The resolv.h header file contains the __res_state structure and the definitions to support the IP Address Resolution functions commonly called the Resolver. It contains the prototypes for the following functions — dn_comp(), dn_expand(), dn_find(), dn_skipname(), res_init(), res_mkquery(), res_query(), res_querydomain(), res_search(), and res_send() — which are used to communicate with a Domain Name Server (DNS).