[Dnsmasq-discuss] negative caching

Simon Kelley simon at thekelleys.org.uk
Wed May 17 22:06:46 BST 2006


Cabrera Erwin-W30163 wrote:
> 
> I am calling DNSMASQ version 2.25 without the "-N" option for turning
> off negative caching.  
>  
> For valid domain names, I see messages in the DNSMASQ log indicating
> that a query was received, forwarded  to the designated DNS server, and
> a reply received from the DNS server.  Also, the correct information
> appears in the cache. The NSlookup application returns the correct
> address(es).
>  
> For invalid domain names, I see log messages indicating that a query was
> received and forwarded, but no replies from the DNS server.  The
> NSlookup application returns Host not found.  tcpdump indicates that a
> response was received from the DNS server.  I was expecting to see
> negative cache entries in the cache, but no entries for the invalid
> domain names appear in the cache.
>  
> I am doing something wrong or missing something?
>  

Use ethereal to examine the responses from the DNS server. Look to see
if there is an SOA record. If it's missing, then dnsmasq can't determine
the time-to-live for a negative cache entry and therefore can't generate
one. (The details of this are in RFC2308)

Many ISPs seem to turn off this information in their DNS servers (mine
is one). I suspect it's a way of reducing server load, but I'm not sure
if they realise that by inhibiting negative caching, they may in fact be
increasing the load in the servers.

Cheers,

Simon.




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