adns

   Advanced, easy to use, asynchronous-capable DNS client library.

   adns is a resolver library for C (and C++) programs. In contrast with
   the existing interfaces, gethostbyname et al and libresolv, it has the
   following features:
     * It is reasonably easy to use for simple programs which just want
       to translate names to addresses, look up MX records, etc.
     * It can be used in an asynchronous, non-blocking, manner. Many
       queries can be handled simultaneously.
     * Responses are decoded automatically into a natural representation
       for a C program - there is no need to deal with DNS packet
       formats.
     * Sanity checking (eg, name syntax checking, reverse/forward
       correspondence, CNAME pointing to CNAME) is performed
       automatically.
     * Time-to-live, CNAME and other similar information is returned in
       an easy-to-use form, without affecting ease-of-use.
     * There is no global state in the library; resolver state is an
       opaque data structure which the client creates explicitly. A
       program can have several instances of the resolver.
     * Errors are reported to the application in a way that distinguishes
       the various causes of failure properly.
     * Understands conventional resolv.conf, but this can overridden by
       environment variables.
     * Flexibility. For example, the application can tell adns to: ignore
       environment variables (for setuid programs), disable sanity checks
       eg to return arbitrary data, override or ignore resolv.conf in
       favour of supplied configuration, change its caching behaviour,
       use a different flow of control model, etc.
     * Believed to be correct ! For example, will correctly back off to
       TCP in case of long replies or queries, or to other nameservers if
       several are available. It has sensible handling of bad responses
       etc.

Forthcoming:
   
   I hope that future versions may also have the following features:
     * The library can be used by threads in a multithreaded program in a
       natural way. It will multiplex many threads' queries through a
       single query socket.
     * Limited caching behaviour.
     * IPv6 support.

   (Technical note: adns requires a real nameserver like BIND running on
   the same system or a nearby one, which must be willing to provide
   `recursive service'. I.e., adns is a `stub resolver'. All properly
   configured UN*X and GNU systems will already have such nameserver(s);
   they are usually listed in /etc/resolv.conf.)