Re: Configuring Exim for mail delivery
Am 01.10.2016 um 14:23 schrieb Mark Fletcher:
On Sat, Oct 01, 2016 at 12:36:10PM +0200, mo wrote:
Did you follow a specific book or guide?
Google "debian exim4 configuration". And look at the upstream
documentation in package exim4-doc-html.
I will do so ;)
Exim configuration has the concept of "routers" and "transports".
Routers basically decide what to do with a message, and transports do
it. One of the routers configured by default in the Debian exim
configuration is for "hubbed hosts". What this means, is machines
capable of sending and receiving email ("hosts" in exim speak) that are
on the same LAN as this machine (connected by a "hub"). Note that this
"hub" could be your local home network router, and for these purposes
machines on WiFi and machines on a wired LAN would be considered on the
same hub, even though that isn't strictly true. The point is that
network packets can be addressed directly between the machines, they
don't require a router in between.
As far as i do understand this is that only machines which are defined as
hubbed hosts can be send mail in the local LAN? Or am i misunderstanding
something here? :)
In essence yes you are right, because a local configuration of exim4
would determine that the addresses are not local and not in
hubbed_hosts, and tell you to eff off, while a smarthost configuration
would attempt to use the smarthost to send to the local machine, which
is doomed to failure, and the internet configuration would attempt to
use DNS to find the local machine, which is also doomed to failure.
Being in hubbed_hosts prevents exim4 for going looking for an MX record
for the receiving host, which is guaranteed to fail for a machine that
isn't registered to the world at large as an email server.
Alright, i finally go it ;) Thanks for the explanation Mark! :)
In Debian, this is achieved with Avahi. This is what allows you, if you
have MachineA and MachineB on your network, to do for example "ping
MachineA.local" from MachineB and expect MachineA.local to be resolved
into an IP address.
I'm not a friend of avahi to be honest, i much rather ignore it :D
All I can do here is echo Brian. But also, even if you aren't its
friend, ignoring it is fine, because it comes already configured and
will just do its job whether it gets any love from you or not. I just
assumed you'd like to know what is going on under the hood.
I sure appreciate to know the inner workings of it all :)
Good to know how avahi is involved here, this paints me a clearer picture :)
In /etc/exim4, create a file owned by root called hubbed_hosts. In the
file, each line maps a "domain" (the part after the @ sign in an email
address) to a "host" (the name of a machine on your network, as it can
be reached from this machine). Put the domain first, then a tab
character (spaces may also be OK) and then the host. So for example I
have a machine on my network called affinity, and so in the hubbed_hosts
file on the machine I am sitting in front of now, I have two lines, one
saying "affinity.local<TAB>affinity.local", and the other saying
"affinity<TAB>affinity.local" (no quotes in the file). This tells the
local exim installation that any email address with @affinity.local as
the domain should be forwarded on to a machine called affinity.local,
and any mail with @affinity as the domain should be forwarded on to a
machine called affinity.local. Exim4 will then say "Connect to
affinity.local!" with no attempt to translate that into an IP address,
and Avahi daemon will answer "that is IP address WW.XX.YY.ZZ!" to which
exim will say "very well, connect to WW.XX.YY.ZZ!" and the exim4 on
affinity will wake up and co-operate to deliver the mail.
I just did that and now mailing works flawlessly :D
Just one questions: Why do i need hubbed_host entries? Should it not be fine
alone to make a entry in /etc/hosts for the machines i want to send mail to
(I do not operate a dedicated DNS server).
This is something i dont really understand...
hubbed_host entries apply only to exim4. I also suspect, but am not
sure, that they are a Debian extension to exim4 in the sense that the
*DEBIAN* exim4 comes configured for them out of the box, while the
upstream exim4 does not. IIRC there is no reference to hubbed_hosts in
the upstream documentation, only in the Debian docs.
They work because the debian config contains a router to handle hubbed
hosts. You can see what it is doing if you search
/var/lib/exim4/config.autogenerated for the text hubbed_hosts.
I will consult the Debian docs, currently reading over the wiki page of
exim :)
If the file is not populated this router is skipped and then exim4
requires either that the address is the local machine, or that there is
a smarthost configured that it can delegate to, or that it can find an
official MX entry for the target domain by doing a DNS lookup. All of
which will fail for a local box that isn't registered to the world as a
mail server.
I see, thanks for the info :)
Mark
Thanks again Mark ;)
Greets
mo
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