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#! PERL_COMMAND -w
# This is a Perl script to demonstrate the possibilities of on-the-fly
# delivery filtering in Exim. It is presented with a message on its standard
# input, and must copy it to the standard output, transforming it as it
# pleases, but of course it must keep to the syntax of RFC 822 for the headers.
# The filter is run before any SMTP-specific processing, such as turning
# \n into \r\n and escaping lines beginning with a dot.
#
# Philip Hazel, May 1997
#############################################################################
# If the filter is called with any arguments, insert them into the message
# as X-Arg headers, just to verify what they are.
for ($ac = 0; $ac < @ARGV; $ac++)
{
printf("X-Arg%d: %s\n", $ac, $ARGV[$ac]);
}
# Now read the header portion of the message; this is easy to do in Perl
$/ = ""; # set paragraph mode
chomp($headers = <STDIN>); # read a paragraph, remove trailing newlines
$/ = "\n"; # unset paragraph mode
# Splitting up a sequence of unique headers is easy to do in Perl, but a
# message may contain duplicate headers of various kinds. It is better
# to extract the headers one wants from the whole paragraph, do any appropriate
# munging, and then put them back (unless removing them altogether). Messing
# with "Received:" headers is not in any case to be encouraged.
# As a demonstration, we extract the "From:" header, add a textual comment
# to it, and put it back.
($pre, $from, $post) =
$headers =~ /^(|(?:.|\n)+\n) (?# Stuff preceding the From header,
which is either null, or any number
of characters, including \n, ending
with \n.)
From:[\s\t]* (?# Header name, with optional space or tab.)
((?:.|\n)*?) (?# Header body, which contains any chars,
including \n, but we want to make it as
short as possible so as not to include
following headers by mistake.)
(|\n\S(?:.|\n)*)$ (?# Header terminates at end or at \n followed
by a non-whitespace character and
remaining headers.)
/ix; # case independent, regular expression,
# use extended features (ignore whitespace)
# Only do something if there was a From: header, of course. It has been
# extracted without the final \n, which is on the front of the $post
# variable.
if ($pre)
{
$headers = $pre . "From: $from (this is an added comment)" . $post;
}
# Add a new header to the end of the headers; remember that the final
# \n isn't there.
$headers .= "\nX-Comment: Message munged";
# Write out the processed headers, plus a blank line to separate them from
# the body.
printf(STDOUT "%s\n\n", $headers);
# As a demonstration of munging the body of a message, reverse all the
# characters in each line.
while (<STDIN>)
{
chomp;
$_ = reverse($_);
printf(STDOUT "%s\n", $_);
}
# End
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