Just one?
Squid.
By default Squid complains if it finds an underscore in a URL hostname. It returns an "Invalid URL" error message and explains that underscores are not allowed in hostnames. Of course you can make Squid accept underscores if you prefer. We felt this was better than returning a "the domain name does not exist" error message. It sucks for the user when a name can be resolved by one machine or by one application, but not by another. Even on FreeBSD you get different answers from different apps: chef-wessels ~ 8> host super_bikes.tripod.com super_bikes.tripod.com has address 209.202.240.100 chef-wessels ~ 9> ping super_bikes.tripod.com ping: cannot resolve super_bikes.tripod.com: Unknown server error Duane W.