COMPUTER #1:
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IP: 192.168.100.241
RUNS: nova-controller, nova-network and one nova-compute
IPTABLES:
iptables -T nat -A POSTROUTING -o eth0 -j MASQUERADE
iptables -A FORWARD -i br100 -o eth0 -j ACCEPT
iptables -A FORWARD -i eth0 -o br100 -m state --state RELATED,ESTABILISHED -j ACCEPT

ROUTE:
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         192.168.100.1   0.0.0.0         UG    0      0        0 br100
10.0.0.0        0.0.0.0         255.255.255.0   U     0      0        0 br100
192.168.100.0   0.0.0.0         255.255.255.0   U     0      0        0 br100
192.168.122.0   0.0.0.0         255.255.255.0   U     0      0        0 virbr0

COMPUTER #2:
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IP: 192.168.100.239
RUNS: nova-compute
IPTABLES:
Didn't set anything.

ROUTE:
0.0.0.0         192.168.100.1   0.0.0.0         UG    0      0        0 br100
192.168.100.0   0.0.0.0         255.255.255.0   U     0      0        0 br100
192.168.122.0   0.0.0.0         255.255.255.0   U     0      0        0 virbr0

OTHERS:
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MY NETWORK DNS: 192.168.100.4

PROBLEM:
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Instances on COMPUTER #2 cant access anything on internet or out of 10.0.0.x network.

TCP DUMP:
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- Instance #1 (on COMPUTER #2) send ping request to COMPUTER #2
- The request was routed to COMPUTER #1 nova-controller/network
- Controller masked the request and sends to COMPUTER #2 correctly
- COMPUTER #2 replied to COMPUTER #1 correctly
- COMPUTER #1 didn't route it again to Instance #1, just drop it.