[dns-operations] UDP fragmentation while not needed/wanted DS www.veilingzaalmelase.be

Thor Spruyt thor.spruyt at telenet.be
Thu Mar 25 01:32:42 UTC 2021


----- On Mar 24, 2021, at 8:12 PM, Viktor Dukhovni ietf-dane at dukhovni.org wrote:

> No.  The server *did* respond with 1 UDP packet, which got fragmented at
> the *IP* layer, into two IPv6 packets, presumably because the IPv6 MTU
> was set to somewhere around 1280.
> 
>> It is also strange that the first fragment has a size of 1232 ...
> 
> Well, that shows 1232 bytes of IP payload, including the 8 byte UDP
> header.  Thus a total of 1298 bytes.  It would perhaps be interesting to
> know how an actual 1232 byte UDP response would be handled from the same
> server.  Would it still be fragmented, or would it generate a single,
> slightly larger first packet.

Ok, that makes sense.
Fragmented response starts from requests with buffer size 1290 apparently.

Shouldn't this server use an edns buffer size of 1289 to avoid fragmentation ?

--
Thor


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