Mod_rewrite Proxy – maintain host headers

My suspicions turned out to be correct. I had to modify the query string with mod_rewrite and then proxy it to my destination. In the end:

1. I enabled mod_rewrite and mod_ssl
2. I added an SSLProxyEngine directive to my vhost definition:
       SSLProxyEngine On

3. I turned on rewrite and added a rule in my vhost def. I also turned on logging for debugging. I learned that I wouldn't be able to rewrite a request for the directory so I had to point to a script. I also had to use the QSA and P options:
       RewriteEngine On
       RewriteRule ^/lookup.php /lookup?key=MYKEY [QSA,P]

4. I set up my proxy rules. Since the query string is constructed, simple proxying rules apply:
       ProxyPass /lookup https://path.to.remote/search.php
       ProxyPassReverse /lookup https://path.to.remote/search.php

5. Don't forget to restart apache! (I did)

6. I now connect to http://localhost/lookup.php?term=SEACRHTERM

My colleague came up with a solution. I had made the mistake of assuming that using SetEnvIf by itself was sufficient. This is now working for us:

RewriteEngine on
# Anything ending with /application/.+ goes to http://apphost:9090
RewriteRule ^/.+(/application/.+)$ http://apphost:9090$1 [P]
# Match SM_USER= in the URI and set an env var with its value.
SetEnvIf Request_URI "SM_USER=([^/]+)" SM_USER=$1
# If the env var is set, put the value into a request header.
RequestHeader set SM_USER "%{SM_USER}e" env=SM_USER

This URI is proxied to http://apphost:9090/application/index.html with the HTTP header SM_USER set to fred@company.com:

/SM_USER=fred@company.com/application/index.html