How do browser cookie domains work?

Question

Due to weird domain/subdomain cookie issues that I'm getting, I'd like to know how browsers handle cookies. If they do it in different ways, it would also be nice to know the differences.

In other words - when a browser receives a cookie, that cookie MAY have a domain and a path attached to it. Or not, in which case the browser probably substitutes some defaults for them. Question 1: what are they?

Later, when the browser is about to make a request, it checks its cookies and filters out the ones it should send for that request. It does so by matching them against the requests path and domain. Question 2: what are the matching rules?

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Added:

The reason I'm asking this is because I'm interested in some edge cases. Like:

• Will a cookie for .example.com be available for www.example.com?
• Will a cookie for .example.com be available for example.com?
• Will a cookie for example.com be available for www.example.com?
• Will a cookie for example.com be available for anotherexample.com?
• Will www.example.com be able to set cookie for example.com?
• Will www.example.com be able to set cookie for www2.example.com?
• Will www.example.com be able to set cookie for .com?
• Etc.

Added 2:

Also, could someone suggest how I should set a cookie so that:

• It can be set by either www.example.com or example.com;
• It is accessible by both www.example.com and example.com.

Answer 1

Although there is the RFC 2965 (Set-Cookie2, had already obsoleted RFC 2109) that should define the cookie nowadays, most browsers don’t fully support that but just comply to the original specification by Netscape.

There is a distinction between the Domain attribute value and the effective domain: the former is taken from the Set-Cookie header field and the latter is the interpretation of that attribute value. According to the RFC 2965, the following should apply:

• If the Set-Cookie header field does not have a Domain attribute, the effective domain is the domain of the request.
• If there is a Domain attribute present, its value will be used as effective domain (if the value does not start with a . it will be added by the client).

Having the effective domain it must also domain-match the current requested domain for being set; otherwise the cookie will be revised. The same rule applies for choosing the cookies to be sent in a request.

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Mapping this knowledge onto your questions, the following should apply:

• Cookie with Domain=.example.com will be available for www.example.com
• Cookie with Domain=.example.com will be available for example.com
• Cookie with Domain=example.com will be converted to .example.com and thus will also be available for www.example.com
• Cookie with Domain=example.com will not be available for anotherexample.com
• www.example.com will be able to set cookie for example.com
• www.example.com will not be able to set cookie for www2.example.com
• www.example.com will not be able to set cookie for .com

And to set and read a cookie for/by www.example.com and example.com, set it for .www.example.com and .example.com respectively. But the first (.www.example.com) will only be accessible for other domains below that domain (e.g. foo.www.example.com or bar.www.example.com) where .example.com can also be accessed by any other domain below example.com (e.g. foo.example.com or bar.example.com).

Answer 2

The previous answers are a little outdated.

RFC 6265 was published in 2011, based on the browser consensus at that time. Since then, there has been some complication with public suffix domains. I've written an article explaining the current situation - http://bayou.io/draft/cookie.domain.html

To summarize, rules to follow regarding cookie domain:

• The origin domain of a cookie is the domain of the originating request.

• If the origin domain is an IP, the cookie's domain attribute must not be set.

• If a cookie's domain attribute is not set, the cookie is only applicable to its origin domain.

• If a cookie's domain attribute is set,

  • the cookie is applicable to that domain and all its subdomains;
  • the cookie's domain must be the same as, or a parent of, the origin domain
  • the cookie's domain must not be a TLD, a public suffix, or a parent of a public suffix.

It can be derived that a cookie is always applicable to its origin domain.

The cookie domain should not have a leading dot, as in .foo.com - simply use foo.com

As an example,

• x.y.z.com can set a cookie domain to itself or parents - x.y.z.com, y.z.com, z.com. But not com, which is a public suffix.
• a cookie with domain=y.z.com is applicable to y.z.com, x.y.z.com, a.x.y.z.com etc.

Examples of public suffixes - com, edu, uk, co.uk, blogspot.com, compute.amazonaws.com