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Configuration

We have a setup that is as follows:

  • Server A that is connected to a large external network (192.168.0.0/24) and a smaller internal network (10.0.0.0/30). It serves as the main interface to connect to different services.
  • Server B connected connected only to the smaller internal network (10.0.0.0/30)
  • Service S1 running on server B on port 5009
  • Forwarding rules of port 5009 from server A to server B

Connecting to the service on port 5009 from a remote server in the larger network is working correctly as expected: Contacting server A on 192.168.0.x:5009 forwards the request to server B and a response is received.

Problem

Another service S2 running on server A that needs to connect to service S1 on server B. To do so, service S2 tries to connect to service S1 on server B using server A's public address 192.168.0.x:5009 (just as done with the remote server) but for some reason this fails (111 - Connection refused). It's as if the port forwarding rules are not applied for the service running on server A.

Note

Connecting to service S1 from server A using server B's address (10.0.0.x:5009) works like a charm. However, due to circumstances, it's not possible to use this address within service S2 so this is not a solution.

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  • Welcome to the site. Please edit your post to indicate how you set up the forwarding rule, by pasting the (possibly anonymized) firewall commands or configuration snippets. Commented Dec 18, 2024 at 13:18
  • Possible explanation is that port-forwarding is added to the nat table in the PREROUTING chain and works for incoming traffic. Traffic from server A to server A does not go through the PREROUTING chain and is not forwarded to server B. If so, traffic should be redirected in the OUTPUT chain (solution depends on the system). Commented Dec 19, 2024 at 5:53

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