On Wed, 28 Feb 2024, Juliusz Chroboczek via Bloat wrote:
But my point is that the OpenWrt router has no way to predict what
address/subnet will be assigned to its WAN port.
In principle, the ISP should assign either a global address, or an address in
the range 100.64.0.0/10 (RFC 6598). This range was deliberately chosen to
not collide with RFC 1918 space, so that the NAT box can choose any RFC 1918
prefix on its downstream interfaces.
In practice, however, ISPs don't necessarily obey the RFCs, and people do
chain NAT boxes, so none of the above is guaranteed.
chaining NAT boxes is very common, too many ISPs don't give you anything other
than a NAT address from their router
Consequently, at boot-time, OpenWrt should simply choose some different
subnet for its LAN subnet(s), and then advertise an mDNS name.
I'm not sure how that could happen at boot time, it would need to happen
whenever a DHCPv4 lease changes. This implies that the router might need
to renumber if the ISP changes its allocation, and there are no
renumbering procedures for IPv4 (I'm not sure if anyone implements RFC 3203).
it's unusual for the network block to change on a renewal, and in that rare case
you could reboot the router.
It would also make addressing non-deterministic, which would make
debugging slightly more difficult. But then, we already have
non-deterministic addressing in IPv6, so I guess that's something we can
live with.
remember, you don't need to randomly pick something, you just need to have a
couple networks, figure out if one is in use by the WAN and if so pick the
other.
I will say that 192.168.0 and 192.168.1 are very commonly used, so anything
other than those a better default
personally, I like 192.168.255 as people tend to forget that's a valid network.
David Lang
-- Juliusz
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