Troubleshooting WireGuard connections

[Riccardo Berto]

Sorry for the late answer, I've been busy with exams this week.

I updated WireGuard to the latest snapshot 20180420 on both server and 
peers.

I use unique key pairs for every host and I'm using the right 
privkey/pubkey combo, I just checked manually via the `wg pubkey` 
command.
I also tried removing all the peers but one Raspberry Pi, I'm still 
getting this weird output on the server from `tcpdump -vv -ni ens3 'port 
51820'`:

15:45:49.138470 IP (tos 0x0, ttl 52, id 27235, offset 0, flags [none], 
proto UDP (17), length 176)
     raspberry-ip.51820 > server-ip.51820: [udp sum ok] UDP, length 148
15:45:49.138883 IP (tos 0x88, ttl 64, id 2728, offset 0, flags [none], 
proto UDP (17), length 120)
     server-ip.51820 > raspberry-ip.51820: [bad udp cksum 0x92e3 -> 
0x0eae!] UDP, length 92
15:46:05.778398 IP (tos 0x0, ttl 52, id 27850, offset 0, flags [none], 
proto UDP (17), length 176)
     raspberry-ip.51820 > server-ip.51820: [udp sum ok] UDP, length 148
15:46:05.778890 IP (tos 0x88, ttl 64, id 5807, offset 0, flags [none], 
proto UDP (17), length 120)
     server-ip.51820 > raspberry-ip.51820: [bad udp cksum 0x92e3 -> 
0x85d0!] UDP, length 92
15:46:22.419043 IP (tos 0x0, ttl 52, id 28761, offset 0, flags [none], 
proto UDP (17), length 176)
     raspberry-ip.51820 > server-ip.51820: [udp sum ok] UDP, length 148
15:46:22.419748 IP (tos 0x88, ttl 64, id 8596, offset 0, flags [none], 
proto UDP (17), length 120)
     server-ip.51820 > raspberry-ip.51820: [bad udp cksum 0x92e3 -> 
0x6d8c!] UDP, length 92

Removing everything and simply adding the "always working" peer (my 
laptop) in the server config makes it working perfectly fine.