{ "mode": "man", "parameter": "resolver", "section": "5", "url": "https://www.chedong.com/phpMan.php/man/resolver/5/json", "generated": "2026-06-02T18:06:57Z", "synopsis": "", "sections": { "NAME": { "content": "resolv.conf - resolver configuration file\n", "subsections": [] }, "SYNOPSIS": { "content": "", "subsections": [ { "name": "/etc/resolv.conf", "content": "" } ] }, "DESCRIPTION": { "content": "The resolver is a set of routines in the C library that provide access to the Internet Domain\nName System (DNS). The resolver configuration file contains information that is read by the\nresolver routines the first time they are invoked by a process. The file is designed to be\nhuman readable and contains a list of keywords with values that provide various types of re‐\nsolver information. The configuration file is considered a trusted source of DNS informa‐\ntion; see the trust-ad option below for details.\n\nIf this file does not exist, only the name server on the local machine will be queried, and\nthe search list contains the local domain name determined from the hostname.\n\nThe different configuration options are:\n\nnameserver Name server IP address\nInternet address of a name server that the resolver should query, either an IPv4 ad‐\ndress (in dot notation), or an IPv6 address in colon (and possibly dot) notation as\nper RFC 2373. Up to MAXNS (currently 3, see ) name servers may be listed,\none per keyword. If there are multiple servers, the resolver library queries them in\nthe order listed. If no nameserver entries are present, the default is to use the\nname server on the local machine. (The algorithm used is to try a name server, and if\nthe query times out, try the next, until out of name servers, then repeat trying all\nthe name servers until a maximum number of retries are made.)\n\nsearch Search list for host-name lookup.\nBy default, the search list contains one entry, the local domain name. It is deter‐\nmined from the local hostname returned by gethostname(2); the local domain name is\ntaken to be everything after the first '.'. Finally, if the hostname does not contain\na '.', the root domain is assumed as the local domain name.\n\nThis may be changed by listing the desired domain search path following the search\nkeyword with spaces or tabs separating the names. Resolver queries having fewer than\nndots dots (default is 1) in them will be attempted using each component of the search\npath in turn until a match is found. For environments with multiple subdomains please\nread options ndots:n below to avoid man-in-the-middle attacks and unnecessary traffic\nfor the root-dns-servers. Note that this process may be slow and will generate a lot\nof network traffic if the servers for the listed domains are not local, and that\nqueries will time out if no server is available for one of the domains.\n\nIf there are multiple search directives, only the search list from the last instance\nis used.\n\nIn glibc 2.25 and earlier, the search list is limited to six domains with a total of\n256 characters. Since glibc 2.26, the search list is unlimited.\n\nThe domain directive is an obsolete name for the search directive that handles one\nsearch list entry only.\n", "subsections": [ { "name": "sortlist", "content": "This option allows addresses returned by gethostbyname(3) to be sorted. A sortlist is\nspecified by IP-address-netmask pairs. The netmask is optional and defaults to the\nnatural netmask of the net. The IP address and optional network pairs are separated\nby slashes. Up to 10 pairs may be specified. Here is an example:\n\nsortlist 130.155.160.0/255.255.240.0 130.155.0.0\n" }, { "name": "options", "content": "Options allows certain internal resolver variables to be modified. The syntax is\n\noptions option ...\n\nwhere option is one of the following:\n\ndebug Sets RESDEBUG in res.options (effective only if glibc was built with debug\nsupport; see resolver(3)).\n\nndots:n\nSets a threshold for the number of dots which must appear in a name given to\nresquery(3) (see resolver(3)) before an initial absolute query will be made.\nThe default for n is 1, meaning that if there are any dots in a name, the name\nwill be tried first as an absolute name before any search list elements are ap‐\npended to it. The value for this option is silently capped to 15.\n\ntimeout:n\nSets the amount of time the resolver will wait for a response from a remote\nname server before retrying the query via a different name server. This may\nnot be the total time taken by any resolver API call and there is no guarantee\nthat a single resolver API call maps to a single timeout. Measured in seconds,\nthe default is RESTIMEOUT (currently 5, see ). The value for this\noption is silently capped to 30.\n\nattempts:n\nSets the number of times the resolver will send a query to its name servers be‐\nfore giving up and returning an error to the calling application. The default\nis RESDFLRETRY (currently 2, see ). The value for this option is\nsilently capped to 5.\n\nrotate Sets RESROTATE in res.options, which causes round-robin selection of name\nservers from among those listed. This has the effect of spreading the query\nload among all listed servers, rather than having all clients try the first\nlisted server first every time.\n\nno-check-names\nSets RESNOCHECKNAME in res.options, which disables the modern BIND checking\nof incoming hostnames and mail names for invalid characters such as underscore\n(), non-ASCII, or control characters.\n\ninet6 Sets RESUSEINET6 in res.options. This has the effect of trying an AAAA\nquery before an A query inside the gethostbyname(3) function, and of mapping\nIPv4 responses in IPv6 \"tunneled form\" if no AAAA records are found but an A\nrecord set exists. Since glibc 2.25, this option is deprecated; applications\nshould use getaddrinfo(3), rather than gethostbyname(3).\n\nSome programs behave strangely when this option is turned on.\n\nip6-bytestring (since glibc 2.3.4 to 2.24)\nSets RESUSEBSTRING in res.options. This causes reverse IPv6 lookups to be\nmade using the bit-label format described in RFC 2673; if this option is not\nset (which is the default), then nibble format is used. This option was re‐\nmoved in glibc 2.25, since it relied on a backward-incompatible DNS extension\nthat was never deployed on the Internet.\n\nip6-dotint/no-ip6-dotint (glibc 2.3.4 to 2.24)\nClear/set RESNOIP6DOTINT in res.options. When this option is clear\n(ip6-dotint), reverse IPv6 lookups are made in the (deprecated) ip6.int zone;\nwhen this option is set (no-ip6-dotint), reverse IPv6 lookups are made in the\nip6.arpa zone by default. These options are available in glibc versions up to\n2.24, where no-ip6-dotint is the default. Since ip6-dotint support long ago\nceased to be available on the Internet, these options were removed in glibc\n2.25.\n\nedns0 (since glibc 2.6)\nSets RESUSEEDNSO in res.options. This enables support for the DNS exten‐\nsions described in RFC 2671.\n\nsingle-request (since glibc 2.10)\nSets RESSNGLKUP in res.options. By default, glibc performs IPv4 and IPv6\nlookups in parallel since version 2.9. Some appliance DNS servers cannot han‐\ndle these queries properly and make the requests time out. This option dis‐\nables the behavior and makes glibc perform the IPv6 and IPv4 requests sequen‐\ntially (at the cost of some slowdown of the resolving process).\n\nsingle-request-reopen (since glibc 2.9)\nSets RESSNGLKUPREOP in res.options. The resolver uses the same socket for\nthe A and AAAA requests. Some hardware mistakenly sends back only one reply.\nWhen that happens the client system will sit and wait for the second reply.\nTurning this option on changes this behavior so that if two requests from the\nsame port are not handled correctly it will close the socket and open a new one\nbefore sending the second request.\n\nno-tld-query (since glibc 2.14)\nSets RESNOTLDQUERY in res.options. This option causes resnsearch() to not\nattempt to resolve an unqualified name as if it were a top level domain (TLD).\nThis option can cause problems if the site has ``localhost'' as a TLD rather\nthan having localhost on one or more elements of the search list. This option\nhas no effect if neither RESDEFNAMES or RESDNSRCH is set.\n\nuse-vc (since glibc 2.14)\nSets RESUSEVC in res.options. This option forces the use of TCP for DNS res‐\nolutions.\n\nno-reload (since glibc 2.26)\nSets RESNORELOAD in res.options. This option disables automatic reloading of\na changed configuration file.\n\ntrust-ad (since glibc 2.31)\nSets RESTRUSTAD in res.options. This option controls the AD bit behavior of\nthe stub resolver. If a validating resolver sets the AD bit in a response, it\nindicates that the data in the response was verified according to the DNSSEC\nprotocol. In order to rely on the AD bit, the local system has to trust both\nthe DNSSEC-validating resolver and the network path to it, which is why an ex‐\nplicit opt-in is required. If the trust-ad option is active, the stub resolver\nsets the AD bit in outgoing DNS queries (to enable AD bit support), and pre‐\nserves the AD bit in responses. Without this option, the AD bit is not set in\nqueries, and it is always removed from responses before they are returned to\nthe application. This means that applications can trust the AD bit in re‐\nsponses if the trust-ad option has been set correctly.\n\nIn glibc version 2.30 and earlier, the AD is not set automatically in queries,\nand is passed through unchanged to applications in responses.\n\nThe search keyword of a system's resolv.conf file can be overridden on a per-process basis by\nsetting the environment variable LOCALDOMAIN to a space-separated list of search domains.\n\nThe options keyword of a system's resolv.conf file can be amended on a per-process basis by\nsetting the environment variable RESOPTIONS to a space-separated list of resolver options as\nexplained above under options.\n\nThe keyword and value must appear on a single line, and the keyword (e.g., nameserver) must\nstart the line. The value follows the keyword, separated by white space.\n\nLines that contain a semicolon (;) or hash character (#) in the first column are treated as\ncomments.\n" } ] }, "FILES": { "content": "/etc/resolv.conf, \n", "subsections": [] }, "SEE ALSO": { "content": "gethostbyname(3), resolver(3), host.conf(5), hosts(5), nsswitch.conf(5), hostname(7),\nnamed(8)\n\nName Server Operations Guide for BIND\n", "subsections": [] }, "COLOPHON": { "content": "This page is part of release 5.10 of the Linux man-pages project. A description of the\nproject, information about reporting bugs, and the latest version of this page, can be found\nat https://www.kernel.org/doc/man-pages/.\n\n\n\n4th Berkeley Distribution 2020-08-13 RESOLV.CONF(5)", "subsections": [] } }, "summary": "resolv.conf - resolver configuration file", "flags": [], "examples": [], "see_also": [ { "name": "gethostbyname", "section": "3", "url": "https://www.chedong.com/phpMan.php/man/gethostbyname/3/json" }, { "name": "host.conf", "section": "5", "url": "https://www.chedong.com/phpMan.php/man/host.conf/5/json" }, { "name": "hosts", "section": "5", "url": "https://www.chedong.com/phpMan.php/man/hosts/5/json" }, { "name": "nsswitch.conf", "section": "5", "url": "https://www.chedong.com/phpMan.php/man/nsswitch.conf/5/json" }, { "name": "hostname", "section": "7", "url": "https://www.chedong.com/phpMan.php/man/hostname/7/json" }, { "name": "named", "section": "8", "url": "https://www.chedong.com/phpMan.php/man/named/8/json" } ] }