don't download filter rules if file already exists

postinstall.sh

@@ -13,15 +13,21 @@ case "$1" in
                     --gecos "<%= project %> user" \
                     --shell /bin/bash "<%= user %>"
         fi
-        URL="https://filters.adtidy.org/android/filters/15.txt"
+
         OUTFILE="/var/lib/dnsfilter/dns.txt"
 
+        if [ ! -f "${OUTFILE}" ]; then
+            echo "${OUTFILE} does not exists. Downloading..."
+
+            URL="https://filters.adtidy.org/android/filters/15.txt"
+
             mkdir -p /var/lib/dnsfilter
             wget -q --timeout=90 "$URL" -O "$OUTFILE"
             if [ $? -ne 0 ]
                 then
                 echo "Filter rules could not be downloaded."
             fi
+        fi
     ;;
     *)
         echo "postinst called with unknown argument \`$1'" >&2