1 |
On 03/28/2013 12:06 PM, Pandu Poluan wrote: |
2 |
> |
3 |
> On Mar 28, 2013 10:38 PM, "Michael Mol" <mikemol@×××××.com |
4 |
> <mailto:mikemol@×××××.com>> wrote: |
5 |
>> |
6 |
>> On 03/28/2013 04:51 AM, Norman Rieß wrote: |
7 |
>> > Hello, |
8 |
>> > |
9 |
>> > i am using pdns recursor to provide a dns server which should be usable |
10 |
>> > for everybody.The problem is, that the server seems to be used in dns |
11 |
>> > amplification attacks. |
12 |
>> > I googled around on how to prevent this but did not really find |
13 |
>> > something usefull. |
14 |
>> > |
15 |
>> > Does anyone got an idea about this? |
16 |
>> |
17 |
>> I'm not sure it can be done. You can't make a resolver available to |
18 |
>> "everybody" without somebody in that "everybody" group abusing it, and |
19 |
>> that's exacly what happens in a DNS amplification attack. |
20 |
>> |
21 |
>> Restrict your resolver to be accessible only to your network or, at |
22 |
>> most, those of the specific group of people you're seeking to help. |
23 |
>> |
24 |
>> You *might* try restricting the resolver to only respond to TCP requests |
25 |
>> rather than UDP requests, but if the resolver sends response data along |
26 |
>> with that first SYN+ACK, then nothing is solved, and you've opened |
27 |
>> yourself up to a SYN flood-based DoS attack. (OTOH, if your resolver |
28 |
>> went offline as a result of a SYN flood, at least it wouldn't be part of |
29 |
>> an amplification attack any longer...) |
30 |
>> |
31 |
> |
32 |
> Can't we rate limit UDP DNS request? |
33 |
> |
34 |
> E.g., limit each source IP to, let's say, 1 UDP per second? |
35 |
> |
36 |
> That should be doable easily using iptables. |
37 |
|
38 |
That makes the resolver highly unreliable for normal use. Many sites |
39 |
trigger resource grabs from 10-15 different domains. If all but the |
40 |
first request is dropped due to rate limiting, you're going to have a |
41 |
very, very broken experience. |