new documentation: IP_TRANSPARENT, is it correct?

[bert hubert <bert.hubert@powerdns.com>]

Hi everyone,

10 years after lartc.org I decided to document a little bit more of Linux
networking, and I hope I got it right. This email asks for your help in
making sure.

Recently I attempted to use IP_TRANSPARENT as outlined in
https://www.kernel.org/doc/Documentation/networking/tproxy.txt but I could
not figure out how it really worked from there (although I could copy paste
my way to some working code).  The web also mostly offered little in the way
of (correct) explanation.

I think I have it figured out by now, but I'm sure there are nuances I have
missed. I'm especially interested in understanding _exactly_ what the
IP_TRANSPARENT socket option does, because it appears somewhat arbitrary
right now:

"The IP_TRANSPARENT socket option enables:

* Binding to addresses that are not (usually) considered local
* Receiving connections and packets from iptables TPROXY redirected sessions"

https://ds9a.nl/tproxy/tproxy.md.html has somewhat prettified Markdown that
requires Javascript, plain Markdown is on
https://github.com/ahupowerdns/tproxydoc/blob/master/tproxy.md

If you could give this a read and a comment on things I got wrong, that
would be most appreciated. Pointers to other relevant documentation are also
very welcome.

Thanks!

	Bert