The simplest form of load balancing is round-robin DNS, where multiple A records are defined for the same name; this results in the servers taking turns responding to any incoming requests. This doesn’t work well if a server fails, though, and it doesn’t take into account any special needs the service may have. With HTTP, for example, we might need to maintain session data such as authentication or cookies and ensure that the same client always connects to the same server. To meet these needs, we’ll get a little more sophisticated and use two tools:

1. IP Virtual Server (IPVS), a transport-level (TCP) load-balancer module that is now a standard Linux component
   
2. ldirectord, a utility that monitors the health of the load-balanced physical servers

The installation instructions are based on the Debian 3.1 (Sarge) Linux distribution.

Since IPVS is already in the Linux kernel, we don’t need to install any software, but we do need to configure it.

On lb, add these lines to /etc/modules.

  ip_vs_d h
  ip_vs_ftp
  ip_vs
  ip_vs_lblc
  ip_vs_lblcr
  ip_vs_lc
  ip_vs_nq
  ip_vs_rr
  ip_vs_sed
  ip_vs_sh
  ip_vs_wlc
  ip_vs_wrr

Then load the modules into the kernel:

  # modprobe ip_vs_dh
  # modprobe ip_vs_ftp
  # modprobe ip_vs
  # modprobe ip_vs_lblc
  # modprobe ip_vs_lblcr
  # modprobe ip_vs_lc
  # modprobe ip_vs_nq
  # modprobe ip_vs_rr
  # modprobe ip_vs_sed
  # modprobe ip_vs_sh
  # modprobe ip_vs_wlc
  # modprobe ip_vs_wrr

To enable packet forwarding in the Linux kernel on lb, edit the file /etc/sysctl.conf and add this line:

  net.ipv4.ip_forward = 1

Then load this setting into the kernel:

  # sysctl -p
  net.ipv4.ip_forward = 1