FAQ The port is open after a start, but closed some hours later

[Silas Mariusz]

Symptoms

• After Rtorrent 16 starts, Network & NAT shows IPv4 TCP port … open.
• Some hours later, it shows IPv4 TCP port … not reachable.

Cause
A router keeps an automatic port mapping only for a limited time. Rtorrent 16 requests a PCP or NAT-PMP mapping for one hour. A UPnP mapping lasts until the router forgets it, for example when the router restarts. Every five minutes, the Rtorrent 16 watchdog checks the mappings. With PCP / NAT-PMP on, it requests every mapping again for another hour. With only UPnP on, it reads the router's UPnP table and requests again any mapping that the router no longer has. A lost mapping therefore returns within five minutes if the router accepts the request.


Resolution

1. Check the switches. In Settings → Port forwarding & VPN, under Router port mappings, enable PCP / NAT-PMP and UPnP if your router supports them. Both are on by default.
   
   
2. Wait up to five minutes. Then open Network & NAT and press Check TCP port. Mapped as in the Network services card shows what the router granted.
3. Read the log. Open Diagnostics and select service under File in the Logs card. With only UPnP on, a lost mapping leaves a line such as "port forwarding: the router no longer had the UPnP mapping for …; mapped again". If the line ends with "not mapped again", the router refused the new request.
4. If the router keeps refusing, forward the ports manually on the router: the BitTorrent port over TCP and UDP, and the DHT port over UDP, to the NAS's LAN address.

Notice & Advise

• When you stop Rtorrent 16, it removes the mappings it made. It requests them again at the next start.
• The QNAP System Logs get a port forwarding line only when something changes, for example when the router starts refusing a mapping.

Ref. links:
Rtorrent 16 guide: Ports