The ifup-aliases script does not check if the gateway (in
/etc/sysconfig/network) is on an aliased interface. While
not common, it turns out to be a major inconvience when the
primary interface is the internal network (ie. 10.x.x.x on
eth0) and the aliased interface (DSL, cable modem, etc) has
the gateway on it.
The "solution" would be to put the gateway interface on eth0
and have eth0:0 be the internal network, but this could
conceivably fail if the gateway interface fails to come up
and then the aliased interface can't be created.
This issue has been forwarded to a developer for further action.
You should be able to change the 'GATEWAYDEV' parameter in
ifup-aliases has been rewritten for the next release,
and it should fix this.