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16 .\" @(#)resolver.5 5.9 (Berkeley) 12/14/89
24 .Nd resolver configuration file
30 is a set of routines in the C library
32 that provide access to the Internet Domain Name System.
35 configuration file contains information that is read
38 routines the first time they are invoked by a process.
39 The file is designed to be human readable and contains a list of
40 keywords with values that provide various types of
44 On a normally configured system, this file should not be necessary.
45 The only name server to be queried will be on the local machine,
46 the domain name is determined from the host name,
47 and the domain search path is constructed from the domain name.
49 The different configuration directives are:
50 .Bl -tag -width "nameser"
52 Internet address (in dot notation) of a name server that the
58 name servers may be listed, one per keyword.
59 If there are multiple servers, the
61 library queries them in the order listed.
64 entries are present, the default is to use the name server on the local machine.
65 (The algorithm used is to try a name server, and if the query times out,
66 try the next, until out of name servers,
67 then repeat trying all the name servers
68 until a maximum number of retries are made).
71 Most queries for names within this domain can use short names
72 relative to the local domain.
75 entry is present, the domain is determined from the local host name returned by
77 the domain part is taken to be everything after the first
79 Finally, if the host name does not contain a domain part, the root
82 Search list for host-name lookup.
83 The search list is normally determined from the local domain name;
84 by default, it contains only the local domain name.
85 This may be changed by listing the desired domain search path
88 keyword with spaces or tabs separating the names.
91 queries will be attempted using each component
92 of the search path in turn until a match is found.
93 Note that this process may be slow and will generate a lot of network
94 traffic if the servers for the listed domains are not local,
95 and that queries will time out if no server is available
96 for one of the domains.
98 The search list is currently limited to six domains
99 with a total of 256 characters.
101 Allows addresses returned by gethostbyname to be sorted.
104 is specified by IP address netmask pairs. The netmask is
105 optional and defaults to the natural netmask of the net. The IP address
106 and optional network pairs are separated by slashes. Up to 10 pairs may
107 be specified. For example:
108 .Bd -literal -offset indent
109 sortlist 130.155.160.0/255.255.240.0 130.155.0.0
112 Allows certain internal
114 variables to be modified.
116 .D1 Li options Ar option ...
119 is one of the following:
120 .Bl -tag -width "ndots:n "
126 .It Li ndots: Ns Ar n
127 sets a threshold for the number of dots which
128 must appear in a name given to
133 .Em initial absolute query
134 will be made. The default for
138 meaning that if there are
140 dots in a name, the name will be tried first as an absolute name before any
142 elements are appended to it.
143 .It Li timeout: Ns Ar n
144 sets the amount of time the resolver will wait for a response from a remote
145 name server before retrying the query via a different name server. Measured in
146 seconds, the default is
150 .It Li attempts: Ns Ar n
151 sets the number of times the resolver will send a query to its name servers
152 before giving up and returning an error to the calling application. The
162 which causes round robin selection of nameservers from among those listed.
163 This has the effect of spreading the query load among all listed servers,
164 rather than having all clients try the first listed server first every time.
165 .It Li no-check-names
170 which disables the modern BIND checking of incoming host names and mail names
171 for invalid characters such as underscore (_), non-ASCII, or control characters.
177 This has the effect of trying a AAAA query before an A query inside the
179 function, and of mapping IPv4 responses in IPv6 ``tunnelled form'' if no
180 AAAA records are found but an A record set exists.
188 keywords are mutually exclusive.
189 If more than one instance of these keywords is present,
190 the last instance wins.
194 keyword of a system's
197 overridden on a per-process basis by setting the environment variable
199 to a space-separated list of search domains.
203 keyword of a system's
205 file can be amended on a per-process basis by setting the environment variable
206 .Dq Ev RES_OPTIONS to a space-separated list of
208 options as explained above under
211 The keyword and value must appear on a single line, and the keyword
214 must start the line. The value follows the keyword, separated by white space.
219 .Xr gethostbyname 3 ,
224 .Dq Name Server Operations Guide for Sy BIND