Beware: Swapping a NIC Card Out Changes the MAC Address

I observed an issue this week where a flapping NIC was replaced by a Dell technician.

When the Dell technician swapped out the NIC card (and left), the interfaces on the card would not come up and go into operation.

They were ABOUT to call Dell back in and swap out the motherboard, when I decided to wander over and take a look and get involved.

It is always important to remember, that when you change out a NIC card, the mac address CHANGES!

And you never know, where that previous mac address might have been used! Here are just a few things a mac address might be used:

• an upstream DHCP server might be assigning an IP address based on mac address
• firewalls might be using the mac address in certain rules and policies
• interfaces in the OS (Linux in particular - especially CentOS) might not come up with a new mac address
• CentOS7 has HWADDR directive in the interface configuration files
• scripts in rc.local or udev may be using the mac address to do certain things
• configure certain interfaces to bridges or bonds

In this particular case, a udev script was renaming a specific interface - based on mac address - and assigning it to a nic teaming configuration (bond).

Don't just swap NICs out! Figure out who might be paying attention to mac addresses before swapping! It can pay dividends.