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
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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 If this file does not exist, only the name server on the local machine
20 will be queried; the domain name is determined from the hostname and
21 the domain search path is constructed from the domain name.
22 The different configuration options are:
24 nameserver Name server IP address
26 Internet address of a name server that the resolver should
27 query, either an IPv4 address (in dot notation), or an IPv6
28 address in colon (and possibly dot) notation as per RFC 2373.
29 Up to MAXNS (currently 3, see <resolv.h>) name servers may be
30 listed, one per keyword. If there are multiple servers, the
31 resolver library queries them in the order listed. If no name‐
32 server entries are present, the default is to use the name
33 server on the local machine. (The algorithm used is to try a
34 name server, and if the query times out, try the next, until out
35 of name servers, then repeat trying all the name servers until a
36 maximum number of retries are 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 set to '.', the root domain is
41 considered. If no domain entry is present, the domain is deter‐
42 mined from the local hostname returned by gethostname(2); the
43 domain part is taken to be everything after the first '.'.
44 Finally, if the hostname does not contain a domain part, the
45 root domain is assumed.
46 search Search list for host-name lookup.
48 The search list is normally determined from the local domain
49 name; by default, it contains only the local domain name. This
50 may be changed by listing the desired domain search path follow‐
51 ing the search keyword with spaces or tabs separating the names.
52 Resolver queries having fewer than ndots dots (default is 1) in
53 them will be attempted using each component of the search path
54 in turn until a match is found. For environments with multiple
55 subdomains please read options ndots:n below to avoid man-in-
56 the-middle attacks and unnecessary traffic for the root-dns-
57 servers. Note that this process may be slow and will generate a
58 lot of network traffic if the servers for the listed domains are
59 not local, and that queries will time out if no server is avail‐
60 able for one of the domains.
61 The search list is currently limited to six domains with a total
63 of 256 characters.
64 sortlist
66 This option allows addresses returned by gethostbyname(3) to be
67 sorted. A sortlist is specified by IP-address-netmask pairs.
68 The netmask is optional and defaults to the natural netmask of
69 the net. The IP address and optional network pairs are sepa‐
70 rated by slashes. Up to 10 pairs may be specified. Here is an
71 example:
72 sortlist 130.155.160.0/255.255.240.0 130.155.0.0
74 options
76 Options allows certain internal resolver variables to be modi‐
77 fied. The syntax is
78 options option ...
80 where option is one of the following:
82 debug sets RES_DEBUG in _res.options (effective only if glibc
84 was built with debug support; see resolver(3)).
85 ndots:n
87 sets a threshold for the number of dots which must appear
88 in a name given to res_query(3) (see resolver(3)) before
89 an initial absolute query will be made. The default for
90 n is 1, meaning that if there are any dots in a name, the
91 name will be tried first as an absolute name before any
92 search list elements are appended to it. The value for
93 this option is silently capped to 15.
94 timeout:n
96 sets the amount of time the resolver will wait for a
97 response from a remote name server before retrying the
98 query via a different name server. Measured in seconds,
99 the default is RES_TIMEOUT (currently 5, see <resolv.h>).
100 The value for this option is silently capped to 30.
101 attempts:n
103 sets the number of times the resolver will send a query
104 to its name servers before giving up and returning an
105 error to the calling application. The default is
106 RES_DFLRETRY (currently 2, see <resolv.h>). The value
107 for this option is silently capped to 5.
108 rotate sets RES_ROTATE in _res.options, which causes round-robin
110 selection of nameservers from among those listed. This
111 has the effect of spreading the query load among all
112 listed servers, rather than having all clients try the
113 first listed server first every time.
114 no-check-names
116 sets RES_NOCHECKNAME in _res.options, which disables the
117 modern BIND checking of incoming hostnames and mail names
118 for invalid characters such as underscore (_), non-ASCII,
119 or control characters.
120 inet6 sets RES_USE_INET6 in _res.options. This has the effect
122 of trying a AAAA query before an A query inside the geth‐
123 ostbyname(3) function, and of mapping IPv4 responses in
124 IPv6 "tunneled form" if no AAAA records are found but an
125 A record set exists.
126 ip6-bytestring (since glibc 2.3.4)
128 sets RES_USE_BSTRING in _res.options. This causes
129 reverse IPv6 lookups to be made using the bit-label for‐
130 mat described in RFC 2673; if this option is not set,
131 then nibble format is used.
132 ip6-dotint/no-ip6-dotint (since glibc 2.3.4)
134 Clear/set RES_NOIP6DOTINT in _res.options. When this
135 option is clear (ip6-dotint), reverse IPv6 lookups are
136 made in the (deprecated) ip6.int zone; when this option
137 is set (no-ip6-dotint), reverse IPv6 lookups are made in
138 the ip6.arpa zone by default. This option is set by
139 default.
140 edns0 (since glibc 2.6)
142 sets RES_USE_EDNSO in _res.options. This enables support
143 for the DNS extensions described in RFC 2671.
144 single-request (since glibc 2.10)
146 sets RES_SNGLKUP in _res.options. By default, glibc per‐
147 forms IPv4 and IPv6 lookups in parallel since version
148 2.9. Some appliance DNS servers cannot handle these
149 queries properly and make the requests time out. This
150 option disables the behavior and makes glibc perform the
151 IPv6 and IPv4 requests sequentially (at the cost of some
152 slowdown of the resolving process).
153 single-request-reopen (since glibc 2.9)
155 The resolver uses the same socket for the A and AAAA
156 requests. Some hardware mistakenly sends back only one
157 reply. When that happens the client system will sit and
158 wait for the second reply. Turning this option on
159 changes this behavior so that if two requests from the
160 same port are not handled correctly it will close the
161 socket and open a new one before sending the second
162 request.
163 The domain and search keywords are mutually exclusive. If more than
165 one instance of these keywords is present, the last instance wins.
166 The search keyword of a system's resolv.conf file can be overridden on
168 a per-process basis by setting the environment variable LOCALDOMAIN to
169 a space-separated list of search domains.
170 The options keyword of a system's resolv.conf file can be amended on a
172 per-process basis by setting the environment variable RES_OPTIONS to a
173 space-separated list of resolver options as explained above under
174 options.
175 The keyword and value must appear on a single line, and the keyword
177 (e.g., nameserver) must start the line. The value follows the keyword,
178 separated by white space.
179 Lines that contain a semicolon (;) or hash character (#) in the first
181 column are treated as comments.
182
184 /etc/resolv.conf, <resolv.h>
185
187 gethostbyname(3), resolver(3), hostname(7), named(8)
188 Name Server Operations Guide for BIND
189
191 This page is part of release 3.53 of the Linux man-pages project. A
192 description of the project, and information about reporting bugs, can
193 be found at http://www.kernel.org/doc/man-pages/.
1944th Berkeley Distribution 2013-07-31 RESOLV.CONF(5)