Annotation of src/usr.bin/ssh/ssh_config.5, Revision 1.128
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
23: .\" notice, this list of conditions and the following disclaimer in the
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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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1.128 ! markus 37: .\" $OpenBSD: ssh_config.5,v 1.127 2010/02/08 10:50:20 markus Exp $
! 38: .Dd $Mdocdate: February 8 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.
435: .It Cm ForwardX11Trusted
1.34 jmc 436: If this option is set to
1.84 jmc 437: .Dq yes ,
438: remote X11 clients will have full access to the original X11 display.
1.42 djm 439: .Pp
1.22 markus 440: If this option is set to
1.84 jmc 441: .Dq no ,
442: remote X11 clients will be considered untrusted and prevented
1.22 markus 443: from stealing or tampering with data belonging to trusted X11
444: clients.
1.42 djm 445: Furthermore, the
446: .Xr xauth 1
447: token used for the session will be set to expire after 20 minutes.
448: Remote clients will be refused access after this time.
1.22 markus 449: .Pp
450: The default is
451: .Dq no .
452: .Pp
453: See the X11 SECURITY extension specification for full details on
454: the restrictions imposed on untrusted clients.
1.1 stevesk 455: .It Cm GatewayPorts
456: Specifies whether remote hosts are allowed to connect to local
457: forwarded ports.
458: By default,
1.84 jmc 459: .Xr ssh 1
1.7 jmc 460: binds local port forwardings to the loopback address.
461: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 462: .Cm GatewayPorts
1.84 jmc 463: can be used to specify that ssh
1.1 stevesk 464: should bind local port forwardings to the wildcard address,
465: thus allowing remote hosts to connect to forwarded ports.
466: The argument must be
467: .Dq yes
468: or
469: .Dq no .
470: The default is
471: .Dq no .
472: .It Cm GlobalKnownHostsFile
473: Specifies a file to use for the global
474: host key database instead of
475: .Pa /etc/ssh/ssh_known_hosts .
1.18 markus 476: .It Cm GSSAPIAuthentication
1.27 markus 477: Specifies whether user authentication based on GSSAPI is allowed.
1.20 jmc 478: The default is
1.21 markus 479: .Dq no .
1.18 markus 480: Note that this option applies to protocol version 2 only.
481: .It Cm GSSAPIDelegateCredentials
482: Forward (delegate) credentials to the server.
483: The default is
484: .Dq no .
485: Note that this option applies to protocol version 2 only.
1.44 djm 486: .It Cm HashKnownHosts
487: Indicates that
1.84 jmc 488: .Xr ssh 1
1.44 djm 489: should hash host names and addresses when they are added to
1.50 djm 490: .Pa ~/.ssh/known_hosts .
1.44 djm 491: These hashed names may be used normally by
1.84 jmc 492: .Xr ssh 1
1.44 djm 493: and
1.84 jmc 494: .Xr sshd 8 ,
1.44 djm 495: but they do not reveal identifying information should the file's contents
496: be disclosed.
497: The default is
498: .Dq no .
1.97 jmc 499: Note that existing names and addresses in known hosts files
500: will not be converted automatically,
501: but may be manually hashed using
1.45 djm 502: .Xr ssh-keygen 1 .
1.1 stevesk 503: .It Cm HostbasedAuthentication
504: Specifies whether to try rhosts based authentication with public key
505: authentication.
506: The argument must be
507: .Dq yes
508: or
509: .Dq no .
510: The default is
511: .Dq no .
512: This option applies to protocol version 2 only and
513: is similar to
514: .Cm RhostsRSAAuthentication .
515: .It Cm HostKeyAlgorithms
516: Specifies the protocol version 2 host key algorithms
517: that the client wants to use in order of preference.
518: The default for this option is:
519: .Dq ssh-rsa,ssh-dss .
520: .It Cm HostKeyAlias
521: Specifies an alias that should be used instead of the
522: real host name when looking up or saving the host key
523: in the host key database files.
1.84 jmc 524: This option is useful for tunneling SSH connections
1.1 stevesk 525: or for multiple servers running on a single host.
526: .It Cm HostName
527: Specifies the real host name to log into.
528: This can be used to specify nicknames or abbreviations for hosts.
1.84 jmc 529: The default is the name given on the command line.
1.1 stevesk 530: Numeric IP addresses are also permitted (both on the command line and in
531: .Cm HostName
532: specifications).
1.29 markus 533: .It Cm IdentitiesOnly
534: Specifies that
1.84 jmc 535: .Xr ssh 1
1.29 markus 536: should only use the authentication identity files configured in the
1.31 jmc 537: .Nm
1.29 markus 538: files,
1.84 jmc 539: even if
540: .Xr ssh-agent 1
1.29 markus 541: offers more identities.
542: The argument to this keyword must be
543: .Dq yes
544: or
545: .Dq no .
1.84 jmc 546: This option is intended for situations where ssh-agent
1.29 markus 547: offers many different identities.
548: The default is
549: .Dq no .
1.67 jmc 550: .It Cm IdentityFile
551: Specifies a file from which the user's RSA or DSA authentication identity
552: is read.
553: The default is
554: .Pa ~/.ssh/identity
555: for protocol version 1, and
556: .Pa ~/.ssh/id_rsa
557: and
558: .Pa ~/.ssh/id_dsa
559: for protocol version 2.
560: Additionally, any identities represented by the authentication agent
561: will be used for authentication.
1.90 djm 562: .Pp
1.67 jmc 563: The file name may use the tilde
1.91 jmc 564: syntax to refer to a user's home directory or one of the following
1.90 djm 565: escape characters:
566: .Ql %d
567: (local user's home directory),
568: .Ql %u
569: (local user name),
570: .Ql %l
571: (local host name),
572: .Ql %h
573: (remote host name) or
1.92 djm 574: .Ql %r
1.90 djm 575: (remote user name).
576: .Pp
1.67 jmc 577: It is possible to have
578: multiple identity files specified in configuration files; all these
579: identities will be tried in sequence.
1.103 djm 580: .It Cm KbdInteractiveAuthentication
581: Specifies whether to use keyboard-interactive authentication.
582: The argument to this keyword must be
583: .Dq yes
584: or
585: .Dq no .
586: The default is
587: .Dq yes .
1.39 djm 588: .It Cm KbdInteractiveDevices
589: Specifies the list of methods to use in keyboard-interactive authentication.
590: Multiple method names must be comma-separated.
591: The default is to use the server specified list.
1.85 jmc 592: The methods available vary depending on what the server supports.
593: For an OpenSSH server,
594: it may be zero or more of:
595: .Dq bsdauth ,
596: .Dq pam ,
597: and
598: .Dq skey .
1.65 reyk 599: .It Cm LocalCommand
600: Specifies a command to execute on the local machine after successfully
601: connecting to the server.
602: The command string extends to the end of the line, and is executed with
1.105 jmc 603: the user's shell.
1.109 dtucker 604: The following escape character substitutions will be performed:
605: .Ql %d
606: (local user's home directory),
607: .Ql %h
608: (remote host name),
609: .Ql %l
610: (local host name),
611: .Ql %n
612: (host name as provided on the command line),
613: .Ql %p
614: (remote port),
615: .Ql %r
616: (remote user name) or
617: .Ql %u
618: (local user name).
1.123 djm 619: .Pp
620: The command is run synchronously and does not have access to the
621: session of the
622: .Xr ssh 1
623: that spawned it.
624: It should not be used for interactive commands.
625: .Pp
1.65 reyk 626: This directive is ignored unless
627: .Cm PermitLocalCommand
628: has been enabled.
1.1 stevesk 629: .It Cm LocalForward
1.74 jmc 630: Specifies that a TCP port on the local machine be forwarded over
1.1 stevesk 631: the secure channel to the specified host and port from the remote machine.
1.49 jmc 632: The first argument must be
1.43 djm 633: .Sm off
1.49 jmc 634: .Oo Ar bind_address : Oc Ar port
1.43 djm 635: .Sm on
1.49 jmc 636: and the second argument must be
637: .Ar host : Ns Ar hostport .
1.46 jmc 638: IPv6 addresses can be specified by enclosing addresses in square brackets or
1.43 djm 639: by using an alternative syntax:
1.49 jmc 640: .Oo Ar bind_address Ns / Oc Ns Ar port
641: and
642: .Ar host Ns / Ns Ar hostport .
1.46 jmc 643: Multiple forwardings may be specified, and additional forwardings can be
1.43 djm 644: given on the command line.
1.1 stevesk 645: Only the superuser can forward privileged ports.
1.43 djm 646: By default, the local port is bound in accordance with the
647: .Cm GatewayPorts
648: setting.
649: However, an explicit
650: .Ar bind_address
651: may be used to bind the connection to a specific address.
652: The
653: .Ar bind_address
654: of
655: .Dq localhost
1.46 jmc 656: indicates that the listening port be bound for local use only, while an
657: empty address or
658: .Sq *
1.43 djm 659: indicates that the port should be available from all interfaces.
1.1 stevesk 660: .It Cm LogLevel
661: Gives the verbosity level that is used when logging messages from
1.84 jmc 662: .Xr ssh 1 .
1.1 stevesk 663: The possible values are:
1.84 jmc 664: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.7 jmc 665: The default is INFO.
666: DEBUG and DEBUG1 are equivalent.
667: DEBUG2 and DEBUG3 each specify higher levels of verbose output.
1.1 stevesk 668: .It Cm MACs
669: Specifies the MAC (message authentication code) algorithms
670: in order of preference.
671: The MAC algorithm is used in protocol version 2
672: for data integrity protection.
673: Multiple algorithms must be comma-separated.
1.84 jmc 674: The default is:
1.101 jmc 675: .Bd -literal -offset indent
676: hmac-md5,hmac-sha1,umac-64@openssh.com,
677: hmac-ripemd160,hmac-sha1-96,hmac-md5-96
678: .Ed
1.1 stevesk 679: .It Cm NoHostAuthenticationForLocalhost
680: This option can be used if the home directory is shared across machines.
681: In this case localhost will refer to a different machine on each of
682: the machines and the user will get many warnings about changed host keys.
683: However, this option disables host authentication for localhost.
684: The argument to this keyword must be
685: .Dq yes
686: or
687: .Dq no .
688: The default is to check the host key for localhost.
689: .It Cm NumberOfPasswordPrompts
690: Specifies the number of password prompts before giving up.
691: The argument to this keyword must be an integer.
1.84 jmc 692: The default is 3.
1.1 stevesk 693: .It Cm PasswordAuthentication
694: Specifies whether to use password authentication.
695: The argument to this keyword must be
696: .Dq yes
697: or
698: .Dq no .
699: The default is
700: .Dq yes .
1.65 reyk 701: .It Cm PermitLocalCommand
702: Allow local command execution via the
703: .Ic LocalCommand
704: option or using the
1.66 jmc 705: .Ic !\& Ns Ar command
1.65 reyk 706: escape sequence in
707: .Xr ssh 1 .
708: The argument must be
709: .Dq yes
710: or
711: .Dq no .
712: The default is
713: .Dq no .
1.127 markus 714: .It Cm PKCS11Provider
715: Specifies which PKCS#11 provider to use.
716: The argument to this keyword is the PKCS#11 shared libary
717: .Xr ssh 1
1.128 ! markus 718: should use to communicate with a PKCS#11 token providing the user's
1.127 markus 719: private RSA key.
1.67 jmc 720: .It Cm Port
721: Specifies the port number to connect on the remote host.
1.84 jmc 722: The default is 22.
1.1 stevesk 723: .It Cm PreferredAuthentications
724: Specifies the order in which the client should try protocol 2
1.11 jmc 725: authentication methods.
1.48 jmc 726: This allows a client to prefer one method (e.g.\&
1.1 stevesk 727: .Cm keyboard-interactive )
1.48 jmc 728: over another method (e.g.\&
1.1 stevesk 729: .Cm password )
730: The default for this option is:
1.94 jmc 731: .Do gssapi-with-mic ,
732: hostbased,
733: publickey,
734: keyboard-interactive,
735: password
736: .Dc .
1.1 stevesk 737: .It Cm Protocol
738: Specifies the protocol versions
1.84 jmc 739: .Xr ssh 1
1.1 stevesk 740: should support in order of preference.
741: The possible values are
1.84 jmc 742: .Sq 1
1.1 stevesk 743: and
1.84 jmc 744: .Sq 2 .
1.1 stevesk 745: Multiple versions must be comma-separated.
1.120 markus 746: When this option is set to
1.121 jmc 747: .Dq 2,1
1.120 markus 748: .Nm ssh
749: will try version 2 and fall back to version 1
750: if version 2 is not available.
1.1 stevesk 751: The default is
1.121 jmc 752: .Sq 2 .
1.1 stevesk 753: .It Cm ProxyCommand
754: Specifies the command to use to connect to the server.
755: The command
756: string extends to the end of the line, and is executed with
1.105 jmc 757: the user's shell.
1.1 stevesk 758: In the command string,
759: .Ql %h
760: will be substituted by the host name to
761: connect and
762: .Ql %p
763: by the port.
764: The command can be basically anything,
765: and should read from its standard input and write to its standard output.
766: It should eventually connect an
767: .Xr sshd 8
768: server running on some machine, or execute
769: .Ic sshd -i
770: somewhere.
771: Host key management will be done using the
772: HostName of the host being connected (defaulting to the name typed by
773: the user).
1.7 jmc 774: Setting the command to
775: .Dq none
1.6 markus 776: disables this option entirely.
1.1 stevesk 777: Note that
778: .Cm CheckHostIP
779: is not available for connects with a proxy command.
1.52 djm 780: .Pp
781: This directive is useful in conjunction with
782: .Xr nc 1
783: and its proxy support.
1.53 jmc 784: For example, the following directive would connect via an HTTP proxy at
1.52 djm 785: 192.0.2.0:
786: .Bd -literal -offset 3n
787: ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
788: .Ed
1.1 stevesk 789: .It Cm PubkeyAuthentication
790: Specifies whether to try public key authentication.
791: The argument to this keyword must be
792: .Dq yes
793: or
794: .Dq no .
795: The default is
796: .Dq yes .
797: This option applies to protocol version 2 only.
1.75 dtucker 798: .It Cm RekeyLimit
799: Specifies the maximum amount of data that may be transmitted before the
1.76 jmc 800: session key is renegotiated.
1.75 dtucker 801: The argument is the number of bytes, with an optional suffix of
1.76 jmc 802: .Sq K ,
803: .Sq M ,
1.75 dtucker 804: or
1.76 jmc 805: .Sq G
1.75 dtucker 806: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
807: The default is between
1.84 jmc 808: .Sq 1G
1.75 dtucker 809: and
1.84 jmc 810: .Sq 4G ,
1.75 dtucker 811: depending on the cipher.
1.76 jmc 812: This option applies to protocol version 2 only.
1.1 stevesk 813: .It Cm RemoteForward
1.74 jmc 814: Specifies that a TCP port on the remote machine be forwarded over
1.1 stevesk 815: the secure channel to the specified host and port from the local machine.
1.49 jmc 816: The first argument must be
1.43 djm 817: .Sm off
1.49 jmc 818: .Oo Ar bind_address : Oc Ar port
1.43 djm 819: .Sm on
1.49 jmc 820: and the second argument must be
821: .Ar host : Ns Ar hostport .
822: IPv6 addresses can be specified by enclosing addresses in square brackets
823: or by using an alternative syntax:
824: .Oo Ar bind_address Ns / Oc Ns Ar port
825: and
826: .Ar host Ns / Ns Ar hostport .
1.1 stevesk 827: Multiple forwardings may be specified, and additional
828: forwardings can be given on the command line.
1.113 stevesk 829: Privileged ports can be forwarded only when
830: logging in as root on the remote machine.
1.118 jmc 831: .Pp
1.117 djm 832: If the
833: .Ar port
834: argument is
835: .Ql 0 ,
836: the listen port will be dynamically allocated on the server and reported
837: to the client at run time.
1.43 djm 838: .Pp
839: If the
840: .Ar bind_address
841: is not specified, the default is to only bind to loopback addresses.
842: If the
843: .Ar bind_address
844: is
845: .Ql *
846: or an empty string, then the forwarding is requested to listen on all
847: interfaces.
848: Specifying a remote
849: .Ar bind_address
1.46 jmc 850: will only succeed if the server's
851: .Cm GatewayPorts
1.43 djm 852: option is enabled (see
1.46 jmc 853: .Xr sshd_config 5 ) .
1.1 stevesk 854: .It Cm RhostsRSAAuthentication
855: Specifies whether to try rhosts based authentication with RSA host
856: authentication.
857: The argument must be
858: .Dq yes
859: or
860: .Dq no .
861: The default is
862: .Dq no .
863: This option applies to protocol version 1 only and requires
1.84 jmc 864: .Xr ssh 1
1.1 stevesk 865: to be setuid root.
866: .It Cm RSAAuthentication
867: Specifies whether to try RSA authentication.
868: The argument to this keyword must be
869: .Dq yes
870: or
871: .Dq no .
872: RSA authentication will only be
873: attempted if the identity file exists, or an authentication agent is
874: running.
875: The default is
876: .Dq yes .
877: Note that this option applies to protocol version 1 only.
1.32 djm 878: .It Cm SendEnv
879: Specifies what variables from the local
880: .Xr environ 7
881: should be sent to the server.
1.84 jmc 882: Note that environment passing is only supported for protocol 2.
883: The server must also support it, and the server must be configured to
1.33 djm 884: accept these environment variables.
1.32 djm 885: Refer to
886: .Cm AcceptEnv
887: in
888: .Xr sshd_config 5
889: for how to configure the server.
1.80 jmc 890: Variables are specified by name, which may contain wildcard characters.
1.33 djm 891: Multiple environment variables may be separated by whitespace or spread
1.32 djm 892: across multiple
893: .Cm SendEnv
894: directives.
895: The default is not to send any environment variables.
1.81 jmc 896: .Pp
897: See
898: .Sx PATTERNS
899: for more information on patterns.
1.28 markus 900: .It Cm ServerAliveCountMax
1.73 jmc 901: Sets the number of server alive messages (see below) which may be
1.28 markus 902: sent without
1.84 jmc 903: .Xr ssh 1
1.28 markus 904: receiving any messages back from the server.
905: If this threshold is reached while server alive messages are being sent,
1.84 jmc 906: ssh will disconnect from the server, terminating the session.
1.28 markus 907: It is important to note that the use of server alive messages is very
908: different from
909: .Cm TCPKeepAlive
910: (below).
911: The server alive messages are sent through the encrypted channel
912: and therefore will not be spoofable.
913: The TCP keepalive option enabled by
914: .Cm TCPKeepAlive
915: is spoofable.
916: The server alive mechanism is valuable when the client or
917: server depend on knowing when a connection has become inactive.
918: .Pp
919: The default value is 3.
920: If, for example,
921: .Cm ServerAliveInterval
1.84 jmc 922: (see below) is set to 15 and
1.28 markus 923: .Cm ServerAliveCountMax
1.84 jmc 924: is left at the default, if the server becomes unresponsive,
925: ssh will disconnect after approximately 45 seconds.
1.89 markus 926: This option applies to protocol version 2 only.
1.67 jmc 927: .It Cm ServerAliveInterval
928: Sets a timeout interval in seconds after which if no data has been received
929: from the server,
1.84 jmc 930: .Xr ssh 1
1.67 jmc 931: will send a message through the encrypted
932: channel to request a response from the server.
933: The default
934: is 0, indicating that these messages will not be sent to the server.
935: This option applies to protocol version 2 only.
1.1 stevesk 936: .It Cm StrictHostKeyChecking
937: If this flag is set to
938: .Dq yes ,
1.84 jmc 939: .Xr ssh 1
1.1 stevesk 940: will never automatically add host keys to the
1.50 djm 941: .Pa ~/.ssh/known_hosts
1.1 stevesk 942: file, and refuses to connect to hosts whose host key has changed.
943: This provides maximum protection against trojan horse attacks,
1.84 jmc 944: though it can be annoying when the
1.1 stevesk 945: .Pa /etc/ssh/ssh_known_hosts
1.84 jmc 946: file is poorly maintained or when connections to new hosts are
1.1 stevesk 947: frequently made.
948: This option forces the user to manually
949: add all new hosts.
950: If this flag is set to
951: .Dq no ,
1.84 jmc 952: ssh will automatically add new host keys to the
1.1 stevesk 953: user known hosts files.
954: If this flag is set to
955: .Dq ask ,
956: new host keys
957: will be added to the user known host files only after the user
958: has confirmed that is what they really want to do, and
1.84 jmc 959: ssh will refuse to connect to hosts whose host key has changed.
1.1 stevesk 960: The host keys of
961: known hosts will be verified automatically in all cases.
962: The argument must be
963: .Dq yes ,
1.84 jmc 964: .Dq no ,
1.1 stevesk 965: or
966: .Dq ask .
967: The default is
968: .Dq ask .
1.26 markus 969: .It Cm TCPKeepAlive
970: Specifies whether the system should send TCP keepalive messages to the
971: other side.
972: If they are sent, death of the connection or crash of one
973: of the machines will be properly noticed.
974: However, this means that
975: connections will die if the route is down temporarily, and some people
976: find it annoying.
977: .Pp
978: The default is
979: .Dq yes
980: (to send TCP keepalive messages), and the client will notice
981: if the network goes down or the remote host dies.
982: This is important in scripts, and many users want it too.
983: .Pp
984: To disable TCP keepalive messages, the value should be set to
985: .Dq no .
1.65 reyk 986: .It Cm Tunnel
1.95 stevesk 987: Request
1.65 reyk 988: .Xr tun 4
1.69 jmc 989: device forwarding between the client and the server.
1.65 reyk 990: The argument must be
1.68 reyk 991: .Dq yes ,
1.95 stevesk 992: .Dq point-to-point
993: (layer 3),
994: .Dq ethernet
995: (layer 2),
1.65 reyk 996: or
997: .Dq no .
1.95 stevesk 998: Specifying
999: .Dq yes
1000: requests the default tunnel mode, which is
1001: .Dq point-to-point .
1.65 reyk 1002: The default is
1003: .Dq no .
1004: .It Cm TunnelDevice
1.95 stevesk 1005: Specifies the
1.65 reyk 1006: .Xr tun 4
1.95 stevesk 1007: devices to open on the client
1008: .Pq Ar local_tun
1009: and the server
1010: .Pq Ar remote_tun .
1011: .Pp
1012: The argument must be
1013: .Sm off
1014: .Ar local_tun Op : Ar remote_tun .
1015: .Sm on
1016: The devices may be specified by numerical ID or the keyword
1017: .Dq any ,
1018: which uses the next available tunnel device.
1019: If
1020: .Ar remote_tun
1021: is not specified, it defaults to
1022: .Dq any .
1023: The default is
1024: .Dq any:any .
1.72 jmc 1025: .It Cm UsePrivilegedPort
1026: Specifies whether to use a privileged port for outgoing connections.
1027: The argument must be
1028: .Dq yes
1029: or
1030: .Dq no .
1031: The default is
1032: .Dq no .
1033: If set to
1.84 jmc 1034: .Dq yes ,
1035: .Xr ssh 1
1.72 jmc 1036: must be setuid root.
1037: Note that this option must be set to
1038: .Dq yes
1039: for
1040: .Cm RhostsRSAAuthentication
1041: with older servers.
1.1 stevesk 1042: .It Cm User
1043: Specifies the user to log in as.
1044: This can be useful when a different user name is used on different machines.
1045: This saves the trouble of
1046: having to remember to give the user name on the command line.
1047: .It Cm UserKnownHostsFile
1048: Specifies a file to use for the user
1049: host key database instead of
1.50 djm 1050: .Pa ~/.ssh/known_hosts .
1.8 jakob 1051: .It Cm VerifyHostKeyDNS
1052: Specifies whether to verify the remote key using DNS and SSHFP resource
1053: records.
1.24 jakob 1054: If this option is set to
1055: .Dq yes ,
1.25 jmc 1056: the client will implicitly trust keys that match a secure fingerprint
1.24 jakob 1057: from DNS.
1058: Insecure fingerprints will be handled as if this option was set to
1059: .Dq ask .
1060: If this option is set to
1061: .Dq ask ,
1062: information on fingerprint match will be displayed, but the user will still
1063: need to confirm new host keys according to the
1064: .Cm StrictHostKeyChecking
1065: option.
1066: The argument must be
1067: .Dq yes ,
1.84 jmc 1068: .Dq no ,
1.25 jmc 1069: or
1070: .Dq ask .
1.8 jakob 1071: The default is
1072: .Dq no .
1.12 jakob 1073: Note that this option applies to protocol version 2 only.
1.84 jmc 1074: .Pp
1075: See also
1076: .Sx VERIFYING HOST KEYS
1077: in
1078: .Xr ssh 1 .
1.111 grunk 1079: .It Cm VisualHostKey
1080: If this flag is set to
1081: .Dq yes ,
1082: an ASCII art representation of the remote host key fingerprint is
1.114 stevesk 1083: printed in addition to the hex fingerprint string at login and
1084: for unknown host keys.
1.111 grunk 1085: If this flag is set to
1086: .Dq no ,
1.114 stevesk 1087: no fingerprint strings are printed at login and
1088: only the hex fingerprint string will be printed for unknown host keys.
1.111 grunk 1089: The default is
1090: .Dq no .
1.1 stevesk 1091: .It Cm XAuthLocation
1.5 stevesk 1092: Specifies the full pathname of the
1.1 stevesk 1093: .Xr xauth 1
1094: program.
1095: The default is
1096: .Pa /usr/X11R6/bin/xauth .
1097: .El
1.86 jmc 1098: .Sh PATTERNS
1099: A
1100: .Em pattern
1101: consists of zero or more non-whitespace characters,
1102: .Sq *
1103: (a wildcard that matches zero or more characters),
1104: or
1105: .Sq ?\&
1106: (a wildcard that matches exactly one character).
1107: For example, to specify a set of declarations for any host in the
1108: .Dq .co.uk
1109: set of domains,
1110: the following pattern could be used:
1111: .Pp
1112: .Dl Host *.co.uk
1113: .Pp
1114: The following pattern
1115: would match any host in the 192.168.0.[0-9] network range:
1116: .Pp
1117: .Dl Host 192.168.0.?
1118: .Pp
1119: A
1120: .Em pattern-list
1121: is a comma-separated list of patterns.
1122: Patterns within pattern-lists may be negated
1123: by preceding them with an exclamation mark
1124: .Pq Sq !\& .
1125: For example,
1126: to allow a key to be used from anywhere within an organisation
1127: except from the
1128: .Dq dialup
1129: pool,
1130: the following entry (in authorized_keys) could be used:
1131: .Pp
1132: .Dl from=\&"!*.dialup.example.com,*.example.com\&"
1.1 stevesk 1133: .Sh FILES
1134: .Bl -tag -width Ds
1.50 djm 1135: .It Pa ~/.ssh/config
1.1 stevesk 1136: This is the per-user configuration file.
1137: The format of this file is described above.
1.84 jmc 1138: This file is used by the SSH client.
1.30 djm 1139: Because of the potential for abuse, this file must have strict permissions:
1140: read/write for the user, and not accessible by others.
1.1 stevesk 1141: .It Pa /etc/ssh/ssh_config
1142: Systemwide configuration file.
1143: This file provides defaults for those
1144: values that are not specified in the user's configuration file, and
1145: for those users who do not have a configuration file.
1146: This file must be world-readable.
1147: .El
1.13 jmc 1148: .Sh SEE ALSO
1149: .Xr ssh 1
1.1 stevesk 1150: .Sh AUTHORS
1151: OpenSSH is a derivative of the original and free
1152: ssh 1.2.12 release by Tatu Ylonen.
1153: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1154: Theo de Raadt and Dug Song
1155: removed many bugs, re-added newer features and
1156: created OpenSSH.
1157: Markus Friedl contributed the support for SSH
1158: protocol versions 1.5 and 2.0.