OpenVPN Server Net30 Error

While trying to set up an OpenVPN client, I received a fatal error message about /30 subnets when I had defined 255.255.255.0 (/24) in the configuration file (see below for the error message).

TUN Error: tun_prop_error: ifconfig addresses are not the same /30 subnet (topology net30)

So I turned to Google to try to find an exact solution or at least a description of the problem. I came across a Stack Overflow post about a similar error, which I think came from a Windows machine.

Each pair of ifconfig-push addresses represents the virtual client and server IP endpoints. They must be taken from successive /30 subnets in order to be compatible with Windows clients and the TAP-Windows driver.

This was not exactly a solution, and I could not find any other solutions. Next, I researched what "net30 topology" meant. From the OpenVPN topology page, I was able to figure out what the problem was. As it turns out, OpenVPN uses net30 by default, even though it is effectively deprecated and subnet mode is recommended. I fixed it by setting OpenVPN to subnet mode in the main configuration file.

topology subnet

Note: I am using configurations generated (but modified) from kylemanna's OpenVPN Docker image

Published: 14/10/2016 UTC

Updated: 18/06/2026 UTC

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