To remove the origin port from the HTTP header forwarded to the backend pool, you must rewrite the header that communicates the original host to the backend. The X-Forwarded-Host header serves this purpose by carrying the original Host header value from the client's request.

By setting the value of this header to the Azure Application Gateway server variable host, you are populating it only with the hostname from the incoming request. The host variable inherently excludes the port number, effectively stripping it before the request is sent to the backend. For example, if the incoming request's Host header is www.contoso.com:8080, setting X-Forwarded-Host to host will result in the header having a value of www.contoso.com.

Microsoft Azure Documentation | Rewrite HTTP headers and URL with Application Gateway: This official documentation details the capabilities of Application Gateway rewrite rules. Under the "Rewrite headers" section, it lists the supported headers, including X-Forwarded-Host. The "Server variables" section explains the available variables.

Section: Server variables

Details: The documentation clarifies how server variables can be used to store information about requests and responses. The host variable specifically extracts the hostname from the Host request header, excluding the port.

Microsoft Azure Documentation | How an application gateway works: This document provides a high-level overview of the request flow. It explains that the gateway acts as a reverse proxy, modifying requests before forwarding them to backend servers. This modification capability includes rewriting headers like X-Forwarded-Host to ensure proper routing and application behavior on the backend.

Section: How a request is routed

Details: The document implies the necessity of header manipulation for seamless communication between the client, gateway, and backend, which justifies the use of rewrite rules.