Obtain client IP address through a network load balancer (TCP socket server, Python)

Question

Hi All,

I run a couple of TCP socket server on EC2, behind a network load balancer (NLB). 
even I've set the target_group to preserve client IP address.  I am not able to obtain the actual client IP address in my Python TCP socket server. All I am getting seems to be a few fixed IP address, likely belong to the load balancer or some edge computing node ?

when I bypass the load balancer, my server app is able to obtain the proper client IP address.

any idea, what I am missing here ?

Answer

More info on this topic:

* For Network Load Balancers, register your targets by instance ID to capture client IP addresses without additional web server configuration. For instructions, see [Target group attributes](https://docs.aws.amazon.com/elasticloadbalancing/latest/network/load-balancer-target-groups.html#target-group-attributes) instead of the following resolutions (refer the link)

* For Network Load Balancers when you can register only IP addresses as targets, enable `proxy protocol version 2` on the load balancer. For instructions, see [Enable proxy protocol](https://docs.aws.amazon.com/elasticloadbalancing/latest/network/load-balancer-target-groups.html#enable-proxy-protocol) instead of the following resolutions.

Reference: https://aws.amazon.com/premiumsupport/knowledge-center/elb-capture-client-ip-addresses/

Answer

Behaviour depends on the type of ELB - NLB preserves Source IP (except for IP Address targets), whereas servers behind CLB & ALB see load balancer IPs as Source IPs in client requests and rely on x-forwarded-for header to indicate true source.

Obtain client IP address through a network load balancer (TCP socket server, Python)

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