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On 28/03/2013 21:38, Michael Mol wrote: |
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> On 03/28/2013 03:16 PM, Alan McKinnon wrote: |
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>> On 28/03/2013 17:38, Michael Mol wrote: |
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>>> On 03/28/2013 04:51 AM, Norman Rieß wrote: |
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>>>> Hello, |
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>>>> |
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>>>> i am using pdns recursor to provide a dns server which should be usable |
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>>>> for everybody.The problem is, that the server seems to be used in dns |
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>>>> amplification attacks. |
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>>>> I googled around on how to prevent this but did not really find |
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>>>> something usefull. |
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>>>> |
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>>>> Does anyone got an idea about this? |
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>>> |
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>>> I'm not sure it can be done. You can't make a resolver available to |
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>>> "everybody" without somebody in that "everybody" group abusing it, and |
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>>> that's exacly what happens in a DNS amplification attack. |
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>>> |
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>>> Restrict your resolver to be accessible only to your network or, at |
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>>> most, those of the specific group of people you're seeking to help. |
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>>> |
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>>> You *might* try restricting the resolver to only respond to TCP requests |
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>>> rather than UDP requests, |
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>> |
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>> NO NO NO NO NO |
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>> |
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>> Under no circumstances ever do this. The service breaks horribly when |
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>> you do this and it has to work even remotely hard. Most likely your ISP |
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>> will outright ban you for that if you use the ISP's caches. I knwo I do, |
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>> and so does every other major ISP in this country. |
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> |
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> Er, what? When we're talking about a recursive resolver requiring |
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> clients connecting to it to use TCP, what does upstream care? He's |
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> talking about running his own open DNS server. |
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Because the list is indexed and archived and Googled forever. Others may |
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get the idea that TCP-only DNS caches are a good idea in general. Have |
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you ever had to deal with the insanity caused when Windows Servers |
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insist on using TCP only, and YOU are the upstream? |
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I understand what the OP was suggesting, but he did not limit the |
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usefulness and scope of the suggestion, so I did. |
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> |
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>> |
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>> but if the resolver sends response data along |
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>>> with that first SYN+ACK, then nothing is solved, and you've opened |
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>>> yourself up to a SYN flood-based DoS attack. (OTOH, if your resolver |
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>>> went offline as a result of a SYN flood, at least it wouldn't be part of |
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>>> an amplification attack any longer...) |
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>> |
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>> |
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>> Or just use the ISP's DNS caches. In the vast majority of cases, the ISP |
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>> knows how to do it right and the user does not. |
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> |
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> Generally true, though I've known people to choose not to use ISP caches |
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> owing to the ISP's implementation of things like '*' records, ISPs |
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> applying safety filters against some hostnames, and concerns about the |
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> persistence of ISP request logs. |
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I get a few of those too every now and again. I know for sure in my case |
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their fears are unfounded, but can't prove it. Those few (and they are |
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few) can go ahead and deploy their own cache. I can't stop them, they |
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are free to do it, they are also free to ignore my advice of they choose. |
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-- |
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Alan McKinnon |
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alan.mckinnon@×××××.com |
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