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

NAME

4     ssh — OpenSSH remote login client
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 [argument ...]]
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 their identity to the
25     remote machine using one of several methods (see below).
26
27     If a command is specified, it will be executed on the remote host instead
28     of a login shell.  A complete command line may be specified as command,
29     or it may have additional arguments.  If supplied, the arguments will be
30     appended to the command, separated by spaces, before it is sent to the
31     server to be executed.
32
33     The options are as follows:
34
35     -4      Forces ssh to use IPv4 addresses only.
36
37     -6      Forces ssh to use IPv6 addresses only.
38
39     -A      Enables forwarding of connections from an authentication agent
40             such as ssh-agent(1).  This can also be specified on a per-host
41             basis in a configuration file.
42
43             Agent forwarding should be enabled with caution.  Users with the
44             ability to bypass file permissions on the remote host (for the
45             agent's UNIX-domain socket) can access the local agent through
46             the forwarded connection.  An attacker cannot obtain key material
47             from the agent, however they can perform operations on the keys
48             that enable them to authenticate using the identities loaded into
49             the agent.  A safer alternative may be to use a jump host (see
50             -J).
51
52     -a      Disables forwarding of the authentication agent connection.
53
54     -B bind_interface
55             Bind to the address of bind_interface before attempting to con‐
56             nect to the destination host.  This is only useful on systems
57             with more than one address.
58
59     -b bind_address
60             Use bind_address on the local machine as the source address of
61             the connection.  Only useful on systems with more than one ad‐
62             dress.
63
64     -C      Requests compression of all data (including stdin, stdout,
65             stderr, and data for forwarded X11, TCP and UNIX-domain connec‐
66             tions).  The compression algorithm is the same used by gzip(1).
67             Compression is desirable on modem lines and other slow connec‐
68             tions, but will only slow down things on fast networks.  The de‐
69             fault value can be set on a host-by-host basis in the configura‐
70             tion files; see the Compression option in ssh_config(5).
71
72     -c cipher_spec
73             Selects the cipher specification for encrypting the session.
74             cipher_spec is a comma-separated list of ciphers listed in order
75             of preference.  See the Ciphers keyword in ssh_config(5) for more
76             information.
77
78     -D [bind_address:]port
79             Specifies a local “dynamic” application-level port forwarding.
80             This works by allocating a socket to listen to port on the local
81             side, optionally bound to the specified bind_address.  Whenever a
82             connection is made to this port, the connection is forwarded over
83             the secure channel, and the application protocol is then used to
84             determine where to connect to from the remote machine.  Currently
85             the SOCKS4 and SOCKS5 protocols are supported, and ssh will act
86             as a SOCKS server.  Only root can forward privileged ports.  Dy‐
87             namic port forwardings can also be specified in the configuration
88             file.
89
90             IPv6 addresses can be specified by enclosing the address in
91             square brackets.  Only the superuser can forward privileged
92             ports.  By default, the local port is bound in accordance with
93             the GatewayPorts setting.  However, an explicit bind_address may
94             be used to bind the connection to a specific address.  The
95             bind_address of “localhost” indicates that the listening port be
96             bound for local use only, while an empty address or ‘*’ indicates
97             that the port should be available from all interfaces.
98
99     -E log_file
100             Append debug logs to log_file instead of standard error.
101
102     -e escape_char
103             Sets the escape character for sessions with a pty (default: ‘~’).
104             The escape character is only recognized at the beginning of a
105             line.  The escape character followed by a dot (‘.’) closes the
106             connection; followed by control-Z suspends the connection; and
107             followed by itself sends the escape character once.  Setting the
108             character to “none” disables any escapes and makes the session
109             fully transparent.
110
111     -F configfile
112             Specifies an alternative per-user configuration file.  If a con‐
113             figuration file is given on the command line, the system-wide
114             configuration file (/etc/ssh/ssh_config) will be ignored.  The
115             default for the per-user configuration file is ~/.ssh/config.  If
116             set to “none”, no configuration files will be read.
117
118     -f      Requests ssh to go to background just before command execution.
119             This is useful if ssh is going to ask for passwords or
120             passphrases, but the user wants it in the background.  This im‐
121             plies -n.  The recommended way to start X11 programs at a remote
122             site is with something like ssh -f host xterm.
123
124             If the ExitOnForwardFailure configuration option is set to “yes”,
125             then a client started with -f will wait for all remote port for‐
126             wards to be successfully established before placing itself in the
127             background.  Refer to the description of ForkAfterAuthentication
128             in ssh_config(5) for details.
129
130     -G      Causes ssh to print its configuration after evaluating Host and
131             Match blocks and exit.
132
133     -g      Allows remote hosts to connect to local forwarded ports.  If used
134             on a multiplexed connection, then this option must be specified
135             on the master process.
136
137     -I pkcs11
138             Specify the PKCS#11 shared library ssh should use to communicate
139             with a PKCS#11 token providing keys for user authentication.
140
141     -i identity_file
142             Selects a file from which the identity (private key) for public
143             key authentication is read.  You can also specify a public key
144             file to use the corresponding private key that is loaded in
145             ssh-agent(1) when the private key file is not present locally.
146             The default is ~/.ssh/id_rsa, ~/.ssh/id_ecdsa,
147             ~/.ssh/id_ecdsa_sk, ~/.ssh/id_ed25519, ~/.ssh/id_ed25519_sk and
148             ~/.ssh/id_dsa.  Identity files may also be specified on a per-
149             host basis in the configuration file.  It is possible to have
150             multiple -i options (and multiple identities specified in config‐
151             uration files).  If no certificates have been explicitly speci‐
152             fied by the CertificateFile directive, ssh will also try to load
153             certificate information from the filename obtained by appending
154             -cert.pub to identity filenames.
155
156     -J destination
157             Connect to the target host by first making a ssh connection to
158             the jump host described by destination and then establishing a
159             TCP forwarding to the ultimate destination from there.  Multiple
160             jump hops may be specified separated by comma characters.  This
161             is a shortcut to specify a ProxyJump configuration directive.
162             Note that configuration directives supplied on the command-line
163             generally apply to the destination host and not any specified
164             jump hosts.  Use ~/.ssh/config to specify configuration for jump
165             hosts.
166
167     -K      Enables GSSAPI-based authentication and forwarding (delegation)
168             of GSSAPI credentials to the server.
169
170     -k      Disables forwarding (delegation) of GSSAPI credentials to the
171             server.
172
173     -L [bind_address:]port:host:hostport
174     -L [bind_address:]port:remote_socket
175     -L local_socket:host:hostport
176     -L local_socket:remote_socket
177             Specifies that connections to the given TCP port or Unix socket
178             on the local (client) host are to be forwarded to the given host
179             and port, or Unix socket, on the remote side.  This works by al‐
180             locating a socket to listen to either a TCP port on the local
181             side, optionally bound to the specified bind_address, or to a
182             Unix socket.  Whenever a connection is made to the local port or
183             socket, the connection is forwarded over the secure channel, and
184             a connection is made to either host port hostport, or the Unix
185             socket remote_socket, from the remote machine.
186
187             Port forwardings can also be specified in the configuration file.
188             Only the superuser can forward privileged ports.  IPv6 addresses
189             can be specified by enclosing the address in square brackets.
190
191             By default, the local port is bound in accordance with the
192             GatewayPorts setting.  However, an explicit bind_address may be
193             used to bind the connection to a specific address.  The
194             bind_address of “localhost” indicates that the listening port be
195             bound for local use only, while an empty address or ‘*’ indicates
196             that the port should be available from all interfaces.
197
198     -l login_name
199             Specifies the user to log in as on the remote machine.  This also
200             may be specified on a per-host basis in the configuration file.
201
202     -M      Places the ssh client into “master” mode for connection sharing.
203             Multiple -M options places ssh into “master” mode but with con‐
204             firmation required using ssh-askpass(1) before each operation
205             that changes the multiplexing state (e.g. opening a new session).
206             Refer to the description of ControlMaster in ssh_config(5) for
207             details.
208
209     -m mac_spec
210             A comma-separated list of MAC (message authentication code) algo‐
211             rithms, specified in order of preference.  See the MACs keyword
212             in ssh_config(5) for more information.
213
214     -N      Do not execute a remote command.  This is useful for just for‐
215             warding ports.  Refer to the description of SessionType in
216             ssh_config(5) for details.
217
218     -n      Redirects stdin from /dev/null (actually, prevents reading from
219             stdin).  This must be used when ssh is run in the background.  A
220             common trick is to use this to run X11 programs on a remote ma‐
221             chine.  For example, ssh -n shadows.cs.hut.fi emacs & will start
222             an emacs on shadows.cs.hut.fi, and the X11 connection will be au‐
223             tomatically forwarded over an encrypted channel.  The ssh program
224             will be put in the background.  (This does not work if ssh needs
225             to ask for a password or passphrase; see also the -f option.)
226             Refer to the description of StdinNull in ssh_config(5) for de‐
227             tails.
228
229     -O ctl_cmd
230             Control an active connection multiplexing master process.  When
231             the -O option is specified, the ctl_cmd argument is interpreted
232             and passed to the master process.  Valid commands are: “check”
233             (check that the master process is running), “forward” (request
234             forwardings without command execution), “cancel” (cancel forward‐
235             ings), “exit” (request the master to exit), and “stop” (request
236             the master to stop accepting further multiplexing requests).
237
238     -o option
239             Can be used to give options in the format used in the configura‐
240             tion file.  This is useful for specifying options for which there
241             is no separate command-line flag.  For full details of the op‐
242             tions listed below, and their possible values, see ssh_config(5).
243
244                   AddKeysToAgent
245                   AddressFamily
246                   BatchMode
247                   BindAddress
248                   BindInterface
249                   CanonicalDomains
250                   CanonicalizeFallbackLocal
251                   CanonicalizeHostname
252                   CanonicalizeMaxDots
253                   CanonicalizePermittedCNAMEs
254                   CASignatureAlgorithms
255                   CertificateFile
256                   CheckHostIP
257                   Ciphers
258                   ClearAllForwardings
259                   Compression
260                   ConnectionAttempts
261                   ConnectTimeout
262                   ControlMaster
263                   ControlPath
264                   ControlPersist
265                   DynamicForward
266                   EnableSSHKeysign
267                   EnableEscapeCommandline
268                   EscapeChar
269                   ExitOnForwardFailure
270                   FingerprintHash
271                   ForkAfterAuthentication
272                   ForwardAgent
273                   ForwardX11
274                   ForwardX11Timeout
275                   ForwardX11Trusted
276                   GatewayPorts
277                   GlobalKnownHostsFile
278                   GSSAPIAuthentication
279                   GSSAPIKeyExchange
280                   GSSAPIClientIdentity
281                   GSSAPIDelegateCredentials
282                   GSSAPIKexAlgorithms
283                   GSSAPIRenewalForcesRekey
284                   GSSAPIServerIdentity
285                   GSSAPITrustDns
286                   HashKnownHosts
287                   Host
288                   HostbasedAcceptedAlgorithms
289                   HostbasedAuthentication
290                   HostKeyAlgorithms
291                   HostKeyAlias
292                   Hostname
293                   IdentitiesOnly
294                   IdentityAgent
295                   IdentityFile
296                   IgnoreUnknown
297                   Include
298                   IPQoS
299                   KbdInteractiveAuthentication
300                   KbdInteractiveDevices
301                   KexAlgorithms
302                   KnownHostsCommand
303                   LocalCommand
304                   LocalForward
305                   LogLevel
306                   LogVerbose
307                   MACs
308                   Match
309                   NoHostAuthenticationForLocalhost
310                   NumberOfPasswordPrompts
311                   PasswordAuthentication
312                   PermitLocalCommand
313                   PermitRemoteOpen
314                   PKCS11Provider
315                   Port
316                   PreferredAuthentications
317                   ProxyCommand
318                   ProxyJump
319                   ProxyUseFdpass
320                   PubkeyAcceptedAlgorithms
321                   PubkeyAuthentication
322                   RekeyLimit
323                   RemoteCommand
324                   RemoteForward
325                   RequestTTY
326                   RevokedHostKeys
327                   SecurityKeyProvider
328                   RequiredRSASize
329                   SendEnv
330                   ServerAliveInterval
331                   ServerAliveCountMax
332                   SessionType
333                   SetEnv
334                   StdinNull
335                   StreamLocalBindMask
336                   StreamLocalBindUnlink
337                   StrictHostKeyChecking
338                   SyslogFacility
339                   TCPKeepAlive
340                   Tunnel
341                   TunnelDevice
342                   UpdateHostKeys
343                   User
344                   UserKnownHostsFile
345                   VerifyHostKeyDNS
346                   VisualHostKey
347                   XAuthLocation
348
349     -p port
350             Port to connect to on the remote host.  This can be specified on
351             a per-host basis in the configuration file.
352
353     -Q query_option
354             Queries for the algorithms supported by one of the following fea‐
355             tures: cipher (supported symmetric ciphers), cipher-auth (sup‐
356             ported symmetric ciphers that support authenticated encryption),
357             help (supported query terms for use with the -Q flag), mac (sup‐
358             ported message integrity codes), kex (key exchange algorithms),
359             kex-gss (GSSAPI key exchange algorithms), key (key types),
360             key-cert (certificate key types), key-plain (non-certificate key
361             types), key-sig (all key types and signature algorithms),
362             protocol-version (supported SSH protocol versions), and sig (sup‐
363             ported signature algorithms).  Alternatively, any keyword from
364             ssh_config(5) or sshd_config(5) that takes an algorithm list may
365             be used as an alias for the corresponding query_option.
366
367     -q      Quiet mode.  Causes most warning and diagnostic messages to be
368             suppressed.
369
370     -R [bind_address:]port:host:hostport
371     -R [bind_address:]port:local_socket
372     -R remote_socket:host:hostport
373     -R remote_socket:local_socket
374     -R [bind_address:]port
375             Specifies that connections to the given TCP port or Unix socket
376             on the remote (server) host are to be forwarded to the local
377             side.
378
379             This works by allocating a socket to listen to either a TCP port
380             or to a Unix socket on the remote side.  Whenever a connection is
381             made to this port or Unix socket, the connection is forwarded
382             over the secure channel, and a connection is made from the local
383             machine to either an explicit destination specified by host port
384             hostport, or local_socket, or, if no explicit destination was
385             specified, ssh will act as a SOCKS 4/5 proxy and forward connec‐
386             tions to the destinations requested by the remote SOCKS client.
387
388             Port forwardings can also be specified in the configuration file.
389             Privileged ports can be forwarded only when logging in as root on
390             the remote machine.  IPv6 addresses can be specified by enclosing
391             the address in square brackets.
392
393             By default, TCP listening sockets on the server will be bound to
394             the loopback interface only.  This may be overridden by specify‐
395             ing a bind_address.  An empty bind_address, or the address ‘*’,
396             indicates that the remote socket should listen on all interfaces.
397             Specifying a remote bind_address will only succeed if the
398             server's GatewayPorts option is enabled (see sshd_config(5)).
399
400             If the port argument is ‘0’, the listen port will be dynamically
401             allocated on the server and reported to the client at run time.
402             When used together with -O forward, the allocated port will be
403             printed to the standard output.
404
405     -S ctl_path
406             Specifies the location of a control socket for connection shar‐
407             ing, or the string “none” to disable connection sharing.  Refer
408             to the description of ControlPath and ControlMaster in
409             ssh_config(5) for details.
410
411     -s      May be used to request invocation of a subsystem on the remote
412             system.  Subsystems facilitate the use of SSH as a secure trans‐
413             port for other applications (e.g. sftp(1)).  The subsystem is
414             specified as the remote command.  Refer to the description of
415             SessionType in ssh_config(5) for details.
416
417     -T      Disable pseudo-terminal allocation.
418
419     -t      Force pseudo-terminal allocation.  This can be used to execute
420             arbitrary screen-based programs on a remote machine, which can be
421             very useful, e.g. when implementing menu services.  Multiple -t
422             options force tty allocation, even if ssh has no local tty.
423
424     -V      Display the version number and exit.
425
426     -v      Verbose mode.  Causes ssh to print debugging messages about its
427             progress.  This is helpful in debugging connection, authentica‐
428             tion, and configuration problems.  Multiple -v options increase
429             the verbosity.  The maximum is 3.
430
431     -W host:port
432             Requests that standard input and output on the client be for‐
433             warded to host on port over the secure channel.  Implies -N, -T,
434             ExitOnForwardFailure and ClearAllForwardings, though these can be
435             overridden in the configuration file or using -o command line op‐
436             tions.
437
438     -w local_tun[:remote_tun]
439             Requests tunnel device forwarding with the specified tun(4) de‐
440             vices between the client (local_tun) and the server (remote_tun).
441
442             The devices may be specified by numerical ID or the keyword
443             “any”, which uses the next available tunnel device.  If
444             remote_tun is not specified, it defaults to “any”.  See also the
445             Tunnel and TunnelDevice directives in ssh_config(5).
446
447             If the Tunnel directive is unset, it will be set to the default
448             tunnel mode, which is “point-to-point”.  If a different Tunnel
449             forwarding mode it desired, then it should be specified before
450             -w.
451
452     -X      Enables X11 forwarding.  This can also be specified on a per-host
453             basis in a configuration file.
454
455             X11 forwarding should be enabled with caution.  Users with the
456             ability to bypass file permissions on the remote host (for the
457             user's X authorization database) can access the local X11 display
458             through the forwarded connection.  An attacker may then be able
459             to perform activities such as keystroke monitoring.
460
461             For this reason, X11 forwarding is subjected to X11 SECURITY ex‐
462             tension restrictions by default.  Refer to the ssh -Y option and
463             the ForwardX11Trusted directive in ssh_config(5) for more infor‐
464             mation.
465
466     -x      Disables X11 forwarding.
467
468     -Y      Enables trusted X11 forwarding.  Trusted X11 forwardings are not
469             subjected to the X11 SECURITY extension controls.
470
471     -y      Send log information using the syslog(3) system module.  By de‐
472             fault this information is sent to stderr.
473
474     ssh may additionally obtain configuration data from a per-user configura‐
475     tion file and a system-wide configuration file.  The file format and con‐
476     figuration options are described in ssh_config(5).
477

AUTHENTICATION

479     The OpenSSH SSH client supports SSH protocol 2.
480
481     The methods available for authentication are: GSSAPI-based authentica‐
482     tion, host-based authentication, public key authentication, keyboard-in‐
483     teractive authentication, and password authentication.  Authentication
484     methods are tried in the order specified above, though
485     PreferredAuthentications can be used to change the default order.
486
487     Host-based authentication works as follows: If the machine the user logs
488     in from is listed in /etc/hosts.equiv or /etc/ssh/shosts.equiv on the re‐
489     mote machine, the user is non-root and the user names are the same on
490     both sides, or if the files ~/.rhosts or ~/.shosts exist in the user's
491     home directory on the remote machine and contain a line containing the
492     name of the client machine and the name of the user on that machine, the
493     user is considered for login.  Additionally, the server must be able to
494     verify the client's host key (see the description of
495     /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts, below) for login to be
496     permitted.  This authentication method closes security holes due to IP
497     spoofing, DNS spoofing, and routing spoofing.  [Note to the administra‐
498     tor: /etc/hosts.equiv, ~/.rhosts, and the rlogin/rsh protocol in general,
499     are inherently insecure and should be disabled if security is desired.]
500
501     Public key authentication works as follows: The scheme is based on pub‐
502     lic-key cryptography, using cryptosystems where encryption and decryption
503     are done using separate keys, and it is unfeasible to derive the decryp‐
504     tion key from the encryption key.  The idea is that each user creates a
505     public/private key pair for authentication purposes.  The server knows
506     the public key, and only the user knows the private key.  ssh implements
507     public key authentication protocol automatically, using one of the DSA,
508     ECDSA, Ed25519 or RSA algorithms.  The HISTORY section of ssl(8) contains
509     a brief discussion of the DSA and RSA algorithms.
510
511     The file ~/.ssh/authorized_keys lists the public keys that are permitted
512     for logging in.  When the user logs in, the ssh program tells the server
513     which key pair it would like to use for authentication.  The client
514     proves that it has access to the private key and the server checks that
515     the corresponding public key is authorized to accept the account.
516
517     The server may inform the client of errors that prevented public key au‐
518     thentication from succeeding after authentication completes using a dif‐
519     ferent method.  These may be viewed by increasing the LogLevel to DEBUG
520     or higher (e.g. by using the -v flag).
521
522     The user creates their key pair by running ssh-keygen(1).  This stores
523     the private key in ~/.ssh/id_dsa (DSA), ~/.ssh/id_ecdsa (ECDSA),
524     ~/.ssh/id_ecdsa_sk (authenticator-hosted ECDSA), ~/.ssh/id_ed25519
525     (Ed25519), ~/.ssh/id_ed25519_sk (authenticator-hosted Ed25519), or
526     ~/.ssh/id_rsa (RSA) and stores the public key in ~/.ssh/id_dsa.pub (DSA),
527     ~/.ssh/id_ecdsa.pub (ECDSA), ~/.ssh/id_ecdsa_sk.pub (authenticator-hosted
528     ECDSA), ~/.ssh/id_ed25519.pub (Ed25519), ~/.ssh/id_ed25519_sk.pub (au‐
529     thenticator-hosted Ed25519), or ~/.ssh/id_rsa.pub (RSA) in the user's
530     home directory.  The user should then copy the public key to
531     ~/.ssh/authorized_keys in their home directory on the remote machine.
532     The authorized_keys file corresponds to the conventional ~/.rhosts file,
533     and has one key per line, though the lines can be very long.  After this,
534     the user can log in without giving the password.
535
536     A variation on public key authentication is available in the form of cer‐
537     tificate authentication: instead of a set of public/private keys, signed
538     certificates are used.  This has the advantage that a single trusted cer‐
539     tification authority can be used in place of many public/private keys.
540     See the CERTIFICATES section of ssh-keygen(1) for more information.
541
542     The most convenient way to use public key or certificate authentication
543     may be with an authentication agent.  See ssh-agent(1) and (optionally)
544     the AddKeysToAgent directive in ssh_config(5) for more information.
545
546     Keyboard-interactive authentication works as follows: The server sends an
547     arbitrary "challenge" text and prompts for a response, possibly multiple
548     times.  Examples of keyboard-interactive authentication include BSD Au‐
549     thentication (see login.conf(5)) and PAM (some non-OpenBSD systems).
550
551     Finally, if other authentication methods fail, ssh prompts the user for a
552     password.  The password is sent to the remote host for checking; however,
553     since all communications are encrypted, the password cannot be seen by
554     someone listening on the network.
555
556     ssh automatically maintains and checks a database containing identifica‐
557     tion for all hosts it has ever been used with.  Host keys are stored in
558     ~/.ssh/known_hosts in the user's home directory.  Additionally, the file
559     /etc/ssh/ssh_known_hosts is automatically checked for known hosts.  Any
560     new hosts are automatically added to the user's file.  If a host's iden‐
561     tification ever changes, ssh warns about this and disables password au‐
562     thentication to prevent server spoofing or man-in-the-middle attacks,
563     which could otherwise be used to circumvent the encryption.  The
564     StrictHostKeyChecking option can be used to control logins to machines
565     whose host key is not known or has changed.
566
567     When the user's identity has been accepted by the server, the server ei‐
568     ther executes the given command in a non-interactive session or, if no
569     command has been specified, logs into the machine and gives the user a
570     normal shell as an interactive session.  All communication with the re‐
571     mote command or shell will be automatically encrypted.
572
573     If an interactive session is requested, ssh by default will only request
574     a pseudo-terminal (pty) for interactive sessions when the client has one.
575     The flags -T and -t can be used to override this behaviour.
576
577     If a pseudo-terminal has been allocated, the user may use the escape
578     characters noted below.
579
580     If no pseudo-terminal has been allocated, the session is transparent and
581     can be used to reliably transfer binary data.  On most systems, setting
582     the escape character to “none” will also make the session transparent
583     even if a tty is used.
584
585     The session terminates when the command or shell on the remote machine
586     exits and all X11 and TCP connections have been closed.
587

ESCAPE CHARACTERS

589     When a pseudo-terminal has been requested, ssh supports a number of func‐
590     tions through the use of an escape character.
591
592     A single tilde character can be sent as ~~ or by following the tilde by a
593     character other than those described below.  The escape character must
594     always follow a newline to be interpreted as special.  The escape charac‐
595     ter can be changed in configuration files using the EscapeChar configura‐
596     tion directive or on the command line by the -e option.
597
598     The supported escapes (assuming the default ‘~’) are:
599
600     ~.      Disconnect.
601
602     ~^Z     Background ssh.
603
604     ~#      List forwarded connections.
605
606     ~&      Background ssh at logout when waiting for forwarded connection /
607             X11 sessions to terminate.
608
609     ~?      Display a list of escape characters.
610
611     ~B      Send a BREAK to the remote system (only useful if the peer sup‐
612             ports it).
613
614     ~C      Open command line.  Currently this allows the addition of port
615             forwardings using the -L, -R and -D options (see above).  It also
616             allows the cancellation of existing port-forwardings with
617             -KL[bind_address:]port for local, -KR[bind_address:]port for re‐
618             mote and -KD[bind_address:]port for dynamic port-forwardings.
619             !command allows the user to execute a local command if the
620             PermitLocalCommand option is enabled in ssh_config(5).  Basic
621             help is available, using the -h option.
622
623     ~R      Request rekeying of the connection (only useful if the peer sup‐
624             ports it).
625
626     ~V      Decrease the verbosity (LogLevel) when errors are being written
627             to stderr.
628
629     ~v      Increase the verbosity (LogLevel) when errors are being written
630             to stderr.
631

TCP FORWARDING

633     Forwarding of arbitrary TCP connections over a secure channel can be
634     specified either on the command line or in a configuration file.  One
635     possible application of TCP forwarding is a secure connection to a mail
636     server; another is going through firewalls.
637
638     In the example below, we look at encrypting communication for an IRC
639     client, even though the IRC server it connects to does not directly sup‐
640     port encrypted communication.  This works as follows: the user connects
641     to the remote host using ssh, specifying the ports to be used to forward
642     the connection.  After that it is possible to start the program locally,
643     and ssh will encrypt and forward the connection to the remote server.
644
645     The following example tunnels an IRC session from the client to an IRC
646     server at “server.example.com”, joining channel “#users”, nickname
647     “pinky”, using the standard IRC port, 6667:
648
649         $ ssh -f -L 6667:localhost:6667 server.example.com sleep 10
650         $ irc -c '#users' pinky IRC/127.0.0.1
651
652     The -f option backgrounds ssh and the remote command “sleep 10” is speci‐
653     fied to allow an amount of time (10 seconds, in the example) to start the
654     program which is going to use the tunnel.  If no connections are made
655     within the time specified, ssh will exit.
656

X11 FORWARDING

658     If the ForwardX11 variable is set to “yes” (or see the description of the
659     -X, -x, and -Y options above) and the user is using X11 (the DISPLAY en‐
660     vironment variable is set), the connection to the X11 display is automat‐
661     ically forwarded to the remote side in such a way that any X11 programs
662     started from the shell (or command) will go through the encrypted chan‐
663     nel, and the connection to the real X server will be made from the local
664     machine.  The user should not manually set DISPLAY.  Forwarding of X11
665     connections can be configured on the command line or in configuration
666     files.
667
668     The DISPLAY value set by ssh will point to the server machine, but with a
669     display number greater than zero.  This is normal, and happens because
670     ssh creates a “proxy” X server on the server machine for forwarding the
671     connections over the encrypted channel.
672
673     ssh will also automatically set up Xauthority data on the server machine.
674     For this purpose, it will generate a random authorization cookie, store
675     it in Xauthority on the server, and verify that any forwarded connections
676     carry this cookie and replace it by the real cookie when the connection
677     is opened.  The real authentication cookie is never sent to the server
678     machine (and no cookies are sent in the plain).
679
680     If the ForwardAgent variable is set to “yes” (or see the description of
681     the -A and -a options above) and the user is using an authentication
682     agent, the connection to the agent is automatically forwarded to the re‐
683     mote side.
684

VERIFYING HOST KEYS

686     When connecting to a server for the first time, a fingerprint of the
687     server's public key is presented to the user (unless the option
688     StrictHostKeyChecking has been disabled).  Fingerprints can be determined
689     using ssh-keygen(1):
690
691           $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
692
693     If the fingerprint is already known, it can be matched and the key can be
694     accepted or rejected.  If only legacy (MD5) fingerprints for the server
695     are available, the ssh-keygen(1) -E option may be used to downgrade the
696     fingerprint algorithm to match.
697
698     Because of the difficulty of comparing host keys just by looking at fin‐
699     gerprint strings, there is also support to compare host keys visually,
700     using random art.  By setting the VisualHostKey option to “yes”, a small
701     ASCII graphic gets displayed on every login to a server, no matter if the
702     session itself is interactive or not.  By learning the pattern a known
703     server produces, a user can easily find out that the host key has changed
704     when a completely different pattern is displayed.  Because these patterns
705     are not unambiguous however, a pattern that looks similar to the pattern
706     remembered only gives a good probability that the host key is the same,
707     not guaranteed proof.
708
709     To get a listing of the fingerprints along with their random art for all
710     known hosts, the following command line can be used:
711
712           $ ssh-keygen -lv -f ~/.ssh/known_hosts
713
714     If the fingerprint is unknown, an alternative method of verification is
715     available: SSH fingerprints verified by DNS.  An additional resource
716     record (RR), SSHFP, is added to a zonefile and the connecting client is
717     able to match the fingerprint with that of the key presented.
718
719     In this example, we are connecting a client to a server,
720     “host.example.com”.  The SSHFP resource records should first be added to
721     the zonefile for host.example.com:
722
723           $ ssh-keygen -r host.example.com.
724
725     The output lines will have to be added to the zonefile.  To check that
726     the zone is answering fingerprint queries:
727
728           $ dig -t SSHFP host.example.com
729
730     Finally the client connects:
731
732           $ ssh -o "VerifyHostKeyDNS ask" host.example.com
733           [...]
734           Matching host key fingerprint found in DNS.
735           Are you sure you want to continue connecting (yes/no)?
736
737     See the VerifyHostKeyDNS option in ssh_config(5) for more information.
738

SSH-BASED VIRTUAL PRIVATE NETWORKS

740     ssh contains support for Virtual Private Network (VPN) tunnelling using
741     the tun(4) network pseudo-device, allowing two networks to be joined se‐
742     curely.  The sshd_config(5) configuration option PermitTunnel controls
743     whether the server supports this, and at what level (layer 2 or 3 traf‐
744     fic).
745
746     The following example would connect client network 10.0.50.0/24 with re‐
747     mote network 10.0.99.0/24 using a point-to-point connection from 10.1.1.1
748     to 10.1.1.2, provided that the SSH server running on the gateway to the
749     remote network, at 192.168.1.15, allows it.
750
751     On the client:
752
753           # ssh -f -w 0:1 192.168.1.15 true
754           # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
755           # route add 10.0.99.0/24 10.1.1.2
756
757     On the server:
758
759           # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
760           # route add 10.0.50.0/24 10.1.1.1
761
762     Client access may be more finely tuned via the /root/.ssh/authorized_keys
763     file (see below) and the PermitRootLogin server option.  The following
764     entry would permit connections on tun(4) device 1 from user “jane” and on
765     tun device 2 from user “john”, if PermitRootLogin is set to
766     “forced-commands-only”:
767
768       tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
769       tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
770
771     Since an SSH-based setup entails a fair amount of overhead, it may be
772     more suited to temporary setups, such as for wireless VPNs.  More perma‐
773     nent VPNs are better provided by tools such as ipsecctl(8) and
774     isakmpd(8).
775

ENVIRONMENT

777     ssh will normally set the following environment variables:
778
779     DISPLAY               The DISPLAY variable indicates the location of the
780                           X11 server.  It is automatically set by ssh to
781                           point to a value of the form “hostname:n”, where
782                           “hostname” indicates the host where the shell runs,
783                           and ‘n’ is an integer ≥ 1.  ssh uses this special
784                           value to forward X11 connections over the secure
785                           channel.  The user should normally not set DISPLAY
786                           explicitly, as that will render the X11 connection
787                           insecure (and will require the user to manually
788                           copy any required authorization cookies).
789
790     HOME                  Set to the path of the user's home directory.
791
792     LOGNAME               Synonym for USER; set for compatibility with sys‐
793                           tems that use this variable.
794
795     MAIL                  Set to the path of the user's mailbox.
796
797     PATH                  Set to the default PATH, as specified when compil‐
798                           ing ssh.
799
800     SSH_ASKPASS           If ssh needs a passphrase, it will read the
801                           passphrase from the current terminal if it was run
802                           from a terminal.  If ssh does not have a terminal
803                           associated with it but DISPLAY and SSH_ASKPASS are
804                           set, it will execute the program specified by
805                           SSH_ASKPASS and open an X11 window to read the
806                           passphrase.  This is particularly useful when call‐
807                           ing ssh from a .xsession or related script.  (Note
808                           that on some machines it may be necessary to redi‐
809                           rect the input from /dev/null to make this work.)
810
811     SSH_ASKPASS_REQUIRE   Allows further control over the use of an askpass
812                           program.  If this variable is set to “never” then
813                           ssh will never attempt to use one.  If it is set to
814                           “prefer”, then ssh will prefer to use the askpass
815                           program instead of the TTY when requesting pass‐
816                           words.  Finally, if the variable is set to “force”,
817                           then the askpass program will be used for all
818                           passphrase input regardless of whether DISPLAY is
819                           set.
820
821     SSH_AUTH_SOCK         Identifies the path of a UNIX-domain socket used to
822                           communicate with the agent.
823
824     SSH_CONNECTION        Identifies the client and server ends of the con‐
825                           nection.  The variable contains four space-sepa‐
826                           rated values: client IP address, client port num‐
827                           ber, server IP address, and server port number.
828
829     SSH_ORIGINAL_COMMAND  This variable contains the original command line if
830                           a forced command is executed.  It can be used to
831                           extract the original arguments.
832
833     SSH_TTY               This is set to the name of the tty (path to the de‐
834                           vice) associated with the current shell or command.
835                           If the current session has no tty, this variable is
836                           not set.
837
838     SSH_TUNNEL            Optionally set by sshd(8) to contain the interface
839                           names assigned if tunnel forwarding was requested
840                           by the client.
841
842     SSH_USER_AUTH         Optionally set by sshd(8), this variable may con‐
843                           tain a pathname to a file that lists the authenti‐
844                           cation methods successfully used when the session
845                           was established, including any public keys that
846                           were used.
847
848     TZ                    This variable is set to indicate the present time
849                           zone if it was set when the daemon was started
850                           (i.e. the daemon passes the value on to new connec‐
851                           tions).
852
853     USER                  Set to the name of the user logging in.
854
855     Additionally, ssh reads ~/.ssh/environment, and adds lines of the format
856     “VARNAME=value” to the environment if the file exists and users are al‐
857     lowed to change their environment.  For more information, see the
858     PermitUserEnvironment option in sshd_config(5).
859

FILES

861     ~/.rhosts
862             This file is used for host-based authentication (see above).  On
863             some machines this file may need to be world-readable if the
864             user's home directory is on an NFS partition, because sshd(8)
865             reads it as root.  Additionally, this file must be owned by the
866             user, and must not have write permissions for anyone else.  The
867             recommended permission for most machines is read/write for the
868             user, and not accessible by others.
869
870     ~/.shosts
871             This file is used in exactly the same way as .rhosts, but allows
872             host-based authentication without permitting login with
873             rlogin/rsh.
874
875     ~/.ssh/
876             This directory is the default location for all user-specific con‐
877             figuration and authentication information.  There is no general
878             requirement to keep the entire contents of this directory secret,
879             but the recommended permissions are read/write/execute for the
880             user, and not accessible by others.
881
882     ~/.ssh/authorized_keys
883             Lists the public keys (DSA, ECDSA, Ed25519, RSA) that can be used
884             for logging in as this user.  The format of this file is de‐
885             scribed in the sshd(8) manual page.  This file is not highly sen‐
886             sitive, but the recommended permissions are read/write for the
887             user, and not accessible by others.
888
889     ~/.ssh/config
890             This is the per-user configuration file.  The file format and
891             configuration options are described in ssh_config(5).  Because of
892             the potential for abuse, this file must have strict permissions:
893             read/write for the user, and not writable by others.
894
895     ~/.ssh/environment
896             Contains additional definitions for environment variables; see
897             ENVIRONMENT, above.
898
899     ~/.ssh/id_dsa
900     ~/.ssh/id_ecdsa
901     ~/.ssh/id_ecdsa_sk
902     ~/.ssh/id_ed25519
903     ~/.ssh/id_ed25519_sk
904     ~/.ssh/id_rsa
905             Contains the private key for authentication.  These files contain
906             sensitive data and should be readable by the user but not acces‐
907             sible by others (read/write/execute).  ssh will simply ignore a
908             private key file if it is accessible by others.  It is possible
909             to specify a passphrase when generating the key which will be
910             used to encrypt the sensitive part of this file using AES-128.
911
912     ~/.ssh/id_dsa.pub
913     ~/.ssh/id_ecdsa.pub
914     ~/.ssh/id_ecdsa_sk.pub
915     ~/.ssh/id_ed25519.pub
916     ~/.ssh/id_ed25519_sk.pub
917     ~/.ssh/id_rsa.pub
918             Contains the public key for authentication.  These files are not
919             sensitive and can (but need not) be readable by anyone.
920
921     ~/.ssh/known_hosts
922             Contains a list of host keys for all hosts the user has logged
923             into that are not already in the systemwide list of known host
924             keys.  See sshd(8) for further details of the format of this
925             file.
926
927     ~/.ssh/rc
928             Commands in this file are executed by ssh when the user logs in,
929             just before the user's shell (or command) is started.  See the
930             sshd(8) manual page for more information.
931
932     /etc/hosts.equiv
933             This file is for host-based authentication (see above).  It
934             should only be writable by root.
935
936     /etc/ssh/shosts.equiv
937             This file is used in exactly the same way as hosts.equiv, but al‐
938             lows host-based authentication without permitting login with
939             rlogin/rsh.
940
941     /etc/ssh/ssh_config
942             Systemwide configuration file.  The file format and configuration
943             options are described in ssh_config(5).
944
945     /etc/ssh/ssh_host_key
946     /etc/ssh/ssh_host_dsa_key
947     /etc/ssh/ssh_host_ecdsa_key
948     /etc/ssh/ssh_host_ed25519_key
949     /etc/ssh/ssh_host_rsa_key
950             These files contain the private parts of the host keys and are
951             used for host-based authentication.
952
953     /etc/ssh/ssh_known_hosts
954             Systemwide list of known host keys.  This file should be prepared
955             by the system administrator to contain the public host keys of
956             all machines in the organization.  It should be world-readable.
957             See sshd(8) for further details of the format of this file.
958
959     /etc/ssh/sshrc
960             Commands in this file are executed by ssh when the user logs in,
961             just before the user's shell (or command) is started.  See the
962             sshd(8) manual page for more information.
963

EXIT STATUS

965     ssh exits with the exit status of the remote command or with 255 if an
966     error occurred.
967

IPV6

969     IPv6 address can be used everywhere where IPv4 address. In all entries
970     must be the IPv6 address enclosed in square brackets. Note: The square
971     brackets are metacharacters for the shell and must be escaped in shell.
972

SEE ALSO

974     scp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), ssh-keyscan(1),
975     tun(4), ssh_config(5), ssh-keysign(8), sshd(8)
976

STANDARDS

978     S. Lehtinen and C. Lonvick, The Secure Shell (SSH) Protocol Assigned
979     Numbers, RFC 4250, January 2006.
980
981     T. Ylonen and C. Lonvick, The Secure Shell (SSH) Protocol Architecture,
982     RFC 4251, January 2006.
983
984     T. Ylonen and C. Lonvick, The Secure Shell (SSH) Authentication Protocol,
985     RFC 4252, January 2006.
986
987     T. Ylonen and C. Lonvick, The Secure Shell (SSH) Transport Layer
988     Protocol, RFC 4253, January 2006.
989
990     T. Ylonen and C. Lonvick, The Secure Shell (SSH) Connection Protocol, RFC
991     4254, January 2006.
992
993     J. Schlyter and W. Griffin, Using DNS to Securely Publish Secure Shell
994     (SSH) Key Fingerprints, RFC 4255, January 2006.
995
996     F. Cusack and M. Forssen, Generic Message Exchange Authentication for the
997     Secure Shell Protocol (SSH), RFC 4256, January 2006.
998
999     J. Galbraith and P. Remaker, The Secure Shell (SSH) Session Channel Break
1000     Extension, RFC 4335, January 2006.
1001
1002     M. Bellare, T. Kohno, and C. Namprempre, The Secure Shell (SSH) Transport
1003     Layer Encryption Modes, RFC 4344, January 2006.
1004
1005     B. Harris, Improved Arcfour Modes for the Secure Shell (SSH) Transport
1006     Layer Protocol, RFC 4345, January 2006.
1007
1008     M. Friedl, N. Provos, and W. Simpson, Diffie-Hellman Group Exchange for
1009     the Secure Shell (SSH) Transport Layer Protocol, RFC 4419, March 2006.
1010
1011     J. Galbraith and R. Thayer, The Secure Shell (SSH) Public Key File
1012     Format, RFC 4716, November 2006.
1013
1014     D. Stebila and J. Green, Elliptic Curve Algorithm Integration in the
1015     Secure Shell Transport Layer, RFC 5656, December 2009.
1016
1017     A. Perrig and D. Song, Hash Visualization: a New Technique to improve
1018     Real-World Security, 1999, International Workshop on Cryptographic
1019     Techniques and E-Commerce (CrypTEC '99).
1020

AUTHORS

1022     OpenSSH is a derivative of the original and free ssh 1.2.12 release by
1023     Tatu Ylonen.  Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
1024     de Raadt and Dug Song removed many bugs, re-added newer features and cre‐
1025     ated OpenSSH.  Markus Friedl contributed the support for SSH protocol
1026     versions 1.5 and 2.0.
1027
1028BSD                            November 28, 2022                           BSD
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