Re: ARP Address Conflict Detection and dhcpcd arping

[Roy Marples <roy%marples.name@localhost>]

Hi Rudolf

On 14/10/2019 11:35, rudolf wrote:
> Thanks for having a look at it!
>
> I've tested it again with the same result, please see the attached logs 
> from tcpdump (where i've edited the mac addresses: NetBSD: 
> aa:aa:aa:aa:aa:aa, Linux: bb:bb:bb:bb:bb:bb)
>
> The testing setup was in both attempts:
> * NetBSD (8.1_STABLE, built on 6th Jul 2019) wm0 has static address 
> 172.31.255.254/30
> * the machines are connected directly with a cable (no switch etc.)
> * the Linux machine is offline (and thus the flags field of NetBSD wm0 
> has value: 0x4<DETACHED>), with attached dhcpcd config amendment
> * tcpdump is launched on NetBSD host
> * the Linux machine is turned on
>
> Looking at the patch you've attached I guess the culprit is probably 
> that due to the direct connect, the wm0 address is tentative at the 
> moment of the arping (the link goes up and down several times before the 
> arping) and thus not being defended?

Well, the culprit is the link going up and down.

dhcpcd ARPing is basically a DaD probe.
The NetBSD host is also performing DaD each time the link comes up and 
it correctly spots dhcpcd ARPing for it and marks it's address as 
duplicated.

The only software fix would be to configure dhcpcd to unicast the ARP 
for the IP address to a specific MAC address that you would need to 
configure.
If this is acceptable, I can work on a patch for dhcpcd.

Let me know.

Roy