Client Computers and ARP Broadcasts


Figure 13-4 shows a client computer sending an ARP broadcast to an LVS-DR cluster. Notice that because the Director and the cluster node (real server 1) are connected to the same network, they will both receive the ARP broadcast asking, "Who owns VIP1?"

image from book
Figure 13-4: An ARP broadcast to an LVS-DR cluster

In Figure 13-4, gray arrows represent the path taken by an ARP broadcast sent from the client computer. The ARP broadcast packet is sent to all nodes connected to the local network (the VLAN or physical network segment), so a gray arrow is shown on the physical wires that connect the Director and real server 1 to the network switch. This is normal network behavior.

However, we want real server 1 to ignore this ARP request and only the LVS-DR Director to respond to it, as shown in Figure 13-5. In the figure, a gray arrow depicts the path of the ARP reply. It should only come from the Director and not real server 1.

image from book
Figure 13-5: An ARP response from the LVS-DR Director

To prevent real servers from replying to ARP broadcasts for the LVS-DR cluster VIP, we need to hide the loopback interface on all of the real servers. Several techniques are available to accomplish this, and they are described in the LVS-HOWTO.

Note 

Starting with Kernel version 2.4.26, the stock Linux kernel contains the code necessary to prevent real servers from replying to ARP broadcasts. This is discussed in Chapter 15.



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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