postinst is in exim4-daemon-heavy 4.90.1-1ubuntu1.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 | #!/bin/sh
set -e
. /usr/share/debconf/confmodule
if [ -n "$EX4DEBUG" ]; then
  echo "now debugging $0 $@"
  set -x
fi
dc_eximconfig_configtype="other"
db_get exim4/dc_eximconfig_configtype || true
if [ "$RET" = "no configuration at this time" ]; then
  dc_eximconfig_configtype="none"
fi
case "$1" in
	configure)
	# || true is needed for succesfull installation with configtype 'none'
	if [ -x /etc/init.d/exim4 ]; then
		db_stop
		if [ "$dc_eximconfig_configtype" = "none" ]; then
			# we may have broken config here, ignore errors
	       		invoke-rc.d exim4 start || true
		else
			# we must have working config here, honor errors
			invoke-rc.d exim4 start
		fi
	fi
	# set up DH-parameter file, update if older than 160 days
	if test -e /var/spool/exim4/gnutls-params-2048 ; then
		if [ `stat --format=%Y /var/spool/exim4/gnutls-params-2048` -le $(( `date +%s` - 13824000 )) ];
			then
			echo "Updating GnuTLS DH parameter file" 1>&2
			/usr/share/exim4/exim4_refresh_gnutls-params
		fi
	else
		echo "Initializing GnuTLS DH parameter file" 1>&2
		tempgnutls=$(tempfile --directory /var/spool/exim4 --mode 644 --prefix  "gnutp")
		chown Debian-exim:Debian-exim $tempgnutls
		if which certtool > /dev/null 2>&1 && \
			timeout --preserve-status --kill-after=15 120 \
			certtool --generate-dh-params --bits 2048 > $tempgnutls ; then
				mv $tempgnutls /var/spool/exim4/gnutls-params-2048
		else
			rm -f $tempgnutls
			install -m 644 -o Debian-exim -g Debian-exim \
				/usr/share/exim4/gnutls-params-2048 \
				/var/spool/exim4/gnutls-params-2048
		fi
	fi
	;;
esac
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