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33 .Nd resolver configuration file
39 is a set of routines in the C library
40 which provide access to the Internet Domain Name System.
41 The resolver configuration file contains information that is read
42 by the resolver routines the first time they are invoked by a process.
43 The file is designed to be human readable and contains a list of
44 keywords with values that provide various types of resolver information.
46 On a normally configured system, setting this file manually should not
48 The only name server(s) to be queried will be on the local machine
49 or automatically configured using DHCP or a similar mechanism,
50 the domain name is determined from the host name,
51 and the domain search path is constructed from the domain name.
53 The different configuration options are:
54 .Bl -tag -width nameserver
56 IPv4 or IPv6 address of a name server
57 that the resolver should query.
60 (currently 3) name servers may be listed,
62 If there are multiple servers,
63 the resolver library queries them in the order listed.
67 the default is to use the name server on the local machine.
68 (The algorithm used is to try a name server, and if the query times out,
69 try the next, until out of name servers,
70 then repeat trying all the name servers
71 until a maximum number of retries are made).
74 Most queries for names within this domain can use short names
75 relative to the local domain.
78 entry is present, the domain is determined
79 from the local host name returned by
81 the domain part is taken to be everything after the first
83 Finally, if the host name does not contain a domain part, the root
86 Search list for host-name lookup.
87 The search list is normally determined from the local domain name;
88 by default, it contains only the local domain name.
89 This may be changed by listing the desired domain search path
92 keyword with spaces or tabs separating
94 Most resolver queries will be attempted using each component
95 of the search path in turn until a match is found.
96 Note that this process may be slow and will generate a lot of network
97 traffic if the servers for the listed domains are not local,
98 and that queries will time out if no server is available
99 for one of the domains.
101 The search list is currently limited to six domains
102 with a total of 256 characters.
104 Sortlist allows addresses returned by gethostbyname to be sorted.
105 A sortlist is specified by IP address netmask pairs.
106 If the netmask is not specified,
107 it defaults to the historical Class A/B/C netmask of the net;
108 this usage is deprecated.
110 and network pairs are separated by slashes.
115 .Dl "sortlist 10.9.1.0/255.255.240.0 10.9.0.0/255.255.0.0"
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 A basic resolv.conf file could be in the following form.
224 .Bd -literal -offset indent
225 # The domain directive is only necessary, if your local
226 # router advertises something like localdomain and you have
227 # set up your hostnames via an external domain.
228 domain localdomain.tld
230 # In case you a running a local dns server or caching name server
231 # like local-unbound(8) for example.
234 # IP address of the local or ISP name service
235 nameserver 192.168.2.1
237 # Fallback nameservers, in this case these from Google.
241 # Attach an OPT pseudo-RR for the EDNS0 extension,
242 # as specified in RFC 2671.
246 .Xr gethostbyname 3 ,
253 file format appeared in