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Networking / Stage 13 of 14

Restore a service from client to answer

Not running Starting usually ~20 s
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Step 1 of 7 · on a machine · 6 min

The filter is gone, yet the site is silent

What you will learn

  • Tell from the kind of failure which stage and which machine to look at.
  • Change the nginx listen address and check that it really changed.
  • Confirm a restore with a real client request and a healthy service.

The site on lab-node2 does not open from lab-node1. A colleague is sure: "It's the filter. Drop all the rules and it will work." Check whether they are right.

The machines are the same lab-node1 and lab-node2 namespaces as in the previous lesson. nc -zv address port checks one thing: whether the port accepts a connection.

Exercise · look

The filter is gone — now what?

Request the site at 192.0.2.12:8080 from lab-node1. Then remove all filter rules on lab-node2, as a colleague suggested, and test the port again — with nc.

First, predict

The filter is gone. What will nc say about port 8080?

Now start the exercise, run the command and compare your prediction with the output.

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Check yourself 1 question

Without the filter nc got Connection refused. What does that mean?

  • nft flush ruleset deletes rules and leaves programs alone. The refusal came from lab-node2 itself: nobody waits on that port and address.

  • It was: with the filter curl waited three seconds and got nothing. Without it lab-node2 answered — if only with a refusal.

  • A site answers an HTTP request, and things never got that far: the connection was refused at the port.

lab-node2 sent the refusal: the packet now gets through, but nobody listens on port 8080 at 192.0.2.12.

Found an inaccuracy? Report an error in the lesson

Ubuntu 24.04
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