RESOLVER

Section: C Library Functions (3)
Updated: November 21, 1987
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NAME

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)
char *exp_dn, *comp_dn;
int length;
char **dnptrs, **lastdnptr;

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

DESCRIPTION

These routines are used for making, sending and interpreting packets for use with Internet domain name servers. 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 stored in _res.options are defined in resolv.h and are as follows. Options are stored a simple bit mask containing the bitwise ``or'' of the options enabled.

RES_INIT: True if the initial name server address and default domain name are initialized (i.e., res_init has been called).
RES_DEBUG: Print debugging messages.
RES_AAONLY: Accept authoritative answers only. With this option, res_send should continue 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 datagrams.
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, i.e., don't 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: If set, res_mkquery will append the default domain name to single-component names (those that do not contain a dot). This is the default.
RES_DNSRCH: If this option is set, the standard host lookup routine gethostbyname(3) will search for host names in the current domain and in parent domains; see hostname(7).

Res_init

reads the initialization file to get the default domain name and the Internet address of the initial hosts running the name server. If this line does not exist, the host running the resolver is tried. Res_mkquery makes a standard query message and places it in buf. Res_mkquery will return 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), the current domain name will be appended to dname. The current domain name is defined by the hostname or is specified in a system file; it 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 will call 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 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, names are not compressed. If lastdnptr is NULL, the list of labels is not updated.

FILES

/etc/resolv.conf see resolver(5)