Website Firewall

[Dave]

What kind of firewall are we talking about?
To prevent bots? To prevent website attacks such as SQL injection and such? To prevent (D)DoS attacks?

I don't use any because I have made my own very basic filtering function in PHP for vBulletin that should prevent any 0day attacks. (D)DoS attacks can not be prevented with a WAF. But besides that there's always some way to bypass a WAF because it simply checks the request sent to the server for certain patterns or strings.

Cloudflare and Incapsula are good examples of DNS providers who also provide WAF and DDoS protection.