# Mounted into the sidecar as /etc/rspamd/local.d/milter_headers.conf.
#
# rspamd writes its verdict into two headers of every message it lets through:
# the action it chose and the score with the symbols behind it. SelfPost's
# inbound journal reads them (Inbound > Log shows them next to the message),
# and the upstream mailbox server receives them with the mail.
#
# rspamd removes headers of these names that a sender wrote himself before it
# adds its own. Without this file the journal still lists accepted mail, with
# no verdict next to it.
#
# The third header is the proof that the other two are rspamd's. The filter is
# attached so that mail still flows when it is down, and then nothing removes
# what a sender wrote: he could send his own verdict. So rspamd signs each
# message - an HMAC-SHA256 of the message's queue id under a key SelfPost
# generates and leaves in the directory it shares with this container - and
# SelfPost reads a verdict only next to a signature it can recompute. The
# signature travels on with the mail and is of no use to whoever sees it: it
# fits that one queue id.
#
# rspamd 3.11.1, src/plugins/lua/milter_headers.lua: a custom routine is called
# as function(task, common_meta) and returns error, headers to add, headers to
# remove, metadata; a remove value of 0 removes every header of that name
# (docs.rspamd.com/modules/milter_headers, "Custom routines").
# src/lua/lua_cryptobox.c, rspamd_lua_hash_create(): "sha256" with a key is
# HMAC-SHA256 over EVP_sha256(), the key being the bytes given - here the hex
# text. task:get_queue_id() is the Queue-Id the proxy took from the milter
# macro "{i}" or "i" (src/libserver/milter.c, rspamd_milter_macro_http(); read
# back in src/libserver/protocol.c), which Postfix sends at
# end-of-message (Postfix 3.7.11, src/global/mail_params.h, DEF_MILT_EOD_MACROS
# "i").
use = ["x-spamd-result", "x-rspamd-action", "selfpost-signature"];

custom {
  selfpost-signature = <<EOD
return function(task, common_meta)
  local remove = { ['X-SelfPost-Filter'] = 0 }
  -- The key is read for every message and kept nowhere: SelfPost may make a
  -- new one (the file was lost, a backup was restored), and a key remembered
  -- here would go on signing what SelfPost no longer accepts until someone
  -- restarted this container. The file is one short line.
  local key
  local f = io.open('/etc/rspamd/selfpost/filter.key', 'r')
  if f then
    key = f:read('*l')
    f:close()
  end
  local qid = task:get_queue_id()
  if not key or key == '' or not qid or qid == '' then
    return nil, {}, remove, {}
  end
  local hash = require "rspamd_cryptobox_hash"
  local sig = hash.create_specific_keyed(key, 'sha256', qid):hex()
  return nil, { ['X-SelfPost-Filter'] = sig }, remove, {}
end
EOD;
}