Setting RequestHeader in Apache

Apache can be used to inject a Request Header in the incoming request that can be either consumed by Apache or forwarded further to another underlying service, in this case Apache works essentially as a reverse proxy.

In a test setup where Apache works as a reverse proxy in front of tomcat, the below Apache configuration is used to implement the reverse proxy functionality and add a Request Header:

<VirtualHost *>
   <Location "/sherif">
      ProxyPass http://127.0.0.1:8080/sherif
      ProxyPassReverse http://127.0.0.1:8080/sherif
      RequestHeader set myh "valueofarequestheader"
   </Location>
</VirtualHost>

In this setup, tomcat is using default port 8080, it has a defined context path for a dummy application defined in tomcat under its context.xml:

<Context>
    <WatchedResource>WEB-INF/web.xml</WatchedResource>
    <WatchedResource>WEB-INF/tomcat-web.xml</WatchedResource>
    <WatchedResource>${catalina.base}/conf/web.xml</WatchedResource>

    <Manager pathname="/sherif" />
</Context>


Under tomcat webapps, we have folder created as sherif and has a simple index.html file to support the test:
[root@localhost conf]# ls -lt ../webapps/sherif/
total 4
-rw-r--r-- 1 root root 14 Mar 19 16:27 index.html
[root@localhost conf]#

To verify the header being added, we configure tomcat to log the head myh.
This is done on tomcat server.xml accesslog value as below:

<Valve className="org.apache.catalina.valves.AccessLogValve" directory="logs"
               prefix="localhost_access_log" suffix=".txt"
               pattern="%h %l %u %t &quot;%r&quot; %s %b %{myh}i" />

Once a request is sent to http://localhost/sherif, tomcat logs the below log showing the request header being added by Apache and reaching tomcat:

127.0.0.1 - - [19/Mar/2019:16:46:47 -0400] "GET /sherif/ HTTP/1.1" 200 17 valueofarequestheader

This configuration is useful in passing headers to backend services in case those are not already sent by the source user agent.