4.2. LVS Cluster via Direct Routing
As mentioned in Section 2.4.2, “Direct Routing”, direct routing allows real servers to process and route packets directly to a requesting user rather than passing outgoing packets through the LVS router. Direct routing requires that the real servers be physically connected to a network segment with the LVS router and be able to process and direct outgoing packets as well.
In a direct routing LVS setup, the LVS router needs to receive incoming requests and route them to the proper real server for processing. The real servers then need to directly route the response to the client. So, for example, if the client is on the Internet, and sends the packet through the LVS router to a real server, the real server must be able to go directly to the client via the Internet. This can be done by configuring a gateway for the real server to pass packets to the Internet. Each real server in the server pool can have its own separate gateway (and each gateway with its own connection to the Internet), allowing for maximum throughput and scalability. For typical LVS setups, however, the real servers can communicate through one gateway (and therefore one network connection).
It is not recommended to use the LVS router as a gateway for the real servers, as that adds unneeded setup complexity as well as network load on the LVS router, which reintroduces the network bottleneck that exists in NAT routing.
The hardware requirements of an LVS system using direct routing is similar to other LVS topologies. While the LVS router needs to be running Red Hat Enterprise Linux to process the incoming requests and perform load-balancing for the real servers, the real servers do not need to be Linux machines to function correctly in the cluster.The LVS routers need one or two NICs each (depending on if there is a back-up router). You can use two NICs for ease of configuration and to distinctly separate traffic — incoming requests are handled by one NIC and routed packets to real servers on the other.
Since the real servers bypass the LVS router and send outgoing packets directly to a client, a gateway to the Internet is required. For maximum performance and availability, each real server can be connected to its own separate gateway which has its own dedicated connection to the carrier network to which the client is connected (such as the Internet or an intranet).
There is some configuration outside of Piranha Configuration Tool that needs to be done, especially for administrators facing ARP issues when using LVS via direct routing. Refer to Section 4.2.1, “Direct Routing and arptables_jf” or Section 4.2.2, “Direct Routing and IPTables” for more information.