From: David Woodhouse <dwmw2@infradead.org> 
Date: Thu, 18 Dec 2003 14:25:47 +0000                                          

Given a domain in DNS of the form...

$ORIGIN vdns.infradead.org.mailtarget.
fish		604800	IN	TXT	dwmw2@infradead.org

(It doesn't _have_ to be in private namespace; you can put it anywhere but I
prefer to have it private)

The following routers use it to implement a virtual domain. You could of course
omit the first and just make sure you have postmaster in all the zones you use
this way...

Rather than hardcoding the DNS domain to use in the router, we can put it into
a flat file with the list of domains for which we should be doing this.

We put this into /etc/exim/dns-virtual-domains:

	vdns.infradead.org: vdns.infradead.org.mailtarget

In the main section of the configuration file we have:

domainlist dns_virtual_domains = lsearch;CONFDIR/dns-virtual-domains

The following routers handle unqualified addresses, multiple TXT records, and
entries of the form '@domain'. Also if we're not primary MX for the virtual
domain in question we'll fall back to forwarding to a higher-priority MX host
if the DNS isn't talking to us....

virtual_postmaster:
  driver = redirect
  domains = +dns_virtual_domains
  local_parts = postmaster:root:abuse:mailer-daemon
  data = postmaster@$primary_hostname

  # For virtual domains, look up the target in DNS and rewrite...

dns_virtual_domains:
  driver = redirect
  domains = +dns_virtual_domains
  check_ancestor
  repeat_use
  one_time
  allow_defer
  allow_fail
  forbid_file
  forbid_pipe
  retry_use_local_part
  qualify_preserve_domain

   # Stash the lookup domain root for use in the next router.
  address_data = ${lookup{$domain}lsearch{CONFDIR/dns-virtual-domains}}

  # The lookup failure won't distinguish between absent record, absent
  # domain, or other temporary failures. So we make this router just
  # give up, and sort out the various failure modes later.

  # The ${sg...} bits turn multiple TXT records (which Exim gives us
  # separated by \n) into a comma-separated list, and also rewrite
  # any element of that list of the form '@domain' (i.e. without a
  # local part) to $local_part@domain, using the original local part
  # from the address being routed, at the newly-provided domain.

  # Addresses containing _only_ a localpart are qualified at the
  # same domain as is being looked up, by qualify_preserve_domain
  # above.
  data = ${sg{\
	    ${sg{\
	       ${lookup dnsdb{txt=$local_part.$address_data}{$value}fail}\
	    }{\n}{,}}\
	 }{(,|^)[ ]*@}{\$1\$local_part@}}

dns_virtual_failed:
  driver = redirect
  domains = +dns_virtual_domains
  allow_fail
  allow_defer
  data = ${lookup dnsdb{ns=$address_data}\
	# If NS lookup succeeded, the domain exists and we can find it.
	# Therefore, the above lookup failure meant that the user
        # just doesn't exist. Fail appropriately:
	{:fail:Unknown user at virtual domain}\
	# NS lookup failed. This means there's a DNS problem -- so we
	# shouldn't fail the delivery; let the following routers handle
	# it... Note "fail" not "{:fail:}". It means 'pass'. :)
	fail}


  # We have DNS problems. If we're actually _delivering_, then try to
  # deliver to a higher-priority MX if one exists. Otherwise, we defer and
  # let it stay on the queue until the problem is fixed.
  # You may prefer to freeze or bounce in this situation; I don't.
dns_virtual_relay:
  driver = dnslookup
  domains = +dns_virtual_domains
  transport = remote_smtp
  self = defer
  no_verify
  no_more

  # On the other hand, if there's a DNS problem and we're only _verifying_,
  # as we do when accepting incoming mail, then accept it for now and
  # it'll get queued for when the DNS works again.
dns_virtual_verify_fallback:
  driver = accept
  domains = +dns_virtual_domains
  verify_only
  no_more

> Now I just need to investigate DDNS and see if it'll let individual
> users update the TXT records for their own aliases in the DNS... :)

This is remarkably simple to set up -- Google is your friend. I'm now
able to set up HMAC-MD5 keys to 'own' certain mail domains, and the
owners of those virtual mail domains can happily change the TXT records
to their hearts content, without bugging me to make changes and roll out
new alias files to all the MX hosts.

A setuid app which is able to read the key file, and which will update
the alias only for the user it's invoked by, is also fairly trivial to
implement -- inspired by the 'cammail' alias system.