waittime="0"
ZONENAME="example.com"
LOGFILE="/home/yourusername/update-local-dns-with-public-ip.log" #PATH TO LOGFILE

[ ! -f "$LOGFILE" ] && touch "$LOGFILE" #CREATE LOGFILE IF NOT EXIST

TIMESPAMP=$(date +'%Y-%m-%d %H:%M:%S')

SCRIPTEVAL=true

if [ "$SCRIPTEVAL" == true ]
then
ZONEFILE="/home/yourusername/example.com.hosts" #WE USE THIS FILE WHILE EVALUATING SCRIPT FUNCTION
else
ZONEFILE="/var/lib/bind/example.com.hosts" #THIS IS PRODUCTION ZONE FILE
fi

if [ "$SCRIPTEVAL" == true ] ;then echo "$ZONEFILE";fi

while true
do
#EXTRACT CURRENT PUBLIC IP
MYPUBLICIP=$(dig +short myip.opendns.com @resolver1.opendns.com)

# EXTRACT CURRENT REGISTRED IP FOR MAIL ENTRY
# THIS WILL SUFFICE SINCE IF THIS DIFFERS FROM CURRENT PUBLIC IP WE WILL THEN REPLACE ALL OCCURENSES OF THIS
# SINCE THIS SCRIPT IS MADE FOR HOME AND THERE IS ONLY ONE PUBLIC IP WE CAN PUBLISH TO
# MYMXIP=$(dig @localhost. mail.example.com +noall +answer | grep -E -o "([0-9]{1,3}[\.]){3}[0-9]{1,3}")
MYMXIP=$(grep "^mail.example.com." "$ZONEFILE" | grep -E -o "([0-9]{1,3}[\.]){3}[0-9]{1,3}")

# COMPARE CURRENT REGISTRED IP WITH CURRENT PUBLIC IP
if [ "$MYPUBLICIP" != "$MYMXIP" ]
then
NEWSERIALDATEPART=$(date +%Y%m%d) # GENERATE DATEPART OF NEW SERIAL NUMBER
NEWSERIALSTART=$(echo $NEWSERIALDATEPART"01") # GENERATE AN INITIAL NEW SERIAL KEY
NEWSERIALCOUNTPART=$(grep -o -P "\d{2}$" <<< $NEWSERIALSTART) # EXTRACT THE COUNT PART FROM SERIAL NUMBER
NEWSERIALCOUNTPART=$(expr $NEWSERIALCOUNTPART + 0) # THIS CONVERTS STRING TO NUMBER

OLDSERIAL=$(grep -o -w -P '(?<!\d)\d{10}(?!\d)' $ZONEFILE) # EXTRACT CURRENT (OLD) SERIAL NUMBER
OLDSERIALDATEPART=$(grep -o -P "(^\d{8})" <<< $OLDSERIAL) # EXTRACT DATEPART FROM OLD SERIAL NUMBER
OLDSERIALCOUNTPART=$(grep -o -P "\d{2}$" <<< $OLDSERIAL) # EXTRACT THE COUNT PART FROM OLD SERIAL NUMBER
OLDSERIALCOUNTPART=$(expr $OLDSERIALCOUNTPART + 0) # THIS CONVERTS FROM STRING TO NUMBER

 

if [ $OLDSERIALDATEPART -lt $NEWSERIALDATEPART ]
then
if [ "$SCRIPTEVAL" == true ] ;then echo "Old date is less than new date";fi
NEWSERIAL=$NEWSERIALSTART
fi

if [ "$OLDSERIALDATEPART" == "$NEWSERIALDATEPART" ]
then
if [ "$SCRIPTEVAL" == true ] ;then echo "Old date is equal to new date";fi
while [ $OLDSERIALCOUNTPART -ge $NEWSERIALCOUNTPART ]
do
if [ $NEWSERIALCOUNTPART -lt 10 ]
then
NEWSERIALCOUNTPART="0"$(expr $NEWSERIALCOUNTPART + 1) # ADD 1 TO COUNT
else
NEWSERIALCOUNTPART=$(expr $NEWSERIALCOUNTPART + 1) # ADD 1 TO COUNT
fi
done
if [ "$SCRIPTEVAL" == true ] ;then echo "Old serial: "$OLDSERIAL" New Serial: "$NEWSERIALDATEPART""$NEWSERIALCOUNTPART;fi
if [ "$SCRIPTEVAL" == true ] ;then echo "Old count: "$OLDSERIALCOUNTPART" New count: "$NEWSERIALCOUNTPART;fi
NEWSERIAL=$NEWSERIALDATEPART$NEWSERIALCOUNTPART
if [ "$SCRIPTEVAL" == true ] ;then echo $NEWSERIAL;fi
fi

# sed -i "s/20[0-9][0-9]\{7\}/$NEWSERIAL/g" "$ZONEFILE"
sed -i "s/[0-9]\{10\}/$NEWSERIAL/g" "$ZONEFILE"
sed -i "s/$MYMXIP/$MYPUBLICIP/g" "$ZONEFILE"
# rndc reload $ZONENAME

echo $(date +'%Y-%m-%d %H:%M:%S')": old IP $MYMXIP replaced with new IP $MYPUBLICIP" >> "$LOGFILE"

else
echo $(date +'%Y-%m-%d %H:%M:%S')": no update needed" >> "$LOGFILE"
fi
if [ "$SCRIPTEVAL" == true ] ;then cat $ZONEFILE;fi
STR="Sleeping $waittime"

echo $STR

if [ "$waittime" == "0" ]
then
break
else
sleep "$waittime"
fi
done