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Linux - IP tunnel

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nimrodm

I have two machines on different subnet. Say one is 1.1.1.1/8 while the other is 2.2.2.2/8. The two are able to talk through a common router.

The 2.2.2.2 machine has an internal interface on a third local network 176.16.1.1/24. An application is listening on this interface. The router is not aware of the 176.16.x.x subnet.

On the 1.1.1.1 I want to be able to send data to the 176.16.1.1 interface.

I want 1.1.1.1 to generate IP packets with a source=1.1.1.1 and destination=2.2.2.2. The packets should encapsulate another packet with the source 1.1.1.1 and destination 172.16.1.1.

I expect the 2.2.2.2 machine to decapsulate the packet and deliver the internal 1.1.1.1->172.16.1.1 packet to the application. Since 1.1.1.1 is a routable IP, the app can respond with a non-encapsulated standard IP packet.

Any hint on how to do that would be greatly appreciated. Thanks!

Edit

On the 1.1.1.1 I used the following

ip tunnel add ipiptun1 mode ipip local 1.1.1.1 remote 2.2.2.2
ip link   set ipiptun1 up
ip addr   add 172.16.0.0/16 dev ipiptun1

The above also creates a routing table entry

172.16.0.0/16 dev ipiptun1 proto kernel scope link src 172.16.0.0

And encapsulated packets do arrive at 2.2.2.2 where the inner packet is 172.16.0.0 -> 172.16.1.1.

However, despite 2.2.2.2 having an eth0:103 interface with the 172.16.1.1 address, the data does not arrive at the application. Instead an ICMP unreachable is returned to 1.1.1.1.

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Tobias K. avatar
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Tobias K.
A few thoughts: 1) 172.16.0.0 itself is a network ID address, and should probably not be used as a client address. 2) If you use TCP I don't think sending one direction via the tunnel and expecting the response via another (the public) interface will work because of the connection-oriented nature. 3) I would just use SSH port forwarding if possible, from 1.1.1.1 `ssh 2.2.2.2 -L 127.0.0.1:1234:176.16.1.1:8000` (using 8000 as example for App-port), and then send your normal packets to 127.0.0.1:1234.
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