Red Hat Cluster Suite, Manual

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Chapter 7. Linux Virtual Server Overview 93
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 topography, the active LVS router receives the request and routes it to the appro-
priate 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.
Using this NAT routing, the real servers may be any kind of machine running various
operating systems. The main disadvantage is that the LVS router may become a bottleneck
in large cluster deployments because it must process outgoing as well as incoming requests.
7.5. Persistence and Firewall Marks
In certain situations, it may be desirable for a client to reconnect repeatedly to the same
real server, rather than have an LVS load balancing algorithm send that request to the best
available server. Examples of such situations include multi-screen web forms, cookies,
SSL, and FTP connections. In these cases, a client may not work properly unless the trans-
actions are being handled by the same server to retain context. LVS provides two different
features to handle this: persistence and firewall marks .
7.5.1. Persistence
When enabled, persistence acts like a timer. When a client connects to a service, LVS
remembers the last connection for a specified period of time. If that same client IP address
connects again within that period, it is sent to the same server it connected to previously
— bypassing the load-balancing mechanisms. When a connection occurs outside the time
window, it is handled according to the scheduling rules in place.
Persistence also allows the administrator to specify a subnet mask to apply to the client
IP address test as a tool for controlling what addresses have a higher level of persistence,
thereby grouping connections to that subnet.
Grouping connections destined for different ports can be important for protocols which
use more than one port to communicate, such as FTP. However, persistence is not the
most efficient way to deal with the problem of grouping together connections destined for
different ports. For these situations, it is best to use firewall marks .
7.5.2. Firewall Marks
Firewall marks are an easy and efficient way to a group ports used for a protocol or group
of related protocols. For instance, if an LVS cluster is deployed to run an e-commerce site,