IP clash (arp: x is using my IP address y on em1!) - recovering?

Karl Pielorz kpielorz_lst at tdx.co.uk
Mon Nov 11 15:03:13 UTC 2013


I've got a couple of FreeBSD boxes (9.2-R). I just accidentally brought 
'up' the same IP address on both.

Sure enough the console logs a bunch of "arp x is using my IP address y on 
..." style messages.

I quickly took down the 'new' IP's on one box (in fact, I shut the whole 
box down).

However the original box with the addresses stopped responding to traffic 
going to those addresses. I flushed the ARP table on another box - and 
tried pinging one of the IP's that clashed - and you get nothing back. You 
don't event get an ARP entry appearing for it - i.e. if both boxes are on 
the same LAN / IP address range.

It'll quite happily respond to other IP's (on the same interface / MAC) - 
but not the one that got 'clobbered'.

In the end I 'ifconfig' deleted the clashed IP's off the original box - and 
added them again to the same box. This seems to have resolved it. Is there 
some way of avoiding this in the future? - I guess it's some kind of 
'defensive' move or something? - We've just never seen it in the past (the 
very occasional times we've brought up two IP's it's been on much older 


More information about the freebsd-net mailing list