See the http headers section of the ELB documentation. Excerpt:
Application Load Balancers and Classic Load Balancers add X-Forwarded-For, X-Forwarded-Proto, and X-Forwarded-Port headers to the request.
For front-end connections that use HTTP/2, the header names are in lowercase. Before the request is sent to the target using HTTP/1.1, the following header names are converted to mixed case: X-Forwarded-For, X-Forwarded-Proto, X-Forwarded-Port, Host, X-Amzn-Trace-Id, Upgrade, and Connection. All other header names are in lowercase.
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