From Newsgroup: comp.protocols.dns.bind

What's the best way to force an A query via UDP to return a TC=1 result:
a really long CNAME chain?

I want to set up a name that can be used in e.g. ping to perform an end
to end resolution check in application context.


The longer version is that there was a thread on postfix-users not too
long ago about the fact that MUSL libc doesn't do TCP (among other
things) and I want a way to test some hardware and statically built
apps. No jumbo frames here.

I was also mildly surprised to discover that glibc doesn't try TCP if
UDP fails to answer; for some reason I thought it did! Instead it
reports "Temporary failure in name resolution" in the ping example.

--

Fred Morris


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From Newsgroup: comp.protocols.dns.bind

On 27 May 2020, at 07:27, John Levine <johnl@iecc.com> wrote:

In article <mailman.442.1590526210.942.bind-users@lists.isc.org> 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.
JohnrCOs example had 187 AAAA records.
Mark
--
Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742 INTERNET: marka@isc.org
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