Routing Methods; Nat Routing - Red Hat ENTERPRISE LINUX 4 - VIRTUAL SERVER ADMINISTRATION Manual

Routing Methods

1.4. Routing Methods
Red Hat Enterprise Linux uses Network Address Translation or NAT routing for LVS, which allows the
administrator tremendous flexibility when utilizing available hardware and integrating the LVS into an
existing network.

1.4.1. NAT Routing

Figure 1.3, "LVS Implemented with NAT Routing", illustrates LVS utilizing NAT routing to move
requests between the Internet and a private network.
Figure 1.3. LVS Implemented with NAT Routing
In the example, there are two NICs in the active LVS router. The NIC for the Internet has a real IP
address on eth0 and has a floating IP address aliased to eth0:1. The NIC for the private network
interface has a real IP address on eth1 and has a floating IP address aliased to eth1:1. In the event of
failover, the virtual interface facing the Internet and the private facing virtual interface are taken-over
by the backup LVS router simultaneously. All of the real servers located on the private network use the
floating IP for the NAT router as their default route to communicate with the active LVS router so that
their abilities to respond to requests from the Internet is not impaired.
In this example, the LVS router's public LVS floating IP address and private NAT floating IP address
are aliased to two physical NICs. While it is possible to associate each floating IP address to its own
physical device on the LVS router nodes, having more than two NICs is not a requirement.
Using this topology, the active LVS router receives the request and routes it to the appropriate server.
The real server then processes the request and returns the packets to the LVS router which uses
network address translation to replace the address of the real server in the packets with the LVS
routers public VIP address. This process is called IP masquerading because the actual IP addresses
of the real servers is hidden from the requesting clients.
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