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If any extra iptables kernel module is loaded then during awall activate awall raises error exception in M.flush() function, line for i, chain in ipairs(M.builtin[tbl]) do

in /usr/share/lua/5.1/awall/iptables.lua:

function M.flush()
    local empty = M.IPTables()
    for family, params in pairs(families) do
       local success, lines = pcall(io.lines, params.procfile)
       if success then
          for tbl in lines do
             for i, chain in ipairs(M.builtin[tbl]) do 
...

since M.builtin has no extra module default and there is no error excpetion handler:

M.builtin = {
    filter={'FORWARD', 'INPUT', 'OUTPUT'},
    mangle={'FORWARD', 'INPUT', 'OUTPUT', 'POSTROUTING', 'PREROUTING'},
    nat={'INPUT', 'OUTPUT', 'POSTROUTING', 'PREROUTING'},
    raw={'OUTPUT', 'PREROUTING'},
    security={'FORWARD', 'INPUT', 'OUTPUT'}
}

This case happened on host with iptable_rawpost module loaded

(from redmine: issue id 3419, created on 2014-10-07, closed on 2017-05-17)