Internet Explorer ignores cookies on some domains (cannot read or set cookies)

Question

Does one of the subdomains use an underscore? IE has problems accepting cookies from subdomains that don't follow the URI RFC.

According to RFC1035 (Domain names - implementation and specification):

[domain names] must start with a letter, end with a letter or digit, and have as interior characters only letters, digits, and hyphen.

Turns out some of the domains had an underscore ( "_" ) in them: some_domain.example.com. Although this is a violation of the RFC, all other browsers work normally.

MSIE 7, on a domain with an underscore, silently drops all cookies for that host and refuses to accept new ones.

The only solution is to use RFC-compliant domains (I've replaced all the "_"s with "-"s and set up a RewriteRule so that traffic is redirected to the compliant domains).

The problem doesn't only apply for underscores in domain names, but also for domain names starting with a numeric digit. So 1aaaaaaa.tld is actually a non-standard domain name, which will cause IE to reject the cookie.

I solved it by using only www2.1aaaaaaa.tld, and then adding rewrite rules for the 1aaaaaaa.tld and www.1aaaaaaa.tld hosts in .htaccess. Don't know if that really qualifies as a standards-compliant solution.... but anyway, it seems to have solved the cookie problem.

Hope that helps someone!