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.\"     @(#)resolver.3  8.1 (Berkeley) 6/4/93

.\"     $Id: resolver.3,v 1.2 2001-09-27 12:01:53 chris Exp $

.\"

.Dd June 4, 1993

.Dt RESOLVER 3

.Os BSD 4.3

.Sh NAME

.Nm res_query ,

.Nm res_search ,

.Nm res_mkquery ,

.Nm res_send ,

.Nm res_init ,

.Nm dn_comp ,

.Nm dn_expand

.Nd resolver routines

.Sh SYNOPSIS

.Fd #include 

.Fd #include 

.Fd #include 

.Fd #include 

.Ft int

.Fo res_query

.Fa "const char *dname"

.Fa "int class"

.Fa "int type"

.Fa "u_char *answer"

.Fa "int anslen"

.Fc

.Ft int

.Fo res_search

.Fa "const char *dname"

.Fa "int class"

.Fa "int type"

.Fa "u_char *answer"

.Fa "int anslen"

.Fc

.Ft int

.Fo res_mkquery

.Fa "int op"

.Fa "const char *dname"

.Fa "int class"

.Fa "int type"

.Fa "const u_char *data"

.Fa "int datalen"

.Fa "const u_char *newrr_in"

.Fa "u_char *buf"

.Fa "int buflen"

.Fc

.Ft int

.Fo res_send

.Fa "const u_char *msg"

.Fa "int msglen"

.Fa "u_char *answer"

.Fa "int anslen"

.Fc

.Ft int

.Fn res_init

.Fo dn_comp

.Fa "const char *exp_dn"

.Fa "u_char *comp_dn"

.Fa "int length"

.Fa "u_char **dnptrs"

.Fa "u_char **lastdnptr"

.Fc

.Ft int

.Fo dn_expand

.Fa "const u_char *msg"

.Fa "const u_char *eomorig"

.Fa "const u_char *comp_dn"

.Fa "char *exp_dn"

.Fa "int length"

.Fc

.Sh DESCRIPTION

These routines are used for making, sending and interpreting

query and reply messages with Internet domain name servers.

.Pp

Global configuration and state information that is used by the

resolver routines is kept in the structure

.Em _res .

Most of the values have reasonable defaults and can be ignored.

Options

stored in

.Em _res.options

are defined in

.Pa resolv.h

and are as follows.

Options are stored as a simple bit mask containing the bitwise ``or''

of the options enabled.

.Bl -tag -width RES_DEFNAMES

.It Dv RES_INIT

True if the initial name server address and default domain name are

initialized (i.e.,

.Fn res_init

has been called).

.It Dv RES_DEBUG

Print debugging messages.

.It Dv RES_AAONLY

Accept authoritative answers only.

With this option,

.Fn res_send

should continue until it finds an authoritative answer or finds an error.

Currently this is not implemented.

.It Dv RES_USEVC

Use

.Tn TCP

connections for queries instead of

.Tn UDP

datagrams.

.It Dv RES_STAYOPEN

Used with

.Dv RES_USEVC

to keep the

.Tn TCP

connection open between

queries.

This is useful only in programs that regularly do many queries.

.Tn UDP

should be the normal mode used.

.It Dv RES_IGNTC

Unused currently (ignore truncation errors, i.e., don't retry with

.Tn TCP ) .

.It Dv RES_RECURSE

Set the recursion-desired bit in queries.

This is the default.

.Pf ( Fn res_send

does not do iterative queries and expects the name server

to handle recursion.)

.It Dv RES_DEFNAMES

If set,

.Fn res_search

will append the default domain name to single-component names

(those that do not contain a dot).

This option is enabled by default.

.It Dv RES_DNSRCH

If this option is set,

.Fn res_search

will search for host names in the current domain and in parent domains; see

.Xr hostname 7 .

This is used by the standard host lookup routine

.Xr gethostbyname 3 .

This option is enabled by default.

.It Dv RES_NOALIASES

This option turns off the user level aliasing feature controlled by the

.Dq Ev HOSTALIASES

environment variable.  Network daemons should set this option.

.El

.Pp

The

.Fn res_init

routine

reads the configuration file (if any; see

.Xr resolver 5 )

to get the default domain name,

search list and

the Internet address of the local name server(s).

If no server is configured, the host running

the resolver is tried.

The current domain name is defined by the hostname

if not specified in the configuration file;

it can be overridden by the environment variable

.Ev LOCALDOMAIN .

This environment variable may contain several blank-separated

tokens if you wish to override the

.Em "search list"

on a per-process basis.  This is similar to the

.Em search

command in the configuration file.

Another environment variable (

.Dq Ev RES_OPTIONS

can be set to

override certain internal resolver options which are otherwise

set by changing fields in the

.Em _res

structure or are inherited from the configuration file's

.Em options

command.  The syntax of the

.Dq Ev RES_OPTIONS

environment variable is explained in

.Xr resolver 5 .

Initialization normally occurs on the first call

to one of the following routines.

.Pp

The

.Fn res_query

function provides an interface to the server query mechanism.

It constructs a query, sends it to the local server,

awaits a response, and makes preliminary checks on the reply.

The query requests information of the specified

.Fa type

and

.Fa class

for the specified fully-qualified domain name

.Fa dname .

The reply message is left in the

.Fa answer

buffer with length

.Fa anslen

supplied by the caller.

.Pp

The

.Fn res_search

routine makes a query and awaits a response like

.Fn res_query ,

but in addition, it implements the default and search rules

controlled by the

.Dv RES_DEFNAMES

and

.Dv RES_DNSRCH

options.

It returns the first successful reply.

.Pp

The remaining routines are lower-level routines used by

.Fn res_query .

The

.Fn res_mkquery

function

constructs a standard query message and places it in

.Fa buf .

It returns the size of the query, or \-1 if the query is

larger than

.Fa buflen .

The query type

.Fa op

is usually

.Dv QUERY ,

but can be any of the query types defined in

.Aq Pa arpa/nameser.h .

The domain name for the query is given by

.Fa dname .

.Fa Newrr

is currently unused but is intended for making update messages.

.Pp

The

.Fn res_send

routine

sends a pre-formatted query and returns an answer.

It will call

.Fn res_init

if

.Dv RES_INIT

is not set, send the query to the local name server, and

handle timeouts and retries.

The length of the reply message is returned, or

\-1 if there were errors.

.Pp

The

.Fn dn_comp

function

compresses the domain name

.Fa exp_dn

and stores it in

.Fa comp_dn .

The size of the compressed name is returned or \-1 if there were errors.

The size of the array pointed to by

.Fa comp_dn

is given by

.Fa length .

The compression uses

an array of pointers

.Fa dnptrs

to previously-compressed names in the current message.

The first pointer points to

to the beginning of the message and the list ends with

.Dv NULL .

The limit to the array is specified by

.Fa lastdnptr .

A side effect of

.Fn dn_comp

is to update the list of pointers for

labels inserted into the message

as the name is compressed.

If

.Em dnptr

is

.Dv NULL, names are not compressed.

If

.Fa lastdnptr

is

.Dv NULL ,

the list of labels is not updated.

.Pp

The

.Fn dn_expand

entry

expands the compressed domain name

.Fa comp_dn

to a full domain name

The compressed name is contained in a query or reply message;

.Fa msg

is a pointer to the beginning of the message.

The uncompressed name is placed in the buffer indicated by

.Fa exp_dn

which is of size

.Fa length .

The size of compressed name is returned or \-1 if there was an error.

.Sh FILES

.Bl -tag -width Pa

/etc/resolv.conf

The configuration file

see

.Xr resolver 5 .

.El

.Sh SEE ALSO

.Xr gethostbyname 3 ,

.Xr resolver 5 ,

.Xr hostname 7 ,

.Xr named 8

.Pp

.%T RFC1032 ,

.%T RFC1033 ,

.%T RFC1034 ,

.%T RFC1035 ,

.%T RFC974

.Rs

.%T "Name Server Operations Guide for BIND"

.Re

.Sh HISTORY

The

.Nm

function appeared in

.Bx 4.3 .