OpenVPN Server Net30 Error

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Published: 14/10/2016 UTC

Updated: 14/10/2016 UTC

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 find an exact solution or a description of the problem. I came across a stackoverflow post about a similar error, that I think was from a Window's machine (see below).

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 started some research into what "net30 topology" was. From this I was able to figure out what the problem was from the OpenVPN Topology page. As it turns out OpenVPN is in net30 by default (even though this is effectively deprecated, and it is recommend to use Subnet mode). So I just set OpenVPN to be in subnet mode by setting it in the master configuration file (see below).

topology subnet

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

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