Red Hat ENTERPRISE LINUX 4 - VIRTUAL SERVER ADMINISTRATION Manual page 12

Chapter 1. Linux Virtual Server Overview
Figure 1.1. A Basic LVS Configuration
Service requests arriving at the LVS routers are addressed to a virtual IP address, or VIP. This is a
publicly-routable address the administrator of the site associates with a fully-qualified domain name,
such as www.example.com, and is assigned to one or more virtual servers. A virtual server is a
Section 4.6, "VIRTUAL SERVERS"
service configured to listen on a specific virtual IP. Refer to for
more information on configuring a virtual server using the Piranha Configuration Tool. A VIP address
migrates from one LVS router to the other during a failover, thus maintaining a presence at that IP
address (also known as floating IP addresses).
VIP addresses may be aliased to the same device which connects the LVS router to the Internet. For
instance, if eth0 is connected to the Internet, than multiple virtual servers can be aliased to eth0:1.
Alternatively, each virtual server can be associated with a separate device per service. For example,
HTTP traffic can be handled on eth0:1, and FTP traffic can be handled on eth0:2.
Only one LVS router is active at a time. The role of the active router is to redirect service requests
from virtual IP addresses to the real servers. The redirection is based on one of eight supported load-
Section 1.3, "LVS Scheduling Overview".
balancing algorithms described further in
The active router also dynamically monitors the overall health of the specific services on the real
servers through simple send/expect scripts. To aid in detecting the health of services that require
dynamic data, such as HTTPS or SSL, the administrator can also call external executables. If a
service on a real server malfunctions, the active router stops sending jobs to that server until it returns
to normal operation.
The backup router performs the role of a standby system. Periodically, the LVS routers exchange
heartbeat messages through the primary external public interface and, in a failover situation, the
private interface. Should the backup node fail to receive a heartbeat message within an expected
interval, it initiates a failover and assumes the role of the active router. During failover, the backup
router takes over the VIP addresses serviced by the failed router using a technique known as ARP
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