UDP messages may be truncated:

https://www.rfc-editor.org/rfc/rfc1035#section-4.2.1

> Messages carried by UDP are restricted to 512 bytes (not counting
> the IP or UDP headers). Longer messages are truncated and the TC
> bit is set in the header.

However, TCP also have a size limitation of 65535 bytes

https://www.rfc-editor.org/rfc/rfc1035#section-4.2.2

> The message is prefixed with a two byte length field which gives
the message length, excluding the two byte length field.

These limitations makes that the maximum possible number of A records
per RRSet is ~ 4090.

There are environments like Kubernetes that may have larger number of
records (5000+) that does not fit in a single message. In this cases,
the DNS server sets the Truncated bit on the message to indicate that
it could not send the full answer despite is using TCP.

We should only retry when the TC bit is set and the connection is UDP,
otherwise, we'll never being able to get an answer and the client will
receive an errNoAnswerFromDNSServer, that is a different behavior than
the existing in the glibc resolver, that returns all the existing
addresses in the TCP truncated response.

Fixes golang#64896

Signed-off-by: Antonio Ojea <aojea@google.com>
Change-Id: I1bc2c85f67668765fa60b5c0378c9e1e1756dff2
Reviewed-on: https://go-review.googlesource.com/c/go/+/552418
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Gudger <ian@iangudger.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Mateusz Poliwczak <mpoliwczak34@gmail.com>
Reviewed-by: Mauri de Souza Meneguzzo <mauri870@gmail.com>
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>