iptables NAT iproute2 – Advanced Routing Problem Solutions

iproute2iptablesnat;

I have 2 internet links using 2 ADSL routers and I need to give access to the Internet for the 192.168.0.0/24 network.

I have to route outgoing traffic based on port number, protocol, …
using iproute2 and iptables on a linux router.

This is my network:

     (ISP-1)                              (ISP-2)
Dynamic public IP                    Dynamic public IP 
        |                                    |
+---------------+                    +---------------+
|ADSL Router (1)|                    |ADSL Router (2)|
+---------------+                    +---------------+
        |                                    |
   192.168.1.1                          192.168.2.1
        |                                    |
        |                                    |
        |                                    |
        |        +------------------+        |
        |        |                  |        |
   192.168.1.2 --|eth1          eth2|-- 192.168.2.2
                 |                  |
                 |   Linux Router   |
                 |                  |
                 |       eth0       |
                 +------------------+
                          |
                     192.168.0.1
                          |
                          |
                    Local Network:
                    192.168.0.0/24

I use the following script to setup the network configuration on the Linux router:

#!/bin/bash

echo 1 >| /proc/sys/net/ipv4/ip_forward
echo 0 >| /proc/sys/net/ipv4/conf/all/rp_filter

# flush all iptables entries
iptables -t filter -F
iptables -t filter -X
iptables -t nat -F
iptables -t nat -X
iptables -t mangle -F
iptables -t mangle -X
iptables -t filter -P INPUT ACCEPT
iptables -t filter -P OUTPUT ACCEPT
iptables -t filter -P FORWARD ACCEPT

# marking packets
iptables -t mangle -A PREROUTING -i eth0 -p icmp -j MARK --set-mark 1
iptables -t mangle -A PREROUTING -i eth0 -p udp  -j MARK --set-mark 1
iptables -t mangle -A PREROUTING -i eth0 -p tcp  -j MARK --set-mark 2

# create routing tables and default routes
echo '1     ISP1' >> /etc/iproute2/rt_tables
echo '2     ISP2' >> /etc/iproute2/rt_tables
ip route add default via 192.168.1.1 dev eth1 table ISP1
ip route add default via 192.168.2.1 dev eth2 table ISP2

# routing based on previous marks
ip rule add fwmark 1 table ISP1
ip rule add fwmark 2 table ISP2

ip route flush cache

# NAT
iptables -t nat -A POSTROUTING -o eth1 -j MASQUERADE
iptables -t nat -A POSTROUTING -o eth2 -j MASQUERADE

The problem is that I can't connect to the internet from the 192.168.0.0/24 network.

When I ping from this network to a remote server I can see (using Wireshark) replies returning back from the remote server to eth1 of the Linux router, but they don't reach out eth0.

Please help. And Thanks in advance.

(EDIT)

I tried troubleshooting this weird issue for a week.

Troubleshooting commands output:

ip rule
0:  from all lookup local 
32764:  from all fwmark 0x1 lookup ISP1
32765:  from all fwmark 0x2 lookup ISP2 
32766:  from all lookup main 
32767:  from all lookup default 

ip route show table ISP1
default via 192.168.1.1 dev eth1

ip route show table ISP2
default via 192.168.2.1 dev eth2

ip route show table main
192.168.2.0/24 dev eth2  proto kernel  scope link  src 192.168.2.2
192.168.1.0/24 dev eth1  proto kernel  scope link  src 192.168.1.2
192.168.0.0/24 dev eth0  proto kernel  scope link  src 192.168.0.1
169.254.0.0/16 dev eth0  scope link  metric 1000 
default via 192.168.1.1 dev eth1  metric 100

I can solve this problem partially by typing the following commands:

ip rule del fwmark 1 table ISP1
ip rule del fwmark 2 table ISP2
ip rule add from 192.168.0.0/24 table ISP1

So I get all traffic from local network routed correctly trough ISP1 link and all PCs get Internet access.

But i am interested in routing based on packets marks.

Best Answer

After a lot of hard work I finally found what was the problem.

In fact it is not a routing problem, the script is correct but something is missing.

This command is not enough to disable rp_filter:

echo 0 >| /proc/sys/net/ipv4/conf/all/rp_filter

So the traffic returning back from Internet was drop at eth1 and eth2.

When I disabled rp_filter explicitly for both interfaces by adding the following commands:

echo 0 >| /proc/sys/net/ipv4/conf/eth1/rp_filter
echo 0 >| /proc/sys/net/ipv4/conf/eth2/rp_filter

The problem was solved and I get everything working perfectly.

The proof that Linux tutorials and documentation are not always completes.

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