I can't undersatand this :

All communication happens using SSH with port forwardings in both directions

Any help?


  • we need more context, without more details it is impossible to see what this is about.
    – JFL
    Mar 2, 2018 at 10:33

1 Answer 1


All communication happens using SSH with port forwardings in both directions

A well-known search engine suggests it's a quote from RIPE describing the network design for Atlas, its internet monitoring network, which has lots of probes out in the network.

It has this diagram (copied per RIPE public-non-profit permissions)

From reading that document, my understanding is:

  • There's a controller, some kind of server, listening on port 8080.
  • There's a client 'probe', which also has some kind of server process listening on port 8080.
  • If there were no privacy or authentication concerns, they'd just connect and communicate.
  • But they are out in the wild
  • So the probe makes an SSH call back to the client (unusually on port 443) with port forwarding

Which means

  • A connection received by the probe on port 8888 is forwarded to the controller on port 8080.
  • A connection received by the controller on port $remote_port is forwarded to the probe on port 8080.

Implicit in this is that SSH is forwarding TCP streams, not UDP.

There are many possibilities with most SSH software, but the usual situation is the incoming stream appears to come from the localhost in the following way. (Assuming this is web traffic ...) A web browser on the client connects from localhost:random to localhost:8888 where SSH (on the client) is listening. SSH sends the traffic, wrapped up in SSH protocol, to the server, which unwraps it. Then the SSH (on the server) makes a connection from its localhost:random to server's localhost:8080.

There is endless material available on SSH port forwarding to cover it in detail. You'll have to dig through more of RIPE's Atlas project if you want the exact details of what happens there.

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