How to set up single client TAP/TUN on the client for my OpenVPN?

Question

My laptop OpenVPN client establishes a TAP device. My client config is TUN, though. This devices shows under ifconfig /ALL with the IP 10.8.0.6

and a subnetmask of 255.255.255.252. (!!!)

This subnetmask is obviously a multi client mask setup which gives only 4 IPs from the subnet to each client.

I have troubles pinging LAN clients inside and outside the tunnel on the remote LAN and suspect the multi client mask to be the reason.

How can I change the mask to a standard 255.255.255.0?

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More details:

I run a Synology Disk Station NAS in my home (remote) network with the IP beeing 192.168.0.8. The router/standard gateway is an Ubiquity ER-X with 192.168.0.1.

This NAS has an OpenVPN Server. I can connect to this server with my client on the LAN/Wifi at work (gateway 192.168.1.1; IP of my laptop in the wifi is 192.168.5.121; tunnel IP is 10.8.0.6).

The tunnel gives my laptop an IP of 10.8.0.6 and my remote server on the NAS has 10.8.0.5

I disabled my laptop firewall (Windows 10).

I can connect via my laptop browser to https://10.8.0.1:5001 which is the web GUI of the Synology NAS Disk Managment System, a GUI of the operating system.

The weird thing: I can not ping this IP 10.8.0.1 although I am connected via the browser on parallel.

Answer 1

Putting topology subnet as one single line into the client configuration file will solve this (only when dev tun is used).

I found this after a discussion with some nice people here.

More details on why multi-client setup was necessary for usage with older versions of windows TAP devices can be found here.