Installation guide

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.