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

[Iñigo Ortiz de Urbina Cazenave]

On 27/08/14 18:48, 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.
> 
> It should at least be safe to have a default value close to ethernet MTU
> sizes minus some for safety (1400-ish).

+1 to this pragmatic approach.

An alternative, working default could also be around the magic number 1280.

> 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.

I believe the threads in question are 'The nameservers for .SOY and .FOO
are broken.' [0] and 'About EDNS' [1]. Sharing the links here for the
interested parties.

> 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

Cheers,

Iñigo Ortiz de Urbina Cazenave

[0]
https://www.mail-archive.com/dnssec-deployment@dnssec-deployment.org/msg00007.html
[1]
https://lists.dns-oarc.net/pipermail/dns-operations/2014-August/012032.html
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