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.
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_dsa, ~/.ssh/id_ecdsa,
147             ~/.ssh/id_ecdsa_sk, ~/.ssh/id_ed25519, ~/.ssh/id_ed25519_sk and
148             ~/.ssh/id_rsa.  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             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                   CanonicalDomains
249                   CanonicalizeFallbackLocal
250                   CanonicalizeHostname
251                   CanonicalizeMaxDots
252                   CanonicalizePermittedCNAMEs
253                   CASignatureAlgorithms
254                   CertificateFile
255                   CheckHostIP
256                   Ciphers
257                   ClearAllForwardings
258                   Compression
259                   ConnectionAttempts
260                   ConnectTimeout
261                   ControlMaster
262                   ControlPath
263                   ControlPersist
264                   DynamicForward
265                   EscapeChar
266                   ExitOnForwardFailure
267                   FingerprintHash
268                   ForkAfterAuthentication
269                   ForwardAgent
270                   ForwardX11
271                   ForwardX11Timeout
272                   ForwardX11Trusted
273                   GatewayPorts
274                   GlobalKnownHostsFile
275                   GSSAPIAuthentication
276                   GSSAPIKeyExchange
277                   GSSAPIClientIdentity
278                   GSSAPIDelegateCredentials
279                   GSSAPIKexAlgorithms
280                   GSSAPIRenewalForcesRekey
281                   GSSAPIServerIdentity
282                   GSSAPITrustDns
283                   HashKnownHosts
284                   Host
285                   HostbasedAcceptedAlgorithms
286                   HostbasedAuthentication
287                   HostKeyAlgorithms
288                   HostKeyAlias
289                   Hostname
290                   IdentitiesOnly
291                   IdentityAgent
292                   IdentityFile
293                   IPQoS
294                   KbdInteractiveAuthentication
295                   KbdInteractiveDevices
296                   KexAlgorithms
297                   KnownHostsCommand
298                   LocalCommand
299                   LocalForward
300                   LogLevel
301                   MACs
302                   Match
303                   NoHostAuthenticationForLocalhost
304                   NumberOfPasswordPrompts
305                   PasswordAuthentication
306                   PermitLocalCommand
307                   PermitRemoteOpen
308                   PKCS11Provider
309                   Port
310                   PreferredAuthentications
311                   ProxyCommand
312                   ProxyJump
313                   ProxyUseFdpass
314                   PubkeyAcceptedAlgorithms
315                   PubkeyAuthentication
316                   RekeyLimit
317                   RemoteCommand
318                   RemoteForward
319                   RequestTTY
320                   SendEnv
321                   ServerAliveInterval
322                   ServerAliveCountMax
323                   SessionType
324                   SetEnv
325                   StdinNull
326                   StreamLocalBindMask
327                   StreamLocalBindUnlink
328                   StrictHostKeyChecking
329                   TCPKeepAlive
330                   Tunnel
331                   TunnelDevice
332                   UpdateHostKeys
333                   User
334                   UserKnownHostsFile
335                   VerifyHostKeyDNS
336                   VisualHostKey
337                   XAuthLocation
338
339     -p port
340             Port to connect to on the remote host.  This can be specified on
341             a per-host basis in the configuration file.
342
343     -Q query_option
344             Queries for the algorithms supported by one of the following fea‐
345             tures: cipher (supported symmetric ciphers), cipher-auth (sup‐
346             ported symmetric ciphers that support authenticated encryption),
347             help (supported query terms for use with the -Q flag), mac (sup‐
348             ported message integrity codes), kex (key exchange algorithms),
349             kex-gss (GSSAPI key exchange algorithms), key (key types),
350             key-cert (certificate key types), key-plain (non-certificate key
351             types), key-sig (all key types and signature algorithms),
352             protocol-version (supported SSH protocol versions), and sig (sup‐
353             ported signature algorithms).  Alternatively, any keyword from
354             ssh_config(5) or sshd_config(5) that takes an algorithm list may
355             be used as an alias for the corresponding query_option.
356
357     -q      Quiet mode.  Causes most warning and diagnostic messages to be
358             suppressed.
359
360     -R [bind_address:]port:host:hostport
361     -R [bind_address:]port:local_socket
362     -R remote_socket:host:hostport
363     -R remote_socket:local_socket
364     -R [bind_address:]port
365             Specifies that connections to the given TCP port or Unix socket
366             on the remote (server) host are to be forwarded to the local
367             side.
368
369             This works by allocating a socket to listen to either a TCP port
370             or to a Unix socket on the remote side.  Whenever a connection is
371             made to this port or Unix socket, the connection is forwarded
372             over the secure channel, and a connection is made from the local
373             machine to either an explicit destination specified by host port
374             hostport, or local_socket, or, if no explicit destination was
375             specified, ssh will act as a SOCKS 4/5 proxy and forward connec‐
376             tions to the destinations requested by the remote SOCKS client.
377
378             Port forwardings can also be specified in the configuration file.
379             Privileged ports can be forwarded only when logging in as root on
380             the remote machine.  IPv6 addresses can be specified by enclosing
381             the address in square brackets.
382
383             By default, TCP listening sockets on the server will be bound to
384             the loopback interface only.  This may be overridden by specify‐
385             ing a bind_address.  An empty bind_address, or the address ‘*’,
386             indicates that the remote socket should listen on all interfaces.
387             Specifying a remote bind_address will only succeed if the
388             server's GatewayPorts option is enabled (see sshd_config(5)).
389
390             If the port argument is ‘0’, the listen port will be dynamically
391             allocated on the server and reported to the client at run time.
392             When used together with -O forward the allocated port will be
393             printed to the standard output.
394
395     -S ctl_path
396             Specifies the location of a control socket for connection shar‐
397             ing, or the string “none” to disable connection sharing.  Refer
398             to the description of ControlPath and ControlMaster in
399             ssh_config(5) for details.
400
401     -s      May be used to request invocation of a subsystem on the remote
402             system.  Subsystems facilitate the use of SSH as a secure trans‐
403             port for other applications (e.g. sftp(1)).  The subsystem is
404             specified as the remote command.  Refer to the description of
405             SessionType in ssh_config(5) for details.
406
407     -T      Disable pseudo-terminal allocation.
408
409     -t      Force pseudo-terminal allocation.  This can be used to execute
410             arbitrary screen-based programs on a remote machine, which can be
411             very useful, e.g. when implementing menu services.  Multiple -t
412             options force tty allocation, even if ssh has no local tty.
413
414     -V      Display the version number and exit.
415
416     -v      Verbose mode.  Causes ssh to print debugging messages about its
417             progress.  This is helpful in debugging connection, authentica‐
418             tion, and configuration problems.  Multiple -v options increase
419             the verbosity.  The maximum is 3.
420
421     -W host:port
422             Requests that standard input and output on the client be for‐
423             warded to host on port over the secure channel.  Implies -N, -T,
424             ExitOnForwardFailure and ClearAllForwardings, though these can be
425             overridden in the configuration file or using -o command line op‐
426             tions.
427
428     -w local_tun[:remote_tun]
429             Requests tunnel device forwarding with the specified tun(4) de‐
430             vices between the client (local_tun) and the server (remote_tun).
431
432             The devices may be specified by numerical ID or the keyword
433             “any”, which uses the next available tunnel device.  If
434             remote_tun is not specified, it defaults to “any”.  See also the
435             Tunnel and TunnelDevice directives in ssh_config(5).
436
437             If the Tunnel directive is unset, it will be set to the default
438             tunnel mode, which is “point-to-point”.  If a different Tunnel
439             forwarding mode it desired, then it should be specified before
440             -w.
441
442     -X      Enables X11 forwarding.  This can also be specified on a per-host
443             basis in a configuration file.
444
445             X11 forwarding should be enabled with caution.  Users with the
446             ability to bypass file permissions on the remote host (for the
447             user's X authorization database) can access the local X11 display
448             through the forwarded connection.  An attacker may then be able
449             to perform activities such as keystroke monitoring.
450
451             For this reason, X11 forwarding is subjected to X11 SECURITY ex‐
452             tension restrictions by default.  Please refer to the ssh -Y op‐
453             tion and the ForwardX11Trusted directive in ssh_config(5) for
454             more information.
455
456     -x      Disables X11 forwarding.
457
458     -Y      Enables trusted X11 forwarding.  Trusted X11 forwardings are not
459             subjected to the X11 SECURITY extension controls.
460
461     -y      Send log information using the syslog(3) system module.  By de‐
462             fault this information is sent to stderr.
463
464     ssh may additionally obtain configuration data from a per-user configura‐
465     tion file and a system-wide configuration file.  The file format and con‐
466     figuration options are described in ssh_config(5).
467

AUTHENTICATION

469     The OpenSSH SSH client supports SSH protocol 2.
470
471     The methods available for authentication are: GSSAPI-based authentica‐
472     tion, host-based authentication, public key authentication, keyboard-in‐
473     teractive authentication, and password authentication.  Authentication
474     methods are tried in the order specified above, though
475     PreferredAuthentications can be used to change the default order.
476
477     Host-based authentication works as follows: If the machine the user logs
478     in from is listed in /etc/hosts.equiv or /etc/ssh/shosts.equiv on the re‐
479     mote machine, the user is non-root and the user names are the same on
480     both sides, or if the files ~/.rhosts or ~/.shosts exist in the user's
481     home directory on the remote machine and contain a line containing the
482     name of the client machine and the name of the user on that machine, the
483     user is considered for login.  Additionally, the server must be able to
484     verify the client's host key (see the description of
485     /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts, below) for login to be
486     permitted.  This authentication method closes security holes due to IP
487     spoofing, DNS spoofing, and routing spoofing.  [Note to the administra‐
488     tor: /etc/hosts.equiv, ~/.rhosts, and the rlogin/rsh protocol in general,
489     are inherently insecure and should be disabled if security is desired.]
490
491     Public key authentication works as follows: The scheme is based on pub‐
492     lic-key cryptography, using cryptosystems where encryption and decryption
493     are done using separate keys, and it is unfeasible to derive the decryp‐
494     tion key from the encryption key.  The idea is that each user creates a
495     public/private key pair for authentication purposes.  The server knows
496     the public key, and only the user knows the private key.  ssh implements
497     public key authentication protocol automatically, using one of the DSA,
498     ECDSA, Ed25519 or RSA algorithms.  The HISTORY section of ssl(8) contains
499     a brief discussion of the DSA and RSA algorithms.
500
501     The file ~/.ssh/authorized_keys lists the public keys that are permitted
502     for logging in.  When the user logs in, the ssh program tells the server
503     which key pair it would like to use for authentication.  The client
504     proves that it has access to the private key and the server checks that
505     the corresponding public key is authorized to accept the account.
506
507     The server may inform the client of errors that prevented public key au‐
508     thentication from succeeding after authentication completes using a dif‐
509     ferent method.  These may be viewed by increasing the LogLevel to DEBUG
510     or higher (e.g. by using the -v flag).
511
512     The user creates their key pair by running ssh-keygen(1).  This stores
513     the private key in ~/.ssh/id_dsa (DSA), ~/.ssh/id_ecdsa (ECDSA),
514     ~/.ssh/id_ecdsa_sk (authenticator-hosted ECDSA), ~/.ssh/id_ed25519
515     (Ed25519), ~/.ssh/id_ed25519_sk (authenticator-hosted Ed25519), or
516     ~/.ssh/id_rsa (RSA) and stores the public key in ~/.ssh/id_dsa.pub (DSA),
517     ~/.ssh/id_ecdsa.pub (ECDSA), ~/.ssh/id_ecdsa_sk.pub (authenticator-hosted
518     ECDSA), ~/.ssh/id_ed25519.pub (Ed25519), ~/.ssh/id_ed25519_sk.pub (au‐
519     thenticator-hosted Ed25519), or ~/.ssh/id_rsa.pub (RSA) in the user's
520     home directory.  The user should then copy the public key to
521     ~/.ssh/authorized_keys in their home directory on the remote machine.
522     The authorized_keys file corresponds to the conventional ~/.rhosts file,
523     and has one key per line, though the lines can be very long.  After this,
524     the user can log in without giving the password.
525
526     A variation on public key authentication is available in the form of cer‐
527     tificate authentication: instead of a set of public/private keys, signed
528     certificates are used.  This has the advantage that a single trusted cer‐
529     tification authority can be used in place of many public/private keys.
530     See the CERTIFICATES section of ssh-keygen(1) for more information.
531
532     The most convenient way to use public key or certificate authentication
533     may be with an authentication agent.  See ssh-agent(1) and (optionally)
534     the AddKeysToAgent directive in ssh_config(5) for more information.
535
536     Keyboard-interactive authentication works as follows: The server sends an
537     arbitrary "challenge" text and prompts for a response, possibly multiple
538     times.  Examples of keyboard-interactive authentication include BSD Au‐
539     thentication (see login.conf(5)) and PAM (some non-OpenBSD systems).
540
541     Finally, if other authentication methods fail, ssh prompts the user for a
542     password.  The password is sent to the remote host for checking; however,
543     since all communications are encrypted, the password cannot be seen by
544     someone listening on the network.
545
546     ssh automatically maintains and checks a database containing identifica‐
547     tion for all hosts it has ever been used with.  Host keys are stored in
548     ~/.ssh/known_hosts in the user's home directory.  Additionally, the file
549     /etc/ssh/ssh_known_hosts is automatically checked for known hosts.  Any
550     new hosts are automatically added to the user's file.  If a host's iden‐
551     tification ever changes, ssh warns about this and disables password au‐
552     thentication to prevent server spoofing or man-in-the-middle attacks,
553     which could otherwise be used to circumvent the encryption.  The
554     StrictHostKeyChecking option can be used to control logins to machines
555     whose host key is not known or has changed.
556
557     When the user's identity has been accepted by the server, the server ei‐
558     ther executes the given command in a non-interactive session or, if no
559     command has been specified, logs into the machine and gives the user a
560     normal shell as an interactive session.  All communication with the re‐
561     mote command or shell will be automatically encrypted.
562
563     If an interactive session is requested ssh by default will only request a
564     pseudo-terminal (pty) for interactive sessions when the client has one.
565     The flags -T and -t can be used to override this behaviour.
566
567     If a pseudo-terminal has been allocated the user may use the escape char‐
568     acters noted below.
569
570     If no pseudo-terminal has been allocated, the session is transparent and
571     can be used to reliably transfer binary data.  On most systems, setting
572     the escape character to “none” will also make the session transparent
573     even if a tty is used.
574
575     The session terminates when the command or shell on the remote machine
576     exits and all X11 and TCP connections have been closed.
577

ESCAPE CHARACTERS

579     When a pseudo-terminal has been requested, ssh supports a number of func‐
580     tions through the use of an escape character.
581
582     A single tilde character can be sent as ~~ or by following the tilde by a
583     character other than those described below.  The escape character must
584     always follow a newline to be interpreted as special.  The escape charac‐
585     ter can be changed in configuration files using the EscapeChar configura‐
586     tion directive or on the command line by the -e option.
587
588     The supported escapes (assuming the default ‘~’) are:
589
590     ~.      Disconnect.
591
592     ~^Z     Background ssh.
593
594     ~#      List forwarded connections.
595
596     ~&      Background ssh at logout when waiting for forwarded connection /
597             X11 sessions to terminate.
598
599     ~?      Display a list of escape characters.
600
601     ~B      Send a BREAK to the remote system (only useful if the peer sup‐
602             ports it).
603
604     ~C      Open command line.  Currently this allows the addition of port
605             forwardings using the -L, -R and -D options (see above).  It also
606             allows the cancellation of existing port-forwardings with
607             -KL[bind_address:]port for local, -KR[bind_address:]port for re‐
608             mote and -KD[bind_address:]port for dynamic port-forwardings.
609             !command allows the user to execute a local command if the
610             PermitLocalCommand option is enabled in ssh_config(5).  Basic
611             help is available, using the -h option.
612
613     ~R      Request rekeying of the connection (only useful if the peer sup‐
614             ports it).
615
616     ~V      Decrease the verbosity (LogLevel) when errors are being written
617             to stderr.
618
619     ~v      Increase the verbosity (LogLevel) when errors are being written
620             to stderr.
621

TCP FORWARDING

623     Forwarding of arbitrary TCP connections over a secure channel can be
624     specified either on the command line or in a configuration file.  One
625     possible application of TCP forwarding is a secure connection to a mail
626     server; another is going through firewalls.
627
628     In the example below, we look at encrypting communication for an IRC
629     client, even though the IRC server it connects to does not directly sup‐
630     port encrypted communication.  This works as follows: the user connects
631     to the remote host using ssh, specifying the ports to be used to forward
632     the connection.  After that it is possible to start the program locally,
633     and ssh will encrypt and forward the connection to the remote server.
634
635     The following example tunnels an IRC session from the client to an IRC
636     server at “server.example.com”, joining channel “#users”, nickname
637     “pinky”, using the standard IRC port, 6667:
638
639         $ ssh -f -L 6667:localhost:6667 server.example.com sleep 10
640         $ irc -c '#users' pinky IRC/127.0.0.1
641
642     The -f option backgrounds ssh and the remote command “sleep 10” is speci‐
643     fied to allow an amount of time (10 seconds, in the example) to start the
644     program which is going to use the tunnel.  If no connections are made
645     within the time specified, ssh will exit.
646

X11 FORWARDING

648     If the ForwardX11 variable is set to “yes” (or see the description of the
649     -X, -x, and -Y options above) and the user is using X11 (the DISPLAY en‐
650     vironment variable is set), the connection to the X11 display is automat‐
651     ically forwarded to the remote side in such a way that any X11 programs
652     started from the shell (or command) will go through the encrypted chan‐
653     nel, and the connection to the real X server will be made from the local
654     machine.  The user should not manually set DISPLAY.  Forwarding of X11
655     connections can be configured on the command line or in configuration
656     files.
657
658     The DISPLAY value set by ssh will point to the server machine, but with a
659     display number greater than zero.  This is normal, and happens because
660     ssh creates a “proxy” X server on the server machine for forwarding the
661     connections over the encrypted channel.
662
663     ssh will also automatically set up Xauthority data on the server machine.
664     For this purpose, it will generate a random authorization cookie, store
665     it in Xauthority on the server, and verify that any forwarded connections
666     carry this cookie and replace it by the real cookie when the connection
667     is opened.  The real authentication cookie is never sent to the server
668     machine (and no cookies are sent in the plain).
669
670     If the ForwardAgent variable is set to “yes” (or see the description of
671     the -A and -a options above) and the user is using an authentication
672     agent, the connection to the agent is automatically forwarded to the re‐
673     mote side.
674

VERIFYING HOST KEYS

676     When connecting to a server for the first time, a fingerprint of the
677     server's public key is presented to the user (unless the option
678     StrictHostKeyChecking has been disabled).  Fingerprints can be determined
679     using ssh-keygen(1):
680
681           $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
682
683     If the fingerprint is already known, it can be matched and the key can be
684     accepted or rejected.  If only legacy (MD5) fingerprints for the server
685     are available, the ssh-keygen(1) -E option may be used to downgrade the
686     fingerprint algorithm to match.
687
688     Because of the difficulty of comparing host keys just by looking at fin‐
689     gerprint strings, there is also support to compare host keys visually,
690     using random art.  By setting the VisualHostKey option to “yes”, a small
691     ASCII graphic gets displayed on every login to a server, no matter if the
692     session itself is interactive or not.  By learning the pattern a known
693     server produces, a user can easily find out that the host key has changed
694     when a completely different pattern is displayed.  Because these patterns
695     are not unambiguous however, a pattern that looks similar to the pattern
696     remembered only gives a good probability that the host key is the same,
697     not guaranteed proof.
698
699     To get a listing of the fingerprints along with their random art for all
700     known hosts, the following command line can be used:
701
702           $ ssh-keygen -lv -f ~/.ssh/known_hosts
703
704     If the fingerprint is unknown, an alternative method of verification is
705     available: SSH fingerprints verified by DNS.  An additional resource
706     record (RR), SSHFP, is added to a zonefile and the connecting client is
707     able to match the fingerprint with that of the key presented.
708
709     In this example, we are connecting a client to a server,
710     “host.example.com”.  The SSHFP resource records should first be added to
711     the zonefile for host.example.com:
712
713           $ ssh-keygen -r host.example.com.
714
715     The output lines will have to be added to the zonefile.  To check that
716     the zone is answering fingerprint queries:
717
718           $ dig -t SSHFP host.example.com
719
720     Finally the client connects:
721
722           $ ssh -o "VerifyHostKeyDNS ask" host.example.com
723           [...]
724           Matching host key fingerprint found in DNS.
725           Are you sure you want to continue connecting (yes/no)?
726
727     See the VerifyHostKeyDNS option in ssh_config(5) for more information.
728

SSH-BASED VIRTUAL PRIVATE NETWORKS

730     ssh contains support for Virtual Private Network (VPN) tunnelling using
731     the tun(4) network pseudo-device, allowing two networks to be joined se‐
732     curely.  The sshd_config(5) configuration option PermitTunnel controls
733     whether the server supports this, and at what level (layer 2 or 3 traf‐
734     fic).
735
736     The following example would connect client network 10.0.50.0/24 with re‐
737     mote network 10.0.99.0/24 using a point-to-point connection from 10.1.1.1
738     to 10.1.1.2, provided that the SSH server running on the gateway to the
739     remote network, at 192.168.1.15, allows it.
740
741     On the client:
742
743           # ssh -f -w 0:1 192.168.1.15 true
744           # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
745           # route add 10.0.99.0/24 10.1.1.2
746
747     On the server:
748
749           # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
750           # route add 10.0.50.0/24 10.1.1.1
751
752     Client access may be more finely tuned via the /root/.ssh/authorized_keys
753     file (see below) and the PermitRootLogin server option.  The following
754     entry would permit connections on tun(4) device 1 from user “jane” and on
755     tun device 2 from user “john”, if PermitRootLogin is set to
756     “forced-commands-only”:
757
758       tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
759       tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
760
761     Since an SSH-based setup entails a fair amount of overhead, it may be
762     more suited to temporary setups, such as for wireless VPNs.  More perma‐
763     nent VPNs are better provided by tools such as ipsecctl(8) and
764     isakmpd(8).
765

ENVIRONMENT

767     ssh will normally set the following environment variables:
768
769     DISPLAY               The DISPLAY variable indicates the location of the
770                           X11 server.  It is automatically set by ssh to
771                           point to a value of the form “hostname:n”, where
772                           “hostname” indicates the host where the shell runs,
773                           and ‘n’ is an integer ≥ 1.  ssh uses this special
774                           value to forward X11 connections over the secure
775                           channel.  The user should normally not set DISPLAY
776                           explicitly, as that will render the X11 connection
777                           insecure (and will require the user to manually
778                           copy any required authorization cookies).
779
780     HOME                  Set to the path of the user's home directory.
781
782     LOGNAME               Synonym for USER; set for compatibility with sys‐
783                           tems that use this variable.
784
785     MAIL                  Set to the path of the user's mailbox.
786
787     PATH                  Set to the default PATH, as specified when compil‐
788                           ing ssh.
789
790     SSH_ASKPASS           If ssh needs a passphrase, it will read the
791                           passphrase from the current terminal if it was run
792                           from a terminal.  If ssh does not have a terminal
793                           associated with it but DISPLAY and SSH_ASKPASS are
794                           set, it will execute the program specified by
795                           SSH_ASKPASS and open an X11 window to read the
796                           passphrase.  This is particularly useful when call‐
797                           ing ssh from a .xsession or related script.  (Note
798                           that on some machines it may be necessary to redi‐
799                           rect the input from /dev/null to make this work.)
800
801     SSH_ASKPASS_REQUIRE   Allows further control over the use of an askpass
802                           program.  If this variable is set to “never” then
803                           ssh will never attempt to use one.  If it is set to
804                           “prefer”, then ssh will prefer to use the askpass
805                           program instead of the TTY when requesting pass‐
806                           words.  Finally, if the variable is set to “force”,
807                           then the askpass program will be used for all
808                           passphrase input regardless of whether DISPLAY is
809                           set.
810
811     SSH_AUTH_SOCK         Identifies the path of a UNIX-domain socket used to
812                           communicate with the agent.
813
814     SSH_CONNECTION        Identifies the client and server ends of the con‐
815                           nection.  The variable contains four space-sepa‐
816                           rated values: client IP address, client port num‐
817                           ber, server IP address, and server port number.
818
819     SSH_ORIGINAL_COMMAND  This variable contains the original command line if
820                           a forced command is executed.  It can be used to
821                           extract the original arguments.
822
823     SSH_TTY               This is set to the name of the tty (path to the de‐
824                           vice) associated with the current shell or command.
825                           If the current session has no tty, this variable is
826                           not set.
827
828     SSH_TUNNEL            Optionally set by sshd(8) to contain the interface
829                           names assigned if tunnel forwarding was requested
830                           by the client.
831
832     SSH_USER_AUTH         Optionally set by sshd(8), this variable may con‐
833                           tain a pathname to a file that lists the authenti‐
834                           cation methods successfully used when the session
835                           was established, including any public keys that
836                           were used.
837
838     TZ                    This variable is set to indicate the present time
839                           zone if it was set when the daemon was started
840                           (i.e. the daemon passes the value on to new connec‐
841                           tions).
842
843     USER                  Set to the name of the user logging in.
844
845     Additionally, ssh reads ~/.ssh/environment, and adds lines of the format
846     “VARNAME=value” to the environment if the file exists and users are al‐
847     lowed to change their environment.  For more information, see the
848     PermitUserEnvironment option in sshd_config(5).
849

FILES

851     ~/.rhosts
852             This file is used for host-based authentication (see above).  On
853             some machines this file may need to be world-readable if the
854             user's home directory is on an NFS partition, because sshd(8)
855             reads it as root.  Additionally, this file must be owned by the
856             user, and must not have write permissions for anyone else.  The
857             recommended permission for most machines is read/write for the
858             user, and not accessible by others.
859
860     ~/.shosts
861             This file is used in exactly the same way as .rhosts, but allows
862             host-based authentication without permitting login with
863             rlogin/rsh.
864
865     ~/.ssh/
866             This directory is the default location for all user-specific con‐
867             figuration and authentication information.  There is no general
868             requirement to keep the entire contents of this directory secret,
869             but the recommended permissions are read/write/execute for the
870             user, and not accessible by others.
871
872     ~/.ssh/authorized_keys
873             Lists the public keys (DSA, ECDSA, Ed25519, RSA) that can be used
874             for logging in as this user.  The format of this file is de‐
875             scribed in the sshd(8) manual page.  This file is not highly sen‐
876             sitive, but the recommended permissions are read/write for the
877             user, and not accessible by others.
878
879     ~/.ssh/config
880             This is the per-user configuration file.  The file format and
881             configuration options are described in ssh_config(5).  Because of
882             the potential for abuse, this file must have strict permissions:
883             read/write for the user, and not writable by others.
884
885     ~/.ssh/environment
886             Contains additional definitions for environment variables; see
887             ENVIRONMENT, above.
888
889     ~/.ssh/id_dsa
890     ~/.ssh/id_ecdsa
891     ~/.ssh/id_ecdsa_sk
892     ~/.ssh/id_ed25519
893     ~/.ssh/id_ed25519_sk
894     ~/.ssh/id_rsa
895             Contains the private key for authentication.  These files contain
896             sensitive data and should be readable by the user but not acces‐
897             sible by others (read/write/execute).  ssh will simply ignore a
898             private key file if it is accessible by others.  It is possible
899             to specify a passphrase when generating the key which will be
900             used to encrypt the sensitive part of this file using AES-128.
901
902     ~/.ssh/id_dsa.pub
903     ~/.ssh/id_ecdsa.pub
904     ~/.ssh/id_ecdsa_sk.pub
905     ~/.ssh/id_ed25519.pub
906     ~/.ssh/id_ed25519_sk.pub
907     ~/.ssh/id_rsa.pub
908             Contains the public key for authentication.  These files are not
909             sensitive and can (but need not) be readable by anyone.
910
911     ~/.ssh/known_hosts
912             Contains a list of host keys for all hosts the user has logged
913             into that are not already in the systemwide list of known host
914             keys.  See sshd(8) for further details of the format of this
915             file.
916
917     ~/.ssh/rc
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
922     /etc/hosts.equiv
923             This file is for host-based authentication (see above).  It
924             should only be writable by root.
925
926     /etc/ssh/shosts.equiv
927             This file is used in exactly the same way as hosts.equiv, but al‐
928             lows host-based authentication without permitting login with
929             rlogin/rsh.
930
931     /etc/ssh/ssh_config
932             Systemwide configuration file.  The file format and configuration
933             options are described in ssh_config(5).
934
935     /etc/ssh/ssh_host_key
936     /etc/ssh/ssh_host_dsa_key
937     /etc/ssh/ssh_host_ecdsa_key
938     /etc/ssh/ssh_host_ed25519_key
939     /etc/ssh/ssh_host_rsa_key
940             These files contain the private parts of the host keys and are
941             used for host-based authentication.
942
943     /etc/ssh/ssh_known_hosts
944             Systemwide list of known host keys.  This file should be prepared
945             by the system administrator to contain the public host keys of
946             all machines in the organization.  It should be world-readable.
947             See sshd(8) for further details of the format of this file.
948
949     /etc/ssh/sshrc
950             Commands in this file are executed by ssh when the user logs in,
951             just before the user's shell (or command) is started.  See the
952             sshd(8) manual page for more information.
953

EXIT STATUS

955     ssh exits with the exit status of the remote command or with 255 if an
956     error occurred.
957

IPV6

959     IPv6 address can be used everywhere where IPv4 address. In all entries
960     must be the IPv6 address enclosed in square brackets. Note: The square
961     brackets are metacharacters for the shell and must be escaped in shell.
962

SEE ALSO

964     scp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), ssh-keyscan(1),
965     tun(4), ssh_config(5), ssh-keysign(8), sshd(8)
966

STANDARDS

968     S. Lehtinen and C. Lonvick, The Secure Shell (SSH) Protocol Assigned
969     Numbers, RFC 4250, January 2006.
970
971     T. Ylonen and C. Lonvick, The Secure Shell (SSH) Protocol Architecture,
972     RFC 4251, January 2006.
973
974     T. Ylonen and C. Lonvick, The Secure Shell (SSH) Authentication Protocol,
975     RFC 4252, January 2006.
976
977     T. Ylonen and C. Lonvick, The Secure Shell (SSH) Transport Layer
978     Protocol, RFC 4253, January 2006.
979
980     T. Ylonen and C. Lonvick, The Secure Shell (SSH) Connection Protocol, RFC
981     4254, January 2006.
982
983     J. Schlyter and W. Griffin, Using DNS to Securely Publish Secure Shell
984     (SSH) Key Fingerprints, RFC 4255, January 2006.
985
986     F. Cusack and M. Forssen, Generic Message Exchange Authentication for the
987     Secure Shell Protocol (SSH), RFC 4256, January 2006.
988
989     J. Galbraith and P. Remaker, The Secure Shell (SSH) Session Channel Break
990     Extension, RFC 4335, January 2006.
991
992     M. Bellare, T. Kohno, and C. Namprempre, The Secure Shell (SSH) Transport
993     Layer Encryption Modes, RFC 4344, January 2006.
994
995     B. Harris, Improved Arcfour Modes for the Secure Shell (SSH) Transport
996     Layer Protocol, RFC 4345, January 2006.
997
998     M. Friedl, N. Provos, and W. Simpson, Diffie-Hellman Group Exchange for
999     the Secure Shell (SSH) Transport Layer Protocol, RFC 4419, March 2006.
1000
1001     J. Galbraith and R. Thayer, The Secure Shell (SSH) Public Key File
1002     Format, RFC 4716, November 2006.
1003
1004     D. Stebila and J. Green, Elliptic Curve Algorithm Integration in the
1005     Secure Shell Transport Layer, RFC 5656, December 2009.
1006
1007     A. Perrig and D. Song, Hash Visualization: a New Technique to improve
1008     Real-World Security, 1999, International Workshop on Cryptographic
1009     Techniques and E-Commerce (CrypTEC '99).
1010

AUTHORS

1012     OpenSSH is a derivative of the original and free ssh 1.2.12 release by
1013     Tatu Ylonen.  Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
1014     de Raadt and Dug Song removed many bugs, re-added newer features and cre‐
1015     ated OpenSSH.  Markus Friedl contributed the support for SSH protocol
1016     versions 1.5 and 2.0.
1017
1018BSD                           September 10, 2021                           BSD
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