--- /dev/null
+#!/bin/sh -eu
+#
+# Short version of this script:
+# curl -f -o /var/cache/exim/opendmarc.tlds https://publicsuffix.org/list/public_suffix_list.dat
+# but run as Exim runtime user, writing to a place it can write to, and with
+# sanity checks and atomic replacement.
+#
+# For now, we deliberately leave the invalid file around for analysis
+# with .<pid> suffix.
+#
+# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~8< cut here >8~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+#
+# Create a cron-job as the Exim run-time user to invoke this daily, with a
+# single parameter, 'cron'. Eg:
+#
+# 3 4 * * * /usr/local/sbin/renew-opendmarc-tlds.sh cron
+#
+# That will, at 3 minutes past the 4th hour (in whatever timezone cron is
+# running it) invoke this script with 'cron'; we will then sleep between 10 and
+# 50 seconds, before continuing.
+#
+# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~8< cut here >8~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+#
+# This should be "pretty portable"; the only things it depends upon are:
+# * a POSIX shell which additionally implements 'local' (dash works)
+# * the 'curl' command; change the fetch_candidate() function to replace that
+# * the 'stat' command, to get the size of a file; change size_of() if need be
+# * the 'hexdump' command and /dev/urandom existing
+# + used when invoked with 'cron', to avoid retrieving on a minute boundary
+# and contending with many other automated systems.
+# + with bash/zsh, can replace with: $(( 10 + ( RANDOM % 40 ) ))
+# + on Debian/Ubuntu systems, hexdump is in the 'bsdmainutils' package.
+
+# Consider putting an email address inside the parentheses, something like
+# noc@example.org or other reachable address, so that if something goes wrong
+# and the server operators need to step in, they can see from logs who to
+# contact instead of just blocking your IP:
+readonly CurlUserAgent='renew-opendmarc-tlds/0.1 (distributed with Exim)'
+
+# change this to your Exim run-time user (exim -n -bP exim_user) :
+readonly RuntimeUser='_exim'
+
+# Do not make this a directory which untrusted users can write to:
+readonly StateDir='/var/cache/exim'
+
+readonly URL='https://publicsuffix.org/list/public_suffix_list.dat'
+
+readonly TargetShortFile='opendmarc.tlds'
+
+# When replacing, new file must be at least this percentage the size of
+# the old one or it's an error:
+readonly MinNewSizeRation=90
+
+# Each of these regexps must be matched by the file, or it's an error:
+readonly MustExistRegexps='
+ ^ac\.uk$
+ ^org$
+ ^tech$
+ '
+
+# =======================8< end of configuration >8=======================
+
+set -eu
+
+readonly FullTargetPath="${StateDir}/${TargetShortFile}"
+readonly WorkingFile="${FullTargetPath}.$$"
+
+progname="$(basename "$0")"
+note() { printf >&2 '%s: %s\n' "$progname" "$*"; }
+die() { note "$@"; exit 1; }
+
+# guard against stomping on file-permissions
+[ ".$(id -un)" = ".${RuntimeUser:?}" ] || \
+ die "must be invoked as ${RuntimeUser}"
+
+fetch_candidate() {
+ curl --user-agent "$CurlUserAgent" -fSs -o "${WorkingFile}" "${URL}"
+}
+
+size_of() {
+ stat -c %s "$1"
+}
+
+sanity_check_candidate() {
+ local new_size prev_size re
+ new_size="$(size_of "$WorkingFile")"
+
+ for re in $MustExistRegexps; do
+ grep -qs "$re" -- "$WorkingFile" || \
+ die "regexp $re not found in $WorkingFile"
+ done
+
+ if ! prev_size="$(size_of "$FullTargetPath")"; then
+ note "missing previous file, can't size-compare: $FullTargetPath"
+ # We're sane by definition, probably initial fetch, and the
+ # stat failure and this note will be printed. That's fine; if
+ # a cron invocation is missing the file then something has gone
+ # badly wrong.
+ return 0
+ fi
+ local ratio
+ ratio=$(expr $new_size \* 100 / $prev_size)
+ if [ $ratio -lt $MinNewSizeRation ]; then
+ die "New $TargetShortFile candidate only ${ratio}% size of old; $new_size vs $prev_size"
+ fi
+}
+
+if [ "${1:-.}" = "cron" ]; then
+ shift
+ # Don't pull on-the-minute, wait for off-cycle-peak
+ sleep $(( ($(dd if=/dev/urandom bs=1 count=1 2>/dev/null | hexdump -e '1/1 "%u"') % 40) + 10))
+fi
+
+umask 022
+fetch_candidate
+sanity_check_candidate
+mv -- "$WorkingFile" "$FullTargetPath"
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