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Performance Impact of OpenVPN Port Sharing

I recently had cause to use OpenVPN on the standard HTTPS port to protect my traffic. This was done as a compromise with administrators who didn’t want to change their egress filtering, but wanted to allow me to continue doing my normal work.

I already run several webservers, including this one, and didn’t want to give up exclusive access to the precious TCP port 443. The recommended way to deal with this is to make use of the port-share option built into OpenVPN. This left me with two choices, run this on an existing server sharing the port with existing websites, or setup a dedicated server just for this instance of OpenVPN.

I couldn’t find any other posts that took a look at how this port sharing effects the performance of the HTTPS server so I felt like doing a quick analysis for other curious parties.

I setup a fresh Nginx server with Let’s Encrypt certificates that mimics my production setup and used ab to bench the service for 30 seconds. The mean measured rate was 288.70 +/- 14.39 requests per second. Mean request fullfillment took 3.47 +/- 0.17 ms.

After enabling port-share on OpenVPN I reran the exact same test. The result was 139.76 +/- 67.68 requests per second. Mean measured request fullfillment 7.44 +/- 4.16 ms.

That is a 51% peak request handling reduction, each request has an additional 4-8ms of latency, and an almost 40x increase of jitter. That is a massive relative impact but the vast majority of the websites I run need won’t be terribly impacted by that additional latency.

I ultimately ended up setting up a seperate server for OpenVPN as I didn’t want to mess with known working systems.