Direct Server Return (DSR) is an implementation approach for load balancing. In traditional ("non-DSR") load balancing architectures, both incoming requests from external clients and the corresponding server responses pass through the load balancer. In contrast, DSR architecture routes client requests through the load balancer, but server responses bypass the load balancer, returning directly to the client.
The main advantage of DSR is that it alleviates load balancer overhead by eliminating the need for the load balancer to process returning packets from servers, thus preventing bottlenecks caused by limited processing capability. However, DSR implementations can only perform Layer 4 load balancing and do not fully support SSL termination. Non-DSR architectures, while increasing load balancer workload, fully support Layer 4 and Layer 7 load balancing and enable complete SSL termination.
F5 BIG-IP supports both DSR and non-DSR configurations. Additionally, F5 provides iApps application templates to simplify configuration and deployment of DSR implementations.