The purpose, presumably, is to make it harder for a target site to detect the OpenProxy. If the target attempts to scan the OpenProxy by asking it to GET a given verification URL, the handler for that URL will not be able to match the incoming IP address to the given OpenProxy being checked.
In practice, however, this is totally useless. All you do is make the verfication URL handler automatically ban the requesting IP, and then start a new OpenProxy scan on that IP. Eventually, you will scan and ban the entire ring.
PROXY-SCAN(ip) IF ip is not already scanned or banned FOR EACH port in PORTS, where PORTS is the set of potential OpenProxy ports make an HTTP proxy request on ip:port for the self-ban URL END FOR EACH END IF End PROXY-SCAN
SELF-BAN, the handler for the self-ban URL ip <-- REMOTE_ADDR Ban(ip) PROXY-SCAN(ip) End SELF-BAN