[getdns-api] EDNS maximum UDP payload size defaults to 512

[Wessels, Duane]

On Aug 28, 2014, at 11:38 AM, Phil Pennock <getdns-api-phil at spodhuis.org> wrote:

> On 2014-08-27 at 16:48 +0000, Wessels, Duane wrote:
>> Hello All,
>> The getdns API description says:
>> 
>>> getdns_return_t getdns_context_set_edns_maximum_udp_payload_size(
>>>    getdns_context *context,
>>>    uint16_t value
>>> );
>>> 
>>> The value is between 512 and 65535; the default is 512.
>> 
>> Can someone explain why 512 should be the default value?  It seems odd
>> that if the library is going to add an EDNS0 OPT record, that it should
>> use the smallest possible value for the buffer size.  Software with long
>> deployment history uses large values, such as 4096 and measurements from
>> root/TLD name servers also indicates that 4096 is a very common value.
> 
> Broken firewalls which don't handle UDP fragment reassembly; broken
> firewalls which limit port 53 UDP to 512 octets.
> 
> A client library which causes breakage, when people switch to it, will
> develop a bad reputation even if the problem lies elsewhere.

Yes, thanks to these broken things we live in a world where DNS clients
must be prepared to retry their queries with different EDNS parameters.
Fine.

But that is not a good reason to make 512 the default.  The default values
should be appropriate for the common case -- for the way things should be.
The default should not cater to the badness.

Although, now that I think about it, perhaps the 512 default will serve
to advance one of my other goals -- increased use of TCP.  If I understand
the spec correctly, if you enable validation but don't know that you should
also increase the payload size, then probably a lot of your responses will
be TC=1 and we'll start to see a lot more TCP...

DW
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