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:

Set-Cookie:
    name=value;
    domain=localhost;
    expires=Thu, 16-Jul-2009 21:25:05 GMT;
    path=/

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

Set-Cookie:
    name=value;
    domain=.localhost;
    expires=Thu, 16-Jul-2009 21:25:05 GMT;
    path=/

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.

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