question about caching of lame servers

Klaus Darilion klaus.mailinglists at
Mon Oct 16 09:47:50 UTC 2006

Hi Tatuya!

Thanks for your answers.

JINMEI Tatuya / 神明達哉 wrote:
>> Further, I not only want to cache lame name servers, but also name 
>> servers which are down. Is this possible?
> Not exactly, but the fact that a server is down is cached as
> a penalized RTT, which makes that server less preferred in subsequent
> server selection.

Penalized RTT works fine if at least one authoritative name server is 
working, but if all authoritative name servers are down, then this is no 

Maybe I should describe the cause of my question. I am using openser as 
SIP proxy. openser is multi threaded (fixed number of threads) and uses 
libresolv for domain resolving. Thus, if openser resolves a domain with 
broken name servers (either by network problems or by intention (DoS 
attack)), openser's thread is busy until a timeout happens.

This can be easily used to make a DoS attack. Probably the best solution 
would be to use asynchronous DNS in openser, but this will not be 
implemented soon.

Do you know a solution to solve this problem in the recursive name server?


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