Global configuration and state information that is used by the resolver routines is kept in the structure _res Most of the values have reasonable defaults and can be ignored. Options stored in _res.options are defined in resolv.h and are as follows. Options are stored as a simple bit mask containing the bitwise ``or'' of the options enabled.
The Fn res_init routine reads the configuration file (if any; see resolv.conf5) 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 Fa search list on a per-process basis. This is similar to the Fa 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 Fa _res structure or are inherited from the configuration file's Fa options command. The syntax of the RES_OPTIONS environment variable is explained in resolv.conf5. Initialization normally occurs on the first call to one of the following routines.
The Fn 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 Fa type and Fa class for the specified fully-qualified domain name Fa dname . The reply message is left in the Fa answer buffer with length Fa anslen supplied by the caller.
The Fn res_search routine makes a query and awaits a response like Fn 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 Fn res_query . The Fn res_mkquery function constructs a standard query message and places it in Fa buf . It returns the size of the query, or -1 if the query is larger than Fa buflen . The query type Fa op is usually QUERY but can be any of the query types defined in Aq Pa arpa/nameser.h . The domain name for the query is given by Fa dname . Fa Newrr is currently unused but is intended for making update messages.
The Fn res_send routine sends a pre-formatted query and returns an answer. It will call Fn 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 Fn dn_comp function compresses the domain name Fa exp_dn and stores it in Fa comp_dn . The size of the compressed name is returned or -1 if there were errors. The size of the array pointed to by Fa comp_dn is given by Fa length . The compression uses an array of pointers Fa dnptrs 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 The limit to the array is specified by Fa lastdnptr . A side effect of Fn 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 Fa lastdnptr is NULL the list of labels is not updated.
The Fn dn_expand entry expands the compressed domain name Fa comp_dn to a full domain name The compressed name is contained in a query or reply message; Fa msg is a pointer to the beginning of the message. The uncompressed name is placed in the buffer indicated by Fa exp_dn which is of size Fa length . The size of compressed name is returned or -1 if there was an error.
/etc/resolv.conf The configuration file see resolv.conf5.
RFC1032 , RFC1033 , RFC1034 , RFC1035 , RFC974