Hi all,


Why changing of mac address isn't recommended ?

The possible reasons that i can find is

1) it can break firewalls
2) it can break some DHCP implementations ?


Is there any other reasons that i have missed ?

I don't think you're likely to break a firewall or dhcp but you could stop your machine, another machine or all machines from communicating within or outside the network.

cheers

it could break the firewall or DHCP on the user end. If static DHCP is used a MAC address change could prevent the proper IP from being assigned, and again some firewalls, and access point use MAC address filtering, changing it would prevent that from working properly, most likely resulting in no Internet connection on the affected computer. But other than for spoofing for penetration testing, security testing, or for nefarious purposes why would someone change the MAC address?

Oh yea forget udev also relies on the MAC address to assign the proper device name to the NIC i.e. eth0, eth1, etc....

An example situation is given in the book "Linux system administration book" by evi nemeth et al as follows


At one time , 3com duplicated ethernet numbers among cards with different types of network connectors; they assumed that customers would order only a single type, this shortcut raised havoc at sites that were transitioning bw media and even caused problems on 3coms own internal network. mac address conflicts are deadly on the same nw

Does that mean if we cahnge the mac address udev will assign a new device name to the NIC ?

Yes it does, trust me I learned that one the hard way. Using a virtual machine the VM description got corrupted so I had to create a new VM description. The MAC address was not the same and when I tried to bring up eth0 the device was not found...WTF After some research and looking through a few files I found that indeed it was because the MAC address was different. When udev ran at boot it didn't find the MAC address associated with eth0 and created a new eth1. This is required or your eth devices would be all mixed up and static entries would end up on the wrong NIC.