1RESOLV.CONF(5) Linux Programmer's Manual RESOLV.CONF(5)
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6 resolv.conf - resolver configuration file
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9 /etc/resolv.conf
10
12 The resolver is a set of routines in the C library that provide access
13 to the Internet Domain Name System (DNS). The resolver configuration
14 file contains information that is read by the resolver routines the
15 first time they are invoked by a process. The file is designed to be
16 human readable and contains a list of keywords with values that provide
17 various types of resolver information.
18 On a normally configured system this file should not be necessary. The
20 only name server to be queried will be on the local machine; the domain
21 name is determined from the hostname and the domain search path is con‐
22 structed from the domain name.
23 The different configuration options are:
25 nameserver Name server IP address
27 Internet address (in dot notation) of a name server that the
28 resolver should query. Up to MAXNS (currently 3, see
29 <resolv.h>) name servers may be listed, one per keyword. If
30 there are multiple servers, the resolver library queries them in
31 the order listed. If no nameserver entries are present, the
32 default is to use the name server on the local machine. (The
33 algorithm used is to try a name server, and if the query times
34 out, try the next, until out of name servers, then repeat trying
35 all the name servers until a maximum number of retries are
36 made.)
37 domain Local domain name.
39 Most queries for names within this domain can use short names
40 relative to the local domain. If no domain entry is present,
41 the domain is determined from the local hostname returned by
42 gethostname(2); the domain part is taken to be everything after
43 the first '.'. Finally, if the hostname does not contain a
44 domain part, the root domain is assumed.
45 search Search list for host-name lookup.
47 The search list is normally determined from the local domain
48 name; by default, it contains only the local domain name. This
49 may be changed by listing the desired domain search path follow‐
50 ing the search keyword with spaces or tabs separating the names.
51 Resolver queries having fewer than ndots dots (default is 1) in
52 them will be attempted using each component of the search path
53 in turn until a match is found. For environments with multiple
54 subdomains please read options ndots:n below to avoid man-in-
55 the-middle attacks and unnecessary traffic for the root-dns-
56 servers. Note that this process may be slow and will generate a
57 lot of network traffic if the servers for the listed domains are
58 not local, and that queries will time out if no server is avail‐
59 able for one of the domains.
60 The search list is currently limited to six domains with a total
62 of 256 characters.
63 sortlist
65 This option allows addresses returned by gethostbyname(3) to be
66 sorted. A sortlist is specified by IP-address-netmask pairs.
67 The netmask is optional and defaults to the natural netmask of
68 the net. The IP address and optional network pairs are sepa‐
69 rated by slashes. Up to 10 pairs may be specified. Here is an
70 example:
71 sortlist 130.155.160.0/255.255.240.0 130.155.0.0
73 options
75 Options allows certain internal resolver variables to be modi‐
76 fied. The syntax is
77 options option ...
79 where option is one of the following:
81 debug sets RES_DEBUG in _res.options.
83 ndots:n
85 sets a threshold for the number of dots which must appear
86 in a name given to res_query(3) (see resolver(3)) before
87 an initial absolute query will be made. The default for
88 n is 1, meaning that if there are any dots in a name, the
89 name will be tried first as an absolute name before any
90 search list elements are appended to it. The value for
91 this option is silently capped to 15.
92 timeout:n
94 sets the amount of time the resolver will wait for a
95 response from a remote name server before retrying the
96 query via a different name server. Measured in seconds,
97 the default is RES_TIMEOUT (currently 5, see <resolv.h>).
98 The value for this option is silently capped to 30.
99 attempts:n
101 sets the number of times the resolver will send a query
102 to its name servers before giving up and returning an
103 error to the calling application. The default is
104 RES_DFLRETRY (currently 2, see <resolv.h>). The value
105 for this option is silently capped to 5.
106 rotate sets RES_ROTATE in _res.options, which causes round robin
108 selection of nameservers from among those listed. This
109 has the effect of spreading the query load among all
110 listed servers, rather than having all clients try the
111 first listed server first every time.
112 no-check-names
114 sets RES_NOCHECKNAME in _res.options, which disables the
115 modern BIND checking of incoming hostnames and mail names
116 for invalid characters such as underscore (_), non-ASCII,
117 or control characters.
118 inet6 sets RES_USE_INET6 in _res.options. This has the effect
120 of trying a AAAA query before an A query inside the geth‐
121 ostbyname(3) function, and of mapping IPv4 responses in
122 IPv6 "tunneled form" if no AAAA records are found but an
123 A record set exists.
124 ip6-bytestring (since glibc 2.3.4)
126 sets RES_USE_BSTRING in _res.options. This causes
127 reverse IPv6 lookups to be made using the bit-label for‐
128 mat described in RFC 2673; if this option is not set,
129 then nibble format is used.
130 ip6-dotint/no-ip6-dotint (since glibc 2.3.4)
132 Clear/set RES_NOIP6DOTINT in _res.options. When this
133 option is clear (ip6-dotint), reverse IPv6 lookups are
134 made in the (deprecated) ip6.int zone; when this option
135 is set (no-ip6-dotint), reverse IPv6 lookups are made in
136 the ip6.arpa zone by default. This option is set by
137 default.
138 edns0 (since glibc 2.6)
140 sets RES_USE_EDNSO in _res.options. This enables support
141 for the DNS extensions described in RFC 2671.
142 The domain and search keywords are mutually exclusive. If more than
144 one instance of these keywords is present, the last instance wins.
145 The search keyword of a system's resolv.conf file can be overridden on
147 a per-process basis by setting the environment variable LOCALDOMAIN to
148 a space-separated list of search domains.
149 The options keyword of a system's resolv.conf file can be amended on a
151 per-process basis by setting the environment variable RES_OPTIONS to a
152 space-separated list of resolver options as explained above under
153 options.
154 The keyword and value must appear on a single line, and the keyword
156 (e.g., nameserver) must start the line. The value follows the keyword,
157 separated by white space.
158
160 /etc/resolv.conf, <resolv.h>
161
163 gethostbyname(3), resolver(3), hostname(7), named(8)
164 Name Server Operations Guide for BIND
165
167 This page is part of release 3.25 of the Linux man-pages project. A
168 description of the project, and information about reporting bugs, can
169 be found at http://www.kernel.org/doc/man-pages/.
1704th Berkeley Distribution 2009-03-01 RESOLV.CONF(5)