Comment 0 for bug 2004145

I have fully reproducible steps to demonstrate this issue on a vanilla DigitalOcean droplet, minimal WireGuard configuration and no firewall rules. I've also seen this issue on other hosting providers.

Testing with `iperf3 -c XXX -P 5`:

- Unencrypted traffic on DigitalOcean's VPC = ~2Gbps
- WireGuard Ubuntu 18.04 = ~1.3Gbps
- WireGuard Ubuntu 20.04 = ~400Mbps
- WireGuard Ubuntu 22.04 = ~400Mbps

htop reported only 20-30% load on the vCPU core so it isn't CPU-bound.

After doing these tests, I did them all again on a different day to rule out temporary network congestion.

Steps to reproduce below. Repeat with each Ubuntu version.

0. Create a DigitalOcean account.
1. Create two $6 droplets (eg, LON1 region) with Regular CPU & 1GB RAM each, called test01 & test02.
2. `apt-get update && DEBIAN_FRONTEND=noninteractive apt-get dist-upgrade -y && reboot`
3. `apt-get install -y wireguard iperf3`

4. On test01, create `/etc/wireguard/test.conf` with these contents. Replace `XXX` with the IP address of the eth1 interface (VPC) on test01.

--------------------
[Interface]
PrivateKey = wOEa8/RS2v065wgYGQn5k7FqOXuZJ9aC/6NDW569c3g=
Address = 192.168.200.10/24
ListenPort = 51820
SaveConfig = false

[Peer]
PublicKey = wdXOzBptLD/QMZjhG475GErrz95Vpj4S7JPEwzcDMV8=
PresharedKey = j5Oeyhu/qDag2LunpVlFqKycp/9CH+Izjza5aq2cYss=
Endpoint = XXX:51820
AllowedIPs = 192.168.200.20/32
--------------------

5. On test02, create `/etc/wireguard/test.conf` with these contents. Replace `YYY` with the IP address of the eth1 interface (VPC) on test02.

--------------------
[Interface]
PrivateKey = kCJ/4rVDTy86HxP9N5wUmgMF1Esqjc051jQPGhrQIGw=
Address = 192.168.200.20/24
ListenPort = 51820
SaveConfig = false

[Peer]
PublicKey = s/GtXkHOtPsqcNDy0BSRoMuxXYb4hK18dsQdkZk20yQ=
PresharedKey = j5Oeyhu/qDag2LunpVlFqKycp/9CH+Izjza5aq2cYss=
Endpoint = YYY:51820
AllowedIPs = 192.168.200.10/32
--------------------

6. On both droplets, run `systemctl start wg-quick@test`
7. On test01, run `iperf3 -s -B XXX.XXX.XXX.XXX`
8. On test02, run `iperf3 -c XXX.XXX.XXX.XXX -P 5 -t 30` and observe ~2Gbps.
9. On test01, run `iperf3 -s -B 192.168.200.10`
10. On test02, run `iperf3 -c 192.168.200.10 -P 5 -t 30`