Evil Data in Orphaned Echo Replies

Install a echocheck
                # ipfwechochk 100
Install a pre-output filter
                icmp {
                        switch icmptype {
                        case echo:
                                call("echochk");
                                break;
                        }
                }
                next;
Install a pre-input filter
                icmp {
                        switch icmptype {
                        case echoreply:
                                ! call("echochk") {
                                        reject;
                                }
                                break;
                        }
                }
                next;