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

[Wessels, Duane]

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.

It should at least be safe to have a default value close to ethernet MTU
sizes minus some for safety (1400-ish).

There was a report recently on the dnsop list (and dnssec-deployment,
archives offline) about some TLDs and authoritative implementations that
don't work properly when EDNS advertises 512.  I believe these came to
light because BIND 9.10 is/was advertising 512 by default.

There is a scheduled release of the getdns library this week I think.  It
would be nice to have clarification on the default EDNS size before then.

DW
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