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InternetDomainNameExplained  
InternetDomainName tips and pitfalls.
Updated Jan 25, 2013 by cbe...@google.com

Introduction

InternetDomainName is a useful tool for parsing and manipulating domain names. It can be used as a validator, a component extractor, and as a value type for passing around domain names in a type-safe way.

However, there are some aspects of InternetDomainName behavior which may be surprising, and which can lead to bugs in calling code. This document addresses these concerns.

Details

Public suffixes and private domains

An InternetDomainName object is guaranteed to be syntactically valid according to relevant RFC specifications, but it is not guaranteed to correspond to an actual addressable domain on the Internet. It is impossible to do that without doing a net lookup of the domain and trying to contact it, and that is unacceptable overhead for most common cases.

Still, it is often very useful to determine whether a given domain name might represent an actual domain on the Internet. For this purpose, we use data from the Public Suffix List (PSL), a list maintained by the Mozilla Foundation. There are methods on InternetDomainName to determine the relationship of a given domain to the PSL. To put it in its most basic terms, if domain.hasPublicSuffix() returns true, then the domain might correspond to a real Internet address; otherwise, it almost certainly does not.

At this point we need to back up and define some terms. There are three terms of interest:

  • Top-Level Domain (TLD): A single-label domain managed by ICANN, such as com or au.
  • Public suffix: A domain under which people can register subdomains, and on which cookies should not be set.
  • Effective Top-Level Domain: A deprecated synonym for "public suffix".

It's worth reading the linked articles carefully before proceeding.

A major source of confusion is that people say "TLD" when they mean "public suffix". These are independent concepts. So, for example,

  • uk is a TLD, but not a public suffix
  • co.uk is a public suffix, but not a TLD
  • squerf is neither a TLD nor a public suffix
  • com is both a TLD and a public suffix

This confusion is especially dangerous because TLD has a crisp, formal definition, while public suffix does not. In the end, a public suffix is something that a credible source has asked the PSL maintainers to add to the list. Credible sources include ICANN and country-domain managers, but also include private companies offering services that share the characteristics that (fuzzily) define a public suffix -- independent subdomains and supercookie suppression. So, for example, many Google-owned domains (e.g. blogspot.com) are included in the PSL.

Getting back to InternetDomainName, as long as we limit ourselves to using hasPublicSuffix() to validate that the domain is a plausible Internet domain, all is well. The danger arises from the methods that identify or extract the "top private domain". From a technical point of view, the top private domain is simply the rightmost superdomain preceding the public suffix. So for example, www.foo.co.uk has a public suffix of co.uk, and a top private domain of foo.co.uk.

As noted in the documentation on isUnderPublicSuffix(), isTopPrivateDomain(), and topPrivateDomain(), the only thing these methods are (mostly) reliable for is determining where one can set cookies. However, what many people are actually trying to do is find the "real" domain, or the "owner" domain, from a subdomain. For example, in mail.google.com they would like to identify google.com as the owner domain. So they write

InternetDomainName owner = InternetDomainName.from("mail.google.com").topPrivateDomain();

...and sure enough, owner ends up with the domain google.com. Indeed, this idiom (and ones like it) work a great deal of the time. It seems intuitive that "the domain under the public suffix" should be semantically equivalent to "the owner domain".

But it's not, and therein lies the problem. Consider blogspot.com, which appears in the PSL. While it has the characteristics of a public suffix -- people can register domains under it (for their blogs), and cookies should not be allowed to be set on it (to prevent cross-blog cookie shenanigans), it is itself an addressable domain on the Internet (which happens to redirect to blogger.com as I write this, but that could easily change).

So if I use the idiom above on foo.blogspot.com, owner will be that same domain, foo.blogspot.com. This is the right answer in the terms we have been discussing, but it is obviously surprising to many people.

The big lessons here are:

  • TLDs and public suffixes are not the same thing.
  • Public suffixes are defined by humans, for strictly limited purposes (mostly domain validation and supercookie prevention), and change unpredictably.
  • There is no defined mapping between the relationship of a given domain to a public suffix, the ability of that domain to respond to web requests, and the "ownership" of that domain.
  • You can use InternetDomainName to determine whether a given string represents a plausibly addressable domain on the Internet, and to determine what portion of a domain is likely to allow cookies to be set.
  • You cannot use InternetDomainName to determine if a domain exists on the Internet as an addressable host, nor what superdomain "owns" that domain in a business or administrative sense.

Remember that if you do not heed this advice, your code will appear to work on a huge variety of inputs...but the failure cases are all bugs just waiting to happen, and the set of failure cases will change as PSL updates are incorporated into the code underlying InternetDomainName.

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