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     dnsres_init, dnsres_gethostbyname, dnsres_gethostbyname2,
     dnsres_gethostbyaddr, dnsres_getaddrinfo - non blocking DNS resolving


     #include <dnsres.h>

     dnsres_init(struct dnsres *_resp);

     dnsres_gethostbyname(struct dnsres* res, const char *name,
             void (*cb)(struct hostent *hp, int error, void *arg, void *arg);

     dnsres_gethostbyname2(const char *name, int af,
             void (*cb)(struct hostent *hp, int error, void *arg, void *arg);

     dnsres_gethostbyaddr(const char *addr, int len, int af,
             void (*cb)(struct hostent *hp, int error, void *arg, void *arg);

     dnsres_getaddrinfo(struct dnsres *, const char *, const char *,
             const struct addrinfo *,
             void (*)(struct addrinfo *, int, void *), void *);;


     The dnsres_init() is used to initialize the dnsres library.  If you are
     developing a multi-threaded application, you need one struct dnsres per

     The dnsres_gethostbyname(), dnsres_gethostbyname2() and
     dnsres_gethostbyaddr() functions each call their provided callback
     function with a pointer to an object with the following structure
     describing an internet host referenced by name or by address,
     respectively.  This structure contains either information obtained from
     the name server (i.e., resolver(3) and named(8)), broken-out fields from
     a line in /etc/hosts, or database entries supplied by the yp(8) system.
     resolv.conf(5) describes how the particular database is chosen.

     struct  dnsres_hostent {
             char    *h_name;        /* official name of host */
             char    **h_aliases;    /* alias list */
             int     h_addrtype;     /* host address type */
             int     h_length;       /* length of address */
             char    **h_addr_list;  /* list of addresses from name server */

     The members of this structure are:

     h_name       Official name of the host.

     h_aliases    A NULL-terminated array of alternate names for the host.

     h_addrtype   The type of address being returned.

     h_length     The length, in bytes, of the address.

     h_addr_list  A zero-terminated array of network addresses for the host.
                  Host addresses are returned in network byte order.

     h_addr       The first address in h_addr_list; this is for backward

     The function dnsres_gethostbyname() will search for the named host in the
     current domain and its parents using the search lookup semantics detailed
     in resolv.conf(5) and hostname(7).

     dnsres_gethostbyname2() is an advanced form of gethostbyname() which
     allows lookups in address families other than AF_INET.  Currently, the
     only supported address family besides AF_INET is AF_INET6.

     The dnsres_gethostbyaddr() function will search for the specified address
     of length len in the address family af.  The only address family
     currently supported is AF_INET.


     HOSTALIASES  A file containing local host aliases.  See hostname(7) for
                  more information.

     RES_OPTIONS  A list of options to override the resolver’s internal
                  defaults.  See resolver(3) for more information.




     Error return status from dnsres_gethostbyname(), dnsres_gethostbyname2(),
     and dnsres_gethostbyaddr() is indicated by a null pointer passed to the
     callback function.  The integer dr_errno may then be checked in struct
     dnsres to see whether this is a temporary failure or an invalid or
     unknown host.

     The variable h_errno can have the following values:

     DNSRES_HOST_NOT_FOUND  No such host is known.

     DNSRES_TRY_AGAIN       This is usually a temporary error and means that
                            the local server did not receive a response from
                            an authoritative server.  A retry at some later
                            time may succeed.

     DNSRES_NO_RECOVERY     Some unexpected server failure was encountered.
                            This is a non-recoverable error.

     DNSRES_NO_DATA         The requested name is valid but does not have an
                            IP address; this is not a temporary error.  This
                            means that the name is known to the name server
                            but there is no address associated with this name.
                            Another type of request to the name server using
                            this domain name will result in an answer; for
                            example, a mail-forwarder may be registered for
                            this domain.

     DNSRES_NETDB_INTERNAL  An internal error occurred.  This may occurs when
                            an address family other than AF_INET or AF_INET6
                            is specified or when a resource is unable to be

     DNSRES_NETDB_SUCCESS   The function completed successfully.


     getnameinfo(3), resolver(3), hosts(5), resolv.conf(5), hostname(7),


     The herror() function appeared in 4.3BSD.  The endhostent(),
     gethostbyaddr(), gethostbyname(), gethostent(), and sethostent()
     functions appeared in 4.2BSD.


     If the search routines in resolv.conf(5) decide to read the /etc/hosts
     file, gethostent() and other functions will read the next line of the
     file, re-opening the file if necessary.

     The sethostent() function opens and/or rewinds the file /etc/hosts.  If
     the stayopen argument is non-zero, the file will not be closed after each
     call to gethostbyname(), gethostbyname2(), or gethostbyaddr().

     The endhostent() function closes the file.


     The dnsres library was hacked together by Niels Provos.  Heavy use was
     made of the exisiting BSD resolver library.