Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.53
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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1.53 ! jmc 37: .\" $OpenBSD: sshd_config.5,v 1.52 2006/02/24 23:43:57 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.
1.49 jmc 104: .Pp
105: See
106: .Sx PATTERNS
107: in
108: .Xr ssh_config 5
109: for more information on patterns.
1.1 stevesk 110: .It Cm AllowTcpForwarding
111: Specifies whether TCP forwarding is permitted.
112: The default is
113: .Dq yes .
114: Note that disabling TCP forwarding does not improve security unless
115: users are also denied shell access, as they can always install their
116: own forwarders.
117: .It Cm AllowUsers
118: This keyword can be followed by a list of user name patterns, separated
119: by spaces.
1.14 jmc 120: If specified, login is allowed only for user names that
1.1 stevesk 121: match one of the patterns.
122: Only user names are valid; a numerical user ID is not recognized.
123: By default, login is allowed for all users.
124: If the pattern takes the form USER@HOST then USER and HOST
125: are separately checked, restricting logins to particular
126: users from particular hosts.
1.49 jmc 127: .Pp
128: See
129: .Sx PATTERNS
130: in
131: .Xr ssh_config 5
132: for more information on patterns.
1.1 stevesk 133: .It Cm AuthorizedKeysFile
134: Specifies the file that contains the public keys that can be used
135: for user authentication.
136: .Cm AuthorizedKeysFile
137: may contain tokens of the form %T which are substituted during connection
1.52 jmc 138: setup.
1.17 jmc 139: The following tokens are defined: %% is replaced by a literal '%',
1.52 jmc 140: %h is replaced by the home directory of the user being authenticated, and
1.1 stevesk 141: %u is replaced by the username of that user.
142: After expansion,
143: .Cm AuthorizedKeysFile
144: is taken to be an absolute path or one relative to the user's home
145: directory.
146: The default is
147: .Dq .ssh/authorized_keys .
148: .It Cm Banner
149: In some jurisdictions, sending a warning message before authentication
150: may be relevant for getting legal protection.
151: The contents of the specified file are sent to the remote user before
152: authentication is allowed.
153: This option is only available for protocol version 2.
154: By default, no banner is displayed.
155: .It Cm ChallengeResponseAuthentication
1.50 jmc 156: Specifies whether challenge-response authentication is allowed.
1.1 stevesk 157: All authentication styles from
158: .Xr login.conf 5
159: are supported.
160: The default is
161: .Dq yes .
162: .It Cm Ciphers
163: Specifies the ciphers allowed for protocol version 2.
164: Multiple ciphers must be comma-separated.
1.34 dtucker 165: The supported ciphers are
166: .Dq 3des-cbc ,
167: .Dq aes128-cbc ,
168: .Dq aes192-cbc ,
169: .Dq aes256-cbc ,
170: .Dq aes128-ctr ,
171: .Dq aes192-ctr ,
172: .Dq aes256-ctr ,
1.43 djm 173: .Dq arcfour128 ,
174: .Dq arcfour256 ,
1.34 dtucker 175: .Dq arcfour ,
176: .Dq blowfish-cbc ,
177: and
178: .Dq cast128-cbc .
1.52 jmc 179: The default is:
180: .Bd -literal -offset 3n
181: aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
182: arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
183: aes192-ctr,aes256-ctr
1.1 stevesk 184: .Ed
185: .It Cm ClientAliveCountMax
1.48 jmc 186: Sets the number of client alive messages (see below) which may be
1.1 stevesk 187: sent without
1.52 jmc 188: .Xr sshd 8
1.17 jmc 189: receiving any messages back from the client.
190: If this threshold is reached while client alive messages are being sent,
1.52 jmc 191: sshd will disconnect the client, terminating the session.
1.17 jmc 192: It is important to note that the use of client alive messages is very
193: different from
1.27 markus 194: .Cm TCPKeepAlive
1.17 jmc 195: (below).
196: The client alive messages are sent through the encrypted channel
197: and therefore will not be spoofable.
198: The TCP keepalive option enabled by
1.27 markus 199: .Cm TCPKeepAlive
1.17 jmc 200: is spoofable.
201: The client alive mechanism is valuable when the client or
1.1 stevesk 202: server depend on knowing when a connection has become inactive.
203: .Pp
1.17 jmc 204: The default value is 3.
205: If
1.1 stevesk 206: .Cm ClientAliveInterval
1.48 jmc 207: (see below) is set to 15, and
1.1 stevesk 208: .Cm ClientAliveCountMax
1.52 jmc 209: is left at the default, unresponsive SSH clients
1.1 stevesk 210: will be disconnected after approximately 45 seconds.
1.42 djm 211: .It Cm ClientAliveInterval
212: Sets a timeout interval in seconds after which if no data has been received
213: from the client,
1.52 jmc 214: .Xr sshd 8
1.42 djm 215: will send a message through the encrypted
216: channel to request a response from the client.
217: The default
218: is 0, indicating that these messages will not be sent to the client.
219: This option applies to protocol version 2 only.
1.3 markus 220: .It Cm Compression
1.44 markus 221: Specifies whether compression is allowed, or delayed until
222: the user has authenticated successfully.
1.3 markus 223: The argument must be
1.44 markus 224: .Dq yes ,
225: .Dq delayed ,
1.3 markus 226: or
227: .Dq no .
228: The default is
1.44 markus 229: .Dq delayed .
1.1 stevesk 230: .It Cm DenyGroups
231: This keyword can be followed by a list of group name patterns, separated
232: by spaces.
233: Login is disallowed for users whose primary group or supplementary
234: group list matches one of the patterns.
235: Only group names are valid; a numerical group ID is not recognized.
236: By default, login is allowed for all groups.
1.49 jmc 237: .Pp
238: See
239: .Sx PATTERNS
240: in
241: .Xr ssh_config 5
242: for more information on patterns.
1.1 stevesk 243: .It Cm DenyUsers
244: This keyword can be followed by a list of user name patterns, separated
245: by spaces.
246: Login is disallowed for user names that match one of the patterns.
247: Only user names are valid; a numerical user ID is not recognized.
248: By default, login is allowed for all users.
249: If the pattern takes the form USER@HOST then USER and HOST
250: are separately checked, restricting logins to particular
251: users from particular hosts.
1.49 jmc 252: .Pp
253: See
254: .Sx PATTERNS
255: in
256: .Xr ssh_config 5
257: for more information on patterns.
1.1 stevesk 258: .It Cm GatewayPorts
259: Specifies whether remote hosts are allowed to connect to ports
260: forwarded for the client.
261: By default,
1.52 jmc 262: .Xr sshd 8
1.15 jmc 263: binds remote port forwardings to the loopback address.
264: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 265: .Cm GatewayPorts
1.52 jmc 266: can be used to specify that sshd
1.39 djm 267: should allow remote port forwardings to bind to non-loopback addresses, thus
268: allowing other hosts to connect.
269: The argument may be
270: .Dq no
271: to force remote port forwardings to be available to the local host only,
1.1 stevesk 272: .Dq yes
1.39 djm 273: to force remote port forwardings to bind to the wildcard address, or
274: .Dq clientspecified
275: to allow the client to select the address to which the forwarding is bound.
1.1 stevesk 276: The default is
277: .Dq no .
1.23 markus 278: .It Cm GSSAPIAuthentication
1.25 markus 279: Specifies whether user authentication based on GSSAPI is allowed.
1.26 djm 280: The default is
1.23 markus 281: .Dq no .
282: Note that this option applies to protocol version 2 only.
283: .It Cm GSSAPICleanupCredentials
284: Specifies whether to automatically destroy the user's credentials cache
285: on logout.
286: The default is
287: .Dq yes .
288: Note that this option applies to protocol version 2 only.
1.1 stevesk 289: .It Cm HostbasedAuthentication
290: Specifies whether rhosts or /etc/hosts.equiv authentication together
291: with successful public key client host authentication is allowed
1.50 jmc 292: (host-based authentication).
1.1 stevesk 293: This option is similar to
294: .Cm RhostsRSAAuthentication
295: and applies to protocol version 2 only.
296: The default is
297: .Dq no .
298: .It Cm HostKey
299: Specifies a file containing a private host key
300: used by SSH.
301: The default is
302: .Pa /etc/ssh/ssh_host_key
303: for protocol version 1, and
304: .Pa /etc/ssh/ssh_host_rsa_key
305: and
306: .Pa /etc/ssh/ssh_host_dsa_key
307: for protocol version 2.
308: Note that
1.52 jmc 309: .Xr sshd 8
1.1 stevesk 310: will refuse to use a file if it is group/world-accessible.
311: It is possible to have multiple host key files.
312: .Dq rsa1
313: keys are used for version 1 and
314: .Dq dsa
315: or
316: .Dq rsa
317: are used for version 2 of the SSH protocol.
318: .It Cm IgnoreRhosts
319: Specifies that
320: .Pa .rhosts
321: and
322: .Pa .shosts
323: files will not be used in
324: .Cm RhostsRSAAuthentication
325: or
326: .Cm HostbasedAuthentication .
327: .Pp
328: .Pa /etc/hosts.equiv
329: and
330: .Pa /etc/shosts.equiv
331: are still used.
332: The default is
333: .Dq yes .
334: .It Cm IgnoreUserKnownHosts
335: Specifies whether
1.52 jmc 336: .Xr sshd 8
1.1 stevesk 337: should ignore the user's
1.41 djm 338: .Pa ~/.ssh/known_hosts
1.1 stevesk 339: during
340: .Cm RhostsRSAAuthentication
341: or
342: .Cm HostbasedAuthentication .
343: The default is
344: .Dq no .
345: .It Cm KerberosAuthentication
1.24 markus 346: Specifies whether the password provided by the user for
1.1 stevesk 347: .Cm PasswordAuthentication
1.24 markus 348: will be validated through the Kerberos KDC.
1.1 stevesk 349: To use this option, the server needs a
350: Kerberos servtab which allows the verification of the KDC's identity.
1.52 jmc 351: The default is
1.29 dtucker 352: .Dq no .
353: .It Cm KerberosGetAFSToken
1.45 djm 354: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
1.29 dtucker 355: an AFS token before accessing the user's home directory.
1.52 jmc 356: The default is
1.1 stevesk 357: .Dq no .
358: .It Cm KerberosOrLocalPasswd
1.52 jmc 359: If password authentication through Kerberos fails then
1.1 stevesk 360: the password will be validated via any additional local mechanism
361: such as
362: .Pa /etc/passwd .
1.52 jmc 363: The default is
1.1 stevesk 364: .Dq yes .
365: .It Cm KerberosTicketCleanup
366: Specifies whether to automatically destroy the user's ticket cache
367: file on logout.
1.52 jmc 368: The default is
1.1 stevesk 369: .Dq yes .
370: .It Cm KeyRegenerationInterval
371: In protocol version 1, the ephemeral server key is automatically regenerated
372: after this many seconds (if it has been used).
373: The purpose of regeneration is to prevent
374: decrypting captured sessions by later breaking into the machine and
375: stealing the keys.
376: The key is never stored anywhere.
377: If the value is 0, the key is never regenerated.
378: The default is 3600 (seconds).
379: .It Cm ListenAddress
380: Specifies the local addresses
1.52 jmc 381: .Xr sshd 8
1.1 stevesk 382: should listen on.
383: The following forms may be used:
384: .Pp
385: .Bl -item -offset indent -compact
386: .It
387: .Cm ListenAddress
388: .Sm off
389: .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
390: .Sm on
391: .It
392: .Cm ListenAddress
393: .Sm off
394: .Ar host No | Ar IPv4_addr No : Ar port
395: .Sm on
396: .It
397: .Cm ListenAddress
398: .Sm off
399: .Oo
400: .Ar host No | Ar IPv6_addr Oc : Ar port
401: .Sm on
402: .El
403: .Pp
404: If
405: .Ar port
406: is not specified,
1.52 jmc 407: sshd will listen on the address and all prior
1.1 stevesk 408: .Cm Port
1.17 jmc 409: options specified.
410: The default is to listen on all local addresses.
1.15 jmc 411: Multiple
1.1 stevesk 412: .Cm ListenAddress
1.17 jmc 413: options are permitted.
414: Additionally, any
1.1 stevesk 415: .Cm Port
1.52 jmc 416: options must precede this option for non-port qualified addresses.
1.1 stevesk 417: .It Cm LoginGraceTime
418: The server disconnects after this time if the user has not
419: successfully logged in.
420: If the value is 0, there is no time limit.
1.12 stevesk 421: The default is 120 seconds.
1.1 stevesk 422: .It Cm LogLevel
423: Gives the verbosity level that is used when logging messages from
1.53 ! jmc 424: .Xr sshd 8 .
1.1 stevesk 425: The possible values are:
1.52 jmc 426: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 427: The default is INFO.
428: DEBUG and DEBUG1 are equivalent.
429: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
430: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 431: .It Cm MACs
432: Specifies the available MAC (message authentication code) algorithms.
433: The MAC algorithm is used in protocol version 2
434: for data integrity protection.
435: Multiple algorithms must be comma-separated.
1.52 jmc 436: The default is:
1.1 stevesk 437: .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
1.33 dtucker 438: .It Cm MaxAuthTries
439: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 440: connection.
441: Once the number of failures reaches half this value,
442: additional failures are logged.
443: The default is 6.
1.1 stevesk 444: .It Cm MaxStartups
445: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 446: SSH daemon.
1.1 stevesk 447: Additional connections will be dropped until authentication succeeds or the
448: .Cm LoginGraceTime
449: expires for a connection.
450: The default is 10.
451: .Pp
452: Alternatively, random early drop can be enabled by specifying
453: the three colon separated values
454: .Dq start:rate:full
1.51 jmc 455: (e.g. "10:30:60").
1.53 ! jmc 456: .Xr sshd 8
1.1 stevesk 457: will refuse connection attempts with a probability of
458: .Dq rate/100
459: (30%)
460: if there are currently
461: .Dq start
462: (10)
463: unauthenticated connections.
464: The probability increases linearly and all connection attempts
465: are refused if the number of unauthenticated connections reaches
466: .Dq full
467: (60).
468: .It Cm PasswordAuthentication
469: Specifies whether password authentication is allowed.
470: The default is
471: .Dq yes .
472: .It Cm PermitEmptyPasswords
473: When password authentication is allowed, it specifies whether the
474: server allows login to accounts with empty password strings.
475: The default is
476: .Dq no .
477: .It Cm PermitRootLogin
1.38 jmc 478: Specifies whether root can log in using
1.1 stevesk 479: .Xr ssh 1 .
480: The argument must be
481: .Dq yes ,
482: .Dq without-password ,
1.52 jmc 483: .Dq forced-commands-only ,
1.1 stevesk 484: or
485: .Dq no .
486: The default is
487: .Dq yes .
488: .Pp
489: If this option is set to
1.52 jmc 490: .Dq without-password ,
1.1 stevesk 491: password authentication is disabled for root.
492: .Pp
493: If this option is set to
1.52 jmc 494: .Dq forced-commands-only ,
1.1 stevesk 495: root login with public key authentication will be allowed,
496: but only if the
497: .Ar command
498: option has been specified
499: (which may be useful for taking remote backups even if root login is
1.17 jmc 500: normally not allowed).
501: All other authentication methods are disabled for root.
1.1 stevesk 502: .Pp
503: If this option is set to
1.52 jmc 504: .Dq no ,
1.38 jmc 505: root is not allowed to log in.
1.46 reyk 506: .It Cm PermitTunnel
507: Specifies whether
508: .Xr tun 4
509: device forwarding is allowed.
1.47 reyk 510: The argument must be
511: .Dq yes ,
512: .Dq point-to-point ,
1.52 jmc 513: .Dq ethernet ,
1.47 reyk 514: or
515: .Dq no .
1.46 reyk 516: The default is
517: .Dq no .
1.6 markus 518: .It Cm PermitUserEnvironment
519: Specifies whether
520: .Pa ~/.ssh/environment
1.9 stevesk 521: and
1.6 markus 522: .Cm environment=
523: options in
524: .Pa ~/.ssh/authorized_keys
1.9 stevesk 525: are processed by
1.52 jmc 526: .Xr sshd 8 .
1.6 markus 527: The default is
528: .Dq no .
1.9 stevesk 529: Enabling environment processing may enable users to bypass access
530: restrictions in some configurations using mechanisms such as
531: .Ev LD_PRELOAD .
1.1 stevesk 532: .It Cm PidFile
1.4 stevesk 533: Specifies the file that contains the process ID of the
1.53 ! jmc 534: SSH daemon.
1.1 stevesk 535: The default is
536: .Pa /var/run/sshd.pid .
537: .It Cm Port
538: Specifies the port number that
1.52 jmc 539: .Xr sshd 8
1.1 stevesk 540: listens on.
541: The default is 22.
542: Multiple options of this type are permitted.
543: See also
544: .Cm ListenAddress .
545: .It Cm PrintLastLog
546: Specifies whether
1.52 jmc 547: .Xr sshd 8
1.36 jaredy 548: should print the date and time of the last user login when a user logs
549: in interactively.
1.1 stevesk 550: The default is
551: .Dq yes .
552: .It Cm PrintMotd
553: Specifies whether
1.52 jmc 554: .Xr sshd 8
1.1 stevesk 555: should print
556: .Pa /etc/motd
557: when a user logs in interactively.
558: (On some systems it is also printed by the shell,
559: .Pa /etc/profile ,
560: or equivalent.)
561: The default is
562: .Dq yes .
563: .It Cm Protocol
564: Specifies the protocol versions
1.52 jmc 565: .Xr sshd 8
1.5 stevesk 566: supports.
1.1 stevesk 567: The possible values are
1.52 jmc 568: .Sq 1
1.1 stevesk 569: and
1.52 jmc 570: .Sq 2 .
1.1 stevesk 571: Multiple versions must be comma-separated.
572: The default is
573: .Dq 2,1 .
1.5 stevesk 574: Note that the order of the protocol list does not indicate preference,
575: because the client selects among multiple protocol versions offered
576: by the server.
577: Specifying
578: .Dq 2,1
579: is identical to
580: .Dq 1,2 .
1.1 stevesk 581: .It Cm PubkeyAuthentication
582: Specifies whether public key authentication is allowed.
583: The default is
584: .Dq yes .
585: Note that this option applies to protocol version 2 only.
586: .It Cm RhostsRSAAuthentication
587: Specifies whether rhosts or /etc/hosts.equiv authentication together
588: with successful RSA host authentication is allowed.
589: The default is
590: .Dq no .
591: This option applies to protocol version 1 only.
592: .It Cm RSAAuthentication
593: Specifies whether pure RSA authentication is allowed.
594: The default is
595: .Dq yes .
596: This option applies to protocol version 1 only.
597: .It Cm ServerKeyBits
598: Defines the number of bits in the ephemeral protocol version 1 server key.
599: The minimum value is 512, and the default is 768.
600: .It Cm StrictModes
601: Specifies whether
1.52 jmc 602: .Xr sshd 8
1.1 stevesk 603: should check file modes and ownership of the
604: user's files and home directory before accepting login.
605: This is normally desirable because novices sometimes accidentally leave their
606: directory or files world-writable.
607: The default is
608: .Dq yes .
609: .It Cm Subsystem
1.51 jmc 610: Configures an external subsystem (e.g. file transfer daemon).
1.1 stevesk 611: Arguments should be a subsystem name and a command to execute upon subsystem
612: request.
613: The command
614: .Xr sftp-server 8
615: implements the
616: .Dq sftp
617: file transfer subsystem.
618: By default no subsystems are defined.
619: Note that this option applies to protocol version 2 only.
620: .It Cm SyslogFacility
621: Gives the facility code that is used when logging messages from
1.53 ! jmc 622: .Xr sshd 8 .
1.1 stevesk 623: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
624: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
625: The default is AUTH.
1.27 markus 626: .It Cm TCPKeepAlive
627: Specifies whether the system should send TCP keepalive messages to the
628: other side.
629: If they are sent, death of the connection or crash of one
630: of the machines will be properly noticed.
631: However, this means that
632: connections will die if the route is down temporarily, and some people
633: find it annoying.
634: On the other hand, if TCP keepalives are not sent,
635: sessions may hang indefinitely on the server, leaving
636: .Dq ghost
637: users and consuming server resources.
638: .Pp
639: The default is
640: .Dq yes
641: (to send TCP keepalive messages), and the server will notice
642: if the network goes down or the client host crashes.
643: This avoids infinitely hanging sessions.
644: .Pp
645: To disable TCP keepalive messages, the value should be set to
646: .Dq no .
1.18 markus 647: .It Cm UseDNS
648: Specifies whether
1.52 jmc 649: .Xr sshd 8
1.40 jmc 650: should look up the remote host name and check that
1.18 markus 651: the resolved host name for the remote IP address maps back to the
652: very same IP address.
653: The default is
654: .Dq yes .
1.1 stevesk 655: .It Cm UseLogin
656: Specifies whether
657: .Xr login 1
658: is used for interactive login sessions.
659: The default is
660: .Dq no .
661: Note that
662: .Xr login 1
663: is never used for remote command execution.
664: Note also, that if this is enabled,
665: .Cm X11Forwarding
666: will be disabled because
667: .Xr login 1
668: does not know how to handle
669: .Xr xauth 1
1.15 jmc 670: cookies.
671: If
1.1 stevesk 672: .Cm UsePrivilegeSeparation
673: is specified, it will be disabled after authentication.
674: .It Cm UsePrivilegeSeparation
675: Specifies whether
1.52 jmc 676: .Xr sshd 8
1.2 stevesk 677: separates privileges by creating an unprivileged child process
1.15 jmc 678: to deal with incoming network traffic.
679: After successful authentication, another process will be created that has
680: the privilege of the authenticated user.
681: The goal of privilege separation is to prevent privilege
1.1 stevesk 682: escalation by containing any corruption within the unprivileged processes.
683: The default is
684: .Dq yes .
685: .It Cm X11DisplayOffset
686: Specifies the first display number available for
1.52 jmc 687: .Xr sshd 8 Ns 's
1.1 stevesk 688: X11 forwarding.
1.52 jmc 689: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 690: The default is 10.
691: .It Cm X11Forwarding
692: Specifies whether X11 forwarding is permitted.
1.13 stevesk 693: The argument must be
694: .Dq yes
695: or
696: .Dq no .
1.1 stevesk 697: The default is
698: .Dq no .
1.13 stevesk 699: .Pp
700: When X11 forwarding is enabled, there may be additional exposure to
701: the server and to client displays if the
1.52 jmc 702: .Xr sshd 8
1.13 stevesk 703: proxy display is configured to listen on the wildcard address (see
704: .Cm X11UseLocalhost
1.52 jmc 705: below), though this is not the default.
1.13 stevesk 706: Additionally, the authentication spoofing and authentication data
707: verification and substitution occur on the client side.
708: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 709: display server may be exposed to attack when the SSH client requests
1.13 stevesk 710: forwarding (see the warnings for
711: .Cm ForwardX11
712: in
1.19 jmc 713: .Xr ssh_config 5 ) .
1.13 stevesk 714: A system administrator may have a stance in which they want to
715: protect clients that may expose themselves to attack by unwittingly
716: requesting X11 forwarding, which can warrant a
717: .Dq no
718: setting.
719: .Pp
720: Note that disabling X11 forwarding does not prevent users from
721: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 722: X11 forwarding is automatically disabled if
723: .Cm UseLogin
724: is enabled.
725: .It Cm X11UseLocalhost
726: Specifies whether
1.52 jmc 727: .Xr sshd 8
1.1 stevesk 728: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 729: the wildcard address.
730: By default,
1.52 jmc 731: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 732: hostname part of the
733: .Ev DISPLAY
734: environment variable to
735: .Dq localhost .
1.8 stevesk 736: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 737: However, some older X11 clients may not function with this
738: configuration.
739: .Cm X11UseLocalhost
740: may be set to
741: .Dq no
742: to specify that the forwarding server should be bound to the wildcard
743: address.
744: The argument must be
745: .Dq yes
746: or
747: .Dq no .
748: The default is
749: .Dq yes .
750: .It Cm XAuthLocation
1.11 stevesk 751: Specifies the full pathname of the
1.1 stevesk 752: .Xr xauth 1
753: program.
754: The default is
755: .Pa /usr/X11R6/bin/xauth .
756: .El
757: .Ss Time Formats
1.53 ! jmc 758: .Xr sshd 8
1.1 stevesk 759: command-line arguments and configuration file options that specify time
760: may be expressed using a sequence of the form:
761: .Sm off
1.7 stevesk 762: .Ar time Op Ar qualifier ,
1.1 stevesk 763: .Sm on
764: where
765: .Ar time
766: is a positive integer value and
767: .Ar qualifier
768: is one of the following:
769: .Pp
770: .Bl -tag -width Ds -compact -offset indent
771: .It Cm <none>
772: seconds
773: .It Cm s | Cm S
774: seconds
775: .It Cm m | Cm M
776: minutes
777: .It Cm h | Cm H
778: hours
779: .It Cm d | Cm D
780: days
781: .It Cm w | Cm W
782: weeks
783: .El
784: .Pp
785: Each member of the sequence is added together to calculate
786: the total time value.
787: .Pp
788: Time format examples:
789: .Pp
790: .Bl -tag -width Ds -compact -offset indent
791: .It 600
792: 600 seconds (10 minutes)
793: .It 10m
794: 10 minutes
795: .It 1h30m
796: 1 hour 30 minutes (90 minutes)
797: .El
798: .Sh FILES
799: .Bl -tag -width Ds
800: .It Pa /etc/ssh/sshd_config
801: Contains configuration data for
1.53 ! jmc 802: .Xr sshd 8 .
1.1 stevesk 803: This file should be writable by root only, but it is recommended
804: (though not necessary) that it be world-readable.
805: .El
1.19 jmc 806: .Sh SEE ALSO
807: .Xr sshd 8
1.1 stevesk 808: .Sh AUTHORS
809: OpenSSH is a derivative of the original and free
810: ssh 1.2.12 release by Tatu Ylonen.
811: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
812: Theo de Raadt and Dug Song
813: removed many bugs, re-added newer features and
814: created OpenSSH.
815: Markus Friedl contributed the support for SSH
816: protocol versions 1.5 and 2.0.
817: Niels Provos and Markus Friedl contributed support
818: for privilege separation.
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