Troubleshooting mac addresses in Ubuntu

29 Nov 2007

As far as /etc/iftab goes, this is not used in Ubuntu 7.10
anymore..this functionality is moved to hotplug,. and maybe causing
your prob.

As you said, the NIC was in when Ubuntu was installed. However, some
NICs for some curious reason do sometimes change MAC address between
powerups.

Checkout the file /etc/udev/rules.d/70-persistent-net.rules . If there
are lots of entries in their for different MAC addresses, there is
your problem.

This file is generated during bootup by
/etc/udev/rules.d/75-persistent-net-generator.rules. The suggestion at
http://ubuntuforums.org/showthread.php?p=3816892 suggests to backup
and delete these two files. The system will then just assign ethernet
devices numbers in the order it finds them, and if it only every finds
one NIC then it will always be eth0

Copied verbatim from Martin’s post on SLUG.

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