Debian/OVH: How to configure multiple Failover IP on the same Xen (Debian) Virtual Machine?

Solution 1:

You do not need to define a broadcast address for secondary IP addresses.

Configuration for "failover" IP adresses is just the following :

# Secondary NIC
auto eth0:0
iface eth0:0 inet static
    address BBB.BBB.BBB.BBB
    netmask 255.255.255.XXX

Example from a live server :

auto eth0:0
iface eth0:0 inet static
    address 178.32.36.20
    netmask 255.255.255.0

On a side note, you are doing this the wrong/overly-complicated way. The simple way to do this is to associate a "Virtual Mac address" to your failover IP addresses. You will find this option in the services tab of your server in the OVH Manager. This allows you to just bridge all the virtual interfaces associated with the given virtual Mac addresses. This way you also don't have to worry about any routing. Simple as it gets.

EDIT : I just thought of something. Make sure you enable routing in you dom0 with net.ipv4.ip_forward=1.

EDIT2 : After your comment, I went and had a look at what OVH recommends to do and you should try it. This is featured in this forum post. I don't know if you understand French but what it says is to put the following in your domU's network configuration.

auto eth0
iface eth0 inet static
    address <IP FAILOVER>        # domU IP address
    netmask 255.255.255.255
    post-up /sbin/ip route add IP.of.your.dom0.254 dev eth0         # IP.of.your.dom0.254 = dom0's gateway
    post-up /sbin/ip route add default via IP.of.your.dom0.254

It seems to me like this would do the trick given that you are in bridged network configuration with virtual MAC addresses.