1RESOLVER(5)                 BSD File Formats Manual                RESOLVER(5)
2

NAME

4     resolver — resolver configuration file
5

SYNOPSIS

7     /etc/resolv.conf
8

DESCRIPTION

10     The resolver is a set of routines in the C library (resolve(3)) that pro‐
11     vide access to the Internet Domain Name System.  The resolver configura‐
12     tion file contains information that is read by the resolver routines the
13     first time they are invoked by a process.  The file is designed to be
14     human readable and contains a list of keywords with values that provide
15     various types of resolver information.
16
17     On a normally configured system, this file should not be necessary.  The
18     only name server to be queried will be on the local machine, the domain
19     name is determined from the host name, and the domain search path is con‐
20     structed from the domain name.
21
22     The different configuration directives are:
23
24     nameserver
25              Internet address (in dot notation) of a name server that the
26              resolver should query.  Up to MAXNS (see <resolv.h>) name
27              servers may be listed, one per keyword.  If there are multiple
28              servers, the resolver library queries them in the order listed.
29              If no nameserver entries are present, the default is to use the
30              name server on the local machine.  (The algorithm used is to try
31              a name server, and if the query times out, try the next, until
32              out of name servers, then repeat trying all the name servers
33              until a maximum number of retries are made).
34
35     domain   Local domain name.  Most queries for names within this domain
36              can use short names relative to the local domain.  If no domain
37              entry is present, the domain is determined from the local host
38              name returned by gethostname; the domain part is taken to be
39              everything after the first ‘.’.  Finally, if the host name does
40              not contain a domain part, the root domain is assumed.
41
42     search   Search list for host-name lookup.  The search list is normally
43              determined from the local domain name; by default, it contains
44              only the local domain name.  This may be changed by listing the
45              desired domain search path following the search keyword with
46              spaces or tabs separating the names.  Most resolver queries will
47              be attempted using each component of the search path in turn
48              until a match is found.  Note that this process may be slow and
49              will generate a lot of network traffic if the servers for the
50              listed domains are not local, and that queries will time out if
51              no server is available for one of the domains.
52
53              The search list is currently limited to six domains with a total
54              of 256 characters.
55
56     sortlist
57              Allows addresses returned by gethostbyname to be sorted.  A
58              sortlist is specified by IP address netmask pairs. The netmask
59              is optional and defaults to the natural netmask of the net. The
60              IP address and optional network pairs are separated by slashes.
61              Up to 10 pairs may be specified.  For example:
62
63                    sortlist 130.155.160.0/255.255.240.0 130.155.0.0
64
65     options  Allows certain internal resolver variables to be modified.  The
66              syntax is
67                    options option ...
68              where option is one of the following:
69
70              debug     sets RES_DEBUG in _res.options.
71
72              ndots:n   sets a threshold for the number of dots which must
73                        appear in a name given to res_query() (see
74                        resolver(3)) before an initial absolute query will be
75                        made.  The default for n is “1”, meaning that if there
76                        are any dots in a name, the name will be tried first
77                        as an absolute name before any search list elements
78                        are appended to it.
79
80              timeout:n
81                        sets the amount of time the resolver will wait for a
82                        response from a remote name server before retrying the
83                        query via a different name server.  Measured in sec‐
84                        onds, the default is RES_TIMEOUT (see <resolv.h>).
85
86              attempts:n
87                        sets the number of times the resolver will send a
88                        query to its name servers before giving up and return‐
89                        ing an error to the calling application.  The default
90                        is RES_DFLRETRY (see <resolv.h>).
91
92              rotate    sets RES_ROTATE in _res.options, which causes round
93                        robin selection of nameservers from among those
94                        listed.  This has the effect of spreading the query
95                        load among all listed servers, rather than having all
96                        clients try the first listed server first every time.
97
98              no-check-names
99                        sets RES_NOCHECKNAME in _res.options, which disables
100                        the modern BIND checking of incoming host names and
101                        mail names for invalid characters such as underscore
102                        (_), non-ASCII, or control characters.
103
104              inet6     sets RES_USE_INET6 in _res.options.  This has the
105                        effect of trying a AAAA query before an A query inside
106                        the gethostbyname function, and of mapping IPv4
107                        responses in IPv6 ``tunnelled form'' if no AAAA
108                        records are found but an A record set exists.
109
110              no-tld-query
111                        sets RES_NOTLDQUERY in _res.options.  This option
112                        causes res_nsearch() to not attempt to resolve a
113                        unqualified name as if it were a top level domain
114                        (TLD).  This option can cause problems if the site has
115                        "localhost" as a TLD rather than having localhost on
116                        one or more elements of the search list.  This option
117                        has no effect if neither RES_DEFNAMES or RES_DNSRCH is
118                        set.
119
120     The domain and search keywords are mutually exclusive.  If more than one
121     instance of these keywords is present, the last instance wins.
122
123     The search keyword of a system's resolv.conf file can be overridden on a
124     per-process basis by setting the environment variable “LOCALDOMAIN” to a
125     space-separated list of search domains.
126
127     The options keyword of a system's resolv.conf file can be amended on a
128     per-process basis by setting the environment variable “RES_OPTIONS to a
129     space-separated list of” resolver options as explained above under
130     options.
131
132     The keyword and value must appear on a single line, and the keyword
133     (e.g., nameserver) must start the line.  The value follows the keyword,
134     separated by white space.
135

FILES

137     /etc/resolv.conf <resolv.h>
138

SEE ALSO

140     gethostbyname(3), hostname(7), resolver(3), resolver(5).  “Name Server
141     Operations Guide for BIND
142
1434th Berkeley Distribution      November 11, 1993     4th Berkeley Distribution
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