[lwip-devel] [bug #61653] PPP Netmask Changed

[Sylvain Rochet]

Follow-up Comment #3, bug #61653 (project lwip):

Hi Ashley,

I took the time to dig a bit more about that, there is quite a bit of history
and legacy on netmask hacks on PPP links.

https://github.com/ppp-project/ppp/blob/eff574c871822c7e2788aa8a928f2480fa7a6cb3/pppd/sys-linux.c#L2766

This is the final answer, for "recent" Linux kernels (Linux 2.1.16 was
released 18-Dec-1996 :-), the netmask is forced to 255.255.255.255, everything
else is legacy code that do nothing. So the subnet hack I supposed to still
exists was only working in the early '90 era…

And you are right, IPCP never exchange netmask between peers, my bad, I
sometimes think there is because I used to see non standard support for that
in the wild but that's all.

Anyway, I am not against allowing the user to set any netmask if necessary,
bypassing the forced 255.255.255.255 value (that is, only setting the netif
netmask at init). After all, the wifi module can still hack around PPP and
create a layer 2 bridge between the PPP link and the Wifi "world" in the same
way cellular modems currently do.

Sylvain

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