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Neither the name of the University nor the names of its contributors 13.\" may be used to endorse or promote products derived from this software 14.\" without specific prior written permission. 15.\" 16.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 17.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 18.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 19.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 20.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 21.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 22.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 23.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 24.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 25.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 26.\" SUCH DAMAGE. 27.\" 28.\" @(#)resolver.5 8.1 (Berkeley) 6/5/93 29.\" $FreeBSD$ 30.\" 31.Dd May 25, 2021 32.Dt RESOLVER 5 33.Os 34.Sh NAME 35.Nm resolver 36.Nd resolver configuration file 37.Sh SYNOPSIS 38.Nm resolv.conf 39.Sh DESCRIPTION 40The 41.Xr resolver 3 42is a set of routines in the C library 43which provide access to the Internet Domain Name System. 44The resolver configuration file contains information that is read 45by the resolver routines the first time they are invoked by a process. 46The file is designed to be human readable and contains a list of 47keywords with values that provide various types of resolver information. 48.Pp 49On a normally configured system this file should not be necessary. 50The only name server to be queried will be on the local machine, 51the domain name is determined from the host name, 52and the domain search path is constructed from the domain name. 53.Pp 54The different configuration options are: 55.Bl -tag -width nameserver 56.It Sy nameserver 57IPv4 or IPv6 address of a name server 58that the resolver should query. 59Up to 60.Dv MAXNS 61(currently 3) name servers may be listed, 62one per keyword. 63If there are multiple servers, 64the resolver library queries them in the order listed. 65If no 66.Sy nameserver 67entries are present, 68the default is to use the name server on the local machine. 69(The algorithm used is to try a name server, and if the query times out, 70try the next, until out of name servers, 71then repeat trying all the name servers 72until a maximum number of retries are made). 73.It Sy domain 74Local domain name. 75Most queries for names within this domain can use short names 76relative to the local domain. 77If no 78.Sy domain 79entry is present, the domain is determined 80from the local host name returned by 81.Xr gethostname 3 ; 82the domain part is taken to be everything after the first 83.Ql \&. . 84Finally, if the host name does not contain a domain part, the root 85domain is assumed. 86.It Sy search 87Search list for host-name lookup. 88The search list is normally determined from the local domain name; 89by default, it contains only the local domain name. 90This may be changed by listing the desired domain search path 91following the 92.Sy search 93keyword with spaces or tabs separating 94the names. 95Most resolver queries will be attempted using each component 96of the search path in turn until a match is found. 97Note that this process may be slow and will generate a lot of network 98traffic if the servers for the listed domains are not local, 99and that queries will time out if no server is available 100for one of the domains. 101.Pp 102The search list is currently limited to six domains 103with a total of 256 characters. 104.It Sy sortlist 105Sortlist allows addresses returned by gethostbyname to be sorted. 106A sortlist is specified by IP address netmask pairs. 107The netmask is 108optional and defaults to the natural netmask of the net. 109The IP address 110and optional network pairs are separated by slashes. 111Up to 10 pairs may 112be specified. 113E.g., 114.Pp 115.Dl "sortlist 130.155.160.0/255.255.240.0 130.155.0.0" 116.It Sy options 117Options allows certain internal resolver variables to be modified. 118The syntax is 119.Pp 120\fBoptions\fP \fIoption\fP \fI...\fP 121.Pp 122where 123.Sy option 124is one of the following: 125.Bl -tag -width no_tld_query 126.It Sy debug 127sets 128.Dv RES_DEBUG 129in _res.options. 130.It Sy usevc 131sets 132.Dv RES_USEVC 133to use TCP instead of UDP for queries. 134.It Sy ndots: Ns Ar n 135sets a threshold for the number of dots which must appear in a name given to 136.Fn res_query 137(see 138.Xr resolver 3 ) 139before an 140.Em initial absolute query 141will be made. 142The default for 143.Em n 144is 145.Dq 1 , 146meaning that if there are any dots in a name, the name 147will be tried first as an absolute name before any 148.Em search list 149elements are appended to it. 150.It Sy timeout: Ns Ar n 151sets the initial amount of time the resolver will wait 152for a response from a remote 153name server before retrying the query via a different name server. 154The resolver may wait longer during subsequent retries 155of the current query since an exponential back-off is applied to 156the timeout value. 157Measured in seconds, the default is 158.Dv RES_TIMEOUT , 159the allowed maximum is 160.Dv RES_MAXRETRANS 161(see 162.In resolv.h ) . 163.It Sy attempts: Ns Ar n 164sets the number of times the resolver will send a query to each of 165its name servers 166before giving up and returning an error to the calling application. 167The default is 168.Dv RES_DFLRETRY , 169the allowed maximum is 170.Dv RES_MAXRETRY 171(see 172.In resolv.h ) . 173.It Sy no_tld_query 174tells the resolver not to attempt to resolve a top level domain name, that 175is, a name that contains no dots. 176Use of this option does not prevent 177the resolver from obeying the standard 178.Sy domain 179and 180.Sy search 181rules with the given name. 182.It Sy reload-period: Ns Ar n 183The resolver checks the modification time of 184.Pa /etc/resolv.conf 185every 186.Ar n 187seconds. 188If 189.Pa /etc/resolv.conf 190has changed, it is automatically reloaded. 191The default for 192.Ar n 193is two seconds. 194Setting it to zero disables the file check. 195.El 196.Pp 197Options may also be specified as a space or tab separated list using the 198.Dv RES_OPTIONS 199environment variable. 200.El 201.Pp 202The 203.Sy domain 204and 205.Sy search 206keywords are mutually exclusive. 207If more than one instance of these keywords is present, 208the last instance will override. 209.Pp 210The keyword and value must appear on a single line, and the keyword 211.Pq for example, Sy nameserver 212must start the line. 213The value follows the keyword, separated by white space. 214.Sh FILES 215.Bl -tag -width /etc/resolv.conf -compact 216.It Pa /etc/resolv.conf 217The file 218.Nm resolv.conf 219resides in 220.Pa /etc . 221.El 222.Sh SEE ALSO 223.Xr gethostbyname 3 , 224.Xr resolver 3 , 225.Xr hostname 7 , 226.Xr resolvconf 8 227.Rs 228.%T "Name Server Operations Guide for BIND" 229.Re 230.Sh HISTORY 231The 232.Nm resolv.conf 233file format appeared in 234.Bx 4.3 . 235