I was able to build a yocto project image with support for tun adapter. Once a connection is established a ip route with destination 0.0.0.0 and gateway 0.0.0.0 is created and my internet connection is broken. My problem is, that I don't know who or what creates this rule. I'm sure, that the software, which establishes the connection does not create any routes (I am the developer). Has anyone an idea how to prevent this route?
Destination Gateway Genmask Flags Metric Ref Use Iface
0.0.0.0 0.0.0.0 0.0.0.0 U 0 0 0 VPN
0.0.0.0 192.168.178.1 0.0.0.0 UG 10 0 0 eth0
169.254.0.0 0.0.0.0 255.255.0.0 U 0 0 0 VPN
192.168.178.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0
192.168.255.0 0.0.0.0 255.255.255.0 U 0 0 0 VPN
Here is my /etc/network/interfaces script
# /etc/network/interfaces -- configuration file for ifup(8), ifdown(8)
# The loopback interface
auto lo
iface lo inet loopback
auto eth0
iface eth0 inet dhcp