Update others arp caches

rickthemick · 12-13-2004, 05:23 AM

Hi!
I want to be able to update other hosts arp caches on my LAN (for Mobile IP purposes). I believe this is sometimes called "gratuitous arp". I have tried implemented this and Ethereal identifies this as an "request/gratuitous arp" but the arp cache on the other Linux boxes (2.4.25) is not beeing updated!

Some where else I read that this task is solved by simply put an unsolicited arp reply on the wire, but this doesnt seem to get the caches updated either.

/Rick

scowles · 12-13-2004, 07:05 AM

I don't have much to add to this thread other than I have always used "arping -U" (request) or "arping -A" (reply) to update my ISP's router arp cache when I changed NIC's (I have a static public IP).

FWIW: The redhat ifup script issues a gratuitious arp when the interface is brought up

Code:

    # update ARP cache of neighboring computers
    arping -q -A -c 1 -I ${REALDEVICE} ${IPADDR}
    ( sleep 2;
      arping -q -U -c 1 -I ${REALDEVICE} ${IPADDR} ) > /dev/null 2>&1 < /dev/null &

Also, using the windows based version of ethereal, windows clients also issue some form of an arping when the interface is brought up. At least thats what I have observed on my XP box.

Good Luck

rickthemick · 12-14-2004, 06:47 AM

I found this:
http://lists.virus.org/fw1-0311/msg00329.html
which says that gratuitous arp seems to be problematic no Debian!
Anyone who knows something about this? I can see my gratuitous arp but the arp table doesnt get updated...