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Neither the name of the University nor the names of its contributors 17.\" may be used to endorse or promote products derived from this software 18.\" without specific prior written permission. 19.\" 20.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 21.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 22.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 23.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 24.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 25.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 26.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 27.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 28.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 29.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 30.\" SUCH DAMAGE. 31.\" 32.\" @(#)resolver.5 8.1 (Berkeley) 6/5/93 33.\" $FreeBSD$ 34.\" 35.Dd December 25, 2013 36.Dt RESOLVER 5 37.Os 38.Sh NAME 39.Nm resolver 40.Nd resolver configuration file 41.Sh SYNOPSIS 42.Nm resolv.conf 43.Sh DESCRIPTION 44The 45.Xr resolver 3 46is a set of routines in the C library 47which provide access to the Internet Domain Name System. 48The resolver configuration file contains information that is read 49by the resolver routines the first time they are invoked by a process. 50The file is designed to be human readable and contains a list of 51keywords with values that provide various types of resolver information. 52.Pp 53On a normally configured system this file should not be necessary. 54The only name server to be queried will be on the local machine, 55the domain name is determined from the host name, 56and the domain search path is constructed from the domain name. 57.Pp 58The different configuration options are: 59.Bl -tag -width nameserver 60.It Sy nameserver 61Internet address (in dot notation) of a name server 62that the resolver should query. 63Up to 64.Dv MAXNS 65(currently 3) name servers may be listed, 66one per keyword. 67If there are multiple servers, 68the resolver library queries them in the order listed. 69If no 70.Sy nameserver 71entries are present, 72the default is to use the name server on the local machine. 73(The algorithm used is to try a name server, and if the query times out, 74try the next, until out of name servers, 75then repeat trying all the name servers 76until a maximum number of retries are made). 77.It Sy domain 78Local domain name. 79Most queries for names within this domain can use short names 80relative to the local domain. 81If no 82.Sy domain 83entry is present, the domain is determined 84from the local host name returned by 85.Xr gethostname 3 ; 86the domain part is taken to be everything after the first 87.Ql \&. . 88Finally, if the host name does not contain a domain part, the root 89domain is assumed. 90.It Sy search 91Search list for host-name lookup. 92The search list is normally determined from the local domain name; 93by default, it contains only the local domain name. 94This may be changed by listing the desired domain search path 95following the 96.Sy search 97keyword with spaces or tabs separating 98the names. 99Most resolver queries will be attempted using each component 100of the search path in turn until a match is found. 101Note that this process may be slow and will generate a lot of network 102traffic if the servers for the listed domains are not local, 103and that queries will time out if no server is available 104for one of the domains. 105.Pp 106The search list is currently limited to six domains 107with a total of 256 characters. 108.It Sy sortlist 109Sortlist allows addresses returned by gethostbyname to be sorted. 110A sortlist is specified by IP address netmask pairs. 111The netmask is 112optional and defaults to the natural netmask of the net. 113The IP address 114and optional network pairs are separated by slashes. 115Up to 10 pairs may 116be specified. 117E.g., 118.Pp 119.Dl "sortlist 130.155.160.0/255.255.240.0 130.155.0.0" 120.It Sy options 121Options allows certain internal resolver variables to be modified. 122The syntax is 123.Pp 124\fBoptions\fP \fIoption\fP \fI...\fP 125.Pp 126where 127.Sy option 128is one of the following: 129.Bl -tag -width no_tld_query 130.It Sy debug 131sets 132.Dv RES_DEBUG 133in _res.options. 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.El 183.Pp 184Options may also be specified as a space or tab separated list using the 185.Dv RES_OPTIONS 186environment variable. 187.El 188.Pp 189The 190.Sy domain 191and 192.Sy search 193keywords are mutually exclusive. 194If more than one instance of these keywords is present, 195the last instance will override. 196.Pp 197The keyword and value must appear on a single line, and the keyword 198(e.g.\& 199.Sy nameserver ) 200must start the line. 201The value follows the keyword, separated by white space. 202.Sh FILES 203.Bl -tag -width /etc/resolv.conf -compact 204.It Pa /etc/resolv.conf 205The file 206.Nm resolv.conf 207resides in 208.Pa /etc . 209.El 210.Sh SEE ALSO 211.Xr gethostbyname 3 , 212.Xr resolver 3 , 213.Xr hostname 7 214.Rs 215.%T "Name Server Operations Guide for BIND" 216.Re 217.Sh HISTORY 218The 219.Nm resolv.conf 220file format appeared in 221.Bx 4.3 . 222