How to create bridges on bonds (with and without VLANs) using NetworkManager

Some production systems you face might make use of bonded network connections that you need to bridge in order to get VMs onto them. That bond may or may not have a native VLAN (in which case you bridge the bond), or it might have VLANs on top (in which case you want to bridge the VLANs), or perhaps you need to do both.

Let’s walk through an example where we have a bond that has a native VLAN, that also has the tagged VLAN 123 on top (and maybe a second VLAN 456), all of which need to be separately bridged. This means we will have the bond (bond0) with a matching bridge (br-bond0), plus a VLAN on the bond (bond0.123) with its matching bridge (br-vlan123). It should look something like this.

+------+   +---------+                           +---------------+
| eth0 |---|         |          +------------+   |  Network one  |
+------+   |         |----------|  br-bond0  |---| (native VLAN) |
           |  bond0  |          +------------+   +---------------+
+------+   |         |                                            
| eth1 |---|         |                                            
+------+   +---------+                           +---------------+
            | |   +---------+   +------------+   |  Network two  |
            | +---| vlan123 |---| br-vlan123 |---| (tagged VLAN) |
            |     +---------+   +------------+   +---------------+
            |                                                     
            |     +---------+   +------------+   +---------------+
            +-----| vlan456 |---| br-vlan456 |---| Network three |
                  +---------+   +------------+   | (tagged VLAN) |
                                                 +---------------+

To make it more complicated, let’s say that the native VLAN on the bond needs a static IP and to operate at an MTU of 1500 while the other uses DHCP and needs MTU of 9000.

OK, so how do we do that?

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How to create Linux bridges and Open vSwitch bridges with NetworkManager

My virtual infrastructure Ansible role supports connecting VMs to both Linux and Open vSwitch bridges, but they must already exist on the KVM host.

Here is how to convert an existing Ethernet device into a bridge. Be careful if doing this on a remote machine with only one connection! Make sure you have some other way to log in (e.g. console), or maybe add additional interfaces instead.

Export interfaces and existing connections

First, export the the device you want to convert so we can easily reference it later (e.g. eth1).

export NET_DEV="eth1"

Now list the current NetworkManager connections for your device exported above, so we know what to disable later.

sudo nmcli con |egrep -w "${NET_DEV}"

This might be something like System eth1 or Wired connection 1, let’s export it too for later reference.

export NM_NAME="Wired connection 1"

Create a Linux bridge

Here is an example of creating a persistent Linux bridge with NetworkManager. It will take a device such as eth1 (substitute as appropriate) and convert it into a bridge. Note that we will be specifically giving it the device name of br0 as that’s the standard convention and what things like libvirt will look for.

Continue reading How to create Linux bridges and Open vSwitch bridges with NetworkManager

Configuring QEMU bridge helper after “access denied by acl file” error

QEMU has a neat bridge-helper utility which allows a non-root user to easily connect a virtual machine to a bridged interface. In Fedora at least, qemu-bridge-helper runs as setuid (any user can run as root) and privileges are immediately dropped to cap_net_admin. It also has a simple white/blacklist ACL mechanism in place which limits connections to virbr0, libvirt’s local area network.

That’s all great, but often you actually want a guest to be a part of your real network. This means it must connect to a bridged interface (often br0) on a physical network device.

Continue reading Configuring QEMU bridge helper after “access denied by acl file” error