Re: [Bloat] mDNS

[David Lang <david@lang.hm>]

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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