The res_ninit() and res_init() routines read the configuration file, if any is present, to get the default domain name, search list and the Internet address of the local name server(s). See resolv.conf(4). If no server is configured, res_init() or res_ninit() will try to obtain name resolution services from the host on which it is running. The current domain name is defined by domainname(1M), or by the hostname if it is not specified in the configuration file. Use the environment variable LOCALDOMAIN to override the domain name. 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. You can set the RES_OPTIONS environment variable to override certain internal resolver options. You can otherwise set them by changing fields in the statp /_res structure. Alternatively, they are inherited from the configuration file's options command. See resolv.conf(4) for information regarding the syntax of the RES_OPTIONS environment variable. Initialization normally occurs on the first call to one of the other resolver routines.

The res_nquery() and res_query() functions provides interfaces to the server query mechanism. They construct a query, send it to the local server, await a response, and make 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. res_nquery() and res_query() return the length of the answer, or -1 upon error.

The res_nquery() and res_query() routines return a length that may be bigger than anslen. In that case, retry the query with a larger buf. The answer to the second query may be larger still], so it is recommended that you supply a buf larger than the answer returned by the previous query. answer must be large enough to receive a maximum UDP response from the server or parts of the answer will be silently discarded. The default maximum UDP response size is 512 bytes.

The res_nsearch() and res_search() routines make a query and await a response, just like like res_nquery() and res_query(). In addition, they implement the default and search rules controlled by the RES_DEFNAMES and RES_DNSRCH options. They return the length of the first successful reply which is stored in answer. On error, they reurn -1.

The res_nsearch() and res_search() routines return a length that may be bigger than anslen. In that case, retry the query with a larger buf. The answer to the second query may be larger still], so it is recommended that you supply a buf larger than the answer returned by the previous query. answer must be large enough to receive a maximum UDP response from the server or parts of the answer will be silently discarded. The default maximum UDP response size is 512 bytes.

These routines are used by res_nquery() and res_query(). The res_nmkquery() and res_mkquery() functions construct a standard query message and place it in buf. The routine 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. newrr is currently unused but is intended for making update messages.

The res_nsend(), res_send(), and res_nsendsigned() routines send a preformatted query that returns an answer. The routine calls res_ninit() or res_init(). If RES_INIT is not set, the routine sends the query to the local name server and handles timeouts and retries. Additionally, the res_nsendsigned() uses TSIG signatures to add authentication to the query and verify the response. In this case, only one name server will be contacted. The routines return the length of the reply message, or -1 if there are errors.

The res_nsend() and res_send() routines return a length that may be bigger than anslen. In that case, retry the query with a larger buf. The answer to the second query may be larger still], so it is recommended that you supply a buf larger than the answer returned by the previous query. answer must be large enough to receive a maximum UDP response from the server or parts of the answer will be silently discarded. The default maximum UDP response size is 512 bytes.

The function fp_resstat() prints out the active flag bits in statp->options preceded by the text ";; res options:" on file.

The function res_hostalias() looks up name in the file referred to by the HOSTALIASES environment variable and returns the fully qualified host name. If name is not found or an error occurs, NULL is returned. res_hostalias() stores the result in buf.

The res_nclose() function closes any open files referenced through statp.

dn_comp() compresses the domain name exp_dn and stores it in comp_dn. dn_comp() returns the size of the compressed name, or -1 if there were errors. length is the size of the array pointed to by comp_dn.

dnptrs is a pointer to the head of the list of pointers to previously compressed names in the current message. The first pointer must point to the beginning of the message. The list ends with NULL. The limit to the array is specified by lastdnptr.

A side effect of calling dn_comp() 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, names are not compressed. If lastdnptr is NULL, dn_comp() does not update the list of labels.

dn_expand() 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 that message. The uncompressed name is placed in the buffer indicated by exp_dn, which is of size length.dn_expand() returns the size of the compressed name, or -1 if there was an error.

The variables statp->res_h_errno and _res.res_h_errno and external variable h_errno are set whenever an error occurs during a resolver operation. The following definitions are given in <netdb.h>:

#define NETDB_INTERNAL -1 /* see errno */  
#define NETDB_SUCCESS  0  /* no problem */ 
#define HOST_NOT_FOUND 1  /* Authoritative Answer Host not found */  
#define TRY_AGAIN      2  /* Non-Authoritative not found, or SERVFAIL */ 
#define NO_RECOVERY    3  /* Non-Recoverable: FORMERR, REFUSED, NOTIMP*/ 
#define NO_DATA        4  /* Valid name, no data for requested type */      

The herror() function writes a message to the diagnostic output consisting of the string parameters, the constant string ": ", and a message corresponding to the value of h_errno.

The hstrerror() function returns a string, which is the message text that corresponds to the value of the err parameter.