Super slow VPN connection with NAS

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Joined: Wed Oct 09, 2019 9:49 am

Post by stu » Wed Oct 09, 2019 9:55 am
Hi guys,
I have set up VPN through Viscosity and a Synology NAS (ds218j) I understand that it is a relatively low end/entry level NAS, but it does all I want, for the majority..

I have created and setup the VPN however it is super slow when I connect to my NAS. Basically unusable.. It'll take roughly 3-5mins to open a 5mb photo. This is done through connecting to hotspot via my iPhone.
I have just run a speed test on the iPhone hotspot and I am getting 26.2mbps down & 8.40mbps up. So the VPN should be running much faster than it currently is.

Anyone have any tips on what I may have done, or what I can do to speed the VPN connection up?


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Post by James » Wed Oct 09, 2019 8:50 pm
Hi stu,

There are a few things I'd recommend checking:

1. Ensure that the CPU on your OpenVPN server is capable of the performance you expect. You can try connecting to it locally and see what performance you can manage. If it's lower than you'd expect trying lowering the cipher strength of the VPN connection (which should be less taxing on the server).

2. Try lowering the MTU and mssfix values of the VPN connection. Many mobile networks use a lower than normal MTU, which can cause packets larger than this to get truncated or dropped, resulting in poor performance.

You can do this from Viscosity's end by editing your VPN connection in Viscosity, and set the "Tun MTU" value under the Networking tab, and adding "mssfix x" on a new line under the Advanced tab. I recommend starting with a Tun MTU value of 1380, and a mssfix value of 1340 (i.e. the command should look like "mssfix 1340" - without the quotes). You may need to make the same adjustment on the server if it doesn't appear to have an affect just setting it client side.

3. Some mobile providers may throttle VPN connections. You can typically work around these attempts by changing the port number your OpenVPN server uses. It may be worth connecting to a different OpenVPN server (e.g. one you set up on a VPS server) to see if you see the same performance: if so throttling may be occurring.

4. The download speed of the VPN connection is limited by the upload speed of the VPN server. If the internet connection where the OpenVPN server is located has a very poor upload speed, that will likely cause the poor download performance on the client end.

James Bekkema
Viscosity Developer

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