[Pdns-users] [Recursor] Resolving large RRsets
bert.hubert at netherlabs.nl
Mon Dec 13 09:42:50 UTC 2010
On Mon, Dec 13, 2010 at 09:35:47AM +0100, Stephane Bortzmeyer wrote:
> On Mon, Dec 13, 2010 at 09:30:18AM +0100,
> bert hubert <bert.hubert at netherlabs.nl> wrote
> a message of 286 lines which said:
> > Dec 13 09:23:54  all-wikileaks.bortzmeyer.fr.: truncated bit set,
> > retrying via TCP
> This is not perfect: with BIND and Unbound, there is no fallback to
> TCP since they use EDNS0 (with a default buffer size of 4096 bytes,
> which is enough for this RRset). Why does PowerDNS do not use EDNS0?
We actually have that code, and it turns out it leads to a lot of fallback
to non-EDNS0 after timeouts. The net effect of EDNS0 usage is heavily
negative, especially when truncated answers are rare.
Since almost no TCP overhead is saved, each timeout caused by EDNS0-probing
is very expensive. Let alone the EDNS0 path MTU probing etc.
In short, for non-DNSSEC workloads, it is not worth it.
A nice middleground is what Nominum does, only try EDNS0 in case a tc=1
answer is seen, that might be worth it.
But, the logic is there in the PowerDNS Recursor, and it will be hooked up
again once we do DNSSEC for validation.
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