You can hava a good read at http://www.grc.com/dos/grcdos.htm

Seems like you are under attack my fren. Same thing here, was down from 21FEB till 26FEB, constantly under attack. Basically it is not your fault. Nothing you can do to prevent an attack. Luckily I got good data center architecture, only effect my service domain, rest is all up and running, email, ftp etc. None of my customers were effected.