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# resolver

    RESOLVER(3)              BSD Library Functions Manual              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>

         int
         res_query(char *dname, int class, int type, u_char *answer, int anslen);

         int
         res_search(char *dname, int class, int type, u_char *answer, int anslen);

         int
         res_mkquery(int op, char *dname, int class, int type, char *data,
                 int datalen, struct rrec *newrr, char *buf, int buflen);

         int
         res_send(char *msg, int msglen, char *answer, int anslen);

         int
         res_init(void);

         int
         dn_comp(char *exp_dn, char *comp_dn, int length, char **dnptrs,
                 char **lastdnptr);

         int
         dn_expand(u_char *msg, u_char *eomorig, u_char *comp_dn, u_char *exp_dn,
                 int length);

    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 reason-
         able 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 imple-
                        mented.

         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_USE_INET6  Enables support for IPv6-only applications.  This causes
                        IPv4 addresses to be returned as an IPv4 mapped address.
                        For example, 10.1.1.1 will be returned as ::ffff:10.1.1.1.
                        The option is not meaningful on OpenBSD.

         The res_init() routine reads the configuration file (if any; see
         resolv.conf(5)) to get the default domain name, search list, and the
         Internet address of the local name server(s).  If no server is config-
         ured, 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 environ-
         ment 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 resolv.conf(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 mecha-
         nism.  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 qual-
         ified 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 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 com-
         pressed.  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 uncom-
         pressed 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.

    FILES
         /etc/resolv.conf configuration file see resolv.conf(5).

    SEE ALSO
         gethostbyname(3), resolv.conf(5), hostname(7), named(8)

         RFC1032, RFC1033, RFC1034, RFC1035, RFC1535, RFC974

         Name Server Operations Guide for BIND.

    HISTORY
         The res_query function appeared in 4.3BSD.

    BSD                              June 4, 1993                              BSD

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