These servers have to be on the same domain as the gadget server. You can configure the container in eXoGadgetServer:/WEB-INF/classes/containers/default/container.js.

"gadgets.content-rewrite" : {
  "include-urls": ".*",
  "exclude-urls": "",
  "include-tags": ["link", "script", "embed", "img", "style"],
  "expires": "86400",
  "proxy-url": "http://localhost:8080/eXoGadgetServer/gadgets/proxy?url=",
  "concat-url": "http://localhost:8080/eXoGadgetServer/gadgets/concat?"
},

Configuring proxy

To allow external gadgets when the server is behind a proxy, add the following code to the beginning of the JVM:

-Dhttp.proxyHost=proxyhostURL -Dhttp.proxyPort=proxyPortNumber -Dhttp.proxyUser=someUserName -Dhttp.proxyPassword=somePassword