Dashing internet host names

I ran into a fun problem today. The HTTP session cookie for one of my websites was not being retained in the Safari web browser. Packet sniffing clearly showed the server was sending a ‘Set-Cookie’ in the HTTP header. Safari accepted the cookie just fine with other virtual hosts having identical code base and similar configurations for Apache webserver and Tomcat. FireFox 3 accepted the cookie from all the virtual hosts. It was quite a perplexing issue.

It turns out that Safari, Version 3.1.1 at least, does not retain cookies from hosts with an underscore (_) in its name and my troubled website host did indeed have one in its name. I changed the underscore to a dash (-) and Safari was happy.

Who woulda thunk?

Related:

Discussion of the issue from the Ruby Forum
Internet Explorer probably would have given me the same behaviour, but I didn’t test it.
Host name specifications are documented in RFC 952 and RFC 1035. The underscore is not among valid characters.