Setting primary interface in ubuntu

Unlike CentOS, ubuntu doesn’t provide a nice tool to set default routes when there are more than 1 ethernet interfaces are connected at the same time. It is possible to set up a static route with post-up but it is still difficult to do if the DHCP range is dynamic and you can’t accurately determine the proper gateway.

I was able to reliably implement it with the following settings.
Open /etc/network/interfaces file and
Add the following line to all other interfaces that does NOT act as a default gateway.

post-up route del default dev $IFACE

This code, instead of setting a default route, basically removes undesired default routes as the device goes up.

Example configuration when you want to use ens160 as default route:

auto ens172
iface ens172 inet dhcp
post-up route del default dev $IFACE

auto ens192
iface ens192 inet dhcp
post-up route del default dev $IFACE

# The primary network interface
auto ens160
iface ens160 inet dhcp

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