Re: [gentoo-user] How to prevent a dns amplification attack

[Michael Mol <mikemol@gmail.com>]

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On 03/28/2013 12:06 PM, Pandu Poluan wrote:
> 
> On Mar 28, 2013 10:38 PM, "Michael Mol" <mikemol@gmail.com
> <mailto:mikemol@gmail.com>> wrote:
>>
>> On 03/28/2013 04:51 AM, Norman Rieß wrote:
>> > Hello,
>> >
>> > i am using pdns recursor to provide a dns server which should be usable
>> > for everybody.The problem is, that the server seems to be used in dns
>> > amplification attacks.
>> > I googled around on how to prevent this but did not really find
>> > something usefull.
>> >
>> > Does anyone got an idea about this?
>>
>> I'm not sure it can be done. You can't make a resolver available to
>> "everybody" without somebody in that "everybody" group abusing it, and
>> that's exacly what happens in a DNS amplification attack.
>>
>> Restrict your resolver to be accessible only to your network or, at
>> most, those of the specific group of people you're seeking to help.
>>
>> You *might* try restricting the resolver to only respond to TCP requests
>> rather than UDP requests, but if the resolver sends response data along
>> with that first SYN+ACK, then nothing is solved, and you've opened
>> yourself up to a SYN flood-based DoS attack. (OTOH, if your resolver
>> went offline as a result of a SYN flood, at least it wouldn't be part of
>> an amplification attack any longer...)
>>
> 
> Can't we rate limit UDP DNS request?
> 
> E.g., limit each source IP to, let's say, 1 UDP per second?
> 
> That should be doable easily using iptables.

That makes the resolver highly unreliable for normal use. Many sites
trigger resource grabs from 10-15 different domains. If all but the
first request is dropped due to rate limiting, you're going to have a
very, very broken experience.



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