Route incoming port:80 connection to other using ipfw.

cx.pow.firewall.plist

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
        <key>Label</key>
        <string>cx.pow.firewall</string>
        <key>ProgramArguments</key>
        <array>
                <string>sh</string>
                <string>-c</string>
                <string>ipfw add fwd 127.0.0.1,20559 tcp from any to me dst-port 80 in &amp;&amp; sysctl -w net.inet.ip.forwarding=1</string>
        </array>
        <key>RunAtLoad</key>
        <true/>
        <key>UserName</key>
        <string>root</string>
</dict>
</plist>