DKIM headers in Exim

gistfile1.txt

acl_check_dkim:
  # Skip this whole acl if header.d contains an @ sign because exim is
  # breaking down the header.i part (which usually is an email address)
  # bit by bit, working towards just the domain name.
  accept  condition      = ${if match{$dkim_cur_signer}{\N@\N}}

  accept  dkim_status    = none
          sender_domains = KNOWN_DKIM_SIGNERS
          dkim_signers   = KNOWN_DKIM_SIGNERS
          condition      = ${if eqi{$sender_address_domain}{$dkim_cur_signer} {yes}{no}}
          log_message    = Possible DKIM Forgery: Unsigned message from $sender_address_domain
          add_header     = :at_start:X-DKIM: Exim $version_number on $primary_hostname (no dkim signature for required domain: $dkim_cur_signer)

  accept  dkim_status    = none
          !sender_domains = KNOWN_DKIM_SIGNERS
          !dkim_signers  = KNOWN_DKIM_SIGNERS
          set acl_m_dkim_hdr = 1
          add_header     = :at_start:X-DKIM: Exim $version_number on $primary_hostname (no dkim signature for $dkim_cur_signer)

  warn    condition      = ${if eq {$acl_m_dkim_hdr}{1} {no}{yes}}
          set acl_m_dkim_hdr = 1
          add_header     = :at_start:X-DKIM: Exim $version_number on $primary_hostname

  accept  dkim_status    = pass
          add_header     = :at_start:Authentication-Results: $primary_hostname; dkim=$dkim_verify_status header.d=$dkim_cur_signer header.i=$dkim_identity header.s=$dkim_selector

  warn    dkim_status    = invalid : fail
          add_header     = :at_start:Authentication-Results: $primary_hostname; dkim=$dkim_verify_status header.d=$dkim_cur_signer header.i=$dkim_identity header.s=$dkim_selector reason="$dkim_verify_reason"

  deny    dkim_status    = fail
          sender_domains = KNOWN_DKIM_SIGNERS
          dkim_signers   = KNOWN_DKIM_SIGNERS
          condition      = ${if eq {$dkim_key_testing}{1} {no}{yes}}
          message        = Rejected: $dkim_verify_reason

  accept

For what it's worth, if like me you want to use a file for the 'known signing domains' then:

DKIM_KNOWN_SIGNERS_FILE=/etc/exim4/dkim_known_signers
# Use a DKIM ACL
acl_smtp_dkim = acl_check_dkim
dkim_verify_signers = $sender_address_domain:$dkim_signers

...

acl_check_dkim:
        # Skip this whole acl if header.d contains an @ sign because exim is
        # breaking down the header.i part (which usually is an email address)
        # bit by bit, working towards just the domain name.
        accept  condition      = ${if match{$dkim_cur_signer}{\N@\N}}

        accept  dkim_status = none
                sender_domains = ${lookup {$sender_address_domain} lsearch,ret=full {DKIM_KNOWN_SIGNERS_FILE} {$sender_address_domain}}
                dkim_signers   = ${lookup {$sender_address_domain} lsearch,ret=full {DKIM_KNOWN_SIGNERS_FILE} {$sender_address_domain}}
                condition      = ${if eqi{$sender_address_domain}{$dkim_cur_signer} {yes}{no}}
                log_message    = Possible DKIM Forgery: Unsigned message from $sender_address_domain
                add_header     = :at_start:X-DKIM: Exim $version_number on $primary_hostname (no dkim signature for required domain: $dkim_cur_signer)

        accept  dkim_status    = none
                !sender_domains = ${lookup {$sender_address_domain} lsearch,ret=full {DKIM_KNOWN_SIGNERS_FILE} {$sender_address_domain}}
                !dkim_signers   = ${lookup {$sender_address_domain} lsearch,ret=full {DKIM_KNOWN_SIGNERS_FILE} {$sender_address_domain}}
                set acl_m_dkim_hdr = 1
                add_header     = :at_start:X-DKIM: Exim $version_number on $primary_hostname (no dkim signature for $dkim_cur_signer)

        warn    condition      = ${if eq {$acl_m_dkim_hdr}{1} {no}{yes}}
                set acl_m_dkim_hdr = 1
                add_header     = :at_start:X-DKIM: Exim $version_number on $primary_hostname

        accept dkim_status     = invalid : pass
                add_header     = :at_start:Authentication-Results: $primary_hostname; dkim=$dkim_verify_status header.d=$dkim_cur_signer header.i=$dkim_identity header.s=$dkim_selector

        accept dkim_status     = invalid : fail
                add_header     = :at_start:Authentication-Results: $primary_hostname; dkim=$dkim_verify_status header.d=$dkim_cur_signer header.i=$dkim_identity header.s=$dkim_selector reason="$dkim_verify_reason"

        accept

You can't just reference the file in dkim_verify_signers , because zero expansion is done on that, ever. Try to set it to a file and you'll end up with $dkim_cur_signer in the ACL literally being that value, i.e. the name of the file. So, work around this by just triggering the ACL for all SMTP-injected email using dkim_verify_signers = $sender_address_domain:$dkim_signers, and then do lookups on the file in the ACL.

The exact form of the ${lookup ...} is so that you can have a file that contains, other than comments, literally just a domain per line.