Broadcast gratuitous ARP and ARP caching...

Discussion in 'Linux Networking' started by TDH1978, Jun 2, 2012.

  1. TDH1978

    TDH1978 Guest

    I have two Linux hosts, X and Y.

    X sends a "broadcast" gratuitous (aka unsolicited) ARP request to Y,
    hoping that Y will refresh its ARP cache:

    X> arping -b -I eth0 -U <ip_addr_of_X>

    Y does receive the ARP request (I verified this with wireshark), and
    the ARP request does contain all the proper information (MAC and IP of
    X), but Y does not update its ARP cache. I verified this by running:

    Y> arp -n


    Y> ip neigh show

    However, this DOES work if X uses a non-broadcast gratuitous ARP
    request (no "-U" option):

    X> arping -b -I eth0 -s <ip_addr_of_X> <ip_addr_of_Y>

    In this scenario, Y does update its ARP cache. However, I cannot use
    this scenario because X does not know about Y (Y is one of many
    neighbours). I have to use broadcast gratuitous ARPs.

    Does anyone know why Y will not update its ARP cache when receiving a
    broadcast gratuitous ARP request? Thanks much.
    TDH1978, Jun 2, 2012
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