There is an industry standard(but not RFC) header available for HTTP called x-forwarded-for, that identifies the originating IP address of an HTTP request, regardless of if it goes through a proxy or load balancer. One workaround is to utilize the x-forwarded-for header option on your proxy. We lose visibility into what the original client IP address is, all traffic appears as it is coming from one single IP address of the web proxy making IPS logs less useful. In an ideal situation you would be able to place the IPS in a position where it would examine the actual source IP address but not all networks may be able to accommodate this. Normally this wouldn’t cause a problem but when you want to setup the IPS appliance to look at all traffic between the inside and firewall it presents an issue. When deploying an IPS appliance I saw a challenge that might come up if you are installing the IPS appliance in addition to a web proxy. One of the by-products of using the default settings of the proxy is that all user traffic going through the proxy ends up being NATted to the IP address of the proxy prior to going to the firewall.

Wireshark http proxy