Re: Just in case you want to know: resolv problems

On 22.06.2005, at 00:01, Linkas wrote:

> Andreas Fink wrote:
>
>
>>
>> On 21.06.2005, at 14:48, Julien Buratto wrote:
>>
>>
>>> I've started kannel on linux redhat 9
>>>
>>> smsbox is calling HTTP dlr urls resolvin ok
>>>
>>> I edited the /etc/resolv.conf and changed the ip address of the  
>>> dns  server
>>>
>>> resolving ok on the linux side
>>>
>>> disabled old dns server
>>>
>>> resolving ok on the linux side
>>>
>>> kannel not resolving correctly
>>>
>>> restarted kannel, now resolving ok.
>>>
>>> Strange, it seems like a resolver in cache...
>>>
>>> J
>>>
>>
>>
>> This is usually the case on most operating systems.
>> If you once resolved a IP it will stay in cache for a while.
>>
>> Andreas Fink
>>
>>
> This is not the case, I'm talking about another dns server changed  
> in the resolv.conf file, so no cached was possible.
> In the case of a cached dns value, the kannel had to keep on  
> resolving correcly with the cached value; in this case the server  
> itself was resolving correcly on the operating system level with  
> the new dns server, so I presume some memory pointer is not garbage- 
> collected correcly.
>
> The kannel has been ip almost 20 hours before being restarted. The  
> target domain was the same between the two dlr-url.

You haver to distinguish DNS Server cache and DNS client cache.
Even while chaning resolv.conf to drop out a DNS Server from your  
query queue, it might be that your CLIENT cache still resolve to  
whatever it resolved before. This means your client (kannel in this  
example) will not query the net for the IP it already resolved a  
while ago. Its implementation specific but whatever library you use  
to resolve DNS might do that. I dont think thats something kannel  
does by itself but the resolver library it uses which is provided by  
the operating system. Killing and restarting kannel then makes this  
cache being lost, resulting in another query next time at what point  
the resolv.conf is queried again.

At least this is the behaviour I have seen with other apps in the  
past on various unix systems.


Andreas Fink
Fink Consulting GmbH

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