IPtables

iptables -L --line-numbers
# Delete rules number 3 in the list
iptables -D 3

Forward packet between vpn client subnet

iptables -A FORWARD -s 10.50.30.0/24 -d 10.50.30.0/24 -j ACCEPT

Block dst IP

iptables -A OUTPUT -d 203.151.31.76 -j DROP

Unblock dst IP

iptables -D OUTPUT -d 203.151.31.76 -j DROP

To block specific port number such tcp port # 5050, enter:

iptables -A OUTPUT -p tcp --dport 5050 -j DROP

To block tcp port # 5050 for an IP address 192.168.1.2 only, enter:

iptables -A OUTPUT -p tcp -d 192.168.1.2 --dport 5050 -j DROP

For unblock just change -A to -D on the above command

IP table src NAT

Ref ​here

All packets leaving eth1 will change source IP to 192.168.20.1

iptables -t nat -A POSTROUTING -o eth1 -j SNAT --to 192.168.20.1

We save iptable as persistent rule when reboot

~]# apt install iptables-persistent
~]# iptables-save > /etc/iptables/rules.v4
OR
~]# ip6tables-save > /etc/iptables/rules.v6
~]# iptable -L -t nat