[Dnsmasq-discuss] negative caching
ecabrera at motorola.com
Thu May 18 16:41:15 BST 2006
You were correct, the response for the invalid URL did not have any
records. That explains the missing negative entry in the cache.
I'm still wondering why the negative response(s) from the DNS server did
not appear in the DNSMASQ log? Tcpdump shows that a message was
Erwin R. Cabrera
From: dnsmasq-discuss-bounces at thekelleys.org.uk
[mailto:dnsmasq-discuss-bounces at thekelleys.org.uk] On Behalf Of Simon
Sent: Wednesday, May 17, 2006 4:07 PM
To: dnsmasq discussion list
Subject: Re: [Dnsmasq-discuss] negative caching
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.
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