Add/Update iptable forwading rule with hostnames

update_iptables.sh

#!/bin/bash

# Usage:
# Add/Update:   ./update_iptables.sh <port_listen> (tcp|udp) <hostname>:<port>
# Delete:       ./update_iptables.sh <port_listen> (tcp|udp) :<port>

PortListen=$1
Protocol=$2
HostName=${3%:*}
PortTarget=${3#*:}

echo $HostName
echo $PortTarget

if [ -n "$HostName" ]; then
IPv4=$(ping -c1 $HostName | grep "bytes of data" | cut -d "(" -f2 | cut -d ")" -f1)
echo $IPv4
fi

# Find the command use to add previous rules
REGEX1="\\-A PREROUTING -p $Protocol -m $Protocol --dport $PortListen"
REGEX2="\\-A POSTROUTING -p $Protocol -m $Protocol --dport $PortTarget"
RULE2DEL1=$(iptables -t nat -S | grep "$REGEX1"  | head -n 1)
RULE2DEL2=$(iptables -t nat -S | grep "$REGEX2"  | head -n 1)

if [ -n "$RULE2DEL1" ]; then
        echo 'Found existing rules'
        # Replace the -A with -D
        RULE2DEL1=${RULE2DEL1/"-A"/"-D"}
        RULE2DEL2=${RULE2DEL2/"-A"/"-D"}
        # Delete them
        iptables -t nat $RULE2DEL1
        iptables -t nat $RULE2DEL2
fi

if [ -n "$HostName" ]; then
        echo 'Adding new rules'
        # Add new rules
        iptables -t nat -A PREROUTING -p $Protocol -m $Protocol --dport $PortListen -j DNAT --to-destination $IPv4:$PortTarget
        iptables -t nat -A POSTROUTING -p $Protocol -m $Protocol --dport $PortTarget -j MASQUERADE
fi