IS CS 1.6 compatiable with reverse proxy based DDOS protection

This seems a little off-topic for this site but depending on where this server is (If you're running it out of your house) You could have a look into messing with the apache module called Mod_Evasive as explained by our sister site Security.stackexchange.com I was going to have you look into a program called Snort as well but do keep in mind that these are based on having administrator access to a linux machine.

If what I'm saying sounds like gibberish then you should request this information from your hosts and see if they are actively protecting against DDoS attacks.

If connecting directly to the IP works, then you could use No-IP for a free domain redirect to the IP address as well.

It also sounds as though the domain name may be misconfigured if you are able to connect to the IP as if you were being DDoS'ed then the ENTIRE SERVER would lag down and falter out on any form of connection.