Linux routing loop

I have to set up a DMZ lab, but I am having issues with the basic network configuration. This is the main configuration:

FW1 - FLAME

Interfaces

eth0 - 192.168.1.3 255.255.255.0 gw 192.168.1.1 (its a virtual environment, this interface is bridged)

eth1 - 192.168.136.1 255.255.255.0

Routing table

0.0.0.0 ----------- 192.168.1.1 ------- eth0

192.168.136.0 --- 0.0.0.0 ------------ eth1

FW2 - ARROW

Interfaces

eth0 - 192.168.136.200 255.255.255.0 gw 192.168.136.1

eth1 - 192.168.30.10 255.255.255.0

Routing table

0.0.0.0 ----------- 192.168.136.1 ------- eth0

192.168.30.0 ---- 0.0.0.0 ---------------- eth1

I also have set to 1 ipv4 forwarding, and set the following IPtables rule in FLAME:

iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE

I tried most of this configuration other times and it worked, but I can't see what am I doing wrong here.

When I try to ping from ARROW to Internet, I can't reach it, but I can ping the eth1 interface on FLAME (it's gateway). Trying to do a traceroute results in a bunch of asterisks, which I interpret as a routing loop.

From FLAME I can reach Internet normally.

This must be a misconfiguration on FLAME, but I cant figure it out. Any ideas?