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RESOLVER(3)              BSD Library Functions Manual              RESOLVER(3)

NAME
     res_query, res_search, res_mkquery, res_send, res_init, dn_comp, dn_expand, dn_skipname, ns_get16,
     ns_get32, ns_put16, ns_put32 -- resolver routines

LIBRARY
     DNS Resolver Library (libresolv, -lresolv)

SYNOPSIS
     #include <sys/types.h>
     #include <netinet/in.h>
     #include <arpa/nameser.h>
     #include <resolv.h>

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

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

     int
     res_mkquery(int op, const char *dname, int class, int type, const u_char *data, int datalen,
         const u_char *newrr_in, u_char *buf, int buflen);

     int
     res_send(const u_char *msg, int msglen, u_char *answer, int anslen);

     int
     res_init();

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

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

     int
     dn_skipname(const u_char *comp_dn, const u_char *eom);

     u_int
     ns_get16(const u_char *src);

     u_long
     ns_get32(const u_char *src);

     void
     ns_put16(u_int src, u_char *dst);

     void
     ns_put32(u_long src, u_char *dst);

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 struc-ture structure
     ture _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 con-taining containing
     taining 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.

     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 meaningful with certain kernel configuration only.

     RES_USE_EDNS0  Enables support for OPT pseudo-RR for EDNS0 extension.  With the option, resolver code
                    will attach OPT pseudo-RR into DNS queries, to inform of our receive buffer size.  The
                    option will allow DNS servers to take advantage of non-default receive buffer size, and
                    to send larger replies.  DNS query packets with EDNS0 extension is not compatible with
                    non-EDNS0 DNS servers.

     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 following 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.  Con-stants Constants
     stants for type and class are defined in <arpa/nameser.h>.

     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 con-structs constructs
     structs 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 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 dn_skipname() function skips over a compressed domain name, which starts at a location pointed to
     by comp_dn.  The compressed name is contained in a query or reply message; eom is a pointer to the end
     of the message.  The size of compressed name is returned or -1 if there was an error.

     The ns_get16() function gets a 16-bit quantity from a buffer pointed to by src.

     The ns_get32() function gets a 32-bit quantity from a buffer pointed to by src.

     The ns_put16() function puts a 16-bit quantity src to a buffer pointed to by dst.

     The ns_put32() function puts a 32-bit quantity src to a buffer pointed to by dst.

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

SEE ALSO
     gethostbyname(3), resolver(5), hostname(7), named(8)

     RFC1_32, RFC1_33, RFC1_34, RFC1_35, RFC974

     Name Server Operations Guide for BIND.

HISTORY
     The res_query function appeared in 4.3BSD.

BSD                              June 4, 1993                              BSD