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Name

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

Synopsis

cc [ flag ... ] file ... -lresolv -lsocket -lnsl [ library ... ] 


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

int res_init(void);

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

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

int res_search(char *dname, int class, int type, uchar *answer, int anslen);

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

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

MT-Level

Unsafe

Description

These routines are used for making, sending and interpreting packets to 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 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() will 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 .
RES_PRIMARY
Query primary server only.
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_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_DNSRCH
Search up local domain tree.

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 <arpa/nameser.h>. dname is the domain name. If dname consists of a single label and the RES_DEFNAMES flag is enabled (the default), dname will be 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. class and type define the class and type of query. *data is the resource record; and datalen is the length of the record.

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. msg is the query sent; msglen is its length. answer is the response returned. The length of the response is stored in anslen. res_send() returns the length of the response or -1 if there were errors.

res_search() formulates and sends a normal query (QUERY ) message, and stores the response in a buffer supplied by the caller. dname is the domain name. class and type define the class and type of query (see <arpa/nameser.h>). The response is returned in the user-supplied buffer answer. res_search returns the length of answer in anslen. res_search() will call res_init() if the RES_INIT flag is not enabled. If dname consists of a single label and the RES_DEFNAMES flag is enabled (the default), dname will be appended with the current domain name. If the RES_DNSRCH flag is enabled, res_search() will search up the local domain tree until an answer has been retrieved or an unrecoverable error has been encountered. res_search() returns the length of answer on success and -1 on error. Note that res_search() is only useful for queries in the same name hierarchy as the local host.

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 dnptrs is NULL , do not try to compress names. If lastdnptr is NULL , do not update the list.

Files

/etc/resolv.conf

See Also

in.named(1M) , nstest(1M) , resolv.conf(4)

Notes

These interfaces are unsafe in multithreaded applications. Unsafe interfaces should be called only from the main thread.

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