Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.55
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.
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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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1.55 ! jmc 37: .\" $OpenBSD: sshd_config.5,v 1.54 2006/02/25 12:28:34 jmc Exp $
1.1 stevesk 38: .Dd September 25, 1999
39: .Dt SSHD_CONFIG 5
40: .Os
41: .Sh NAME
42: .Nm sshd_config
43: .Nd OpenSSH SSH daemon configuration file
44: .Sh SYNOPSIS
45: .Bl -tag -width Ds -compact
46: .It Pa /etc/ssh/sshd_config
47: .El
48: .Sh DESCRIPTION
1.53 jmc 49: .Xr sshd 8
1.1 stevesk 50: reads configuration data from
51: .Pa /etc/ssh/sshd_config
52: (or the file specified with
53: .Fl f
54: on the command line).
55: The file contains keyword-argument pairs, one per line.
56: Lines starting with
57: .Ql #
58: and empty lines are interpreted as comments.
59: .Pp
60: The possible
61: keywords and their meanings are as follows (note that
62: keywords are case-insensitive and arguments are case-sensitive):
63: .Bl -tag -width Ds
1.30 djm 64: .It Cm AcceptEnv
65: Specifies what environment variables sent by the client will be copied into
66: the session's
67: .Xr environ 7 .
68: See
69: .Cm SendEnv
70: in
71: .Xr ssh_config 5
72: for how to configure the client.
1.31 djm 73: Note that environment passing is only supported for protocol 2.
1.30 djm 74: Variables are specified by name, which may contain the wildcard characters
1.51 jmc 75: .Ql *
1.30 djm 76: and
77: .Ql \&? .
1.31 djm 78: Multiple environment variables may be separated by whitespace or spread
1.30 djm 79: across multiple
80: .Cm AcceptEnv
81: directives.
1.31 djm 82: Be warned that some environment variables could be used to bypass restricted
1.30 djm 83: user environments.
84: For this reason, care should be taken in the use of this directive.
85: The default is not to accept any environment variables.
1.37 djm 86: .It Cm AddressFamily
87: Specifies which address family should be used by
1.53 jmc 88: .Xr sshd 8 .
1.37 djm 89: Valid arguments are
90: .Dq any ,
91: .Dq inet
1.52 jmc 92: (use IPv4 only), or
1.37 djm 93: .Dq inet6
94: (use IPv6 only).
95: The default is
96: .Dq any .
1.1 stevesk 97: .It Cm AllowGroups
98: This keyword can be followed by a list of group name patterns, separated
99: by spaces.
100: If specified, login is allowed only for users whose primary
101: group or supplementary group list matches one of the patterns.
102: Only group names are valid; a numerical group ID is not recognized.
103: By default, login is allowed for all groups.
1.54 jmc 104: The allow/deny directives are processed in the following order:
105: .Cm DenyUsers ,
106: .Cm AllowUsers ,
107: .Cm DenyGroups ,
108: and finally
109: .Cm AllowGroups .
1.49 jmc 110: .Pp
111: See
112: .Sx PATTERNS
113: in
114: .Xr ssh_config 5
115: for more information on patterns.
1.1 stevesk 116: .It Cm AllowTcpForwarding
117: Specifies whether TCP forwarding is permitted.
118: The default is
119: .Dq yes .
120: Note that disabling TCP forwarding does not improve security unless
121: users are also denied shell access, as they can always install their
122: own forwarders.
123: .It Cm AllowUsers
124: This keyword can be followed by a list of user name patterns, separated
125: by spaces.
1.14 jmc 126: If specified, login is allowed only for user names that
1.1 stevesk 127: match one of the patterns.
128: Only user names are valid; a numerical user ID is not recognized.
129: By default, login is allowed for all users.
130: If the pattern takes the form USER@HOST then USER and HOST
131: are separately checked, restricting logins to particular
132: users from particular hosts.
1.54 jmc 133: The allow/deny directives are processed in the following order:
134: .Cm DenyUsers ,
135: .Cm AllowUsers ,
136: .Cm DenyGroups ,
137: and finally
138: .Cm AllowGroups .
1.49 jmc 139: .Pp
140: See
141: .Sx PATTERNS
142: in
143: .Xr ssh_config 5
144: for more information on patterns.
1.1 stevesk 145: .It Cm AuthorizedKeysFile
146: Specifies the file that contains the public keys that can be used
147: for user authentication.
148: .Cm AuthorizedKeysFile
149: may contain tokens of the form %T which are substituted during connection
1.52 jmc 150: setup.
1.17 jmc 151: The following tokens are defined: %% is replaced by a literal '%',
1.52 jmc 152: %h is replaced by the home directory of the user being authenticated, and
1.1 stevesk 153: %u is replaced by the username of that user.
154: After expansion,
155: .Cm AuthorizedKeysFile
156: is taken to be an absolute path or one relative to the user's home
157: directory.
158: The default is
159: .Dq .ssh/authorized_keys .
160: .It Cm Banner
161: In some jurisdictions, sending a warning message before authentication
162: may be relevant for getting legal protection.
163: The contents of the specified file are sent to the remote user before
164: authentication is allowed.
165: This option is only available for protocol version 2.
166: By default, no banner is displayed.
167: .It Cm ChallengeResponseAuthentication
1.50 jmc 168: Specifies whether challenge-response authentication is allowed.
1.1 stevesk 169: All authentication styles from
170: .Xr login.conf 5
171: are supported.
172: The default is
173: .Dq yes .
174: .It Cm Ciphers
175: Specifies the ciphers allowed for protocol version 2.
176: Multiple ciphers must be comma-separated.
1.34 dtucker 177: The supported ciphers are
178: .Dq 3des-cbc ,
179: .Dq aes128-cbc ,
180: .Dq aes192-cbc ,
181: .Dq aes256-cbc ,
182: .Dq aes128-ctr ,
183: .Dq aes192-ctr ,
184: .Dq aes256-ctr ,
1.43 djm 185: .Dq arcfour128 ,
186: .Dq arcfour256 ,
1.34 dtucker 187: .Dq arcfour ,
188: .Dq blowfish-cbc ,
189: and
190: .Dq cast128-cbc .
1.52 jmc 191: The default is:
192: .Bd -literal -offset 3n
193: aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
194: arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
195: aes192-ctr,aes256-ctr
1.1 stevesk 196: .Ed
197: .It Cm ClientAliveCountMax
1.48 jmc 198: Sets the number of client alive messages (see below) which may be
1.1 stevesk 199: sent without
1.52 jmc 200: .Xr sshd 8
1.17 jmc 201: receiving any messages back from the client.
202: If this threshold is reached while client alive messages are being sent,
1.52 jmc 203: sshd will disconnect the client, terminating the session.
1.17 jmc 204: It is important to note that the use of client alive messages is very
205: different from
1.27 markus 206: .Cm TCPKeepAlive
1.17 jmc 207: (below).
208: The client alive messages are sent through the encrypted channel
209: and therefore will not be spoofable.
210: The TCP keepalive option enabled by
1.27 markus 211: .Cm TCPKeepAlive
1.17 jmc 212: is spoofable.
213: The client alive mechanism is valuable when the client or
1.1 stevesk 214: server depend on knowing when a connection has become inactive.
215: .Pp
1.17 jmc 216: The default value is 3.
217: If
1.1 stevesk 218: .Cm ClientAliveInterval
1.48 jmc 219: (see below) is set to 15, and
1.1 stevesk 220: .Cm ClientAliveCountMax
1.52 jmc 221: is left at the default, unresponsive SSH clients
1.1 stevesk 222: will be disconnected after approximately 45 seconds.
1.42 djm 223: .It Cm ClientAliveInterval
224: Sets a timeout interval in seconds after which if no data has been received
225: from the client,
1.52 jmc 226: .Xr sshd 8
1.42 djm 227: will send a message through the encrypted
228: channel to request a response from the client.
229: The default
230: is 0, indicating that these messages will not be sent to the client.
231: This option applies to protocol version 2 only.
1.3 markus 232: .It Cm Compression
1.44 markus 233: Specifies whether compression is allowed, or delayed until
234: the user has authenticated successfully.
1.3 markus 235: The argument must be
1.44 markus 236: .Dq yes ,
237: .Dq delayed ,
1.3 markus 238: or
239: .Dq no .
240: The default is
1.44 markus 241: .Dq delayed .
1.1 stevesk 242: .It Cm DenyGroups
243: This keyword can be followed by a list of group name patterns, separated
244: by spaces.
245: Login is disallowed for users whose primary group or supplementary
246: group list matches one of the patterns.
247: Only group names are valid; a numerical group ID is not recognized.
248: By default, login is allowed for all groups.
1.54 jmc 249: The allow/deny directives are processed in the following order:
250: .Cm DenyUsers ,
251: .Cm AllowUsers ,
252: .Cm DenyGroups ,
253: and finally
254: .Cm AllowGroups .
1.49 jmc 255: .Pp
256: See
257: .Sx PATTERNS
258: in
259: .Xr ssh_config 5
260: for more information on patterns.
1.1 stevesk 261: .It Cm DenyUsers
262: This keyword can be followed by a list of user name patterns, separated
263: by spaces.
264: Login is disallowed for user names that match one of the patterns.
265: Only user names are valid; a numerical user ID is not recognized.
266: By default, login is allowed for all users.
267: If the pattern takes the form USER@HOST then USER and HOST
268: are separately checked, restricting logins to particular
269: users from particular hosts.
1.54 jmc 270: The allow/deny directives are processed in the following order:
271: .Cm DenyUsers ,
272: .Cm AllowUsers ,
273: .Cm DenyGroups ,
274: and finally
275: .Cm AllowGroups .
1.49 jmc 276: .Pp
277: See
278: .Sx PATTERNS
279: in
280: .Xr ssh_config 5
281: for more information on patterns.
1.1 stevesk 282: .It Cm GatewayPorts
283: Specifies whether remote hosts are allowed to connect to ports
284: forwarded for the client.
285: By default,
1.52 jmc 286: .Xr sshd 8
1.15 jmc 287: binds remote port forwardings to the loopback address.
288: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 289: .Cm GatewayPorts
1.52 jmc 290: can be used to specify that sshd
1.39 djm 291: should allow remote port forwardings to bind to non-loopback addresses, thus
292: allowing other hosts to connect.
293: The argument may be
294: .Dq no
295: to force remote port forwardings to be available to the local host only,
1.1 stevesk 296: .Dq yes
1.39 djm 297: to force remote port forwardings to bind to the wildcard address, or
298: .Dq clientspecified
299: to allow the client to select the address to which the forwarding is bound.
1.1 stevesk 300: The default is
301: .Dq no .
1.23 markus 302: .It Cm GSSAPIAuthentication
1.25 markus 303: Specifies whether user authentication based on GSSAPI is allowed.
1.26 djm 304: The default is
1.23 markus 305: .Dq no .
306: Note that this option applies to protocol version 2 only.
307: .It Cm GSSAPICleanupCredentials
308: Specifies whether to automatically destroy the user's credentials cache
309: on logout.
310: The default is
311: .Dq yes .
312: Note that this option applies to protocol version 2 only.
1.1 stevesk 313: .It Cm HostbasedAuthentication
314: Specifies whether rhosts or /etc/hosts.equiv authentication together
315: with successful public key client host authentication is allowed
1.50 jmc 316: (host-based authentication).
1.1 stevesk 317: This option is similar to
318: .Cm RhostsRSAAuthentication
319: and applies to protocol version 2 only.
320: The default is
321: .Dq no .
322: .It Cm HostKey
323: Specifies a file containing a private host key
324: used by SSH.
325: The default is
326: .Pa /etc/ssh/ssh_host_key
327: for protocol version 1, and
328: .Pa /etc/ssh/ssh_host_rsa_key
329: and
330: .Pa /etc/ssh/ssh_host_dsa_key
331: for protocol version 2.
332: Note that
1.52 jmc 333: .Xr sshd 8
1.1 stevesk 334: will refuse to use a file if it is group/world-accessible.
335: It is possible to have multiple host key files.
336: .Dq rsa1
337: keys are used for version 1 and
338: .Dq dsa
339: or
340: .Dq rsa
341: are used for version 2 of the SSH protocol.
342: .It Cm IgnoreRhosts
343: Specifies that
344: .Pa .rhosts
345: and
346: .Pa .shosts
347: files will not be used in
348: .Cm RhostsRSAAuthentication
349: or
350: .Cm HostbasedAuthentication .
351: .Pp
352: .Pa /etc/hosts.equiv
353: and
354: .Pa /etc/shosts.equiv
355: are still used.
356: The default is
357: .Dq yes .
358: .It Cm IgnoreUserKnownHosts
359: Specifies whether
1.52 jmc 360: .Xr sshd 8
1.1 stevesk 361: should ignore the user's
1.41 djm 362: .Pa ~/.ssh/known_hosts
1.1 stevesk 363: during
364: .Cm RhostsRSAAuthentication
365: or
366: .Cm HostbasedAuthentication .
367: The default is
368: .Dq no .
369: .It Cm KerberosAuthentication
1.24 markus 370: Specifies whether the password provided by the user for
1.1 stevesk 371: .Cm PasswordAuthentication
1.24 markus 372: will be validated through the Kerberos KDC.
1.1 stevesk 373: To use this option, the server needs a
374: Kerberos servtab which allows the verification of the KDC's identity.
1.52 jmc 375: The default is
1.29 dtucker 376: .Dq no .
377: .It Cm KerberosGetAFSToken
1.45 djm 378: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
1.29 dtucker 379: an AFS token before accessing the user's home directory.
1.52 jmc 380: The default is
1.1 stevesk 381: .Dq no .
382: .It Cm KerberosOrLocalPasswd
1.52 jmc 383: If password authentication through Kerberos fails then
1.1 stevesk 384: the password will be validated via any additional local mechanism
385: such as
386: .Pa /etc/passwd .
1.52 jmc 387: The default is
1.1 stevesk 388: .Dq yes .
389: .It Cm KerberosTicketCleanup
390: Specifies whether to automatically destroy the user's ticket cache
391: file on logout.
1.52 jmc 392: The default is
1.1 stevesk 393: .Dq yes .
394: .It Cm KeyRegenerationInterval
395: In protocol version 1, the ephemeral server key is automatically regenerated
396: after this many seconds (if it has been used).
397: The purpose of regeneration is to prevent
398: decrypting captured sessions by later breaking into the machine and
399: stealing the keys.
400: The key is never stored anywhere.
401: If the value is 0, the key is never regenerated.
402: The default is 3600 (seconds).
403: .It Cm ListenAddress
404: Specifies the local addresses
1.52 jmc 405: .Xr sshd 8
1.1 stevesk 406: should listen on.
407: The following forms may be used:
408: .Pp
409: .Bl -item -offset indent -compact
410: .It
411: .Cm ListenAddress
412: .Sm off
413: .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
414: .Sm on
415: .It
416: .Cm ListenAddress
417: .Sm off
418: .Ar host No | Ar IPv4_addr No : Ar port
419: .Sm on
420: .It
421: .Cm ListenAddress
422: .Sm off
423: .Oo
424: .Ar host No | Ar IPv6_addr Oc : Ar port
425: .Sm on
426: .El
427: .Pp
428: If
429: .Ar port
430: is not specified,
1.52 jmc 431: sshd will listen on the address and all prior
1.1 stevesk 432: .Cm Port
1.17 jmc 433: options specified.
434: The default is to listen on all local addresses.
1.15 jmc 435: Multiple
1.1 stevesk 436: .Cm ListenAddress
1.17 jmc 437: options are permitted.
438: Additionally, any
1.1 stevesk 439: .Cm Port
1.52 jmc 440: options must precede this option for non-port qualified addresses.
1.1 stevesk 441: .It Cm LoginGraceTime
442: The server disconnects after this time if the user has not
443: successfully logged in.
444: If the value is 0, there is no time limit.
1.12 stevesk 445: The default is 120 seconds.
1.1 stevesk 446: .It Cm LogLevel
447: Gives the verbosity level that is used when logging messages from
1.53 jmc 448: .Xr sshd 8 .
1.1 stevesk 449: The possible values are:
1.52 jmc 450: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 451: The default is INFO.
452: DEBUG and DEBUG1 are equivalent.
453: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
454: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 455: .It Cm MACs
456: Specifies the available MAC (message authentication code) algorithms.
457: The MAC algorithm is used in protocol version 2
458: for data integrity protection.
459: Multiple algorithms must be comma-separated.
1.52 jmc 460: The default is:
1.1 stevesk 461: .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
1.33 dtucker 462: .It Cm MaxAuthTries
463: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 464: connection.
465: Once the number of failures reaches half this value,
466: additional failures are logged.
467: The default is 6.
1.1 stevesk 468: .It Cm MaxStartups
469: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 470: SSH daemon.
1.1 stevesk 471: Additional connections will be dropped until authentication succeeds or the
472: .Cm LoginGraceTime
473: expires for a connection.
474: The default is 10.
475: .Pp
476: Alternatively, random early drop can be enabled by specifying
477: the three colon separated values
478: .Dq start:rate:full
1.51 jmc 479: (e.g. "10:30:60").
1.53 jmc 480: .Xr sshd 8
1.1 stevesk 481: will refuse connection attempts with a probability of
482: .Dq rate/100
483: (30%)
484: if there are currently
485: .Dq start
486: (10)
487: unauthenticated connections.
488: The probability increases linearly and all connection attempts
489: are refused if the number of unauthenticated connections reaches
490: .Dq full
491: (60).
492: .It Cm PasswordAuthentication
493: Specifies whether password authentication is allowed.
494: The default is
495: .Dq yes .
496: .It Cm PermitEmptyPasswords
497: When password authentication is allowed, it specifies whether the
498: server allows login to accounts with empty password strings.
499: The default is
500: .Dq no .
501: .It Cm PermitRootLogin
1.38 jmc 502: Specifies whether root can log in using
1.1 stevesk 503: .Xr ssh 1 .
504: The argument must be
505: .Dq yes ,
506: .Dq without-password ,
1.52 jmc 507: .Dq forced-commands-only ,
1.1 stevesk 508: or
509: .Dq no .
510: The default is
511: .Dq yes .
512: .Pp
513: If this option is set to
1.52 jmc 514: .Dq without-password ,
1.1 stevesk 515: password authentication is disabled for root.
516: .Pp
517: If this option is set to
1.52 jmc 518: .Dq forced-commands-only ,
1.1 stevesk 519: root login with public key authentication will be allowed,
520: but only if the
521: .Ar command
522: option has been specified
523: (which may be useful for taking remote backups even if root login is
1.17 jmc 524: normally not allowed).
525: All other authentication methods are disabled for root.
1.1 stevesk 526: .Pp
527: If this option is set to
1.52 jmc 528: .Dq no ,
1.38 jmc 529: root is not allowed to log in.
1.46 reyk 530: .It Cm PermitTunnel
531: Specifies whether
532: .Xr tun 4
533: device forwarding is allowed.
1.47 reyk 534: The argument must be
535: .Dq yes ,
536: .Dq point-to-point ,
1.52 jmc 537: .Dq ethernet ,
1.47 reyk 538: or
539: .Dq no .
1.46 reyk 540: The default is
541: .Dq no .
1.6 markus 542: .It Cm PermitUserEnvironment
543: Specifies whether
544: .Pa ~/.ssh/environment
1.9 stevesk 545: and
1.6 markus 546: .Cm environment=
547: options in
548: .Pa ~/.ssh/authorized_keys
1.9 stevesk 549: are processed by
1.52 jmc 550: .Xr sshd 8 .
1.6 markus 551: The default is
552: .Dq no .
1.9 stevesk 553: Enabling environment processing may enable users to bypass access
554: restrictions in some configurations using mechanisms such as
555: .Ev LD_PRELOAD .
1.1 stevesk 556: .It Cm PidFile
1.4 stevesk 557: Specifies the file that contains the process ID of the
1.53 jmc 558: SSH daemon.
1.1 stevesk 559: The default is
560: .Pa /var/run/sshd.pid .
561: .It Cm Port
562: Specifies the port number that
1.52 jmc 563: .Xr sshd 8
1.1 stevesk 564: listens on.
565: The default is 22.
566: Multiple options of this type are permitted.
567: See also
568: .Cm ListenAddress .
569: .It Cm PrintLastLog
570: Specifies whether
1.52 jmc 571: .Xr sshd 8
1.36 jaredy 572: should print the date and time of the last user login when a user logs
573: in interactively.
1.1 stevesk 574: The default is
575: .Dq yes .
576: .It Cm PrintMotd
577: Specifies whether
1.52 jmc 578: .Xr sshd 8
1.1 stevesk 579: should print
580: .Pa /etc/motd
581: when a user logs in interactively.
582: (On some systems it is also printed by the shell,
583: .Pa /etc/profile ,
584: or equivalent.)
585: The default is
586: .Dq yes .
587: .It Cm Protocol
588: Specifies the protocol versions
1.52 jmc 589: .Xr sshd 8
1.5 stevesk 590: supports.
1.1 stevesk 591: The possible values are
1.52 jmc 592: .Sq 1
1.1 stevesk 593: and
1.52 jmc 594: .Sq 2 .
1.1 stevesk 595: Multiple versions must be comma-separated.
596: The default is
597: .Dq 2,1 .
1.5 stevesk 598: Note that the order of the protocol list does not indicate preference,
599: because the client selects among multiple protocol versions offered
600: by the server.
601: Specifying
602: .Dq 2,1
603: is identical to
604: .Dq 1,2 .
1.1 stevesk 605: .It Cm PubkeyAuthentication
606: Specifies whether public key authentication is allowed.
607: The default is
608: .Dq yes .
609: Note that this option applies to protocol version 2 only.
610: .It Cm RhostsRSAAuthentication
611: Specifies whether rhosts or /etc/hosts.equiv authentication together
612: with successful RSA host authentication is allowed.
613: The default is
614: .Dq no .
615: This option applies to protocol version 1 only.
616: .It Cm RSAAuthentication
617: Specifies whether pure RSA authentication is allowed.
618: The default is
619: .Dq yes .
620: This option applies to protocol version 1 only.
621: .It Cm ServerKeyBits
622: Defines the number of bits in the ephemeral protocol version 1 server key.
623: The minimum value is 512, and the default is 768.
624: .It Cm StrictModes
625: Specifies whether
1.52 jmc 626: .Xr sshd 8
1.1 stevesk 627: should check file modes and ownership of the
628: user's files and home directory before accepting login.
629: This is normally desirable because novices sometimes accidentally leave their
630: directory or files world-writable.
631: The default is
632: .Dq yes .
633: .It Cm Subsystem
1.51 jmc 634: Configures an external subsystem (e.g. file transfer daemon).
1.1 stevesk 635: Arguments should be a subsystem name and a command to execute upon subsystem
636: request.
637: The command
638: .Xr sftp-server 8
639: implements the
640: .Dq sftp
641: file transfer subsystem.
642: By default no subsystems are defined.
643: Note that this option applies to protocol version 2 only.
644: .It Cm SyslogFacility
645: Gives the facility code that is used when logging messages from
1.53 jmc 646: .Xr sshd 8 .
1.1 stevesk 647: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
648: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
649: The default is AUTH.
1.27 markus 650: .It Cm TCPKeepAlive
651: Specifies whether the system should send TCP keepalive messages to the
652: other side.
653: If they are sent, death of the connection or crash of one
654: of the machines will be properly noticed.
655: However, this means that
656: connections will die if the route is down temporarily, and some people
657: find it annoying.
658: On the other hand, if TCP keepalives are not sent,
659: sessions may hang indefinitely on the server, leaving
660: .Dq ghost
661: users and consuming server resources.
662: .Pp
663: The default is
664: .Dq yes
665: (to send TCP keepalive messages), and the server will notice
666: if the network goes down or the client host crashes.
667: This avoids infinitely hanging sessions.
668: .Pp
669: To disable TCP keepalive messages, the value should be set to
670: .Dq no .
1.18 markus 671: .It Cm UseDNS
672: Specifies whether
1.52 jmc 673: .Xr sshd 8
1.40 jmc 674: should look up the remote host name and check that
1.18 markus 675: the resolved host name for the remote IP address maps back to the
676: very same IP address.
677: The default is
678: .Dq yes .
1.1 stevesk 679: .It Cm UseLogin
680: Specifies whether
681: .Xr login 1
682: is used for interactive login sessions.
683: The default is
684: .Dq no .
685: Note that
686: .Xr login 1
687: is never used for remote command execution.
688: Note also, that if this is enabled,
689: .Cm X11Forwarding
690: will be disabled because
691: .Xr login 1
692: does not know how to handle
693: .Xr xauth 1
1.15 jmc 694: cookies.
695: If
1.1 stevesk 696: .Cm UsePrivilegeSeparation
697: is specified, it will be disabled after authentication.
698: .It Cm UsePrivilegeSeparation
699: Specifies whether
1.52 jmc 700: .Xr sshd 8
1.2 stevesk 701: separates privileges by creating an unprivileged child process
1.15 jmc 702: to deal with incoming network traffic.
703: After successful authentication, another process will be created that has
704: the privilege of the authenticated user.
705: The goal of privilege separation is to prevent privilege
1.1 stevesk 706: escalation by containing any corruption within the unprivileged processes.
707: The default is
708: .Dq yes .
709: .It Cm X11DisplayOffset
710: Specifies the first display number available for
1.52 jmc 711: .Xr sshd 8 Ns 's
1.1 stevesk 712: X11 forwarding.
1.52 jmc 713: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 714: The default is 10.
715: .It Cm X11Forwarding
716: Specifies whether X11 forwarding is permitted.
1.13 stevesk 717: The argument must be
718: .Dq yes
719: or
720: .Dq no .
1.1 stevesk 721: The default is
722: .Dq no .
1.13 stevesk 723: .Pp
724: When X11 forwarding is enabled, there may be additional exposure to
725: the server and to client displays if the
1.52 jmc 726: .Xr sshd 8
1.13 stevesk 727: proxy display is configured to listen on the wildcard address (see
728: .Cm X11UseLocalhost
1.52 jmc 729: below), though this is not the default.
1.13 stevesk 730: Additionally, the authentication spoofing and authentication data
731: verification and substitution occur on the client side.
732: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 733: display server may be exposed to attack when the SSH client requests
1.13 stevesk 734: forwarding (see the warnings for
735: .Cm ForwardX11
736: in
1.19 jmc 737: .Xr ssh_config 5 ) .
1.13 stevesk 738: A system administrator may have a stance in which they want to
739: protect clients that may expose themselves to attack by unwittingly
740: requesting X11 forwarding, which can warrant a
741: .Dq no
742: setting.
743: .Pp
744: Note that disabling X11 forwarding does not prevent users from
745: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 746: X11 forwarding is automatically disabled if
747: .Cm UseLogin
748: is enabled.
749: .It Cm X11UseLocalhost
750: Specifies whether
1.52 jmc 751: .Xr sshd 8
1.1 stevesk 752: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 753: the wildcard address.
754: By default,
1.52 jmc 755: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 756: hostname part of the
757: .Ev DISPLAY
758: environment variable to
759: .Dq localhost .
1.8 stevesk 760: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 761: However, some older X11 clients may not function with this
762: configuration.
763: .Cm X11UseLocalhost
764: may be set to
765: .Dq no
766: to specify that the forwarding server should be bound to the wildcard
767: address.
768: The argument must be
769: .Dq yes
770: or
771: .Dq no .
772: The default is
773: .Dq yes .
774: .It Cm XAuthLocation
1.11 stevesk 775: Specifies the full pathname of the
1.1 stevesk 776: .Xr xauth 1
777: program.
778: The default is
779: .Pa /usr/X11R6/bin/xauth .
780: .El
1.55 ! jmc 781: .Sh TIME FORMATS
1.53 jmc 782: .Xr sshd 8
1.1 stevesk 783: command-line arguments and configuration file options that specify time
784: may be expressed using a sequence of the form:
785: .Sm off
1.7 stevesk 786: .Ar time Op Ar qualifier ,
1.1 stevesk 787: .Sm on
788: where
789: .Ar time
790: is a positive integer value and
791: .Ar qualifier
792: is one of the following:
793: .Pp
794: .Bl -tag -width Ds -compact -offset indent
795: .It Cm <none>
796: seconds
797: .It Cm s | Cm S
798: seconds
799: .It Cm m | Cm M
800: minutes
801: .It Cm h | Cm H
802: hours
803: .It Cm d | Cm D
804: days
805: .It Cm w | Cm W
806: weeks
807: .El
808: .Pp
809: Each member of the sequence is added together to calculate
810: the total time value.
811: .Pp
812: Time format examples:
813: .Pp
814: .Bl -tag -width Ds -compact -offset indent
815: .It 600
816: 600 seconds (10 minutes)
817: .It 10m
818: 10 minutes
819: .It 1h30m
820: 1 hour 30 minutes (90 minutes)
821: .El
822: .Sh FILES
823: .Bl -tag -width Ds
824: .It Pa /etc/ssh/sshd_config
825: Contains configuration data for
1.53 jmc 826: .Xr sshd 8 .
1.1 stevesk 827: This file should be writable by root only, but it is recommended
828: (though not necessary) that it be world-readable.
829: .El
1.19 jmc 830: .Sh SEE ALSO
831: .Xr sshd 8
1.1 stevesk 832: .Sh AUTHORS
833: OpenSSH is a derivative of the original and free
834: ssh 1.2.12 release by Tatu Ylonen.
835: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
836: Theo de Raadt and Dug Song
837: removed many bugs, re-added newer features and
838: created OpenSSH.
839: Markus Friedl contributed the support for SSH
840: protocol versions 1.5 and 2.0.
841: Niels Provos and Markus Friedl contributed support
842: for privilege separation.
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