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I have exactly the same problem with Woody (Skolelinux) and webmin
1.080:

From webmin (with ACL extended to manage network interface) I can
create a virtual interface eth0:1. This interface is activated without any
problem.

Afer that when I go to the webmin gateway tool I can change the default
gateway to the eth0:1 interface with an appropriate IP.

After that when I visit the list of available network interface, the eth0:1 ones
is gone.

It looks like the webmin gateway tool does manage well the virtual interface.

Indeed, when looking at /etc/networks/interfaces, the eth0:1 definition is
gone.