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Old 12-13-2004, 05:23 AM   #1
Registered: Aug 2003
Location: Sweden
Distribution: Debian
Posts: 52

Rep: Reputation: 15
Update others arp caches

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.

Old 12-13-2004, 07:05 AM   #2
Registered: Sep 2004
Location: Texas, USA
Distribution: Fedora
Posts: 620

Rep: Reputation: 31
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
    # 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
Old 12-14-2004, 06:47 AM   #3
Registered: Aug 2003
Location: Sweden
Distribution: Debian
Posts: 52

Original Poster
Rep: Reputation: 15
Gratuitous arp

I found this:
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...


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