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1SSH(1)                      General Commands Manual                     SSH(1)
2
3NAME
4     ssh M-bM-^@M-^S OpenSSH SSH client (remote login program)
5
6SYNOPSIS
7     ssh [-1246AaCfGgKkMNnqsTtVvXxYy] [-b bind_address] [-c cipher_spec]
8         [-D [bind_address:]port] [-E log_file] [-e escape_char]
9         [-F configfile] [-I pkcs11] [-i identity_file]
10         [-L [bind_address:]port:host:hostport] [-l login_name] [-m mac_spec]
11         [-O ctl_cmd] [-o option] [-p port]
12         [-Q cipher | cipher-auth | mac | kex | key | protocol-version]
13         [-R [bind_address:]port:host:hostport] [-S ctl_path] [-W host:port]
14         [-w local_tun[:remote_tun]] [user@]hostname [command]
15
16DESCRIPTION
17     ssh (SSH client) is a program for logging into a remote machine and for
18     executing commands on a remote machine.  It is intended to replace rlogin
19     and rsh, and provide secure encrypted communications between two
20     untrusted hosts over an insecure network.  X11 connections, arbitrary TCP
21     ports and UNIX-domain sockets can also be forwarded over the secure
22     channel.
23
24     ssh connects and logs into the specified hostname (with optional user
25     name).  The user must prove his/her identity to the remote machine using
26     one of several methods depending on the protocol version used (see
27     below).
28
29     If command is specified, it is executed on the remote host instead of a
30     login shell.
31
32     The options are as follows:
33
34     -1      Forces ssh to try protocol version 1 only.
35
36     -2      Forces ssh to try protocol version 2 only.
37
38     -4      Forces ssh to use IPv4 addresses only.
39
40     -6      Forces ssh to use IPv6 addresses only.
41
42     -A      Enables forwarding of the authentication agent connection.  This
43             can also be specified on a per-host basis in a configuration
44             file.
45
46             Agent forwarding should be enabled with caution.  Users with the
47             ability to bypass file permissions on the remote host (for the
48             agent's UNIX-domain socket) can access the local agent through
49             the forwarded connection.  An attacker cannot obtain key material
50             from the agent, however they can perform operations on the keys
51             that enable them to authenticate using the identities loaded into
52             the agent.
53
54     -a      Disables forwarding of the authentication agent connection.
55
56     -b bind_address
57             Use bind_address on the local machine as the source address of
58             the connection.  Only useful on systems with more than one
59             address.
60
61     -C      Requests compression of all data (including stdin, stdout,
62             stderr, and data for forwarded X11, TCP and UNIX-domain
63             connections).  The compression algorithm is the same used by
64             gzip(1), and the M-bM-^@M-^\levelM-bM-^@M-^] can be controlled by the
65             CompressionLevel option for protocol version 1.  Compression is
66             desirable on modem lines and other slow connections, but will
67             only slow down things on fast networks.  The default value can be
68             set on a host-by-host basis in the configuration files; see the
69             Compression option.
70
71     -c cipher_spec
72             Selects the cipher specification for encrypting the session.
73
74             Protocol version 1 allows specification of a single cipher.  The
75             supported values are M-bM-^@M-^\3desM-bM-^@M-^], M-bM-^@M-^\blowfishM-bM-^@M-^], and M-bM-^@M-^\desM-bM-^@M-^].  For protocol
76             version 2, cipher_spec is a comma-separated list of ciphers
77             listed in order of preference.  See the Ciphers keyword in
78             ssh_config(5) for more information.
79
80     -D [bind_address:]port
81             Specifies a local M-bM-^@M-^\dynamicM-bM-^@M-^] application-level port forwarding.
82             This works by allocating a socket to listen to port on the local
83             side, optionally bound to the specified bind_address.  Whenever a
84             connection is made to this port, the connection is forwarded over
85             the secure channel, and the application protocol is then used to
86             determine where to connect to from the remote machine.  Currently
87             the SOCKS4 and SOCKS5 protocols are supported, and ssh will act
88             as a SOCKS server.  Only root can forward privileged ports.
89             Dynamic port forwardings can also be specified in the
90             configuration file.
91
92             IPv6 addresses can be specified by enclosing the address in
93             square brackets.  Only the superuser can forward privileged
94             ports.  By default, the local port is bound in accordance with
95             the GatewayPorts setting.  However, an explicit bind_address may
96             be used to bind the connection to a specific address.  The
97             bind_address of M-bM-^@M-^\localhostM-bM-^@M-^] indicates that the listening port be
98             bound for local use only, while an empty address or M-bM-^@M-^X*M-bM-^@M-^Y indicates
99             that the port should be available from all interfaces.
100
101     -E log_file
102             Append debug logs to log_file instead of standard error.
103
104     -e escape_char
105             Sets the escape character for sessions with a pty (default: M-bM-^@M-^X~M-bM-^@M-^Y).
106             The escape character is only recognized at the beginning of a
107             line.  The escape character followed by a dot (M-bM-^@M-^X.M-bM-^@M-^Y) closes the
108             connection; followed by control-Z suspends the connection; and
109             followed by itself sends the escape character once.  Setting the
110             character to M-bM-^@M-^\noneM-bM-^@M-^] disables any escapes and makes the session
111             fully transparent.
112
113     -F configfile
114             Specifies an alternative per-user configuration file.  If a
115             configuration file is given on the command line, the system-wide
116             configuration file (/etc/ssh/ssh_config) will be ignored.  The
117             default for the per-user configuration file is ~/.ssh/config.
118
119     -f      Requests ssh to go to background just before command execution.
120             This is useful if ssh is going to ask for passwords or
121             passphrases, but the user wants it in the background.  This
122             implies -n.  The recommended way to start X11 programs at a
123             remote site is with something like ssh -f host xterm.
124
125             If the ExitOnForwardFailure configuration option is set to M-bM-^@M-^\yesM-bM-^@M-^],
126             then a client started with -f will wait for all remote port
127             forwards to be successfully established before placing itself in
128             the background.
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 the user's private RSA key.
140
141     -i identity_file
142             Selects a file from which the identity (private key) for public
143             key authentication is read.  The default is ~/.ssh/identity for
144             protocol version 1, and ~/.ssh/id_dsa, ~/.ssh/id_ecdsa,
145             ~/.ssh/id_ed25519 and ~/.ssh/id_rsa for protocol version 2.
146             Identity files may also be specified on a per-host basis in the
147             configuration file.  It is possible to have multiple -i options
148             (and multiple identities specified in configuration files).  ssh
149             will also try to load certificate information from the filename
150             obtained by appending -cert.pub to identity filenames.
151
152     -K      Enables GSSAPI-based authentication and forwarding (delegation)
153             of GSSAPI credentials to the server.
154
155     -k      Disables forwarding (delegation) of GSSAPI credentials to the
156             server.
157
158     -L [bind_address:]port:host:hostport
159             Specifies that the given port on the local (client) host is to be
160             forwarded to the given host and port on the remote side.  This
161             works by allocating a socket to listen to port on the local side,
162             optionally bound to the specified bind_address.  Whenever a
163             connection is made to this port, the connection is forwarded over
164             the secure channel, and a connection is made to host port
165             hostport from the remote machine.  Port forwardings can also be
166             specified in the configuration file.  IPv6 addresses can be
167             specified by enclosing the address in square brackets.  Only the
168             superuser can forward privileged ports.  By default, the local
169             port is bound in accordance with the GatewayPorts setting.
170             However, an explicit bind_address may be used to bind the
171             connection to a specific address.  The bind_address of
172             M-bM-^@M-^\localhostM-bM-^@M-^] indicates that the listening port be bound for local
173             use only, while an empty address or M-bM-^@M-^X*M-bM-^@M-^Y indicates that the port
174             should be available from all interfaces.
175
176     -l login_name
177             Specifies the user to log in as on the remote machine.  This also
178             may be specified on a per-host basis in the configuration file.
179
180     -M      Places the ssh client into M-bM-^@M-^\masterM-bM-^@M-^] mode for connection sharing.
181             Multiple -M options places ssh into M-bM-^@M-^\masterM-bM-^@M-^] mode with
182             confirmation required before slave connections are accepted.
183             Refer to the description of ControlMaster in ssh_config(5) for
184             details.
185
186     -m mac_spec
187             Additionally, for protocol version 2 a comma-separated list of
188             MAC (message authentication code) algorithms can be specified in
189             order of preference.  See the MACs keyword for more information.
190
191     -N      Do not execute a remote command.  This is useful for just
192             forwarding ports (protocol version 2 only).
193
194     -n      Redirects stdin from /dev/null (actually, prevents reading from
195             stdin).  This must be used when ssh is run in the background.  A
196             common trick is to use this to run X11 programs on a remote
197             machine.  For example, ssh -n shadows.cs.hut.fi emacs & will
198             start an emacs on shadows.cs.hut.fi, and the X11 connection will
199             be automatically forwarded over an encrypted channel.  The ssh
200             program will be put in the background.  (This does not work if
201             ssh needs to ask for a password or passphrase; see also the -f
202             option.)
203
204     -O ctl_cmd
205             Control an active connection multiplexing master process.  When
206             the -O option is specified, the ctl_cmd argument is interpreted
207             and passed to the master process.  Valid commands are: M-bM-^@M-^\checkM-bM-^@M-^]
208             (check that the master process is running), M-bM-^@M-^\forwardM-bM-^@M-^] (request
209             forwardings without command execution), M-bM-^@M-^\cancelM-bM-^@M-^] (cancel
210             forwardings), M-bM-^@M-^\exitM-bM-^@M-^] (request the master to exit), and M-bM-^@M-^\stopM-bM-^@M-^]
211             (request the master to stop accepting further multiplexing
212             requests).
213
214     -o option
215             Can be used to give options in the format used in the
216             configuration file.  This is useful for specifying options for
217             which there is no separate command-line flag.  For full details
218             of the options listed below, and their possible values, see
219             ssh_config(5).
220
221                   AddressFamily
222                   BatchMode
223                   BindAddress
224                   CanonicalDomains
225                   CanonicalizeFallbackLocal
226                   CanonicalizeHostname
227                   CanonicalizeMaxDots
228                   CanonicalizePermittedCNAMEs
229                   ChallengeResponseAuthentication
230                   CheckHostIP
231                   Cipher
232                   Ciphers
233                   ClearAllForwardings
234                   Compression
235                   CompressionLevel
236                   ConnectionAttempts
237                   ConnectTimeout
238                   ControlMaster
239                   ControlPath
240                   ControlPersist
241                   DynamicForward
242                   EscapeChar
243                   ExitOnForwardFailure
244                   FingerprintHash
245                   ForwardAgent
246                   ForwardX11
247                   ForwardX11Timeout
248                   ForwardX11Trusted
249                   GatewayPorts
250                   GlobalKnownHostsFile
251                   GSSAPIAuthentication
252                   GSSAPIDelegateCredentials
253                   HashKnownHosts
254                   Host
255                   HostbasedAuthentication
256                   HostbasedKeyTypes
257                   HostKeyAlgorithms
258                   HostKeyAlias
259                   HostName
260                   IdentityFile
261                   IdentitiesOnly
262                   IPQoS
263                   KbdInteractiveAuthentication
264                   KbdInteractiveDevices
265                   KexAlgorithms
266                   LocalCommand
267                   LocalForward
268                   LogLevel
269                   MACs
270                   Match
271                   NoHostAuthenticationForLocalhost
272                   NumberOfPasswordPrompts
273                   PasswordAuthentication
274                   PermitLocalCommand
275                   PKCS11Provider
276                   Port
277                   PreferredAuthentications
278                   Protocol
279                   ProxyCommand
280                   ProxyUseFdpass
281                   PubkeyAuthentication
282                   RekeyLimit
283                   RemoteForward
284                   RequestTTY
285                   RhostsRSAAuthentication
286                   RSAAuthentication
287                   SendEnv
288                   ServerAliveInterval
289                   ServerAliveCountMax
290                   StreamLocalBindMask
291                   StreamLocalBindUnlink
292                   StrictHostKeyChecking
293                   TCPKeepAlive
294                   Tunnel
295                   TunnelDevice
296                   UpdateHostKeys
297                   UsePrivilegedPort
298                   User
299                   UserKnownHostsFile
300                   VerifyHostKeyDNS
301                   VisualHostKey
302                   XAuthLocation
303
304     -p port
305             Port to connect to on the remote host.  This can be specified on
306             a per-host basis in the configuration file.
307
308     -Q cipher | cipher-auth | mac | kex | key | protocol-version
309             Queries ssh for the algorithms supported for the specified
310             version 2.  The available features are: cipher (supported
311             symmetric ciphers), cipher-auth (supported symmetric ciphers that
312             support authenticated encryption), mac (supported message
313             integrity codes), kex (key exchange algorithms), key (key types)
314             and protocol-version (supported SSH protocol versions).
315
316     -q      Quiet mode.  Causes most warning and diagnostic messages to be
317             suppressed.
318
319     -R [bind_address:]port:host:hostport
320             Specifies that the given port on the remote (server) host is to
321             be forwarded to the given host and port on the local side.  This
322             works by allocating a socket to listen to port on the remote
323             side, and whenever a connection is made to this port, the
324             connection is forwarded over the secure channel, and a connection
325             is made to host port hostport from the local machine.
326
327             Port forwardings can also be specified in the configuration file.
328             Privileged ports can be forwarded only when logging in as root on
329             the remote machine.  IPv6 addresses can be specified by enclosing
330             the address in square brackets.
331
332             By default, the listening socket on the server will be bound to
333             the loopback interface only.  This may be overridden by
334             specifying a bind_address.  An empty bind_address, or the address
335             M-bM-^@M-^X*M-bM-^@M-^Y, indicates that the remote socket should listen on all
336             interfaces.  Specifying a remote bind_address will only succeed
337             if the server's GatewayPorts option is enabled (see
338             sshd_config(5)).
339
340             If the port argument is M-bM-^@M-^X0M-bM-^@M-^Y, the listen port will be dynamically
341             allocated on the server and reported to the client at run time.
342             When used together with -O forward the allocated port will be
343             printed to the standard output.
344
345     -S ctl_path
346             Specifies the location of a control socket for connection
347             sharing, or the string M-bM-^@M-^\noneM-bM-^@M-^] to disable connection sharing.
348             Refer to the description of ControlPath and ControlMaster in
349             ssh_config(5) for details.
350
351     -s      May be used to request invocation of a subsystem on the remote
352             system.  Subsystems are a feature of the SSH2 protocol which
353             facilitate the use of SSH as a secure transport for other
354             applications (eg. sftp(1)).  The subsystem is specified as the
355             remote command.
356
357     -T      Disable pseudo-tty allocation.
358
359     -t      Force pseudo-tty allocation.  This can be used to execute
360             arbitrary screen-based programs on a remote machine, which can be
361             very useful, e.g. when implementing menu services.  Multiple -t
362             options force tty allocation, even if ssh has no local tty.
363
364     -V      Display the version number and exit.
365
366     -v      Verbose mode.  Causes ssh to print debugging messages about its
367             progress.  This is helpful in debugging connection,
368             authentication, and configuration problems.  Multiple -v options
369             increase the verbosity.  The maximum is 3.
370
371     -W host:port
372             Requests that standard input and output on the client be
373             forwarded to host on port over the secure channel.  Implies -N,
374             -T, ExitOnForwardFailure and ClearAllForwardings.  Works with
375             Protocol version 2 only.
376
377     -w local_tun[:remote_tun]
378             Requests tunnel device forwarding with the specified tun(4)
379             devices between the client (local_tun) and the server
380             (remote_tun).
381
382             The devices may be specified by numerical ID or the keyword
383             M-bM-^@M-^\anyM-bM-^@M-^], which uses the next available tunnel device.  If
384             remote_tun is not specified, it defaults to M-bM-^@M-^\anyM-bM-^@M-^].  See also the
385             Tunnel and TunnelDevice directives in ssh_config(5).  If the
386             Tunnel directive is unset, it is set to the default tunnel mode,
387             which is M-bM-^@M-^\point-to-pointM-bM-^@M-^].
388
389     -X      Enables X11 forwarding.  This can also be specified on a per-host
390             basis in a configuration file.
391
392             X11 forwarding should be enabled with caution.  Users with the
393             ability to bypass file permissions on the remote host (for the
394             user's X authorization database) can access the local X11 display
395             through the forwarded connection.  An attacker may then be able
396             to perform activities such as keystroke monitoring.
397
398             For this reason, X11 forwarding is subjected to X11 SECURITY
399             extension restrictions by default.  Please refer to the ssh -Y
400             option and the ForwardX11Trusted directive in ssh_config(5) for
401             more information.
402
403     -x      Disables X11 forwarding.
404
405     -Y      Enables trusted X11 forwarding.  Trusted X11 forwardings are not
406             subjected to the X11 SECURITY extension controls.
407
408     -y      Send log information using the syslog(3) system module.  By
409             default this information is sent to stderr.
410
411     ssh may additionally obtain configuration data from a per-user
412     configuration file and a system-wide configuration file.  The file format
413     and configuration options are described in ssh_config(5).
414
415AUTHENTICATION
416     The OpenSSH SSH client supports SSH protocols 1 and 2.  The default is to
417     use protocol 2 only, though this can be changed via the Protocol option
418     in ssh_config(5) or the -1 and -2 options (see above).  Both protocols
419     support similar authentication methods, but protocol 2 is the default
420     since it provides additional mechanisms for confidentiality (the traffic
421     is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour) and
422     integrity (hmac-md5, hmac-sha1, hmac-sha2-256, hmac-sha2-512, umac-64,
423     umac-128, hmac-ripemd160).  Protocol 1 lacks a strong mechanism for
424     ensuring the integrity of the connection.
425
426     The methods available for authentication are: GSSAPI-based
427     authentication, host-based authentication, public key authentication,
428     challenge-response authentication, and password authentication.
429     Authentication methods are tried in the order specified above, though
430     protocol 2 has a configuration option to change the default order:
431     PreferredAuthentications.
432
433     Host-based authentication works as follows: If the machine the user logs
434     in from is listed in /etc/hosts.equiv or /etc/shosts.equiv on the remote
435     machine, and the user names are the same on both sides, or if the files
436     ~/.rhosts or ~/.shosts exist in the user's home directory on the remote
437     machine and contain a line containing the name of the client machine and
438     the name of the user on that machine, the user is considered for login.
439     Additionally, the server must be able to verify the client's host key
440     (see the description of /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts,
441     below) for login to be permitted.  This authentication method closes
442     security holes due to IP spoofing, DNS spoofing, and routing spoofing.
443     [Note to the administrator: /etc/hosts.equiv, ~/.rhosts, and the
444     rlogin/rsh protocol in general, are inherently insecure and should be
445     disabled if security is desired.]
446
447     Public key authentication works as follows: The scheme is based on
448     public-key cryptography, using cryptosystems where encryption and
449     decryption are done using separate keys, and it is unfeasible to derive
450     the decryption key from the encryption key.  The idea is that each user
451     creates a public/private key pair for authentication purposes.  The
452     server knows the public key, and only the user knows the private key.
453     ssh implements public key authentication protocol automatically, using
454     one of the DSA, ECDSA, Ed25519 or RSA algorithms.  Protocol 1 is
455     restricted to using only RSA keys, but protocol 2 may use any.  The
456     HISTORY section of ssl(8) contains a brief discussion of the DSA and RSA
457     algorithms.
458
459     The file ~/.ssh/authorized_keys lists the public keys that are permitted
460     for logging in.  When the user logs in, the ssh program tells the server
461     which key pair it would like to use for authentication.  The client
462     proves that it has access to the private key and the server checks that
463     the corresponding public key is authorized to accept the account.
464
465     The user creates his/her key pair by running ssh-keygen(1).  This stores
466     the private key in ~/.ssh/identity (protocol 1), ~/.ssh/id_dsa (protocol
467     2 DSA), ~/.ssh/id_ecdsa (protocol 2 ECDSA), ~/.ssh/id_ed25519 (protocol 2
468     Ed25519), or ~/.ssh/id_rsa (protocol 2 RSA) and stores the public key in
469     ~/.ssh/identity.pub (protocol 1), ~/.ssh/id_dsa.pub (protocol 2 DSA),
470     ~/.ssh/id_ecdsa.pub (protocol 2 ECDSA), ~/.ssh/id_ed25519.pub (protocol 2
471     Ed25519), or ~/.ssh/id_rsa.pub (protocol 2 RSA) in the user's home
472     directory.  The user should then copy the public key to
473     ~/.ssh/authorized_keys in his/her home directory on the remote machine.
474     The authorized_keys file corresponds to the conventional ~/.rhosts file,
475     and has one key per line, though the lines can be very long.  After this,
476     the user can log in without giving the password.
477
478     A variation on public key authentication is available in the form of
479     certificate authentication: instead of a set of public/private keys,
480     signed certificates are used.  This has the advantage that a single
481     trusted certification authority can be used in place of many
482     public/private keys.  See the CERTIFICATES section of ssh-keygen(1) for
483     more information.
484
485     The most convenient way to use public key or certificate authentication
486     may be with an authentication agent.  See ssh-agent(1) for more
487     information.
488
489     Challenge-response authentication works as follows: The server sends an
490     arbitrary "challenge" text, and prompts for a response.  Protocol 2
491     allows multiple challenges and responses; protocol 1 is restricted to
492     just one challenge/response.  Examples of challenge-response
493     authentication include BSD Authentication (see login.conf(5)) and PAM
494     (some non-OpenBSD systems).
495
496     Finally, if other authentication methods fail, ssh prompts the user for a
497     password.  The password is sent to the remote host for checking; however,
498     since all communications are encrypted, the password cannot be seen by
499     someone listening on the network.
500
501     ssh automatically maintains and checks a database containing
502     identification for all hosts it has ever been used with.  Host keys are
503     stored in ~/.ssh/known_hosts in the user's home directory.  Additionally,
504     the file /etc/ssh/ssh_known_hosts is automatically checked for known
505     hosts.  Any new hosts are automatically added to the user's file.  If a
506     host's identification ever changes, ssh warns about this and disables
507     password authentication to prevent server spoofing or man-in-the-middle
508     attacks, which could otherwise be used to circumvent the encryption.  The
509     StrictHostKeyChecking option can be used to control logins to machines
510     whose host key is not known or has changed.
511
512     When the user's identity has been accepted by the server, the server
513     either executes the given command, or logs into the machine and gives the
514     user a normal shell on the remote machine.  All communication with the
515     remote command or shell will be automatically encrypted.
516
517     If a pseudo-terminal has been allocated (normal login session), the user
518     may use the escape characters noted below.
519
520     If no pseudo-tty has been allocated, the session is transparent and can
521     be used to reliably transfer binary data.  On most systems, setting the
522     escape character to M-bM-^@M-^\noneM-bM-^@M-^] will also make the session transparent even if
523     a tty is used.
524
525     The session terminates when the command or shell on the remote machine
526     exits and all X11 and TCP connections have been closed.
527
528ESCAPE CHARACTERS
529     When a pseudo-terminal has been requested, ssh supports a number of
530     functions through the use of an escape character.
531
532     A single tilde character can be sent as ~~ or by following the tilde by a
533     character other than those described below.  The escape character must
534     always follow a newline to be interpreted as special.  The escape
535     character can be changed in configuration files using the EscapeChar
536     configuration directive or on the command line by the -e option.
537
538     The supported escapes (assuming the default M-bM-^@M-^X~M-bM-^@M-^Y) are:
539
540     ~.      Disconnect.
541
542     ~^Z     Background ssh.
543
544     ~#      List forwarded connections.
545
546     ~&      Background ssh at logout when waiting for forwarded connection /
547             X11 sessions to terminate.
548
549     ~?      Display a list of escape characters.
550
551     ~B      Send a BREAK to the remote system (only useful for SSH protocol
552             version 2 and if the peer supports it).
553
554     ~C      Open command line.  Currently this allows the addition of port
555             forwardings using the -L, -R and -D options (see above).  It also
556             allows the cancellation of existing port-forwardings with
557             -KL[bind_address:]port for local, -KR[bind_address:]port for
558             remote and -KD[bind_address:]port for dynamic port-forwardings.
559             !command allows the user to execute a local command if the
560             PermitLocalCommand option is enabled in ssh_config(5).  Basic
561             help is available, using the -h option.
562
563     ~R      Request rekeying of the connection (only useful for SSH protocol
564             version 2 and if the peer supports it).
565
566     ~V      Decrease the verbosity (LogLevel) when errors are being written
567             to stderr.
568
569     ~v      Increase the verbosity (LogLevel) when errors are being written
570             to stderr.
571
572TCP FORWARDING
573     Forwarding of arbitrary TCP connections over the secure channel can be
574     specified either on the command line or in a configuration file.  One
575     possible application of TCP forwarding is a secure connection to a mail
576     server; another is going through firewalls.
577
578     In the example below, we look at encrypting communication between an IRC
579     client and server, even though the IRC server does not directly support
580     encrypted communications.  This works as follows: the user connects to
581     the remote host using ssh, specifying a port to be used to forward
582     connections to the remote server.  After that it is possible to start the
583     service which is to be encrypted on the client machine, connecting to the
584     same local port, and ssh will encrypt and forward the connection.
585
586     The following example tunnels an IRC session from client machine
587     M-bM-^@M-^\127.0.0.1M-bM-^@M-^] (localhost) to remote server M-bM-^@M-^\server.example.comM-bM-^@M-^]:
588
589         $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
590         $ irc -c '#users' -p 1234 pinky 127.0.0.1
591
592     This tunnels a connection to IRC server M-bM-^@M-^\server.example.comM-bM-^@M-^], joining
593     channel M-bM-^@M-^\#usersM-bM-^@M-^], nickname M-bM-^@M-^\pinkyM-bM-^@M-^], using port 1234.  It doesn't matter
594     which port is used, as long as it's greater than 1023 (remember, only
595     root can open sockets on privileged ports) and doesn't conflict with any
596     ports already in use.  The connection is forwarded to port 6667 on the
597     remote server, since that's the standard port for IRC services.
598
599     The -f option backgrounds ssh and the remote command M-bM-^@M-^\sleep 10M-bM-^@M-^] is
600     specified to allow an amount of time (10 seconds, in the example) to
601     start the service which is to be tunnelled.  If no connections are made
602     within the time specified, ssh will exit.
603
604X11 FORWARDING
605     If the ForwardX11 variable is set to M-bM-^@M-^\yesM-bM-^@M-^] (or see the description of the
606     -X, -x, and -Y options above) and the user is using X11 (the DISPLAY
607     environment variable is set), the connection to the X11 display is
608     automatically forwarded to the remote side in such a way that any X11
609     programs started from the shell (or command) will go through the
610     encrypted channel, and the connection to the real X server will be made
611     from the local machine.  The user should not manually set DISPLAY.
612     Forwarding of X11 connections can be configured on the command line or in
613     configuration files.
614
615     The DISPLAY value set by ssh will point to the server machine, but with a
616     display number greater than zero.  This is normal, and happens because
617     ssh creates a M-bM-^@M-^\proxyM-bM-^@M-^] X server on the server machine for forwarding the
618     connections over the encrypted channel.
619
620     ssh will also automatically set up Xauthority data on the server machine.
621     For this purpose, it will generate a random authorization cookie, store
622     it in Xauthority on the server, and verify that any forwarded connections
623     carry this cookie and replace it by the real cookie when the connection
624     is opened.  The real authentication cookie is never sent to the server
625     machine (and no cookies are sent in the plain).
626
627     If the ForwardAgent variable is set to M-bM-^@M-^\yesM-bM-^@M-^] (or see the description of
628     the -A and -a options above) and the user is using an authentication
629     agent, the connection to the agent is automatically forwarded to the
630     remote side.
631
632VERIFYING HOST KEYS
633     When connecting to a server for the first time, a fingerprint of the
634     server's public key is presented to the user (unless the option
635     StrictHostKeyChecking has been disabled).  Fingerprints can be determined
636     using ssh-keygen(1):
637
638           $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
639
640     If the fingerprint is already known, it can be matched and the key can be
641     accepted or rejected.  Because of the difficulty of comparing host keys
642     just by looking at fingerprint strings, there is also support to compare
643     host keys visually, using random art.  By setting the VisualHostKey
644     option to M-bM-^@M-^\yesM-bM-^@M-^], a small ASCII graphic gets displayed on every login to a
645     server, no matter if the session itself is interactive or not.  By
646     learning the pattern a known server produces, a user can easily find out
647     that the host key has changed when a completely different pattern is
648     displayed.  Because these patterns are not unambiguous however, a pattern
649     that looks similar to the pattern remembered only gives a good
650     probability that the host key is the same, not guaranteed proof.
651
652     To get a listing of the fingerprints along with their random art for all
653     known hosts, the following command line can be used:
654
655           $ ssh-keygen -lv -f ~/.ssh/known_hosts
656
657     If the fingerprint is unknown, an alternative method of verification is
658     available: SSH fingerprints verified by DNS.  An additional resource
659     record (RR), SSHFP, is added to a zonefile and the connecting client is
660     able to match the fingerprint with that of the key presented.
661
662     In this example, we are connecting a client to a server,
663     M-bM-^@M-^\host.example.comM-bM-^@M-^].  The SSHFP resource records should first be added to
664     the zonefile for host.example.com:
665
666           $ ssh-keygen -r host.example.com.
667
668     The output lines will have to be added to the zonefile.  To check that
669     the zone is answering fingerprint queries:
670
671           $ dig -t SSHFP host.example.com
672
673     Finally the client connects:
674
675           $ ssh -o "VerifyHostKeyDNS ask" host.example.com
676           [...]
677           Matching host key fingerprint found in DNS.
678           Are you sure you want to continue connecting (yes/no)?
679
680     See the VerifyHostKeyDNS option in ssh_config(5) for more information.
681
682SSH-BASED VIRTUAL PRIVATE NETWORKS
683     ssh contains support for Virtual Private Network (VPN) tunnelling using
684     the tun(4) network pseudo-device, allowing two networks to be joined
685     securely.  The sshd_config(5) configuration option PermitTunnel controls
686     whether the server supports this, and at what level (layer 2 or 3
687     traffic).
688
689     The following example would connect client network 10.0.50.0/24 with
690     remote network 10.0.99.0/24 using a point-to-point connection from
691     10.1.1.1 to 10.1.1.2, provided that the SSH server running on the gateway
692     to the remote network, at 192.168.1.15, allows it.
693
694     On the client:
695
696           # ssh -f -w 0:1 192.168.1.15 true
697           # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
698           # route add 10.0.99.0/24 10.1.1.2
699
700     On the server:
701
702           # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
703           # route add 10.0.50.0/24 10.1.1.1
704
705     Client access may be more finely tuned via the /root/.ssh/authorized_keys
706     file (see below) and the PermitRootLogin server option.  The following
707     entry would permit connections on tun(4) device 1 from user M-bM-^@M-^\janeM-bM-^@M-^] and on
708     tun device 2 from user M-bM-^@M-^\johnM-bM-^@M-^], if PermitRootLogin is set to
709     M-bM-^@M-^\forced-commands-onlyM-bM-^@M-^]:
710
711       tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
712       tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
713
714     Since an SSH-based setup entails a fair amount of overhead, it may be
715     more suited to temporary setups, such as for wireless VPNs.  More
716     permanent VPNs are better provided by tools such as ipsecctl(8) and
717     isakmpd(8).
718
719ENVIRONMENT
720     ssh will normally set the following environment variables:
721
722     DISPLAY               The DISPLAY variable indicates the location of the
723                           X11 server.  It is automatically set by ssh to
724                           point to a value of the form M-bM-^@M-^\hostname:nM-bM-^@M-^], where
725                           M-bM-^@M-^\hostnameM-bM-^@M-^] indicates the host where the shell runs,
726                           and M-bM-^@M-^XnM-bM-^@M-^Y is an integer M-bM-^IM-% 1.  ssh uses this special
727                           value to forward X11 connections over the secure
728                           channel.  The user should normally not set DISPLAY
729                           explicitly, as that will render the X11 connection
730                           insecure (and will require the user to manually
731                           copy any required authorization cookies).
732
733     HOME                  Set to the path of the user's home directory.
734
735     LOGNAME               Synonym for USER; set for compatibility with
736                           systems that use this variable.
737
738     MAIL                  Set to the path of the user's mailbox.
739
740     PATH                  Set to the default PATH, as specified when
741                           compiling ssh.
742
743     SSH_ASKPASS           If ssh needs a passphrase, it will read the
744                           passphrase from the current terminal if it was run
745                           from a terminal.  If ssh does not have a terminal
746                           associated with it but DISPLAY and SSH_ASKPASS are
747                           set, it will execute the program specified by
748                           SSH_ASKPASS and open an X11 window to read the
749                           passphrase.  This is particularly useful when
750                           calling ssh from a .xsession or related script.
751                           (Note that on some machines it may be necessary to
752                           redirect the input from /dev/null to make this
753                           work.)
754
755     SSH_AUTH_SOCK         Identifies the path of a UNIX-domain socket used to
756                           communicate with the agent.
757
758     SSH_CONNECTION        Identifies the client and server ends of the
759                           connection.  The variable contains four space-
760                           separated values: client IP address, client port
761                           number, server IP address, and server port number.
762
763     SSH_ORIGINAL_COMMAND  This variable contains the original command line if
764                           a forced command is executed.  It can be used to
765                           extract the original arguments.
766
767     SSH_TTY               This is set to the name of the tty (path to the
768                           device) associated with the current shell or
769                           command.  If the current session has no tty, this
770                           variable is not set.
771
772     TZ                    This variable is set to indicate the present time
773                           zone if it was set when the daemon was started
774                           (i.e. the daemon passes the value on to new
775                           connections).
776
777     USER                  Set to the name of the user logging in.
778
779     Additionally, ssh reads ~/.ssh/environment, and adds lines of the format
780     M-bM-^@M-^\VARNAME=valueM-bM-^@M-^] to the environment if the file exists and users are
781     allowed to change their environment.  For more information, see the
782     PermitUserEnvironment option in sshd_config(5).
783
784FILES
785     ~/.rhosts
786             This file is used for host-based authentication (see above).  On
787             some machines this file may need to be world-readable if the
788             user's home directory is on an NFS partition, because sshd(8)
789             reads it as root.  Additionally, this file must be owned by the
790             user, and must not have write permissions for anyone else.  The
791             recommended permission for most machines is read/write for the
792             user, and not accessible by others.
793
794     ~/.shosts
795             This file is used in exactly the same way as .rhosts, but allows
796             host-based authentication without permitting login with
797             rlogin/rsh.
798
799     ~/.ssh/
800             This directory is the default location for all user-specific
801             configuration and authentication information.  There is no
802             general requirement to keep the entire contents of this directory
803             secret, but the recommended permissions are read/write/execute
804             for the user, and not accessible by others.
805
806     ~/.ssh/authorized_keys
807             Lists the public keys (DSA, ECDSA, Ed25519, RSA) that can be used
808             for logging in as this user.  The format of this file is
809             described in the sshd(8) manual page.  This file is not highly
810             sensitive, but the recommended permissions are read/write for the
811             user, and not accessible by others.
812
813     ~/.ssh/config
814             This is the per-user configuration file.  The file format and
815             configuration options are described in ssh_config(5).  Because of
816             the potential for abuse, this file must have strict permissions:
817             read/write for the user, and not writable by others.
818
819     ~/.ssh/environment
820             Contains additional definitions for environment variables; see
821             ENVIRONMENT, above.
822
823     ~/.ssh/identity
824     ~/.ssh/id_dsa
825     ~/.ssh/id_ecdsa
826     ~/.ssh/id_ed25519
827     ~/.ssh/id_rsa
828             Contains the private key for authentication.  These files contain
829             sensitive data and should be readable by the user but not
830             accessible by others (read/write/execute).  ssh will simply
831             ignore a private key file if it is accessible by others.  It is
832             possible to specify a passphrase when generating the key which
833             will be used to encrypt the sensitive part of this file using
834             3DES.
835
836     ~/.ssh/identity.pub
837     ~/.ssh/id_dsa.pub
838     ~/.ssh/id_ecdsa.pub
839     ~/.ssh/id_ed25519.pub
840     ~/.ssh/id_rsa.pub
841             Contains the public key for authentication.  These files are not
842             sensitive and can (but need not) be readable by anyone.
843
844     ~/.ssh/known_hosts
845             Contains a list of host keys for all hosts the user has logged
846             into that are not already in the systemwide list of known host
847             keys.  See sshd(8) for further details of the format of this
848             file.
849
850     ~/.ssh/rc
851             Commands in this file are executed by ssh when the user logs in,
852             just before the user's shell (or command) is started.  See the
853             sshd(8) manual page for more information.
854
855     /etc/hosts.equiv
856             This file is for host-based authentication (see above).  It
857             should only be writable by root.
858
859     /etc/shosts.equiv
860             This file is used in exactly the same way as hosts.equiv, but
861             allows host-based authentication without permitting login with
862             rlogin/rsh.
863
864     /etc/ssh/ssh_config
865             Systemwide configuration file.  The file format and configuration
866             options are described in ssh_config(5).
867
868     /etc/ssh/ssh_host_key
869     /etc/ssh/ssh_host_dsa_key
870     /etc/ssh/ssh_host_ecdsa_key
871     /etc/ssh/ssh_host_ed25519_key
872     /etc/ssh/ssh_host_rsa_key
873             These files contain the private parts of the host keys and are
874             used for host-based authentication.  If protocol version 1 is
875             used, ssh must be setuid root, since the host key is readable
876             only by root.  For protocol version 2, ssh uses ssh-keysign(8) to
877             access the host keys, eliminating the requirement that ssh be
878             setuid root when host-based authentication is used.  By default
879             ssh is not setuid root.
880
881     /etc/ssh/ssh_known_hosts
882             Systemwide list of known host keys.  This file should be prepared
883             by the system administrator to contain the public host keys of
884             all machines in the organization.  It should be world-readable.
885             See sshd(8) for further details of the format of this file.
886
887     /etc/ssh/sshrc
888             Commands in this file are executed by ssh when the user logs in,
889             just before the user's shell (or command) is started.  See the
890             sshd(8) manual page for more information.
891
892EXIT STATUS
893     ssh exits with the exit status of the remote command or with 255 if an
894     error occurred.
895
896SEE ALSO
897     scp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), ssh-keyscan(1),
898     tun(4), ssh_config(5), ssh-keysign(8), sshd(8)
899
900STANDARDS
901     S. Lehtinen and C. Lonvick, The Secure Shell (SSH) Protocol Assigned
902     Numbers, RFC 4250, January 2006.
903
904     T. Ylonen and C. Lonvick, The Secure Shell (SSH) Protocol Architecture,
905     RFC 4251, January 2006.
906
907     T. Ylonen and C. Lonvick, The Secure Shell (SSH) Authentication Protocol,
908     RFC 4252, January 2006.
909
910     T. Ylonen and C. Lonvick, The Secure Shell (SSH) Transport Layer
911     Protocol, RFC 4253, January 2006.
912
913     T. Ylonen and C. Lonvick, The Secure Shell (SSH) Connection Protocol, RFC
914     4254, January 2006.
915
916     J. Schlyter and W. Griffin, Using DNS to Securely Publish Secure Shell
917     (SSH) Key Fingerprints, RFC 4255, January 2006.
918
919     F. Cusack and M. Forssen, Generic Message Exchange Authentication for the
920     Secure Shell Protocol (SSH), RFC 4256, January 2006.
921
922     J. Galbraith and P. Remaker, The Secure Shell (SSH) Session Channel Break
923     Extension, RFC 4335, January 2006.
924
925     M. Bellare, T. Kohno, and C. Namprempre, The Secure Shell (SSH) Transport
926     Layer Encryption Modes, RFC 4344, January 2006.
927
928     B. Harris, Improved Arcfour Modes for the Secure Shell (SSH) Transport
929     Layer Protocol, RFC 4345, January 2006.
930
931     M. Friedl, N. Provos, and W. Simpson, Diffie-Hellman Group Exchange for
932     the Secure Shell (SSH) Transport Layer Protocol, RFC 4419, March 2006.
933
934     J. Galbraith and R. Thayer, The Secure Shell (SSH) Public Key File
935     Format, RFC 4716, November 2006.
936
937     D. Stebila and J. Green, Elliptic Curve Algorithm Integration in the
938     Secure Shell Transport Layer, RFC 5656, December 2009.
939
940     A. Perrig and D. Song, Hash Visualization: a New Technique to improve
941     Real-World Security, 1999, International Workshop on Cryptographic
942     Techniques and E-Commerce (CrypTEC '99).
943
944AUTHORS
945     OpenSSH is a derivative of the original and free ssh 1.2.12 release by
946     Tatu Ylonen.  Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
947     de Raadt and Dug Song removed many bugs, re-added newer features and
948     created OpenSSH.  Markus Friedl contributed the support for SSH protocol
949     versions 1.5 and 2.0.
950
951OpenBSD 5.7                      March 3, 2015                     OpenBSD 5.7
952