1RESOLV.CONF(5)             Linux Programmer's Manual            RESOLV.CONF(5)
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NAME

6       resolv.conf - resolver configuration file
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SYNOPSIS

9       /etc/resolv.conf
10

DESCRIPTION

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.  The configuration file is  con‐
18       sidered  a  trusted  source of DNS information; see the trust-ad option
19       below for details.
20
21       If this file does not exist, only the name server on the local  machine
22       will be queried, and the search list contains the local domain name de‐
23       termined from the hostname.
24
25       The different configuration options are:
26
27       nameserver Name server IP address
28              Internet address of a  name  server  that  the  resolver  should
29              query,  either an IPv4 address (in dot notation), or an IPv6 ad‐
30              dress in colon (and possibly dot) notation as per RFC 2373.   Up
31              to  MAXNS  (currently  3,  see  <resolv.h>)  name servers may be
32              listed, one per keyword.  If there are multiple servers, the re‐
33              solver  library  queries  them in the order listed.  If no name‐
34              server entries are present, the  default  is  to  use  the  name
35              server  on  the  local machine.  (The algorithm used is to try a
36              name server, and if the query times out, try the next, until out
37              of name servers, then repeat trying all the name servers until a
38              maximum number of retries are made.)
39
40       search Search list for host-name lookup.
41              By default, the search list contains one entry, the local domain
42              name.   It  is  determined  from  the local hostname returned by
43              gethostname(2); the local domain name is taken to be  everything
44              after  the first '.'.  Finally, if the hostname does not contain
45              a '.', the root domain is assumed as the local domain name.
46
47              This may be changed by listing the desired  domain  search  path
48              following  the search keyword with spaces or tabs separating the
49              names.  Resolver queries having fewer than ndots  dots  (default
50              is  1)  in  them  will  be attempted using each component of the
51              search path in turn until a match is  found.   For  environments
52              with  multiple  subdomains  please read options ndots:n below to
53              avoid man-in-the-middle attacks and unnecessary traffic for  the
54              root-dns-servers.   Note  that this process may be slow and will
55              generate a lot of network traffic if the servers for the  listed
56              domains  are  not  local,  and  that queries will time out if no
57              server is available for one of the domains.
58
59              If there are multiple search directives, only  the  search  list
60              from the last instance is used.
61
62              In glibc 2.25 and earlier, the search list is limited to six do‐
63              mains with a total of 256 characters.   Since  glibc  2.26,  the
64              search list is unlimited.
65
66              The  domain  directive is an obsolete name for the search direc‐
67              tive that handles one search list entry only.
68
69       sortlist
70              This option allows addresses returned by gethostbyname(3) to  be
71              sorted.   A  sortlist  is specified by IP-address-netmask pairs.
72              The netmask is optional and defaults to the natural  netmask  of
73              the  net.   The  IP address and optional network pairs are sepa‐
74              rated by slashes.  Up to 10 pairs may be specified.  Here is  an
75              example:
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77                  sortlist 130.155.160.0/255.255.240.0 130.155.0.0
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79       options
80              Options  allows  certain internal resolver variables to be modi‐
81              fied.  The syntax is
82
83                     options option ...
84
85              where option is one of the following:
86
87              debug  Sets RES_DEBUG in _res.options (effective only  if  glibc
88                     was built with debug support; see resolver(3)).
89
90              ndots:n
91                     Sets a threshold for the number of dots which must appear
92                     in a name given to res_query(3) (see resolver(3))  before
93                     an  initial absolute query will be made.  The default for
94                     n is 1, meaning that if there are any dots in a name, the
95                     name  will  be tried first as an absolute name before any
96                     search list elements are appended to it.  The  value  for
97                     this option is silently capped to 15.
98
99              timeout:n
100                     Sets  the amount of time the resolver will wait for a re‐
101                     sponse from a remote  name  server  before  retrying  the
102                     query  via  a different name server.  This may not be the
103                     total time taken by any resolver API call and there is no
104                     guarantee  that a single resolver API call maps to a sin‐
105                     gle  timeout.   Measured  in  seconds,  the  default   is
106                     RES_TIMEOUT (currently 5, see <resolv.h>).  The value for
107                     this option is silently capped to 30.
108
109              attempts:n
110                     Sets the number of times the resolver will send  a  query
111                     to its name servers before giving up and returning an er‐
112                     ror  to  the  calling  application.    The   default   is
113                     RES_DFLRETRY  (currently  2,  see <resolv.h>).  The value
114                     for this option is silently capped to 5.
115
116              rotate Sets RES_ROTATE in _res.options, which causes round-robin
117                     selection  of name servers from among those listed.  This
118                     has the effect of spreading  the  query  load  among  all
119                     listed  servers,  rather  than having all clients try the
120                     first listed server first every time.
121
122              no-check-names
123                     Sets RES_NOCHECKNAME in _res.options, which disables  the
124                     modern BIND checking of incoming hostnames and mail names
125                     for invalid characters such as underscore (_), non-ASCII,
126                     or control characters.
127
128              inet6  Sets  RES_USE_INET6 in _res.options.  This has the effect
129                     of trying an AAAA query before  an  A  query  inside  the
130                     gethostbyname(3)  function, and of mapping IPv4 responses
131                     in IPv6 "tunneled form" if no AAAA records are found  but
132                     an A record set exists.  Since glibc 2.25, this option is
133                     deprecated;  applications  should   use   getaddrinfo(3),
134                     rather than gethostbyname(3).
135
136              ip6-bytestring (since glibc 2.3.4 to 2.24)
137                     Sets RES_USEBSTRING in _res.options.  This causes reverse
138                     IPv6 lookups to be made using the  bit-label  format  de‐
139                     scribed  in RFC 2673; if this option is not set (which is
140                     the default), then nibble format is  used.   This  option
141                     was removed in glibc 2.25, since it relied on a backward-
142                     incompatible DNS extension that was never deployed on the
143                     Internet.
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145              ip6-dotint/no-ip6-dotint (glibc 2.3.4 to 2.24)
146                     Clear/set RES_NOIP6DOTINT in _res.options.  When this op‐
147                     tion is clear (ip6-dotint), reverse IPv6 lookups are made
148                     in the (deprecated) ip6.int zone; when this option is set
149                     (no-ip6-dotint), reverse IPv6 lookups  are  made  in  the
150                     ip6.arpa zone by default.  These options are available in
151                     glibc versions up to 2.24, where no-ip6-dotint is the de‐
152                     fault.   Since  ip6-dotint  support long ago ceased to be
153                     available on the Internet, these options were removed  in
154                     glibc 2.25.
155
156              edns0 (since glibc 2.6)
157                     Sets RES_USE_EDNS0 in _res.options.  This enables support
158                     for the DNS extensions described in RFC 2671.
159
160              single-request (since glibc 2.10)
161                     Sets RES_SNGLKUP in _res.options.  By default, glibc per‐
162                     forms  IPv4  and  IPv6  lookups in parallel since version
163                     2.9.  Some appliance  DNS  servers  cannot  handle  these
164                     queries  properly  and  make the requests time out.  This
165                     option disables the behavior and makes glibc perform  the
166                     IPv6  and IPv4 requests sequentially (at the cost of some
167                     slowdown of the resolving process).
168
169              single-request-reopen (since glibc 2.9)
170                     Sets RES_SNGLKUPREOP in _res.options.  The resolver  uses
171                     the  same socket for the A and AAAA requests.  Some hard‐
172                     ware mistakenly sends back only  one  reply.   When  that
173                     happens  the client system will sit and wait for the sec‐
174                     ond reply.  Turning this option on changes this  behavior
175                     so  that  if two requests from the same port are not han‐
176                     dled correctly it will close the socket and  open  a  new
177                     one before sending the second request.
178
179              no-tld-query (since glibc 2.14)
180                     Sets  RES_NOTLDQUERY in _res.options.  This option causes
181                     res_nsearch() to not attempt to  resolve  an  unqualified
182                     name as if it were a top level domain (TLD).  This option
183                     can cause problems if the site has ``localhost'' as a TLD
184                     rather  than  having localhost on one or more elements of
185                     the search list.  This option has no  effect  if  neither
186                     RES_DEFNAMES or RES_DNSRCH is set.
187
188              use-vc (since glibc 2.14)
189                     Sets  RES_USEVC  in _res.options.  This option forces the
190                     use of TCP for DNS resolutions.
191
192              no-reload (since glibc 2.26)
193                     Sets RES_NORELOAD in _res.options.  This option  disables
194                     automatic reloading of a changed configuration file.
195
196              trust-ad (since glibc 2.31)
197                     Sets  RES_TRUSTAD  in _res.options.  This option controls
198                     the AD bit behavior of the stub resolver.  If a  validat‐
199                     ing  resolver sets the AD bit in a response, it indicates
200                     that the data in the response was verified  according  to
201                     the DNSSEC protocol.  In order to rely on the AD bit, the
202                     local system has to trust both the DNSSEC-validating  re‐
203                     solver  and  the  network path to it, which is why an ex‐
204                     plicit opt-in is required.  If the trust-ad option is ac‐
205                     tive,  the  stub resolver sets the AD bit in outgoing DNS
206                     queries (to enable AD bit support), and preserves the  AD
207                     bit in responses.  Without this option, the AD bit is not
208                     set in queries, and it is always removed  from  responses
209                     before  they are returned to the application.  This means
210                     that applications can trust the AD bit  in  responses  if
211                     the trust-ad option has been set correctly.
212
213                     In  glibc version 2.30 and earlier, the AD is not set au‐
214                     tomatically in queries, and is passed  through  unchanged
215                     to applications in responses.
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217       The  search keyword of a system's resolv.conf file can be overridden on
218       a per-process basis by setting the environment variable LOCALDOMAIN  to
219       a space-separated list of search domains.
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221       The  options keyword of a system's resolv.conf file can be amended on a
222       per-process basis by setting the environment variable RES_OPTIONS to  a
223       space-separated  list  of resolver options as explained above under op‐
224       tions.
225
226       The keyword and value must appear on a single  line,  and  the  keyword
227       (e.g., nameserver) must start the line.  The value follows the keyword,
228       separated by white space.
229
230       Lines that contain a semicolon (;) or hash character (#) in  the  first
231       column are treated as comments.
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FILES

234       /etc/resolv.conf, <resolv.h>
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SEE ALSO

237       gethostbyname(3),    resolver(3),    host.conf(5),    hosts(5),    nss‐
238       witch.conf(5), hostname(7), named(8)
239
240       Name Server Operations Guide for BIND
241

COLOPHON

243       This page is part of release 5.13 of the Linux  man-pages  project.   A
244       description  of  the project, information about reporting bugs, and the
245       latest    version    of    this    page,    can     be     found     at
246       https://www.kernel.org/doc/man-pages/.
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2504th Berkeley Distribution         2021-03-22                    RESOLV.CONF(5)
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