iptables -t nat -A POSTROUTING -s 172.16.1.0/24 -d 172.16.1.0/24 -j ACCEPT
iptables -t nat -A POSTROUTING -s 172.16.1.0/24 -j MASQUERADE

echo install br_netfilter /bin/true > /etc/modprobe.d/disable-br-netfilter.conf

sysctl -w net.bridge.bridge-nf-call-iptables=0

iptables -t nat -A POSTROUTING -s 172.18.0.0/16 -d 172.18.0.0/16 -j ACCEPT

iptables -A FORWARD -i br0 -o br0 -j ACCEPT

for ns in host1 host2 router; do
    ip netns del $ns 2>/dev/null || :
    ip netns add $ns
    ip -n $ns link set lo up
done

ip netns exec router sysctl -q -w net.ipv4.conf.default.forwarding=1

ip -n router link add bridge0 type bridge
ip -n router link set bridge0 up
ip -n router address add 192.168.0.1/24 dev bridge0

for i in 1 2; do
    ip -n host$i link add eth0 type veth peer netns router port$i
    ip -n host$i link set eth0 up
    ip -n host$i address add 192.168.0.10$i/24 dev eth0
    ip -n host$i route add default via 192.168.0.1
    ip -n router link set port$i up master bridge0
done

#to mimic a standard NAT router, iptables rule voluntarily made as it is to show the last "effect"
ip -n router link add name eth0 type dummy
ip -n router link set eth0 up
ip -n router address add 192.0.2.100/24 dev eth0
ip -n router route add default via 192.0.2.1
ip netns exec router iptables -t nat -A POSTROUTING -s 192.168.0.0/24 -j MASQUERADE

modprobe br_netfilter
sysctl -w net.bridge.bridge-nf-call-iptables=0

ip netns exec router iptables -A FORWARD -p icmp --icmp-type echo-request
ip netns exec router iptables -A FORWARD -p icmp --icmp-type echo-reply

# ip netns exec host1 ping -n -c2 192.168.0.102
PING 192.168.0.102 (192.168.0.102) 56(84) bytes of data.
64 bytes from 192.168.0.102: icmp_seq=1 ttl=64 time=0.047 ms
64 bytes from 192.168.0.102: icmp_seq=2 ttl=64 time=0.058 ms

--- 192.168.0.102 ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 1017ms
rtt min/avg/max/mdev = 0.047/0.052/0.058/0.009 ms

# ip netns exec router iptables -v -S FORWARD
-P FORWARD ACCEPT -c 0 0
-A FORWARD -p icmp -m icmp --icmp-type 8 -c 0 0
-A FORWARD -p icmp -m icmp --icmp-type 0 -c 0 0

# sysctl -w net.bridge.bridge-nf-call-iptables=1
net.bridge.bridge-nf-call-iptables = 1
# ip netns exec host1 ping -n -c2 192.168.0.102
PING 192.168.0.102 (192.168.0.102) 56(84) bytes of data.
64 bytes from 192.168.0.102: icmp_seq=1 ttl=64 time=0.094 ms
64 bytes from 192.168.0.102: icmp_seq=2 ttl=64 time=0.163 ms

--- 192.168.0.102 ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 1006ms
rtt min/avg/max/mdev = 0.094/0.128/0.163/0.036 ms

# ip netns exec router iptables -v -S FORWARD
-P FORWARD ACCEPT -c 4 336
-A FORWARD -p icmp -m icmp --icmp-type 8 -c 2 168
-A FORWARD -p icmp -m icmp --icmp-type 0 -c 2 168

# ip netns exec host1 ping -n -c2 192.168.0.102
PING 192.168.0.102 (192.168.0.102) 56(84) bytes of data.

--- 192.168.0.102 ping statistics ---
2 packets transmitted, 0 received, 100% packet loss, time 1008ms

# ip netns exec router iptables -v -S FORWARD
-P FORWARD DROP -c 2 168
-A FORWARD -p icmp -m icmp --icmp-type 8 -c 4 336
-A FORWARD -p icmp -m icmp --icmp-type 0 -c 2 168

# ip netns exec router iptables -A FORWARD -i bridge0 -o bridge0 -j ACCEPT
# ip netns exec host1 ping -n -c2 192.168.0.102
PING 192.168.0.102 (192.168.0.102) 56(84) bytes of data.
64 bytes from 192.168.0.102: icmp_seq=1 ttl=64 time=0.132 ms
64 bytes from 192.168.0.102: icmp_seq=2 ttl=64 time=0.123 ms

--- 192.168.0.102 ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 1024ms
rtt min/avg/max/mdev = 0.123/0.127/0.132/0.012 ms

# ip netns exec router iptables -v -S FORWARD
-P FORWARD DROP -c 0 0
-A FORWARD -p icmp -m icmp --icmp-type 8 -c 6 504
-A FORWARD -p icmp -m icmp --icmp-type 0 -c 4 336
-A FORWARD -i bridge0 -o bridge0 -c 4 336 -j ACCEPT

# ip netns exec host2 tcpdump -l -n -s0 -i eth0 -p icmp
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 262144 bytes
02:16:11.068795 IP 192.168.0.1 > 192.168.0.102: ICMP echo request, id 9496, seq 1, length 64
02:16:11.068817 IP 192.168.0.102 > 192.168.0.1: ICMP echo reply, id 9496, seq 1, length 64
02:16:12.088002 IP 192.168.0.1 > 192.168.0.102: ICMP echo request, id 9496, seq 2, length 64
02:16:12.088063 IP 192.168.0.102 > 192.168.0.1: ICMP echo reply, id 9496, seq 2, length 64