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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

# 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?

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up vote 11 down vote accepted

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: simply 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.

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