MAC Verification

All traffic from an interface or from a subnet on an interface can be verified to originate from a defined set of MAC addresses. Furthermore, each MAC address may be optionally associated with one or more IP addresses.

Your kernel must include MAC match support (CONFIG_IP_NF_MATCH_MAC - module name ipt_mac.o).

There are four components to this facility.

The maclist interface option in /etc/shorewall/interfaces. When this option is specified, all traffic arriving on the interface is subjet to MAC verification.
The maclist option in /etc/shorewall/hosts. When this option is specified for a subnet, all traffic from that subnet is subject to MAC verification.
The /etc/shorewall/maclist file. This file is used to associate MAC addresses with interfaces and to optionally associate IP addresses with MAC addresses.
The MACLIST_DISPOSITION and MACLIST_LOG_LEVEL variables in /etc/shorewall/shorewall.conf. The MACLIST_DISPOSITION variable has the value DROP, REJECT or ACCEPT and determines the disposition of connection requests that fail MAC verification. The MACLIST_LOG_LEVEL variable gives the syslogd level at which connection requests that fail verification are to be logged. If set the the empty value (e.g., MACLIST_LOG_LEVEL="") then failing connection requests are not logged.

The columns in /etc/shorewall/maclist are:

INTERFACE - The name of an ethernet interface on the Shorewall system.
MAC - The MAC address of a device on the ethernet segment connected by INTERFACE. It is not necessary to use the Shorewall MAC format in this column although you may use that format if you so choose.
IP Address - An optional comma-separated list of IP addresses for the device whose MAC is listed in the MAC column.

Example 1: Here are my files:

/etc/shorewall/shorewall.conf:

     MACLIST_DISPOSITION=REJECT
     MACLIST_LOG_LEVEL=info

/etc/shorewall/interfaces:

#ZONE   INTERFACE        BROADCAST       OPTIONS
net     eth0            206.124.146.255 dhcp,norfc1918,routefilter,blacklist,tcpflags
loc     eth2            192.168.1.255   dhcp
dmz     eth1            192.168.2.255
wap     eth3            192.168.3.255   dhcp,maclist
-       texas           192.168.9.255

/etc/shorewall/maclist:

#INTERFACE              MAC                     IP ADDRESSES (Optional)
eth3                    00:A0:CC:A2:0C:A0       192.168.3.7     #Work Laptop
eth3                    00:04:5a:fe:85:b9       192.168.3.250   #WAP11
eth3                    00:06:25:56:33:3c                       #WET11
eth3                    00:0b:cd:C4:cc:97       192.168.3.8     #TIPPER

As shown above, I use MAC Verification on my wireless zone.

Note: The WET11 is a somewhat curious device; when forwarding DHCP traffic, it uses the MAC address of the host (TIPPER) but for other forwarded traffic it uses it's own MAC address. Consequently, I don't assign the WET11 a fixed IP address in /etc/shorewall/maclist.

Example 2: Router in Local Zone

Suppose now that I add a second wireless segment to my wireless zone and gateway that segment via a router with MAC address 00:06:43:45:C6:15 and IP address 192.168.3.253. Hosts in the second segment have IP addresses in the subnet 192.168.4.0/24. I would add the following entry to my /etc/shorewall/maclist file:

     eth3                     00:06:43:45:C6:15       192.168.3.253,192.168.4.0/24

This entry accomodates traffic from the router itself (192.168.3.253) and from the second wireless segment (192.168.4.0/24). Remember that all traffic being sent to my firewall from the 192.168.4.0/24 segment will be forwarded by the router so that traffic's MAC address will be that of the router (00:06:43:45:C6:15) and not that of the host sending the traffic.

Updated 6/10/2002 - Tom Eastep