The Tunnel That Wasn't There: Diagnosing a Curio Startup Failure in a Distributed Proving System

Introduction

In distributed systems, the most frustrating failures are often the simplest: a service can't reach another service because a port isn't open, a tunnel isn't configured, or a configuration file was never updated. Message 1409 of this opencode session captures one such moment—a diagnostic pivot point where the assistant, faced with a Curio daemon crash on a newly deployed GPU proving instance, begins tracing the failure through the system's network topology. The message is deceptively brief: a single paragraph of reasoning followed by a bash command to check a systemd service. But within that brevity lies a rich example of systematic debugging, architectural knowledge in action, and the kind of "thin thread" reasoning that experienced operators use to navigate complex systems.

The Message

The assistant writes:

The issue is that the curio daemon can't reach the lotus API at 127.0.0.1:1234. This goes through the portavail tunnel — port 1234 is in the forwarded ports list. Let me check if the tunnel is up on that instance and if portavaild is forwarding 1234.

``bash ssh 10.1.2.104 'sudo systemctl status portavaild --no-pager -l | tail -20' ``

`` ● portavaild.service - PortAvail Daemon - Secure TCP Port Forwarding Loaded: loaded (/etc/systemd/system/portavaild.service; enabled; preset: enabled) Active: active (running) since Wed 2026-03-11 23:21:23 UTC; 11h ago Main PID: 103499 (portavaild) Tasks: 28 (limit: 35761) Memory: 9.3M (peak: 33.4M) CPU: 1min 56.305s CGroup: /system.slice/portavaild.service └─103499 /usr/local/bin/portavaild --listen 0.0.0.0:22222 --ports 1234,1235,5433,904... ``

The output confirms that the portavaild daemon on the controller host (10.1.2.104) is running and configured to forward port 1234. But as we will see, this is only half the story—and the wrong half at that.

The Architecture: Why Port 1234 Matters

To understand the diagnostic reasoning in this message, one must grasp the system's network architecture. The proving infrastructure consists of a controller host (10.1.2.104) and multiple worker instances rented from Vast.ai, a GPU marketplace. Each worker runs a curio daemon that performs Filecoin proving operations. Crucially, curio depends on a Lotus API endpoint—a Filecoin node that provides chain data—which listens on port 1234.

But the workers do not have direct network access to the Lotus node. Instead, the system uses a custom tunnel mechanism called portavail: a portavaild server runs on the controller, and each worker runs a portavailc client that establishes tunnels back to the controller. The controller's portavaild is configured with --ports 1234,1235,5433,904... to forward those ports. The workers' portavailc clients connect to the controller and expose those ports locally, making services like the Lotus API appear to be running on 127.0.0.1:1234 from the worker's perspective.

This is a classic "hub-and-spoke" tunneling pattern, common in cloud and edge deployments where workers are behind NAT or have no public IPs. The tunnel is the single point of coordination—and the single point of failure.

WHY This Message Was Written: The Diagnostic Trigger

The message is a direct response to a user report ([msg 1408]) showing that Curio failed to start with a clear error:

[curio]WARN  curio/chain  deps/apiinfo.go:65  Not able to establish connection 
to node with addr: ws://127.0.0.1:1234/rpc/v1, Reason: cannot dial address 
ws://127.0.0.1:1234/rpc/v1 for dial tcp 127.0.0.1:1234: connect: connection 
refused
[curio]ERROR: failed to establish connection with all chain nodes

This error is unambiguous: connection refused means nothing is listening on port 1234 at localhost. The assistant's first move is to connect this symptom to the known architecture. The reasoning chain is:

Symptom: Curio cannot reach 127.0.0.1:1234
Architecture knowledge: Port 1234 hosts the Lotus API, which is accessed through the portavail tunnel
Hypothesis: The tunnel is not forwarding port 1234 to this worker
Test: Check the portavaild server configuration on the controller This is textbook differential diagnosis: map the symptom to a component in the architecture, form a hypothesis about that component's failure mode, and test it. The assistant does not waste time checking Curio's configuration, the Lotus node's health, or network routing—it goes straight to the tunnel, because the architecture tells it that the tunnel is the only path from the worker to the Lotus API.## The Assumption and Its Subtlety The assistant's assumption is worth examining closely. The error comes from a worker instance—a Vast.ai GPU machine running Curio. But the bash command in the message checks portavaild on the controller host (10.1.2.104). The assistant assumes that if the controller's portavaild is running and configured to forward port 1234, then the tunnel should work. This is a reasonable first check, but it contains a hidden assumption: that the problem is on the server side of the tunnel, not the client side. In fact, the portavaild output confirms the daemon is running and has been up for 11 hours. It shows --ports 1234,1235,5433,904... in its command line, meaning the controller is ready to forward those ports. The assistant's hypothesis—that the tunnel configuration might have been wrong—is partially disproven by this result. The controller side looks healthy. But this creates a puzzle. If the controller is forwarding port 1234, why can't the worker reach it? The answer must lie on the worker side: either the portavailc client is not running, or it was started without port 1234 in its forwarded ports list. The assistant does not yet check this—the message ends with the controller check—but the seed of that next diagnostic step is planted. The reader familiar with the conversation's history (specifically [msg 1396] and the chunk 0 summary mentioning "port 1234 tunnel forwarding in entrypoint for curio startup") knows that this exact issue was anticipated and supposedly fixed in a previous round. The entrypoint script was updated to include port 1234 in the portavailc command. The fact that the error is recurring suggests either the fix was not deployed, the new instance was created from an older image, or there is a deeper race condition in the startup sequence.

Input Knowledge Required

To fully understand this message, a reader needs several pieces of context:

The portavail system: Knowledge that portavaild is a server-side daemon and portavailc is a client-side tunnel tool, and that they work together to forward TCP ports from the controller to workers.
The Lotus API role: Understanding that Curio depends on a Lotus Filecoin node for chain data, and that this node listens on port 1234. Without this API, Curio cannot perform proving operations.
The deployment architecture: Awareness that workers are Vast.ai instances with no direct access to internal services, and that all such access goes through tunnels established at startup.
The previous fix: Knowledge from [msg 1396] that the entrypoint script was updated to include 1234 in the portavailc forwarded ports list. Without this context, the message appears to be a routine check; with it, the message becomes a "did the fix actually work?" verification.
The controller vs. worker distinction: The bash command checks the controller (10.1.2.104), not the failing worker. The reader must understand that this is a server-side check, and that a positive result here shifts suspicion to the client side.

Output Knowledge Created

This message produces several valuable outputs:

Confirmation of controller health: The portavaild daemon is running, has been stable for 11 hours, and is configured with the correct port list. This rules out a server-side configuration error.
Refined hypothesis space: Since the controller is healthy, the failure must be on the worker side. The next diagnostic step is to check the worker's portavailc process, its command-line arguments, and the entrypoint script that launched it.
A documented debugging trace: The message serves as a permanent record of the diagnostic path. Any future operator seeing a similar Curio startup failure can follow this chain: check the controller's portavaild, then check the worker's portavailc, then check the entrypoint script.
Evidence of systematic methodology: The assistant does not guess or try random fixes. It follows the data: symptom → architecture → hypothesis → test. This methodology is itself a form of output knowledge, modeling good debugging practice for anyone reading the conversation.

The Thinking Process Visible in the Reasoning

The assistant's reasoning, while compressed into a few sentences, reveals a clear mental model. The phrase "This goes through the portavail tunnel" is the key insight—it connects the symptom (connection refused on port 1234) to the architectural component (the tunnel). The assistant does not say "maybe the Lotus node is down" or "maybe the network is broken." It immediately identifies the tunnel as the critical path and checks it.

The reasoning also shows an understanding of the system's control flow. The assistant knows that Curio's startup sequence includes a network call to the Lotus API, and that this call traverses the tunnel. If the tunnel is not forwarding port 1234, the call fails with exactly the error shown. This is not just debugging—it's tracing causality through the system architecture.

The use of systemctl status portavaild with --no-pager -l and tail -20 is also telling. The assistant knows exactly what to look for: the --ports argument in the process command line. It does not need to check logs or configuration files—the running process's command line is the ground truth. This is a sign of deep familiarity with the system: the assistant knows that portavaild's configuration is baked into its systemd service file and visible in ps, so checking the status output is the fastest way to verify.

Mistakes and Incorrect Assumptions

The message contains no outright mistakes, but it does reflect an assumption that is subtly incomplete. The assistant assumes that checking the controller's portavaild is the right first step. In a system where the controller is shared and the worker is the new variable, this is logical. But it means the assistant is checking the stable component (the controller, which has been running for 11 hours) rather than the unstable component (the newly deployed worker). A more aggressive diagnostic might have gone straight to the worker to check its portavailc process.

However, this is not really a mistake—it's a prioritization choice. Checking the controller first is faster (one SSH command) and rules out a whole class of failures. If the controller's portavaild had been misconfigured or down, the fix would be on the controller. Only after ruling that out does the worker become the focus. This is efficient, not wrong.

The deeper assumption—that the tunnel fix from the previous round was correctly deployed—is also worth noting. The assistant does not explicitly re-verify that the Docker image pushed to the workers includes the updated entrypoint. It assumes the fix is in place and that the failure must be something else. This turns out to be a reasonable assumption given that the fix was verified at the time, but in distributed systems, "it worked in testing" does not guarantee "it works in production." The new instance might have been created from an older image, or the entrypoint might have failed silently.

Conclusion

Message 1409 is a masterclass in targeted diagnostic reasoning. In two sentences and one bash command, the assistant maps a concrete symptom to an architectural component, tests the most likely failure point, and narrows the search space for the next round. The message demonstrates that effective debugging is not about knowing all the answers—it's about knowing which question to ask next. By checking the controller's portavaild first, the assistant eliminates a whole category of potential causes and focuses attention where it belongs: on the worker instance and its tunnel client. This is the essence of systematic troubleshooting in complex distributed systems, and this message captures it perfectly.