Re: [Exim] fallback_transport?

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Author: Dave C.
To: Philip Hazel
CC: exim-users
Subject: Re: [Exim] fallback_transport?
Ok.. Ive come up with something which might be worth including in the
cookbook. Credit where it is due, the idea for this came from Nigel's

I have a setup to support ETRN for a few small (ok, two) domains.
Currently it just leaves all the mail in the exim spool, and uses the
default exim etrn response to flush it out.

I don't like that - I agree with the opinion expressed on the list that
mail should be delivered somewhere else, and then shoved down an SMTP
session by some other program. Ive searched far and wide for something
suitable to do that shoving, and finally hit upon the only program I
trust to do that, handling errors and rejections correctly - exim

Nigel's solution for 'situation where a site I MX for has a known
outage', combined with a bit of bash scriptery, seems to form a neat
solution. (An intermittently connected host sort of falls under the
'known outage' category ;)

Without any further fluff, here are the details. Additional comments
appear below..

Either the real (intermittently connected) destination host needs to be
listed as the lowest MX (with the exim server as a less preferred) , or
the exim server needs to be the lowest MX, but have a router before the
lookuphost router which uses route_list or something appropriate to
normally deliver mail to the dialup host. The former is probably better
for a host which is usually connected and is only occasionally
disconnected (since other hosts would be able to delivery directly most
of the time, skipping an extra relay), while the latter would probably
work better for the converse ;) This paragraph actually applies anytime
you are using ETRN..

In either case, the routers below must precede whatever router handles
the normal direct-to-dialup-destination..


smtp_etrn_command = /etc/exim/etrn_script $domain

[- Content of /etc/exim/etrn_script: -]

# Where exim lives

# Something appropriate to generate a temporary unique string
UNIQ=`head -c100 /dev/urandom | md5sum | cut -f 1 -d" "`

domain=`echo $arg | sed 's/^\#//g'`

if ( test -f /var/spool/etrn/${domain} ); then
exim_lock -q /var/spool/etrn/${domain} "mv /var/spool/etrn/${domain} /tmp/etrn-bsmtp-${UNIQ}"
( cat /tmp/etrn-bsmtp-${UNIQ}
echo "QUIT" ) | $EXIM -bS -oMr etrn_requeue
rm -f /tmp/etrn-bsmtp-${UNIQ}

$EXIM -R $domain

# If you use smtp_etrn_serialize, the following ensures that the
# serialize hint is removed for the argument exactly as specified by
# the client (which might have an # prepended if they are issuing an
# ETRN argument as required by exim's default ETRN support

$EXIM -R $arg

[- end of etrn_script -]

[- exim transport -]

check_string = "."
escape_string = ".."
prefix = ""
suffix = ""

[- routers -]
[- You probably would want to put the domains in a file or a dbm and
[- adjused the 'domains' setting appropriately for both of these..

# If any message has already been delivered to the bsmtp file,
# this will detect the existence of the file and all messages will
# go there, regardless of age..
driver = domainlist
transport = bsmtp_for_etrn
require_files = /var/spool/etrn/${domain}
domains =
route_list = *

# If a message has been on the queue for over the specified amount of
# time, this will catch it and drop it into the bsmtp file
driver = domainlist
transport = bsmtp_for_etrn
condition = "${if >{$message_age}{1800} {yes}{no}}"
domains =
route_list = *

[- -]

Basically, this setup lets exim try to deliver to the real host for up
to whatever time is specified in the etrn_delay router. (1800 seconds =
30 minutes), and then delivers all waiting messages, and any further
messages, directly to a bsmtp file.. This setup uses one big BSMTP
file, it probably wouldnt be too complex to have it use seperate

When the etrn_script runs, it locks and renames the bsmtp file, and
reinjects the messages to exim, which (presumably) will now be able to
deliver them. If it can't, then once they are too old they will again
be sent off to the bsmtp file.. (If for som reason this occurs over and
over without exim being able to deliver them, eventually the messages
will be returned with "too many Received headers"; this is a good
thing, since their age will never get high enough for them to be
returned by any retry rules)

On Tue, 21 Nov 2000, Philip Hazel wrote:

> On Tue, 21 Nov 2000, Dave C. wrote:
> > ${if {$first_delivery} {foo} {bar}}
> >
> > Should it have been
> >
> > ${if first_delivery {foo} {bar}}
> Yes.
> (There is no form of ${if that has a { immediately after "$if{". There's
> always a condition name (which might be a symbol such as > )).
> > Anyway, another question.. Is there a variable available to tell how
> > long a message has been in exim's queue?
> $message_age