Issues with VERP in local part and reusing local part for a pipe transport (tainted arg)

Hello,

(Sent from my private Gmail address as our uni mail server isn’t fond of the CNAME for lists.exim.org)

We are using exim with a sympa mailing list server and have mostly managed to use a dynamic exim configuration for this, i.e. we do not generate a list of addresses out of sympa up front but check the validity of list email addresses on reception with require-files checking the existence of a mailing list at that address.

We have not yet managed to get bounce addresses with VERP working. Those are of the form

[email protected] <mailto:[email protected]>

Unfortunately I have not been able to create a config in which the local_part is untainted and can be reused for a pipe transport.

In broad strokes, our router and transport currently look like this:

sympa_list_bounce:
debug_print = "R: sympa_list_bounce for ${local_part_prefix}${local_part}@$domain"
driver = accept
domains = +sympa_domains
local_parts = ${if bool_lax{${lookup {${if match{$local_part}{\N^.+==([^=]+)$\N} {$1}{$local_part}}} dsearch {SYMPA_EXPL/${domain_data}}}} {${local_part}}}
local_part_prefix = bounce+
require_files = +SYMPA_EXPL/${lc::$domain_data}/${if match{$local_part}{\N^.+==([^=]+)$\N} {$1}{$local_part}}/config
retry_use_local_part
transport = sympa_bounce

sympa_bounce:
driver = pipe
command = "SYMPA_BOUNCE ${local_part_prefix}${local_part_data}\@${domain_data}"
return_fail_output
user = sympa

A few details:
- SYMPA_BOUNCE is the executable to which the message should be passed
- SYMPA+EXPL is the folder in which sympa keeps list configs in the form lists.ourdomain/thelist/config and some other files in the same folder
- sympa_domains has a list of domains that are, well, domains where sympa mailing lists exist

I now am a little stuck. Extending the prefix with prefix until the list name with for example local_part_prefix = *== would give me a local part that I don’t need to slice up before using it, but I don’t know how I then could ensure that the local part starts with bounce+ and it would be similarly tainted, correct?

The local_parts stanza is theoretically not necessary as the validity of the address is already checked in the require_files. We however use local_parts = dsearch;SYMPA_EXPL/${domain_data} or something similar in other routers to untaint the local_part. Here though the local_parts test returns a tainted string which I guess is the reason why the transport returns with the tainted arg error?

Does anyone have a suggestion how to solve this?

Thank you,

Felix Schäfer
Admin der FSInfo der TU Dortmund

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