iptables on Debian 10 Router

*nat
:PREROUTING ACCEPT [0:0]
:INPUT ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
:POSTROUTING ACCEPT [0:0]

# ens18 is WAN interface, #ens19 is LAN interface
-A POSTROUTING -o ens18 -j MASQUERADE

COMMIT

*filter
:INPUT DROP [0:0]
:FORWARD DROP [0:0]
:OUTPUT ACCEPT [0:0]

# Service rules
# basic global accept rules - ICMP, loopback, traceroute, established all accepted
-A INPUT -s 127.0.0.0/8 -d 127.0.0.0/8 -i lo -j ACCEPT
-A INPUT -p icmp -j ACCEPT
-A INPUT -m state --state ESTABLISHED -j ACCEPT

# enable traceroute rejections to get sent out
-A INPUT -p udp -m udp --dport 33434:33523 -j REJECT --reject-with icmp-port-unreachable

# DNS - accept from LAN
-A INPUT -i ens19 -p tcp --dport 53 -j ACCEPT
-A INPUT -i ens19 -p udp --dport 53 -j ACCEPT

# BGP - accept from WAN
-A INPUT -i ens18 tcp --dport 179 -j ACCEPT

# SSH - accept from WAN
-A INPUT -i ens18 -p tcp --dport 22 -j ACCEPT

# SSH - accept from LAN
-A INPUT -i ens19 -p tcp --dport 22 -j ACCEPT

# DHCP client requests - accept from LAN
-A INPUT -i ens19 -p udp --dport 67:68 -j ACCEPT

# drop all other inbound traffic
-A INPUT -j DROP

# Forwarding rules
# forward packets along established/related connections
-A FORWARD -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT

# forward from LAN (ens19) to WAN (ens18)
-A FORWARD -i ens19 -o ens18 -j ACCEPT

# forward from WAN (ens18) to LAN (ens19)
-A FORWARD -i ens18 -o ens19 -j ACCEPT

# drop all other forwarded traffic
-A FORWARD -j DROP

COMMIT