The is no real good way, unless you block all incoming traffic. (ICMP,TCP,UDP).
Obviously if this is a server, that may not be feasible. You could attempt to block all ports except the one service port you need open, but that service may provide enough information to "fingerprint" your machine. In which case you could attempt to customize the service (say Apache) to report differently.
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...