Annotation of src/usr.bin/ssh/ssh_config.5, Revision 1.134
1.1 stevesk 1: .\" -*- nroff -*-
2: .\"
3: .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
4: .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
5: .\" All rights reserved
6: .\"
7: .\" As far as I am concerned, the code I have written for this software
8: .\" can be used freely for any purpose. Any derived versions of this
9: .\" software must be clearly marked as such, and if the derived work is
10: .\" incompatible with the protocol description in the RFC file, it must be
11: .\" called by a name other than "ssh" or "Secure Shell".
12: .\"
13: .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
14: .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
15: .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
16: .\"
17: .\" Redistribution and use in source and binary forms, with or without
18: .\" modification, are permitted provided that the following conditions
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20: .\" 1. Redistributions of source code must retain the above copyright
21: .\" notice, this list of conditions and the following disclaimer.
22: .\" 2. Redistributions in binary form must reproduce the above copyright
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26: .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
27: .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
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1.134 ! djm 37: .\" $OpenBSD: ssh_config.5,v 1.133 2010/04/16 06:45:01 jmc Exp $
! 38: .Dd $Mdocdate: April 16 2010 $
1.1 stevesk 39: .Dt SSH_CONFIG 5
40: .Os
41: .Sh NAME
42: .Nm ssh_config
43: .Nd OpenSSH SSH client configuration files
44: .Sh SYNOPSIS
1.98 jmc 45: .Nm ~/.ssh/config
46: .Nm /etc/ssh/ssh_config
1.1 stevesk 47: .Sh DESCRIPTION
1.84 jmc 48: .Xr ssh 1
1.1 stevesk 49: obtains configuration data from the following sources in
50: the following order:
1.79 jmc 51: .Pp
1.2 stevesk 52: .Bl -enum -offset indent -compact
53: .It
54: command-line options
55: .It
56: user's configuration file
1.50 djm 57: .Pq Pa ~/.ssh/config
1.2 stevesk 58: .It
59: system-wide configuration file
60: .Pq Pa /etc/ssh/ssh_config
61: .El
1.1 stevesk 62: .Pp
63: For each parameter, the first obtained value
64: will be used.
1.41 jmc 65: The configuration files contain sections separated by
1.1 stevesk 66: .Dq Host
67: specifications, and that section is only applied for hosts that
68: match one of the patterns given in the specification.
69: The matched host name is the one given on the command line.
70: .Pp
71: Since the first obtained value for each parameter is used, more
72: host-specific declarations should be given near the beginning of the
73: file, and general defaults at the end.
1.80 jmc 74: .Pp
1.1 stevesk 75: The configuration file has the following format:
76: .Pp
77: Empty lines and lines starting with
78: .Ql #
79: are comments.
80: Otherwise a line is of the format
81: .Dq keyword arguments .
82: Configuration options may be separated by whitespace or
83: optional whitespace and exactly one
84: .Ql = ;
85: the latter format is useful to avoid the need to quote whitespace
86: when specifying configuration options using the
87: .Nm ssh ,
1.87 jmc 88: .Nm scp ,
1.1 stevesk 89: and
90: .Nm sftp
91: .Fl o
92: option.
1.88 dtucker 93: Arguments may optionally be enclosed in double quotes
94: .Pq \&"
95: in order to represent arguments containing spaces.
1.1 stevesk 96: .Pp
97: The possible
98: keywords and their meanings are as follows (note that
99: keywords are case-insensitive and arguments are case-sensitive):
100: .Bl -tag -width Ds
101: .It Cm Host
102: Restricts the following declarations (up to the next
103: .Cm Host
104: keyword) to be only for those hosts that match one of the patterns
105: given after the keyword.
1.112 krw 106: If more than one pattern is provided, they should be separated by whitespace.
1.1 stevesk 107: A single
1.83 jmc 108: .Ql *
1.1 stevesk 109: as a pattern can be used to provide global
110: defaults for all hosts.
111: The host is the
112: .Ar hostname
1.83 jmc 113: argument given on the command line (i.e. the name is not converted to
1.1 stevesk 114: a canonicalized host name before matching).
1.81 jmc 115: .Pp
116: See
117: .Sx PATTERNS
118: for more information on patterns.
1.10 djm 119: .It Cm AddressFamily
1.11 jmc 120: Specifies which address family to use when connecting.
121: Valid arguments are
1.10 djm 122: .Dq any ,
123: .Dq inet
1.84 jmc 124: (use IPv4 only), or
1.10 djm 125: .Dq inet6
1.40 jmc 126: (use IPv6 only).
1.1 stevesk 127: .It Cm BatchMode
128: If set to
129: .Dq yes ,
130: passphrase/password querying will be disabled.
131: This option is useful in scripts and other batch jobs where no user
132: is present to supply the password.
133: The argument must be
134: .Dq yes
135: or
136: .Dq no .
137: The default is
138: .Dq no .
139: .It Cm BindAddress
1.60 dtucker 140: Use the specified address on the local machine as the source address of
1.61 jmc 141: the connection.
142: Only useful on systems with more than one address.
1.1 stevesk 143: Note that this option does not work if
144: .Cm UsePrivilegedPort
145: is set to
146: .Dq yes .
147: .It Cm ChallengeResponseAuthentication
1.82 jmc 148: Specifies whether to use challenge-response authentication.
1.1 stevesk 149: The argument to this keyword must be
150: .Dq yes
151: or
152: .Dq no .
153: The default is
154: .Dq yes .
155: .It Cm CheckHostIP
156: If this flag is set to
157: .Dq yes ,
1.84 jmc 158: .Xr ssh 1
159: will additionally check the host IP address in the
1.1 stevesk 160: .Pa known_hosts
161: file.
162: This allows ssh to detect if a host key changed due to DNS spoofing.
1.107 grunk 163: If the option is set to
1.1 stevesk 164: .Dq no ,
165: the check will not be executed.
166: The default is
167: .Dq yes .
168: .It Cm Cipher
169: Specifies the cipher to use for encrypting the session
170: in protocol version 1.
171: Currently,
172: .Dq blowfish ,
173: .Dq 3des ,
174: and
175: .Dq des
176: are supported.
177: .Ar des
178: is only supported in the
1.84 jmc 179: .Xr ssh 1
1.1 stevesk 180: client for interoperability with legacy protocol 1 implementations
181: that do not support the
182: .Ar 3des
1.7 jmc 183: cipher.
184: Its use is strongly discouraged due to cryptographic weaknesses.
1.1 stevesk 185: The default is
186: .Dq 3des .
187: .It Cm Ciphers
188: Specifies the ciphers allowed for protocol version 2
189: in order of preference.
190: Multiple ciphers must be comma-separated.
1.35 dtucker 191: The supported ciphers are
192: .Dq 3des-cbc ,
193: .Dq aes128-cbc ,
194: .Dq aes192-cbc ,
195: .Dq aes256-cbc ,
196: .Dq aes128-ctr ,
197: .Dq aes192-ctr ,
198: .Dq aes256-ctr ,
1.54 djm 199: .Dq arcfour128 ,
200: .Dq arcfour256 ,
1.35 dtucker 201: .Dq arcfour ,
202: .Dq blowfish-cbc ,
203: and
204: .Dq cast128-cbc .
1.84 jmc 205: The default is:
206: .Bd -literal -offset 3n
1.116 naddy 207: aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
208: aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
209: aes256-cbc,arcfour
1.1 stevesk 210: .Ed
211: .It Cm ClearAllForwardings
1.84 jmc 212: Specifies that all local, remote, and dynamic port forwardings
1.1 stevesk 213: specified in the configuration files or on the command line be
1.7 jmc 214: cleared.
215: This option is primarily useful when used from the
1.84 jmc 216: .Xr ssh 1
1.1 stevesk 217: command line to clear port forwardings set in
218: configuration files, and is automatically set by
219: .Xr scp 1
220: and
221: .Xr sftp 1 .
222: The argument must be
223: .Dq yes
224: or
225: .Dq no .
226: The default is
227: .Dq no .
228: .It Cm Compression
229: Specifies whether to use compression.
230: The argument must be
231: .Dq yes
232: or
233: .Dq no .
234: The default is
235: .Dq no .
236: .It Cm CompressionLevel
237: Specifies the compression level to use if compression is enabled.
238: The argument must be an integer from 1 (fast) to 9 (slow, best).
239: The default level is 6, which is good for most applications.
240: The meaning of the values is the same as in
241: .Xr gzip 1 .
242: Note that this option applies to protocol version 1 only.
243: .It Cm ConnectionAttempts
244: Specifies the number of tries (one per second) to make before exiting.
245: The argument must be an integer.
246: This may be useful in scripts if the connection sometimes fails.
247: The default is 1.
1.9 djm 248: .It Cm ConnectTimeout
1.84 jmc 249: Specifies the timeout (in seconds) used when connecting to the
250: SSH server, instead of using the default system TCP timeout.
1.11 jmc 251: This value is used only when the target is down or really unreachable,
252: not when it refuses the connection.
1.36 djm 253: .It Cm ControlMaster
254: Enables the sharing of multiple sessions over a single network connection.
255: When set to
1.84 jmc 256: .Dq yes ,
257: .Xr ssh 1
1.36 djm 258: will listen for connections on a control socket specified using the
259: .Cm ControlPath
260: argument.
261: Additional sessions can connect to this socket using the same
262: .Cm ControlPath
263: with
264: .Cm ControlMaster
265: set to
266: .Dq no
1.38 jmc 267: (the default).
1.64 jmc 268: These sessions will try to reuse the master instance's network connection
1.63 djm 269: rather than initiating new ones, but will fall back to connecting normally
270: if the control socket does not exist, or is not listening.
271: .Pp
1.37 djm 272: Setting this to
273: .Dq ask
1.84 jmc 274: will cause ssh
1.37 djm 275: to listen for control connections, but require confirmation using the
276: .Ev SSH_ASKPASS
277: program before they are accepted (see
278: .Xr ssh-add 1
1.38 jmc 279: for details).
1.51 jakob 280: If the
281: .Cm ControlPath
1.84 jmc 282: cannot be opened,
283: ssh will continue without connecting to a master instance.
1.58 djm 284: .Pp
285: X11 and
1.59 jmc 286: .Xr ssh-agent 1
1.58 djm 287: forwarding is supported over these multiplexed connections, however the
1.70 stevesk 288: display and agent forwarded will be the one belonging to the master
1.59 jmc 289: connection i.e. it is not possible to forward multiple displays or agents.
1.56 djm 290: .Pp
291: Two additional options allow for opportunistic multiplexing: try to use a
292: master connection but fall back to creating a new one if one does not already
293: exist.
294: These options are:
295: .Dq auto
296: and
297: .Dq autoask .
298: The latter requires confirmation like the
299: .Dq ask
300: option.
1.36 djm 301: .It Cm ControlPath
1.55 djm 302: Specify the path to the control socket used for connection sharing as described
303: in the
1.36 djm 304: .Cm ControlMaster
1.57 djm 305: section above or the string
306: .Dq none
307: to disable connection sharing.
1.55 djm 308: In the path,
1.77 djm 309: .Ql %l
310: will be substituted by the local host name,
1.55 djm 311: .Ql %h
312: will be substituted by the target host name,
313: .Ql %p
1.84 jmc 314: the port, and
1.55 djm 315: .Ql %r
316: by the remote login username.
1.56 djm 317: It is recommended that any
318: .Cm ControlPath
319: used for opportunistic connection sharing include
1.78 jmc 320: at least %h, %p, and %r.
1.56 djm 321: This ensures that shared connections are uniquely identified.
1.38 jmc 322: .It Cm DynamicForward
1.74 jmc 323: Specifies that a TCP port on the local machine be forwarded
1.38 jmc 324: over the secure channel, and the application
325: protocol is then used to determine where to connect to from the
326: remote machine.
1.62 djm 327: .Pp
328: The argument must be
329: .Sm off
330: .Oo Ar bind_address : Oc Ar port .
331: .Sm on
332: IPv6 addresses can be specified by enclosing addresses in square brackets or
333: by using an alternative syntax:
334: .Oo Ar bind_address Ns / Oc Ns Ar port .
335: By default, the local port is bound in accordance with the
336: .Cm GatewayPorts
337: setting.
338: However, an explicit
339: .Ar bind_address
340: may be used to bind the connection to a specific address.
341: The
342: .Ar bind_address
343: of
344: .Dq localhost
345: indicates that the listening port be bound for local use only, while an
346: empty address or
347: .Sq *
348: indicates that the port should be available from all interfaces.
349: .Pp
1.38 jmc 350: Currently the SOCKS4 and SOCKS5 protocols are supported, and
1.84 jmc 351: .Xr ssh 1
1.38 jmc 352: will act as a SOCKS server.
353: Multiple forwardings may be specified, and
354: additional forwardings can be given on the command line.
355: Only the superuser can forward privileged ports.
1.14 markus 356: .It Cm EnableSSHKeysign
357: Setting this option to
358: .Dq yes
359: in the global client configuration file
360: .Pa /etc/ssh/ssh_config
361: enables the use of the helper program
362: .Xr ssh-keysign 8
363: during
364: .Cm HostbasedAuthentication .
365: The argument must be
366: .Dq yes
367: or
368: .Dq no .
369: The default is
370: .Dq no .
1.23 jmc 371: This option should be placed in the non-hostspecific section.
1.14 markus 372: See
373: .Xr ssh-keysign 8
374: for more information.
1.1 stevesk 375: .It Cm EscapeChar
376: Sets the escape character (default:
377: .Ql ~ ) .
378: The escape character can also
379: be set on the command line.
380: The argument should be a single character,
381: .Ql ^
382: followed by a letter, or
383: .Dq none
384: to disable the escape
385: character entirely (making the connection transparent for binary
386: data).
1.96 markus 387: .It Cm ExitOnForwardFailure
388: Specifies whether
389: .Xr ssh 1
390: should terminate the connection if it cannot set up all requested
1.102 stevesk 391: dynamic, tunnel, local, and remote port forwardings.
1.96 markus 392: The argument must be
393: .Dq yes
394: or
395: .Dq no .
396: The default is
397: .Dq no .
1.1 stevesk 398: .It Cm ForwardAgent
399: Specifies whether the connection to the authentication agent (if any)
400: will be forwarded to the remote machine.
401: The argument must be
402: .Dq yes
403: or
404: .Dq no .
405: The default is
406: .Dq no .
1.3 stevesk 407: .Pp
1.7 jmc 408: Agent forwarding should be enabled with caution.
409: Users with the ability to bypass file permissions on the remote host
410: (for the agent's Unix-domain socket)
411: can access the local agent through the forwarded connection.
412: An attacker cannot obtain key material from the agent,
1.3 stevesk 413: however they can perform operations on the keys that enable them to
414: authenticate using the identities loaded into the agent.
1.1 stevesk 415: .It Cm ForwardX11
416: Specifies whether X11 connections will be automatically redirected
417: over the secure channel and
418: .Ev DISPLAY
419: set.
420: The argument must be
421: .Dq yes
422: or
423: .Dq no .
424: The default is
425: .Dq no .
1.3 stevesk 426: .Pp
1.7 jmc 427: X11 forwarding should be enabled with caution.
428: Users with the ability to bypass file permissions on the remote host
1.22 markus 429: (for the user's X11 authorization database)
1.7 jmc 430: can access the local X11 display through the forwarded connection.
1.22 markus 431: An attacker may then be able to perform activities such as keystroke monitoring
432: if the
433: .Cm ForwardX11Trusted
434: option is also enabled.
1.134 ! djm 435: .It Cm ForwardX11Timeout
! 436: Specify a timeout for untrusted X11 forwarding using the format described in
! 437: .Sx TIME FORMATS
! 438: section of
! 439: .Xr sshd_config 5 .
! 440: X11 connections received by
! 441: .Xr ssh 1
! 442: after this time will be refused.
! 443: The default is to disable untrusted X11 forwarding after twenty minutes has
! 444: elapsed.
1.22 markus 445: .It Cm ForwardX11Trusted
1.34 jmc 446: If this option is set to
1.84 jmc 447: .Dq yes ,
448: remote X11 clients will have full access to the original X11 display.
1.42 djm 449: .Pp
1.22 markus 450: If this option is set to
1.84 jmc 451: .Dq no ,
452: remote X11 clients will be considered untrusted and prevented
1.22 markus 453: from stealing or tampering with data belonging to trusted X11
454: clients.
1.42 djm 455: Furthermore, the
456: .Xr xauth 1
457: token used for the session will be set to expire after 20 minutes.
458: Remote clients will be refused access after this time.
1.22 markus 459: .Pp
460: The default is
461: .Dq no .
462: .Pp
463: See the X11 SECURITY extension specification for full details on
464: the restrictions imposed on untrusted clients.
1.1 stevesk 465: .It Cm GatewayPorts
466: Specifies whether remote hosts are allowed to connect to local
467: forwarded ports.
468: By default,
1.84 jmc 469: .Xr ssh 1
1.7 jmc 470: binds local port forwardings to the loopback address.
471: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 472: .Cm GatewayPorts
1.84 jmc 473: can be used to specify that ssh
1.1 stevesk 474: should bind local port forwardings to the wildcard address,
475: thus allowing remote hosts to connect to forwarded ports.
476: The argument must be
477: .Dq yes
478: or
479: .Dq no .
480: The default is
481: .Dq no .
482: .It Cm GlobalKnownHostsFile
483: Specifies a file to use for the global
484: host key database instead of
485: .Pa /etc/ssh/ssh_known_hosts .
1.18 markus 486: .It Cm GSSAPIAuthentication
1.27 markus 487: Specifies whether user authentication based on GSSAPI is allowed.
1.20 jmc 488: The default is
1.21 markus 489: .Dq no .
1.18 markus 490: Note that this option applies to protocol version 2 only.
491: .It Cm GSSAPIDelegateCredentials
492: Forward (delegate) credentials to the server.
493: The default is
494: .Dq no .
495: Note that this option applies to protocol version 2 only.
1.44 djm 496: .It Cm HashKnownHosts
497: Indicates that
1.84 jmc 498: .Xr ssh 1
1.44 djm 499: should hash host names and addresses when they are added to
1.50 djm 500: .Pa ~/.ssh/known_hosts .
1.44 djm 501: These hashed names may be used normally by
1.84 jmc 502: .Xr ssh 1
1.44 djm 503: and
1.84 jmc 504: .Xr sshd 8 ,
1.44 djm 505: but they do not reveal identifying information should the file's contents
506: be disclosed.
507: The default is
508: .Dq no .
1.97 jmc 509: Note that existing names and addresses in known hosts files
510: will not be converted automatically,
511: but may be manually hashed using
1.45 djm 512: .Xr ssh-keygen 1 .
1.1 stevesk 513: .It Cm HostbasedAuthentication
514: Specifies whether to try rhosts based authentication with public key
515: authentication.
516: The argument must be
517: .Dq yes
518: or
519: .Dq no .
520: The default is
521: .Dq no .
522: This option applies to protocol version 2 only and
523: is similar to
524: .Cm RhostsRSAAuthentication .
525: .It Cm HostKeyAlgorithms
526: Specifies the protocol version 2 host key algorithms
527: that the client wants to use in order of preference.
528: The default for this option is:
529: .Dq ssh-rsa,ssh-dss .
530: .It Cm HostKeyAlias
531: Specifies an alias that should be used instead of the
532: real host name when looking up or saving the host key
533: in the host key database files.
1.84 jmc 534: This option is useful for tunneling SSH connections
1.1 stevesk 535: or for multiple servers running on a single host.
536: .It Cm HostName
537: Specifies the real host name to log into.
538: This can be used to specify nicknames or abbreviations for hosts.
1.84 jmc 539: The default is the name given on the command line.
1.1 stevesk 540: Numeric IP addresses are also permitted (both on the command line and in
541: .Cm HostName
542: specifications).
1.29 markus 543: .It Cm IdentitiesOnly
544: Specifies that
1.84 jmc 545: .Xr ssh 1
1.29 markus 546: should only use the authentication identity files configured in the
1.31 jmc 547: .Nm
1.29 markus 548: files,
1.84 jmc 549: even if
550: .Xr ssh-agent 1
1.29 markus 551: offers more identities.
552: The argument to this keyword must be
553: .Dq yes
554: or
555: .Dq no .
1.84 jmc 556: This option is intended for situations where ssh-agent
1.29 markus 557: offers many different identities.
558: The default is
559: .Dq no .
1.67 jmc 560: .It Cm IdentityFile
561: Specifies a file from which the user's RSA or DSA authentication identity
562: is read.
563: The default is
564: .Pa ~/.ssh/identity
565: for protocol version 1, and
566: .Pa ~/.ssh/id_rsa
567: and
568: .Pa ~/.ssh/id_dsa
569: for protocol version 2.
570: Additionally, any identities represented by the authentication agent
571: will be used for authentication.
1.129 djm 572: .Xr ssh 1
573: will try to load certificate information from the filename obtained by
574: appending
575: .Pa -cert.pub
576: to the path of a specified
577: .Cm IdentityFile .
1.90 djm 578: .Pp
1.67 jmc 579: The file name may use the tilde
1.91 jmc 580: syntax to refer to a user's home directory or one of the following
1.90 djm 581: escape characters:
582: .Ql %d
583: (local user's home directory),
584: .Ql %u
585: (local user name),
586: .Ql %l
587: (local host name),
588: .Ql %h
589: (remote host name) or
1.92 djm 590: .Ql %r
1.90 djm 591: (remote user name).
592: .Pp
1.67 jmc 593: It is possible to have
594: multiple identity files specified in configuration files; all these
595: identities will be tried in sequence.
1.103 djm 596: .It Cm KbdInteractiveAuthentication
597: Specifies whether to use keyboard-interactive authentication.
598: The argument to this keyword must be
599: .Dq yes
600: or
601: .Dq no .
602: The default is
603: .Dq yes .
1.39 djm 604: .It Cm KbdInteractiveDevices
605: Specifies the list of methods to use in keyboard-interactive authentication.
606: Multiple method names must be comma-separated.
607: The default is to use the server specified list.
1.85 jmc 608: The methods available vary depending on what the server supports.
609: For an OpenSSH server,
610: it may be zero or more of:
611: .Dq bsdauth ,
612: .Dq pam ,
613: and
614: .Dq skey .
1.65 reyk 615: .It Cm LocalCommand
616: Specifies a command to execute on the local machine after successfully
617: connecting to the server.
618: The command string extends to the end of the line, and is executed with
1.105 jmc 619: the user's shell.
1.109 dtucker 620: The following escape character substitutions will be performed:
621: .Ql %d
622: (local user's home directory),
623: .Ql %h
624: (remote host name),
625: .Ql %l
626: (local host name),
627: .Ql %n
628: (host name as provided on the command line),
629: .Ql %p
630: (remote port),
631: .Ql %r
632: (remote user name) or
633: .Ql %u
634: (local user name).
1.123 djm 635: .Pp
636: The command is run synchronously and does not have access to the
637: session of the
638: .Xr ssh 1
639: that spawned it.
640: It should not be used for interactive commands.
641: .Pp
1.65 reyk 642: This directive is ignored unless
643: .Cm PermitLocalCommand
644: has been enabled.
1.1 stevesk 645: .It Cm LocalForward
1.74 jmc 646: Specifies that a TCP port on the local machine be forwarded over
1.1 stevesk 647: the secure channel to the specified host and port from the remote machine.
1.49 jmc 648: The first argument must be
1.43 djm 649: .Sm off
1.49 jmc 650: .Oo Ar bind_address : Oc Ar port
1.43 djm 651: .Sm on
1.49 jmc 652: and the second argument must be
653: .Ar host : Ns Ar hostport .
1.46 jmc 654: IPv6 addresses can be specified by enclosing addresses in square brackets or
1.43 djm 655: by using an alternative syntax:
1.49 jmc 656: .Oo Ar bind_address Ns / Oc Ns Ar port
657: and
658: .Ar host Ns / Ns Ar hostport .
1.46 jmc 659: Multiple forwardings may be specified, and additional forwardings can be
1.43 djm 660: given on the command line.
1.1 stevesk 661: Only the superuser can forward privileged ports.
1.43 djm 662: By default, the local port is bound in accordance with the
663: .Cm GatewayPorts
664: setting.
665: However, an explicit
666: .Ar bind_address
667: may be used to bind the connection to a specific address.
668: The
669: .Ar bind_address
670: of
671: .Dq localhost
1.46 jmc 672: indicates that the listening port be bound for local use only, while an
673: empty address or
674: .Sq *
1.43 djm 675: indicates that the port should be available from all interfaces.
1.1 stevesk 676: .It Cm LogLevel
677: Gives the verbosity level that is used when logging messages from
1.84 jmc 678: .Xr ssh 1 .
1.1 stevesk 679: The possible values are:
1.84 jmc 680: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.7 jmc 681: The default is INFO.
682: DEBUG and DEBUG1 are equivalent.
683: DEBUG2 and DEBUG3 each specify higher levels of verbose output.
1.1 stevesk 684: .It Cm MACs
685: Specifies the MAC (message authentication code) algorithms
686: in order of preference.
687: The MAC algorithm is used in protocol version 2
688: for data integrity protection.
689: Multiple algorithms must be comma-separated.
1.84 jmc 690: The default is:
1.101 jmc 691: .Bd -literal -offset indent
692: hmac-md5,hmac-sha1,umac-64@openssh.com,
693: hmac-ripemd160,hmac-sha1-96,hmac-md5-96
694: .Ed
1.1 stevesk 695: .It Cm NoHostAuthenticationForLocalhost
696: This option can be used if the home directory is shared across machines.
697: In this case localhost will refer to a different machine on each of
698: the machines and the user will get many warnings about changed host keys.
699: However, this option disables host authentication for localhost.
700: The argument to this keyword must be
701: .Dq yes
702: or
703: .Dq no .
704: The default is to check the host key for localhost.
705: .It Cm NumberOfPasswordPrompts
706: Specifies the number of password prompts before giving up.
707: The argument to this keyword must be an integer.
1.84 jmc 708: The default is 3.
1.1 stevesk 709: .It Cm PasswordAuthentication
710: Specifies whether to use password authentication.
711: The argument to this keyword must be
712: .Dq yes
713: or
714: .Dq no .
715: The default is
716: .Dq yes .
1.65 reyk 717: .It Cm PermitLocalCommand
718: Allow local command execution via the
719: .Ic LocalCommand
720: option or using the
1.66 jmc 721: .Ic !\& Ns Ar command
1.65 reyk 722: escape sequence in
723: .Xr ssh 1 .
724: The argument must be
725: .Dq yes
726: or
727: .Dq no .
728: The default is
729: .Dq no .
1.127 markus 730: .It Cm PKCS11Provider
731: Specifies which PKCS#11 provider to use.
732: The argument to this keyword is the PKCS#11 shared libary
733: .Xr ssh 1
1.128 markus 734: should use to communicate with a PKCS#11 token providing the user's
1.127 markus 735: private RSA key.
1.67 jmc 736: .It Cm Port
737: Specifies the port number to connect on the remote host.
1.84 jmc 738: The default is 22.
1.1 stevesk 739: .It Cm PreferredAuthentications
740: Specifies the order in which the client should try protocol 2
1.11 jmc 741: authentication methods.
1.48 jmc 742: This allows a client to prefer one method (e.g.\&
1.1 stevesk 743: .Cm keyboard-interactive )
1.48 jmc 744: over another method (e.g.\&
1.131 jmc 745: .Cm password ) .
746: The default is:
747: .Bd -literal -offset indent
748: gssapi-with-mic,hostbased,publickey,
749: keyboard-interactive,password
750: .Ed
1.1 stevesk 751: .It Cm Protocol
752: Specifies the protocol versions
1.84 jmc 753: .Xr ssh 1
1.1 stevesk 754: should support in order of preference.
755: The possible values are
1.84 jmc 756: .Sq 1
1.1 stevesk 757: and
1.84 jmc 758: .Sq 2 .
1.1 stevesk 759: Multiple versions must be comma-separated.
1.120 markus 760: When this option is set to
1.121 jmc 761: .Dq 2,1
1.120 markus 762: .Nm ssh
763: will try version 2 and fall back to version 1
764: if version 2 is not available.
1.1 stevesk 765: The default is
1.121 jmc 766: .Sq 2 .
1.1 stevesk 767: .It Cm ProxyCommand
768: Specifies the command to use to connect to the server.
769: The command
770: string extends to the end of the line, and is executed with
1.105 jmc 771: the user's shell.
1.133 jmc 772: In the command string, any occurrence of
1.1 stevesk 773: .Ql %h
774: will be substituted by the host name to
1.132 djm 775: connect,
1.1 stevesk 776: .Ql %p
1.133 jmc 777: by the port, and
778: .Ql %r
1.132 djm 779: by the remote user name.
1.1 stevesk 780: The command can be basically anything,
781: and should read from its standard input and write to its standard output.
782: It should eventually connect an
783: .Xr sshd 8
784: server running on some machine, or execute
785: .Ic sshd -i
786: somewhere.
787: Host key management will be done using the
788: HostName of the host being connected (defaulting to the name typed by
789: the user).
1.7 jmc 790: Setting the command to
791: .Dq none
1.6 markus 792: disables this option entirely.
1.1 stevesk 793: Note that
794: .Cm CheckHostIP
795: is not available for connects with a proxy command.
1.52 djm 796: .Pp
797: This directive is useful in conjunction with
798: .Xr nc 1
799: and its proxy support.
1.53 jmc 800: For example, the following directive would connect via an HTTP proxy at
1.52 djm 801: 192.0.2.0:
802: .Bd -literal -offset 3n
803: ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
804: .Ed
1.1 stevesk 805: .It Cm PubkeyAuthentication
806: Specifies whether to try public key authentication.
807: The argument to this keyword must be
808: .Dq yes
809: or
810: .Dq no .
811: The default is
812: .Dq yes .
813: This option applies to protocol version 2 only.
1.75 dtucker 814: .It Cm RekeyLimit
815: Specifies the maximum amount of data that may be transmitted before the
1.76 jmc 816: session key is renegotiated.
1.75 dtucker 817: The argument is the number of bytes, with an optional suffix of
1.76 jmc 818: .Sq K ,
819: .Sq M ,
1.75 dtucker 820: or
1.76 jmc 821: .Sq G
1.75 dtucker 822: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
823: The default is between
1.84 jmc 824: .Sq 1G
1.75 dtucker 825: and
1.84 jmc 826: .Sq 4G ,
1.75 dtucker 827: depending on the cipher.
1.76 jmc 828: This option applies to protocol version 2 only.
1.1 stevesk 829: .It Cm RemoteForward
1.74 jmc 830: Specifies that a TCP port on the remote machine be forwarded over
1.1 stevesk 831: the secure channel to the specified host and port from the local machine.
1.49 jmc 832: The first argument must be
1.43 djm 833: .Sm off
1.49 jmc 834: .Oo Ar bind_address : Oc Ar port
1.43 djm 835: .Sm on
1.49 jmc 836: and the second argument must be
837: .Ar host : Ns Ar hostport .
838: IPv6 addresses can be specified by enclosing addresses in square brackets
839: or by using an alternative syntax:
840: .Oo Ar bind_address Ns / Oc Ns Ar port
841: and
842: .Ar host Ns / Ns Ar hostport .
1.1 stevesk 843: Multiple forwardings may be specified, and additional
844: forwardings can be given on the command line.
1.113 stevesk 845: Privileged ports can be forwarded only when
846: logging in as root on the remote machine.
1.118 jmc 847: .Pp
1.117 djm 848: If the
849: .Ar port
850: argument is
851: .Ql 0 ,
852: the listen port will be dynamically allocated on the server and reported
853: to the client at run time.
1.43 djm 854: .Pp
855: If the
856: .Ar bind_address
857: is not specified, the default is to only bind to loopback addresses.
858: If the
859: .Ar bind_address
860: is
861: .Ql *
862: or an empty string, then the forwarding is requested to listen on all
863: interfaces.
864: Specifying a remote
865: .Ar bind_address
1.46 jmc 866: will only succeed if the server's
867: .Cm GatewayPorts
1.43 djm 868: option is enabled (see
1.46 jmc 869: .Xr sshd_config 5 ) .
1.1 stevesk 870: .It Cm RhostsRSAAuthentication
871: Specifies whether to try rhosts based authentication with RSA host
872: authentication.
873: The argument must be
874: .Dq yes
875: or
876: .Dq no .
877: The default is
878: .Dq no .
879: This option applies to protocol version 1 only and requires
1.84 jmc 880: .Xr ssh 1
1.1 stevesk 881: to be setuid root.
882: .It Cm RSAAuthentication
883: Specifies whether to try RSA authentication.
884: The argument to this keyword must be
885: .Dq yes
886: or
887: .Dq no .
888: RSA authentication will only be
889: attempted if the identity file exists, or an authentication agent is
890: running.
891: The default is
892: .Dq yes .
893: Note that this option applies to protocol version 1 only.
1.32 djm 894: .It Cm SendEnv
895: Specifies what variables from the local
896: .Xr environ 7
897: should be sent to the server.
1.84 jmc 898: Note that environment passing is only supported for protocol 2.
899: The server must also support it, and the server must be configured to
1.33 djm 900: accept these environment variables.
1.32 djm 901: Refer to
902: .Cm AcceptEnv
903: in
904: .Xr sshd_config 5
905: for how to configure the server.
1.80 jmc 906: Variables are specified by name, which may contain wildcard characters.
1.33 djm 907: Multiple environment variables may be separated by whitespace or spread
1.32 djm 908: across multiple
909: .Cm SendEnv
910: directives.
911: The default is not to send any environment variables.
1.81 jmc 912: .Pp
913: See
914: .Sx PATTERNS
915: for more information on patterns.
1.28 markus 916: .It Cm ServerAliveCountMax
1.73 jmc 917: Sets the number of server alive messages (see below) which may be
1.28 markus 918: sent without
1.84 jmc 919: .Xr ssh 1
1.28 markus 920: receiving any messages back from the server.
921: If this threshold is reached while server alive messages are being sent,
1.84 jmc 922: ssh will disconnect from the server, terminating the session.
1.28 markus 923: It is important to note that the use of server alive messages is very
924: different from
925: .Cm TCPKeepAlive
926: (below).
927: The server alive messages are sent through the encrypted channel
928: and therefore will not be spoofable.
929: The TCP keepalive option enabled by
930: .Cm TCPKeepAlive
931: is spoofable.
932: The server alive mechanism is valuable when the client or
933: server depend on knowing when a connection has become inactive.
934: .Pp
935: The default value is 3.
936: If, for example,
937: .Cm ServerAliveInterval
1.84 jmc 938: (see below) is set to 15 and
1.28 markus 939: .Cm ServerAliveCountMax
1.84 jmc 940: is left at the default, if the server becomes unresponsive,
941: ssh will disconnect after approximately 45 seconds.
1.89 markus 942: This option applies to protocol version 2 only.
1.67 jmc 943: .It Cm ServerAliveInterval
944: Sets a timeout interval in seconds after which if no data has been received
945: from the server,
1.84 jmc 946: .Xr ssh 1
1.67 jmc 947: will send a message through the encrypted
948: channel to request a response from the server.
949: The default
950: is 0, indicating that these messages will not be sent to the server.
951: This option applies to protocol version 2 only.
1.1 stevesk 952: .It Cm StrictHostKeyChecking
953: If this flag is set to
954: .Dq yes ,
1.84 jmc 955: .Xr ssh 1
1.1 stevesk 956: will never automatically add host keys to the
1.50 djm 957: .Pa ~/.ssh/known_hosts
1.1 stevesk 958: file, and refuses to connect to hosts whose host key has changed.
959: This provides maximum protection against trojan horse attacks,
1.84 jmc 960: though it can be annoying when the
1.1 stevesk 961: .Pa /etc/ssh/ssh_known_hosts
1.84 jmc 962: file is poorly maintained or when connections to new hosts are
1.1 stevesk 963: frequently made.
964: This option forces the user to manually
965: add all new hosts.
966: If this flag is set to
967: .Dq no ,
1.84 jmc 968: ssh will automatically add new host keys to the
1.1 stevesk 969: user known hosts files.
970: If this flag is set to
971: .Dq ask ,
972: new host keys
973: will be added to the user known host files only after the user
974: has confirmed that is what they really want to do, and
1.84 jmc 975: ssh will refuse to connect to hosts whose host key has changed.
1.1 stevesk 976: The host keys of
977: known hosts will be verified automatically in all cases.
978: The argument must be
979: .Dq yes ,
1.84 jmc 980: .Dq no ,
1.1 stevesk 981: or
982: .Dq ask .
983: The default is
984: .Dq ask .
1.26 markus 985: .It Cm TCPKeepAlive
986: Specifies whether the system should send TCP keepalive messages to the
987: other side.
988: If they are sent, death of the connection or crash of one
989: of the machines will be properly noticed.
990: However, this means that
991: connections will die if the route is down temporarily, and some people
992: find it annoying.
993: .Pp
994: The default is
995: .Dq yes
996: (to send TCP keepalive messages), and the client will notice
997: if the network goes down or the remote host dies.
998: This is important in scripts, and many users want it too.
999: .Pp
1000: To disable TCP keepalive messages, the value should be set to
1001: .Dq no .
1.65 reyk 1002: .It Cm Tunnel
1.95 stevesk 1003: Request
1.65 reyk 1004: .Xr tun 4
1.69 jmc 1005: device forwarding between the client and the server.
1.65 reyk 1006: The argument must be
1.68 reyk 1007: .Dq yes ,
1.95 stevesk 1008: .Dq point-to-point
1009: (layer 3),
1010: .Dq ethernet
1011: (layer 2),
1.65 reyk 1012: or
1013: .Dq no .
1.95 stevesk 1014: Specifying
1015: .Dq yes
1016: requests the default tunnel mode, which is
1017: .Dq point-to-point .
1.65 reyk 1018: The default is
1019: .Dq no .
1020: .It Cm TunnelDevice
1.95 stevesk 1021: Specifies the
1.65 reyk 1022: .Xr tun 4
1.95 stevesk 1023: devices to open on the client
1024: .Pq Ar local_tun
1025: and the server
1026: .Pq Ar remote_tun .
1027: .Pp
1028: The argument must be
1029: .Sm off
1030: .Ar local_tun Op : Ar remote_tun .
1031: .Sm on
1032: The devices may be specified by numerical ID or the keyword
1033: .Dq any ,
1034: which uses the next available tunnel device.
1035: If
1036: .Ar remote_tun
1037: is not specified, it defaults to
1038: .Dq any .
1039: The default is
1040: .Dq any:any .
1.72 jmc 1041: .It Cm UsePrivilegedPort
1042: Specifies whether to use a privileged port for outgoing connections.
1043: The argument must be
1044: .Dq yes
1045: or
1046: .Dq no .
1047: The default is
1048: .Dq no .
1049: If set to
1.84 jmc 1050: .Dq yes ,
1051: .Xr ssh 1
1.72 jmc 1052: must be setuid root.
1053: Note that this option must be set to
1054: .Dq yes
1055: for
1056: .Cm RhostsRSAAuthentication
1057: with older servers.
1.1 stevesk 1058: .It Cm User
1059: Specifies the user to log in as.
1060: This can be useful when a different user name is used on different machines.
1061: This saves the trouble of
1062: having to remember to give the user name on the command line.
1063: .It Cm UserKnownHostsFile
1064: Specifies a file to use for the user
1065: host key database instead of
1.50 djm 1066: .Pa ~/.ssh/known_hosts .
1.8 jakob 1067: .It Cm VerifyHostKeyDNS
1068: Specifies whether to verify the remote key using DNS and SSHFP resource
1069: records.
1.24 jakob 1070: If this option is set to
1071: .Dq yes ,
1.25 jmc 1072: the client will implicitly trust keys that match a secure fingerprint
1.24 jakob 1073: from DNS.
1074: Insecure fingerprints will be handled as if this option was set to
1075: .Dq ask .
1076: If this option is set to
1077: .Dq ask ,
1078: information on fingerprint match will be displayed, but the user will still
1079: need to confirm new host keys according to the
1080: .Cm StrictHostKeyChecking
1081: option.
1082: The argument must be
1083: .Dq yes ,
1.84 jmc 1084: .Dq no ,
1.25 jmc 1085: or
1086: .Dq ask .
1.8 jakob 1087: The default is
1088: .Dq no .
1.12 jakob 1089: Note that this option applies to protocol version 2 only.
1.84 jmc 1090: .Pp
1091: See also
1092: .Sx VERIFYING HOST KEYS
1093: in
1094: .Xr ssh 1 .
1.111 grunk 1095: .It Cm VisualHostKey
1096: If this flag is set to
1097: .Dq yes ,
1098: an ASCII art representation of the remote host key fingerprint is
1.114 stevesk 1099: printed in addition to the hex fingerprint string at login and
1100: for unknown host keys.
1.111 grunk 1101: If this flag is set to
1102: .Dq no ,
1.114 stevesk 1103: no fingerprint strings are printed at login and
1104: only the hex fingerprint string will be printed for unknown host keys.
1.111 grunk 1105: The default is
1106: .Dq no .
1.1 stevesk 1107: .It Cm XAuthLocation
1.5 stevesk 1108: Specifies the full pathname of the
1.1 stevesk 1109: .Xr xauth 1
1110: program.
1111: The default is
1112: .Pa /usr/X11R6/bin/xauth .
1113: .El
1.86 jmc 1114: .Sh PATTERNS
1115: A
1116: .Em pattern
1117: consists of zero or more non-whitespace characters,
1118: .Sq *
1119: (a wildcard that matches zero or more characters),
1120: or
1121: .Sq ?\&
1122: (a wildcard that matches exactly one character).
1123: For example, to specify a set of declarations for any host in the
1124: .Dq .co.uk
1125: set of domains,
1126: the following pattern could be used:
1127: .Pp
1128: .Dl Host *.co.uk
1129: .Pp
1130: The following pattern
1131: would match any host in the 192.168.0.[0-9] network range:
1132: .Pp
1133: .Dl Host 192.168.0.?
1134: .Pp
1135: A
1136: .Em pattern-list
1137: is a comma-separated list of patterns.
1138: Patterns within pattern-lists may be negated
1139: by preceding them with an exclamation mark
1140: .Pq Sq !\& .
1141: For example,
1142: to allow a key to be used from anywhere within an organisation
1143: except from the
1144: .Dq dialup
1145: pool,
1146: the following entry (in authorized_keys) could be used:
1147: .Pp
1148: .Dl from=\&"!*.dialup.example.com,*.example.com\&"
1.1 stevesk 1149: .Sh FILES
1150: .Bl -tag -width Ds
1.50 djm 1151: .It Pa ~/.ssh/config
1.1 stevesk 1152: This is the per-user configuration file.
1153: The format of this file is described above.
1.84 jmc 1154: This file is used by the SSH client.
1.30 djm 1155: Because of the potential for abuse, this file must have strict permissions:
1156: read/write for the user, and not accessible by others.
1.1 stevesk 1157: .It Pa /etc/ssh/ssh_config
1158: Systemwide configuration file.
1159: This file provides defaults for those
1160: values that are not specified in the user's configuration file, and
1161: for those users who do not have a configuration file.
1162: This file must be world-readable.
1163: .El
1.13 jmc 1164: .Sh SEE ALSO
1165: .Xr ssh 1
1.1 stevesk 1166: .Sh AUTHORS
1167: OpenSSH is a derivative of the original and free
1168: ssh 1.2.12 release by Tatu Ylonen.
1169: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1170: Theo de Raadt and Dug Song
1171: removed many bugs, re-added newer features and
1172: created OpenSSH.
1173: Markus Friedl contributed the support for SSH
1174: protocol versions 1.5 and 2.0.
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