What is SNAT in terms of Load Balancing?

When we are in the early stages of our career and started working with Load Balancers, the SNAT concept is a little tricky one to understand. In this post, I have discussed when we go for SNAT and what it means to the traffic.

Lets Start..!!

SNAT - Source Network Address Translation, is the method used to change the Source IP address of the incoming packet and send it to the destination server.
Consider the scenario, where a load balancer is configured in between the client and server. and, there is a also a route avaliable for the Server to reach client witout the Loadbalancer say via a switch.


In this case, lets say the client sent a request for a web page that is hosted on the servers via the Load balancer. As the VIP(virtual IP) of the webpage is configured on the loadbalancer, the request reaches the load balancer and then sent to the server.

Now, if the connection speed between the Client-Switch-Server is faster than Client - Load Balancer- Server, then the server will send back the response to the client system via the switch.

If a firewall is configured in the system or before the system, the packet sent by the server to the client will appear as a "new request" instead to look like a response and will be dropped.

The client can never view the web-page.

To aovide this, we go for S-NAT. Now what happens is, when client sends request to VIP on load balancer, it forwards it to the server by changing the source ip in the header of the request packet. (The packet upon reaching the server will appear as it has originated from the Load Balancer.) Now the server sends its response to the load balancer as it never knew that the original requestor is the client system.

The Load balancer then assigns the original ip to the header and sends it to the client.