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Re: DNS problems to RoadRunner - tcp vs udp
On 15/06/2008, at 9:18 AM, Scott McGrath wrote:
Yes - we are blocking TCP too many problems with drone armies and we started about a year ago when our DNS servers became unresponsive for no apparent reason. Investigation showed TCP flows of hundreds of megabits/sec and connection table overflows from tens of thousands of bots all trying to simultaneously do zone transfers and failing tried active denial systems and shunning with limited effectiveness.
There's really two problems here - one is packet/bit rate causing problems for your network, that's not necessarily an end system thing. Not really DNS specific, and blocking 53/TCP doesn't really help here as people could just send 53/UDP your way and get the same effect.
Connection table overflowing is a bit of a different issue, obvious way to overcome that is to whack a load balancer in there to share the load around. It's not immediately obvious to me why your connection table would be filling up - what state were connections stuck in?
Anyway, one thought that comes to me would be to split off UDP and TCP services to different servers - if some TCP attack kills your TCP DNS server you:
a) don't have to worry about UDP services failing.
b) can turn it off for the duration of the attack, and are no worse off than you are right now, then turn it back on when you see the high volume of SYN messages disappear.
c) as TCP DNS service recovery isn't super time critical (I'm assuming this, because you're not running it at all right now) you have time to look at the anatomy of the attack and figure out how to filter it more precisely if possible, instead of simply dropping all TCP.
Obviously, you'd want to make sure TCP from your other name servers always goes to the UDP one, etc. etc.
-- Nathan Ward