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I have a server running Proxmox (a virtualization platform), and created a KVM virtual machine with Ubuntu Server 12.10. After configuring the network, everything works fine, but if I add a second network adapter to that virtual machine, it is not recognized. dmesg | grep -i eth just shows one interface (eth0).

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Finally found the solution. Rebooting the machine is not enough, stopping the VM and starting it again is needed.

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Load the following kernel modules in the guest to enable hot-plugging of devices in KVM:

modprobe acpiphp
modprobe pci_hotplug

Add them as two separate lines in /etc/modules (without the modprobe part) to enable them permanently. This is Debian/Ubuntu specific as most other distributions (like CentOS/RedHat/Fedora/SUSE) build the kernel with these modules built-in. For Debian/Ubuntu you'll always have to load these modules yourself, unfortunately.

See also (block devices and network devices aren't really much different in this aspect):

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