Force a static HWaddr to tinc interface

[Nick Hibma]

On 11 Jul 2013, at 06:26, Ville Mattila <ville at mattila.fi> wrote:

> Hi,
> 
> I have been working with tinc network where one host will provide the IP addresses over DHCP to each connected node. Tinc runs in switch mode.
> 
> It seems that the HWaddr of the tinc interface is regenerated every time when tinc is (re)started. As a result, DHCP server will assign a new IP for the host. Is there any configuration variable we could use to avoid HWaddr changing?
> 
> Currently, I have added a short snippet into tinc-up script that persists the (once given) HWaddr to a file and tries to reset it before starting dhclient. This works, but I feel it a bit hacky solution...
> 
> -- tinc-up --
> 
> #!/bin/sh
> set -e
> 
> MACFILE="/etc/tinc/mynet/mac"
> if [ -e $MACFILE ]; then
>   MAC=`cat $MACFILE`
>   ifconfig $INTERFACE hw ether $MAC
> fi
> 
> dhclient $INTERFACE &
> MAC=`ip link show mynet | awk '/ether/ {print $2}'`
> echo $MAC > $MACFILE

In my configuration we do:

	AWMAC1="$(printf '42:00:00:00:01:%02x' $AWSUBNET)"
	AWMAC2="$(printf '42:00:00:00:02:%02x' $AWSUBNET)"
	AWMAC3="$(printf '42:00:00:00:03:%02x' $AWSUBNET)"

AWSUBNET is a number in the range of 190  - 199 in our case. Notice how 42 sets b2 in top byte, the 'locally administered' bit (see http://en.wikipedia.org/wiki/MAC_address). Isn't it the answer to everything?

tinc-up looks like:

	/sbin/ifconfig $INTERFACE ether '@@AWMAC2@@'
	/sbin/ifconfig $INTERFACE up

Background: The culprit is not so much tinc as is the underlying OS that has to generate a globally unique MAC address. for generated interfaces The same problem occurs on bridges on FreeBSD (very annoying when trying to use static IP through DHCP there). I guess that we applied this approach for the same reason that you do, but not sure.

Nick


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