Re: [exim] ACL filtering 3 dashes in host name

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Author: Eli Sand
Date:  
To: 'Exim Users'
Subject: Re: [exim] ACL filtering 3 dashes in host name
Paul wrote:

> I've read that twice before and had a quick glance at it when Mike
> posted the link.


Many of the questions you have been asking (more specifically, directly in
this thread), are indeed answered in the Exim docs posted online. I (and
many of the others lending a hand as well) are not trying to be rude or
condescending when we point you to the documentation, so please don't take
it as such - not that you have, but many newcomers do seem to get offended
by such actions.

Without trying to re-ignite the older thread, the Exim documentation is,
from a functional standpoint, extremely well documented such that every
feature, variable, function, etc... is documented and organized in its
proper place within the document. However, as you may have encountered,
there is a *lot* of information to cover before you may figure out many
things, and you may not get all the examples in the world... but you *do*
get a good starting point if you are looking at the default provided Exim
config file, looking at it as you read the documentation.

I have been using Exim for quite possibly around 6-8 years now or something
(unbelievable to think it's been that long) and I have read that online
documentation more than twice and I can still come up with questions once in
a while. If you would truly like to get a grasp of how to use Exim, I would
suggest you ensure you read the whole documentation from start to finish
(you can skim over some of the places which just cover variables and things
like that... but reading over the whole thing gives you an understanding of
the "flow" in Exim which is crucial to understanding how to modify its
configuration).

> I had to do a Google to discover what PCRE is. Its plain old Pearl. My
> very large text book on it, purchased circa 1998, remains unread. Its
> one more job I have to find time for.


One additional area to study up on, and which can come in *extreme* handy in
practically any "walk of life" as you use computers with any slight
inclination to program/script for them, is how to use Regular Expressions.
Additionally, as a bit of a history lesson, the original head programmer for
Exim, Philip Hazel (now retired from the Exim development), was also the
creator of PCRE which is Perl compatible Regular Expressions.

PCRE is quite *not* "plain old Perl", rather it is an amazing extension of
POSIX Regular Expressions. Unfortunately, you have now got in to a whole
new area which will require more reading than the entire Exim documentation
can provide. I am grateful that I have been blessed with the ability to
actually understand, write, and even sometimes optimize RE's and not break a
sweat. However, this "talent" didn't come from questions and answers, but
rather something like 15 years of trying to wrap my head around using RE's
in Linux, programming, etc... almost daily.

If you have access to Linux, a good place to start to try to just figure out
the basics of RE's is to "man regex". I've read that man page something
like 50 times at least. Another oddly good resource is the PHP
documentation on their PCRE implementation. I find that this documentation
is very well put together (it could possibly be taken from somewhere else -
perhaps even the PCRE documentation itself... I'm not sure), and should be
read several times and referenced constantly before trying to ask questions:
http://www.php.net/manual/en/pcre.pattern.php

Unfortunately, as is your case, I do not have time to try and help you with
your actual RE issue and create some for you to match hostnames with
multiple dashes in them, though safe to say that you're in for a world of
(brain-related) hurt when you try to figure out why your patterns are
horribly, horribly wrong (I'll give you a hint - the wildcard * is meant to
match anything, right? How does it know you do NOT want it to also match a
dash? This is where the term "greedy" comes in to play... the asterisk
wildcard, especially in RE's, is a "greedy" match, meaning it will take
everything you got - even when you don't want it to).

Although I am unsure of your actual intentions in using Exim, from the
sounds of things you may have tried to dive in head first in to a little
pile of mess to try and fix something or implement something for a superior?
In any case - that's just not how it works I'm afraid, and you really do
have to "study" the subject before you can try and use it.

Eli.