adns - advanced, alternative, asynchronous resolver
Keywords: adns

# GNU adns

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 and RRDATA formats.
- Sanity checking (eg, name syntax checking, reverse/forward correspondence, CNAME pointing to CNAME) is performed automatically by default.
- 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.
- adns 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.

## 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.

Bug reports should be reported to the GNU Debbugs. Send an email to `submit@debbugs.gnu.org` and at the top of your email, in a paragraph of its own, write the single line

    Package: adns

Your bug report will be published via to the adns-discuss list.

Feedback and discussion takes place on the `adns-discuss` list. You can mail me privately at `ijackson@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.

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`.

## Documentation

- adns.h API header file with documentation comments
- usage message for adnshost

## Download and source code

- The current release as a gzipped tarfile.
- Previous versions and other files (including OpenPGP signatures).
- master git (version control) repository browser.

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

## Installation 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'.

adns requires that your real nameserver is on the same machine, or connected via a secure network, so that an attacker cannot fake the replies to adns's queries.

## References and related projects

- Python bindings by Andy Dustman.
- liboop event loop library has a built-in binding for adns.
- port to MS Visual Studio 6 C++ by Jarle Aase.

## Copyright and licensing

`adns` is Copyright 1997-2000,2003,2006,2014-2016,2020 Ian Jackson, Copyright 2014 Mark Wooding, Copyright 1999-2000,2003,2006 Tony Finch, and Copyright 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 3 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 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 Free Software Foundation or email `ijackson@chiark.greenend.org.uk`.

Ian Jackson / `ijackson@chiark.greenend.org.uk`.

GNU home page; chiark home page; site or mirror home page

This web page is Copyright (C)1996-2005,2014-2016 Ian Jackson. See the Copyright/acknowledgements.