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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, setting this file manually should not
51 The only name server(s) to be queried will be on the local machine
52 or automatically configured using DHCP or a similar mechanism,
53 the domain name is determined from the host name,
54 and the domain search path is constructed from the domain name.
56 The different configuration options are:
57 .Bl -tag -width nameserver
59 IPv4 or IPv6 address of a name server
60 that the resolver should query.
63 (currently 3) name servers may be listed,
65 If there are multiple servers,
66 the resolver library queries them in the order listed.
70 the default is to use the name server on the local machine.
71 (The algorithm used is to try a name server, and if the query times out,
72 try the next, until out of name servers,
73 then repeat trying all the name servers
74 until a maximum number of retries are made).
77 Most queries for names within this domain can use short names
78 relative to the local domain.
81 entry is present, the domain is determined
82 from the local host name returned by
84 the domain part is taken to be everything after the first
86 Finally, if the host name does not contain a domain part, the root
89 Search list for host-name lookup.
90 The search list is normally determined from the local domain name;
91 by default, it contains only the local domain name.
92 This may be changed by listing the desired domain search path
95 keyword with spaces or tabs separating
97 Most resolver queries will be attempted using each component
98 of the search path in turn until a match is found.
99 Note that this process may be slow and will generate a lot of network
100 traffic if the servers for the listed domains are not local,
101 and that queries will time out if no server is available
102 for one of the domains.
104 The search list is currently limited to six domains
105 with a total of 256 characters.
107 Sortlist allows addresses returned by gethostbyname to be sorted.
108 A sortlist is specified by IP address netmask pairs.
109 If the netmask is not specified,
110 it defaults to the historical Class A/B/C netmask of the net;
111 this usage is deprecated.
113 and network pairs are separated by slashes.
118 .Dl "sortlist 10.9.1.0/255.255.240.0 10.9.0.0/255.255.0.0"
120 Options allows certain internal resolver variables to be modified.
123 \fBoptions\fP \fIoption\fP \fI...\fP
127 is one of the following:
128 .Bl -tag -width no_tld_query
136 to use TCP instead of UDP for queries.
137 .It Sy ndots : Ns Ar n
138 sets a threshold for the number of dots which must appear in a name given to
143 .Em initial absolute query
149 meaning that if there are any dots in a name, the name
150 will be tried first as an absolute name before any
152 elements are appended to it.
153 .It Sy timeout : Ns Ar n
154 sets the initial amount of time the resolver will wait
155 for a response from a remote
156 name server before retrying the query via a different name server.
157 The resolver may wait longer during subsequent retries
158 of the current query since an exponential back-off is applied to
160 Measured in seconds, the default is
162 the allowed maximum is
166 .It Sy attempts : Ns Ar n
167 sets the number of times the resolver will send a query to each of
169 before giving up and returning an error to the calling application.
172 the allowed maximum is
177 tells the resolver not to attempt to resolve a top level domain name, that
178 is, a name that contains no dots.
179 Use of this option does not prevent
180 the resolver from obeying the standard
184 rules with the given name.
185 .It Sy reload-period : Ns Ar n
186 The resolver checks the modification time of
193 has changed, it is automatically reloaded.
197 Setting it to zero disables the file check.
200 Options may also be specified as a space or tab separated list using the
202 environment variable.
209 keywords are mutually exclusive.
210 If more than one instance of these keywords is present,
211 the last instance will override.
213 The keyword and value must appear on a single line, and the keyword
214 .Pq for example, Sy nameserver
216 The value follows the keyword, separated by white space.
218 .Bl -tag -width /etc/resolv.conf -compact
219 .It Pa /etc/resolv.conf
226 A basic resolv.conf file could be in the following form.
227 .Bd -literal -offset indent
228 # The domain directive is only necessary, if your local
229 # router advertises something like localdomain and you have
230 # set up your hostnames via an external domain.
231 domain localdomain.tld
233 # In case you a running a local dns server or caching name server
234 # like local-unbound(8) for example.
237 # IP address of the local or ISP name service
238 nameserver 192.168.2.1
240 # Fallback nameservers, in this case these from Google.
244 # Attach an OPT pseudo-RR for the EDNS0 extension,
245 # as specified in RFC 2671.
249 .Xr gethostbyname 3 ,
256 file format appeared in