[dns-operations] latest bind, EDNS & TCP

Simon Munton Simon.Munton at cdns.net
Sat Oct 11 09:21:22 UTC 2014

> Which one(s) have been recently updated and are suspect?  Would they really have overwritten previously-configured options, or blithely added new ones which were enabled by default?

Previously, bind has started with bufsize=4096 and reduced it if the 
queried server says it can only provide smaller packets.

My understand is that the new behaviour is that it will start with 
bufsize=512 and raise it.

But what we are seeing is that some servers are failing to do this and 
so making excessive use of TCP.

Although I can't directly this effect with a code change - the timing is 
right and its the same area of the code.

And its been a very sudden change (we can identify the day it started 
happening) - which suggests code change, as opposed to a config change 
(i.e. it happened in a number of places at about the same time).

I said before, the fact remains that servers that are clearly capable of 
a larger bufsize but are sticking with a minimum sized one, and so 
re-issuing *ALL* their queries over UDP, then immediately again over TCP 
(where the target zone is signed).

At a minimum this doubles the workload on the target server, without 
taking into account the much higher workload of TCP over UDP.

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