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28 .\" @(#)resolver.5 8.1 (Berkeley) 6/5/93
36 .Nd resolver configuration file
42 is a set of routines in the C library
43 which provide access to the Internet Domain Name System.
44 The resolver configuration file contains information that is read
45 by the resolver routines the first time they are invoked by a process.
46 The file is designed to be human readable and contains a list of
47 keywords with values that provide various types of resolver information.
49 On a normally configured system this file should not be necessary.
50 The only name server to be queried will be on the local machine,
51 the domain name is determined from the host name,
52 and the domain search path is constructed from the domain name.
54 The different configuration options are:
55 .Bl -tag -width nameserver
57 Internet address (in dot notation) of a name server
58 that the resolver should query.
61 (currently 3) name servers may be listed,
63 If there are multiple servers,
64 the resolver library queries them in the order listed.
68 the default is to use the name server on the local machine.
69 (The algorithm used is to try a name server, and if the query times out,
70 try the next, until out of name servers,
71 then repeat trying all the name servers
72 until a maximum number of retries are made).
75 Most queries for names within this domain can use short names
76 relative to the local domain.
79 entry is present, the domain is determined
80 from the local host name returned by
82 the domain part is taken to be everything after the first
84 Finally, if the host name does not contain a domain part, the root
87 Search list for host-name lookup.
88 The search list is normally determined from the local domain name;
89 by default, it contains only the local domain name.
90 This may be changed by listing the desired domain search path
93 keyword with spaces or tabs separating
95 Most resolver queries will be attempted using each component
96 of the search path in turn until a match is found.
97 Note that this process may be slow and will generate a lot of network
98 traffic if the servers for the listed domains are not local,
99 and that queries will time out if no server is available
100 for one of the domains.
102 The search list is currently limited to six domains
103 with a total of 256 characters.
105 Sortlist allows addresses returned by gethostbyname to be sorted.
106 A sortlist is specified by IP address netmask pairs.
108 optional and defaults to the natural netmask of the net.
110 and optional network pairs are separated by slashes.
115 .Dl "sortlist 130.155.160.0/255.255.240.0 130.155.0.0"
117 Options allows certain internal resolver variables to be modified.
120 \fBoptions\fP \fIoption\fP \fI...\fP
124 is one of the following:
125 .Bl -tag -width no_tld_query
133 to use TCP instead of UDP for queries.
134 .It Sy ndots: Ns Ar n
135 sets a threshold for the number of dots which must appear in a name given to
140 .Em initial absolute query
146 meaning that if there are any dots in a name, the name
147 will be tried first as an absolute name before any
149 elements are appended to it.
150 .It Sy timeout: Ns Ar n
151 sets the initial amount of time the resolver will wait
152 for a response from a remote
153 name server before retrying the query via a different name server.
154 The resolver may wait longer during subsequent retries
155 of the current query since an exponential back-off is applied to
157 Measured in seconds, the default is
159 the allowed maximum is
163 .It Sy attempts: Ns Ar n
164 sets the number of times the resolver will send a query to each of
166 before giving up and returning an error to the calling application.
169 the allowed maximum is
174 tells the resolver not to attempt to resolve a top level domain name, that
175 is, a name that contains no dots.
176 Use of this option does not prevent
177 the resolver from obeying the standard
181 rules with the given name.
182 .It Sy reload-period: Ns Ar n
183 The resolver checks the modification time of
190 has changed, it is automatically reloaded.
194 Setting it to zero disables the file check.
197 Options may also be specified as a space or tab separated list using the
199 environment variable.
206 keywords are mutually exclusive.
207 If more than one instance of these keywords is present,
208 the last instance will override.
210 The keyword and value must appear on a single line, and the keyword
211 .Pq for example, Sy nameserver
213 The value follows the keyword, separated by white space.
215 .Bl -tag -width /etc/resolv.conf -compact
216 .It Pa /etc/resolv.conf
223 .Xr gethostbyname 3 ,
228 .%T "Name Server Operations Guide for BIND"
233 file format appeared in