Cookies on localhost with explicit domain


I must be missing some basic thing about cookies. On localhost, when I set a cookie on server side and specify the domain explicitly as localhost (or .localhost). the cookie does not seem to be accepted by some browsers.

Firefox 3.5: I checked the HTTP request in Firebug. What I see is:

    expires=Thu, 16-Jul-2009 21:25:05 GMT;

or (when I set the domain to .localhost):

    expires=Thu, 16-Jul-2009 21:25:05 GMT;

In either case, the cookie is not stored.

IE8: I did not use any extra tool, but the cookie does not seem to be stored as well, because it’s not being sent back in subsequent requests.

Opera 9.64: Both localhost and .localhost work, but when I check the list of cookies in Preferences, the domain is set to localhost.local even though it’s listed under localhost (in the list grouping).

Safari 4: Both localhost and .localhost work, but they are always listed as .localhost in Preferences. On the other hand, a cookie without an explicit domain, it being shown as just localhost (no dot).

What is the problem with localhost? Because of such a number of inconsistencies, there must be some special rules involving localhost. Also, it’s not completely clear to me why domains must be prefixed by a dot? RFC 2109 explicitly states that:

The value for the Domain attribute
contains no embedded dots or does not
start with a dot.

Why? The document indicates that it has to do something with security. I have to admit that I have not read the entire specification (may do it later), but it sounds a bit strange. Based on this, setting cookies on localhost would be impossible.

Best Solution

By design, domain names must have at least two dots; otherwise the browser will consider them invalid. (See reference on

When working on localhost, the cookie domain must be omitted entirely. You should not set it to "" or NULL or FALSE instead of "localhost". It is not enough.

For PHP, see comments on

If working with the Java Servlet API, don't call the cookie.setDomain("...") method at all.