Zabbix server behind firewall

[pancu]

Hi guys,
this is my first time using zabbix.

I have installed zabbix server on linux.
The work station with zabbix server is hidden by a firewall with iptables.

This is my setup

Internet -> Firewall -> LAN ( zabbix server )

The firewall has two network interfaces:
- eth0 192.168.1.2 ( connected to the internet )
- eth1 192.168.0.1 ( connected to the LAN )

Zabbix server has ip 192.168.0.80

On the firewall I have configured iptables like this ( forwarding is enabled ):

# Default policy to drop all incoming packets.
iptables -P INPUT DROP
iptables -P FORWARD DROP


# Accept incoming packets from localhost and the LAN interface.
iptables -A INPUT -i lo -j ACCEPT
iptables -A INPUT -i eth1 -j ACCEPT


# Accept incoming packets from the WAN if the router initiated the connection.
iptables -A INPUT -i eth0 -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT

# Forward LAN packets to the WAN.
iptables -A FORWARD -i eth1 -o eth0 -j ACCEPT

# Forward WAN packets to the LAN if the LAN initiated the connection.
iptables -A FORWARD -i eth0 -o eth1 -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT

# Zabbix
iptables -t nat -A PREROUTING -p tcp -i eth0 --dport 10050 -j DNAT --to-destination 192.168.0.80
iptables -A FORWARD -s 161.97.136.20/32 -p tcp -d 192.168.0.80 --dport 10050 -m state --state NEW,ESTABLISHED,RELATED -j ACCEPT
iptables -t nat -A PREROUTING -p tcp -i eth0 --dport 10051 -j DNAT --to-destination 192.168.0.80
iptables -A FORWARD -s 161.97.136.20/32 -p tcp -d 192.168.0.80 --dport 10051 -m state --state NEW,ESTABLISHED,RELATED -j ACCEPT

Now, in the Zabbix Server ( 192.168.0.80 ) i run the command to monitor calls on port 10051:

sudo tcpdump -i any port 10051

I installed a zabbix agent on the server with ip 161.97.136.20.
If I try to execute the command

zabbix_sender -z <hostname server> -s "<source>" -p 10051 -vv -k <key> -o <value>

i expect to see something from the tcpdump, but I don't see anything.

If instead I execute the same command from another server inside the LAN (example with IP 192.168.0.2), I see that tcpdump capture traffic.

So, it looks like the issue is with the firewall's iptables commands.

I hope I have been clear.

Can you help me?