GNU adns

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

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
* Sanity checking (eg, name syntax checking, reverse/forward
correspondence, CNAME pointing to CNAME) is performed
* 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

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 [1]adnshost
usage message for a summary of its capabilities.


I'm afraid there is no manual yet. However, competent C programmers
should be able to use the library based on the [2]commented adns.h
header file, and the usage messages for the programs should be


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 If you'd prefer that your
message wasn't forwarded to the adns-bugreports list, send it to


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 [3]archives and subscription web pages, or you can subscribe
by sending mail containing the word `subscribe' to or


Available for download from [4] are:
* The [5]current release as a gzipped tarfile.
* [6]adns.h API header file with comments, and [7]usage message for
adnshost (currently there is no manual, sorry).
* All versions released so far are also available via [8]anonymous
FTP and [9]HTTP,
* A mirror of my CVS repository is available via rsync from (use FTP
first to find your way around), or via [10]cvsweb.

adns is also available from the [11]GNU Project FTP servers and their

Technical note

adns requires a real nameserver like [13]BIND or [14]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.

Copyright and licensing

adns is Copyright 1997-2000 Ian Jackson, Copyright 1999-2000 Tony
Finch, and Copyright (C) 1991 Massachusetts Institute of Technology.

adns is free software; you can redistribute it and/or modify it under
the terms of the GNU General Public License as published by the Free
Software Foundation; either version 2 of the License, or (at your
option) any later version.

This program and documentation 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
[15]GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with adns, or one should be available above; if not, write to
the [16]Free Software Foundation, 59 Temple Place - Suite 330, Boston,
MA 02111-1307, USA, or email

[17]Ian Jackson / [18]; more [19]free
software by me.

[20]GNU home page; [21]chiark home page; [22]site or mirror home page

This web page is Copyright (C)1996-2000 Ian Jackson. See the

Use any browser - [24]Campaign for a non-browser-specific WWW


22. file://localhost/