[exim] ARC signing and verification

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Autor: Tom Crane
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Dla: exim-users
CC: Tom Crane
Temat: [exim] ARC signing and verification
Dear All,
     I am attempting to configure my server (exim-4.92.3) to ARC 
(Authenticated Received Chain) sign and possibly also verify messages.


I already DKIM sign messages which verify on external receiving MTAs when
the sender address is in my local domain. The server also maintains a set
of distribution lists expanded from aliases. Some of the senders to these
distribution lists are outside my local domain which breaks DKIM
verification, which is why I want to try ARC signing.

I accept ARC is a new, experimental feature in EXIM but the documentation
in doc/experimental-spec.txt is very brief. The thing I really need is
some simple worked example configurations.

Ideally I just need to check whether an incoming message was for one of my
distribution lists and if so ARC sign it.

Currently I have tried this simple configuration,

remote_smtp:
driver = smtp
dkim_domain = $sender_address_domain
dkim_selector = selector3
dkim_private_key = /etc/exim/dkim/selector3.pem
dkim_canon = relaxed

EXPERIMENTAL_ARC=yes
arc_sign = $primary_hostname : selector3 : /etc/exim/dkim/selector3.pem

but exim fails to start with "Exim configuration error in line XXX"
"transport name missing"

where line XXX is the 'arc_sign = ' line.

What am I doing wrong?

Please help.
Thanks
Tom Crane