Introduction

Direct Server Return (DSR) is a method whereby traffic hits the LoadMaster on the way in and bypasses the LoadMaster on the way out.

The primary advantage of DSR is that the LoadMaster only handles a portion of the work associated with load balancing, specifically the inbound traffic. The servers respond directly to the clients, bypassing the LoadMaster on the way out.

If the particular traffic profile for a site is for every packet in, eight packets are sent out, this would result in the LoadMaster handling around 87% less traffic than it would without DSR.

For DSR to work, the Virtual IP (VIP) address on a Real Server must be configured so that the server does not respond to ARP requests on the VIP address.

For Linux with a recent 2.4 kernel, this can be done by creating the VIP as an IP alias on the loopback interface. On Windows this involves creating a loopback adapter with specific configuration parameters. Refer to the Configuring a VIP on the loopback interface on Linux and DSR Configuration on Windows sections for detailed steps on how to do this in both operating systems.

When you create the Virtual Service, enable Force L4 in Standard Options and select Direct return as the Forwarding method when adding the Real Server. This means that the LoadMaster just routes the packets from a client to a Real Server without modifying the IP addresses. The Real Server accepts requests for the VIP destination address because it has configured the VIP as an IP alias. The Real Server will then reply to the IP address of the requesting client with the source IP address of the reply set to the VIP.

The table below shows an example of DSR steps.