This needs more fleshing out, but see: http://jira.codehaus.org/browse/GEOS-598

You can now configure GeoServer to have the capabilities document properly report a proxy. We'd like to get this so it is easier to configure, but figured it's better to have the option at all. To configure a proxy, add this line to your web.xml

<context-param>
  <param-name>PROXY_BASE_URL</param-name>
  <param-value>http://wms.externalhost.com/</param-value>
</context-param>

Where the value is the url to have GeoServer report in the capabilities document.

In the 1.6.x series and later, this setting has moved to $GEOSERVER_DATA_DIR/services.xml .  You can configure it by browsing to http://localhost:8080/geoserver/config/server.do; (replace localhost:8080 with the appropriate hostname and port for you your geoserver instance) and entering the desired value in the field labelled (Proxy base URL).

This is not working for me using geoserver-1.6.0-RC1.exe Posted by yecarrillo at Dec 17, 2007 16:39