NTPClient::forceUpdate() treated any non-empty datagram arriving on its fixed
local port 1337 as a time response: it ignored the read length, the version,
the mode, the stratum, the leap indicator and any request/response correlation,
then handed the bytes at offset 40 to the bridge. The bridge only checked that
the derived epoch was at least 2026-01-01 before calling settimeofday() and
writing the RTC. A host probe against the installed library source accepted a
one-byte non-NTP datagram and produced epoch 2085978496. Anyone able to land a
UDP datagram during a query window could set a bogus clock, which then feeds JWT
issuance, certificate validity and packet timestamps.
Replace that path with probeNtpServer(): one exchange per server on a fresh
ephemeral socket, closed again on every exit. A reply is accepted only if it is
a full 48 bytes, from the address and port queried, NTPv3/v4 mode 4, from a
synchronised server (no leap alarm, stratum 1-15), echoing the random transmit
timestamp of the request, with an epoch inside a plausible range. Rejected
datagrams leave the clock alone and do not end the wait, so an early bogus
packet cannot pre-empt the real answer. Era-1 (post-2036) timestamps convert
forward instead of wrapping into 1900.
The diagnostic now runs the same validated probe, so `get mqtt.ntp.diag`
answers the question it is asked — would this server be trusted? — instead of
reporting a datagram nothing checked. It also no longer leaves its UDP socket
open after the probe, and each server's per-probe result carries the reason it
failed ("DNS failed", "unsolicited reply", "server unsynced", ...). The
diagnostic still blocks its caller; making it asynchronous is a separate change.
The acceptance rules live in NtpValidation.h (pure, host-tested), including the
review's one-byte-datagram reproduction.
