IPTABLES

https://wiki.merionet.ru/servernye-resheniya/14/pogruzhenie-v-iptables-teoriya-i-nastrojka/
https://losst.ru/nastrojka-iptables-dlya-chajnikov
https://linuxconfig.org/how-to-turn-on-off-ip-forwarding-in-linux
https://help.ubuntu.com/community/IptablesHowTo

# for rerouting
cat /proc/sys/net/ipv4/ip_forward
echo 1 >/proc/sys/net/ipv4/ip_forward
sudo sysctl -w net.ipv4.ip_forward=1

# list
iptables -L -v
iptables -nLv INPUT/OUTPUT/FORWARD -t filter/nat/

# Reject all incoming tcp requests
iptables -t filter -A INPUT -p tcp --dport 0:9999 -j DROP

# Reroute all incoming requests on port 80 to internal lan ip:port 
iptables -t nat -A PREROUTING -i enp0s3 -p tcp --dport 80 -j DNAT --to 192.168.2.19:80

# Reroute all outcoming requests from port 80 to internal lan ip:port
iptables -t nat -A POSTROUTING -i enp0s3 -p tcp --dport 80 -j SNAT --to 192.168.2.19:80



UFW

ufw allow 80
ufw deny 80