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I need help troubleshooting slow wireguard tunnel (VPS to home network) (feddit.dk)
submitted 3 days ago* (last edited 1 day ago) by stavefajl@feddit.dk to c/selfhosted@lemmy.world
12 comments fedilink hide all child comments
 

Hello I hope one of you can help point me in the right direction.

I have a VPS with a static IP and a wireguard tunnel from VPS to home network (no bridging in the router, just point-to-point with specific devices).

I found an abysmal connection speed with bandwidth on the order of 50-100 kbps tested via iperf. Connection between the same devices outside the wireguard tunnel is 10-20 mbps, which is 100-400 times slower, which I don't understand since wireguard usually has very little overhead.

I have tried different MTU settings on both VPS and devices on my home network (both cabled and via wi-fi) in the range from 1360 to 1460, and above speeds are the best I have reached with MTU 1420 and 1440. I have tried both with and without iptables rules setting the mss correspondingly.

The above speeds are acceptable for incremental backups and document synchronization, but completely unsuitable for media streaming.

Where would I start diagnosing the bottleneck?

Thanks in advance.

UPDATE 2025-10-09:
I have not figured the issue out yet, and I do not know where to go from here.
I found a single similar issue (asymmetric wireguard speeds) here: https://forum.openwrt.org/t/wireguard-client-upload-slowness/190772, but that was resolved by changing MTU.

It must be a VPS issue, since when testing from multiple clients on different networks and locations, the bandwidth from client to server does not vary.
I have tried MTU from 1280 to 1460. Current best settings, MTU = 1440 on client and server, and an iptables rule:
iptables -t mangle -A FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu.

Via tcpdump I have confirmed that the two endpoints negotiates an mss of 1460 (outside wireguard), making an MTU of 1420 to 1440 appropriate for the wireguard tunnel.

UDP without wireguard can saturate network connection.
TCP without wireguard has about 50-60% slowdown compared to UDP.
Bandwidth from vps to client is decent inside wireguard tunnel.
Bandwidth from client to vps inside wireguard tunnel is still abysmal.

Current speeds UDP:

  • UDP, no wireguard: 16 mbps (client to vps) (at the time of testing, saturates client connection upload)
  • UDP, no wireguard: 270 mbps (vps to client) (at the time of testing, saturates client connection download)
  • UDP, wireguard: 80 kbps (client to vps)
  • UDP, wireguard: 106 mbps (vps to client) Current speeds TCP:
  • TCP, no wireguard: 8 mbps (client to vps)
  • TCP, no wireguard: 117 mbps (vps to client)
  • TCP, wireguard: 280 kbps (client to vps)
  • TCP, wireguard: 4 mbps (vps to client)
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[–] tychosmoose@lemmy.world 5 points 3 days ago (1 children)

Are you specifying bandwidth (-b) on the iperf UDP test? It defaults to 1M if I recall correctly, which would explain the result.

If not, try -b 10M or -b 0 for unlimited (the behavior used for TCP).

[–] stavefajl@feddit.dk 4 points 2 days ago* (last edited 2 days ago) (1 children)

Thanks! I had not read the manpage close enough, I guess.
When specifying the bandwidth, I can saturate the connection using UDP to around 15 Mbits/s. (That this speed is much much lower than the 300-500 Mbits my connection and the VPS is capable of is a problem for a different time, I think).

What I also realized is, that I had not put the iperf server in UDP-mode, so my results reported in another comment are wrong.
I read the results from the client, but the server did not respond.
When running the iperf server in UDP-mode, I get 15 Mbits/s outside the wireguard tunnel and 180 Kbits/s inside the tunnel. With TCP-mode I get 10-15 Mbits/s outside the thunnel and 250 Kbits/s inside the tunnel.

[–] tychosmoose@lemmy.world 3 points 2 days ago (1 children)

Just throwing out more ideas:

Is there a CPU spike on the VPS?

Anything weird about Wireguard on either end? Using kernel mode WG everywhere and not a user mode version, right?

As a test I would be inclined to try a very small mtu to see if it makes a difference. 1280 is a failsafe that I use when on unknown networks and trying to wg out.

Maybe try with a smaller packet size, like 1KB which I think is -l 1K

[–] stavefajl@feddit.dk 1 points 1 day ago

Thanks for the suggestions.

I see no CPU spike on the VPS, and no CPU spikes on the clients.
I use WG started by root using wg-quick via systemctl on all devices.
I tried setting the MTU to 1280, with no significant changes, apart from slight slowdown compared to MTU of 1420 or 1440.
Smaller packet size also resulted in slightly lower speeds.

I used tcpdump on both client and server to find the negotiated MSS, and it shows an MSS of 1460 outside wg tunnel, so by following the calculations shown in this article https://www.procustodibus.com/blog/2022/12/wireguard-performance-tuning/, 1440 is the correct MTU for the wireguard interface when using IPv4 inside the tunnel.

[–] Brkdncr@lemmy.world 2 points 3 days ago (2 children)

What iperf speeds are you getting with UDP?

On windows you can use the test-connection -MTUSize powershell command to figure out the correct mtu.

If your udp speeds are normal but your tcp speeds are not, it could be fragmentation, or out of order packets, or high latency.

[–] stavefajl@feddit.dk 1 points 3 days ago* (last edited 3 days ago)

I'm on linux on all devices. I tested packet fragmentation using:
ping -M do -s [packet size] [ip]
No fragmentation using packet size 1472 and below (before wireguard overhead).

[–] stavefajl@feddit.dk 1 points 3 days ago (1 children)

I just tested again:

  • no wireguard, tcp: 9.75 Mbits/s
  • no wireguard, udp: 1.05 Mbits/s
  • wireguard, tcp: 248 Kbits/s
  • wireguard, udp: 1.05 Mbits/s

So, I get the "full" udp speed, but I get some errors / warnings about 'connection refused' and 'did not receive ack'. Obviously not correctly configured, when it is 10 times slower than tcp.

[–] Brkdncr@lemmy.world 1 points 3 days ago (1 children)

What’s your mtu inside the vpn?

[–] stavefajl@feddit.dk 1 points 3 days ago

MTU is 1440.

[–] stavefajl@feddit.dk 1 points 3 days ago (1 children)

I am currently running mtr from multiple devices:

  • in wireguard tunnel: single hop 10.1.1.1, loss% 0,1, avg ping 27.4 ms
  • outside wg between same devices:
    • ISP supplied modem/router 55% loss, avg ping 1.6 ms
    • multiple hops without loss, avg ping 16-20 ms
    • random intermediary 30 % loss, avg ping 20 ms
    • endpoint, 5% loss, avg ping 25 ms

It looks like across the board my ISP modem / router is dropping 50-80 % of packets, and that packet loss is ramping up from 4% to 80 percent after a few minutes of running mtr.
It also looks like my VPS endpoint climbs to 20% packet loss over time (5-15 minutes of testing).

Can I use this information to probe further into the devices I have access to (ISP modem and VPS)?

[–] Brkdncr@lemmy.world 1 points 3 days ago (1 children)

Some devices will drop icmp packets. Does iperf show the same amount of loss?

Can you try a different internet connection such as mobile hotspot just to see if you have similar results? That could help identify it as a vps issue.

[–] stavefajl@feddit.dk 1 points 2 days ago

I had to borrow a phone to set up a mobile hotspot.
It has the same speeds inside the wireguard tunnel as when I tested from my wired connection (250 kbps TCP, 170 kbps UDP).
The loss reported by iperf is dependent on the bandwidth that i test with. But as I increase bandwidth from the client the loss grows towards 100%.

I tried testing in reverse (sending from VPS to devices on different networks) with surprising results:

  • TCP, wireguard: 5-10 mbps
  • UDP, wireguard: 50 mbps
  • TCP, no wireguard: 45 mbps
  • UDP, no wireguard: 250 mbps (saturates download speed on client when compared to speedtest.net)