| resolver(3) | Library Functions Manual | resolver(3) |
res_ninit, res_nquery, res_nsearch, res_nquerydomain, res_nmkquery, res_nsend, res_nclose, res_init, res_query, res_search, res_querydomain, res_mkquery, res_send, dn_comp, dn_expand - resolver routines
Resolver library (libresolv, -lresolv)
#include <netinet/in.h> #include <arpa/nameser.h> #include <resolv.h>
struct __res_state; typedef struct __res_state *res_state;
int res_ninit(res_state statep);
void res_nclose(res_state statep);
int res_nquery(res_state statep,
const char *dname, int class, int type,
unsigned char answer[.anslen], int anslen);
int res_nsearch(res_state statep,
const char *dname, int class, int type,
unsigned char answer[.anslen], int anslen);
int res_nquerydomain(res_state statep,
const char *name, const char *domain,
int class, int type, unsigned char answer[.anslen],
int anslen);
int res_nmkquery(res_state statep,
int op, const char *dname, int class,
int type, const unsigned char data[.datalen], int datalen,
const unsigned char *newrr,
unsigned char buf[.buflen], int buflen);
int res_nsend(res_state statep,
const unsigned char msg[.msglen], int msglen,
unsigned char answer[.anslen], int anslen);
int dn_comp(const char *exp_dn, unsigned char comp_dn[.length],
int length, unsigned char **dnptrs,
unsigned char **lastdnptr);
int dn_expand(const unsigned char *msg,
const unsigned char *eomorig,
const unsigned char *comp_dn, char exp_dn[.length],
int length);
[[deprecated]] extern struct __res_state _res;
[[deprecated]] int res_init(void);
[[deprecated]]
int res_query(const char *dname, int class, int type,
unsigned char answer[.anslen], int anslen);
[[deprecated]]
int res_search(const char *dname, int class, int type,
unsigned char answer[.anslen], int anslen);
[[deprecated]]
int res_querydomain(const char *name, const char *domain,
int class, int type, unsigned char answer[.anslen],
int anslen);
[[deprecated]]
int res_mkquery(int op, const char *dname, int class,
int type, const unsigned char data[.datalen], int datalen,
const unsigned char *newrr,
unsigned char buf[.buflen], int buflen);
[[deprecated]]
int res_send(const unsigned char msg[.msglen], int msglen,
unsigned char answer[.anslen], int anslen);
Note: This page is incomplete (various resolver functions provided by glibc are not described) and likely out of date.
The functions described below make queries to and interpret the responses from Internet domain name servers.
The API consists of a set of more modern, reentrant functions and an older set of nonreentrant functions that have been superseded. The traditional resolver interfaces such as res_init() and res_query() use some static (global) state stored in the _res structure, rendering these functions non-thread-safe. BIND 8.2 introduced a set of new interfaces res_ninit(), res_nquery(), and so on, which take a res_state as their first argument, so you can use a per-thread resolver state.
The res_ninit() and res_init() functions read the configuration files (see resolv.conf(5)) to get the default domain name and name server address(es). If no server is given, the local host is tried. If no domain is given, that associated with the local host is used. It can be overridden with the environment variable LOCALDOMAIN. res_ninit() or res_init() is normally executed by the first call to one of the other functions. Every call to res_ninit() requires a corresponding call to res_nclose() to free memory allocated by res_ninit() and subsequent calls to res_nquery().
The res_nquery() and res_query() functions query the name server for the fully qualified domain name name of specified type and class. The reply is left in the buffer answer of length anslen supplied by the caller.
The res_nsearch() and res_search() functions make a query and waits for the response like res_nquery() and res_query(), but in addition they implement the default and search rules controlled by RES_DEFNAMES and RES_DNSRCH (see description of _res options below).
The res_nquerydomain() and res_querydomain() functions make a query using res_nquery()/res_query() on the concatenation of name and domain.
The following functions are lower-level routines used by res_nquery()/res_query().
The res_nmkquery() and res_mkquery() functions construct a query message in buf of length buflen for the domain name dname. The query type op is one of the following (typically QUERY):
newrr is currently unused.
The res_nsend() and res_send() function send a preformatted query given in msg of length msglen and returns the answer in answer which is of length anslen. They will call res_ninit()/res_init() if it has not already been called.
The dn_comp() function compresses the domain name exp_dn and stores it in the buffer comp_dn of length length. The compression uses an array of pointers dnptrs to previously compressed names in the current message. The first pointer points to the beginning of the message and the list ends with NULL. The limit of the array is specified by lastdnptr. If dnptr is NULL, domain names are not compressed. If lastdnptr is NULL, the list of labels is not updated.
The dn_expand() function expands the compressed domain name comp_dn to a full domain name, which is placed in the buffer exp_dn of size length. The compressed name is contained in a query or reply message, and msg points to the beginning of the message.
The resolver routines use configuration and state information contained in a __res_state structure (either passed as the statep argument, or in the global variable _res, in the case of the older nonreentrant functions). The only field of this structure that is normally manipulated by the user is the options field. This field can contain the bitwise "OR" of the following options:
The res_ninit() and res_init() functions return 0 on success, or -1 if an error occurs.
The res_nquery(), res_query(), res_nsearch(), res_search(), res_nquerydomain(), res_querydomain(), res_nmkquery(), res_mkquery(), res_nsend(), and res_send() functions return the length of the response, or -1 if an error occurs.
The dn_comp() and dn_expand() functions return the length of the compressed name, or -1 if an error occurs.
In the case of an error return from res_nquery(), res_query(), res_nsearch(), res_search(), res_nquerydomain(), or res_querydomain(), the global variable h_errno (see gethostbyname(3)) can be consulted to determine the cause of the error.
For an explanation of the terms used in this section, see attributes(7).
| Interface | Attribute | Value |
| res_ninit (), res_nclose (), res_nquery (), res_nsearch (), res_nquerydomain (), res_nsend () | Thread safety | MT-Safe locale |
| res_nmkquery (), dn_comp (), dn_expand () | Thread safety | MT-Safe |
None.
4.3BSD.
gethostbyname(3), resolv.conf(5), resolver(5), hostname(7), named(8)
The GNU C library source file resolv/README.
| 2023-07-20 | Linux man-pages 6.05.01 |