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NAME    [Toc]    [Back]

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

LIBRARY    [Toc]    [Back]

     Standard C Library (libc, -lc)

SYNOPSIS    [Toc]    [Back]

     #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 char *data, int datalen, struct rrec *newrr, 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, u_char *exp_dn, int length);

DESCRIPTION    [Toc]    [Back]

     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

     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

     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
     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 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 resolv.conf(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.  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

FILES    [Toc]    [Back]

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

SEE ALSO    [Toc]    [Back]

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

     RFC974, RFC1032, RFC1033, RFC1034, RFC1035, RFC1535

     Name Server Operations Guide for BIND.

HISTORY    [Toc]    [Back]

     The res_query function appeared in 4.3BSD.

BSD                              June 4, 1993                              BSD
[ Back ]
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