bind9/lib/lwres/man/resolver.5

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.Dd Jun 30, 2000
.Dt RESOLVER 5
.Os BIND9 9
.ds vT BIND9 Programmer's Manual
.Sh NAME
.Nm resolver 
.Nd resolver configuration file
.Sh SYNOPSIS
.Pa /etc/resolv.conf
.Sh DESCRIPTION
The
.Nm resolver
and
.Nm "BIND9 lightwreight resolver"
are sets of routines in the C library and BIND9 library respectively
that provide access to the Internet Domain Name System.
The resolver configuration file,
.Pa /etc/resolv.conf ,
is normally gets read
by the resolver and lightweight resolver
routines to initialise low-level resolver data structures the first time
that a process invokes those routines.
The file contains a list of
keywords with values that provide various types of 
information, such as the IP addresses of the servers that should be
used when making lookup requests.
.Pp
Each keyword and value must appear on a single line.
The value follows the keyword, separated by white space.
The hash character '#'  or semi-colon ';' introduce comments.
Anything from either of these characters to the end of the line
is treated as a comment and ignored.
.\" XXXJR - Don't think the next para is relevant. Not every host will run
.\" a name server or lightweight resolver. Those that don't will need
.\" a resolv.conf so they can send their lookups somewhere sensible.
.\".Pp
.\"On a normally configured system, this file should not be necessary.
.\"The only name server to be queried will be on the local machine,
.\"the domain name is determined from the host name,
.\"and the domain search path is constructed from the domain name.
.Pp
The available configuration directives are:
.Bl -tag -width "nameserver"
.It Li nameserver
Internet address of a name server that the resolver should query.
The IP address of the server can be supplied in the usual notation:
either in dotted-decimal form for an IPv4 addess or in double colon
form described in RFC1884 for an IPv6 address.
Up to three nameserver directives can be listed.
See
.Dv LWRES_CONFMAXNAMESERVERS
in 
.Pa <lwres/lwres.h> .
If more than
.Dv LWRES_CONFMAXNAMESERVERS 
nameserver directives are present in the file,
only the first three are used.
Any others are ignored.
.Pp
When there are multiple servers, the resolver 
library queries them in the order listed.
If no 
.Li nameserver 
entries are present, the default is to attempt to use the name server or
lightweight resolver daemon on the local machine.
The algorithm used is to try a 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.
.It Li domain
Local domain name.
Most queries for names within this domain can use short names
relative to the local domain.
When no 
.Li domain 
entry is present, the domain is determined from the local host name returned by
.Xr gethostname 2 ;
the domain part is taken to be everything after the first 
.Sq \&. .
Finally, if the host name does not contain a domain part, the root
domain is assumed.
.It Li search
Search list for host-name lookup.
The search list is normally determined from the local domain name;
by default, it contains only the local domain name.
This may be changed by listing the desired domain search path
following the 
.Li search 
keyword with spaces or tabs separating the names.
Most resolver 
queries will be attempted using each component
of the search path in turn until a match is found.
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.
.Pp
The search list is currently limited to eight domains
with a total of 256 characters.
See
.Dv LWRES_CONFMAXSEARCH
and
.Dv LWRES_CONFMAXLINELEN
in
.Pa lwres/lwres.h .
.It Li sortlist
Allows addresses returned by gethostbyname to be sorted.
A 
.Li 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.
For example:
.Bd -literal -offset indent
sortlist 130.155.160.0/255.255.240.0 130.155.0.0
.Ed
.It Li options
Allows certain internal 
.Nm resolver 
variables to be modified.
The syntax is
.D1 Li options Ar option ...
where 
.Ar option 
is one of the following:
.Bl -tag -width "no_tld_query"
.It Li ndots: Ns Ar n 
sets a threshold for the number of dots which
must appear in a name given to 
.Fn res_query 
(see 
.Xr resolver 3 )
before an 
.Em initial absolute query 
will be made.
The default for
.Ar n 
is 
.Dq 1 , 
meaning that if there are 
.Em any 
dots in a name, the name will be tried first as an absolute name before any 
.Em search list
elements are appended to it.
Acceptable values of 
.Ar n
are between 0 and 255 inclusive.
.It debug
enables debugging traces for the resolver
XXXJR if anything actually looked at
.Dv confdata->resdebug .
At present the option is just recognised but not used by
the BIND9 implementation.
.It no_tld_query
tells the resolver not to attempt to resolve a
top level domain name, i.e. a name containing no dots.
Use of this option does not prevent the resolver from obeying the
standard 
.Ar domain
and
.Ar search
rules with the given name.
XXXJR At present this option is recognised but is otherwise unused
in BIND9.
.It missing
XXXJR what about the other BIND8 resolver options like rotate and
timeout? Shouldn't these be provided for backwards compatibility?
.El
.El
.Pp
The 
.Li domain 
and 
.Li search 
keywords are mutually exclusive.
If more than one instance of these keywords is present,
the last instance wins.
.Sh FILES
.Pa /etc/resolv.conf
.Pa <lwres/lwres.h>
.Sh SEE ALSO
.Xr gethostbyname 3 ,
.Xr lwres_config 3 ,
.Xr lwres 3 ,
.Xr resolver 3 ,
.Xr hostname 7 ,
.Xr lwresd 8 ,
.Xr named 8 ,
"BIND9 Administrators Manual".
.Sh BUGS
Unrecognised directives and syntax errors in
.Pa /etc/resolv.conf
are usually silently ignored, though 
.Er LWRES_R_FAILURE
can be returned by the lightweight resolver functions described in
.Xr lwres_config 3
which parse this file.