Direct Server Return (DSR) is a method whereby traffic hits the LoadMaster on the way in and bypasses the LoadMaster on the way out.
This feature was created to deal with a specific problem. When response traffic is greater than the request traffic, a load balancer could become a bottleneck. This is particularly relevant when using sensitive real-time applications like UDP services delivering real-time video and audio content.
Other reasons can include the need for transparency when the client is in the same subnet as the Real Server.
After configuring DSR, the traffic flow might leave you scratching your head because a packet capture can look strange. The LoadMaster forwards the request from the client to the loopback adapter. The client's IP and Virtual IP always remains the same. However, the MAC addresses will be changed.
The below diagram shows the traffic flow. The client preforms a three-way handshake with the Virtual IP. Each client request is forwarded to the loop back address on the Real Server (blue lines). The response from the Real Server is returned directly to the client (red lines). This traffic will not flow back through the LoadMaster as it is routed through the Real Server's gateway.