Filtering by Hwaddr With Port Forwarding

Hello,

I appear to have a router (Linksys RV042) that doesn't give any options
to filter traffic based on Mac Address.

I use that router to port forward, pushing the traffic towards an
appropriate server behind the firewall.

Once the traffic gets to a server, I have IP-Tables on there to further
classify (and possibly reject) the traffic. Most things work this way,
but unfortunately, the router port forwarding loses the MAC Address of
the person who origianlly made the request (the SRC becomes the routers
MAC Address instead).

I need to filter by Mac Address on one of my servers, but the port
forwarding is preventing me from doing that.

Does anybody have a hunch as to whether I might be able to work around
this problem and filter by Mac Address without having to replace my
router or using one of the servers for routing instead? I get the
feeling there is no way IPCHains will be able to do it unless I come up
with some sort of router configuration change so it can do something
different than standard forwarding.

~Mark