Screaming Penguin

Lookout, IP masquerading in the 2.2.x kernels has some exploitation possibilities. It turns out that UDP masquerading can be used to create a tunnel INTO the masq machine and thus the internal network. This requires the successful takeover of UDP ports used in the masquerade, but as the article points out, it is not too terribly difficult to determine those ports. Currently there is not much of a real solution other than "disable UDP masquerading." Check the securityFocus article and note that securityFocus is a mighty fine site.   Linux 2.2.x Masquerading Exploits: SecurityFocus