Port Forward Rule
Make sure the rule’s protocol is UDP.
nmap
Check if the port is open.
sudo nmap -sU -p 51820 <ip-or-endpoint>
tcpdump
Check is traffic is flowing on the interface.
sudo tcpdump -nn -i wg0
Last Handshake
On the main server, check if the peer has ever, or recently connected, by looking at the latest handshake.
$ sudo wg show
interface: wg0
public key: <value>
private key: (hidden)
listening port: 51820
peer: <value>
endpoint: <public-ip>:37451
allowed ips: 10.8.0.2/32
latest handshake: 7 seconds ago
transfer: 68.22 KiB received, 267.18 KiB sent