I'm new to Linux.
On Windows I'm used to have the firewall triggered automatically by an application and then I authorize whether or not to let the application access real world.

Linux using UFW has more manual approach which is OK by me.

What I found is an interesting phenomenon.
I opened port 51413 for Transmission on GUFW (Both TCP and UDP):

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Yet still on Transmission it seems the port isn't open:

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I thought maybe something isn't working with the UPNP (Though the same computer with Windows configure the Router with UPNP perfectly on Transmission for Windows). So I downloaded small utility to see the UPNP configuration - UPnP Router Control.

What I saw was very strange:

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Skype configured ports using UPNP which means UPNP works.
Now the questions:

  1. It seems the UPNP is working, how come Transmission isn't able to properly open the port? Or even do it like Skype?
  2. How the hell Skype could bypass the UFW and open port to the outside world without any permission?
  3. What's the UPNP policy? Can any UPNP application just bypass the UFW?

Thank You.

  • I don't know about UPNP, but should be able to open the port manually on the router, should solve the transmission problem for now... – Xen2050 Jul 27 '17 at 20:38
  • @Xen2050, Yes, I know. But I want to understand why Skype could open port without my knowledge and bypass UFW. This is security thing, isn't it? – Royi Jul 27 '17 at 20:44
  • Just noticed - try checking the "Use UPnP..." checkbox in Transmission's options and see what happens then (maybe after a transmission quit & restart). And was it Skype for ubuntu that opened the port? Maybe Transmission just doesn't know how (or didn't try because of the unchecked option above) – Xen2050 Jul 27 '17 at 20:58
  • @Xen2050, I played with the UPNP option in Transmission yet what I'm more curious about is the port policy of Linux. I thought something like that can never happe, application doing something with no permission. On Windows the firewall will ask the user permission. – Royi Jul 27 '17 at 21:08
  • I'm not sure, could try turning off the firewall (ufw) and then see if Transmission & skype can still open router ports - could be that ufw doesn't stop outgoing traffic at all & should have no effect. Reading up on ufw should provide much more detail, I haven't investigated it much but I'd start with man/info pages & google. I see a "logging on|off|LEVEL" option that might be handy, or "ufw app info <name>" – Xen2050 Jul 27 '17 at 21:34

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