Debian: 2 network devices on the same network

Question

I have a host running virtual machines using KVM. Each additional IP I get from the hosting company is bound to a specific virtual MAC address and will only work with this MAC (their router configuration).

I got vmbr0 which is a eth0 bridge. Now I need to give one VM two IPs which are bound to two different MACs. The only way I can think of is to give the VM two network devices (eth0 and eth1) which different MACs. The problem is that they use the same network.

Here /etc/network/interfaces configuration (111.111.111.x and b:e:e:f:: are placeholders and a 255.255.255.255 netmask is required by my ISP)

auto eth0
iface eth0 inet static
        address 111.111.111.36
        broadcast 111.111.111.63
        netmask 255.255.255.255
        pointopoint 111.111.111.1
        gateway 111.111.111.1
iface eth0 inet6 static
        address b:ee:f:301::2
        netmask 64
        up ip -6 route add b:ee:f:300::1 dev eth0
        down ip -6 route del b:ee:f:300::1 dev eth0
        up ip -6 route add default via b:ee:f:300::1 dev eth0
        down ip -6 route del default via b:ee:f:300::1 dev eth0

auto eth1
iface eth1 inet static
        address 111.111.111.43
        broadcast 111.111.111.63
        netmask 255.255.255.255
        pointopoint 111.111.111.1
        gateway 111.111.111.1
iface eth1 inet6 static
        address b:ee:f:301:30::2
        netmask 64
        up ip -6 route add b:ee:f:300::1 dev eth1
        down ip -6 route del b:ee:f:300::1 dev eth1
        up ip -6 route add default via b:ee:f:300::1 dev eth1
        down ip -6 route del default via b:ee:f:300::1 dev eth1

However with this setup the network doesn't work and when I remove the gateway part from eth1 I get these messages in syslog (and the second IP does not work):

Apr 14 13:50:36 VM kernel: [  907.043036] martian source 111.111.111.43 from some.ip, on dev eth1
Apr 14 13:50:36 VM kernel: [  907.043084] ll header: 00:50:56:00:04:98:00:26:88:76:18:18:08:00

Is there any way to get this working? After some research I found out that you can't use two network interfaces on the same network segment. But I need to find a way to set 2 IP addresses (preferable using two network devices for easier firewalling).

Answer 1

I am using 3 IPs for my box, but with virtual interfaces as shown below. Don't know if that is appropriate for your case...

auto eth0 eth0:1
iface eth0 inet static
        metric 100
        address 111.111.111.36
        broadcast 111.111.111.63
        netmask 255.255.255.255
        pointopoint 111.111.111.1
        gateway 111.111.111.1
        hwaddress ether 11:11:11:11:11:11     //updated

iface eth0 inet6 static
        address b:ee:f:301::2
        netmask 64
        up ip -6 route add b:ee:f:300::1 dev eth0
        down ip -6 route del b:ee:f:300::1 dev eth0
        up ip -6 route add default via b:ee:f:300::1 dev eth0
        down ip -6 route del default via b:ee:f:300::1 dev eth0

iface eth0:1 inet static
        metric 200
        address 111.111.111.43
        broadcast 111.111.111.63
        netmask 255.255.255.255
        pointopoint 111.111.111.1
        gateway 111.111.111.1
        hwaddress ether 22:22:22:22:22:22   //updated

iface eth0:1 inet6 static
        address b:ee:f:301:30::2
        netmask 64
        up ip -6 route add b:ee:f:300::1 dev eth1
        down ip -6 route del b:ee:f:300::1 dev eth1
        up ip -6 route add default via b:ee:f:300::1 dev eth1
        down ip -6 route del default via b:ee:f:300::1 dev eth1