Setting X-REMOTE_USER in Apache proxied request

gistfile1.txt

RequestHeader unset X-REMOTE_USER
RequestHeader set X-REMOTE_USER "%{REMOTE_USER}e" env=REMOTE_USER

# see http://httpd.apache.org/docs/current/mod/mod_headers.html#header

https://stackoverflow.com/questions/33368653/how-do-i-set-remote-user-in-a-http-header/47873815#47873815

The example above is incorrect, because it assumes REMOTE_USER is an environment variable. It is not (unless some non-standard additional configuration has additionally defined such a variable).

Instead it is a special property of the request, best accessed via expr= functionality (2.4.10 or later required)

RequestHeader set X-Remote-User "expr=%{REMOTE_USER}" "expr=-n %{REMOTE_USER}"

Specifically when Apache is running a reverse proxy together with local authentication within Apache (for example mod_webauth), there is a legitimate use case for the config above to pass the username set by authentication through to the proxied app. I can imagine without this context it would not seem a very useful configuration.