Getting a lot of hits on my web server and it's taking down my httpd. Possible DDoS? [duplicate]
Solution 1:
Are these domains you are hosting? I suspect not.
I've seen a big increase lately in scanning for open http proxies on my machines - it looks like you may running an open http proxy (which is just as bad as running an open mail relay - worse even, since most people now implement carious mitigations like RBL and SPF).
Disable proxying / add authentication / restrict to your LAN addresses.
OTOH if you really are webmaster for all these domains then have a look at mod_evasive and mod_security.