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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 November 23, 2022 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, setting this file manually should not 50be necessary. 51The only name server(s) to be queried will be on the local machine 52or automatically configured using DHCP or a similar mechanism, 53the domain name is determined from the host name, 54and the domain search path is constructed from the domain name. 55.Pp 56The different configuration options are: 57.Bl -tag -width nameserver 58.It Sy nameserver 59IPv4 or IPv6 address of a name server 60that the resolver should query. 61Up to 62.Dv MAXNS 63(currently 3) name servers may be listed, 64one per keyword. 65If there are multiple servers, 66the resolver library queries them in the order listed. 67If no 68.Sy nameserver 69entries are present, 70the default is to use the name server on the local machine. 71(The algorithm used is to try a name server, and if the query times out, 72try the next, until out of name servers, 73then repeat trying all the name servers 74until a maximum number of retries are made). 75.It Sy domain 76Local domain name. 77Most queries for names within this domain can use short names 78relative to the local domain. 79If no 80.Sy domain 81entry is present, the domain is determined 82from the local host name returned by 83.Xr gethostname 3 ; 84the domain part is taken to be everything after the first 85.Ql \&. . 86Finally, if the host name does not contain a domain part, the root 87domain is assumed. 88.It Sy search 89Search list for host-name lookup. 90The search list is normally determined from the local domain name; 91by default, it contains only the local domain name. 92This may be changed by listing the desired domain search path 93following the 94.Sy search 95keyword with spaces or tabs separating 96the names. 97Most resolver queries will be attempted using each component 98of the search path in turn until a match is found. 99Note that this process may be slow and will generate a lot of network 100traffic if the servers for the listed domains are not local, 101and that queries will time out if no server is available 102for one of the domains. 103.Pp 104The search list is currently limited to six domains 105with a total of 256 characters. 106.It Sy sortlist 107Sortlist allows addresses returned by gethostbyname to be sorted. 108A sortlist is specified by IP address netmask pairs. 109If the netmask is not specified, 110it defaults to the historical Class A/B/C netmask of the net; 111this usage is deprecated. 112The IP address 113and network pairs are separated by slashes. 114Up to 10 pairs may 115be specified. 116E.g., 117.Pp 118.Dl "sortlist 10.9.1.0/255.255.240.0 10.9.0.0/255.255.0.0" 119.It Sy options 120Options allows certain internal resolver variables to be modified. 121The syntax is 122.Pp 123\fBoptions\fP \fIoption\fP \fI...\fP 124.Pp 125where 126.Sy option 127is one of the following: 128.Bl -tag -width no_tld_query 129.It Sy debug 130sets 131.Dv RES_DEBUG 132in _res.options. 133.It Sy usevc 134sets 135.Dv RES_USEVC 136to use TCP instead of UDP for queries. 137.It Sy ndots : Ns Ar n 138sets a threshold for the number of dots which must appear in a name given to 139.Fn res_query 140(see 141.Xr resolver 3 ) 142before an 143.Em initial absolute query 144will be made. 145The default for 146.Em n 147is 148.Dq 1 , 149meaning that if there are any dots in a name, the name 150will be tried first as an absolute name before any 151.Em search list 152elements are appended to it. 153.It Sy timeout : Ns Ar n 154sets the initial amount of time the resolver will wait 155for a response from a remote 156name server before retrying the query via a different name server. 157The resolver may wait longer during subsequent retries 158of the current query since an exponential back-off is applied to 159the timeout value. 160Measured in seconds, the default is 161.Dv RES_TIMEOUT , 162the allowed maximum is 163.Dv RES_MAXRETRANS 164(see 165.In resolv.h ) . 166.It Sy attempts : Ns Ar n 167sets the number of times the resolver will send a query to each of 168its name servers 169before giving up and returning an error to the calling application. 170The default is 171.Dv RES_DFLRETRY , 172the allowed maximum is 173.Dv RES_MAXRETRY 174(see 175.In resolv.h ) . 176.It Sy no_tld_query 177tells the resolver not to attempt to resolve a top level domain name, that 178is, a name that contains no dots. 179Use of this option does not prevent 180the resolver from obeying the standard 181.Sy domain 182and 183.Sy search 184rules with the given name. 185.It Sy reload-period : Ns Ar n 186The resolver checks the modification time of 187.Pa /etc/resolv.conf 188every 189.Ar n 190seconds. 191If 192.Pa /etc/resolv.conf 193has changed, it is automatically reloaded. 194The default for 195.Ar n 196is two seconds. 197Setting it to zero disables the file check. 198.El 199.Pp 200Options may also be specified as a space or tab separated list using the 201.Dv RES_OPTIONS 202environment variable. 203.El 204.Pp 205The 206.Sy domain 207and 208.Sy search 209keywords are mutually exclusive. 210If more than one instance of these keywords is present, 211the last instance will override. 212.Pp 213The keyword and value must appear on a single line, and the keyword 214.Pq for example, Sy nameserver 215must start the line. 216The value follows the keyword, separated by white space. 217.Sh FILES 218.Bl -tag -width /etc/resolv.conf -compact 219.It Pa /etc/resolv.conf 220The file 221.Nm resolv.conf 222resides in 223.Pa /etc . 224.El 225.Sh EXAMPLES 226A basic resolv.conf file could be in the following form. 227.Bd -literal -offset indent 228# The domain directive is only necessary, if your local 229# router advertises something like localdomain and you have 230# set up your hostnames via an external domain. 231domain localdomain.tld 232 233# In case you a running a local dns server or caching name server 234# like local-unbound(8) for example. 235nameserver 127.0.0.1 236 237# IP address of the local or ISP name service 238nameserver 192.168.2.1 239 240# Fallback nameservers, in this case these from Google. 241nameserver 8.8.8.8 242nameserver 4.4.4.4 243 244# Attach an OPT pseudo-RR for the EDNS0 extension, 245# as specified in RFC 2671. 246options edns0 247.Ed 248.Sh SEE ALSO 249.Xr gethostbyname 3 , 250.Xr resolver 3 , 251.Xr hostname 7 , 252.Xr resolvconf 8 253.Sh HISTORY 254The 255.Nm resolv.conf 256file format appeared in 257.Bx 4.3 . 258