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My router's configuration app (browser-interface) has a page for Virtual Server. It has entries I did not create, at least not intentionally using the interface, for Teredo, Skype, Spotify, uTorrent.

I have two questions about this:

1) how did these entries get there in the first place?

2) I changed the IP address of the gateway from 192.168.1.1 to 192.168.2.1 because we just got FIOS internet in addition to COMCAST, intending to phase out the COMCAST in the very near future; the FIOS router was defaulted to 192.168.1.1 so I changed my router's IP address to ...2.1 and now some though not all of these entries for Teredo, Skype, Spotify, and uTorrent still show the 192.168.1.x IP address. Can these "stale" addresses be deleted?

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  • This is probably a better question for SuperUser.SE, since it's not actually related to security.
    – Xander
    Feb 17, 2013 at 15:33

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Several protocols exist, which allow applications running behind NAT-aware routers to configure the router to make some redirections, i.e. to allow these applications to be contacted from the outside. See e.g. IGD and NAT-PMP. Without such specific protocols, applications which want to do direct client-to-client communications (e.g. Skype) need to resort to cruder tools like UDP hole punching.

Thus, your entries were added automatically, on behalf of the applications which asked for them. How to remove stale entries is entirely device-dependent. Simply rebooting your router might do the trick; or not.

From a security point of view, this highlights the fact that NAT is not totally equivalent to a firewall which blocks incoming connections. Applications which run on the machines behind the NAT and intent on doing some server-type actions can still do them.

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  • Thank you @Thomas Pornin for the lucid and informative answer.
    – user20874
    Feb 17, 2013 at 15:53

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