possible issue with IPv4 mapped address and $REMOTE_ADDR in CGI

[itojun@iijlab.net]

        because of the difference in http server in handling IPv4 mapped
        address (IPv4 traffic goes into the system into AF_INET6 socket),
        the peer's address passed by $REMOTE_ADDR to CGI script can vary
        between the http server.
        some http server would pass IPv4 mapped address as is (::ffff:10.1.1.1),
        and some http server would pass IPv4 mapped address converted into
        IPv4 address (10.1.1.1).  this complexity could confuse CGI program
        writers and could open up vulnerability if CGI program uses
        $REMOTE_ADDR for authentication.

        solution: do not accept IPv4 traffic by using AF_INET6 socket.  use
        AF_INET socket (http server should listen to AF_INET and AF_INET6
        socket explicitly).  draft-cmetz-v6ops-v4mapped-api-harmful-01.txt
        talks more about this issue.

itojun