[Exim] Sender and receiver verification can cause mail loops…

In case it affects anyone else; be careful with callouts.

I used to have this in my check_recipient acl:

  require verify = sender/callout=20s,random,postmaster
  require verify = recipient/callout=20s,random
  accept  domains = +local_domains
  accept  domains = +relay_domains
  accept  hosts = +relay_hosts
  deny    message = relay not permitted

Then I got a MAIL FROM:<somelocalpart@???>

Exim did a callout to the primary (only) MX for that domain, 127.0.0.3,
and spoke to another instance of itself. It sent MAIL FROM:<> and RCPT
TO:<somelocalpoart@???>

The second instance of Exim did a callout to 127.0.0.3 again to verify
the recipient...

Repeat until Exim stops accepting connections because there are too many
concurrent SMTP sessions. :)

Exim probably shouldn't do callouts to the local host. Doesn't it
already have logic to prevent sending mail when a higher priority MX
host turns out to be the local host?

For now, I've adjusted the ACL as follows:

  require verify = sender/callout=20s,random,postmaster
  accept  domains = +local_domains
          verify = recipient
  deny    domains = +local_domains
          message = Unknown recipient
  accept  domains = +relay_domains
          verify = recipient/defer_ok/callout=20s,random,defer_ok
  deny    domains = +relay_domains
          message = Unknown recipient at target domain
  accept  hosts = +relay_hosts
          verify = recipient/callout=20s,random
  deny    hosts = +relay_hosts
          message = Failed to verify recipient
  deny    message = relay not permitted

--
dwmw2