[lwip-devel] Netmask and GW when setting IPv6 static address?

[Erik Slagter]

Hi there,

I am posting this here instead of on lwip-users, as it appears nobody's 
there.

I am using LWIP from the ESP-IDF (Espressif) but it doesn't seem to be 
an ESP-IDF thing, so I am asking here. They're using LWIP from git 
commit f79221431fa9042b3572d271d687de66da7560c4.

This is my situation. I have a client (running LWIP) configured with 
IPv6 enabled and also autoconfigure (SLAAC) enabled.

The client is connected over wifi to a switch acting as IPv6 router, 
connecting to other clients in other subnets and VRF's. The switch sends 
router advertisements with the proper prefix configured, with a netmask 
of /64 (which seems to be a requirement).

I have it working using SLAAC, no problem there. But I'd rather use a 
static address and so I did. I was looking for a way to set the netmask 
and the default gateway for this address but I found no way to do so, 
there is just the address and that's it.

So I guessed the netmask and the default gw are taken from the router 
advertisement packets. Ok, I get that, but I'd rather not have SLAAC at 
all, so I configured only a static address and disabled router 
advertisements from the switch. And indeed, no SLAAC address was 
created, fine.

Now what puzzles me, is that I can still reach this client running LWIP 
from a different subnet/vlan/broadcast domain. How can LWIP know what's 
the netmask (i.e. what's local/non-local) for a global address and how 
can LWIP know what address to use for the default gw, if no router 
advertisements are used?

I need to know this, because probably some assumptions are made about 
the network which may not be valid in my network.

Why can't I just configure a default gw and a netmask for a global 
static interface address? Or even better, use the "token" concept as 
found in the linux "ip" command, where a statically configured number is 
used for lower 64 bits of SLAAC instead of the mac address. Shouldn't be 
that hard to implement I think?

Thanks for your information!
Erik.