libasyncns 0.8

Copyright 2005-2009 Lennart Poettering <mznflapaf (at) 0pointer (dot) de>


This program is free software; you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation, either version 2.1 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more details.


Thu 15 Oct 2009:

Version 0.8 released; changes include: various smaller fixes.

Tue 28 Oct 2008:

Version 0.7 released; changes include: rework thread shutdown logic.

Fri 24 Oct 2008:

Version 0.6 released; changes include: save and restore h_errno in addition to errno; drop usage of pthread's cancelling since it is problematic on some platforms.

Sun 17 Aug 2008:

Version 0.5 released; changes include: fix getaddrinfo() serialization.

Sun 27 Jul 2008:

Version 0.4 released; changes include: rework error handling logic, add asyncns_freeanswer(), a lot of fixes

Wed 23 May 2007:

Version 0.3 released; changes include: build fixes: properly detect res_query() on Linux/AMD64, support older autoconf versions

Fri 16 Feb 2007:

Version 0.2 released; changes include: res_query() support; minor bug fixes; doc updates

Sat 8 Jan 2005:

Version 0.1 released


libasyncns is a C library for Linux/Unix for executing name service queries asynchronously. It is an asynchronous wrapper around getaddrinfo(3), getnameinfo(3), res_query(3) and res_search(3) from libc and libresolv.

In contrast to GNU's asynchronous name resolving API getaddrinfo_a(), libasyncns does not make use of UNIX signals for reporting completion of name queries. Instead, the API exports a standard UNIX file descriptor which may be integerated cleanly into custom main loops.

In contrast to asynchronous DNS resolving libraries like libdenise, skadns, adns, libasyncns is just an asynchronous wrapper around the libc's synchronous getaddrinfo() API, which has the advantage of allowing name resolution using techniques like Multicast DNS, LDAP or NIS using standard libc NSS (Name Service Switch) modules. libasyncns is compatible with IPv6 if the underlying libc is.

libasyncns is very tiny, consisting of just one header and one source file. It has no dependencies besides libc.

By default libasyncns spawns a number of worker threads (LWPs) to process name queries. Alternatively or when POSIX Threads are not supported, libasyncns can fork() off worker processes instead.

Current Status

Version 0.8 is quite stable and feature complete.

Damien Thébault has written a Python module libasyncns-python that wraps the libasyncns C API.


You may browse the Doxygen generated programing documentation the API. (Run make doxygen to generate this documentation from the source tree)


Currently, libasyncns is tested on Linux and Solaris (sparc-sun-solaris2.8) only.

libasyncns was developed and tested on Debian GNU/Linux "testing" from January 2005, it should work on most other Linux distributions (and maybe Unix versions) since it uses GNU autoconf and GNU libtool for source code configuration and shared library management.

libasyncns has no dependencies besides a libc that supports getaddrinfo() and res_query().


As this package is made with the GNU autotools you should run ./configure inside the distribution directory for configuring the source tree. After that you should run make for compilation and make install (as root) for installation of libasyncns.


Sjoerd Simons for the res_query() support.


The newest release is always available from

The current release is 0.8

Get libasyncns's development sources from the GIT repository (gitweb):

git clone git://

You may find an up to date Debian package of libasyncns on the Debian package repository.

If you want to be notified whenever I release a new version of this software use the subscription feature of Freshmeat.

Lennart Poettering <mznflapaf (at) 0pointer (dot) de>, October 2009