netdev - iproute2: unexpected expansion of IPv4 addresses

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Message-ID: <8c354331845a3ac599e84c3759da5944@sdomi.pl>
Date: Fri, 18 Oct 2024 13:27:26 +0100
From: sdomi <ja@...mi.pl>
To: netdev@...r.kernel.org
Subject: iproute2: unexpected expansion of IPv4 addresses

Hi,

I'm running iproute2-6.10.0 (from Alpine edge repositories), and I 
noticed that when manipulating the routes using short-hand addresses, 
the expansion is conducted in an unusual manner:

`ip r a 1.1 via 127.1` will result in `1.1.0.0 via 127.1.0.0 dev lo`, 
while I'd expect it to be equivalent to `ip r a 1.0.0.1 via 127.0.0.1`.

The same issue is present when manipulating addresses (e.g. `ip a a 
127.1.1 dev lo` results in `127.1.1.0` instead of `127.1.0.1`)

FWIW, I wasn't able to find an RFC which specified how this should be 
implemented, and the deeper I look into it, the more it feels like a 
40-year old BSD4.2 quirk that somehow lives on. Nonetheless, I think it 
would be nice to either validate against using shorthands (as it can 
cause confusion), or mimic what `ping` and other such utilities do.

~sdomi