Best way to force a TC=1 response?

Mark Andrews marka at
Tue May 26 23:28:47 UTC 2020

> On 27 May 2020, at 07:27, John Levine <johnl at> wrote:
> In article <mailman.442.1590526210.942.bind-users at> you write:
>> What's the best way to force an A query via UDP to return a TC=1 result:
>> a really long CNAME chain?
> I'd suggest lots of AAAA records.  You could do it with A records but you'd
> need four times as many

Well ~2 times as many.  Each additional A record requires 16 bytes and each
addition AAAA records requires 28 bytes.  That means ~256 A records and ~146
AAAA records to force TCP with a 4096 byte UDP buffer size.

John’s example had 187 AAAA records.

Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742              INTERNET: marka at

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