Update iptables firewall with dynamic DNS updates from cron job

iptables-ddns.sh

#!/bin/bash

# Set as cronjob
# *	*	*	*	*	/root/scripts/iptables-ddns.sh >> /root/logs/iptables-ddns.log 2>&1

log () {
    echo "[$(date "+%F +%T")] [$1] $2" >> "$LOGS/changes.log"
}

HOSTS="mydynamichost.ddns.net"
LOGS="/root/logs/iptables-ddns/"
PORT=22

if [ ! -d "$LOGS" ]; then
    install -d "$LOGS"
fi

for host in $HOSTS; do
    LOG="$LOGS/$host"
    CURRENT=$(getent hosts "$host" | awk '{print $1}')

    if [ "$CURRENT" == "" ]; then
        log "$host" "[EMPTY] Current address empty"
        continue
    fi

    if [ -f "$LOG" ]; then
        PREVIOUS=$(cat "$LOG")
    else
        PREVIOUS=""
    fi

    if [ "$CURRENT" == "$PREVIOUS" ]; then
        log "$host" "[SAME] Current and Previous are same ($CURRENT)"
        continue
    fi

    if [ "$PREVIOUS" != "" ]; then
        iptables -D INPUT -s "$PREVIOUS" -p tcp -m tcp --dport "$PORT" -j ACCEPT
    fi

    iptables -A INPUT -s "$CURRENT" -p tcp -m tcp --dport "$PORT" -j ACCEPT

    echo "$CURRENT" > $LOG

    log "$host" "[UPDATED] $PREVIOUS > $CURRENT"
done

exit 0

Can you correct line 22 and 33?

22: if [ "$CURRENT" = "" ]; then
33: if [ "$CURRENT" = "$PREVIOUS" ]; then

(with one '=' in stead of '==')
because with '==' you'll get an error: unexpected operator

---- Edit 1 -----
And one more thing.

This script works very well. But if you do a restart of this device or a restart of the service iptables, then all of the updates by this script will be deleted from iptables as expected, as long the ddns address stays unchanged this script don't add the 'old' ddns address to the iptables.

So I have added the line between 34-35:
iptables -D INPUT -s "$PREVIOUS" -p tcp -m tcp --dport "$PORT" -j ACCEPT
iptables -A INPUT -s "$PREVIOUS" -p tcp -m tcp --dport "$PORT" -j ACCEPT

After those "corrections" all of this works perfectly
(sorry for my broken English)

Thanks for this script anyway <3

Thanks, @eusonlito and @Dieterm5. Here's full fixed that opens ALL ports and protocols for your DDNS ip:

#!/bin/bash

# Set as cronjob
# 10	*	*	*	*	/root/scripts/iptables-ddns.sh >> /root/logs/iptables-ddns.log 2>&1

log () {
    echo "[$(date "+%F +%T")] [$1] $2" >> "$LOGS/ddns-changes.log"
}

HOSTS="your-ip.ddns.net"
LOGS="/var/log/"
PORT=22

if [ ! -d "$LOGS" ]; then
    install -d "$LOGS"
fi

for host in $HOSTS; do
    LOG="$LOGS/$host"
    CURRENT=$(getent hosts "$host" | awk '{print $1}')

    if [ "$CURRENT" = "" ]; then
        log "$host" "[EMPTY] Current address empty"
        continue
    fi

    if [ -f "$LOG" ]; then
        PREVIOUS=$(cat "$LOG")
    else
        PREVIOUS=""
    fi

    if [ "$CURRENT" = "$PREVIOUS" ]; then
        echo "$host" "[SAME] Current and Previous are same ($CURRENT)"
        iptables -D INPUT -s "$PREVIOUS" -j ACCEPT
        iptables -A INPUT -s "$PREVIOUS" -j ACCEPT
        continue
    fi

    if [ "$PREVIOUS" != "" ]; then
        iptables -D INPUT -s "$PREVIOUS" -j ACCEPT
    fi

    iptables -A INPUT -s "$CURRENT" -j ACCEPT

    echo "$CURRENT" > $LOG

    log "$host" "[UPDATED] $PREVIOUS > $CURRENT"
done

exit 0