Port forwarding checkers don't work until AFTER you connect through the app/device

[decker]

Using RT-AC86U on merlin (second to latest update). I opened a port to use with a p2p app. Then I verified with a port checker (https://canyouseeme.org/) to make sure it was working. It kept saying error it could not see the port. I was a bit confused since opening a port is pretty straightforward. I tried with different port numbers, different protocols (TCP/UDP/both) and they all failed. Finally I opened the app and tried to connect anyway. It connected fine. I went back and retested the port checker and now it said it can see the port. This happens with every port you open -- you have to actually make a connection through the port with the app or device before the port checker will say it is open which makes the port checker useless. I tried a different port checker with the same result. I'm pretty sure it wasn't like this with my last router (non-Asus).

Is this some security measure with the RT-AC86U or a bug?

 

[ColinTaylor]

What "p2p app" are you talking about? Is it something that runs on your LAN, or is a client app (like on a phone) that connects to a server somewhere?

If this "app" runs on your LAN then it must be running for a port checker to see that the port is open.

 

[decker]

What "p2p app" are you talking about? Is it something that runs on your LAN, or is a client app (like on a phone) that connects to a server somewhere?

If this "app" runs on your LAN then it must be running for a port checker to see that the port is open.

It's a torrent client app called Tixati, similar to Qbittorrent. But it happens on every client app.

 

[ColinTaylor]

It's a torrent client app called Tixati, similar to Qbittorrent. But it happens on every client app.

OK. So as I said above, an app (or service) must be running for any port checker to work. This is always the case.

P.S. You said you "opened a port". I assume you meant that you created a port forwarding rule. Correct? Most BitTorrent clients don't need a port to be manually forwarded because they have an option to use UPnP to do it automatically (assuming UPnP is enabled on the router).

 

[decker]

OK. So as I said above, an app (or service) must be running for any port checker to work. This is always the case.

P.S. You said you "opened a port". I assume you meant that you created a port forwarding rule. Correct? Most BitTorrent clients don't need a port to be manually forwarded because they have an option to use UPnP to do it automatically (assuming UPnP is enabled on the router).

Hmm I could have sworn you were able to check the port at any time once it was forwarded. How does the router know if I configure a port in an app vs configuring a port in a port checker?

I know I can use UPnP but I heard it was less safe than port forwarding so I use that instead.

 

[ColinTaylor]

Hmm I thought once you forwarded a port it was considered always open and you could check to see if it was open at any time.

No, simply forwarding a port is not enough. For a port to be detected as "open" there has to be something listening on the destination port to acknowledge the connection.

How does the router know if I configure a port in an app vs configuring a port in a port checker?

I know I can use UPnP but I heard it was less safe than port forwarding so I use that instead.

Your choice. Either way will work.

 

[decker]

No, simply forwarding a port is not enough. For a port to be detected as "open" there has to be something listening on the destination port to acknowledge the connection.

Oh that's what I thought a port checker was doing. My mistake then. Thanks for clearing that up