Linux Virtual Server

Linux Virtual Server (LVS) is load balancing software for Linux kernel–based operating systems.
LVS is a free and open-source project started by Wensong Zhang in May 1998, subject to the requirements of the GNU General Public License (GPL), version 2. The mission of the project is to build a high-performance and highly available server for Linux using clustering technology, which provides good scalability, reliability and serviceability.
Overview.
The major work of the LVS project is now to develop advanced IP load balancing software (IPVS), application-level load balancing software (KTCPVS), and cluster management components.
LVS can be used for building highly scalable and highly available network services, such as web, email, media and VoIP services, and integrating scalable network services into large-scale reliable e-commerce or e-government applications. LVS-based solutions already have been deployed in many real applications throughout the world, including Wikipedia.
The LVS components depend upon the Linux Netfilter framework, and its source code is available in the codice_1 subdirectory within the Linux kernel source. LVS is able to handle UDP, TCP layer-4 protocols as well as FTP passive connection by inspecting layer-7 packets. It provides a hierarchy of counters in the codice_2 directory.
The userland utility program used to configure LVS is called ipvsadm, which requires superuser privileges to run.
Schedulers.
LVS implements several balancing schedulers, listed below with the relevant source files:
Glossary.
Commonly used terms include the following:
Examples.
Setting up a virtual HTTP server with two real servers:
ipvsadm -A -t 192.168.0.1:80 -s rr
ipvsadm -a -t 192.168.0.1:80 -r 172.16.0.1:80 -m
ipvsadm -a -t 192.168.0.1:80 -r 172.16.0.2:80 -m
The first command assigns TCP port 80 on IP address 192.168.0.1 to the virtual server. The chosen scheduling algorithm for load balancing is round-robin (codice_14). The second and third commands are adding IP addresses of real servers to the LVS setup. The forwarded network packets shall be masked (codice_15).
Querying the status of the above configured LVS setup:
IP Virtual Server version 1.0.8 (size=65536)
Prot LocalAddress:Port Scheduler Flags
 -> RemoteAddress:Port Forward Weight ActiveConn InActConn
TCP 192.168.0.1:80 rr
 -> 172.16.0.2:80 Masq 1 3 1
 -> 172.16.0.1:80 Masq 1 4 0