Annotation of src/usr.bin/ssh/ssh_config.5, Revision 1.102
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: .\"
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22: .\" 2. Redistributions in binary form must reproduce the above copyright
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1.102 ! stevesk 37: .\" $OpenBSD: ssh_config.5,v 1.101 2007/06/08 07:43:46 jmc Exp $
! 38: .Dd $Mdocdate: June 8 2007 $
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.
106: A single
1.83 jmc 107: .Ql *
1.1 stevesk 108: as a pattern can be used to provide global
109: defaults for all hosts.
110: The host is the
111: .Ar hostname
1.83 jmc 112: argument given on the command line (i.e. the name is not converted to
1.1 stevesk 113: a canonicalized host name before matching).
1.81 jmc 114: .Pp
115: See
116: .Sx PATTERNS
117: for more information on patterns.
1.10 djm 118: .It Cm AddressFamily
1.11 jmc 119: Specifies which address family to use when connecting.
120: Valid arguments are
1.10 djm 121: .Dq any ,
122: .Dq inet
1.84 jmc 123: (use IPv4 only), or
1.10 djm 124: .Dq inet6
1.40 jmc 125: (use IPv6 only).
1.1 stevesk 126: .It Cm BatchMode
127: If set to
128: .Dq yes ,
129: passphrase/password querying will be disabled.
130: This option is useful in scripts and other batch jobs where no user
131: is present to supply the password.
132: The argument must be
133: .Dq yes
134: or
135: .Dq no .
136: The default is
137: .Dq no .
138: .It Cm BindAddress
1.60 dtucker 139: Use the specified address on the local machine as the source address of
1.61 jmc 140: the connection.
141: Only useful on systems with more than one address.
1.1 stevesk 142: Note that this option does not work if
143: .Cm UsePrivilegedPort
144: is set to
145: .Dq yes .
146: .It Cm ChallengeResponseAuthentication
1.82 jmc 147: Specifies whether to use challenge-response authentication.
1.1 stevesk 148: The argument to this keyword must be
149: .Dq yes
150: or
151: .Dq no .
152: The default is
153: .Dq yes .
154: .It Cm CheckHostIP
155: If this flag is set to
156: .Dq yes ,
1.84 jmc 157: .Xr ssh 1
158: will additionally check the host IP address in the
1.1 stevesk 159: .Pa known_hosts
160: file.
161: This allows ssh to detect if a host key changed due to DNS spoofing.
162: If the option is set to
163: .Dq no ,
164: the check will not be executed.
165: The default is
166: .Dq yes .
167: .It Cm Cipher
168: Specifies the cipher to use for encrypting the session
169: in protocol version 1.
170: Currently,
171: .Dq blowfish ,
172: .Dq 3des ,
173: and
174: .Dq des
175: are supported.
176: .Ar des
177: is only supported in the
1.84 jmc 178: .Xr ssh 1
1.1 stevesk 179: client for interoperability with legacy protocol 1 implementations
180: that do not support the
181: .Ar 3des
1.7 jmc 182: cipher.
183: Its use is strongly discouraged due to cryptographic weaknesses.
1.1 stevesk 184: The default is
185: .Dq 3des .
186: .It Cm Ciphers
187: Specifies the ciphers allowed for protocol version 2
188: in order of preference.
189: Multiple ciphers must be comma-separated.
1.35 dtucker 190: The supported ciphers are
191: .Dq 3des-cbc ,
192: .Dq aes128-cbc ,
193: .Dq aes192-cbc ,
194: .Dq aes256-cbc ,
195: .Dq aes128-ctr ,
196: .Dq aes192-ctr ,
197: .Dq aes256-ctr ,
1.54 djm 198: .Dq arcfour128 ,
199: .Dq arcfour256 ,
1.35 dtucker 200: .Dq arcfour ,
201: .Dq blowfish-cbc ,
202: and
203: .Dq cast128-cbc .
1.84 jmc 204: The default is:
205: .Bd -literal -offset 3n
206: aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
207: arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
208: aes192-ctr,aes256-ctr
1.1 stevesk 209: .Ed
210: .It Cm ClearAllForwardings
1.84 jmc 211: Specifies that all local, remote, and dynamic port forwardings
1.1 stevesk 212: specified in the configuration files or on the command line be
1.7 jmc 213: cleared.
214: This option is primarily useful when used from the
1.84 jmc 215: .Xr ssh 1
1.1 stevesk 216: command line to clear port forwardings set in
217: configuration files, and is automatically set by
218: .Xr scp 1
219: and
220: .Xr sftp 1 .
221: The argument must be
222: .Dq yes
223: or
224: .Dq no .
225: The default is
226: .Dq no .
227: .It Cm Compression
228: Specifies whether to use compression.
229: The argument must be
230: .Dq yes
231: or
232: .Dq no .
233: The default is
234: .Dq no .
235: .It Cm CompressionLevel
236: Specifies the compression level to use if compression is enabled.
237: The argument must be an integer from 1 (fast) to 9 (slow, best).
238: The default level is 6, which is good for most applications.
239: The meaning of the values is the same as in
240: .Xr gzip 1 .
241: Note that this option applies to protocol version 1 only.
242: .It Cm ConnectionAttempts
243: Specifies the number of tries (one per second) to make before exiting.
244: The argument must be an integer.
245: This may be useful in scripts if the connection sometimes fails.
246: The default is 1.
1.9 djm 247: .It Cm ConnectTimeout
1.84 jmc 248: Specifies the timeout (in seconds) used when connecting to the
249: SSH server, instead of using the default system TCP timeout.
1.11 jmc 250: This value is used only when the target is down or really unreachable,
251: not when it refuses the connection.
1.36 djm 252: .It Cm ControlMaster
253: Enables the sharing of multiple sessions over a single network connection.
254: When set to
1.84 jmc 255: .Dq yes ,
256: .Xr ssh 1
1.36 djm 257: will listen for connections on a control socket specified using the
258: .Cm ControlPath
259: argument.
260: Additional sessions can connect to this socket using the same
261: .Cm ControlPath
262: with
263: .Cm ControlMaster
264: set to
265: .Dq no
1.38 jmc 266: (the default).
1.64 jmc 267: These sessions will try to reuse the master instance's network connection
1.63 djm 268: rather than initiating new ones, but will fall back to connecting normally
269: if the control socket does not exist, or is not listening.
270: .Pp
1.37 djm 271: Setting this to
272: .Dq ask
1.84 jmc 273: will cause ssh
1.37 djm 274: to listen for control connections, but require confirmation using the
275: .Ev SSH_ASKPASS
276: program before they are accepted (see
277: .Xr ssh-add 1
1.38 jmc 278: for details).
1.51 jakob 279: If the
280: .Cm ControlPath
1.84 jmc 281: cannot be opened,
282: ssh will continue without connecting to a master instance.
1.58 djm 283: .Pp
284: X11 and
1.59 jmc 285: .Xr ssh-agent 1
1.58 djm 286: forwarding is supported over these multiplexed connections, however the
1.70 stevesk 287: display and agent forwarded will be the one belonging to the master
1.59 jmc 288: connection i.e. it is not possible to forward multiple displays or agents.
1.56 djm 289: .Pp
290: Two additional options allow for opportunistic multiplexing: try to use a
291: master connection but fall back to creating a new one if one does not already
292: exist.
293: These options are:
294: .Dq auto
295: and
296: .Dq autoask .
297: The latter requires confirmation like the
298: .Dq ask
299: option.
1.36 djm 300: .It Cm ControlPath
1.55 djm 301: Specify the path to the control socket used for connection sharing as described
302: in the
1.36 djm 303: .Cm ControlMaster
1.57 djm 304: section above or the string
305: .Dq none
306: to disable connection sharing.
1.55 djm 307: In the path,
1.77 djm 308: .Ql %l
309: will be substituted by the local host name,
1.55 djm 310: .Ql %h
311: will be substituted by the target host name,
312: .Ql %p
1.84 jmc 313: the port, and
1.55 djm 314: .Ql %r
315: by the remote login username.
1.56 djm 316: It is recommended that any
317: .Cm ControlPath
318: used for opportunistic connection sharing include
1.78 jmc 319: at least %h, %p, and %r.
1.56 djm 320: This ensures that shared connections are uniquely identified.
1.38 jmc 321: .It Cm DynamicForward
1.74 jmc 322: Specifies that a TCP port on the local machine be forwarded
1.38 jmc 323: over the secure channel, and the application
324: protocol is then used to determine where to connect to from the
325: remote machine.
1.62 djm 326: .Pp
327: The argument must be
328: .Sm off
329: .Oo Ar bind_address : Oc Ar port .
330: .Sm on
331: IPv6 addresses can be specified by enclosing addresses in square brackets or
332: by using an alternative syntax:
333: .Oo Ar bind_address Ns / Oc Ns Ar port .
334: By default, the local port is bound in accordance with the
335: .Cm GatewayPorts
336: setting.
337: However, an explicit
338: .Ar bind_address
339: may be used to bind the connection to a specific address.
340: The
341: .Ar bind_address
342: of
343: .Dq localhost
344: indicates that the listening port be bound for local use only, while an
345: empty address or
346: .Sq *
347: indicates that the port should be available from all interfaces.
348: .Pp
1.38 jmc 349: Currently the SOCKS4 and SOCKS5 protocols are supported, and
1.84 jmc 350: .Xr ssh 1
1.38 jmc 351: will act as a SOCKS server.
352: Multiple forwardings may be specified, and
353: additional forwardings can be given on the command line.
354: Only the superuser can forward privileged ports.
1.14 markus 355: .It Cm EnableSSHKeysign
356: Setting this option to
357: .Dq yes
358: in the global client configuration file
359: .Pa /etc/ssh/ssh_config
360: enables the use of the helper program
361: .Xr ssh-keysign 8
362: during
363: .Cm HostbasedAuthentication .
364: The argument must be
365: .Dq yes
366: or
367: .Dq no .
368: The default is
369: .Dq no .
1.23 jmc 370: This option should be placed in the non-hostspecific section.
1.14 markus 371: See
372: .Xr ssh-keysign 8
373: for more information.
1.1 stevesk 374: .It Cm EscapeChar
375: Sets the escape character (default:
376: .Ql ~ ) .
377: The escape character can also
378: be set on the command line.
379: The argument should be a single character,
380: .Ql ^
381: followed by a letter, or
382: .Dq none
383: to disable the escape
384: character entirely (making the connection transparent for binary
385: data).
1.96 markus 386: .It Cm ExitOnForwardFailure
387: Specifies whether
388: .Xr ssh 1
389: should terminate the connection if it cannot set up all requested
1.102 ! stevesk 390: dynamic, tunnel, local, and remote port forwardings.
1.96 markus 391: The argument must be
392: .Dq yes
393: or
394: .Dq no .
395: The default is
396: .Dq no .
1.1 stevesk 397: .It Cm ForwardAgent
398: Specifies whether the connection to the authentication agent (if any)
399: will be forwarded to the remote machine.
400: The argument must be
401: .Dq yes
402: or
403: .Dq no .
404: The default is
405: .Dq no .
1.3 stevesk 406: .Pp
1.7 jmc 407: Agent forwarding should be enabled with caution.
408: Users with the ability to bypass file permissions on the remote host
409: (for the agent's Unix-domain socket)
410: can access the local agent through the forwarded connection.
411: An attacker cannot obtain key material from the agent,
1.3 stevesk 412: however they can perform operations on the keys that enable them to
413: authenticate using the identities loaded into the agent.
1.1 stevesk 414: .It Cm ForwardX11
415: Specifies whether X11 connections will be automatically redirected
416: over the secure channel and
417: .Ev DISPLAY
418: set.
419: The argument must be
420: .Dq yes
421: or
422: .Dq no .
423: The default is
424: .Dq no .
1.3 stevesk 425: .Pp
1.7 jmc 426: X11 forwarding should be enabled with caution.
427: Users with the ability to bypass file permissions on the remote host
1.22 markus 428: (for the user's X11 authorization database)
1.7 jmc 429: can access the local X11 display through the forwarded connection.
1.22 markus 430: An attacker may then be able to perform activities such as keystroke monitoring
431: if the
432: .Cm ForwardX11Trusted
433: option is also enabled.
434: .It Cm ForwardX11Trusted
1.34 jmc 435: If this option is set to
1.84 jmc 436: .Dq yes ,
437: remote X11 clients will have full access to the original X11 display.
1.42 djm 438: .Pp
1.22 markus 439: If this option is set to
1.84 jmc 440: .Dq no ,
441: remote X11 clients will be considered untrusted and prevented
1.22 markus 442: from stealing or tampering with data belonging to trusted X11
443: clients.
1.42 djm 444: Furthermore, the
445: .Xr xauth 1
446: token used for the session will be set to expire after 20 minutes.
447: Remote clients will be refused access after this time.
1.22 markus 448: .Pp
449: The default is
450: .Dq no .
451: .Pp
452: See the X11 SECURITY extension specification for full details on
453: the restrictions imposed on untrusted clients.
1.1 stevesk 454: .It Cm GatewayPorts
455: Specifies whether remote hosts are allowed to connect to local
456: forwarded ports.
457: By default,
1.84 jmc 458: .Xr ssh 1
1.7 jmc 459: binds local port forwardings to the loopback address.
460: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 461: .Cm GatewayPorts
1.84 jmc 462: can be used to specify that ssh
1.1 stevesk 463: should bind local port forwardings to the wildcard address,
464: thus allowing remote hosts to connect to forwarded ports.
465: The argument must be
466: .Dq yes
467: or
468: .Dq no .
469: The default is
470: .Dq no .
471: .It Cm GlobalKnownHostsFile
472: Specifies a file to use for the global
473: host key database instead of
474: .Pa /etc/ssh/ssh_known_hosts .
1.18 markus 475: .It Cm GSSAPIAuthentication
1.27 markus 476: Specifies whether user authentication based on GSSAPI is allowed.
1.20 jmc 477: The default is
1.21 markus 478: .Dq no .
1.18 markus 479: Note that this option applies to protocol version 2 only.
480: .It Cm GSSAPIDelegateCredentials
481: Forward (delegate) credentials to the server.
482: The default is
483: .Dq no .
484: Note that this option applies to protocol version 2 only.
1.44 djm 485: .It Cm HashKnownHosts
486: Indicates that
1.84 jmc 487: .Xr ssh 1
1.44 djm 488: should hash host names and addresses when they are added to
1.50 djm 489: .Pa ~/.ssh/known_hosts .
1.44 djm 490: These hashed names may be used normally by
1.84 jmc 491: .Xr ssh 1
1.44 djm 492: and
1.84 jmc 493: .Xr sshd 8 ,
1.44 djm 494: but they do not reveal identifying information should the file's contents
495: be disclosed.
496: The default is
497: .Dq no .
1.97 jmc 498: Note that existing names and addresses in known hosts files
499: will not be converted automatically,
500: but may be manually hashed using
1.45 djm 501: .Xr ssh-keygen 1 .
1.1 stevesk 502: .It Cm HostbasedAuthentication
503: Specifies whether to try rhosts based authentication with public key
504: authentication.
505: The argument must be
506: .Dq yes
507: or
508: .Dq no .
509: The default is
510: .Dq no .
511: This option applies to protocol version 2 only and
512: is similar to
513: .Cm RhostsRSAAuthentication .
514: .It Cm HostKeyAlgorithms
515: Specifies the protocol version 2 host key algorithms
516: that the client wants to use in order of preference.
517: The default for this option is:
518: .Dq ssh-rsa,ssh-dss .
519: .It Cm HostKeyAlias
520: Specifies an alias that should be used instead of the
521: real host name when looking up or saving the host key
522: in the host key database files.
1.84 jmc 523: This option is useful for tunneling SSH connections
1.1 stevesk 524: or for multiple servers running on a single host.
525: .It Cm HostName
526: Specifies the real host name to log into.
527: This can be used to specify nicknames or abbreviations for hosts.
1.84 jmc 528: The default is the name given on the command line.
1.1 stevesk 529: Numeric IP addresses are also permitted (both on the command line and in
530: .Cm HostName
531: specifications).
1.29 markus 532: .It Cm IdentitiesOnly
533: Specifies that
1.84 jmc 534: .Xr ssh 1
1.29 markus 535: should only use the authentication identity files configured in the
1.31 jmc 536: .Nm
1.29 markus 537: files,
1.84 jmc 538: even if
539: .Xr ssh-agent 1
1.29 markus 540: offers more identities.
541: The argument to this keyword must be
542: .Dq yes
543: or
544: .Dq no .
1.84 jmc 545: This option is intended for situations where ssh-agent
1.29 markus 546: offers many different identities.
547: The default is
548: .Dq no .
1.67 jmc 549: .It Cm IdentityFile
550: Specifies a file from which the user's RSA or DSA authentication identity
551: is read.
552: The default is
553: .Pa ~/.ssh/identity
554: for protocol version 1, and
555: .Pa ~/.ssh/id_rsa
556: and
557: .Pa ~/.ssh/id_dsa
558: for protocol version 2.
559: Additionally, any identities represented by the authentication agent
560: will be used for authentication.
1.90 djm 561: .Pp
1.67 jmc 562: The file name may use the tilde
1.91 jmc 563: syntax to refer to a user's home directory or one of the following
1.90 djm 564: escape characters:
565: .Ql %d
566: (local user's home directory),
567: .Ql %u
568: (local user name),
569: .Ql %l
570: (local host name),
571: .Ql %h
572: (remote host name) or
1.92 djm 573: .Ql %r
1.90 djm 574: (remote user name).
575: .Pp
1.67 jmc 576: It is possible to have
577: multiple identity files specified in configuration files; all these
578: identities will be tried in sequence.
1.39 djm 579: .It Cm KbdInteractiveDevices
580: Specifies the list of methods to use in keyboard-interactive authentication.
581: Multiple method names must be comma-separated.
582: The default is to use the server specified list.
1.85 jmc 583: The methods available vary depending on what the server supports.
584: For an OpenSSH server,
585: it may be zero or more of:
586: .Dq bsdauth ,
587: .Dq pam ,
588: and
589: .Dq skey .
1.65 reyk 590: .It Cm LocalCommand
591: Specifies a command to execute on the local machine after successfully
592: connecting to the server.
593: The command string extends to the end of the line, and is executed with
594: .Pa /bin/sh .
595: This directive is ignored unless
596: .Cm PermitLocalCommand
597: has been enabled.
1.1 stevesk 598: .It Cm LocalForward
1.74 jmc 599: Specifies that a TCP port on the local machine be forwarded over
1.1 stevesk 600: the secure channel to the specified host and port from the remote machine.
1.49 jmc 601: The first argument must be
1.43 djm 602: .Sm off
1.49 jmc 603: .Oo Ar bind_address : Oc Ar port
1.43 djm 604: .Sm on
1.49 jmc 605: and the second argument must be
606: .Ar host : Ns Ar hostport .
1.46 jmc 607: IPv6 addresses can be specified by enclosing addresses in square brackets or
1.43 djm 608: by using an alternative syntax:
1.49 jmc 609: .Oo Ar bind_address Ns / Oc Ns Ar port
610: and
611: .Ar host Ns / Ns Ar hostport .
1.46 jmc 612: Multiple forwardings may be specified, and additional forwardings can be
1.43 djm 613: given on the command line.
1.1 stevesk 614: Only the superuser can forward privileged ports.
1.43 djm 615: By default, the local port is bound in accordance with the
616: .Cm GatewayPorts
617: setting.
618: However, an explicit
619: .Ar bind_address
620: may be used to bind the connection to a specific address.
621: The
622: .Ar bind_address
623: of
624: .Dq localhost
1.46 jmc 625: indicates that the listening port be bound for local use only, while an
626: empty address or
627: .Sq *
1.43 djm 628: indicates that the port should be available from all interfaces.
1.1 stevesk 629: .It Cm LogLevel
630: Gives the verbosity level that is used when logging messages from
1.84 jmc 631: .Xr ssh 1 .
1.1 stevesk 632: The possible values are:
1.84 jmc 633: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.7 jmc 634: The default is INFO.
635: DEBUG and DEBUG1 are equivalent.
636: DEBUG2 and DEBUG3 each specify higher levels of verbose output.
1.1 stevesk 637: .It Cm MACs
638: Specifies the MAC (message authentication code) algorithms
639: in order of preference.
640: The MAC algorithm is used in protocol version 2
641: for data integrity protection.
642: Multiple algorithms must be comma-separated.
1.84 jmc 643: The default is:
1.101 jmc 644: .Bd -literal -offset indent
645: hmac-md5,hmac-sha1,umac-64@openssh.com,
646: hmac-ripemd160,hmac-sha1-96,hmac-md5-96
647: .Ed
1.1 stevesk 648: .It Cm NoHostAuthenticationForLocalhost
649: This option can be used if the home directory is shared across machines.
650: In this case localhost will refer to a different machine on each of
651: the machines and the user will get many warnings about changed host keys.
652: However, this option disables host authentication for localhost.
653: The argument to this keyword must be
654: .Dq yes
655: or
656: .Dq no .
657: The default is to check the host key for localhost.
658: .It Cm NumberOfPasswordPrompts
659: Specifies the number of password prompts before giving up.
660: The argument to this keyword must be an integer.
1.84 jmc 661: The default is 3.
1.1 stevesk 662: .It Cm PasswordAuthentication
663: Specifies whether to use password authentication.
664: The argument to this keyword must be
665: .Dq yes
666: or
667: .Dq no .
668: The default is
669: .Dq yes .
1.65 reyk 670: .It Cm PermitLocalCommand
671: Allow local command execution via the
672: .Ic LocalCommand
673: option or using the
1.66 jmc 674: .Ic !\& Ns Ar command
1.65 reyk 675: escape sequence in
676: .Xr ssh 1 .
677: The argument must be
678: .Dq yes
679: or
680: .Dq no .
681: The default is
682: .Dq no .
1.67 jmc 683: .It Cm Port
684: Specifies the port number to connect on the remote host.
1.84 jmc 685: The default is 22.
1.1 stevesk 686: .It Cm PreferredAuthentications
687: Specifies the order in which the client should try protocol 2
1.11 jmc 688: authentication methods.
1.48 jmc 689: This allows a client to prefer one method (e.g.\&
1.1 stevesk 690: .Cm keyboard-interactive )
1.48 jmc 691: over another method (e.g.\&
1.1 stevesk 692: .Cm password )
693: The default for this option is:
1.94 jmc 694: .Do gssapi-with-mic ,
695: hostbased,
696: publickey,
697: keyboard-interactive,
698: password
699: .Dc .
1.1 stevesk 700: .It Cm Protocol
701: Specifies the protocol versions
1.84 jmc 702: .Xr ssh 1
1.1 stevesk 703: should support in order of preference.
704: The possible values are
1.84 jmc 705: .Sq 1
1.1 stevesk 706: and
1.84 jmc 707: .Sq 2 .
1.1 stevesk 708: Multiple versions must be comma-separated.
709: The default is
710: .Dq 2,1 .
1.84 jmc 711: This means that ssh
1.1 stevesk 712: tries version 2 and falls back to version 1
713: if version 2 is not available.
714: .It Cm ProxyCommand
715: Specifies the command to use to connect to the server.
716: The command
717: string extends to the end of the line, and is executed with
718: .Pa /bin/sh .
719: In the command string,
720: .Ql %h
721: will be substituted by the host name to
722: connect and
723: .Ql %p
724: by the port.
725: The command can be basically anything,
726: and should read from its standard input and write to its standard output.
727: It should eventually connect an
728: .Xr sshd 8
729: server running on some machine, or execute
730: .Ic sshd -i
731: somewhere.
732: Host key management will be done using the
733: HostName of the host being connected (defaulting to the name typed by
734: the user).
1.7 jmc 735: Setting the command to
736: .Dq none
1.6 markus 737: disables this option entirely.
1.1 stevesk 738: Note that
739: .Cm CheckHostIP
740: is not available for connects with a proxy command.
1.52 djm 741: .Pp
742: This directive is useful in conjunction with
743: .Xr nc 1
744: and its proxy support.
1.53 jmc 745: For example, the following directive would connect via an HTTP proxy at
1.52 djm 746: 192.0.2.0:
747: .Bd -literal -offset 3n
748: ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
749: .Ed
1.1 stevesk 750: .It Cm PubkeyAuthentication
751: Specifies whether to try public key authentication.
752: The argument to this keyword must be
753: .Dq yes
754: or
755: .Dq no .
756: The default is
757: .Dq yes .
758: This option applies to protocol version 2 only.
1.75 dtucker 759: .It Cm RekeyLimit
760: Specifies the maximum amount of data that may be transmitted before the
1.76 jmc 761: session key is renegotiated.
1.75 dtucker 762: The argument is the number of bytes, with an optional suffix of
1.76 jmc 763: .Sq K ,
764: .Sq M ,
1.75 dtucker 765: or
1.76 jmc 766: .Sq G
1.75 dtucker 767: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
768: The default is between
1.84 jmc 769: .Sq 1G
1.75 dtucker 770: and
1.84 jmc 771: .Sq 4G ,
1.75 dtucker 772: depending on the cipher.
1.76 jmc 773: This option applies to protocol version 2 only.
1.1 stevesk 774: .It Cm RemoteForward
1.74 jmc 775: Specifies that a TCP port on the remote machine be forwarded over
1.1 stevesk 776: the secure channel to the specified host and port from the local machine.
1.49 jmc 777: The first argument must be
1.43 djm 778: .Sm off
1.49 jmc 779: .Oo Ar bind_address : Oc Ar port
1.43 djm 780: .Sm on
1.49 jmc 781: and the second argument must be
782: .Ar host : Ns Ar hostport .
783: IPv6 addresses can be specified by enclosing addresses in square brackets
784: or by using an alternative syntax:
785: .Oo Ar bind_address Ns / Oc Ns Ar port
786: and
787: .Ar host Ns / Ns Ar hostport .
1.1 stevesk 788: Multiple forwardings may be specified, and additional
789: forwardings can be given on the command line.
790: Only the superuser can forward privileged ports.
1.43 djm 791: .Pp
792: If the
793: .Ar bind_address
794: is not specified, the default is to only bind to loopback addresses.
795: If the
796: .Ar bind_address
797: is
798: .Ql *
799: or an empty string, then the forwarding is requested to listen on all
800: interfaces.
801: Specifying a remote
802: .Ar bind_address
1.46 jmc 803: will only succeed if the server's
804: .Cm GatewayPorts
1.43 djm 805: option is enabled (see
1.46 jmc 806: .Xr sshd_config 5 ) .
1.1 stevesk 807: .It Cm RhostsRSAAuthentication
808: Specifies whether to try rhosts based authentication with RSA host
809: authentication.
810: The argument must be
811: .Dq yes
812: or
813: .Dq no .
814: The default is
815: .Dq no .
816: This option applies to protocol version 1 only and requires
1.84 jmc 817: .Xr ssh 1
1.1 stevesk 818: to be setuid root.
819: .It Cm RSAAuthentication
820: Specifies whether to try RSA authentication.
821: The argument to this keyword must be
822: .Dq yes
823: or
824: .Dq no .
825: RSA authentication will only be
826: attempted if the identity file exists, or an authentication agent is
827: running.
828: The default is
829: .Dq yes .
830: Note that this option applies to protocol version 1 only.
1.32 djm 831: .It Cm SendEnv
832: Specifies what variables from the local
833: .Xr environ 7
834: should be sent to the server.
1.84 jmc 835: Note that environment passing is only supported for protocol 2.
836: The server must also support it, and the server must be configured to
1.33 djm 837: accept these environment variables.
1.32 djm 838: Refer to
839: .Cm AcceptEnv
840: in
841: .Xr sshd_config 5
842: for how to configure the server.
1.80 jmc 843: Variables are specified by name, which may contain wildcard characters.
1.33 djm 844: Multiple environment variables may be separated by whitespace or spread
1.32 djm 845: across multiple
846: .Cm SendEnv
847: directives.
848: The default is not to send any environment variables.
1.81 jmc 849: .Pp
850: See
851: .Sx PATTERNS
852: for more information on patterns.
1.28 markus 853: .It Cm ServerAliveCountMax
1.73 jmc 854: Sets the number of server alive messages (see below) which may be
1.28 markus 855: sent without
1.84 jmc 856: .Xr ssh 1
1.28 markus 857: receiving any messages back from the server.
858: If this threshold is reached while server alive messages are being sent,
1.84 jmc 859: ssh will disconnect from the server, terminating the session.
1.28 markus 860: It is important to note that the use of server alive messages is very
861: different from
862: .Cm TCPKeepAlive
863: (below).
864: The server alive messages are sent through the encrypted channel
865: and therefore will not be spoofable.
866: The TCP keepalive option enabled by
867: .Cm TCPKeepAlive
868: is spoofable.
869: The server alive mechanism is valuable when the client or
870: server depend on knowing when a connection has become inactive.
871: .Pp
872: The default value is 3.
873: If, for example,
874: .Cm ServerAliveInterval
1.84 jmc 875: (see below) is set to 15 and
1.28 markus 876: .Cm ServerAliveCountMax
1.84 jmc 877: is left at the default, if the server becomes unresponsive,
878: ssh will disconnect after approximately 45 seconds.
1.89 markus 879: This option applies to protocol version 2 only.
1.67 jmc 880: .It Cm ServerAliveInterval
881: Sets a timeout interval in seconds after which if no data has been received
882: from the server,
1.84 jmc 883: .Xr ssh 1
1.67 jmc 884: will send a message through the encrypted
885: channel to request a response from the server.
886: The default
887: is 0, indicating that these messages will not be sent to the server.
888: This option applies to protocol version 2 only.
1.1 stevesk 889: .It Cm SmartcardDevice
1.11 jmc 890: Specifies which smartcard device to use.
891: The argument to this keyword is the device
1.84 jmc 892: .Xr ssh 1
1.1 stevesk 893: should use to communicate with a smartcard used for storing the user's
1.11 jmc 894: private RSA key.
895: By default, no device is specified and smartcard support is not activated.
1.1 stevesk 896: .It Cm StrictHostKeyChecking
897: If this flag is set to
898: .Dq yes ,
1.84 jmc 899: .Xr ssh 1
1.1 stevesk 900: will never automatically add host keys to the
1.50 djm 901: .Pa ~/.ssh/known_hosts
1.1 stevesk 902: file, and refuses to connect to hosts whose host key has changed.
903: This provides maximum protection against trojan horse attacks,
1.84 jmc 904: though it can be annoying when the
1.1 stevesk 905: .Pa /etc/ssh/ssh_known_hosts
1.84 jmc 906: file is poorly maintained or when connections to new hosts are
1.1 stevesk 907: frequently made.
908: This option forces the user to manually
909: add all new hosts.
910: If this flag is set to
911: .Dq no ,
1.84 jmc 912: ssh will automatically add new host keys to the
1.1 stevesk 913: user known hosts files.
914: If this flag is set to
915: .Dq ask ,
916: new host keys
917: will be added to the user known host files only after the user
918: has confirmed that is what they really want to do, and
1.84 jmc 919: ssh will refuse to connect to hosts whose host key has changed.
1.1 stevesk 920: The host keys of
921: known hosts will be verified automatically in all cases.
922: The argument must be
923: .Dq yes ,
1.84 jmc 924: .Dq no ,
1.1 stevesk 925: or
926: .Dq ask .
927: The default is
928: .Dq ask .
1.26 markus 929: .It Cm TCPKeepAlive
930: Specifies whether the system should send TCP keepalive messages to the
931: other side.
932: If they are sent, death of the connection or crash of one
933: of the machines will be properly noticed.
934: However, this means that
935: connections will die if the route is down temporarily, and some people
936: find it annoying.
937: .Pp
938: The default is
939: .Dq yes
940: (to send TCP keepalive messages), and the client will notice
941: if the network goes down or the remote host dies.
942: This is important in scripts, and many users want it too.
943: .Pp
944: To disable TCP keepalive messages, the value should be set to
945: .Dq no .
1.65 reyk 946: .It Cm Tunnel
1.95 stevesk 947: Request
1.65 reyk 948: .Xr tun 4
1.69 jmc 949: device forwarding between the client and the server.
1.65 reyk 950: The argument must be
1.68 reyk 951: .Dq yes ,
1.95 stevesk 952: .Dq point-to-point
953: (layer 3),
954: .Dq ethernet
955: (layer 2),
1.65 reyk 956: or
957: .Dq no .
1.95 stevesk 958: Specifying
959: .Dq yes
960: requests the default tunnel mode, which is
961: .Dq point-to-point .
1.65 reyk 962: The default is
963: .Dq no .
964: .It Cm TunnelDevice
1.95 stevesk 965: Specifies the
1.65 reyk 966: .Xr tun 4
1.95 stevesk 967: devices to open on the client
968: .Pq Ar local_tun
969: and the server
970: .Pq Ar remote_tun .
971: .Pp
972: The argument must be
973: .Sm off
974: .Ar local_tun Op : Ar remote_tun .
975: .Sm on
976: The devices may be specified by numerical ID or the keyword
977: .Dq any ,
978: which uses the next available tunnel device.
979: If
980: .Ar remote_tun
981: is not specified, it defaults to
982: .Dq any .
983: The default is
984: .Dq any:any .
1.72 jmc 985: .It Cm UsePrivilegedPort
986: Specifies whether to use a privileged port for outgoing connections.
987: The argument must be
988: .Dq yes
989: or
990: .Dq no .
991: The default is
992: .Dq no .
993: If set to
1.84 jmc 994: .Dq yes ,
995: .Xr ssh 1
1.72 jmc 996: must be setuid root.
997: Note that this option must be set to
998: .Dq yes
999: for
1000: .Cm RhostsRSAAuthentication
1001: with older servers.
1.1 stevesk 1002: .It Cm User
1003: Specifies the user to log in as.
1004: This can be useful when a different user name is used on different machines.
1005: This saves the trouble of
1006: having to remember to give the user name on the command line.
1007: .It Cm UserKnownHostsFile
1008: Specifies a file to use for the user
1009: host key database instead of
1.50 djm 1010: .Pa ~/.ssh/known_hosts .
1.8 jakob 1011: .It Cm VerifyHostKeyDNS
1012: Specifies whether to verify the remote key using DNS and SSHFP resource
1013: records.
1.24 jakob 1014: If this option is set to
1015: .Dq yes ,
1.25 jmc 1016: the client will implicitly trust keys that match a secure fingerprint
1.24 jakob 1017: from DNS.
1018: Insecure fingerprints will be handled as if this option was set to
1019: .Dq ask .
1020: If this option is set to
1021: .Dq ask ,
1022: information on fingerprint match will be displayed, but the user will still
1023: need to confirm new host keys according to the
1024: .Cm StrictHostKeyChecking
1025: option.
1026: The argument must be
1027: .Dq yes ,
1.84 jmc 1028: .Dq no ,
1.25 jmc 1029: or
1030: .Dq ask .
1.8 jakob 1031: The default is
1032: .Dq no .
1.12 jakob 1033: Note that this option applies to protocol version 2 only.
1.84 jmc 1034: .Pp
1035: See also
1036: .Sx VERIFYING HOST KEYS
1037: in
1038: .Xr ssh 1 .
1.1 stevesk 1039: .It Cm XAuthLocation
1.5 stevesk 1040: Specifies the full pathname of the
1.1 stevesk 1041: .Xr xauth 1
1042: program.
1043: The default is
1044: .Pa /usr/X11R6/bin/xauth .
1045: .El
1.86 jmc 1046: .Sh PATTERNS
1047: A
1048: .Em pattern
1049: consists of zero or more non-whitespace characters,
1050: .Sq *
1051: (a wildcard that matches zero or more characters),
1052: or
1053: .Sq ?\&
1054: (a wildcard that matches exactly one character).
1055: For example, to specify a set of declarations for any host in the
1056: .Dq .co.uk
1057: set of domains,
1058: the following pattern could be used:
1059: .Pp
1060: .Dl Host *.co.uk
1061: .Pp
1062: The following pattern
1063: would match any host in the 192.168.0.[0-9] network range:
1064: .Pp
1065: .Dl Host 192.168.0.?
1066: .Pp
1067: A
1068: .Em pattern-list
1069: is a comma-separated list of patterns.
1070: Patterns within pattern-lists may be negated
1071: by preceding them with an exclamation mark
1072: .Pq Sq !\& .
1073: For example,
1074: to allow a key to be used from anywhere within an organisation
1075: except from the
1076: .Dq dialup
1077: pool,
1078: the following entry (in authorized_keys) could be used:
1079: .Pp
1080: .Dl from=\&"!*.dialup.example.com,*.example.com\&"
1.1 stevesk 1081: .Sh FILES
1082: .Bl -tag -width Ds
1.50 djm 1083: .It Pa ~/.ssh/config
1.1 stevesk 1084: This is the per-user configuration file.
1085: The format of this file is described above.
1.84 jmc 1086: This file is used by the SSH client.
1.30 djm 1087: Because of the potential for abuse, this file must have strict permissions:
1088: read/write for the user, and not accessible by others.
1.1 stevesk 1089: .It Pa /etc/ssh/ssh_config
1090: Systemwide configuration file.
1091: This file provides defaults for those
1092: values that are not specified in the user's configuration file, and
1093: for those users who do not have a configuration file.
1094: This file must be world-readable.
1095: .El
1.13 jmc 1096: .Sh SEE ALSO
1097: .Xr ssh 1
1.1 stevesk 1098: .Sh AUTHORS
1099: OpenSSH is a derivative of the original and free
1100: ssh 1.2.12 release by Tatu Ylonen.
1101: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1102: Theo de Raadt and Dug Song
1103: removed many bugs, re-added newer features and
1104: created OpenSSH.
1105: Markus Friedl contributed the support for SSH
1106: protocol versions 1.5 and 2.0.
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