The following is a small, but complete, Metaproxy configuration file (included in the distribution as metaproxy/etc/config1.xml). This file defines a very simple configuration that simply proxies to whatever back-end server the client requests, but logs each request and response. This can be useful for debugging complex client-server dialogues.

<?xml version="1.0"?>
<metaproxy xmlns="http://indexdata.com/metaproxy" version="1.0">
  <dlpath>/usr/lib/metaproxy/modules</dlpath>
  <start route="start"/>
  <filters>
    <filter id="frontend" type="frontend_net">
      <port>@:9000</port>
    </filter>
    <filter id="backend" type="z3950_client">
    </filter>
  </filters>
  <routes>
    <route id="start">
      <filter refid="frontend"/>
      <filter type="log"/>
      <filter refid="backend"/>
      <filter type="bounce"/>
    </route>
  </routes>
</metaproxy>

It works by defining a single route, called start, which consists of a sequence of four filters. The first and last of these are included by reference: their <filter> elements have refid attributes that refer to filters defined within the prior <filters> section. The middle filter is included inline in the route.

The four filters in the route are as follows: first, a frontend_net filter accepts Z39.50 requests from any host on port 9000; then these requests are passed through a log filter that emits a message for each request; they are then fed into a z3950_client filter, which forwards all Z39.50 requests to the client-specified back-end Z39.50 server. Those Z39.50 packages are returned by the z3950_client filter, with the response data filled by the external Z39.50 server targeted. All non-Z39.50 packages are passed through to the bounce filter, which definitely bounces everything, including fish, bananas, cold pyjamas, mutton, beef and trout packages. When the response arrives, it is handed back to the log filter, which emits another message; and then to the frontend_net filter, which returns the response to the client.