CentOS 7 KVM Macvtap Loses All Connectivity

System Configuration

I have a CentOS 7 host machine with two KVM guests:

  1. Smoothwall 3.1
  2. Ubuntu 14.04

The Smoothwall is configured RED+GREEN+ORANGE with three separate network interfaces. The RED and GREEN interfaces are connected via bridged Macvtap with virtio drivers to two separate host interfaces (enp5s0 and enp6s0) which, in turn, are connected to two physical network cards on the machine. ORANGE is connected to a guest-only virtual switch. RED is connected to another network / gateway that connects the Smoothwall to the Internet, same as the host interface on the same card (but with different IP addresses – the RED is static, entered manually and the host is dynamic – DHCP).

After a restart of the host, all machines, host and guests, have all of the expected network connectivity, and everything just works.

The Problem

Where I live we experience periodic Internet outages lasting from a second or two to several minutes. Every time there is an outage the RED interface loses all connectivity, both inbound and outbound, once connectivity to the host has been restored (and possibly during the outage).

Following the loss of connectivity, in the host Settings -> Network control panel the interface associate with RED (Macvtap0), which is normally visible and editable, is still visible but it’s options cannot be changed as the Options button is missing. Also, the ON/OFF button is missing too. Furthermore, it no longer has anything more than a MAC address (normally it has both a MAC and an IPV6 address).

Control Panel Before Outage

Macvtap working properly

Control Panel After Outage

Macvtap failing

I’ve tried changing the driver (virtio, rtl8139, e1000, etc.) and I’ve tried changing the physical hardware but neither change has altered┬áthe behavior so I’m pretty sure it’s something in the Macvtap software that’s failing (or misconfigured).

The Solution?

There are multiple articles and bug reports that seem to indicate that this can be repaired by enabling promiscuous mode on the interface, but I tried that (sudo ifconfig Macvtap0 promisc) to no avail. Maybe I needed to toggle the interface (ifup or something like that?)

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