RESOLVER(3)                 Library Functions Manual               RESOLVER(3)

     res_query, res_search, res_mkquery, res_send, res_init, dn_comp,
     dn_expand — resolver routines

     #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);


     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);

     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.

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

              Print debugging messages.

              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.

              Use TCP connections for queries instead of UDP datagrams.

              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.

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

              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.)

              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.

              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.

              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.

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

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

4th Berkeley Distribution      December 11, 1995     4th Berkeley Distribution