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Re: ARP Address Conflict Detection and dhcpcd arping

On Fri, Oct 11, 2019 at 5:23 AM Andy Ruhl <> wrote:
> On Fri, Oct 11, 2019 at 4:41 AM rudolf <> wrote:
> >
> > Hi,
> >
> > I have a NetBSD 8.1_STABLE (july 2019) host with a static IP address and
> > another machine (Linux) with dhcpcd "arping" configuration for this IP
> > address. With the default value of sysctl net.inet.ip.dad_count (=3), I
> > see no ARP reply ("ARP, Reply <IP addr> is-at <MAC>") messages from the
> > NetBSD host on the arping queries ("ARP, Request who-has <IP addr> tell
> >") and eventually the NetBSD host marks its IP address as
> > duplicated (and the "arping" dhcpcd configuration of the second host fails).
> >
> > This seems contradictory to what I read in "man 4 arp":
> > ``ARP will defend the host's active address when a conflicting message
> > is received.''
> >
> > With "sysctl -w net.inet.ip.dad_count=0", the NetBSD host sends the ARP
> > reply and the "arping" configuration of the second host works.
> >
> > Is this expected behavior?
> I did a manual arping -D <ip> from an already configured Linux machine
> to my 8.0 NetBSD machine and the arp was replied to right away. Can
> you try the same thing? It did say reply so it should be
> pretty similar to what you're doing.

I just noticed that the flag to make arping set the reply to
is different on different Linuxes. On the one I was on, it was -D. On
another one, it's -0. I tried it on a new-ish 8.1 machine and it
worked as well, both using the standard net.inet.ip.dad_count setting


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