The Connection Refused: A Microcosm of Remote Infrastructure Debugging

Message in Full

[assistant] [bash] ssh -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null -o ConnectTimeout=10 -p 20381 root@154.42.3.18 'echo CONNECTED; tail -5 /var/log/entrypoint.log' 2>&1 | head -15
ssh: connect to host 154.42.3.18 port 20381: Connection refused

This single message ([msg 1186]) is deceptively simple. On its surface, it is a failed SSH connection attempt — a bash command that tried to reach a remote server and was met with a terse "Connection refused." But to understand why this message exists, what it reveals, and what it cost to produce, one must trace the tangled thread of inference, failure, and adaptation that led to this precise moment. This message is not a mistake; it is a deliberate diagnostic probe, born from a chain of reasoning that spans dozens of prior messages, and it encodes a wealth of assumptions about network topology, instance lifecycle, and the behavior of cloud GPU rental platforms.

The Context: A Cascade of Failures

To understand [msg 1186], we must first understand the situation the assistant found itself in. The broader session (Segment 8 of the opencode conversation) was focused on resolving Out-of-Memory (OOM) crashes in low-RAM GPU instances running the CuZK proving engine for Filecoin PoRep proofs. Two instances were of particular concern: a "Belgium" instance (2× NVIDIA A40, 2 TB RAM, instance ID 32715193) and a "Czechia" instance (2× RTX 3090, 251 GB RAM, instance ID 32715618). Both had been created on the Vast.ai marketplace — a peer-to-peer GPU rental platform where anyone can offer their hardware for cloud compute.

The assistant had already weathered a storm of failures. The original Belgium instance (32714146) was killed by the vast-manager's monitor because its benchmark exceeded a 20-minute timeout — a timeout the assistant later increased to 45 minutes after calculating that the full benchmark pipeline (warmup proof with PCE extraction, daemon restart, SRS preload, and 12 proofs at concurrency 6) would take approximately 25–30 minutes. The original Czechia instance (32714145) failed with a gRPC "transport error" — a broken pipe on the first batch proof after the daemon restart — and was subsequently killed by the manager for having a bench_rate of 0, falling below the 50 proofs/hour minimum.

The assistant responded tactically. It increased the benchmark timeout in the vast-manager's Go source code, rebuilt the binary, deployed it to the controller host (10.1.2.104), and restarted the systemd service. It then created replacement instances — the new Belgium (32715193) and the new Czechia (32715618) — using the vastai create instance CLI command. It also added a "post-restart warmup" proof to benchmark.sh to mitigate the gRPC timeout issue, rebuilt the Docker image, and pushed it to Docker Hub.

But then came the access problem.

The SSH Authentication Wall

When the assistant tried to check on the new instances, it hit a wall. The first attempt to reach Belgium via the Vast.ai SSH proxy (ssh4.vast.ai:35192) resulted in "Permission denied (publickey)" ([msg 1183]). The second attempt, routed through the controller host to the same proxy, also failed with the same error ([msg 1184]). The assistant hypothesized that the SSH key might not be set up correctly on the Vast.ai side — a common issue when provisioning instances on a marketplace where SSH key configuration depends on the provider's setup.

This is where the assistant's reasoning took a specific turn. It checked the instance details via vastai show instances --raw and discovered that both instances had "direct" port ranges — a feature Vast.ai offers where instances can be accessed directly via their public IP address and a mapped port, bypassing the SSH proxy entirely. The Belgium instance showed direct_start: 20381 and public_ipaddr: 154.42.3.18. The Czechia instance showed direct_start: 41821 and public_ipaddr: 77.48.24.153 ([msg 1185]).

The assistant then made an inference: if the SSH proxy was rejecting the key, perhaps direct SSH would work. The direct connection does not go through Vast.ai's proxy infrastructure — it connects straight to the provider's machine. The SSH key authentication might behave differently on the direct path, or the proxy might have a key configuration issue that direct access avoids. This inference is what produced [msg 1186].## The Reasoning Behind the Probe

The command in [msg 1186] is not a random attempt. It is a carefully constructed diagnostic probe. The assistant chose specific SSH options: StrictHostKeyChecking=no and UserKnownHostsFile=/dev/null to bypass host key verification (since the remote host is ephemeral and may have a new key each time), and ConnectTimeout=10 to avoid hanging indefinitely on an unresponsive port. The command to execute on the remote host — echo CONNECTED; tail -5 /var/log/entrypoint.log — was designed to produce a quick, unambiguous signal: either "CONNECTED" appears and log lines follow, or the connection fails. The head -15 on the local side ensured that even if the remote command produced voluminous output, only the first 15 lines would be shown, preventing terminal noise.

The assistant was operating under a specific hypothesis: that direct SSH access would work where proxy SSH had failed. This hypothesis was grounded in the architecture of Vast.ai, which supports two distinct access paths. The proxy path (ssh4.vast.ai:35192) routes through Vast.ai's own SSH servers, which authenticate against the user's Vast.ai SSH key. The direct path (154.42.3.18:20381) connects directly to the provider's machine, which authenticates against the SSH key baked into the instance at creation time. The assistant assumed that the proxy's key rejection was a proxy-specific issue, not a fundamental authentication failure.

What "Connection Refused" Actually Means

The result — "Connection refused" — is a specific TCP error. It means that the remote host (154.42.3.18) actively rejected the connection attempt on port 20381. This is different from "Connection timed out" (which would indicate a firewall or network issue) and different from "No route to host" (which would indicate a routing problem). Connection refused means the destination host received the TCP SYN packet and responded with a RST (reset) because nothing was listening on that port.

This is a critical diagnostic signal. It tells us that either:

  1. The instance is not yet running. When a Vast.ai instance is first created, there is a provisioning delay while the provider's machine downloads the Docker image, starts the container, and sets up the port mappings. During this window, the direct port is not yet open. The Belgium instance (32715193) had been created only a few minutes earlier ([msg 1163]), and the Docker image was large enough that the provider might still be pulling it.
  2. The instance's direct port mapping is different from direct_start. The direct_start value (20381) is the beginning of a port range, but the actual SSH port might be at a different offset. Vast.ai documentation is not entirely clear on this mapping, and the assistant may have assumed a one-to-one correspondence that does not hold.
  3. The instance has already failed and been destroyed. Given the pattern of failures in this session — instances being killed by the monitor for timeout or low bench_rate — it is possible that the Belgium instance failed during its benchmark and was destroyed before the assistant could connect. The assistant, however, could not immediately distinguish between these possibilities. It needed more information.

The Contrast with Czechia

The very next message ([msg 1187]) provides a striking contrast. The assistant tried the same approach for the Czechia instance, connecting to 77.48.24.153:41821:

Warning: Permanently added '[77.48.24.153]:41821' (ED25519) to the list of known hosts.
Welcome to vast.ai. If authentication fails, try again after a few seconds, and double check your ssh key.
Have fun!
CONNECTED
cf6c62|OK  |    10MiB/s|/var/tmp/filecoin-proof-parameters/v28-fil-inner-product-v1.srs

This connection succeeded immediately. The assistant could see that the Czechia instance was alive and actively downloading SRS parameters — the large structured reference strings needed for the proving system. The CONNECTED echo appeared, followed by output from tail -5 /var/log/entrypoint.log showing a paramfetch download in progress.

This contrast is revealing. The Czechia instance (on host 135723, a provider in the Czech Republic) was provisioned and ready. The Belgium instance (on host 178156, a different provider) was not. The difference could be due to:

Assumptions Made and Their Validity

The assistant made several assumptions in [msg 1186] that deserve scrutiny:

  1. Direct SSH would work where proxy SSH failed. This was a reasonable assumption given the architectural difference between the two paths, but it proved only partially correct — it worked for Czechia but not for Belgium. The assumption did not account for provisioning delays that could differ between providers.
  2. The direct port is direct_start (20381). Vast.ai assigns a range of direct ports (direct_start to direct_end), and the SSH service typically listens on the first port in the range. However, this mapping is not guaranteed, and some providers may use a different port within the range. The assistant did not attempt other ports in the range.
  3. The instance was still alive. Given the history of instances being killed by the monitor, the assistant assumed that the new Belgium instance, having been created after the timeout fix was deployed, would survive long enough to be reachable. This assumption may have been incorrect if the instance encountered a different failure mode.
  4. The SSH key would be accepted on direct connection. The assistant assumed that the key rejection on the proxy was a proxy issue, not a key configuration issue. The success with Czechia partially validates this assumption, but the Belgium failure could still be due to key issues if the provider's SSH configuration differs.

The Knowledge Required to Interpret This Message

To fully understand [msg 1186], a reader needs:

The Thinking Process Visible in This Message

The assistant's thinking process, visible across the surrounding messages, follows a pattern of iterative hypothesis testing:

  1. Observe failure: The proxy SSH fails with "Permission denied."
  2. Form hypothesis: The proxy has a key configuration issue; direct SSH might bypass it.
  3. Gather data: Query instance details to find direct port and public IP.
  4. Test hypothesis: Attempt direct SSH to Belgium ([msg 1186]).
  5. Evaluate result: "Connection refused" — the hypothesis is not confirmed for Belgium.
  6. Test alternative: Attempt direct SSH to Czechia ([msg 1187]).
  7. Refine understanding: Czechia works, Belgium doesn't — the difference is likely provisioning state, not a fundamental access issue. This is classic scientific method applied to debugging: form a hypothesis, design an experiment, run it, interpret the results, and iterate. The assistant does not conclude from the Belgium failure that direct SSH is broken; instead, it immediately tests the other instance to disambiguate between "direct SSH doesn't work" and "this particular instance isn't ready yet."

Output Knowledge Created by This Message

[msg 1186] produced a negative result, but negative results are valuable. It told the assistant that:

  1. The Belgium instance (32715193) was not yet reachable via direct SSH on port 20381.
  2. The provisioning delay for this provider (host 178156) was longer than expected.
  3. The direct SSH approach was not a panacea — it worked for some providers but not others, depending on lifecycle state. This knowledge shaped the assistant's subsequent actions. It did not retry the Belgium connection immediately; instead, it proceeded to check Czechia, which succeeded, and then continued monitoring both instances over time. The negative result also implicitly validated the decision to increase the benchmark timeout — if the instance was still provisioning, it would need the extra time once it finally started.

Conclusion

[msg 1186] is a small message that tells a big story. It is a single SSH command that failed, but it sits at the intersection of multiple debugging threads: the network topology of cloud GPU rentals, the lifecycle management of ephemeral instances, the authentication quirks of SSH proxies, and the relentless iteration of hypothesis testing that characterizes effective remote debugging. The "Connection refused" was not the end of the story — it was a data point that, combined with the successful Czechia connection, helped the assistant build a more accurate model of the system's state. In the complex dance of distributed systems debugging, even a refusal is a form of communication.