Error RTNETLINK answers: File exists

On 12.04, running as a guest in VirtualBox using two adapters, I was getting this error in response to ifup eth1

RTNETLINK answers: File exists
Failed to bring up eth1

This was the contents of /etc/network/interfaces

# The loopback network interface
auto lo
iface lo inet loopback
# Host-only interface
auto eth1
iface eth1 inet static address 192.168.56.20 netmask 255.255.255.0 network 192.168.56.0 broadcast 192.168.56.255 gateway 192.168.56.1
# NAT interface
auto eth2
iface eth2 inet dhcp

See this question for more about why I configured it this way.
In VirtualBox, how do I set up host-only virtual machines that can access the Internet?

1 Answer

The problem was that two default gateways were being set, one static and one by dhcp. You can't have two default gateways with the same metric.

The solution: remove the gateway line from the configuration of eth1.

Or, add an explicit metric 100 line to eth1, to give the eth1 gateway a lower priority than the eth2 gateway.