Third-party / 3rd-party DNS (3pdns) & Dynamic DNS (DDNS) Service Providers

Online Tools

dnsreport.com
Relatively extensive checks on DNS correctness with easy-to-understand results. Includes citations to sections of relevant RFCs in the event of problems. Also checks SMTP server behavior and looks for www within the zone.

checkdns.net
Relatively extensive checks on DNS correctness with easy-to-understand results. Less detail than dnsreport.com Also checks SMTP and HTTP server behavior, based on MX records and www names.

squish.net dns checker
Enumerates all the possible paths that a query could take, starting from the roots to final resolution. Tests one query type at a time. Generates quite a lot of output that is difficult to parse. The "Servers involved in resolving" table at the end is neat.

dnscheck.se
A persistent database of zones that are periodically re-checked. Associates a color-coded status with each zone. The logfiles are a little difficult to read. For example, its not immediately clear where it stops querying one nameserver and begins querying the next.

DNS Bajaj
Graphically displays the relationships between a zone and its nameserver delgations, starting at one of the roots. Also includes some textual information and warnings about the zone.

ZoneCheck
This is the online demo of the ZoneCheck software. In addition to a few standard DNS checks, it also tests for TCP, UDP, and ICMP connectivity.

Pingability
This tool checks nameservers for lame delegations, geographic location, matching reverse entries, open MX relays, and more.

DNSDoctor
This tool also performs a suite of checks against the authoritative nameservers for a zone.

Validation Software

dns-package.pl
The Perl script used by dnscheck.se.

dnstracer
Textually displays the hierarchy of nameservers involved in a target name. Reports on certain errors, such as lame delegation. Here is an example.

ZoneCheck
The ZoneCheck software uses ruby and provides three interfaces: CGI, CLI, and X11.

DoC (Domain Obscenity Control)
This software was formerly maintained by Brad Knowles. His site hosts version 2.1.4. It seems that others continued its development, e.g. version 2.2.3.

dnswalk
This program debugs DNS problems by making zone transfers from live servers and then validating the records that it finds. Here is an example.

dlint
This /bin/sh script calls dig and other utilities to perform consistency checks upon a zone. Here is an example.

Other interesting software

fpdns
A DNS fingerprinter in Perl. Uses an extensive list of heuristics to determine what is behind a particular nameserver. Here is an example.

dns-check
A nifty /bin/bash script from William Stearns that you can run periodically to detect changes in DNS records. 

Pages like this one

Tools to manage DNS
From the dns.net site.

DNS Diagnostic Tools
From netsys.com.

DNS ports
DNS-related software from the FreeBSD Ports collection.