RESOLVER(3)

NAME
res_query, res_search, res_mkquery, res_send, res_init, dn_comp, dn_expand - resolver routines

SYNOPSIS
#include <sys/types.h>
#include <netinet/in.h>
#include <arpa/nameser.h>
#include <resolv.h>
res_query(const char *dname, int class, type, u_char *answer, int anslen);
res_search(const char *dname, int class, type, u_char *answer, int anslen);
res_mkquery(int op, const char *dname, int class, type, const char *data, int datalen, struct rrec *newrr, u_char *buf, int buflen);
res_send(const u_char *msg, int msglen, u_char *answer, int anslen);
res_init();
dn_comp(const char *exp_dn, u_char *comp_dn, int length, u_char **dnptrs, **lastdnptr);
dn_expand(const u_char *msg, *eomorig, *comp_dn, char *exp_dn, int length);
herror(const char *s);
hstrerror(int err);

DESCRIPTION
These routines are used for making, sending and interpreting query and reply messages with Internet domain name servers.

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.

RES_INIT
    True if the initial name server address and default domain name are initialized (i.e., res_init() has been called).

RES_DEBUG
    Print debugging messages.

RES_AAONLY
    Accept authoritative answers only. With this option, res_send() should continue until it finds an authoritative answer or finds an error. Currently this is not implemented.

RES_USEVC
    Use TCP connections for queries instead of UDP datagrams.

RES_STAYOPEN
    Used with RES_USEVC to keep the TCP connection open between queries. This is useful only in programs that regularly do many queries. UDP should be the normal mode used.

RES_IGNTC
    Unused currently (ignore truncation errors, i.e., don't retry with TCP).

RES_RECURSE
    Set the recursion-desired bit in queries. This is the default. (res_send() does not do iterative queries and expects the name server to handle recursion.)

RES_DEFNAMES
    If set, res_search() will append the default domain name to single-component names (those that do not contain a dot). This option is enabled by default.

RES_DNSRCH
    If this option is set, res_search() will search for host names in the current domain and in parent domains; see hostname(7). This is used by the standard host lookup routine gethostbyname(3). This option is enabled by default.

RES_NOALIASES
    This option turns off the user level aliasing feature controlled by the HOSTALIASES environment variable. Network daemons should set this option.

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 other resolver 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. Newrr 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 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 external variable h_errno is set whenever an error occurs during 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-Authoritive not found, or SERVFAIL */
    #define NO_RECOVERY    3   /* Nonrecoverable: 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 parameter s, the constant string ": ", and a message corresponding to the value of h_errno.

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

FILES

/etc/resolv.conf
    See resolver(5).

SEE ALSO
gethostbyname(3), hostname(7), named(8), resolver(5); RFC1032, RFC1033, RFC1034, RFC1035, RFC974; SMM:11, “Name Server Operations Guide for BIND”