When the IP Recover Kit software switches an IP resource from one server to another, the MAC address associated with the switched IP address changes because the interface changes. Each router and user system on the LAN must reflect this change in its ARP table before it can contact the IP address at its new location. In certain operating systems, when a new IP address is added to a network interface, an ARP packet is automatically sent out by the operating system to update all clients’ ARP tables on the subnet. This feature does not exist in Linux. LifeKeeper therefore must send out an ARP packet after adding a switchable IP address to an interface to force this client ARP cache update.
TCP/IP implementations differ in their ability to implement the required ARP updates in response to this ARP packet. The following list describes some important cases:
- Full Linux TCP/IP implementation. Fully functional TCP implementations in Linux and most other operating systems support ARP cache updates when the systems receive an ARP request packet. LifeKeeper uses this feature, as described above, to force ARP cache updates on such systems.
- ARP cache. User systems that do not support the ARP refinements but do support an ARP cache usually have a timer associated with the cache to maintain some level of currency. For some implementations, decreasing the timer value can minimize the time required for that particular user system to reflect the changed address mapping. If the number of users on the LAN is small, this option may be acceptable. For other systems, decreasing the timer value may not be necessary. For example, the TCP implementation shipped with Windows NT uses a ten second timer value, so no change in timer value would be needed.
- Static address mapping. For systems without a dynamic ARP cache or those where cache timing is not tunable, routers can be used to handle mapping changes. Such user systems would access the IP resource subnet by way of a router (gateway). In this configuration, cache update is needed only for the routers directly connected to the resource subnet and no changes are needed on the user systems themselves.