ARP Broadcasts and the LVS-DR Cluster


As we've just seen, placing VIP addresses on the loopback (lo) device on each cluster node allows the cluster nodes in an LVS-DR cluster to accept packets that are destined for the VIP address. However, this has one dangerous side effect: the real servers inside the cluster will try to reply to ARP broadcasts from client computers that are looking for the VIP. Unless special precautions are taken, the real servers will claim to own the VIP address, and client computers will send their packets directly to real servers, thus circumventing the cluster load-balancing method and destroying the integrity of network communication with the Director (where packets that use the VIP as their destination address are supposed to go).

To understand this problem (called "The ARP Problem" in the LVS- HOWTO), let's look at how a client computer uses the VIP address to find the correct MAC address by using ARP.



The Linux Enterprise Cluster. Build a Highly Available Cluster with Commodity Hardware and Free Software
Linux Enterprise Cluster: Build a Highly Available Cluster with Commodity Hardware and Free Software
ISBN: 1593270364
EAN: 2147483647
Year: 2003
Pages: 219
Authors: Karl Kopper

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