How Client Computers Access Resources

Normally client computers know the name of the server offering the resource they want to use (mail.mydomain.com or www.mydomain.com, for example), and they use the Domain Name System (DNS) to look up the proper IP address of the server. Once the routing of the packet through the Internet or WAN is done, however, this IP address has to be converted into a physical network card address called the Media Access Control (MAC) address.

At this point, the router or the locally connected client computers use the Address Resolution Protocol (ARP) to ask: "Who owns this IP address?" When the computer using the IP address responds, the router or client computer adds the IP address and its corresponding MAC address to a table in its memory called the ARP table so it won't have to ask each time it needs to use the IP address. After a few minutes, most computers let unused ARP table entries expire, to make sure they do not hold unused (or inaccurate) addresses in memory.^[9]

Failover using IP Address Takeover (IPAT)

In order to move a resource (a service or daemon and its associated IP address) from one computer to another, we need a way to move the IP address from the primary computer to the backup computer. The method normally used by Heartbeat is called IP address takeover (sometimes called IPAT). To accomplish IPAT, Heartbeat uses secondary IP addresses (formerly called IP aliases in older Linux kernels) and gratuitous ARP broadcasts.

FAILING OVER MULTIPLE IP ALIASES

In theory, you can add as many secondary IP addresses (or IP aliases) as your network IP address numbering scheme will allow (depending upon your kernel version). However, if you want Heartbeat to move more than a few IP addresses from a primary server to a backup server when the primary server fails, you may want to use a different method for IP address failover than the method described in this chapter. In fact, Heartbeat versions prior to version 0.4.9 were limited to eight IP addresses per network interface card (NIC) unless you used this method. (See the file /usr/share/doc/packages/heartbeat/faqntips.html after installing the Heartbeat RPM for instructions on how to configure your router for this alternative method.)

^[9]For more information see RFC 826, "Ethernet Address Resolution Protocol."