# resolv.conf

## NAME



           resolv.conf - resolver configuration file
    


## SYNOPSIS



           **/etc/resolv.conf**
    


## DESCRIPTION



           The _resolver_ is a set of routines in the C library that provide
           access to the Internet Domain Name System (DNS).  The resolver
           configuration file contains information that is read by the
           resolver routines the first time they are invoked by a process.
           The file is designed to be human readable and contains a list of
           keywords with values that provide various types of resolver
           information.  The configuration file is considered a trusted
           source of DNS information; see the **trust-ad** option below for
           details.
    
           If this file does not exist, only the name server on the local
           machine will be queried, and the search list contains the local
           domain name determined from the hostname.
    
           The different configuration options are:
    
           **nameserver** Name server IP address
                  Internet address of a name server that the resolver should
                  query, either an IPv4 address (in dot notation), or an IPv6
                  address in colon (and possibly dot) notation as per RFC
                  2373.  Up to **MAXNS**(currently 3, see _< resolv.h>_) name
                  servers may be listed, one per keyword.  If there are
                  multiple servers, the resolver library queries them in the
                  order listed.  If no **nameserver** entries are present, the
                  default is to use the name server on the local machine.
                  (The algorithm used is to try a name server, and if the
                  query times out, try the next, until out of name servers,
                  then repeat trying all the name servers until a maximum
                  number of retries are made.)
    
           **search** Search list for host-name lookup.
                  By default, the search list contains one entry, the local
                  domain name.  It is determined from the local hostname
                  returned by gethostname(2); the local domain name is taken
                  to be everything after the first '.'.  Finally, if the
                  hostname does not contain a '.', the root domain is assumed
                  as the local domain name.
    
                  This may be changed by listing the desired domain search
                  path following the _search_ keyword with spaces or tabs
                  separating the names.  Resolver queries having fewer than
                  _ndots_ dots (default is 1) in them will be attempted using
                  each component of the search path in turn until a match is
                  found.  For environments with multiple subdomains please
                  read **options ndots:**_n_ below to avoid man-in-the-middle
                  attacks and unnecessary traffic for the root-dns-servers.
                  Note that this process may be slow and will generate a lot
                  of network traffic if the servers for the listed domains
                  are not local, and that queries will time out if no server
                  is available for one of the domains.
    
                  If there are multiple **search** directives, only the search
                  list from the last instance is used.
    
                  In glibc 2.25 and earlier, the search list is limited to
                  six domains with a total of 256 characters.  Since glibc
                  2.26, the search list is unlimited.
    
                  The **domain** directive is an obsolete name for the **search**
                  directive that handles one search list entry only.
    
           **sortlist**
                  This option allows addresses returned by gethostbyname(3)
                  to be sorted.  A sortlist is specified by IP-address-
                  netmask pairs.  The netmask is optional and defaults to the
                  natural netmask of the net.  The IP address and optional
                  network pairs are separated by slashes.  Up to 10 pairs may
                  be specified.  Here is an example:
    
                      sortlist 130.155.160.0/255.255.240.0 130.155.0.0
    
           **options**
                  Options allows certain internal resolver variables to be
                  modified.  The syntax is
    
                         **options** _option ..._
    
                  where _option_ is one of the following:
    
                  **debug** Sets **RES_DEBUG** in __res.options_ (effective only if
                         glibc was built with debug support; see
                         resolver(3)).
    
                  **ndots:**_n_
                         Sets a threshold for the number of dots which must
                         appear in a name given to res_query(3) (see
                         resolver(3)) before an _initial absolute query_ will
                         be made.  The default for _n_ is 1, meaning that if
                         there are any dots in a name, the name will be tried
                         first as an absolute name before any _search list_
                         elements are appended to it.  The value for this
                         option is silently capped to 15.
    
                  **timeout:**_n_
                         Sets the amount of time the resolver will wait for a
                         response from a remote name server before retrying
                         the query via a different name server.  This may **not**
                         be the total time taken by any resolver API call and
                         there is no guarantee that a single resolver API
                         call maps to a single timeout.  Measured in seconds,
                         the default is **RES_TIMEOUT**(currently 5, see
                         _< resolv.h>_).  The value for this option is silently
                         capped to 30.
    
                  **attempts:**_n_
                         Sets the number of times the resolver will send a
                         query to its name servers before giving up and
                         returning an error to the calling application.  The
                         default is **RES_DFLRETRY**(currently 2, see
                         _< resolv.h>_).  The value for this option is silently
                         capped to 5.
    
                  **rotate** Sets **RES_ROTATE** in __res.options_ , which causes round-
                         robin selection of name servers from among those
                         listed.  This has the effect of spreading the query
                         load among all listed servers, rather than having
                         all clients try the first listed server first every
                         time.
    
                  **no-aaaa (since glibc 2.36)**
                         Sets **RES_NOAAAA** in __res.options_ , which suppresses
                         AAAA queries made by the stub resolver, including
                         AAAA lookups triggered by NSS-based interfaces such
                         as getaddrinfo(3).  Only DNS lookups are affected:
                         IPv6 data in hosts(5) is still used, getaddrinfo(3)
                         with **AI_PASSIVE** will still produce IPv6 addresses,
                         and configured IPv6 name servers are still used.  To
                         produce correct Name Error (NXDOMAIN) results, AAAA
                         queries are translated to A queries.  This option is
                         intended preliminary for diagnostic purposes, to
                         rule out that AAAA DNS queries have adverse impact.
                         It is incompatible with EDNS0 usage and DNSSEC
                         validation by applications.
    
                  **no-check-names**
                         Sets **RES_NOCHECKNAME** in __res.options_ , which disables
                         the modern BIND checking of incoming hostnames and
                         mail names for invalid characters such as underscore
                         (_), non-ASCII, or control characters.
    
                  **inet6** Sets **RES_USE_INET6** in __res.options_.  This has the
                         effect of trying an AAAA query before an A query
                         inside the gethostbyname(3) function, and of mapping
                         IPv4 responses in IPv6 "tunneled form" if no AAAA
                         records are found but an A record set exists.  Since
                         glibc 2.25, this option is deprecated; applications
                         should use getaddrinfo(3), rather than
                         gethostbyname(3).
    
                  **ip6-bytestring**(since glibc 2.3.4 to glibc 2.24)
                         Sets **RES_USEBSTRING** in __res.options_.  This causes
                         reverse IPv6 lookups to be made using the bit-label
                         format described in RFC 2673; if this option is not
                         set (which is the default), then nibble format is
                         used.  This option was removed in glibc 2.25, since
                         it relied on a backward-incompatible DNS extension
                         that was never deployed on the Internet.
    
                  **ip6-dotint** /**no-ip6-dotint**(glibc 2.3.4 to glibc 2.24)
                         Clear/set **RES_NOIP6DOTINT** in __res.options_.  When
                         this option is clear (**ip6-dotint**), reverse IPv6
                         lookups are made in the (deprecated) _ip6.int_ zone;
                         when this option is set (**no-ip6-dotint**), reverse
                         IPv6 lookups are made in the _ip6.arpa_ zone by
                         default.  These options are available up to glibc
                         2.24, where **no-ip6-dotint** is the default.  Since
                         **ip6-dotint** support long ago ceased to be available
                         on the Internet, these options were removed in glibc
                         2.25.
    
                  **edns0**(since glibc 2.6)
                         Sets **RES_USE_EDNS0** in __res.options_.  This enables
                         support for the DNS extensions described in
                         RFC 2671.
    
                  **single-request**(since glibc 2.10)
                         Sets **RES_SNGLKUP** in __res.options_.  By default, glibc
                         performs IPv4 and IPv6 lookups in parallel since
                         glibc 2.9.  Some appliance DNS servers cannot handle
                         these queries properly and make the requests time
                         out.  This option disables the behavior and makes
                         glibc perform the IPv6 and IPv4 requests
                         sequentially (at the cost of some slowdown of the
                         resolving process).
    
                  **single-request-reopen**(since glibc 2.9)
                         Sets **RES_SNGLKUPREOP** in __res.options_.  The resolver
                         uses the same socket for the A and AAAA requests.
                         Some hardware mistakenly sends back only one reply.
                         When that happens the client system will sit and
                         wait for the second reply.  Turning this option on
                         changes this behavior so that if two requests from
                         the same port are not handled correctly it will
                         close the socket and open a new one before sending
                         the second request.
    
                  **no-tld-query**(since glibc 2.14)
                         Sets **RES_NOTLDQUERY** in __res.options_.  This option
                         causes **res_nsearch**() to not attempt to resolve an
                         unqualified name as if it were a top level domain
                         (TLD).  This option can cause problems if the site
                         has ``localhost'' as a TLD rather than having
                         localhost on one or more elements of the search
                         list.  This option has no effect if neither
                         RES_DEFNAMES or RES_DNSRCH is set.
    
                  **use-vc**(since glibc 2.14)
                         Sets **RES_USEVC** in __res.options_.  This option forces
                         the use of TCP for DNS resolutions.
    
                  **no-reload**(since glibc 2.26)
                         Sets **RES_NORELOAD** in __res.options_.  This option
                         disables automatic reloading of a changed
                         configuration file.
    
                  **trust-ad**(since glibc 2.31)
                         Sets **RES_TRUSTAD** in __res.options_.  This option
                         controls the AD bit behavior of the stub resolver.
                         If a validating resolver sets the AD bit in a
                         response, it indicates that the data in the response
                         was verified according to the DNSSEC protocol.  In
                         order to rely on the AD bit, the local system has to
                         trust both the DNSSEC-validating resolver and the
                         network path to it, which is why an explicit opt-in
                         is required.  If the **trust-ad** option is active, the
                         stub resolver sets the AD bit in outgoing DNS
                         queries (to enable AD bit support), and preserves
                         the AD bit in responses.  Without this option, the
                         AD bit is not set in queries, and it is always
                         removed from responses before they are returned to
                         the application.  This means that applications can
                         trust the AD bit in responses if the **trust-ad** option
                         has been set correctly.
    
                         In glibc 2.30 and earlier, the AD is not set
                         automatically in queries, and is passed through
                         unchanged to applications in responses.
    
           The _search_ keyword of a system's _resolv.conf_ file can be
           overridden on a per-process basis by setting the environment
           variable **LOCALDOMAIN** to a space-separated list of search domains.
    
           The _options_ keyword of a system's _resolv.conf_ file can be amended
           on a per-process basis by setting the environment variable
           **RES_OPTIONS** to a space-separated list of resolver options as
           explained above under **options**.
    
           The keyword and value must appear on a single line, and the
           keyword (e.g., **nameserver**) must start the line.  The value follows
           the keyword, separated by white space.
    
           Lines that contain a semicolon (;) or hash character (#) in the
           first column are treated as comments.
    


## FILES



           _/etc/resolv.conf_ , _< resolv.h>_
    


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