We are aware of a small number of cases where the upgrade to CentOS 5.4 has resulted in customers not being able to start networking on their Linode. The customers in question installed CentOS 5.2 while it was available, and either upgraded directly to 5.4 or stepped through from 5.3.

The underlying cause is an oversight in our build of 5.2 which left the MAC address of the virtual container we built CentOS in being placed in a networking script. For some reason, CentOS 5.4 now cares about this, and will refuse to start your networking if it is present.

The symptom to look for will be the lack of eth0 in a casual ifconfig, and no network connectivity on the Linode.

Look for this line in /etc/sysconfig/network-scripts/ifcfg-eth0:

HWADDR=00:1C:42:50:9C:35

If that line is present, remove it completely and restart your networking stack using the following command:

service network restart

After that, your Linode's networking should return to normal. If not, let us know.

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Disclaimer: I am no longer employed by Linode; opinions are my own alone.