I spent a few hours this weekend fiddling with networking things at home. One of the things I ran into was that the DHCP server provided by my ISP was behaving erratically. Specifically, it was being very fussy about giving out a new lease. It would give out a lease to a Windows 7 system I was using for testing, but not to my Gentoo server. At some point, having spent the day with this kind of frustration, I was ready to put up with almost any hack to get things running. Someone on the #gentoo IRC channel suggested that spoofing the MAC address that already had a lease might be a solution. Their solution was to do this: 

ifconfig eth0 down
ifconfig eth0 hw ether 08:07:99:66:12:01
ifconfig eth0 up

Here, you have to imagine that eth0 is the name of the interface, although on my system it isn't any more. (Another thing I learned this weekend was about predictable interface names.) You should also imagine that 08:07:99:66:12:01 is the mac address of the network interface on my Win7 system. 

The trouble with this is that it doesn't integrate very well in the standard init scripts that get things going on a Gentoo system. Network interfaces are started by running /etc/init.d/net.eth0 (although that's just a link to another script). The configuration is to be found in /etc/init.d/net where you can add directives that control the way your network interfaces are configured. The most important of these are the ones that begin with "config_". For example, to set up a static IP for eth0, you might say something like: 

config_eth0="192.168.0.99 netmask 255.255.255.0 brd 192.168.0.255"

or for DHCP it's much simpler: 

config_eth0="dhcp"

So my obvious first try for setting up a spoofed MAC address was something like this:

config_eth0="dhcp hw ether 08:07:99:66:12:01"

but this didn't work at all. Anyway - after a bit of fiddling and more Googling (sorry - I can't remember where I found this) it turned out that there's a specific directive just for this purpose. I tried this

mac_eth0="08:07:99:66:12:01"
config_eth0="dhcp"

It works a treat. Note that the order is important, which is obvious once you know it I suppose, but wasn't obvious to me until I'd got it wrong once. 

The good news after that was that for an established lease, everything worked rather better.