Manual Pages  — RESOLVER

The res_init() routine reads the configuration file (if any; see resolver(5)) to get the default domain name, search list and the Internet address of the local name server(s). If no server is configured, the host running the resolver is tried. The current domain name is defined by the hostname if not specified in the configuration file; it can be overridden by the environment variable LOCALDOMAIN. This environment variable may contain several blank-separated tokens if you wish to override the search list on a per-process basis. This is similar to the search command in the configuration file. Another environment variable " RES_OPTIONS" can be set to override certain internal resolver options which are otherwise set by changing fields in the _res structure or are inherited from the configuration file's options command. The syntax of the " RES_OPTIONS" environment variable is explained in resolver(5). Initialization normally occurs on the first call to one of the following routines.

The res_query() function provides an interface to the server query mechanism. It constructs a query, sends it to the local server, awaits a response, and makes preliminary checks on the reply. The query requests information of the specified type and class for the specified fully-qualified domain name dname. The reply message is left in the answer buffer with length anslen supplied by the caller.

The res_search() routine makes a query and awaits a response like res_query(), but in addition, it implements the default and search rules controlled by the RES_DEFNAMES and RES_DNSRCH options. It returns the first successful reply.

The remaining routines are lower-level routines used by res_query(). The res_mkquery() function constructs a standard query message and places it in buf. It returns the size of the query, or -1 if the query is larger than buflen. The query type op is usually QUERY, but can be any of the query types defined in <arpa/nameser.h>. The domain name for the query is given by dname. The newrr_in argument is currently unused but is intended for making update messages.

The res_send() routine sends a pre-formatted query 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 reply message is returned, or -1 if there were errors.

The dn_comp() function 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 size of the array pointed to by comp_dn is given by 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 to the array is specified by lastdnptr. A side effect of dn_comp() is to update the list of pointers for labels inserted into the message as the name is compressed. If dnptr is NULL, names are not compressed. If lastdnptr is NULL, the list of labels is not updated.

The dn_expand() entry expands the compressed domain name comp_dn to a full domain name The compressed name is contained in a query or reply message; msg is a pointer to the beginning of the message. The uncompressed name is placed in the buffer indicated by exp_dn which is of size length. The size of compressed name is returned or -1 if there was an error.

The dn_skipname() function skips over a compressed domain name, which starts at a location pointed to by comp_dn. The compressed name is contained in a query or reply message; eom is a pointer to the end of the message. The size of compressed name is returned or -1 if there was an error.

The ns_get16() function gets a 16-bit quantity from a buffer pointed to by src.

The ns_get32() function gets a 32-bit quantity from a buffer pointed to by src.

The ns_put16() function puts a 16-bit quantity src to a buffer pointed to by dst.

The ns_put32() function puts a 32-bit quantity src to a buffer pointed to by dst.

The res_mkquery(), res_search(), and res_query() functions return the size of the response on success, or -1 if an error occurs. The integer h_errno may be checked to determine the reason for error. See gethostbyname(3) for more information.

RFC1032, RFC1033, RFC1034, RFC1035, RFC974

Name Server Operations Guide for BIND,