Import OpenSSH-5.9p1.
[dragonfly.git] / crypto / openssh / ssh_config.5
CommitLineData
16c343f1
PA
1.\"
2.\" Author: Tatu Ylonen <ylo@cs.hut.fi>
3.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
4.\" All rights reserved
5.\"
6.\" As far as I am concerned, the code I have written for this software
7.\" can be used freely for any purpose. Any derived versions of this
8.\" software must be clearly marked as such, and if the derived work is
9.\" incompatible with the protocol description in the RFC file, it must be
10.\" called by a name other than "ssh" or "Secure Shell".
11.\"
12.\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
13.\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
14.\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
15.\"
16.\" Redistribution and use in source and binary forms, with or without
17.\" modification, are permitted provided that the following conditions
18.\" are met:
19.\" 1. Redistributions of source code must retain the above copyright
20.\" notice, this list of conditions and the following disclaimer.
21.\" 2. Redistributions in binary form must reproduce the above copyright
22.\" notice, this list of conditions and the following disclaimer in the
23.\" documentation and/or other materials provided with the distribution.
24.\"
25.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
26.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
27.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
28.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
29.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
30.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
31.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
32.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
33.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
34.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
35.\"
1c188a7f
PA
36.\" $OpenBSD: ssh_config.5,v 1.153 2011/08/02 01:22:11 djm Exp $
37.Dd $Mdocdate: August 2 2011 $
16c343f1
PA
38.Dt SSH_CONFIG 5
39.Os
40.Sh NAME
41.Nm ssh_config
42.Nd OpenSSH SSH client configuration files
43.Sh SYNOPSIS
44.Nm ~/.ssh/config
45.Nm /etc/ssh/ssh_config
46.Sh DESCRIPTION
47.Xr ssh 1
48obtains configuration data from the following sources in
49the following order:
50.Pp
51.Bl -enum -offset indent -compact
52.It
53command-line options
54.It
55user's configuration file
56.Pq Pa ~/.ssh/config
57.It
58system-wide configuration file
59.Pq Pa /etc/ssh/ssh_config
60.El
61.Pp
62For each parameter, the first obtained value
63will be used.
64The configuration files contain sections separated by
65.Dq Host
66specifications, and that section is only applied for hosts that
67match one of the patterns given in the specification.
68The matched host name is the one given on the command line.
69.Pp
70Since the first obtained value for each parameter is used, more
71host-specific declarations should be given near the beginning of the
72file, and general defaults at the end.
73.Pp
74The configuration file has the following format:
75.Pp
76Empty lines and lines starting with
77.Ql #
78are comments.
79Otherwise a line is of the format
80.Dq keyword arguments .
81Configuration options may be separated by whitespace or
82optional whitespace and exactly one
83.Ql = ;
84the latter format is useful to avoid the need to quote whitespace
85when specifying configuration options using the
86.Nm ssh ,
87.Nm scp ,
88and
89.Nm sftp
90.Fl o
91option.
92Arguments may optionally be enclosed in double quotes
93.Pq \&"
94in order to represent arguments containing spaces.
95.Pp
96The possible
97keywords and their meanings are as follows (note that
98keywords are case-insensitive and arguments are case-sensitive):
99.Bl -tag -width Ds
100.It Cm Host
101Restricts the following declarations (up to the next
102.Cm Host
103keyword) to be only for those hosts that match one of the patterns
104given after the keyword.
cb5eb4f1 105If more than one pattern is provided, they should be separated by whitespace.
16c343f1
PA
106A single
107.Ql *
108as a pattern can be used to provide global
109defaults for all hosts.
110The host is the
111.Ar hostname
112argument given on the command line (i.e. the name is not converted to
113a canonicalized host name before matching).
114.Pp
1c188a7f
PA
115A pattern entry may be negated by prefixing it with an exclamation mark
116.Pq Sq !\& .
117If a negated entry is matched, then the
118.Cm Host
119entry is ignored, regardless of whether any other patterns on the line
120match.
121Negated matches are therefore useful to provide exceptions for wildcard
122matches.
123.Pp
16c343f1
PA
124See
125.Sx PATTERNS
126for more information on patterns.
127.It Cm AddressFamily
128Specifies which address family to use when connecting.
129Valid arguments are
130.Dq any ,
131.Dq inet
132(use IPv4 only), or
133.Dq inet6
134(use IPv6 only).
135.It Cm BatchMode
136If set to
137.Dq yes ,
138passphrase/password querying will be disabled.
139This option is useful in scripts and other batch jobs where no user
140is present to supply the password.
141The argument must be
142.Dq yes
143or
144.Dq no .
145The default is
146.Dq no .
147.It Cm BindAddress
148Use the specified address on the local machine as the source address of
149the connection.
150Only useful on systems with more than one address.
151Note that this option does not work if
152.Cm UsePrivilegedPort
153is set to
154.Dq yes .
155.It Cm ChallengeResponseAuthentication
156Specifies whether to use challenge-response authentication.
157The argument to this keyword must be
158.Dq yes
159or
160.Dq no .
161The default is
162.Dq yes .
163.It Cm CheckHostIP
164If this flag is set to
165.Dq yes ,
166.Xr ssh 1
167will additionally check the host IP address in the
168.Pa known_hosts
169file.
170This allows ssh to detect if a host key changed due to DNS spoofing.
171If the option is set to
172.Dq no ,
173the check will not be executed.
174The default is
175.Dq yes .
176.It Cm Cipher
177Specifies the cipher to use for encrypting the session
178in protocol version 1.
179Currently,
180.Dq blowfish ,
181.Dq 3des ,
182and
183.Dq des
184are supported.
185.Ar des
186is only supported in the
187.Xr ssh 1
188client for interoperability with legacy protocol 1 implementations
189that do not support the
190.Ar 3des
191cipher.
192Its use is strongly discouraged due to cryptographic weaknesses.
193The default is
194.Dq 3des .
195.It Cm Ciphers
196Specifies the ciphers allowed for protocol version 2
197in order of preference.
198Multiple ciphers must be comma-separated.
199The supported ciphers are
200.Dq 3des-cbc ,
201.Dq aes128-cbc ,
202.Dq aes192-cbc ,
203.Dq aes256-cbc ,
204.Dq aes128-ctr ,
205.Dq aes192-ctr ,
206.Dq aes256-ctr ,
207.Dq arcfour128 ,
208.Dq arcfour256 ,
209.Dq arcfour ,
210.Dq blowfish-cbc ,
211and
212.Dq cast128-cbc .
213The default is:
214.Bd -literal -offset 3n
cb5eb4f1
PA
215aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
216aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
217aes256-cbc,arcfour
16c343f1
PA
218.Ed
219.It Cm ClearAllForwardings
220Specifies that all local, remote, and dynamic port forwardings
221specified in the configuration files or on the command line be
222cleared.
223This option is primarily useful when used from the
224.Xr ssh 1
225command line to clear port forwardings set in
226configuration files, and is automatically set by
227.Xr scp 1
228and
229.Xr sftp 1 .
230The argument must be
231.Dq yes
232or
233.Dq no .
234The default is
235.Dq no .
236.It Cm Compression
237Specifies whether to use compression.
238The argument must be
239.Dq yes
240or
241.Dq no .
242The default is
243.Dq no .
244.It Cm CompressionLevel
245Specifies the compression level to use if compression is enabled.
246The argument must be an integer from 1 (fast) to 9 (slow, best).
247The default level is 6, which is good for most applications.
248The meaning of the values is the same as in
249.Xr gzip 1 .
250Note that this option applies to protocol version 1 only.
251.It Cm ConnectionAttempts
252Specifies the number of tries (one per second) to make before exiting.
253The argument must be an integer.
254This may be useful in scripts if the connection sometimes fails.
255The default is 1.
256.It Cm ConnectTimeout
257Specifies the timeout (in seconds) used when connecting to the
258SSH server, instead of using the default system TCP timeout.
259This value is used only when the target is down or really unreachable,
260not when it refuses the connection.
261.It Cm ControlMaster
262Enables the sharing of multiple sessions over a single network connection.
263When set to
264.Dq yes ,
265.Xr ssh 1
266will listen for connections on a control socket specified using the
267.Cm ControlPath
268argument.
269Additional sessions can connect to this socket using the same
270.Cm ControlPath
271with
272.Cm ControlMaster
273set to
274.Dq no
275(the default).
276These sessions will try to reuse the master instance's network connection
277rather than initiating new ones, but will fall back to connecting normally
278if the control socket does not exist, or is not listening.
279.Pp
280Setting this to
281.Dq ask
282will cause ssh
283to listen for control connections, but require confirmation using the
284.Ev SSH_ASKPASS
285program before they are accepted (see
286.Xr ssh-add 1
287for details).
288If the
289.Cm ControlPath
290cannot be opened,
291ssh will continue without connecting to a master instance.
292.Pp
293X11 and
294.Xr ssh-agent 1
295forwarding is supported over these multiplexed connections, however the
296display and agent forwarded will be the one belonging to the master
297connection i.e. it is not possible to forward multiple displays or agents.
298.Pp
299Two additional options allow for opportunistic multiplexing: try to use a
300master connection but fall back to creating a new one if one does not already
301exist.
302These options are:
303.Dq auto
304and
305.Dq autoask .
306The latter requires confirmation like the
307.Dq ask
308option.
309.It Cm ControlPath
310Specify the path to the control socket used for connection sharing as described
311in the
312.Cm ControlMaster
313section above or the string
314.Dq none
315to disable connection sharing.
316In the path,
1c188a7f
PA
317.Ql %L
318will be substituted by the first component of the local host name,
16c343f1 319.Ql %l
1c188a7f 320will be substituted by the local host name (including any domain name),
16c343f1
PA
321.Ql %h
322will be substituted by the target host name,
1c188a7f
PA
323.Ql %n
324will be substituted by the original target host name
325specified on the command line,
16c343f1 326.Ql %p
1c188a7f 327the port,
16c343f1 328.Ql %r
1c188a7f
PA
329by the remote login username, and
330.Ql %u
331by the username of the user running
332.Xr ssh 1 .
16c343f1
PA
333It is recommended that any
334.Cm ControlPath
335used for opportunistic connection sharing include
336at least %h, %p, and %r.
337This ensures that shared connections are uniquely identified.
856ea928
PA
338.It Cm ControlPersist
339When used in conjunction with
340.Cm ControlMaster ,
341specifies that the master connection should remain open
342in the background (waiting for future client connections)
343after the initial client connection has been closed.
344If set to
345.Dq no ,
346then the master connection will not be placed into the background,
347and will close as soon as the initial client connection is closed.
348If set to
349.Dq yes ,
350then the master connection will remain in the background indefinitely
351(until killed or closed via a mechanism such as the
352.Xr ssh 1
353.Dq Fl O No exit
354option).
355If set to a time in seconds, or a time in any of the formats documented in
356.Xr sshd_config 5 ,
357then the backgrounded master connection will automatically terminate
358after it has remained idle (with no client connections) for the
359specified time.
16c343f1
PA
360.It Cm DynamicForward
361Specifies that a TCP port on the local machine be forwarded
362over the secure channel, and the application
363protocol is then used to determine where to connect to from the
364remote machine.
365.Pp
366The argument must be
367.Sm off
368.Oo Ar bind_address : Oc Ar port .
369.Sm on
856ea928 370IPv6 addresses can be specified by enclosing addresses in square brackets.
16c343f1
PA
371By default, the local port is bound in accordance with the
372.Cm GatewayPorts
373setting.
374However, an explicit
375.Ar bind_address
376may be used to bind the connection to a specific address.
377The
378.Ar bind_address
379of
380.Dq localhost
381indicates that the listening port be bound for local use only, while an
382empty address or
383.Sq *
384indicates that the port should be available from all interfaces.
385.Pp
386Currently the SOCKS4 and SOCKS5 protocols are supported, and
387.Xr ssh 1
388will act as a SOCKS server.
389Multiple forwardings may be specified, and
390additional forwardings can be given on the command line.
391Only the superuser can forward privileged ports.
392.It Cm EnableSSHKeysign
393Setting this option to
394.Dq yes
395in the global client configuration file
396.Pa /etc/ssh/ssh_config
397enables the use of the helper program
398.Xr ssh-keysign 8
399during
400.Cm HostbasedAuthentication .
401The argument must be
402.Dq yes
403or
404.Dq no .
405The default is
406.Dq no .
407This option should be placed in the non-hostspecific section.
408See
409.Xr ssh-keysign 8
410for more information.
411.It Cm EscapeChar
412Sets the escape character (default:
413.Ql ~ ) .
414The escape character can also
415be set on the command line.
416The argument should be a single character,
417.Ql ^
418followed by a letter, or
419.Dq none
420to disable the escape
421character entirely (making the connection transparent for binary
422data).
423.It Cm ExitOnForwardFailure
424Specifies whether
425.Xr ssh 1
426should terminate the connection if it cannot set up all requested
427dynamic, tunnel, local, and remote port forwardings.
428The argument must be
429.Dq yes
430or
431.Dq no .
432The default is
433.Dq no .
434.It Cm ForwardAgent
435Specifies whether the connection to the authentication agent (if any)
436will be forwarded to the remote machine.
437The argument must be
438.Dq yes
439or
440.Dq no .
441The default is
442.Dq no .
443.Pp
444Agent forwarding should be enabled with caution.
445Users with the ability to bypass file permissions on the remote host
446(for the agent's Unix-domain socket)
447can access the local agent through the forwarded connection.
448An attacker cannot obtain key material from the agent,
449however they can perform operations on the keys that enable them to
450authenticate using the identities loaded into the agent.
451.It Cm ForwardX11
452Specifies whether X11 connections will be automatically redirected
453over the secure channel and
454.Ev DISPLAY
455set.
456The argument must be
457.Dq yes
458or
459.Dq no .
460The default is
461.Dq no .
462.Pp
463X11 forwarding should be enabled with caution.
464Users with the ability to bypass file permissions on the remote host
465(for the user's X11 authorization database)
466can access the local X11 display through the forwarded connection.
467An attacker may then be able to perform activities such as keystroke monitoring
468if the
469.Cm ForwardX11Trusted
470option is also enabled.
856ea928
PA
471.It Cm ForwardX11Timeout
472Specify a timeout for untrusted X11 forwarding
473using the format described in the
474.Sx TIME FORMATS
475section of
476.Xr sshd_config 5 .
477X11 connections received by
478.Xr ssh 1
479after this time will be refused.
480The default is to disable untrusted X11 forwarding after twenty minutes has
481elapsed.
16c343f1
PA
482.It Cm ForwardX11Trusted
483If this option is set to
484.Dq yes ,
485remote X11 clients will have full access to the original X11 display.
486.Pp
487If this option is set to
488.Dq no ,
489remote X11 clients will be considered untrusted and prevented
490from stealing or tampering with data belonging to trusted X11
491clients.
492Furthermore, the
493.Xr xauth 1
494token used for the session will be set to expire after 20 minutes.
495Remote clients will be refused access after this time.
496.Pp
497The default is
498.Dq no .
499.Pp
500See the X11 SECURITY extension specification for full details on
501the restrictions imposed on untrusted clients.
502.It Cm GatewayPorts
503Specifies whether remote hosts are allowed to connect to local
504forwarded ports.
505By default,
506.Xr ssh 1
507binds local port forwardings to the loopback address.
508This prevents other remote hosts from connecting to forwarded ports.
509.Cm GatewayPorts
510can be used to specify that ssh
511should bind local port forwardings to the wildcard address,
512thus allowing remote hosts to connect to forwarded ports.
513The argument must be
514.Dq yes
515or
516.Dq no .
517The default is
518.Dq no .
519.It Cm GlobalKnownHostsFile
1c188a7f
PA
520Specifies one or more files to use for the global
521host key database, separated by whitespace.
522The default is
523.Pa /etc/ssh/ssh_known_hosts ,
524.Pa /etc/ssh/ssh_known_hosts2 .
16c343f1
PA
525.It Cm GSSAPIAuthentication
526Specifies whether user authentication based on GSSAPI is allowed.
527The default is
528.Dq no .
529Note that this option applies to protocol version 2 only.
530.It Cm GSSAPIDelegateCredentials
531Forward (delegate) credentials to the server.
532The default is
533.Dq no .
534Note that this option applies to protocol version 2 only.
535.It Cm HashKnownHosts
536Indicates that
537.Xr ssh 1
538should hash host names and addresses when they are added to
539.Pa ~/.ssh/known_hosts .
540These hashed names may be used normally by
541.Xr ssh 1
542and
543.Xr sshd 8 ,
544but they do not reveal identifying information should the file's contents
545be disclosed.
546The default is
547.Dq no .
548Note that existing names and addresses in known hosts files
549will not be converted automatically,
550but may be manually hashed using
551.Xr ssh-keygen 1 .
552.It Cm HostbasedAuthentication
553Specifies whether to try rhosts based authentication with public key
554authentication.
555The argument must be
556.Dq yes
557or
558.Dq no .
559The default is
560.Dq no .
561This option applies to protocol version 2 only and
562is similar to
563.Cm RhostsRSAAuthentication .
564.It Cm HostKeyAlgorithms
565Specifies the protocol version 2 host key algorithms
566that the client wants to use in order of preference.
567The default for this option is:
9f304aaf
PA
568.Bd -literal -offset 3n
569ecdsa-sha2-nistp256-cert-v01@openssh.com,
570ecdsa-sha2-nistp384-cert-v01@openssh.com,
571ecdsa-sha2-nistp521-cert-v01@openssh.com,
572ssh-rsa-cert-v01@openssh.com,ssh-dss-cert-v01@openssh.com,
573ssh-rsa-cert-v00@openssh.com,ssh-dss-cert-v00@openssh.com,
574ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
575ssh-rsa,ssh-dss
576.Ed
577.Pp
578If hostkeys are known for the destination host then this default is modified
579to prefer their algorithms.
16c343f1
PA
580.It Cm HostKeyAlias
581Specifies an alias that should be used instead of the
582real host name when looking up or saving the host key
583in the host key database files.
584This option is useful for tunneling SSH connections
585or for multiple servers running on a single host.
586.It Cm HostName
587Specifies the real host name to log into.
588This can be used to specify nicknames or abbreviations for hosts.
856ea928
PA
589If the hostname contains the character sequence
590.Ql %h ,
1c188a7f 591then this will be replaced with the host name specified on the command line
856ea928 592(this is useful for manipulating unqualified names).
16c343f1
PA
593The default is the name given on the command line.
594Numeric IP addresses are also permitted (both on the command line and in
595.Cm HostName
596specifications).
597.It Cm IdentitiesOnly
598Specifies that
599.Xr ssh 1
600should only use the authentication identity files configured in the
601.Nm
602files,
603even if
604.Xr ssh-agent 1
605offers more identities.
606The argument to this keyword must be
607.Dq yes
608or
609.Dq no .
610This option is intended for situations where ssh-agent
611offers many different identities.
612The default is
613.Dq no .
614.It Cm IdentityFile
9f304aaf
PA
615Specifies a file from which the user's DSA, ECDSA or DSA authentication
616identity is read.
16c343f1
PA
617The default is
618.Pa ~/.ssh/identity
619for protocol version 1, and
9f304aaf
PA
620.Pa ~/.ssh/id_dsa ,
621.Pa ~/.ssh/id_ecdsa
16c343f1 622and
9f304aaf 623.Pa ~/.ssh/id_rsa
16c343f1
PA
624for protocol version 2.
625Additionally, any identities represented by the authentication agent
626will be used for authentication.
856ea928
PA
627.Xr ssh 1
628will try to load certificate information from the filename obtained by
629appending
630.Pa -cert.pub
631to the path of a specified
632.Cm IdentityFile .
16c343f1
PA
633.Pp
634The file name may use the tilde
635syntax to refer to a user's home directory or one of the following
636escape characters:
637.Ql %d
638(local user's home directory),
639.Ql %u
640(local user name),
641.Ql %l
642(local host name),
643.Ql %h
644(remote host name) or
645.Ql %r
646(remote user name).
647.Pp
648It is possible to have
649multiple identity files specified in configuration files; all these
650identities will be tried in sequence.
1c188a7f
PA
651Multiple
652.Cm IdentityFile
653directives will add to the list of identities tried (this behaviour
654differs from that of other configuration directives).
9f304aaf
PA
655.It Cm IPQoS
656Specifies the IPv4 type-of-service or DSCP class for connections.
657Accepted values are
658.Dq af11 ,
659.Dq af12 ,
660.Dq af13 ,
661.Dq af14 ,
662.Dq af22 ,
663.Dq af23 ,
664.Dq af31 ,
665.Dq af32 ,
666.Dq af33 ,
667.Dq af41 ,
668.Dq af42 ,
669.Dq af43 ,
670.Dq cs0 ,
671.Dq cs1 ,
672.Dq cs2 ,
673.Dq cs3 ,
674.Dq cs4 ,
675.Dq cs5 ,
676.Dq cs6 ,
677.Dq cs7 ,
678.Dq ef ,
679.Dq lowdelay ,
680.Dq throughput ,
681.Dq reliability ,
682or a numeric value.
683This option may take one or two arguments, separated by whitespace.
684If one argument is specified, it is used as the packet class unconditionally.
685If two values are specified, the first is automatically selected for
686interactive sessions and the second for non-interactive sessions.
687The default is
688.Dq lowdelay
689for interactive sessions and
690.Dq throughput
691for non-interactive sessions.
16c343f1
PA
692.It Cm KbdInteractiveAuthentication
693Specifies whether to use keyboard-interactive authentication.
694The argument to this keyword must be
695.Dq yes
696or
697.Dq no .
698The default is
699.Dq yes .
700.It Cm KbdInteractiveDevices
701Specifies the list of methods to use in keyboard-interactive authentication.
702Multiple method names must be comma-separated.
703The default is to use the server specified list.
704The methods available vary depending on what the server supports.
705For an OpenSSH server,
706it may be zero or more of:
707.Dq bsdauth ,
708.Dq pam ,
709and
710.Dq skey .
9f304aaf
PA
711.It Cm KexAlgorithms
712Specifies the available KEX (Key Exchange) algorithms.
713Multiple algorithms must be comma-separated.
714The default is:
715.Bd -literal -offset indent
716ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
717diffie-hellman-group-exchange-sha256,
718diffie-hellman-group-exchange-sha1,
719diffie-hellman-group14-sha1,
720diffie-hellman-group1-sha1
721.Ed
16c343f1
PA
722.It Cm LocalCommand
723Specifies a command to execute on the local machine after successfully
724connecting to the server.
725The command string extends to the end of the line, and is executed with
726the user's shell.
3b69e377
PA
727The following escape character substitutions will be performed:
728.Ql %d
729(local user's home directory),
730.Ql %h
731(remote host name),
732.Ql %l
733(local host name),
734.Ql %n
735(host name as provided on the command line),
736.Ql %p
737(remote port),
738.Ql %r
739(remote user name) or
740.Ql %u
741(local user name).
856ea928
PA
742.Pp
743The command is run synchronously and does not have access to the
744session of the
745.Xr ssh 1
746that spawned it.
747It should not be used for interactive commands.
748.Pp
16c343f1
PA
749This directive is ignored unless
750.Cm PermitLocalCommand
751has been enabled.
752.It Cm LocalForward
753Specifies that a TCP port on the local machine be forwarded over
754the secure channel to the specified host and port from the remote machine.
755The first argument must be
756.Sm off
757.Oo Ar bind_address : Oc Ar port
758.Sm on
759and the second argument must be
760.Ar host : Ns Ar hostport .
856ea928 761IPv6 addresses can be specified by enclosing addresses in square brackets.
16c343f1
PA
762Multiple forwardings may be specified, and additional forwardings can be
763given on the command line.
764Only the superuser can forward privileged ports.
765By default, the local port is bound in accordance with the
766.Cm GatewayPorts
767setting.
768However, an explicit
769.Ar bind_address
770may be used to bind the connection to a specific address.
771The
772.Ar bind_address
773of
774.Dq localhost
775indicates that the listening port be bound for local use only, while an
776empty address or
777.Sq *
778indicates that the port should be available from all interfaces.
779.It Cm LogLevel
780Gives the verbosity level that is used when logging messages from
781.Xr ssh 1 .
782The possible values are:
783QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
784The default is INFO.
785DEBUG and DEBUG1 are equivalent.
786DEBUG2 and DEBUG3 each specify higher levels of verbose output.
787.It Cm MACs
788Specifies the MAC (message authentication code) algorithms
789in order of preference.
790The MAC algorithm is used in protocol version 2
791for data integrity protection.
792Multiple algorithms must be comma-separated.
793The default is:
794.Bd -literal -offset indent
795hmac-md5,hmac-sha1,umac-64@openssh.com,
1c188a7f
PA
796hmac-ripemd160,hmac-sha1-96,hmac-md5-96,
797hmac-sha2-256,hmac-sha2-256-96,hmac-sha2-512,
798hmac-sha2-512-96
16c343f1
PA
799.Ed
800.It Cm NoHostAuthenticationForLocalhost
801This option can be used if the home directory is shared across machines.
802In this case localhost will refer to a different machine on each of
803the machines and the user will get many warnings about changed host keys.
804However, this option disables host authentication for localhost.
805The argument to this keyword must be
806.Dq yes
807or
808.Dq no .
809The default is to check the host key for localhost.
810.It Cm NumberOfPasswordPrompts
811Specifies the number of password prompts before giving up.
812The argument to this keyword must be an integer.
813The default is 3.
814.It Cm PasswordAuthentication
815Specifies whether to use password authentication.
816The argument to this keyword must be
817.Dq yes
818or
819.Dq no .
820The default is
821.Dq yes .
822.It Cm PermitLocalCommand
823Allow local command execution via the
824.Ic LocalCommand
825option or using the
826.Ic !\& Ns Ar command
827escape sequence in
828.Xr ssh 1 .
829The argument must be
830.Dq yes
831or
832.Dq no .
833The default is
834.Dq no .
856ea928
PA
835.It Cm PKCS11Provider
836Specifies which PKCS#11 provider to use.
9f304aaf 837The argument to this keyword is the PKCS#11 shared library
856ea928
PA
838.Xr ssh 1
839should use to communicate with a PKCS#11 token providing the user's
840private RSA key.
16c343f1
PA
841.It Cm Port
842Specifies the port number to connect on the remote host.
843The default is 22.
844.It Cm PreferredAuthentications
845Specifies the order in which the client should try protocol 2
846authentication methods.
847This allows a client to prefer one method (e.g.\&
848.Cm keyboard-interactive )
849over another method (e.g.\&
856ea928
PA
850.Cm password ) .
851The default is:
852.Bd -literal -offset indent
853gssapi-with-mic,hostbased,publickey,
854keyboard-interactive,password
855.Ed
16c343f1
PA
856.It Cm Protocol
857Specifies the protocol versions
858.Xr ssh 1
859should support in order of preference.
860The possible values are
861.Sq 1
862and
863.Sq 2 .
864Multiple versions must be comma-separated.
856ea928
PA
865When this option is set to
866.Dq 2,1
867.Nm ssh
868will try version 2 and fall back to version 1
16c343f1 869if version 2 is not available.
856ea928
PA
870The default is
871.Sq 2 .
16c343f1
PA
872.It Cm ProxyCommand
873Specifies the command to use to connect to the server.
874The command
875string extends to the end of the line, and is executed with
876the user's shell.
856ea928 877In the command string, any occurrence of
16c343f1
PA
878.Ql %h
879will be substituted by the host name to
856ea928 880connect,
16c343f1 881.Ql %p
856ea928
PA
882by the port, and
883.Ql %r
884by the remote user name.
16c343f1
PA
885The command can be basically anything,
886and should read from its standard input and write to its standard output.
887It should eventually connect an
888.Xr sshd 8
889server running on some machine, or execute
890.Ic sshd -i
891somewhere.
892Host key management will be done using the
893HostName of the host being connected (defaulting to the name typed by
894the user).
895Setting the command to
896.Dq none
897disables this option entirely.
898Note that
899.Cm CheckHostIP
900is not available for connects with a proxy command.
901.Pp
902This directive is useful in conjunction with
903.Xr nc 1
904and its proxy support.
905For example, the following directive would connect via an HTTP proxy at
906192.0.2.0:
907.Bd -literal -offset 3n
908ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
909.Ed
910.It Cm PubkeyAuthentication
911Specifies whether to try public key authentication.
912The argument to this keyword must be
913.Dq yes
914or
915.Dq no .
916The default is
917.Dq yes .
918This option applies to protocol version 2 only.
919.It Cm RekeyLimit
920Specifies the maximum amount of data that may be transmitted before the
921session key is renegotiated.
922The argument is the number of bytes, with an optional suffix of
923.Sq K ,
924.Sq M ,
925or
926.Sq G
927to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
928The default is between
929.Sq 1G
930and
931.Sq 4G ,
932depending on the cipher.
933This option applies to protocol version 2 only.
934.It Cm RemoteForward
935Specifies that a TCP port on the remote machine be forwarded over
936the secure channel to the specified host and port from the local machine.
937The first argument must be
938.Sm off
939.Oo Ar bind_address : Oc Ar port
940.Sm on
941and the second argument must be
942.Ar host : Ns Ar hostport .
856ea928 943IPv6 addresses can be specified by enclosing addresses in square brackets.
16c343f1
PA
944Multiple forwardings may be specified, and additional
945forwardings can be given on the command line.
cb5eb4f1
PA
946Privileged ports can be forwarded only when
947logging in as root on the remote machine.
948.Pp
949If the
950.Ar port
951argument is
952.Ql 0 ,
953the listen port will be dynamically allocated on the server and reported
954to the client at run time.
16c343f1
PA
955.Pp
956If the
957.Ar bind_address
958is not specified, the default is to only bind to loopback addresses.
959If the
960.Ar bind_address
961is
962.Ql *
963or an empty string, then the forwarding is requested to listen on all
964interfaces.
965Specifying a remote
966.Ar bind_address
967will only succeed if the server's
968.Cm GatewayPorts
969option is enabled (see
970.Xr sshd_config 5 ) .
1c188a7f
PA
971.It Cm RequestTTY
972Specifies whether to request a pseudo-tty for the session.
973The argument may be one of:
974.Dq no
975(never request a TTY),
976.Dq yes
977(always request a TTY when standard input is a TTY),
978.Dq force
979(always request a TTY) or
980.Dq auto
981(request a TTY when opening a login session).
982This option mirrors the
983.Fl t
984and
985.Fl T
986flags for
987.Xr ssh 1 .
16c343f1
PA
988.It Cm RhostsRSAAuthentication
989Specifies whether to try rhosts based authentication with RSA host
990authentication.
991The argument must be
992.Dq yes
993or
994.Dq no .
995The default is
996.Dq no .
997This option applies to protocol version 1 only and requires
998.Xr ssh 1
999to be setuid root.
1000.It Cm RSAAuthentication
1001Specifies whether to try RSA authentication.
1002The argument to this keyword must be
1003.Dq yes
1004or
1005.Dq no .
1006RSA authentication will only be
1007attempted if the identity file exists, or an authentication agent is
1008running.
1009The default is
1010.Dq yes .
1011Note that this option applies to protocol version 1 only.
1012.It Cm SendEnv
1013Specifies what variables from the local
1014.Xr environ 7
1015should be sent to the server.
1016Note that environment passing is only supported for protocol 2.
1017The server must also support it, and the server must be configured to
1018accept these environment variables.
1019Refer to
1020.Cm AcceptEnv
1021in
1022.Xr sshd_config 5
1023for how to configure the server.
1024Variables are specified by name, which may contain wildcard characters.
1025Multiple environment variables may be separated by whitespace or spread
1026across multiple
1027.Cm SendEnv
1028directives.
1029The default is not to send any environment variables.
1030.Pp
1031See
1032.Sx PATTERNS
1033for more information on patterns.
1034.It Cm ServerAliveCountMax
1035Sets the number of server alive messages (see below) which may be
1036sent without
1037.Xr ssh 1
1038receiving any messages back from the server.
1039If this threshold is reached while server alive messages are being sent,
1040ssh will disconnect from the server, terminating the session.
1041It is important to note that the use of server alive messages is very
1042different from
1043.Cm TCPKeepAlive
1044(below).
1045The server alive messages are sent through the encrypted channel
1046and therefore will not be spoofable.
1047The TCP keepalive option enabled by
1048.Cm TCPKeepAlive
1049is spoofable.
1050The server alive mechanism is valuable when the client or
1051server depend on knowing when a connection has become inactive.
1052.Pp
1053The default value is 3.
1054If, for example,
1055.Cm ServerAliveInterval
1056(see below) is set to 15 and
1057.Cm ServerAliveCountMax
1058is left at the default, if the server becomes unresponsive,
1059ssh will disconnect after approximately 45 seconds.
1060This option applies to protocol version 2 only.
1061.It Cm ServerAliveInterval
1062Sets a timeout interval in seconds after which if no data has been received
1063from the server,
1064.Xr ssh 1
1065will send a message through the encrypted
1066channel to request a response from the server.
1067The default
1068is 0, indicating that these messages will not be sent to the server.
1069This option applies to protocol version 2 only.
16c343f1
PA
1070.It Cm StrictHostKeyChecking
1071If this flag is set to
1072.Dq yes ,
1073.Xr ssh 1
1074will never automatically add host keys to the
1075.Pa ~/.ssh/known_hosts
1076file, and refuses to connect to hosts whose host key has changed.
1077This provides maximum protection against trojan horse attacks,
1078though it can be annoying when the
1079.Pa /etc/ssh/ssh_known_hosts
1080file is poorly maintained or when connections to new hosts are
1081frequently made.
1082This option forces the user to manually
1083add all new hosts.
1084If this flag is set to
1085.Dq no ,
1086ssh will automatically add new host keys to the
1087user known hosts files.
1088If this flag is set to
1089.Dq ask ,
1090new host keys
1091will be added to the user known host files only after the user
1092has confirmed that is what they really want to do, and
1093ssh will refuse to connect to hosts whose host key has changed.
1094The host keys of
1095known hosts will be verified automatically in all cases.
1096The argument must be
1097.Dq yes ,
1098.Dq no ,
1099or
1100.Dq ask .
1101The default is
1102.Dq ask .
1103.It Cm TCPKeepAlive
1104Specifies whether the system should send TCP keepalive messages to the
1105other side.
1106If they are sent, death of the connection or crash of one
1107of the machines will be properly noticed.
1108However, this means that
1109connections will die if the route is down temporarily, and some people
1110find it annoying.
1111.Pp
1112The default is
1113.Dq yes
1114(to send TCP keepalive messages), and the client will notice
1115if the network goes down or the remote host dies.
1116This is important in scripts, and many users want it too.
1117.Pp
1118To disable TCP keepalive messages, the value should be set to
1119.Dq no .
1120.It Cm Tunnel
1121Request
1122.Xr tun 4
1123device forwarding between the client and the server.
1124The argument must be
1125.Dq yes ,
1126.Dq point-to-point
1127(layer 3),
1128.Dq ethernet
1129(layer 2),
1130or
1131.Dq no .
1132Specifying
1133.Dq yes
1134requests the default tunnel mode, which is
1135.Dq point-to-point .
1136The default is
1137.Dq no .
1138.It Cm TunnelDevice
1139Specifies the
1140.Xr tun 4
1141devices to open on the client
1142.Pq Ar local_tun
1143and the server
1144.Pq Ar remote_tun .
1145.Pp
1146The argument must be
1147.Sm off
1148.Ar local_tun Op : Ar remote_tun .
1149.Sm on
1150The devices may be specified by numerical ID or the keyword
1151.Dq any ,
1152which uses the next available tunnel device.
1153If
1154.Ar remote_tun
1155is not specified, it defaults to
1156.Dq any .
1157The default is
1158.Dq any:any .
1159.It Cm UsePrivilegedPort
1160Specifies whether to use a privileged port for outgoing connections.
1161The argument must be
1162.Dq yes
1163or
1164.Dq no .
1165The default is
1166.Dq no .
1167If set to
1168.Dq yes ,
1169.Xr ssh 1
1170must be setuid root.
1171Note that this option must be set to
1172.Dq yes
1173for
1174.Cm RhostsRSAAuthentication
1175with older servers.
1176.It Cm User
1177Specifies the user to log in as.
1178This can be useful when a different user name is used on different machines.
1179This saves the trouble of
1180having to remember to give the user name on the command line.
1181.It Cm UserKnownHostsFile
1c188a7f
PA
1182Specifies one or more files to use for the user
1183host key database, separated by whitespace.
1184The default is
1185.Pa ~/.ssh/known_hosts ,
1186.Pa ~/.ssh/known_hosts2 .
16c343f1
PA
1187.It Cm VerifyHostKeyDNS
1188Specifies whether to verify the remote key using DNS and SSHFP resource
1189records.
1190If this option is set to
1191.Dq yes ,
1192the client will implicitly trust keys that match a secure fingerprint
1193from DNS.
1194Insecure fingerprints will be handled as if this option was set to
1195.Dq ask .
1196If this option is set to
1197.Dq ask ,
1198information on fingerprint match will be displayed, but the user will still
1199need to confirm new host keys according to the
1200.Cm StrictHostKeyChecking
1201option.
1202The argument must be
1203.Dq yes ,
1204.Dq no ,
1205or
1206.Dq ask .
1207The default is
1208.Dq no .
1209Note that this option applies to protocol version 2 only.
1210.Pp
1211See also
1212.Sx VERIFYING HOST KEYS
1213in
1214.Xr ssh 1 .
3b69e377
PA
1215.It Cm VisualHostKey
1216If this flag is set to
1217.Dq yes ,
1218an ASCII art representation of the remote host key fingerprint is
cb5eb4f1
PA
1219printed in addition to the hex fingerprint string at login and
1220for unknown host keys.
3b69e377
PA
1221If this flag is set to
1222.Dq no ,
cb5eb4f1
PA
1223no fingerprint strings are printed at login and
1224only the hex fingerprint string will be printed for unknown host keys.
3b69e377
PA
1225The default is
1226.Dq no .
16c343f1
PA
1227.It Cm XAuthLocation
1228Specifies the full pathname of the
1229.Xr xauth 1
1230program.
1231The default is
1232.Pa /usr/X11R6/bin/xauth .
1233.El
1234.Sh PATTERNS
1235A
1236.Em pattern
1237consists of zero or more non-whitespace characters,
1238.Sq *
1239(a wildcard that matches zero or more characters),
1240or
1241.Sq ?\&
1242(a wildcard that matches exactly one character).
1243For example, to specify a set of declarations for any host in the
1244.Dq .co.uk
1245set of domains,
1246the following pattern could be used:
1247.Pp
1248.Dl Host *.co.uk
1249.Pp
1250The following pattern
1251would match any host in the 192.168.0.[0-9] network range:
1252.Pp
1253.Dl Host 192.168.0.?
1254.Pp
1255A
1256.Em pattern-list
1257is a comma-separated list of patterns.
1258Patterns within pattern-lists may be negated
1259by preceding them with an exclamation mark
1260.Pq Sq !\& .
1261For example,
1262to allow a key to be used from anywhere within an organisation
1263except from the
1264.Dq dialup
1265pool,
1266the following entry (in authorized_keys) could be used:
1267.Pp
1268.Dl from=\&"!*.dialup.example.com,*.example.com\&"
1269.Sh FILES
1270.Bl -tag -width Ds
1271.It Pa ~/.ssh/config
1272This is the per-user configuration file.
1273The format of this file is described above.
1274This file is used by the SSH client.
1275Because of the potential for abuse, this file must have strict permissions:
1276read/write for the user, and not accessible by others.
1277.It Pa /etc/ssh/ssh_config
1278Systemwide configuration file.
1279This file provides defaults for those
1280values that are not specified in the user's configuration file, and
1281for those users who do not have a configuration file.
1282This file must be world-readable.
1283.El
1284.Sh SEE ALSO
1285.Xr ssh 1
1286.Sh AUTHORS
1287OpenSSH is a derivative of the original and free
1288ssh 1.2.12 release by Tatu Ylonen.
1289Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1290Theo de Raadt and Dug Song
1291removed many bugs, re-added newer features and
1292created OpenSSH.
1293Markus Friedl contributed the support for SSH
1294protocol versions 1.5 and 2.0.