To enable remote assistance from Internet of a WinXP PC on a LAN using NAT one must have a UP&P NAT compatible router.
UPnP(tm) is basically SNMP done with SOAP instead of ASN.1, with some bits of ZeroConf thrown in for good measure.
It involves resource discovery, service discovery and property manipulation.
If you're interested, the specification for an "Internet Gateway Device" is available from the UPnP website. Basically, the IGD allows (authorised) machines to request the gateway to do things such as: - Connect to the internet - report statistics - create a forwarded port
The "create a forwarded port" part is used by Remote Assistance as well as MSN Messenger.
These are not things that can be emulated in netfilter - you'll need a SOAP service, and a whole bunch of other software to implement the IGD specification.
Last time I looked, some people had in fact implemented UPnP services for Linux, but I'm not sure whether IGD was implemented.
Regards Alex
