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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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|
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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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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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