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

adns is a resolver library for C (and C++) programs, and a collection of useful DNS resolver utilities.

C library

In contrast with the standard 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 getting in the way.
• 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 hostname syntax sanity checks to return arbitrary data, override or ignore resolv.conf in favour of supplied configuration, 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.

DNS utility programs

adns also comes with a number of utility programs for use from the command line and in scripts:

• adnslogres is a much faster version of Apache's logresolv program.
• adnsresfilter is a filter which copies its input to its output, replacing IP addresses by the corresponding names, without unduly delaying the output. For example, you can usefully pipe the output of netstat -n, tcpdump -ln, and the like, into it.
• adnshost is a general-purpose DNS lookup utility which can be used easily in from the command line and from shell scripts to do simple lookups. In a more advanced mode it can be used as a general-purpose DNS helper program for scripting languages which can invoke and communicate with subprocesses. See the adnshost usage message for a summary of its capabilities.

Forthcoming:

I hope that future versions may also have the following features:

• The library should be useable by threads in a multithreaded program in a natural way. It should multiplex many threads' queries through a single query socket.
• IPv6 support.
• Some kind of awareness of DNSSEC.
• Possibly some very limited caching behaviour.
• There was, at one point, a draft native Perl interface from Tony Finch.

Documentation

I'm afraid there is no manual yet. However, competent C programmers should be able to use the library based on the commented adns.h header file, and the usage messages for the programs should be sufficient.

Feedback

I'd be pleased if you would let me know if you're using my library in your project, and what you think of it.

If you are subscribed to adns-discuss please send feedback, including bug reports, there; otherwise send mail to adns-bugreports@chiark.greenend.org.uk. If you'd prefer that your message wasn't forwarded to the adns-bugreports list, send it to adns-maint@chiark.greenend.org.uk.

Mailinglists

I have set up mailinglists adns-announce and adns-discuss. The announcements list is moderated and will contain only announcements of important bugs, new versions, etc. The bug reports address mentioned above is also a mailing list; feel free to subscribe to it.

There are archives and subscription web pages, or you can subscribe by sending mail containing the word `subscribe' to adns-announce-REQUEST@chiark.greenend.org.uk or adns-discuss-REQUEST@chiark.greenend.org.uk.

Download

Available for download from chiark.greenend.org.uk are:

• The current release as a gzipped tarfile.
• adns.h API header file with comments, and usage message for adnshost (currently there is no manual, sorry).
• All versions released so far are also available via anonymous FTP and HTTP.

You can also access the project CVS repositories:

• A mirror of the source code repository is available via rsync from rsync.chiark.greenend.org.uk::ftp/users/ian/cvs-pub/adns (use FTP first to find your way around), or via cvsweb.
• This web page is maintained in Savannah, where you can browse the webpage CVS.

adns is also available from the GNU Project FTP servers and their mirrors.

Technical note

adns requires a real nameserver like BIND or Dents 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.

References and related projects

There is a port to MS Visual Studio 6 C++ by Jarle Aase.

Copyright and licensing