Nice writeup. Alway put my mDNS-capable server devices behind a fire-walled sub-subnet or disable server’s Avahi/Bonjour if not needed.

Clients can run Avahi/Bonjour just fine, provided those ports are firewall-limited to its non-public subnet.

Since mDNS works over multicast, it will not leave their respective subnet. Getting that traffic over subnet boundaries is a problem of it's own (see also mDNS reflector)

(devices that advertise mDNS as a server should also not communicate via unicast IP out to the Internet, block those kinds).

Looking at you, both HP and Epson printers. Buy a Brothers printer but without their “Refresh” option (Refresh is a dial-home leaky privacy thing).

is the Unix Network Programming series still in print? A new copy of the book is extremely expensive on Amazon.

Here's a link to the book on the publisher's web site:

"Zero Configuration Networking: The Definitive Guide" by Daniel H Steinberg, Stuart Cheshire (2005) https://www.oreilly.com/library/view/zero-configuration-netw...

There's a link on that page to Amazon and other sellers.

ISBN: 9780596101008

That's different book; the OP meant the book on the second photo, "Unix Network Programming", ISBN 9780131411555.