What does "UPnP/NAT-PMP: Port mapping failure" mean in qBittorrent?

Question

There is an error UPnP/NAT-PMP: Port mapping failure, message: could not map port using UPnP: no router found (always appears thrice with the same timestamp) in execution log (view > log > show). No other errors. The message appears when the client connects. I haven't noticed any speed or connection issues, however.
qBittorrent 3.3.12.

Solution

If you are not facing any issues you can safely ignore this message. As can be seen here.

UPnP/NAT-PMP is used here to Forward/Open ports in an upstream router to provide a connection to your machine from outside.

This port-forwarding can fail in following scenarios:

1. Upstream router does not support UPnP/NAT-PMP
2. UPnP/NAT-PMP is disabled on upstream router.
3. No upstream router. (You are directly connected to Internet)

Port-forwarding can succeed but still in fail in following scenario:

1. Double-NAT/Multiple-NAT. First level up supports UPnP/NAT-PMP but further routers do not Port-Forward to you.

Comments

[AEonAX]

It means that there is no UPnP enabled router accessible directly from your machine. Here UPnP is used to configure Port Forwarding to your machine through the router.

[user598527]

@AEonAX: Can I ignore this error? Are there drawbacks?

[AEonAX]

If it works!, you can ignore.