On Sat, Oct 01, 2016 at 01:56:40PM +0100, Brian wrote:
The thing for Mo to grasp is that exim *always* does an MX lookup, often
using the ISP's DNS server. user@server will fail (as has been found
out) because the domain "server" is not in the DNS.
/etc/hosts is not consulted when the lookup is done. exim can be made to
look at /etc/hosts but for such a simple setup it is not worth the
effort and would likely lead to a world of pain.
With great respect I think that confuses the issue. A DNS lookup will
*not* explicitly get done by exim4 if the target domain is in
hubbed_hosts, or if exim4 has been told it is the local domain. The
point is one wishes to PREVENT a DNS lookup that is doomed to fail, and
one does so for machines that are not the machine-local domain by
putting them in hubbed_hosts, which will cause exim4 to throw out a
connection request for the local network infrastructure (starting with
the network infra on the local machine) to handle, without doing an
explicit DNS lookup. That is handled by /etc/hosts if it is populated,
and by Avahi if it is present and /etc/hosts is not populated, and no
doubt by a few other alternatives I am not familiar with as well. If
none of them succeed, the delivery will fail, because if I recall
correctly hubbed_hosts router config ends with no_more which prevents
other routers having a go.
However, for domains NOT listed in hubbed_hosts, if not in smarthost
mode, yes exim4's next move will be an explicit DNS lookup on the
domain. It is trying to find out what *machine* it should contact to
make the delivery to that domain. If it is in smarthost mode, it throws
its hands in the air and passes the problem to the smarthost to solve.