Azure : What does the Direct Server Return option do for a Load Balancer?

Introduction

When setting up a load balancing rule in Azure, you’ll be given the opportunity to enable/disable “Direct Server Return”.

2016-08-18 16_06_29-Add load balancing rule - Microsoft Azure

 

So what does it do?

Apart from disabling the “backend port” input field, what does it do? Clicking on the “?” gives us a start…

2016-08-18 16_06_00-Add load balancing rule - Microsoft Azure

Basically, DSR (Direct Server Return) will disable any NAT involved. So the targetted VM should be aware of the loadbalancer IP, or the network flow will break.

So it’s usefull to use as a cluster IP address (for example, when using a cluster IP), though do NOT use it for typical load balancing scenario’s where the nodes aren’t aware of the cluster address.

 

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2 thoughts on “Azure : What does the Direct Server Return option do for a Load Balancer?

    1. Hey Roger,
      DSR is a common feature, present in many load balancers out there. The main goal is not having the return traffic processed by the LB, and thus spare LB CPU cycles (and eventually reduce latency and eliminate the LB as possible bandwidth bottleneck).
      If you google for DSR you will find articles by many ADC vendors explaining use cases in detail, but the one I have seen the most is heavily asymmetric traffic, such as video serving apps.
      Hth,

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