Command Section
RESOLVER(3)            FreeBSD Library Functions Manual            RESOLVER(3)

     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

     Standard C Library (libc, -lc)

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

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

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

     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);

     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,
         u_char **lastdnptr);

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

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

     ns_get16(const u_char *src);

     ns_get32(const u_char *src);

     ns_put16(u_int src, u_char *dst);

     ns_put32(u_long src, u_char *dst);

     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

     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

     RES_IGNTC      Unused currently (ignore truncation errors, i.e., do not
                    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

     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, will be returned as ::ffff:
                    The option is meaningful with certain kernel configuration

     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

     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.  The newrr_in argument 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

     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.

     This implementation of the resolver is thread-safe, but it will not
     function properly if the programmer attempts to declare his or her own
     _res structure in an attempt to replace the per-thread version referred
     to by that macro.

     The following compile-time option can be specified to change the default
     behavior of resolver routines when necessary.

     RES_ENFORCE_RFC1034  If this symbol is defined during compile-time,
                          res_search() will enforce RFC 1034 check, namely,
                          disallow using of underscore character within host
                          names.  This is used by the standard host lookup
                          routines like gethostbyname(3).  For compatibility
                          reasons this option is not enabled by default.

     The res_init() function will return 0 on success, or -1 in a threaded
     program if per-thread storage could not be allocated.

     The res_mkquery(), res_search(), and res_query() functions return the
     size of the response on success, or -1 if an error occurs.  The integer
     h_errno may be checked to determine the reason for error.  See
     gethostbyname(3) for more information.

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

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

     RFC1032, RFC1033, RFC1034, RFC1035, RFC974

     Name Server Operations Guide for BIND.

     The res_query function appeared in 4.3BSD.

FreeBSD 11.1-RELEASE-p4          May 29, 2009          FreeBSD 11.1-RELEASE-p4
Command Section