Rspamd Lua - check if DKIM domain matches header from domain [Fosdem 2024 talk question]

rspamd.lua

-- Rspamd Lua - check if DKIM domain matches header from domain [Fosdem 2024 talk question]
-- https://fosdem.org/2024/schedule/event/fosdem-2024-2661--operations-enterprise-mail-security-with-open-source-/
-- set symbol DKIM_EQ_HFROM when at least one of the validated DKIM signature domains
-- match the header from domain

local lua_util = require "lua_util"
local E = {}

rspamd_config:register_symbol{
  type = 'callback', -- or virtual, callback, prefilter or postfilter
  name = 'DKIM_EQ_HFROM',
  score = 0.0, -- Metric score
  flags = 'empty,explicit_disable,ignore_passthrough,nostat',

  callback = function(task)
    local symbol = "DKIM_EQ_HFROM"

    local from_domain = (((task:get_from('mime') or E)[1] or E).domain or nil)

    if from_domain then

      local dkim_options = (((task:get_symbol('R_DKIM_ALLOW') or E)[1] or E)['options'] or E)
      local dkim_domain_match = false

      for _,s in ipairs(dkim_options) do
        local dkim_split = lua_util.str_split(s, ':')
        if from_domain == dkim_split[1] then
          dkim_domain_match = true
        end
      end

      if dkim_domain_match then
        task:insert_result(symbol, 1.0, from_domain)
        return true
      end

    end
    return false
  end,
}

rspamd_config:register_dependency("DKIM_EQ_HFROM","R_DKIM_ALLOW")