Name

res_mkquery, res_send, res_init, dn_comp, dn_expand − resolver routines

Syntax

#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

The resolver routines are used for making, sending, and interpreting packets to BIND servers.  Global information that is used by the resolver routines is kept in the variable _res. Most of the values have reasonable defaults and you need not be concerned with them. The options are a simple bit mask and are or’ed in to enable. The options stored in _res.options are defined in /usr/include/resolv.h and are as follows:

RES_INITTrue if the initial name server address and default domain name are initialized, for example if res_init has been called.

RES_DEBUGPrint debugging messages. 

RES_AAONLYAccept authoritative answers only. 

RES_USEVCUse TCP connections for queries instead of UDP. 

RES_STAYOPENThis is used with RES_USEVC to keep the TCP connection open between queries. This is useful only in programs that regularly do many queries. You should normally use UDP.

RES_RECURSESet the recursion desired bit in queries.  This is the default.  The res_send routine does not do iterative queries and expects the BIND server to handle recursion.

RES_DEFNAMESAppend the default domain name to single label queries.  This is the default. 

The following lists the routines found in /usr/lib/libc.a

res_initThis routine 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_mkqueryThis routine makes a standard query message and places it in buf. The res_mkquery routine returns the size of the query or −1 if the query is larger than buflen.

opThe opcode is usually QUERY, but can be any of the query types defined in nameser.h.

Dname
This variable is the domain name. If dname consists of a single label and the RES_DEFNAMES flag is enabled, which is the default, dname is appended with the current domain name.  The current domain name is defined in a system file, but you can override it by using the environment variable LOCALDOMAIN.

res_sendThis routine sends a query to the BIND servers and returns an answer.  It calls the res_init routine. If RES_INIT is not set, res_send sends 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_compThis routine 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. The length is the size of the array pointed to by comp_dn.

dnptrsThis variable 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
This is a pointer to the end of the array pointed to by 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, the names are not compressed.  If lastdnptr is NULL, the list is not updated. 

dn_expandThis routine expands the compressed domain name comp_dn to a full BIND domain name. Expanded names are converted to upper case.

msgThis variable is a pointer to the beginning of the message. 

exp_dn
This variable is a pointer to a buffer of size length for the result.  The size of the compressed name is returned or -1 if there was an error. 

Files

/etc/resolv.conf
/usr/include/resolv.h
/usr/include/arpa/nameser.h

See Also

named(8), resolv.conf(5)
Guide to the BIND/Hesiod Service