[LibreQoS] Tracking unknown IPs (maybe for 1.4?)

[Herbert Wolverson]

As we approach the v1.3 pre-release feature freeze, I've been thinking a
little bit about nice things to have. One thing I found useful in both
BracketQoS and Preseem was the ability to grab a list of IP addresses that
had been through the shaper, but weren't mapped to a queue (obviously, only
from within the "allowed IP" range - we're not trying to map the Internet!).

In Preseem, there's a link to download a CSV file containing all the
unmapped IP addresses and how much traffic they have consumed. BracketQoS
(pre cpumap-pping) has a report showing the IPs (no traffic).

*Why is this useful?*

Knowing which local IP addresses were processed but not mapped lets you
find:

* the times that a device was installed, but the on-boarding process wasn't
completed. Yes, that shouldn't happen. And - unfortunately - it
occasionally does. If you're using RADIUS-based authentication, it's really
difficult for this to happen - but not everyone is.
* If there's a bug in your shaper integration, it's helpful to see "oops, I
put X on the default"
* Just occasionally, you get a customer who needs a special setup; it's
helpful to see that it worked.

*Current Status*

Before cpumap-pping, Bracket was grabbing them by reading the pping output
and listing addresses that didn't match a shaping rule. That doesn't work
now:

* xdp_pping is spitting out TC handles, rather than IP addresses.
* With a default rule in place, and handling for IPv6 and IPv4 subnets, an
IP address might not exactly match an entry (requires an LPM trie lookup) -
and IPs matching a default rule (::/0 or 0.0.0.0/0) will always come back
with the "default" handle.

It's currently pretty tricky to do.

So I'm curious; would others like to see this? I have a few ideas for how
to make it work, but don't want to start serious planning/design if I'm the
only one who wants the feature.
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