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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 September 9, 2004 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.Pp 130.Bl -tag -width no_tld_query 131.It Sy debug 132sets 133.Dv RES_DEBUG 134in _res.options. 135.It Sy ndots: Ns Ar n 136sets a threshold for the number of dots which must appear in a name given to 137.Fn res_query 138(see 139.Xr resolver 3 ) 140before an 141.Em initial absolute query 142will be made. 143The default for 144.Em n 145is 146.Dq 1 , 147meaning that if there are any dots in a name, the name 148will be tried first as an absolute name before any 149.Em search list 150elements are appended to it. 151.It Sy timeout: Ns Ar n 152sets the initial amount of time the resolver will wait 153for a response from a remote 154name server before retrying the query via a different name server. 155The resolver may wait longer during subsequent retries 156of the current query since an exponential back-off is applied to 157the timeout value. 158Measured in seconds, the default is 159.Dv RES_TIMEOUT , 160the allowed maximum is 161.Dv RES_MAXRETRANS 162(see 163.In resolv.h ) . 164.It Sy attempts: Ns Ar n 165sets the number of times the resolver will send a query to each of 166its name servers 167before giving up and returning an error to the calling application. 168The default is 169.Dv RES_DFLRETRY , 170the allowed maximum is 171.Dv RES_MAXRETRY 172(see 173.In resolv.h ) . 174.It Sy no_tld_query 175tells the resolver not to attempt to resolve a top level domain name, that 176is, a name that contains no dots. 177Use of this option does not prevent 178the resolver from obeying the standard 179.Sy domain 180and 181.Sy search 182rules with the given name. 183.El 184.Pp 185Options may also be specified as a space or tab separated list using the 186.Dv RES_OPTIONS 187environment variable. 188.El 189.Pp 190The 191.Sy domain 192and 193.Sy search 194keywords are mutually exclusive. 195If more than one instance of these keywords is present, 196the last instance will override. 197.Pp 198The keyword and value must appear on a single line, and the keyword 199(e.g.\& 200.Sy nameserver ) 201must start the line. 202The value follows the keyword, separated by white space. 203.Sh FILES 204.Bl -tag -width /etc/resolv.conf -compact 205.It Pa /etc/resolv.conf 206The file 207.Nm resolv.conf 208resides in 209.Pa /etc . 210.El 211.Sh SEE ALSO 212.Xr gethostbyname 3 , 213.Xr resolver 3 , 214.Xr hostname 7 , 215.Xr named 8 216.Rs 217.%T "Name Server Operations Guide for BIND" 218.Re 219.Sh HISTORY 220The 221.Nm resolv.conf 222file format appeared in 223.Bx 4.3 . 224