RESOLVER(3)

NAME
resolver, res_mkquery, res_send, res_init, dn_comp, dn_expand - resolver routines

SYNOPSIS
#include <sys/types.h>
#include <netinet/in.h>
#include <arpa/nameser.h>
#include <resolv.h>

res_mkquery(op, dname, class, type, data, datalen, newrr, buf, buflen)
int op;
char *dname;
int class, type;
char *data;
int datalen;
struct rrec *newrr;
char *buf;
int buflen;

res_send(msg, msglen, answer, anslen)
char *msg;
int msglen;
char *answer;
int anslen;

res_init()

dn_comp(exp_dn, comp_dn, length, dnptrs, lastdnptr)
u_char *exp_dn, *comp_dn;
int length;
u_char **dnptrs, **lastdnptr;

dn_expand(msg, msglen, comp_dn, exp_dn, length)
u_char *msg, *eomorig, *comp_dn, exp_dn;
int length;

DESCRIPTION

These routines are used for making, sending and interpreting packets to Internet domain name servers. You can link a program with the resolver library using the -lresolv argument on the linking command line.

Global information that is used by the resolver routines is kept in the variable _res. Most of the values have reasonable defaults and can be ignored. Options are a simple bit mask and are OR'ed in to enable. Options stored in _res.options are defined in <resolv.h> and are as follows.

RES_INIT
    True if the initial name server address and default domain name are initialized (that is, res_init() has been called).

RES_DEBUG
    Print debugging messages.

RES_AAONLY
    Accept authoritative answers only. res_send() continues until it finds an authoritative answer or finds an error. Currently this is not implemented.

RES_USEVC
    Use TCP connections for queries instead of UDP.

RES_STAYOPEN
    Used with RES_USEVC to keep the TCP connection open between queries. This is useful only in programs that regularly do many queries. UDP should be the normal mode used.

RES_IGNTC
    Unused currently (ignore truncation errors, that is, do not retry with TCP).

RES_RECURSE
    Set the recursion desired bit in queries. This is the default. res_send() does not do iterative queries and expects the name server to handle recursion.

RES_DEFNAMES
    Append the default domain name to single label queries. This is the default.

RES_DNSRCH
    Search up the domain tree from the default domain, in all but the top level. This is the default.

res_init() reads the initialization file to get the default domain name and the Internet addresses of the initial name servers. If no nameserver line exists, the host running the resolver is tried. res_mkquery() makes a standard query message and places it in buf. res_mkquery() returns the size of the query or -1 if the query is larger than buflen. op is usually QUERY but can be any of the query types defined in <nameser.h>. dname is the domain name. If dname consists of a single label and the RES_DEFNAMES flag is enabled (the default), dname is appended with the current domain name. The current domain name is defined in a system file and can be overridden by the environment variable LOCALDOMAIN. newrr is currently unused but is intended for making update messages.

res_send() sends a query to name servers and returns an answer. It calls res_init() if RES_INIT is not set, send the query to the local name server, and handle timeouts and retries. The length of the message is returned or -1 if there were errors.

dn_expand() Expands the compressed domain name comp_dn to a full domain name. Expanded names are converted to upper case. msg is a pointer to the beginning of the message, exp_dn is a pointer to a buffer of size length for the result. The size of compressed name is returned or -1 if there was an error.

dn_comp() Compresses the domain name exp_dn and stores it in comp_dn. The size of the compressed name is returned or -1 if there were errors. length is the size of the array pointed to by comp_dn. dnptrs is a list of pointers to previously compressed names in the current message. The first pointer points to the beginning of the message and the list ends with NULL. lastdnptr is a pointer to the end of the array pointed to dnptrs. A side effect is to update the list of pointers for labels inserted into the message by dn_comp() as the name is compressed. If dnptr is NULL, do not try to compress names. If lastdnptr is NULL, do not update the list.

FILES
/etc/resolv.conf
    see resolv.conf(5)
/usr/lib/libresolv.a

SEE ALSO
resolv.conf(5), named(8C)

NOTES

/usr/lib/libresolv.a is necessary for compiling programs.