I'm pretty new to KVM and have a KVM CentOS 7.1 hypervisor running a few VMs. I'm moving all my VMs from an ESXi host as I want to use KVM in future. Most of my VMs are working except for one which is running a Sophos UTM router (Sophos UTM is similar to products like pfSense, Smoothwall etc).

The host has 3 physical NICs which are configured on the Sophos VM as:
1) LAN (fixed IP)
2) DMZ (fixed IP)
3) WAN (which is directly plugged into a cable modem for the internet connection and is configured DHCP).

I have imported the settings from the "old" Sophos machine so I know the configuration of the new one is identical to the old one. I have even tried configuring the NICs to have the same MAC addresses as the old one.

The problem is that no matter what I try I cannot get the WAN NIC to get an internet link up and running with my cable modem. I have re-installed the VM countless times, turned off the modem and VM, done a factory reset of the modem, and, as I mentioned, ensured the MAC addresses are the same. Nothing I try has been successful.

After hours and hours of trying to get this to work and failing, this is what I did:

1) Overwrote the CentOS KVM installation with Citrix Xenserver to see if a Sophos UTM VM on XenServer would connect to the modem - it did.

2) So then I re-installed CentOS and KVM using instructions from http://linux.dell.com/files/whitepapers/KVM_Virtualization_in_RHEL_7_Made_Easy.pdf.

3) Re-installed the Sophos UTM VM (making sure I used the same MAC addresses for the NICs as the original), turned the modem off and back on, and hey presto everything worked!

Not sure why all that was necessary but anyway, glad to have it working now.