.NET domain name parsing library (uses http://publicsuffix.org rules)

A domain name has 3 major parts:

• The top level domain, or TLD (like .com, .net, .info)
• The domain name, or SLD (like google, microsoft, ebay)
• The subdomain (like www, photos)

Parsing a domain name into it's 3 major parts sounds easy, but is no trivial task. What happens when you come across hosts like test.co.uk? What about hosts like www.parliament.uk?

From http://publicsuffix.org :

"Since there is no algorithmic method of finding the highest level at which a domain may be registered for a particular top-level domain (the policies differ with each registry), the only method is to create a list. This is the aim of the Public Suffix List."

The domain name parsing component uses the list of rules at www.publicsuffix.org to parse a domain name into 3 component parts.

There are 3 types of rules: 'Normal' domain rules, 'Wildcard' rules, and 'Exception' rules.

Download the latest release from the NuGet repository

Install-Package domainname-parser

Download the latest rules file from https://publicsuffix.org/list/ and configure your app.config to point to the rules file you just downloaded (see the sample app.config)

Using the component is simple. Just use the constructor or the static TryParse method and pass in the complete host name string. The component will return the parsed domain in a DomainName component. It's as simple as that:

// Try parsing a 'wildcard' domain 
if (DomainName.TryParse("photos.verybritish.co.uk", out outDomain)) 
{ 
  // The domain should be parsed as 'verybritish' 
  Assert.AreEqual("verybritish", outDomain.Domain);
  
  // The TLD is 'co.uk' 
  Assert.AreEqual("co.uk", outDomain.TLD);
  
  // The SLD is just an alias for 'Domain': 
  Assert.AreEqual(outDomain.Domain, outDomain.SLD);
  
  // The subdomain is everything else to the left of the domain: 
  Assert.AreEqual("photos", outDomain.SubDomain); 
} 
else 
{ 
  Debug.WriteLine("Apparently, we couldn't parse photos.verybritish.co.uk"); 
}

You can download the latest rules from the Public Suffix site: http://publicsuffix.org/list/