Having your Skype client open on your PC also means that your network can be used for relaying other Skype calls if the system detects incoming TCP/UDP connections can go through your NAT firewall (e.g. due to UPnP opening a few ports). It seems this doesn't happen very often as I haven't observed a relay session over my PC until recently, when I suddenly had a 200 kbit/s data stream in both direction running over my PC for several minutes.
Wireshark is always close at hand so I could quickly confirm that the data stream was indeed to and from the Skype client. A Reverse DNS lookup then revealed that the call was between two other PCs in Germany in networks of different providers. That probably means that the DSL routers in those networks do not have UPnP enabled, otherwise the IP packets could have been exchanged directly between the two parties. Also interesting to note was that the relaying went over my Wi-Fi. So unknowingly to those on the call their data packets went over a wireless link at least twice, probably even four times if you assume that both parties used Wi-Fi at their end as well.
3 thoughts on “Being a Skype Relay – Twice Over Wi-Fi”
Can you clarify a point please. You state that the PC was behind a wifi router. Was it NaTted? If so, then apparently even NATted PCs can be a relay node, contrary to generally believed model.
UPnP opens the ports as stated in the blog post. Otherwise, as you say, it would not work due to the NATing.
I didn’t know this about Skype relaying over Wi-Fi connected notebook computers. I just assumed that it would “choose” hard wired devices due to latency issues. So what about smart phones with Skype Apps. also being relays?
My experience is that Skype is good with PC to PC connections (i.e. better/faster hardware), but not as good when using mobile devices with Skype applications. Recent mobile network call quality from Ireland (O2 GSM) to USA (T-Mobile GSM or UMA) was superior to Skype calls (Notebook PC Wi-Fi to N900 Wi-Fi). Maybe your example of call routing has something to do with it?
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