Diagnosing a Phantom Network Failure: The Art of Isolating Service vs. Connectivity Issues

The Message

[assistant] kuri2 is listening on 7011. The issue may be a firewall or port mapping configuration. Let me test from the head node (internal network):
[bash] # Test from head node (internal)
ssh 10.1.232.82 "curl -s -o /dev/null -w 'Internal kuri2 7011: %{http_code}\n' --connect-timeout 5 http://10.1.232.84:7011/"

# Test external again with more detail
echo ""
echo "Testing external 7011..."
curl -v --connect-timeout 5 http://45.33.141.226:7011/ 2>&1 | head -20
Internal kuri2 7011: 401

Testing external 7011...
  % Total    % Received % Xferd  Average Speed  Time    Time    Time   Current
                                 Dload  Upload  Total   Spent   Left   Speed

  0      0   0      0   0      0      0      0                              0*   Trying 45.33.141.226:7011...
* Established connection to 45.33.141.226 (45.33.141.226 port 7011) from 10.0.0.1 port 32790 
* using HTTP/1.x
> GET / HTTP/1.1
> Host: 45.33.141.226:7011
> User-Agent: curl/8.18.0
>...

Context and Motivation

This message sits at a critical juncture in a production deployment of the Filecoin Gateway (FGW) distributed S3 storage system. The team had just completed a multi-phase implementation spanning cluster topology, load testing, and cross-node communication fixes. The immediate preceding events form a tight diagnostic chain:

  1. The network team added public port mappings from 45.33.141.226:7010/7011 to the two Kuri storage nodes.
  2. The assistant updated the EXTERNAL_LOCALWEB_URL configuration on both nodes to point to these public addresses.
  3. An initial connectivity test showed kuri1 (port 7010) returning a 401 response (expected—these CAR staging endpoints require authentication), but kuri2 (port 7011) returned a 000 status code, which in curl indicates a connection failure at the transport level.
  4. The assistant then checked whether kuri2 was actually running and listening, confirming via systemctl status and ss -tlnp that the service was active and bound to port 7011. This sets the stage for message 2130, where the assistant must answer a single, focused question: Is kuri2's external endpoint genuinely unreachable, or was the initial failure a false negative?

The Reasoning and Diagnostic Strategy

The assistant's opening line—"kuri2 is listening on 7011. The issue may be a firewall or port mapping configuration"—reveals a critical diagnostic assumption. Having confirmed that the service is running and bound to the correct port, the assistant correctly narrows the possible failure modes to two categories: either the port mapping from the public IP to the internal node is misconfigured, or a firewall is blocking the external connection. The service itself has been ruled out as the cause.

What makes this message particularly elegant is the two-pronged testing strategy the assistant employs. Rather than simply retrying the external curl command and hoping for a different result, the assistant designs a controlled experiment with an internal control group:

  1. Internal test: SSH into the head node (10.1.232.82) and curl kuri2's LocalWeb endpoint from within the trusted network. This tests whether the service is actually serving HTTP responses correctly, bypassing any external firewall or NAT issues.
  2. External test with verbose output: Retry the external curl against the public IP, but this time with the -v (verbose) flag to capture the full TCP connection handshake and HTTP request details. This is textbook network troubleshooting: isolate variables one at a time. The internal test answers "does the service work at all?" while the external verbose test answers "at what point does the external connection fail?"

Assumptions Made

The assistant makes several implicit assumptions in this message:

Mistakes and Incorrect Assumptions

The most notable aspect of this message is what it does not contain: a definitive conclusion. The verbose curl output is truncated with >..., showing only the HTTP request headers being sent. The reader never sees the server's response (or lack thereof). This truncation is likely an artifact of the head -20 command limiting output, but it means the diagnostic remains incomplete within this message itself.

Looking at the subsequent message (msg 2131), we learn that both endpoints are indeed working externally, both returning 401. This means the initial 000 failure for kuri2 was a false negative—possibly caused by a transient network issue, a race condition where the port mapping wasn't fully propagated when the first test ran, or a timing issue where the service hadn't finished initializing after the config update and restart.

The assistant's assumption that "the issue may be a firewall or port mapping configuration" turned out to be incorrect in the specific sense—there was no actual issue. But it was the correct diagnostic hypothesis to pursue, because those were indeed the only remaining variables that could explain the observed behavior. The mistake was not in the reasoning but in the premature conclusion that a problem existed at all, when in fact the initial test result was anomalous.

Input Knowledge Required

To fully understand this message, the reader needs knowledge of:

Output Knowledge Created

This message produces several valuable pieces of knowledge:

The Thinking Process Visible in the Reasoning

The assistant's reasoning follows a clear logical chain:

  1. Observation: External test of kuri2 returned 000 (connection failure).
  2. Hypothesis generation: Either the service is down, or the network path is broken.
  3. Test hypothesis 1 (service): Check systemctl status and socket bindings → service is running and listening on 7011.
  4. Test hypothesis 1b (service behavior): Internal curl test → returns 401, service is working correctly.
  5. Refine hypothesis: Since the service is healthy, the problem must be in the network path (firewall, NAT, port mapping).
  6. Test hypothesis 2 (network): External verbose curl → TCP connection establishes successfully.
  7. Implicit conclusion: The TCP connection works, so the port mapping and firewall are likely correct at the transport layer. The issue may be a slow response, a timeout, or the head -20 truncating the output before the response arrives. This is a textbook example of hypothesis-driven debugging. Each test eliminates one variable, narrowing the search space. The assistant does not jump to conclusions or make unfounded assumptions—it lets the evidence guide the investigation.

Broader Significance

While this message appears to be a simple diagnostic step, it illustrates a deeper principle of distributed systems debugging: the importance of testing from multiple vantage points. A single test from one network location can produce misleading results due to asymmetric routing, firewall rules that differ by source IP, or NAT behavior that varies by protocol. By testing both internally (from the head node) and externally (from the public internet), the assistant builds a complete picture of the system's behavior from different perspectives.

The message also demonstrates the value of verbose tool output. The initial test used -s -o /dev/null -w '%{http_code}', which provides a single numeric result but no diagnostic information. The retest with -v reveals the TCP handshake details, giving the operator visibility into exactly where in the connection lifecycle the failure occurs. This is a lesson applicable to any network debugging scenario: when a simple test fails, the next step is almost always to increase verbosity and capture the raw protocol exchange.

In the end, the "failure" was a phantom—both endpoints worked correctly, and the initial 000 was a transient anomaly. But the diagnostic process was sound, and the message stands as a clean example of methodical network troubleshooting in a production environment.