Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.120
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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18: .\" modification, are permitted provided that the following conditions
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22: .\" 2. Redistributions in binary form must reproduce the above copyright
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27: .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
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1.120 ! djm 37: .\" $OpenBSD: sshd_config.5,v 1.119 2010/03/04 12:51:25 jmc Exp $
1.119 jmc 38: .Dd $Mdocdate: March 4 2010 $
1.1 stevesk 39: .Dt SSHD_CONFIG 5
40: .Os
41: .Sh NAME
42: .Nm sshd_config
43: .Nd OpenSSH SSH daemon configuration file
44: .Sh SYNOPSIS
1.71 jmc 45: .Nm /etc/ssh/sshd_config
1.1 stevesk 46: .Sh DESCRIPTION
1.53 jmc 47: .Xr sshd 8
1.1 stevesk 48: reads configuration data from
49: .Pa /etc/ssh/sshd_config
50: (or the file specified with
51: .Fl f
52: on the command line).
53: The file contains keyword-argument pairs, one per line.
54: Lines starting with
55: .Ql #
56: and empty lines are interpreted as comments.
1.56 dtucker 57: Arguments may optionally be enclosed in double quotes
58: .Pq \&"
59: in order to represent arguments containing spaces.
1.1 stevesk 60: .Pp
61: The possible
62: keywords and their meanings are as follows (note that
63: keywords are case-insensitive and arguments are case-sensitive):
64: .Bl -tag -width Ds
1.30 djm 65: .It Cm AcceptEnv
66: Specifies what environment variables sent by the client will be copied into
67: the session's
68: .Xr environ 7 .
69: See
70: .Cm SendEnv
71: in
72: .Xr ssh_config 5
73: for how to configure the client.
1.31 djm 74: Note that environment passing is only supported for protocol 2.
1.30 djm 75: Variables are specified by name, which may contain the wildcard characters
1.51 jmc 76: .Ql *
1.30 djm 77: and
78: .Ql \&? .
1.31 djm 79: Multiple environment variables may be separated by whitespace or spread
1.30 djm 80: across multiple
81: .Cm AcceptEnv
82: directives.
1.31 djm 83: Be warned that some environment variables could be used to bypass restricted
1.30 djm 84: user environments.
85: For this reason, care should be taken in the use of this directive.
86: The default is not to accept any environment variables.
1.37 djm 87: .It Cm AddressFamily
88: Specifies which address family should be used by
1.53 jmc 89: .Xr sshd 8 .
1.37 djm 90: Valid arguments are
91: .Dq any ,
92: .Dq inet
1.52 jmc 93: (use IPv4 only), or
1.37 djm 94: .Dq inet6
95: (use IPv6 only).
96: The default is
97: .Dq any .
1.89 jmc 98: .It Cm AllowAgentForwarding
99: Specifies whether
100: .Xr ssh-agent 1
101: forwarding is permitted.
102: The default is
103: .Dq yes .
104: Note that disabling agent forwarding does not improve security
105: unless users are also denied shell access, as they can always install
106: their own forwarders.
1.1 stevesk 107: .It Cm AllowGroups
108: This keyword can be followed by a list of group name patterns, separated
109: by spaces.
110: If specified, login is allowed only for users whose primary
111: group or supplementary group list matches one of the patterns.
112: Only group names are valid; a numerical group ID is not recognized.
113: By default, login is allowed for all groups.
1.54 jmc 114: The allow/deny directives are processed in the following order:
115: .Cm DenyUsers ,
116: .Cm AllowUsers ,
117: .Cm DenyGroups ,
118: and finally
119: .Cm AllowGroups .
1.49 jmc 120: .Pp
121: See
122: .Sx PATTERNS
123: in
124: .Xr ssh_config 5
125: for more information on patterns.
1.1 stevesk 126: .It Cm AllowTcpForwarding
127: Specifies whether TCP forwarding is permitted.
128: The default is
129: .Dq yes .
130: Note that disabling TCP forwarding does not improve security unless
131: users are also denied shell access, as they can always install their
132: own forwarders.
133: .It Cm AllowUsers
134: This keyword can be followed by a list of user name patterns, separated
135: by spaces.
1.14 jmc 136: If specified, login is allowed only for user names that
1.1 stevesk 137: match one of the patterns.
138: Only user names are valid; a numerical user ID is not recognized.
139: By default, login is allowed for all users.
140: If the pattern takes the form USER@HOST then USER and HOST
141: are separately checked, restricting logins to particular
142: users from particular hosts.
1.54 jmc 143: The allow/deny directives are processed in the following order:
144: .Cm DenyUsers ,
145: .Cm AllowUsers ,
146: .Cm DenyGroups ,
147: and finally
148: .Cm AllowGroups .
1.49 jmc 149: .Pp
150: See
151: .Sx PATTERNS
152: in
153: .Xr ssh_config 5
154: for more information on patterns.
1.1 stevesk 155: .It Cm AuthorizedKeysFile
156: Specifies the file that contains the public keys that can be used
157: for user authentication.
158: .Cm AuthorizedKeysFile
159: may contain tokens of the form %T which are substituted during connection
1.52 jmc 160: setup.
1.17 jmc 161: The following tokens are defined: %% is replaced by a literal '%',
1.52 jmc 162: %h is replaced by the home directory of the user being authenticated, and
1.1 stevesk 163: %u is replaced by the username of that user.
164: After expansion,
165: .Cm AuthorizedKeysFile
166: is taken to be an absolute path or one relative to the user's home
167: directory.
168: The default is
169: .Dq .ssh/authorized_keys .
170: .It Cm Banner
171: The contents of the specified file are sent to the remote user before
172: authentication is allowed.
1.78 djm 173: If the argument is
174: .Dq none
175: then no banner is displayed.
1.1 stevesk 176: This option is only available for protocol version 2.
177: By default, no banner is displayed.
178: .It Cm ChallengeResponseAuthentication
1.50 jmc 179: Specifies whether challenge-response authentication is allowed.
1.1 stevesk 180: All authentication styles from
181: .Xr login.conf 5
182: are supported.
183: The default is
184: .Dq yes .
1.80 djm 185: .It Cm ChrootDirectory
1.113 stevesk 186: Specifies the pathname of a directory to
1.80 djm 187: .Xr chroot 2
188: to after authentication.
1.113 stevesk 189: All components of the pathname must be root-owned directories that are
1.80 djm 190: not writable by any other user or group.
1.106 stevesk 191: After the chroot,
192: .Xr sshd 8
193: changes the working directory to the user's home directory.
1.80 djm 194: .Pp
1.113 stevesk 195: The pathname may contain the following tokens that are expanded at runtime once
1.80 djm 196: the connecting user has been authenticated: %% is replaced by a literal '%',
197: %h is replaced by the home directory of the user being authenticated, and
198: %u is replaced by the username of that user.
199: .Pp
200: The
201: .Cm ChrootDirectory
202: must contain the necessary files and directories to support the
1.103 stevesk 203: user's session.
1.80 djm 204: For an interactive session this requires at least a shell, typically
205: .Xr sh 1 ,
206: and basic
207: .Pa /dev
208: nodes such as
209: .Xr null 4 ,
210: .Xr zero 4 ,
211: .Xr stdin 4 ,
212: .Xr stdout 4 ,
213: .Xr stderr 4 ,
214: .Xr arandom 4
215: and
216: .Xr tty 4
217: devices.
218: For file transfer sessions using
1.105 jmc 219: .Dq sftp ,
1.80 djm 220: no additional configuration of the environment is necessary if the
1.105 jmc 221: in-process sftp server is used,
222: though sessions which use logging do require
1.104 stevesk 223: .Pa /dev/log
224: inside the chroot directory (see
225: .Xr sftp-server 8
1.81 jmc 226: for details).
1.80 djm 227: .Pp
228: The default is not to
229: .Xr chroot 2 .
1.1 stevesk 230: .It Cm Ciphers
231: Specifies the ciphers allowed for protocol version 2.
232: Multiple ciphers must be comma-separated.
1.34 dtucker 233: The supported ciphers are
234: .Dq 3des-cbc ,
235: .Dq aes128-cbc ,
236: .Dq aes192-cbc ,
237: .Dq aes256-cbc ,
238: .Dq aes128-ctr ,
239: .Dq aes192-ctr ,
240: .Dq aes256-ctr ,
1.43 djm 241: .Dq arcfour128 ,
242: .Dq arcfour256 ,
1.34 dtucker 243: .Dq arcfour ,
244: .Dq blowfish-cbc ,
245: and
246: .Dq cast128-cbc .
1.52 jmc 247: The default is:
248: .Bd -literal -offset 3n
1.100 naddy 249: aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
250: aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
251: aes256-cbc,arcfour
1.1 stevesk 252: .Ed
253: .It Cm ClientAliveCountMax
1.48 jmc 254: Sets the number of client alive messages (see below) which may be
1.1 stevesk 255: sent without
1.52 jmc 256: .Xr sshd 8
1.17 jmc 257: receiving any messages back from the client.
258: If this threshold is reached while client alive messages are being sent,
1.52 jmc 259: sshd will disconnect the client, terminating the session.
1.17 jmc 260: It is important to note that the use of client alive messages is very
261: different from
1.27 markus 262: .Cm TCPKeepAlive
1.17 jmc 263: (below).
264: The client alive messages are sent through the encrypted channel
265: and therefore will not be spoofable.
266: The TCP keepalive option enabled by
1.27 markus 267: .Cm TCPKeepAlive
1.17 jmc 268: is spoofable.
269: The client alive mechanism is valuable when the client or
1.1 stevesk 270: server depend on knowing when a connection has become inactive.
271: .Pp
1.17 jmc 272: The default value is 3.
273: If
1.1 stevesk 274: .Cm ClientAliveInterval
1.48 jmc 275: (see below) is set to 15, and
1.1 stevesk 276: .Cm ClientAliveCountMax
1.52 jmc 277: is left at the default, unresponsive SSH clients
1.1 stevesk 278: will be disconnected after approximately 45 seconds.
1.57 markus 279: This option applies to protocol version 2 only.
1.42 djm 280: .It Cm ClientAliveInterval
281: Sets a timeout interval in seconds after which if no data has been received
282: from the client,
1.52 jmc 283: .Xr sshd 8
1.42 djm 284: will send a message through the encrypted
285: channel to request a response from the client.
286: The default
287: is 0, indicating that these messages will not be sent to the client.
288: This option applies to protocol version 2 only.
1.3 markus 289: .It Cm Compression
1.44 markus 290: Specifies whether compression is allowed, or delayed until
291: the user has authenticated successfully.
1.3 markus 292: The argument must be
1.44 markus 293: .Dq yes ,
294: .Dq delayed ,
1.3 markus 295: or
296: .Dq no .
297: The default is
1.44 markus 298: .Dq delayed .
1.1 stevesk 299: .It Cm DenyGroups
300: This keyword can be followed by a list of group name patterns, separated
301: by spaces.
302: Login is disallowed for users whose primary group or supplementary
303: group list matches one of the patterns.
304: Only group names are valid; a numerical group ID is not recognized.
305: By default, login is allowed for all groups.
1.54 jmc 306: The allow/deny directives are processed in the following order:
307: .Cm DenyUsers ,
308: .Cm AllowUsers ,
309: .Cm DenyGroups ,
310: and finally
311: .Cm AllowGroups .
1.49 jmc 312: .Pp
313: See
314: .Sx PATTERNS
315: in
316: .Xr ssh_config 5
317: for more information on patterns.
1.1 stevesk 318: .It Cm DenyUsers
319: This keyword can be followed by a list of user name patterns, separated
320: by spaces.
321: Login is disallowed for user names that match one of the patterns.
322: Only user names are valid; a numerical user ID is not recognized.
323: By default, login is allowed for all users.
324: If the pattern takes the form USER@HOST then USER and HOST
325: are separately checked, restricting logins to particular
326: users from particular hosts.
1.54 jmc 327: The allow/deny directives are processed in the following order:
328: .Cm DenyUsers ,
329: .Cm AllowUsers ,
330: .Cm DenyGroups ,
331: and finally
332: .Cm AllowGroups .
1.49 jmc 333: .Pp
334: See
335: .Sx PATTERNS
336: in
337: .Xr ssh_config 5
338: for more information on patterns.
1.67 dtucker 339: .It Cm ForceCommand
340: Forces the execution of the command specified by
341: .Cm ForceCommand ,
1.84 djm 342: ignoring any command supplied by the client and
343: .Pa ~/.ssh/rc
344: if present.
1.67 dtucker 345: The command is invoked by using the user's login shell with the -c option.
346: This applies to shell, command, or subsystem execution.
347: It is most useful inside a
348: .Cm Match
349: block.
350: The command originally supplied by the client is available in the
351: .Ev SSH_ORIGINAL_COMMAND
352: environment variable.
1.82 djm 353: Specifying a command of
354: .Dq internal-sftp
355: will force the use of an in-process sftp server that requires no support
356: files when used with
357: .Cm ChrootDirectory .
1.1 stevesk 358: .It Cm GatewayPorts
359: Specifies whether remote hosts are allowed to connect to ports
360: forwarded for the client.
361: By default,
1.52 jmc 362: .Xr sshd 8
1.15 jmc 363: binds remote port forwardings to the loopback address.
364: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 365: .Cm GatewayPorts
1.52 jmc 366: can be used to specify that sshd
1.39 djm 367: should allow remote port forwardings to bind to non-loopback addresses, thus
368: allowing other hosts to connect.
369: The argument may be
370: .Dq no
371: to force remote port forwardings to be available to the local host only,
1.1 stevesk 372: .Dq yes
1.39 djm 373: to force remote port forwardings to bind to the wildcard address, or
374: .Dq clientspecified
375: to allow the client to select the address to which the forwarding is bound.
1.1 stevesk 376: The default is
377: .Dq no .
1.23 markus 378: .It Cm GSSAPIAuthentication
1.25 markus 379: Specifies whether user authentication based on GSSAPI is allowed.
1.26 djm 380: The default is
1.23 markus 381: .Dq no .
382: Note that this option applies to protocol version 2 only.
383: .It Cm GSSAPICleanupCredentials
384: Specifies whether to automatically destroy the user's credentials cache
385: on logout.
386: The default is
387: .Dq yes .
388: Note that this option applies to protocol version 2 only.
1.1 stevesk 389: .It Cm HostbasedAuthentication
390: Specifies whether rhosts or /etc/hosts.equiv authentication together
391: with successful public key client host authentication is allowed
1.50 jmc 392: (host-based authentication).
1.1 stevesk 393: This option is similar to
394: .Cm RhostsRSAAuthentication
395: and applies to protocol version 2 only.
1.70 dtucker 396: The default is
397: .Dq no .
398: .It Cm HostbasedUsesNameFromPacketOnly
399: Specifies whether or not the server will attempt to perform a reverse
400: name lookup when matching the name in the
401: .Pa ~/.shosts ,
402: .Pa ~/.rhosts ,
403: and
404: .Pa /etc/hosts.equiv
405: files during
406: .Cm HostbasedAuthentication .
407: A setting of
408: .Dq yes
409: means that
410: .Xr sshd 8
411: uses the name supplied by the client rather than
412: attempting to resolve the name from the TCP connection itself.
1.1 stevesk 413: The default is
414: .Dq no .
1.117 djm 415: .It Cm HostCertificate
416: Specifies a file containing a public host certificate.
417: The certificate's public key must match a private host key already specified
418: by
419: .Cm HostKey .
420: The default behaviour of
421: .Xr sshd 8
422: is not to load any certificates.
1.1 stevesk 423: .It Cm HostKey
424: Specifies a file containing a private host key
425: used by SSH.
426: The default is
427: .Pa /etc/ssh/ssh_host_key
428: for protocol version 1, and
429: .Pa /etc/ssh/ssh_host_rsa_key
430: and
431: .Pa /etc/ssh/ssh_host_dsa_key
432: for protocol version 2.
433: Note that
1.52 jmc 434: .Xr sshd 8
1.1 stevesk 435: will refuse to use a file if it is group/world-accessible.
436: It is possible to have multiple host key files.
437: .Dq rsa1
438: keys are used for version 1 and
439: .Dq dsa
440: or
441: .Dq rsa
442: are used for version 2 of the SSH protocol.
443: .It Cm IgnoreRhosts
444: Specifies that
445: .Pa .rhosts
446: and
447: .Pa .shosts
448: files will not be used in
449: .Cm RhostsRSAAuthentication
450: or
451: .Cm HostbasedAuthentication .
452: .Pp
453: .Pa /etc/hosts.equiv
454: and
455: .Pa /etc/shosts.equiv
456: are still used.
457: The default is
458: .Dq yes .
459: .It Cm IgnoreUserKnownHosts
460: Specifies whether
1.52 jmc 461: .Xr sshd 8
1.1 stevesk 462: should ignore the user's
1.41 djm 463: .Pa ~/.ssh/known_hosts
1.1 stevesk 464: during
465: .Cm RhostsRSAAuthentication
466: or
467: .Cm HostbasedAuthentication .
468: The default is
469: .Dq no .
470: .It Cm KerberosAuthentication
1.24 markus 471: Specifies whether the password provided by the user for
1.1 stevesk 472: .Cm PasswordAuthentication
1.24 markus 473: will be validated through the Kerberos KDC.
1.1 stevesk 474: To use this option, the server needs a
475: Kerberos servtab which allows the verification of the KDC's identity.
1.52 jmc 476: The default is
1.29 dtucker 477: .Dq no .
478: .It Cm KerberosGetAFSToken
1.45 djm 479: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
1.29 dtucker 480: an AFS token before accessing the user's home directory.
1.52 jmc 481: The default is
1.1 stevesk 482: .Dq no .
483: .It Cm KerberosOrLocalPasswd
1.52 jmc 484: If password authentication through Kerberos fails then
1.1 stevesk 485: the password will be validated via any additional local mechanism
486: such as
487: .Pa /etc/passwd .
1.52 jmc 488: The default is
1.1 stevesk 489: .Dq yes .
490: .It Cm KerberosTicketCleanup
491: Specifies whether to automatically destroy the user's ticket cache
492: file on logout.
1.52 jmc 493: The default is
1.1 stevesk 494: .Dq yes .
495: .It Cm KeyRegenerationInterval
496: In protocol version 1, the ephemeral server key is automatically regenerated
497: after this many seconds (if it has been used).
498: The purpose of regeneration is to prevent
499: decrypting captured sessions by later breaking into the machine and
500: stealing the keys.
501: The key is never stored anywhere.
502: If the value is 0, the key is never regenerated.
503: The default is 3600 (seconds).
504: .It Cm ListenAddress
505: Specifies the local addresses
1.52 jmc 506: .Xr sshd 8
1.1 stevesk 507: should listen on.
508: The following forms may be used:
509: .Pp
510: .Bl -item -offset indent -compact
511: .It
512: .Cm ListenAddress
513: .Sm off
514: .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
515: .Sm on
516: .It
517: .Cm ListenAddress
518: .Sm off
519: .Ar host No | Ar IPv4_addr No : Ar port
520: .Sm on
521: .It
522: .Cm ListenAddress
523: .Sm off
524: .Oo
525: .Ar host No | Ar IPv6_addr Oc : Ar port
526: .Sm on
527: .El
528: .Pp
529: If
530: .Ar port
531: is not specified,
1.52 jmc 532: sshd will listen on the address and all prior
1.1 stevesk 533: .Cm Port
1.17 jmc 534: options specified.
535: The default is to listen on all local addresses.
1.15 jmc 536: Multiple
1.1 stevesk 537: .Cm ListenAddress
1.17 jmc 538: options are permitted.
539: Additionally, any
1.1 stevesk 540: .Cm Port
1.52 jmc 541: options must precede this option for non-port qualified addresses.
1.1 stevesk 542: .It Cm LoginGraceTime
543: The server disconnects after this time if the user has not
544: successfully logged in.
545: If the value is 0, there is no time limit.
1.12 stevesk 546: The default is 120 seconds.
1.1 stevesk 547: .It Cm LogLevel
548: Gives the verbosity level that is used when logging messages from
1.53 jmc 549: .Xr sshd 8 .
1.1 stevesk 550: The possible values are:
1.52 jmc 551: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 552: The default is INFO.
553: DEBUG and DEBUG1 are equivalent.
554: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
555: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 556: .It Cm MACs
557: Specifies the available MAC (message authentication code) algorithms.
558: The MAC algorithm is used in protocol version 2
559: for data integrity protection.
560: Multiple algorithms must be comma-separated.
1.52 jmc 561: The default is:
1.77 jmc 562: .Bd -literal -offset indent
563: hmac-md5,hmac-sha1,umac-64@openssh.com,
564: hmac-ripemd160,hmac-sha1-96,hmac-md5-96
565: .Ed
1.60 dtucker 566: .It Cm Match
1.61 jmc 567: Introduces a conditional block.
1.65 dtucker 568: If all of the criteria on the
1.60 dtucker 569: .Cm Match
1.65 dtucker 570: line are satisfied, the keywords on the following lines override those
571: set in the global section of the config file, until either another
1.60 dtucker 572: .Cm Match
1.65 dtucker 573: line or the end of the file.
1.91 djm 574: .Pp
1.61 jmc 575: The arguments to
1.60 dtucker 576: .Cm Match
1.65 dtucker 577: are one or more criteria-pattern pairs.
1.60 dtucker 578: The available criteria are
579: .Cm User ,
1.69 dtucker 580: .Cm Group ,
1.60 dtucker 581: .Cm Host ,
582: and
583: .Cm Address .
1.91 djm 584: The match patterns may consist of single entries or comma-separated
585: lists and may use the wildcard and negation operators described in the
1.92 djm 586: .Sx PATTERNS
1.91 djm 587: section of
1.92 djm 588: .Xr ssh_config 5 .
1.91 djm 589: .Pp
590: The patterns in an
591: .Cm Address
592: criteria may additionally contain addresses to match in CIDR
1.93 jmc 593: address/masklen format, e.g.\&
1.91 djm 594: .Dq 192.0.2.0/24
595: or
596: .Dq 3ffe:ffff::/32 .
597: Note that the mask length provided must be consistent with the address -
598: it is an error to specify a mask length that is too long for the address
1.93 jmc 599: or one with bits set in this host portion of the address.
600: For example,
1.91 djm 601: .Dq 192.0.2.0/33
602: and
1.93 jmc 603: .Dq 192.0.2.0/8
1.91 djm 604: respectively.
605: .Pp
1.60 dtucker 606: Only a subset of keywords may be used on the lines following a
607: .Cm Match
608: keyword.
609: Available keywords are
1.99 okan 610: .Cm AllowAgentForwarding ,
1.62 dtucker 611: .Cm AllowTcpForwarding ,
1.72 dtucker 612: .Cm Banner ,
1.85 djm 613: .Cm ChrootDirectory ,
1.67 dtucker 614: .Cm ForceCommand ,
1.62 dtucker 615: .Cm GatewayPorts ,
1.87 djm 616: .Cm GSSAPIAuthentication ,
617: .Cm HostbasedAuthentication ,
1.74 jmc 618: .Cm KbdInteractiveAuthentication ,
1.72 dtucker 619: .Cm KerberosAuthentication ,
1.95 dtucker 620: .Cm MaxAuthTries ,
1.94 dtucker 621: .Cm MaxSessions ,
1.72 dtucker 622: .Cm PasswordAuthentication ,
1.97 djm 623: .Cm PermitEmptyPasswords ,
1.66 dtucker 624: .Cm PermitOpen ,
1.79 dtucker 625: .Cm PermitRootLogin ,
1.107 dtucker 626: .Cm PubkeyAuthentication ,
1.72 dtucker 627: .Cm RhostsRSAAuthentication ,
628: .Cm RSAAuthentication ,
1.66 dtucker 629: .Cm X11DisplayOffset ,
1.101 djm 630: .Cm X11Forwarding
1.60 dtucker 631: and
1.102 djm 632: .Cm X11UseLocalHost .
1.33 dtucker 633: .It Cm MaxAuthTries
634: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 635: connection.
636: Once the number of failures reaches half this value,
637: additional failures are logged.
638: The default is 6.
1.90 djm 639: .It Cm MaxSessions
640: Specifies the maximum number of open sessions permitted per network connection.
641: The default is 10.
1.1 stevesk 642: .It Cm MaxStartups
643: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 644: SSH daemon.
1.1 stevesk 645: Additional connections will be dropped until authentication succeeds or the
646: .Cm LoginGraceTime
647: expires for a connection.
648: The default is 10.
649: .Pp
650: Alternatively, random early drop can be enabled by specifying
651: the three colon separated values
652: .Dq start:rate:full
1.51 jmc 653: (e.g. "10:30:60").
1.53 jmc 654: .Xr sshd 8
1.1 stevesk 655: will refuse connection attempts with a probability of
656: .Dq rate/100
657: (30%)
658: if there are currently
659: .Dq start
660: (10)
661: unauthenticated connections.
662: The probability increases linearly and all connection attempts
663: are refused if the number of unauthenticated connections reaches
664: .Dq full
665: (60).
666: .It Cm PasswordAuthentication
667: Specifies whether password authentication is allowed.
668: The default is
669: .Dq yes .
670: .It Cm PermitEmptyPasswords
671: When password authentication is allowed, it specifies whether the
672: server allows login to accounts with empty password strings.
673: The default is
674: .Dq no .
1.62 dtucker 675: .It Cm PermitOpen
676: Specifies the destinations to which TCP port forwarding is permitted.
677: The forwarding specification must be one of the following forms:
678: .Pp
679: .Bl -item -offset indent -compact
680: .It
681: .Cm PermitOpen
682: .Sm off
683: .Ar host : port
684: .Sm on
685: .It
686: .Cm PermitOpen
687: .Sm off
688: .Ar IPv4_addr : port
689: .Sm on
690: .It
691: .Cm PermitOpen
692: .Sm off
693: .Ar \&[ IPv6_addr \&] : port
694: .Sm on
695: .El
696: .Pp
1.68 dtucker 697: Multiple forwards may be specified by separating them with whitespace.
1.62 dtucker 698: An argument of
699: .Dq any
700: can be used to remove all restrictions and permit any forwarding requests.
1.63 jmc 701: By default all port forwarding requests are permitted.
1.1 stevesk 702: .It Cm PermitRootLogin
1.38 jmc 703: Specifies whether root can log in using
1.1 stevesk 704: .Xr ssh 1 .
705: The argument must be
706: .Dq yes ,
707: .Dq without-password ,
1.52 jmc 708: .Dq forced-commands-only ,
1.1 stevesk 709: or
710: .Dq no .
711: The default is
712: .Dq yes .
713: .Pp
714: If this option is set to
1.52 jmc 715: .Dq without-password ,
1.1 stevesk 716: password authentication is disabled for root.
717: .Pp
718: If this option is set to
1.52 jmc 719: .Dq forced-commands-only ,
1.1 stevesk 720: root login with public key authentication will be allowed,
721: but only if the
722: .Ar command
723: option has been specified
724: (which may be useful for taking remote backups even if root login is
1.17 jmc 725: normally not allowed).
726: All other authentication methods are disabled for root.
1.1 stevesk 727: .Pp
728: If this option is set to
1.52 jmc 729: .Dq no ,
1.38 jmc 730: root is not allowed to log in.
1.46 reyk 731: .It Cm PermitTunnel
732: Specifies whether
733: .Xr tun 4
734: device forwarding is allowed.
1.47 reyk 735: The argument must be
736: .Dq yes ,
1.58 stevesk 737: .Dq point-to-point
738: (layer 3),
739: .Dq ethernet
740: (layer 2), or
1.47 reyk 741: .Dq no .
1.58 stevesk 742: Specifying
743: .Dq yes
744: permits both
745: .Dq point-to-point
746: and
747: .Dq ethernet .
1.46 reyk 748: The default is
749: .Dq no .
1.6 markus 750: .It Cm PermitUserEnvironment
751: Specifies whether
752: .Pa ~/.ssh/environment
1.9 stevesk 753: and
1.6 markus 754: .Cm environment=
755: options in
756: .Pa ~/.ssh/authorized_keys
1.9 stevesk 757: are processed by
1.52 jmc 758: .Xr sshd 8 .
1.6 markus 759: The default is
760: .Dq no .
1.9 stevesk 761: Enabling environment processing may enable users to bypass access
762: restrictions in some configurations using mechanisms such as
763: .Ev LD_PRELOAD .
1.1 stevesk 764: .It Cm PidFile
1.4 stevesk 765: Specifies the file that contains the process ID of the
1.53 jmc 766: SSH daemon.
1.1 stevesk 767: The default is
768: .Pa /var/run/sshd.pid .
769: .It Cm Port
770: Specifies the port number that
1.52 jmc 771: .Xr sshd 8
1.1 stevesk 772: listens on.
773: The default is 22.
774: Multiple options of this type are permitted.
775: See also
776: .Cm ListenAddress .
777: .It Cm PrintLastLog
778: Specifies whether
1.52 jmc 779: .Xr sshd 8
1.36 jaredy 780: should print the date and time of the last user login when a user logs
781: in interactively.
1.1 stevesk 782: The default is
783: .Dq yes .
784: .It Cm PrintMotd
785: Specifies whether
1.52 jmc 786: .Xr sshd 8
1.1 stevesk 787: should print
788: .Pa /etc/motd
789: when a user logs in interactively.
790: (On some systems it is also printed by the shell,
791: .Pa /etc/profile ,
792: or equivalent.)
793: The default is
794: .Dq yes .
795: .It Cm Protocol
796: Specifies the protocol versions
1.52 jmc 797: .Xr sshd 8
1.5 stevesk 798: supports.
1.1 stevesk 799: The possible values are
1.52 jmc 800: .Sq 1
1.1 stevesk 801: and
1.52 jmc 802: .Sq 2 .
1.1 stevesk 803: Multiple versions must be comma-separated.
804: The default is
1.109 jmc 805: .Sq 2 .
1.5 stevesk 806: Note that the order of the protocol list does not indicate preference,
807: because the client selects among multiple protocol versions offered
808: by the server.
809: Specifying
810: .Dq 2,1
811: is identical to
812: .Dq 1,2 .
1.1 stevesk 813: .It Cm PubkeyAuthentication
814: Specifies whether public key authentication is allowed.
815: The default is
816: .Dq yes .
817: Note that this option applies to protocol version 2 only.
1.118 djm 818: .It Cm RevokedKeys
819: Specifies a list of revoked public keys.
820: Keys listed in this file will be refused for public key authentication.
821: Note that if this file is not readable, then public key authentication will
822: be refused for all users.
1.1 stevesk 823: .It Cm RhostsRSAAuthentication
824: Specifies whether rhosts or /etc/hosts.equiv authentication together
825: with successful RSA host authentication is allowed.
826: The default is
827: .Dq no .
828: This option applies to protocol version 1 only.
829: .It Cm RSAAuthentication
830: Specifies whether pure RSA authentication is allowed.
831: The default is
832: .Dq yes .
833: This option applies to protocol version 1 only.
834: .It Cm ServerKeyBits
835: Defines the number of bits in the ephemeral protocol version 1 server key.
1.96 djm 836: The minimum value is 512, and the default is 1024.
1.1 stevesk 837: .It Cm StrictModes
838: Specifies whether
1.52 jmc 839: .Xr sshd 8
1.1 stevesk 840: should check file modes and ownership of the
841: user's files and home directory before accepting login.
842: This is normally desirable because novices sometimes accidentally leave their
843: directory or files world-writable.
844: The default is
845: .Dq yes .
1.112 djm 846: Note that this does not apply to
847: .Cm ChrootDirectory ,
848: whose permissions and ownership are checked unconditionally.
1.1 stevesk 849: .It Cm Subsystem
1.51 jmc 850: Configures an external subsystem (e.g. file transfer daemon).
1.59 djm 851: Arguments should be a subsystem name and a command (with optional arguments)
852: to execute upon subsystem request.
1.80 djm 853: .Pp
1.1 stevesk 854: The command
855: .Xr sftp-server 8
856: implements the
857: .Dq sftp
858: file transfer subsystem.
1.80 djm 859: .Pp
860: Alternately the name
861: .Dq internal-sftp
862: implements an in-process
863: .Dq sftp
864: server.
865: This may simplify configurations using
866: .Cm ChrootDirectory
867: to force a different filesystem root on clients.
868: .Pp
1.1 stevesk 869: By default no subsystems are defined.
870: Note that this option applies to protocol version 2 only.
871: .It Cm SyslogFacility
872: Gives the facility code that is used when logging messages from
1.53 jmc 873: .Xr sshd 8 .
1.1 stevesk 874: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
875: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
876: The default is AUTH.
1.27 markus 877: .It Cm TCPKeepAlive
878: Specifies whether the system should send TCP keepalive messages to the
879: other side.
880: If they are sent, death of the connection or crash of one
881: of the machines will be properly noticed.
882: However, this means that
883: connections will die if the route is down temporarily, and some people
884: find it annoying.
885: On the other hand, if TCP keepalives are not sent,
886: sessions may hang indefinitely on the server, leaving
887: .Dq ghost
888: users and consuming server resources.
889: .Pp
890: The default is
891: .Dq yes
892: (to send TCP keepalive messages), and the server will notice
893: if the network goes down or the client host crashes.
894: This avoids infinitely hanging sessions.
895: .Pp
896: To disable TCP keepalive messages, the value should be set to
897: .Dq no .
1.118 djm 898: .It Cm TrustedUserCAKeys
899: Specifies a file containing public keys of certificate authorities that are
1.120 ! djm 900: trusted to sign user certificates for authentication.
1.119 jmc 901: Keys are listed one per line; empty lines and comments starting with
1.118 djm 902: .Ql #
903: are allowed.
904: If a certificate is presented for authentication and has its signing CA key
905: listed in this file, then it may be used for authentication for any user
906: listed in the certificate's principals list.
907: Note that certificates that lack a list of principals will not be permitted
908: for authentication using
909: .Cm TrustedUserCAKeys .
1.119 jmc 910: For more details on certificates, see the
1.118 djm 911: .Sx CERTIFICATES
912: section in
913: .Xr ssh-keygen 1 .
1.18 markus 914: .It Cm UseDNS
915: Specifies whether
1.52 jmc 916: .Xr sshd 8
1.40 jmc 917: should look up the remote host name and check that
1.18 markus 918: the resolved host name for the remote IP address maps back to the
919: very same IP address.
920: The default is
921: .Dq yes .
1.1 stevesk 922: .It Cm UseLogin
923: Specifies whether
924: .Xr login 1
925: is used for interactive login sessions.
926: The default is
927: .Dq no .
928: Note that
929: .Xr login 1
930: is never used for remote command execution.
931: Note also, that if this is enabled,
932: .Cm X11Forwarding
933: will be disabled because
934: .Xr login 1
935: does not know how to handle
936: .Xr xauth 1
1.15 jmc 937: cookies.
938: If
1.1 stevesk 939: .Cm UsePrivilegeSeparation
940: is specified, it will be disabled after authentication.
941: .It Cm UsePrivilegeSeparation
942: Specifies whether
1.52 jmc 943: .Xr sshd 8
1.2 stevesk 944: separates privileges by creating an unprivileged child process
1.15 jmc 945: to deal with incoming network traffic.
946: After successful authentication, another process will be created that has
947: the privilege of the authenticated user.
948: The goal of privilege separation is to prevent privilege
1.1 stevesk 949: escalation by containing any corruption within the unprivileged processes.
950: The default is
951: .Dq yes .
952: .It Cm X11DisplayOffset
953: Specifies the first display number available for
1.52 jmc 954: .Xr sshd 8 Ns 's
1.1 stevesk 955: X11 forwarding.
1.52 jmc 956: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 957: The default is 10.
958: .It Cm X11Forwarding
959: Specifies whether X11 forwarding is permitted.
1.13 stevesk 960: The argument must be
961: .Dq yes
962: or
963: .Dq no .
1.1 stevesk 964: The default is
965: .Dq no .
1.13 stevesk 966: .Pp
967: When X11 forwarding is enabled, there may be additional exposure to
968: the server and to client displays if the
1.52 jmc 969: .Xr sshd 8
1.13 stevesk 970: proxy display is configured to listen on the wildcard address (see
971: .Cm X11UseLocalhost
1.52 jmc 972: below), though this is not the default.
1.13 stevesk 973: Additionally, the authentication spoofing and authentication data
974: verification and substitution occur on the client side.
975: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 976: display server may be exposed to attack when the SSH client requests
1.13 stevesk 977: forwarding (see the warnings for
978: .Cm ForwardX11
979: in
1.19 jmc 980: .Xr ssh_config 5 ) .
1.13 stevesk 981: A system administrator may have a stance in which they want to
982: protect clients that may expose themselves to attack by unwittingly
983: requesting X11 forwarding, which can warrant a
984: .Dq no
985: setting.
986: .Pp
987: Note that disabling X11 forwarding does not prevent users from
988: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 989: X11 forwarding is automatically disabled if
990: .Cm UseLogin
991: is enabled.
992: .It Cm X11UseLocalhost
993: Specifies whether
1.52 jmc 994: .Xr sshd 8
1.1 stevesk 995: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 996: the wildcard address.
997: By default,
1.52 jmc 998: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 999: hostname part of the
1000: .Ev DISPLAY
1001: environment variable to
1002: .Dq localhost .
1.8 stevesk 1003: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 1004: However, some older X11 clients may not function with this
1005: configuration.
1006: .Cm X11UseLocalhost
1007: may be set to
1008: .Dq no
1009: to specify that the forwarding server should be bound to the wildcard
1010: address.
1011: The argument must be
1012: .Dq yes
1013: or
1014: .Dq no .
1015: The default is
1016: .Dq yes .
1017: .It Cm XAuthLocation
1.11 stevesk 1018: Specifies the full pathname of the
1.1 stevesk 1019: .Xr xauth 1
1020: program.
1021: The default is
1022: .Pa /usr/X11R6/bin/xauth .
1023: .El
1.55 jmc 1024: .Sh TIME FORMATS
1.53 jmc 1025: .Xr sshd 8
1.1 stevesk 1026: command-line arguments and configuration file options that specify time
1027: may be expressed using a sequence of the form:
1028: .Sm off
1.7 stevesk 1029: .Ar time Op Ar qualifier ,
1.1 stevesk 1030: .Sm on
1031: where
1032: .Ar time
1033: is a positive integer value and
1034: .Ar qualifier
1035: is one of the following:
1036: .Pp
1037: .Bl -tag -width Ds -compact -offset indent
1.64 jmc 1038: .It Aq Cm none
1.1 stevesk 1039: seconds
1040: .It Cm s | Cm S
1041: seconds
1042: .It Cm m | Cm M
1043: minutes
1044: .It Cm h | Cm H
1045: hours
1046: .It Cm d | Cm D
1047: days
1048: .It Cm w | Cm W
1049: weeks
1050: .El
1051: .Pp
1052: Each member of the sequence is added together to calculate
1053: the total time value.
1054: .Pp
1055: Time format examples:
1056: .Pp
1057: .Bl -tag -width Ds -compact -offset indent
1058: .It 600
1059: 600 seconds (10 minutes)
1060: .It 10m
1061: 10 minutes
1062: .It 1h30m
1063: 1 hour 30 minutes (90 minutes)
1064: .El
1065: .Sh FILES
1066: .Bl -tag -width Ds
1067: .It Pa /etc/ssh/sshd_config
1068: Contains configuration data for
1.53 jmc 1069: .Xr sshd 8 .
1.1 stevesk 1070: This file should be writable by root only, but it is recommended
1071: (though not necessary) that it be world-readable.
1072: .El
1.19 jmc 1073: .Sh SEE ALSO
1074: .Xr sshd 8
1.1 stevesk 1075: .Sh AUTHORS
1076: OpenSSH is a derivative of the original and free
1077: ssh 1.2.12 release by Tatu Ylonen.
1078: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1079: Theo de Raadt and Dug Song
1080: removed many bugs, re-added newer features and
1081: created OpenSSH.
1082: Markus Friedl contributed the support for SSH
1083: protocol versions 1.5 and 2.0.
1084: Niels Provos and Markus Friedl contributed support
1085: for privilege separation.
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