Ciscoのiosの話でよければALGっていうのがあって、こいつがアプリケーション層を書き換えてる。 ALG is an Application Layer Gateway (ALG). NAT performs translation service on any Transmission Control Protocol/User Datagram Protocol (TCP/UDP) traffic that does not carry source and/or destination IP addresses in the application data stream. These protocols include FTP, HTTP, SKINNY, H232, DNS, RAS, SIP ,TFTP, telnet, archie, finger, NTP, NFS, rlogin, rsh, rcp. Specific protocols that embed IP address information within the payload require support of an Application Level Gateway (ALG).
