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

NAME

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

SYNOPSIS

7     ssh [-46AaCfGgKkMNnqsTtVvXxYy] [-B bind_interface] [-b bind_address]
8         [-c cipher_spec] [-D [bind_address:]port] [-E log_file]
9         [-e escape_char] [-F configfile] [-I pkcs11] [-i identity_file]
10         [-J destination] [-L address] [-l login_name] [-m mac_spec]
11         [-O ctl_cmd] [-o option] [-p port] [-Q query_option] [-R address]
12         [-S ctl_path] [-W host:port] [-w local_tun[:remote_tun]] destination
13         [command]
14

DESCRIPTION

16     ssh (SSH client) is a program for logging into a remote machine and for
17     executing commands on a remote machine.  It is intended to provide secure
18     encrypted communications between two untrusted hosts over an insecure
19     network.  X11 connections, arbitrary TCP ports and UNIX-domain sockets
20     can also be forwarded over the secure channel.
21
22     ssh connects and logs into the specified destination, which may be speci‐
23     fied as either [user@]hostname or a URI of the form
24     ssh://[user@]hostname[:port].  The user must prove his/her identity to
25     the remote machine using one of several methods (see below).
26
27     If a command is specified, it is executed on the remote host instead of a
28     login shell.
29
30     The options are as follows:
31
32     -4      Forces ssh to use IPv4 addresses only.
33
34     -6      Forces ssh to use IPv6 addresses only.
35
36     -A      Enables forwarding of the authentication agent connection.  This
37             can also be specified on a per-host basis in a configuration
38             file.
39
40             Agent forwarding should be enabled with caution.  Users with the
41             ability to bypass file permissions on the remote host (for the
42             agent's UNIX-domain socket) can access the local agent through
43             the forwarded connection.  An attacker cannot obtain key material
44             from the agent, however they can perform operations on the keys
45             that enable them to authenticate using the identities loaded into
46             the agent.
47
48     -a      Disables forwarding of the authentication agent connection.
49
50     -B bind_interface
51             Bind to the address of bind_interface before attempting to con‐
52             nect to the destination host.  This is only useful on systems
53             with more than one address.
54
55     -b bind_address
56             Use bind_address on the local machine as the source address of
57             the connection.  Only useful on systems with more than one
58             address.
59
60     -C      Requests compression of all data (including stdin, stdout,
61             stderr, and data for forwarded X11, TCP and UNIX-domain connec‐
62             tions).  The compression algorithm is the same used by gzip(1).
63             Compression is desirable on modem lines and other slow connec‐
64             tions, but will only slow down things on fast networks.  The
65             default value can be set on a host-by-host basis in the configu‐
66             ration files; see the Compression option.
67
68     -c cipher_spec
69             Selects the cipher specification for encrypting the session.
70             cipher_spec is a comma-separated list of ciphers listed in order
71             of preference.  See the Ciphers keyword in ssh_config(5) for more
72             information.
73
74     -D [bind_address:]port
75             Specifies a local “dynamic” application-level port forwarding.
76             This works by allocating a socket to listen to port on the local
77             side, optionally bound to the specified bind_address.  Whenever a
78             connection is made to this port, the connection is forwarded over
79             the secure channel, and the application protocol is then used to
80             determine where to connect to from the remote machine.  Currently
81             the SOCKS4 and SOCKS5 protocols are supported, and ssh will act
82             as a SOCKS server.  Only root can forward privileged ports.
83             Dynamic port forwardings can also be specified in the configura‐
84             tion file.
85
86             IPv6 addresses can be specified by enclosing the address in
87             square brackets.  Only the superuser can forward privileged
88             ports.  By default, the local port is bound in accordance with
89             the GatewayPorts setting.  However, an explicit bind_address may
90             be used to bind the connection to a specific address.  The
91             bind_address of “localhost” indicates that the listening port be
92             bound for local use only, while an empty address or ‘*’ indicates
93             that the port should be available from all interfaces.
94
95     -E log_file
96             Append debug logs to log_file instead of standard error.
97
98     -e escape_char
99             Sets the escape character for sessions with a pty (default: ‘~’).
100             The escape character is only recognized at the beginning of a
101             line.  The escape character followed by a dot (‘.’) closes the
102             connection; followed by control-Z suspends the connection; and
103             followed by itself sends the escape character once.  Setting the
104             character to “none” disables any escapes and makes the session
105             fully transparent.
106
107     -F configfile
108             Specifies an alternative per-user configuration file.  If a con‐
109             figuration file is given on the command line, the system-wide
110             configuration file (/etc/ssh/ssh_config) will be ignored.  The
111             default for the per-user configuration file is ~/.ssh/config.
112
113     -f      Requests ssh to go to background just before command execution.
114             This is useful if ssh is going to ask for passwords or
115             passphrases, but the user wants it in the background.  This
116             implies -n.  The recommended way to start X11 programs at a
117             remote site is with something like ssh -f host xterm.
118
119             If the ExitOnForwardFailure configuration option is set to “yes”,
120             then a client started with -f will wait for all remote port for‐
121             wards to be successfully established before placing itself in the
122             background.
123
124     -G      Causes ssh to print its configuration after evaluating Host and
125             Match blocks and exit.
126
127     -g      Allows remote hosts to connect to local forwarded ports.  If used
128             on a multiplexed connection, then this option must be specified
129             on the master process.
130
131     -I pkcs11
132             Specify the PKCS#11 shared library ssh should use to communicate
133             with a PKCS#11 token providing keys for user authentication.
134
135     -i identity_file
136             Selects a file from which the identity (private key) for public
137             key authentication is read.  The default is ~/.ssh/id_dsa,
138             ~/.ssh/id_ecdsa, ~/.ssh/id_ed25519 and ~/.ssh/id_rsa.  Identity
139             files may also be specified on a per-host basis in the configura‐
140             tion file.  It is possible to have multiple -i options (and mul‐
141             tiple identities specified in configuration files).  If no cer‐
142             tificates have been explicitly specified by the CertificateFile
143             directive, ssh will also try to load certificate information from
144             the filename obtained by appending -cert.pub to identity file‐
145             names.
146
147     -J destination
148             Connect to the target host by first making a ssh connection to
149             the jump host described by destination and then establishing a
150             TCP forwarding to the ultimate destination from there.  Multiple
151             jump hops may be specified separated by comma characters.  This
152             is a shortcut to specify a ProxyJump configuration directive.
153             Note that configuration directives supplied on the command-line
154             generally apply to the destination host and not any specified
155             jump hosts.  Use ~/.ssh/config to specify configuration for jump
156             hosts.
157
158     -K      Enables GSSAPI-based authentication and forwarding (delegation)
159             of GSSAPI credentials to the server.
160
161     -k      Disables forwarding (delegation) of GSSAPI credentials to the
162             server.
163
164     -L [bind_address:]port:host:hostport
165     -L [bind_address:]port:remote_socket
166     -L local_socket:host:hostport
167     -L local_socket:remote_socket
168             Specifies that connections to the given TCP port or Unix socket
169             on the local (client) host are to be forwarded to the given host
170             and port, or Unix socket, on the remote side.  This works by
171             allocating a socket to listen to either a TCP port on the local
172             side, optionally bound to the specified bind_address, or to a
173             Unix socket.  Whenever a connection is made to the local port or
174             socket, the connection is forwarded over the secure channel, and
175             a connection is made to either host port hostport, or the Unix
176             socket remote_socket, from the remote machine.
177
178             Port forwardings can also be specified in the configuration file.
179             Only the superuser can forward privileged ports.  IPv6 addresses
180             can be specified by enclosing the address in square brackets.
181
182             By default, the local port is bound in accordance with the
183             GatewayPorts setting.  However, an explicit bind_address may be
184             used to bind the connection to a specific address.  The
185             bind_address of “localhost” indicates that the listening port be
186             bound for local use only, while an empty address or ‘*’ indicates
187             that the port should be available from all interfaces.
188
189     -l login_name
190             Specifies the user to log in as on the remote machine.  This also
191             may be specified on a per-host basis in the configuration file.
192
193     -M      Places the ssh client into “master” mode for connection sharing.
194             Multiple -M options places ssh into “master” mode but with con‐
195             firmation required using ssh-askpass(1) before each operation
196             that changes the multiplexing state (e.g. opening a new session).
197             Refer to the description of ControlMaster in ssh_config(5) for
198             details.
199
200     -m mac_spec
201             A comma-separated list of MAC (message authentication code) algo‐
202             rithms, specified in order of preference.  See the MACs keyword
203             for more information.
204
205     -N      Do not execute a remote command.  This is useful for just for‐
206             warding ports.
207
208     -n      Redirects stdin from /dev/null (actually, prevents reading from
209             stdin).  This must be used when ssh is run in the background.  A
210             common trick is to use this to run X11 programs on a remote
211             machine.  For example, ssh -n shadows.cs.hut.fi emacs & will
212             start an emacs on shadows.cs.hut.fi, and the X11 connection will
213             be automatically forwarded over an encrypted channel.  The ssh
214             program will be put in the background.  (This does not work if
215             ssh needs to ask for a password or passphrase; see also the -f
216             option.)
217
218     -O ctl_cmd
219             Control an active connection multiplexing master process.  When
220             the -O option is specified, the ctl_cmd argument is interpreted
221             and passed to the master process.  Valid commands are: “check”
222             (check that the master process is running), “forward” (request
223             forwardings without command execution), “cancel” (cancel forward‐
224             ings), “exit” (request the master to exit), and “stop” (request
225             the master to stop accepting further multiplexing requests).
226
227     -o option
228             Can be used to give options in the format used in the configura‐
229             tion file.  This is useful for specifying options for which there
230             is no separate command-line flag.  For full details of the
231             options listed below, and their possible values, see
232             ssh_config(5).
233
234                   AddKeysToAgent
235                   AddressFamily
236                   BatchMode
237                   BindAddress
238                   CanonicalDomains
239                   CanonicalizeFallbackLocal
240                   CanonicalizeHostname
241                   CanonicalizeMaxDots
242                   CanonicalizePermittedCNAMEs
243                   CASignatureAlgorithms
244                   CertificateFile
245                   ChallengeResponseAuthentication
246                   CheckHostIP
247                   Ciphers
248                   ClearAllForwardings
249                   Compression
250                   ConnectionAttempts
251                   ConnectTimeout
252                   ControlMaster
253                   ControlPath
254                   ControlPersist
255                   DynamicForward
256                   EscapeChar
257                   ExitOnForwardFailure
258                   FingerprintHash
259                   ForwardAgent
260                   ForwardX11
261                   ForwardX11Timeout
262                   ForwardX11Trusted
263                   GatewayPorts
264                   GlobalKnownHostsFile
265                   GSSAPIAuthentication
266                   GSSAPIKeyExchange
267                   GSSAPIClientIdentity
268                   GSSAPIDelegateCredentials
269                   GSSAPIKexAlgorithms
270                   GSSAPIRenewalForcesRekey
271                   GSSAPIServerIdentity
272                   GSSAPITrustDns
273                   HashKnownHosts
274                   Host
275                   HostbasedAuthentication
276                   HostbasedKeyTypes
277                   HostKeyAlgorithms
278                   HostKeyAlias
279                   Hostname
280                   IdentitiesOnly
281                   IdentityAgent
282                   IdentityFile
283                   IPQoS
284                   KbdInteractiveAuthentication
285                   KbdInteractiveDevices
286                   KexAlgorithms
287                   LocalCommand
288                   LocalForward
289                   LogLevel
290                   MACs
291                   Match
292                   NoHostAuthenticationForLocalhost
293                   NumberOfPasswordPrompts
294                   PasswordAuthentication
295                   PermitLocalCommand
296                   PKCS11Provider
297                   Port
298                   PreferredAuthentications
299                   ProxyCommand
300                   ProxyJump
301                   ProxyUseFdpass
302                   PubkeyAcceptedKeyTypes
303                   PubkeyAuthentication
304                   RekeyLimit
305                   RemoteCommand
306                   RemoteForward
307                   RequestTTY
308                   SendEnv
309                   ServerAliveInterval
310                   ServerAliveCountMax
311                   SetEnv
312                   StreamLocalBindMask
313                   StreamLocalBindUnlink
314                   StrictHostKeyChecking
315                   TCPKeepAlive
316                   Tunnel
317                   TunnelDevice
318                   UpdateHostKeys
319                   User
320                   UserKnownHostsFile
321                   VerifyHostKeyDNS
322                   VisualHostKey
323                   XAuthLocation
324
325     -p port
326             Port to connect to on the remote host.  This can be specified on
327             a per-host basis in the configuration file.
328
329     -Q query_option
330             Queries ssh for the algorithms supported for the specified ver‐
331             sion 2.  The available features are: cipher (supported symmetric
332             ciphers), cipher-auth (supported symmetric ciphers that support
333             authenticated encryption), help (supported query terms for use
334             with the -Q flag), mac (supported message integrity codes), kex
335             (key exchange algorithms), kex-gss (GSSAPI key exchange algo‐
336             rithms), key (key types), key-cert (certificate key types),
337             key-plain (non-certificate key types), protocol-version (sup‐
338             ported SSH protocol versions), and sig (supported signature algo‐
339             rithms).
340
341     -q      Quiet mode.  Causes most warning and diagnostic messages to be
342             suppressed.
343
344     -R [bind_address:]port:host:hostport
345     -R [bind_address:]port:local_socket
346     -R remote_socket:host:hostport
347     -R remote_socket:local_socket
348     -R [bind_address:]port
349             Specifies that connections to the given TCP port or Unix socket
350             on the remote (server) host are to be forwarded to the local
351             side.
352
353             This works by allocating a socket to listen to either a TCP port
354             or to a Unix socket on the remote side.  Whenever a connection is
355             made to this port or Unix socket, the connection is forwarded
356             over the secure channel, and a connection is made from the local
357             machine to either an explicit destination specified by host port
358             hostport, or local_socket, or, if no explicit destination was
359             specified, ssh will act as a SOCKS 4/5 proxy and forward connec‐
360             tions to the destinations requested by the remote SOCKS client.
361
362             Port forwardings can also be specified in the configuration file.
363             Privileged ports can be forwarded only when logging in as root on
364             the remote machine.  IPv6 addresses can be specified by enclosing
365             the address in square brackets.
366
367             By default, TCP listening sockets on the server will be bound to
368             the loopback interface only.  This may be overridden by specify‐
369             ing a bind_address.  An empty bind_address, or the address ‘*’,
370             indicates that the remote socket should listen on all interfaces.
371             Specifying a remote bind_address will only succeed if the
372             server's GatewayPorts option is enabled (see sshd_config(5)).
373
374             If the port argument is ‘0’, the listen port will be dynamically
375             allocated on the server and reported to the client at run time.
376             When used together with -O forward the allocated port will be
377             printed to the standard output.
378
379     -S ctl_path
380             Specifies the location of a control socket for connection shar‐
381             ing, or the string “none” to disable connection sharing.  Refer
382             to the description of ControlPath and ControlMaster in
383             ssh_config(5) for details.
384
385     -s      May be used to request invocation of a subsystem on the remote
386             system.  Subsystems facilitate the use of SSH as a secure trans‐
387             port for other applications (e.g. sftp(1)).  The subsystem is
388             specified as the remote command.
389
390     -T      Disable pseudo-terminal allocation.
391
392     -t      Force pseudo-terminal allocation.  This can be used to execute
393             arbitrary screen-based programs on a remote machine, which can be
394             very useful, e.g. when implementing menu services.  Multiple -t
395             options force tty allocation, even if ssh has no local tty.
396
397     -V      Display the version number and exit.
398
399     -v      Verbose mode.  Causes ssh to print debugging messages about its
400             progress.  This is helpful in debugging connection, authentica‐
401             tion, and configuration problems.  Multiple -v options increase
402             the verbosity.  The maximum is 3.
403
404     -W host:port
405             Requests that standard input and output on the client be for‐
406             warded to host on port over the secure channel.  Implies -N, -T,
407             ExitOnForwardFailure and ClearAllForwardings, though these can be
408             overridden in the configuration file or using -o command line
409             options.
410
411     -w local_tun[:remote_tun]
412             Requests tunnel device forwarding with the specified tun(4)
413             devices between the client (local_tun) and the server
414             (remote_tun).
415
416             The devices may be specified by numerical ID or the keyword
417             “any”, which uses the next available tunnel device.  If
418             remote_tun is not specified, it defaults to “any”.  See also the
419             Tunnel and TunnelDevice directives in ssh_config(5).
420
421             If the Tunnel directive is unset, it will be set to the default
422             tunnel mode, which is “point-to-point”.  If a different Tunnel
423             forwarding mode it desired, then it should be specified before
424             -w.
425
426     -X      Enables X11 forwarding.  This can also be specified on a per-host
427             basis in a configuration file.
428
429             X11 forwarding should be enabled with caution.  Users with the
430             ability to bypass file permissions on the remote host (for the
431             user's X authorization database) can access the local X11 display
432             through the forwarded connection.  An attacker may then be able
433             to perform activities such as keystroke monitoring.
434
435             For this reason, X11 forwarding is subjected to X11 SECURITY
436             extension restrictions by default.  Please refer to the ssh -Y
437             option and the ForwardX11Trusted directive in ssh_config(5) for
438             more information.
439
440     -x      Disables X11 forwarding.
441
442     -Y      Enables trusted X11 forwarding.  Trusted X11 forwardings are not
443             subjected to the X11 SECURITY extension controls.
444
445     -y      Send log information using the syslog(3) system module.  By
446             default this information is sent to stderr.
447
448     ssh may additionally obtain configuration data from a per-user configura‐
449     tion file and a system-wide configuration file.  The file format and con‐
450     figuration options are described in ssh_config(5).
451

AUTHENTICATION

453     The OpenSSH SSH client supports SSH protocol 2.
454
455     The methods available for authentication are: GSSAPI-based authentica‐
456     tion, host-based authentication, public key authentication, challenge-
457     response authentication, and password authentication.  Authentication
458     methods are tried in the order specified above, though
459     PreferredAuthentications can be used to change the default order.
460
461     Host-based authentication works as follows: If the machine the user logs
462     in from is listed in /etc/hosts.equiv or /etc/ssh/shosts.equiv on the
463     remote machine, and the user names are the same on both sides, or if the
464     files ~/.rhosts or ~/.shosts exist in the user's home directory on the
465     remote machine and contain a line containing the name of the client
466     machine and the name of the user on that machine, the user is considered
467     for login.  Additionally, the server must be able to verify the client's
468     host key (see the description of /etc/ssh/ssh_known_hosts and
469     ~/.ssh/known_hosts, below) for login to be permitted.  This authentica‐
470     tion method closes security holes due to IP spoofing, DNS spoofing, and
471     routing spoofing.  [Note to the administrator: /etc/hosts.equiv,
472     ~/.rhosts, and the rlogin/rsh protocol in general, are inherently inse‐
473     cure and should be disabled if security is desired.]
474
475     Public key authentication works as follows: The scheme is based on pub‐
476     lic-key cryptography, using cryptosystems where encryption and decryption
477     are done using separate keys, and it is unfeasible to derive the decryp‐
478     tion key from the encryption key.  The idea is that each user creates a
479     public/private key pair for authentication purposes.  The server knows
480     the public key, and only the user knows the private key.  ssh implements
481     public key authentication protocol automatically, using one of the DSA,
482     ECDSA, Ed25519 or RSA algorithms.  The HISTORY section of ssl(8) contains
483     a brief discussion of the DSA and RSA algorithms.
484
485     The file ~/.ssh/authorized_keys lists the public keys that are permitted
486     for logging in.  When the user logs in, the ssh program tells the server
487     which key pair it would like to use for authentication.  The client
488     proves that it has access to the private key and the server checks that
489     the corresponding public key is authorized to accept the account.
490
491     The server may inform the client of errors that prevented public key
492     authentication from succeeding after authentication completes using a
493     different method.  These may be viewed by increasing the LogLevel to
494     DEBUG or higher (e.g. by using the -v flag).
495
496     The user creates his/her key pair by running ssh-keygen(1).  This stores
497     the private key in ~/.ssh/id_dsa (DSA), ~/.ssh/id_ecdsa (ECDSA),
498     ~/.ssh/id_ed25519 (Ed25519), or ~/.ssh/id_rsa (RSA) and stores the public
499     key in ~/.ssh/id_dsa.pub (DSA), ~/.ssh/id_ecdsa.pub (ECDSA),
500     ~/.ssh/id_ed25519.pub (Ed25519), or ~/.ssh/id_rsa.pub (RSA) in the user's
501     home directory.  The user should then copy the public key to
502     ~/.ssh/authorized_keys in his/her home directory on the remote machine.
503     The authorized_keys file corresponds to the conventional ~/.rhosts file,
504     and has one key per line, though the lines can be very long.  After this,
505     the user can log in without giving the password.
506
507     A variation on public key authentication is available in the form of cer‐
508     tificate authentication: instead of a set of public/private keys, signed
509     certificates are used.  This has the advantage that a single trusted cer‐
510     tification authority can be used in place of many public/private keys.
511     See the CERTIFICATES section of ssh-keygen(1) for more information.
512
513     The most convenient way to use public key or certificate authentication
514     may be with an authentication agent.  See ssh-agent(1) and (optionally)
515     the AddKeysToAgent directive in ssh_config(5) for more information.
516
517     Challenge-response authentication works as follows: The server sends an
518     arbitrary "challenge" text, and prompts for a response.  Examples of
519     challenge-response authentication include BSD Authentication (see
520     login.conf(5)) and PAM (some non-OpenBSD systems).
521
522     Finally, if other authentication methods fail, ssh prompts the user for a
523     password.  The password is sent to the remote host for checking; however,
524     since all communications are encrypted, the password cannot be seen by
525     someone listening on the network.
526
527     ssh automatically maintains and checks a database containing identifica‐
528     tion for all hosts it has ever been used with.  Host keys are stored in
529     ~/.ssh/known_hosts in the user's home directory.  Additionally, the file
530     /etc/ssh/ssh_known_hosts is automatically checked for known hosts.  Any
531     new hosts are automatically added to the user's file.  If a host's iden‐
532     tification ever changes, ssh warns about this and disables password
533     authentication to prevent server spoofing or man-in-the-middle attacks,
534     which could otherwise be used to circumvent the encryption.  The
535     StrictHostKeyChecking option can be used to control logins to machines
536     whose host key is not known or has changed.
537
538     When the user's identity has been accepted by the server, the server
539     either executes the given command in a non-interactive session or, if no
540     command has been specified, logs into the machine and gives the user a
541     normal shell as an interactive session.  All communication with the
542     remote command or shell will be automatically encrypted.
543
544     If an interactive session is requested ssh by default will only request a
545     pseudo-terminal (pty) for interactive sessions when the client has one.
546     The flags -T and -t can be used to override this behaviour.
547
548     If a pseudo-terminal has been allocated the user may use the escape char‐
549     acters noted below.
550
551     If no pseudo-terminal has been allocated, the session is transparent and
552     can be used to reliably transfer binary data.  On most systems, setting
553     the escape character to “none” will also make the session transparent
554     even if a tty is used.
555
556     The session terminates when the command or shell on the remote machine
557     exits and all X11 and TCP connections have been closed.
558

ESCAPE CHARACTERS

560     When a pseudo-terminal has been requested, ssh supports a number of func‐
561     tions through the use of an escape character.
562
563     A single tilde character can be sent as ~~ or by following the tilde by a
564     character other than those described below.  The escape character must
565     always follow a newline to be interpreted as special.  The escape charac‐
566     ter can be changed in configuration files using the EscapeChar configura‐
567     tion directive or on the command line by the -e option.
568
569     The supported escapes (assuming the default ‘~’) are:
570
571     ~.      Disconnect.
572
573     ~^Z     Background ssh.
574
575     ~#      List forwarded connections.
576
577     ~&      Background ssh at logout when waiting for forwarded connection /
578             X11 sessions to terminate.
579
580     ~?      Display a list of escape characters.
581
582     ~B      Send a BREAK to the remote system (only useful if the peer sup‐
583             ports it).
584
585     ~C      Open command line.  Currently this allows the addition of port
586             forwardings using the -L, -R and -D options (see above).  It also
587             allows the cancellation of existing port-forwardings with
588             -KL[bind_address:]port for local, -KR[bind_address:]port for
589             remote and -KD[bind_address:]port for dynamic port-forwardings.
590             !command allows the user to execute a local command if the
591             PermitLocalCommand option is enabled in ssh_config(5).  Basic
592             help is available, using the -h option.
593
594     ~R      Request rekeying of the connection (only useful if the peer sup‐
595             ports it).
596
597     ~V      Decrease the verbosity (LogLevel) when errors are being written
598             to stderr.
599
600     ~v      Increase the verbosity (LogLevel) when errors are being written
601             to stderr.
602

TCP FORWARDING

604     Forwarding of arbitrary TCP connections over a secure channel can be
605     specified either on the command line or in a configuration file.  One
606     possible application of TCP forwarding is a secure connection to a mail
607     server; another is going through firewalls.
608
609     In the example below, we look at encrypting communication for an IRC
610     client, even though the IRC server it connects to does not directly sup‐
611     port encrypted communication.  This works as follows: the user connects
612     to the remote host using ssh, specifying the ports to be used to forward
613     the connection.  After that it is possible to start the program locally,
614     and ssh will encrypt and forward the connection to the remote server.
615
616     The following example tunnels an IRC session from the client to an IRC
617     server at “server.example.com”, joining channel “#users”, nickname
618     “pinky”, using the standard IRC port, 6667:
619
620         $ ssh -f -L 6667:localhost:6667 server.example.com sleep 10
621         $ irc -c '#users' pinky IRC/127.0.0.1
622
623     The -f option backgrounds ssh and the remote command “sleep 10” is speci‐
624     fied to allow an amount of time (10 seconds, in the example) to start the
625     program which is going to use the tunnel.  If no connections are made
626     within the time specified, ssh will exit.
627

X11 FORWARDING

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

VERIFYING HOST KEYS

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

SSH-BASED VIRTUAL PRIVATE NETWORKS

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

ENVIRONMENT

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

FILES

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

EXIT STATUS

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

IPV6

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

SEE ALSO

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

STANDARDS

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

AUTHORS

979     OpenSSH is a derivative of the original and free ssh 1.2.12 release by
980     Tatu Ylonen.  Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
981     de Raadt and Dug Song removed many bugs, re-added newer features and cre‐
982     ated OpenSSH.  Markus Friedl contributed the support for SSH protocol
983     versions 1.5 and 2.0.
984
985BSD                            December 31, 2019                           BSD
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