JDNS

Date: October 1st, 2005 Author: Justin Karneges justin@karneges.com

JDNS is a simple DNS implementation that can perform normal DNS queries of any record type (notably SRV), as well as Multicast DNS queries and advertising. Multicast support is based on Jeremie Miller's "mdnsd" implementation.

For maximum flexibility, JDNS is written in C with no direct dependencies, and is licensed under the MIT license. Your application must supply functionality to JDNS, such as UDP sending/receiving, via callbacks.

For Qt users there is a wrapper available called QJDns. jdns.pri can be used to include everything into a qmake project. CMake-project will build the sample Qt-based commandline tool 'jdns'.

Features:

• DNS client "stub" resolver
• Can fetch any record type, but provides handy decoding for many known types: A, AAAA, SRV, MX, TXT, etc.
• Performs retries, caching/expiration, and CNAME following
• Algorithm logic adapted from Q3Dns
• Multicast queries
• Multicast advertising

Why?

• Trolltech is phasing out the Qt DNS implementation, which in Qt 4 has been relegated to the Qt3Support module. A replacement was desired.

• While there are many DNS libraries available, at the time of this writing it was (and still may be) hard to find one that satisfies three essential conditions: cross-platform friendliness (and this includes Windows 9x!), the ability to integrate into existing eventloops, sensible licensing (ie, not GPL).

How to use:

• Prepare callbacks and call jdns_session_new()
• Call jdns_init_unicast() or jdns_init_multicast(), depending on if you want regular or multicast DNS. If you want both kinds, you can always make two sessions.
• Make queries and have fun
• Call jdns_step() at the right times to advance JDNS processing

What is left to you:

• The callback functions, obviously.
• Querying for several "qualified" names. Here is what Q3Dns does: Query for name as provided Query for name + '.domain' (for every domain the computer is in)
• Detecting for '.local' in a name to be queried, and using that to decide whether to query via Multicast or normal DNS.
• Recognition of IP addresses. If you want an IP address to resolve to itself, then do that yourself. Passing an IP address as a DNS name to JDNS won't work (especially since it wouldn't make any sense in some contexts, like SRV).
• Recognition of known hosts. If you want this, compare inputs against jdns_system_dnsparams().
• For zeroconf/Bonjour, keep in mind that JDNS only provides Multicast DNS capability. DNS-SD and any higher layers would be your job.

Using a custom DNS implementation has the drawback that it is difficult to take advantage of platform-specific features (for example, an OS-wide DNS cache or LDAP integration).

An application strategy for normal DNS should probably be:

• If an A or AAAA record is desired, use a native lookup.
• Else, if the platform has advanced DNS features already (ie, res_query), use those.
• Else, use JDNS.

However, it may not be a bad idea at first to use JDNS for all occasions, so that it can be debugged.

For Multicast DNS, awareness of the platform is doubly important. There should only be one Multicast DNS "Responder" per computer, and using JDNS at the same time could result in a conflict.

An application strategy for Multicast DNS should be:

• If the platform has a Multicast DNS daemon installed already, use it somehow.
• Else, use JDNS.

Have fun!