In this directory, you can create the following filenames to add
custom rules at each section of the firewall script:

ipv4_custom_allowedports   ipv4_custom_mssclamp     ipv4_custom_proto
ipv4_custom_blockincoming  ipv4_custom_nat          ipv4_custom_routing
ipv4_custom_blockports     ipv4_custom_notrack
ipv4_custom_conntrack      ipv4_custom_portforward