Butter Mountain

Recently while attending a networking course which covered redundant multipathing for high availability systems I got to thinking - could this be applied to my wired / wireless network at home? The end result being that removing the wired network from my laptop by unplugging the cable would instantly fail over to my wireless connection. I’m sure many people are thinking “but gnome network manager already does that” well, not quite.

With redundant bonding or multi-pathing both interfaces are connected to the same network and put into a special group. These interfaces are constantly connected to the network and are ready to work, however only one of them is active at any one time. Should the active interface fail (such as a network cable being removed) the mac address, ip address and all other configuration is almost instantly assigned to one of the other available network interfaces. This allows you to continue working as if nothing had happened, songs playing from network shares will continue playing, instant messenger conversations will continue to work, downloads will not be interrupted… the list goes on.

It turns out that under Linux this works incredibly well, read on for details.

Requirements.

In terms of networking you must have your wired and wireless networks on the same physical network segment, in this post I am also assuming that your wireless network is setup and that you have knowledge of networking.
As for packages you will need to install the ifenslave package under ubuntu.

# apt-get install ifenslave-2.6

Step One.

Warning: if you follow the steps I’ve included here, reboot and anything goes wrong, I accept no responsibility - thanks.

Under Linux high availability and link aggregation is handled by the bonding module, this can be enabled by adding the following line to your /etc/modules configuration file.

bonding mode=1 miimon=100 downdelay=200 updelay=200

This will load the bonding module upon next boot, however, it can be loaded at any time using the modprobe command the options are as follows.

• mode=1 - This enables the active backup mode, this will provide link level redundancy but it does not allow for any kind of link aggregation.

• miimon=100 - This enables link level monitoring of the connection (the default value is 0 - which disables link monitoring). The value passed is the frequency in milliseconds that the link is checked. Link level monitoring only takes into account the physical connection, not if the network is correctly configured or not.

• downdelay=200 - This is the delay in milliseconds before the link is marked as failed, it must be a multiple of miimon

• updelay=200 - This is the delay in milliseconds before the link is marked as active, it must be a multiple of miimon

Step Two.

Next the bonding interface (bond0) must be configured, this can be done in the /etc/network/interfaces file an entry such as this needs to be added:

auto bond0
iface bond0 inet static
address 192.168.1.34
netmask 255.255.255.0
gateway 192.168.1.24
broadcast 192.168.1.255
post-up ifenslave bond0 eth0 eth1
post-up echo “eth1″ > /sys/class/net/bond0/bonding/primary
pre-down ifenslave -d bond0 eth0 eth1

The network address, netmask, gateway and broadcast should be configured for your network. This is the one single address that your machine will be known as, assuming that one of the connections to your laptop is available. You should be able to use dhcp to configure this interface, however I have chosen a static address.

• ‘post-up ifenslave bond0 eth0 eth1′ - This line assigns my wired (eth1) and wireless (eth0) interfaces into my failover group bond0. You should replace these with the interfaces you want to use.

• ‘post-up echo “eth1″ > /sys/class/net/bond0/bonding/primary’ - This line specifies a primary slave, that is if this interface is available it will always be used in preference to the others. In this case eth1 is my wired interface, this ensures that when I return to my desk and plug my network cable back in I will be using the faster ethernet network and not my slower wireless.

• ‘pre-down ifenslave -d bond0 eth0 eth1′ - This line removes the eth0 and eth1 interfaces from the bond group when networking is stopped.

Step Three.

Now reboot for the settings to take effect. It is of course possible to do all of this without rebooting, one would simply perform the following steps:

1. modprobe the bonding driver
2. ifconfig the bonding interface
3. add the default route for the bonding interface
4. ifenslave the network devices
5. set the primary bonding interface

To Test that everything is working simply ping a host on your local network then once its going unplug the wired interface it should fail over to the wireless interface without dropping any packets. You can then re-insert the network cable and instantly switch back to the wired interface.
Help, its not working.

I had a few problems getting this working, firstly ifenslave would assign the bonding interface the mac address of my wired interface. As part of my wireless network security I have mac filtering enabled, however, only the mac address of the wireless card was allowed. Adding the mac of the bonding interface to my mac filter list cleared up that problem.

My other problem involved gnome network applet, I believe that this process is called nm-monitor. This would attempt to reconfigure my wireless network or wired network whenever I removed the network cable to my laptop. This appeared to prevent the failover from working as quickly as it should have. killing off the nm-monitor fixed this little issue.

I’ve now been using this for roughly two weeks without any issues, being able to unplug my laptop and take it to another room of the house without having to even consider whats going to happen to my network has been a real step forward.