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[ih] Why UDP/DNS limited to 512 exactly?

I'm curious what the reason was for the specific limit of 512 bytes
for a UDP/DNS message.

Was it a convenient value that fell on a word boundary and close
enough to the 576 IP datagram limit?

I would note in theory you could end up with a 512 byte DNS message
that wouldn't fit into a 576 byte datagram (after adding 8 bytes for
UDP and 60 bytes for a maximum IPv4 header with a full set of options).