Own IPv6 broadcast domain

[ilario]

@pony1k proposed to have a LibreMesh-specific IPv6 broadcast domain.

Background:
Now one can do ping6 ff02::1%name_of_interface and get answers from all directly reachable devices.

Proposal:
One could ping another address for getting answers from all and only the LibreMesh devices, something like ping6 ff12::bee%name_of_interface

This would be convenient for general network maintenance, and specifically it would help shared-state to detect other LibreMesh nodes replacing the generic IPv6 broadcast address here:

lime-packages/packages/shared-state/files/usr/bin/shared-state-get_candidates_neigh

Lines 36 to 65 in 4c51c7e

	ping -i 0.1 -c 2 ff02::1%${iface} 2> /dev/null | \
	awk '{if ($3 == "from") print substr($4, 1, length($4)-1)'"\"%${iface}\""'}'
	done | sort -u -r)"
	babelCandidateAddresses="$([ -e /var/etc/babeld.conf ] &&
	for iface in $(grep interface /var/etc/babeld.conf | awk '{print $2}'); do
	ping -i 0.1 -c 2 ff02::1%${iface} 2> /dev/null | \
	awk '{if ($3 == "from") print substr($4, 1, length($4)-1)'"\"%${iface}\""'}'
	done | sort -u -r)"
	wifi_get_macs="
	wireless = require('lime.wireless')
	iwinfo = require('iwinfo')
	utils = require('lime.utils')
	for _, ifc in ipairs(wireless.mesh_ifaces()) do
	for mac, sta in pairs(iwinfo.nl80211.assoclist(ifc)) do
	if sta.inactive < 5000 then
	print(utils.mac2ipv6linklocal(mac) .. '%' .. ifc)
	end
	end
	end
	"
	wirelessCandidateAddresses=$(echo "$wifi_get_macs" | lua - | sort -u -r)
	candidateAddresses="$batmanNonMeshCandidateAddresses
	$babelCandidateAddresses
	$wirelessCandidateAddresses
	$(ping -i 0.1 -c 2 ff02::1%br-lan | \

The original proposal here, for easing the identification of the devices reachable from a specific interface:
https://lists.autistici.org/message/20240621.153757.1eab7dbb.en.html

A proposal of implementation here:
https://lists.autistici.org/message/20240726.154215.73440c31.en.html

[pony1k]

I also wanted to share the idea that shared-state could use multicast-addresses for communication. Currently, it uses multicast ping to discover neighbors, then communicates with them via TCP. But I'm not qualified to decide whether that makes sense.

[ilario]

@javierbrk @G10h4ck opinions?