/etc/freeradius/modules/mac2ip is in freeradius 2.2.5+dfsg-0.2+deb8u1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 | # -*- text -*-
#
#  $Id: 793d5690e1d4520bb3db1d9900d6be09da2587ae $
######################################################################
#
#  This next section is a sample configuration for the "passwd"
#  module, that reads flat-text files.
#
#  The file is in the format <mac>,<ip>
#
#	00:01:02:03:04:05,192.168.1.100
#	01:01:02:03:04:05,192.168.1.101
#	02:01:02:03:04:05,192.168.1.102
#
#  This lets you perform simple static IP assignments from a flat-text
#  file.  You will have to define lease times yourself.
#
######################################################################
passwd mac2ip {
	filename = ${confdir}/mac2ip
	format = "*DHCP-Client-Hardware-Address:=DHCP-Your-IP-Address"
	delimiter = ","
}
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