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.
> On 27 May 2020, at 07:27, John Levine <[hidden email]> wrote:
> In article <[hidden email]> 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: [hidden email]