/usr/share/fwbuilder-5.1.0.3599/configlets/freebsd/ifconfig_interface is in fwbuilder-common 5.1.0-4.
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 | ## -*- mode: shell-script; -*-
##
## Lines that start with "##" will be removed before this code is
## added to the generated script. Regular shell comments can be added
## using single "#", these will appear in the script.
##
$IFCONFIG {{$interface_name}} {{if have_mtu}}mtu {{$mtu}}{{endif}} {{$options}}
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