forces ssh to use IPv6 addresses only.
35
36     -A      Enables forwarding of the authentication agent connection.  This
37             can also be specified on a per-host basis in a configuration
38             file.
39
40             Agent forwarding should be enabled with caution.  Users with the
41             ability to bypass file permissions on the remote host (for the
42             agent's UNIX-domain socket) can access the local agent through
43             the forwarded connection.  An attacker cannot obtain key material
44             from the agent, however they can perform operations on the keys
45             that enable them to authenticate using the identities loaded into
46             the agent.
47
48     -a      Disables forwarding of the authentication agent connection.
49
50     -B bind_interface
51             Bind to the address of bind_interface before attempting to con‐
52             nect to the destination host.  This is only useful on systems
53             with more than one address.
54
55     -b bind_address
56             Use bind_address on the local machine as the source address of
57             the connection.  Only useful on systems with more than one
58             address.
59
60     -C      Requests compression of all data (including stdin, stdout,
61             stderr, and data for forwarded X11, TCP and UNIX-domain connec‐
62             tions).  The compression algorithm is the same used by gzip(1).
63             Compression is desirable on modem lines and other slow connec‐
64             tions, but will only slow down things on fast networks.  The
65             default value can be set on a host-by-host basis in the configu‐
66             ration files; see the Compression option.
67
68     -c cipher_spec
69             Selects the cipher specification for encrypting the session.
70             cipher_spec is a comma-separated list of ciphers listed in order
71             of preference.  See the Ciphers keyword in ssh_config(5) for more
72             information.
73
74     -D [bind_address:]port
75             Specifies a local “dynamic” application-level port forwarding.
76             This works by allocating a socket to listen to port on the local
77             side, optionally bound to the specified bind_address.  Whenever a
78             connection is made to this port, the connection is forwarded over
79             the secure channel, and the application protocol is then used to
80             determine where to connect to from the remote machine.  Currently
81             the SOCKS4 and SOCKS5 protocols are supported, and ssh will act
82             as a SOCKS server.  Only root can forward privileged ports.
83             Dynamic port forwardings can also be specified in the configura‐
84             tion file.
85
86             IPv6 addresses can be specified by enclosing the address in
87             square brackets.  Only the superuser can forward privileged
88             ports.  By default, the local port is bound in accordance with
89             the GatewayPorts setting.  However, an explicit bind_address may
90             be used to bind the connection to a specific address.  The
91             bind_address of “localhost” indicates that the listening port be
92             bound for local use only, while an empty address or ‘*’ indicates
93             that the port should be available from all interfaces.
94
95     -E log_file
96             Append debug logs to log_file instead of standard error.
97
98     -e escape_char
99             Sets the escape character for sessions with a pty (default: ‘~’).
100             The escape character is only recognized at the beginning of a
101             line.  The escape character followed by a dot (‘.’) closes the
102             connection; followed by control-Z suspends the connection; and
103             followed by itself sends the escape character once.  Setting the
104             character to “none” disables any escapes and makes the session
105             fully transparent.
106
107     -F configfile
108             Specifies an alternative per-user configuration file.  If a con‐
109             figuration file is given on the command line, the system-wide
110             configuration file (/etc/ssh/ssh_config) will be ignored.  The
111             default for the per-user configuration file is ~/.ssh/config.
112
113     -f      Requests ssh to go to background just before command execution.
114             This is useful if ssh is going to ask for passwords or
115             passphrases, but the user wants it in the background.  This
116             implies -n.  The recommended way to start X11 programs at a
117             remote site is with something like ssh -f host xterm.
118
119             If the ExitOnForwardFailure configuration option is set to “yes”,
120             then a client started with -f will wait for all remote port for‐
121             wards to be successfully established before placing itself in the
122             background.
123
124     -G      Causes ssh to print its configuration after evaluating Host and
125             Match blocks and exit.
126
127     -g      Allows remote hosts to connect to local forwarded ports.  If used
128             on a multiplexed connection, then this option must be specified
129             on the master process.
130
131     -I pkcs11
132             Specify the PKCS#11 shared library ssh should use to communicate
133             with a PKCS#11 token providing the user's private RSA key.
134
135     -i identity_file
136             Selects a file from which the identity (private key) for public
137             key authentication is read.  The default is ~/.ssh/id_dsa,
138             ~/.ssh/id_ecdsa, ~/.ssh/id_ed25519 and ~/.ssh/id_rsa.  Identity
139             files may also be specified on a per-host basis in the configura‐
140             tion file.  It is possible to have multiple -i options (and mul‐
141             tiple identities specified in configuration files).  If no cer‐
142             tificates have been explicitly specified by the CertificateFile
143             directive, ssh will also try to load certificate information from
144             the filename obtained by appending -cert.pub to identity file‐
145             names.
146
147     -J destination
148             Connect to the target host by first making a ssh connection to
149             the jump host described by destination and then establishing a
150             TCP forwarding to the ultimate destination from there.  Multiple
151             jump hops may be specified separated by comma characters.  This
152             is a shortcut to specify a ProxyJump configuration directive.
153
154     -K      Enables GSSAPI-based authentication and forwarding (delegation)
155             of GSSAPI credentials to the server.
156
157     -k      Disables forwarding (delegation) of GSSAPI credentials to the
158             server.
159
160     -L [bind_address:]port:host:hostport
161     -L [bind_address:]port:remote_socket
162     -L local_socket:host:hostport
163     -L local_socket:remote_socket
164             Specifies that connections to the given TCP port or Unix socket
165             on the local (client) host are to be forwarded to the given host
166             and port, or Unix socket, on the remote side.  This works by
167             allocating a socket to listen to either a TCP port on the local
168             side, optionally bound to the specified bind_address, or to a
169             Unix socket.  Whenever a connection is made to the local port or
170             socket, the connection is forwarded over the secure channel, and
171             a connection is made to either host port hostport, or the Unix
172             socket remote_socket, from the remote machine.
173
174             Port forwardings can also be specified in the configuration file.
175             Only the superuser can forward privileged ports.  IPv6 addresses
176             can be specified by enclosing the address in square brackets.
177
178             By default, the local port is bound in accordance with the
179             GatewayPorts setting.  However, an explicit bind_address may be
180             used to bind the connection to a specific address.  The
181             bind_address of “localhost” indicates that the listening port be
182             bound for local use only, while an empty address or ‘*’ indicates
183             that the port should be available from all interfaces.
184
185     -l login_name
186             Specifies the user to log in as on the remote machine.  This also
187             may be specified on a per-host basis in the configuration file.
188
189     -M      Places the ssh client into “master” mode for connection sharing.
190             Multiple -M options places ssh into “master” mode but with con‐
191             firmation required using ssh-askpass(1) before each operation
192             that changes the multiplexing state (e.g. opening a new session).
193             Refer to the description of ControlMaster in ssh_config(5) for
194             details.
195
196     -m mac_spec
197             A comma-separated list of MAC (message authentication code) algo‐
198             rithms, specified in order of preference.  See the MACs keyword
199             for more information.
200
201     -N      Do not execute a remote command.  This is useful for just for‐
202             warding ports.
203
204     -n      Redirects stdin from /dev/null (actually, prevents reading from
205             stdin).  This must be used when ssh is run in the background.  A
206             common trick is to use this to run X11 programs on a remote
207             machine.  For example, ssh -n shadows.cs.hut.fi emacs & will
208             start an emacs on shadows.cs.hut.fi, and the X11 connection will
209             be automatically forwarded over an encrypted channel.  The ssh
210             program will be put in the background.  (This does not work if
211             ssh needs to ask for a password or passphrase; see also the -f
212             option.)
213
214     -O ctl_cmd
215             Control an active connection multiplexing master process.  When
216             the -O option is specified, the ctl_cmd argument is interpreted
217             and passed to the master process.  Valid commands are: “check”
218             (check that the master process is running), “forward” (request
219             forwardings without command execution), “cancel” (cancel forward‐
220             ings), “exit” (request the master to exit), and “stop” (request
221             the master to stop accepting further multiplexing requests).
222
223     -o option
224             Can be used to give options in the format used in the configura‐
225             tion file.  This is useful for specifying options for which there
226             is no separate command-line flag.  For full details of the
227             options listed below, and their possible values, see
228             ssh_config(5).
229
230                   AddKeysToAgent
231                   AddressFamily
232                   BatchMode
233                   BindAddress
234                   CanonicalDomains
235                   CanonicalizeFallbackLocal
236                   CanonicalizeHostname
237                   CanonicalizeMaxDots
238                   CanonicalizePermittedCNAMEs
239                   CASignatureAlgorithms
240                   CertificateFile
241                   ChallengeResponseAuthentication
242                   CheckHostIP
243                   Ciphers
244                   ClearAllForwardings
245                   Compression
246                   ConnectionAttempts
247                   ConnectTimeout
248                   ControlMaster
249                   ControlPath
250                   ControlPersist
251                   DynamicForward
252                   EscapeChar
253                   ExitOnForwardFailure
254                   FingerprintHash
255                   ForwardAgent
256                   ForwardX11
257                   ForwardX11Timeout
258                   ForwardX11Trusted
259                   GatewayPorts
260                   GlobalKnownHostsFile
261                   GSSAPIAuthentication
262                   GSSAPIKeyExchange
263                   GSSAPIClientIdentity
264                   GSSAPIDelegateCredentials
265                   GSSAPIRenewalForcesRekey
266                   GSSAPITrustDNS
267                   GSSAPIKexAlgorithms
268                   HashKnownHosts
269                   Host
270                   HostbasedAuthentication
271                   HostbasedKeyTypes
272                   HostKeyAlgorithms
273                   HostKeyAlias
274                   HostName
275                   IdentitiesOnly
276                   IdentityAgent
277                   IdentityFile
278                   IPQoS
279                   KbdInteractiveAuthentication
280                   KbdInteractiveDevices
281                   KexAlgorithms
282                   LocalCommand
283                   LocalForward
284                   LogLevel
285                   MACs
286                   Match
287                   NoHostAuthenticationForLocalhost
288                   NumberOfPasswordPrompts
289                   PasswordAuthentication
290                   PermitLocalCommand
291                   PKCS11Provider
292                   Port
293                   PreferredAuthentications
294                   ProxyCommand
295                   ProxyJump
296                   ProxyUseFdpass
297                   PubkeyAcceptedKeyTypes
298                   PubkeyAuthentication
299                   RekeyLimit
300                   RemoteCommand
301                   RemoteForward
302                   RequestTTY
303                   SendEnv
304                   ServerAliveInterval
305                   ServerAliveCountMax
306                   SetEnv
307                   StreamLocalBindMask
308                   StreamLocalBindUnlink
309                   StrictHostKeyChecking
310                   TCPKeepAlive
311                   Tunnel
312                   TunnelDevice
313                   UpdateHostKeys
314                   User
315                   UserKnownHostsFile
316                   VerifyHostKeyDNS
317                   VisualHostKey
318                   XAuthLocation
319
320     -p port
321             Port to connect to on the remote host.  This can be specified on
322             a per-host basis in the configuration file.
323
324     -Q query_option
325             Queries ssh for the algorithms supported for the specified ver‐
326             sion 2.  The available features are: cipher (supported symmetric
327             ciphers), cipher-auth (supported symmetric ciphers that support
328             authenticated encryption), help (supported query terms for use
329             with the -Q flag), mac (supported message integrity codes), kex
330             (key exchange algorithms), key (key types), key-cert (certificate
331             key types), key-plain (non-certificate key types),
332             protocol-version (supported SSH protocol versions), and sig (sup‐
333             ported signature algorithms).
334
335     -q      Quiet mode.  Causes most warning and diagnostic messages to be
336             suppressed.
337
338     -R [bind_address:]port:host:hostport
339     -R [bind_address:]port:local_socket
340     -R remote_socket:host:hostport
341     -R remote_socket:local_socket
342     -R [bind_address:]port
343             Specifies that connections to the given TCP port or Unix socket
344             on the remote (server) host are to be forwarded to the local
345             side.
346
347             This works by allocating a socket to listen to either a TCP port
348             or to a Unix socket on the remote side.  Whenever a connection is
349             made to this port or Unix socket, the connection is forwarded
350             over the secure channel, and a connection is made from the local
351             machine to either an explicit destination specified by host port
352             hostport, or local_socket, or, if no explicit destination was
353             specified, ssh will act as a SOCKS 4/5 proxy and forward connec‐
354             tions to the destinations requested by the remote SOCKS client.
355
356             Port forwardings can also be specified in the configuration file.
357             Privileged ports can be forwarded only when logging in as root on
358             the remote machine.  IPv6 addresses can be specified by enclosing
359             the address in square brackets.
360
361             By default, TCP listening sockets on the server will be bound to
362             the loopback interface only.  This may be overridden by specify‐
363             ing a bind_address.  An empty bind_address, or the address ‘*’,
364             indicates that the remote socket should listen on all interfaces.
365             Specifying a remote bind_address will only succeed if the
366             server's GatewayPorts option is enabled (see sshd_config(5)).
367
368             If the port argument is ‘0’, the listen port will be dynamically
369             allocated on the server and reported to the client at run time.
370             When used together with -O forward the allocated port will be
371             printed to the standard output.
372
373     -S ctl_path
374             Specifies the location of a control socket for connection shar‐
375             ing, or the string “none” to disable connection sharing.  Refer
376             to the description of ControlPath and ControlMaster in
377             ssh_config(5) for details.
378
379     -s      May be used to request invocation of a subsystem on the remote
380             system.  Subsystems facilitate the use of SSH as a secure trans‐
381             port for other applications (e.g. sftp(1)).  The subsystem is
382             specified as the remote command.
383
384     -T      Disable pseudo-terminal allocation.
385
386     -t      Force pseudo-terminal allocation.  This can be used to execute
387             arbitrary screen-based programs on a remote machine, which can be
388             very useful, e.g. when implementing menu services.  Multiple -t
389             options force tty allocation, even if ssh has no local tty.
390
391     -V      Display the version number and exit.
392
393     -v      Verbose mode.  Causes ssh to print debugging messages about its
394             progress.  This is helpful in debugging connection, authentica‐
395             tion, and configuration problems.  Multiple -v options increase
396             the verbosity.  The maximum is 3.
397
398     -W host:port
399             Requests that standard input and output on the client be for‐
400             warded to host on port over the secure channel.  Implies -N, -T,
401             ExitOnForwardFailure and ClearAllForwardings, though these can be
402             overridden in the configuration file or using -o command line
403             options.
404
405     -w local_tun[:remote_tun]
406             Requests tunnel device forwarding with the specified tun(4)
407             devices between the client (local_tun) and the server
408             (remote_tun).
409
410             The devices may be specified by numerical ID or the keyword
411             “any”, which uses the next available tunnel device.  If
412             remote_tun is not specified, it defaults to “any”.  See also the
413             Tunnel and TunnelDevice directives in ssh_config(5).
414
415             If the Tunnel directive is unset, it will be set to the default
416             tunnel mode, which is “point-to-point”.  If a different Tunnel
417             forwarding mode it desired, then it should be specified before
418             -w.
419
420     -X      Enables X11 forwarding.  This can also be specified on a per-host
421             basis in a configuration file.
422
423             X11 forwarding should be enabled with caution.  Users with the
424             ability to bypass file permissions on the remote host (for the
425             user's X authorization database) can access the local X11 display
426             through the forwarded connection.  An attacker may then be able
427             to perform activities such as keystroke monitoring.
428
429             For this reason, X11 forwarding is subjected to X11 SECURITY
430             extension restrictions by default.  Please refer to the ssh -Y
431             option and the ForwardX11Trusted directive in ssh_config(5) for
432             more information.
433
434     -x      Disables X11 forwarding.
435
436     -Y      Enables trusted X11 forwarding.  Trusted X11 forwardings are not
437             subjected to the X11 SECURITY extension controls.
438
439     -y      Send log information using the syslog(3) system module.  By
440             default this information is sent to stderr.
441
442     ssh may additionally obtain configuration data from a per-user configura‐
443     tion file and a system-wide configuration file.  The file format and con‐
444     figuration options are described in ssh_config(5).
445

AUTHENTICATION

447     The OpenSSH SSH client supports SSH protocol 2.
448
449     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
455     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
469     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
479     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
485     The server may inform the client of errors that prevented public key
486     authentication from succeeding after authentication completes using a
487     different method.  These may be viewed by increasing the LogLevel to
488     DEBUG or higher (e.g. by using the -v flag).
489
490     The user creates his/her key pair by running ssh-keygen(1).  This stores
491     the private key in ~/.ssh/id_dsa (DSA), ~/.ssh/id_ecdsa (ECDSA),
492     ~/.ssh/id_ed25519 (Ed25519), or ~/.ssh/id_rsa (RSA) and stores the public
493     key in ~/.ssh/id_dsa.pub (DSA), ~/.ssh/id_ecdsa.pub (ECDSA),
494     ~/.ssh/id_ed25519.pub (Ed25519), or ~/.ssh/id_rsa.pub (RSA) in the user's
495     home directory.  The user should then copy the public key to
496     ~/.ssh/authorized_keys in his/her home directory on the remote machine.
497     The authorized_keys file corresponds to the conventional ~/.rhosts file,
498     and has one key per line, though the lines can be very long.  After this,
499     the user can log in without giving the password.
500
501     A variation on public key authentication is available in the form of cer‐
502     tificate authentication: instead of a set of public/private keys, signed
503     certificates are used.  This has the advantage that a single trusted cer‐
504     tification authority can be used in place of many public/private keys.
505     See the CERTIFICATES section of ssh-keygen(1) for more information.
506
507     The most convenient way to use public key or certificate authentication
508     may be with an authentication agent.  See ssh-agent(1) and (optionally)
509     the AddKeysToAgent directive in ssh_config(5) for more information.
510
511     Challenge-response authentication works as follows: The server sends an
512     arbitrary "challenge" text, and prompts for a response.  Examples of
513     challenge-response authentication include BSD Authentication (see
514     login.conf(5)) and PAM (some non-OpenBSD systems).
515
516     Finally, if other authentication methods fail, ssh prompts the user for a
517     password.  The password is sent to the remote host for checking; however,
518     since all communications are encrypted, the password cannot be seen by
519     someone listening on the network.
520
521     ssh automatically maintains and checks a database containing identifica‐
522     tion for all hosts it has ever been used with.  Host keys are stored in
523     ~/.ssh/known_hosts in the user's home directory.  Additionally, the file
524     /etc/ssh/ssh_known_hosts is automatically checked for known hosts.  Any
525     new hosts are automatically added to the user's file.  If a host's iden‐
526     tification ever changes, ssh warns about this and disables password
527     authentication to prevent server spoofing or man-in-the-middle attacks,
528     which could otherwise be used to circumvent the encryption.  The
529     StrictHostKeyChecking option can be used to control logins to machines
530     whose host key is not known or has changed.
531
532     When the user's identity has been accepted by the server, the server
533     either executes the given command in a non-interactive session or, if no
534     command has been specified, logs into the machine and gives the user a
535     normal shell as an interactive session.  All communication with the
536     remote command or shell will be automatically encrypted.
537
538     If an interactive session is requested ssh by default will only request a
539     pseudo-terminal (pty) for interactive sessions when the client has one.
540     The flags -T and -t can be used to override this behaviour.
541
542     If a pseudo-terminal has been allocated the user may use the escape char‐
543     acters noted below.
544
545     If no pseudo-terminal has been allocated, the session is transparent and
546     can be used to reliably transfer binary data.  On most systems, setting
547     the escape character to “none” will also make the session transparent
548     even if a tty is used.
549
550     The session terminates when the command or shell on the remote machine
551     exits and all X11 and TCP connections have been closed.
552

ESCAPE CHARACTERS

554     When a pseudo-terminal has been requested, ssh supports a number of func‐
555     tions through the use of an escape character.
556
557     A single tilde character can be sent as ~~ or by following the tilde by a
558     character other than those described below.  The escape character must
559     always follow a newline to be interpreted as special.  The escape charac‐
560     ter can be changed in configuration files using the EscapeChar configura‐
561     tion directive or on the command line by the -e option.
562
563     The supported escapes (assuming the default ‘~’) are:
564
565     ~.      Disconnect.
566
567     ~^Z     Background ssh.
568
569     ~#      List forwarded connections.
570
571     ~&      Background ssh at logout when waiting for forwarded connection /
572             X11 sessions to terminate.
573
574     ~?      Display a list of escape characters.
575
576     ~B      Send a BREAK to the remote system (only useful if the peer sup‐
577             ports it).
578
579     ~C      Open command line.  Currently this allows the addition of port
580             forwardings using the -L, -R and -D options (see above).  It also
581             allows the cancellation of existing port-forwardings with
582             -KL[bind_address:]port for local, -KR[bind_address:]port for
583             remote and -KD[bind_address:]port for dynamic port-forwardings.
584             !command allows the user to execute a local command if the
585             PermitLocalCommand option is enabled in ssh_config(5).  Basic
586             help is available, using the -h option.
587
588     ~R      Request rekeying of the connection (only useful if the peer sup‐
589             ports it).
590
591     ~V      Decrease the verbosity (LogLevel) when errors are being written
592             to stderr.
593
594     ~v      Increase the verbosity (LogLevel) when errors are being written
595             to stderr.
596

TCP FORWARDING

598     Forwarding of arbitrary TCP connections over the secure channel can be
599     specified either on the command line or in a configuration file.  One
600     possible application of TCP forwarding is a secure connection to a mail
601     server; another is going through firewalls.
602
603     In the example below, we look at encrypting communication between an IRC
604     client and server, even though the IRC server does not directly support
605     encrypted communications.  This works as follows: the user connects to
606     the remote host using ssh, specifying a port to be used to forward con‐
607     nections to the remote server.  After that it is possible to start the
608     service which is to be encrypted on the client machine, connecting to the
609     same local port, and ssh will encrypt and forward the connection.
610
611     The following example tunnels an IRC session from client machine
612     “127.0.0.1” (localhost) to remote server “server.example.com”:
613
614         $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
615         $ irc -c '#users' -p 1234 pinky 127.0.0.1
616
617     This tunnels a connection to IRC server “server.example.com”, joining
618     channel “#users”, nickname “pinky”, using port 1234.  It doesn't matter
619     which port is used, as long as it's greater than 1023 (remember, only
620     root can open sockets on privileged ports) and doesn't conflict with any
621     ports already in use.  The connection is forwarded to port 6667 on the
622     remote server, since that's the standard port for IRC services.
623
624     The -f option backgrounds ssh and the remote command “sleep 10” is speci‐
625     fied to allow an amount of time (10 seconds, in the example) to start the
626     service which is to be tunnelled.  If no connections are made within the
627     time specified, ssh will exit.
628

X11 FORWARDING

630     If the ForwardX11 variable is set to “yes” (or see the description of the
631     -X, -x, and -Y options above) and the user is using X11 (the DISPLAY
632     environment variable is set), the connection to the X11 display is auto‐
633     matically forwarded to the remote side in such a way that any X11 pro‐
634     grams started from the shell (or command) will go through the encrypted
635     channel, and the connection to the real X server will be made from the
636     local machine.  The user should not manually set DISPLAY.  Forwarding of
637     X11 connections can be configured on the command line or in configuration
638     files.
639
640     The DISPLAY value set by ssh will point to the server machine, but with a
641     display number greater than zero.  This is normal, and happens because
642     ssh creates a “proxy” X server on the server machine for forwarding the
643     connections over the encrypted channel.
644
645     ssh will also automatically set up Xauthority data on the server machine.
646     For this purpose, it will generate a random authorization cookie, store
647     it in Xauthority on the server, and verify that any forwarded connections
648     carry this cookie and replace it by the real cookie when the connection
649     is opened.  The real authentication cookie is never sent to the server
650     machine (and no cookies are sent in the plain).
651
652     If the ForwardAgent variable is set to “yes” (or see the description of
653     the -A and -a options above) and the user is using an authentication
654     agent, the connection to the agent is automatically forwarded to the
655     remote side.
656

VERIFYING HOST KEYS

658     When connecting to a server for the first time, a fingerprint of the
659     server's public key is presented to the user (unless the option
660     StrictHostKeyChecking has been disabled).  Fingerprints can be determined
661     using ssh-keygen(1):
662
663           $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
664
665     If the fingerprint is already known, it can be matched and the key can be
666     accepted or rejected.  If only legacy (MD5) fingerprints for the server
667     are available, the ssh-keygen(1) -E option may be used to downgrade the
668     fingerprint algorithm to match.
669
670     Because of the difficulty of comparing host keys just by looking at fin‐
671     gerprint strings, there is also support to compare host keys visually,
672     using random art.  By setting the VisualHostKey option to “yes”, a small
673     ASCII graphic gets displayed on every login to a server, no matter if the
674     session itself is interactive or not.  By learning the pattern a known
675     server produces, a user can easily find out that the host key has changed
676     when a completely different pattern is displayed.  Because these patterns
677     are not unambiguous however, a pattern that looks similar to the pattern
678     remembered only gives a good probability that the host key is the same,
679     not guaranteed proof.
680
681     To get a listing of the fingerprints along with their random art for all
682     known hosts, the following command line can be used:
683
684           $ ssh-keygen -lv -f ~/.ssh/known_hosts
685
686     If the fingerprint is unknown, an alternative method of verification is
687     available: SSH fingerprints verified by DNS.  An additional resource
688     record (RR), SSHFP, is added to a zonefile and the connecting client is
689     able to match the fingerprint with that of the key presented.
690
691     In this example, we are connecting a client to a server,
692     “host.example.com”.  The SSHFP resource records should first be added to
693     the zonefile for host.example.com:
694
695           $ ssh-keygen -r host.example.com.
696
697     The output lines will have to be added to the zonefile.  To check that
698     the zone is answering fingerprint queries:
699
700           $ dig -t SSHFP host.example.com
701
702     Finally the client connects:
703
704           $ ssh -o "VerifyHostKeyDNS ask" host.example.com
705           [...]
706           Matching host key fingerprint found in DNS.
707           Are you sure you want to continue connecting (yes/no)?
708
709     See the VerifyHostKeyDNS option in ssh_config(5) for more information.
710

SSH-BASED VIRTUAL PRIVATE NETWORKS

712     ssh contains support for Virtual Private Network (VPN) tunnelling using
713     the tun(4) network pseudo-device, allowing two networks to be joined
714     securely.  The sshd_config(5) configuration option PermitTunnel controls
715     whether the server supports this, and at what level (layer 2 or 3 traf‐
716     fic).
717
718     The following example would connect client network 10.0.50.0/24 with
719     remote network 10.0.99.0/24 using a point-to-point connection from
720     10.1.1.1 to 10.1.1.2, provided that the SSH server running on the gateway
721     to the remote network, at 192.168.1.15, allows it.
722
723     On the client:
724
725           # ssh -f -w 0:1 192.168.1.15 true
726           # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
727           # route add 10.0.99.0/24 10.1.1.2
728
729     On the server:
730
731           # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
732           # route add 10.0.50.0/24 10.1.1.1
733
734     Client access may be more finely tuned via the /root/.ssh/authorized_keys
735     file (see below) and the PermitRootLogin server option.  The following
736     entry would permit connections on tun(4) device 1 from user “jane” and on
737     tun device 2 from user “john”, if PermitRootLogin is set to
738     “forced-commands-only”:
739
740       tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
741       tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
742
743     Since an SSH-based setup entails a fair amount of overhead, it may be
744     more suited to temporary setups, such as for wireless VPNs.  More perma‐
745     nent VPNs are better provided by tools such as ipsecctl(8) and
746     isakmpd(8).
747

ENVIRONMENT

749     ssh will normally set the following environment variables:
750
751     DISPLAY               The DISPLAY variable indicates the location of the
752                           X11 server.  It is automatically set by ssh to
753                           point to a value of the form “hostname:n”, where
754                           “hostname” indicates the host where the shell runs,
755                           and ‘n’ is an integer ≥ 1.  ssh uses this special
756                           value to forward X11 connections over the secure
757                           channel.  The user should normally not set DISPLAY
758                           explicitly, as that will render the X11 connection
759                           insecure (and will require the user to manually
760                           copy any required authorization cookies).
761
762     HOME                  Set to the path of the user's home directory.
763
764     LOGNAME               Synonym for USER; set for compatibility with sys‐
765                           tems that use this variable.
766
767     MAIL                  Set to the path of the user's mailbox.
768
769     PATH                  Set to the default PATH, as specified when compil‐
770                           ing ssh.
771
772     SSH_ASKPASS           If ssh needs a passphrase, it will read the
773                           passphrase from the current terminal if it was run
774                           from a terminal.  If ssh does not have a terminal
775                           associated with it but DISPLAY and SSH_ASKPASS are
776                           set, it will execute the program specified by
777                           SSH_ASKPASS and open an X11 window to read the
778                           passphrase.  This is particularly useful when call‐
779                           ing ssh from a .xsession or related script.  (Note
780                           that on some machines it may be necessary to redi‐
781                           rect the input from /dev/null to make this work.)
782
783     SSH_AUTH_SOCK         Identifies the path of a UNIX-domain socket used to
784                           communicate with the agent.
785
786     SSH_CONNECTION        Identifies the client and server ends of the con‐
787                           nection.  The variable contains four space-sepa‐
788                           rated values: client IP address, client port num‐
789                           ber, server IP address, and server port number.
790
791     SSH_ORIGINAL_COMMAND  This variable contains the original command line if
792                           a forced command is executed.  It can be used to
793                           extract the original arguments.
794
795     SSH_TTY               This is set to the name of the tty (path to the
796                           device) associated with the current shell or com‐
797                           mand.  If the current session has no tty, this
798                           variable is not set.
799
800     SSH_TUNNEL            Optionally set by sshd(8) to contain the interface
801                           names assigned if tunnel forwarding was requested
802                           by the client.
803
804     SSH_USER_AUTH         Optionally set by sshd(8), this variable may con‐
805                           tain a pathname to a file that lists the authenti‐
806                           cation methods successfully used when the session
807                           was established, including any public keys that
808                           were used.
809
810     TZ                    This variable is set to indicate the present time
811                           zone if it was set when the daemon was started
812                           (i.e. the daemon passes the value on to new connec‐
813                           tions).
814
815     USER                  Set to the name of the user logging in.
816
817     Additionally, ssh reads ~/.ssh/environment, and adds lines of the format
818     “VARNAME=value” to the environment if the file exists and users are
819     allowed to change their environment.  For more information, see the
820     PermitUserEnvironment option in sshd_config(5).
821

FILES

823     ~/.rhosts
824             This file is used for host-based authentication (see above).  On
825             some machines this file may need to be world-readable if the
826             user's home directory is on an NFS partition, because sshd(8)
827             reads it as root.  Additionally, this file must be owned by the
828             user, and must not have write permissions for anyone else.  The
829             recommended permission for most machines is read/write for the
830             user, and not accessible by others.
831
832     ~/.shosts
833             This file is used in exactly the same way as .rhosts, but allows
834             host-based authentication without permitting login with
835             rlogin/rsh.
836
837     ~/.ssh/
838             This directory is the default location for all user-specific con‐
839             figuration and authentication information.  There is no general
840             requirement to keep the entire contents of this directory secret,
841             but the recommended permissions are read/write/execute for the
842             user, and not accessible by others.
843
844     ~/.ssh/authorized_keys
845             Lists the public keys (DSA, ECDSA, Ed25519, RSA) that can be used
846             for logging in as this user.  The format of this file is
847             described in the sshd(8) manual page.  This file is not highly
848             sensitive, but the recommended permissions are read/write for the
849             user, and not accessible by others.
850
851     ~/.ssh/config
852             This is the per-user configuration file.  The file format and
853             configuration options are described in ssh_config(5).  Because of
854             the potential for abuse, this file must have strict permissions:
855             read/write for the user, and not writable by others.
856
857     ~/.ssh/environment
858             Contains additional definitions for environment variables; see
859             ENVIRONMENT, above.
860
861     ~/.ssh/id_dsa
862     ~/.ssh/id_ecdsa
863     ~/.ssh/id_ed25519
864     ~/.ssh/id_rsa
865             Contains the private key for authentication.  These files contain
866             sensitive data and should be readable by the user but not acces‐
867             sible by others (read/write/execute).  ssh will simply ignore a
868             private key file if it is accessible by others.  It is possible
869             to specify a passphrase when generating the key which will be
870             used to encrypt the sensitive part of this file using AES-128.
871
872     ~/.ssh/id_dsa.pub
873     ~/.ssh/id_ecdsa.pub
874     ~/.ssh/id_ed25519.pub
875     ~/.ssh/id_rsa.pub
876             Contains the public key for authentication.  These files are not
877             sensitive and can (but need not) be readable by anyone.
878
879     ~/.ssh/known_hosts
880             Contains a list of host keys for all hosts the user has logged
881             into that are not already in the systemwide list of known host
882             keys.  See sshd(8) for further details of the format of this
883             file.
884
885     ~/.ssh/rc
886             Commands in this file are executed by ssh when the user logs in,
887             just before the user's shell (or command) is started.  See the
888             sshd(8) manual page for more information.
889
890     /etc/hosts.equiv
891             This file is for host-based authentication (see above).  It
892             should only be writable by root.
893
894     /etc/ssh/shosts.equiv
895             This file is used in exactly the same way as hosts.equiv, but
896             allows host-based authentication without permitting login with
897             rlogin/rsh.
898
899     /etc/ssh/ssh_config
900             Systemwide configuration file.  The file format and configuration
901             options are described in ssh_config(5).
902
903     /etc/ssh/ssh_host_key
904     /etc/ssh/ssh_host_dsa_key
905     /etc/ssh/ssh_host_ecdsa_key
906     /etc/ssh/ssh_host_ed25519_key
907     /etc/ssh/ssh_host_rsa_key
908             These files contain the private parts of the host keys and are
909             used for host-based authentication.
910
911     /etc/ssh/ssh_known_hosts
912             Systemwide list of known host keys.  This file should be prepared
913             by the system administrator to contain the public host keys of
914             all machines in the organization.  It should be world-readable.
915             See sshd(8) for further details of the format of this file.
916
917     /etc/ssh/sshrc
918             Commands in this file are executed by ssh when the user logs in,
919             just before the user's shell (or command) is started.  See the
920             sshd(8) manual page for more information.
921

EXIT STATUS

923     ssh exits with the exit status of the remote command or with 255 if an
924     error occurred.
925

IPV6

927     IPv6 address can be used everywhere where IPv4 address. In all entries
928     must be the IPv6 address enclosed in square brackets. Note: The square
929     brackets are metacharacters for the shell and must be escaped in shell.
930

SEE ALSO

932     scp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), ssh-keyscan(1),
933     tun(4), ssh_config(5), ssh-keysign(8), sshd(8)
934

STANDARDS

936     S. Lehtinen and C. Lonvick, The Secure Shell (SSH) Protocol Assigned
937     Numbers, RFC 4250, January 2006.
938
939     T. Ylonen and C. Lonvick, The Secure Shell (SSH) Protocol Architecture,
940     RFC 4251, January 2006.
941
942     T. Ylonen and C. Lonvick, The Secure Shell (SSH) Authentication Protocol,
943     RFC 4252, January 2006.
944
945     T. Ylonen and C. Lonvick, The Secure Shell (SSH) Transport Layer
946     Protocol, RFC 4253, January 2006.
947
948     T. Ylonen and C. Lonvick, The Secure Shell (SSH) Connection Protocol, RFC
949     4254, January 2006.
950
951     J. Schlyter and W. Griffin, Using DNS to Securely Publish Secure Shell
952     (SSH) Key Fingerprints, RFC 4255, January 2006.
953
954     F. Cusack and M. Forssen, Generic Message Exchange Authentication for the
955     Secure Shell Protocol (SSH), RFC 4256, January 2006.
956
957     J. Galbraith and P. Remaker, The Secure Shell (SSH) Session Channel Break
958     Extension, RFC 4335, January 2006.
959
960     M. Bellare, T. Kohno, and C. Namprempre, The Secure Shell (SSH) Transport
961     Layer Encryption Modes, RFC 4344, January 2006.
962
963     B. Harris, Improved Arcfour Modes for the Secure Shell (SSH) Transport
964     Layer Protocol, RFC 4345, January 2006.
965
966     M. Friedl, N. Provos, and W. Simpson, Diffie-Hellman Group Exchange for
967     the Secure Shell (SSH) Transport Layer Protocol, RFC 4419, March 2006.
968
969     J. Galbraith and R. Thayer, The Secure Shell (SSH) Public Key File
970     Format, RFC 4716, November 2006.
971
972     D. Stebila and J. Green, Elliptic Curve Algorithm Integration in the
973     Secure Shell Transport Layer, RFC 5656, December 2009.
974
975     A. Perrig and D. Song, Hash Visualization: a New Technique to improve
976     Real-World Security, 1999, International Workshop on Cryptographic
977     Techniques and E-Commerce (CrypTEC '99).
978

AUTHORS

980     OpenSSH is a derivative of the original and free ssh 1.2.12 release by
981     Tatu Ylonen.  Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
982     de Raadt and Dug Song removed many bugs, re-added newer features and cre‐
983     ated OpenSSH.  Markus Friedl contributed the support for SSH protocol
984     versions 1.5 and 2.0.
985
986BSD                              June 20, 2019                             BSD
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