The Port Conflict That Broke the Cluster: Debugging Host Network Mode in a Distributed S3 Test Environment

In any complex distributed system, the difference between a working test cluster and a broken one often comes down to a single port number. Message 1312 in this coding session captures exactly such a moment — a turning point where an assistant debugging a horizontally scalable S3 architecture realizes that switching to Docker's host network mode has introduced a cascade of port conflicts that are silently killing the storage nodes. The message is brief but dense with diagnostic reasoning, revealing the thought process behind one of the most frustrating classes of infrastructure bugs: the silent port collision.

The Subject Message

The message reads:

[assistant] Something else is using port 8080 on the host. In host network mode, all ports conflict with the host. The IPFS gateway defaults to 8080. I need to configure different ports for IPFS too.

>

Let me check the settings.env to see what ports are being set: [bash] cat /data/fgw2/config/kuri-1/settings.env | grep -i port # Host network mode - all ports bind directly to host export RIBS_DATA="/data/ribs" export RIBS_MAX_LOCAL_GROUP_COUNT="4" export RIBS_FILECOIN_API_ENDPOINT="https://pac-l-gw.devtty.eu/rpc/v1" # Database settings (host network - use 127.0.0.1 with custom ports) export RIBS_YUGABYTE_CQL_HOSTS="127.0.0.1" export RIBS_YUGABYTE_CQL_PORT="19042" export RIBS_YUGABYTE_CQL_KEYSPACE="filecoingw" export RIBS_YUGABYTE_SQL_HOST="127.0.0.1" export RIBS_YUGABYTE_SQL_PORT="25433" export RIBS_YUGABYTE_SQL_DB="fi..."

Context and Motivation

To understand why this message was written, we need to step back and look at the broader debugging session. The assistant had been building a horizontally scalable S3-compatible storage system using the Filecoin Gateway (FGW) architecture. The test cluster consists of multiple components: a stateless S3 frontend proxy, two Kuri storage nodes (which handle the actual data storage and retrieval), a shared YugabyteDB instance for metadata, and various supporting services.

Earlier in the session, the assistant had switched from Docker's default bridge networking mode to host network mode. The motivation was performance: bridge networking introduces NAT overhead as Docker maps container ports to host ports through iptables rules. For a storage system being load-tested for throughput, every microsecond of latency matters. Host network mode eliminates this translation layer by having containers bind directly to the host's network interfaces.

However, this optimization came with a hidden cost. In bridge mode, port conflicts are largely invisible because Docker handles the mapping — a container can bind to port 8080 internally while exposing it on the host as port 8081 or any other available port. In host network mode, the container's port bindings are direct: if the container tries to bind to port 8080, it must be free on the host, or the container fails to start.

The Diagnostic Trail

The message is the culmination of a diagnostic chain that began when the assistant noticed that kuri-1 had exited shortly after starting. The previous messages show the assistant checking container status with docker ps, finding only kuri-2 and YugabyteDB running, then examining kuri-1's logs. The logs revealed the critical error: serveHTTPGateway: manet.Listen(/ip4/127.0.0.1/tcp/8080) failed: listen tcp4 127.0.0.1:8080: bind: address already in use.

This error tells us that the Kuri node's embedded IPFS gateway — a component that provides HTTP access to IPFS content — defaults to listening on port 8080. On a typical Linux development machine, port 8080 is commonly used by web servers, development proxies, or other services. The assistant confirmed this with ss -tlnp | grep 8080, which showed a listener on 0.0.0.0:8080 — something else was already occupying that port.

The key insight in message 1312 is the generalization: "In host network mode, all ports conflict with the host." This is the moment where the assistant connects the specific symptom (kuri-1 failing on port 8080) to the systemic cause (host network mode exposes every container port to host availability requirements). The IPFS gateway port is just one of many ports that could conflict — the S3 API ports, the LocalWeb ports, the YugabyteDB ports, and any other default port used by the software stack are all now subject to host-level availability.

Assumptions Made

The assistant made several assumptions in this message, some of which proved incorrect. First, the assistant assumed that the port conflict was the only issue preventing kuri-1 from starting. In reality, as the subsequent messages show, there were multiple compounding problems: dirty database migration states, an IPFS configuration that persisted across container restarts, and a startup command that used && instead of ; so that a failed init step prevented the daemon from running at all. The port conflict was the symptom that led the assistant down the right path, but it was not the sole root cause.

Second, the assistant assumed that the solution was to configure different ports for IPFS — "I need to configure different ports for IPFS too." This assumption is visible in the grep command, which searches for port-related environment variables in the settings file. The assistant is looking for where IPFS ports might be configured, expecting to find an environment variable like IPFS_GATEWAY_PORT or similar. However, as message 1313 reveals, no such environment variable exists in the codebase. The IPFS gateway address is configured through IPFS's own configuration file, not through the application's environment variables.

Third, the assistant assumed that the settings.env file would contain all the relevant port configurations. The grep output shows database ports (CQL on 19042, SQL on 25433) and various RIBS settings, but no IPFS gateway port. This negative result is itself informative — it tells the assistant that the IPFS port is not configurable through the application's standard configuration mechanism, which means a different approach is needed.

Input Knowledge Required

To fully understand this message, the reader needs several pieces of contextual knowledge. First, an understanding of Docker networking modes — specifically the difference between bridge mode (where Docker manages port mapping through NAT) and host mode (where containers share the host's network stack directly). The trade-off between performance and isolation is central to why the assistant made the choice to use host mode and why it's now causing problems.

Second, knowledge of the IPFS (InterPlanetary File System) protocol and its default port assignments. IPFS nodes typically listen on port 4001 for swarm connections, port 5001 for the RPC API, and port 8080 for the HTTP gateway. The Kuri storage node embeds an IPFS node for content-addressed storage, so it inherits these default ports.

Third, familiarity with the Filecoin Gateway architecture and the Kuri storage node's role. Kuri nodes are the storage layer in this distributed S3 system, handling data persistence, retrieval, and content addressing through IPFS. They expose multiple interfaces: an S3-compatible API for object storage operations, a LocalWeb endpoint for CAR file serving, and the embedded IPFS node for content routing.

Fourth, understanding of the test cluster's configuration system. The gen-config.sh script generates per-node settings.env files that are loaded by the Kuri nodes at startup. These files contain database connection parameters, storage paths, network bind addresses, and other operational settings.

Output Knowledge Created

This message creates several important pieces of knowledge for the ongoing debugging session. First, it establishes that the IPFS gateway port (8080) is not configurable through the application's environment variable system — at least not through the variables that are currently documented or exposed. This negative finding directs future investigation toward either finding the hidden configuration mechanism or changing the approach entirely.

Second, the message documents the fundamental tension between host network mode and the software's default port assignments. The assistant now understands that every default port in the stack — from YugabyteDB's 15433 (already encountered and worked around) to IPFS's 8080 — becomes a potential conflict point. This systemic understanding is more valuable than fixing any single port conflict.

Third, the message captures the assistant's diagnostic methodology: observe the symptom (container exit), check the logs (port bind failure), verify the host state (ss/lsof to check port usage), and then examine the configuration to understand the full scope of the problem. This pattern of "symptom → log → host state → configuration audit" is a reusable debugging workflow.

The Thinking Process

The reasoning visible in this message shows a progression from specific to general. The assistant starts with the concrete observation: "Something else is using port 8080 on the host." This is a factual statement based on the log output and the ss command. The next sentence generalizes: "In host network mode, all ports conflict with the host." This is the key insight — the assistant is recognizing that this isn't an isolated issue with port 8080, but a systemic property of the networking mode they've chosen.

The third sentence connects the specific port to the specific component: "The IPFS gateway defaults to 8080." This links the observed failure (kuri-1 can't bind 8080) to the component responsible (the embedded IPFS gateway inside the Kuri node). The assistant now has a causal chain: host network mode → container binds directly to host ports → IPFS gateway tries port 8080 → port is occupied → container fails.

The final sentence — "I need to configure different ports for IPFS too" — reveals the assistant's planned remediation strategy. The "too" is telling: it references the earlier workaround where the assistant changed YugabyteDB's YSQL port from 15433 to 25433 to avoid a conflict with YugabyteDB's own web UI. The assistant is now planning to apply the same pattern to the IPFS gateway port.

The grep command that follows is the first step in executing this plan: find where ports are configured so they can be changed. The assistant expects to find an IPFS_GATEWAY_PORT or similar variable in the settings file. The grep output shows that the settings file contains database ports (CQL on 19042, SQL on 25433), storage settings, and API endpoints, but no IPFS port configuration. This negative result will force the assistant to reconsider the approach.

The Broader Implications

What makes this message significant is not the port conflict itself — port conflicts are mundane infrastructure problems — but what it reveals about the complexity of running distributed systems in development environments. Each component in the stack (YugabyteDB, Kuri, IPFS, S3 proxy) has its own set of default ports, and these defaults are chosen independently by different development teams. When you assemble them into a single test cluster, the probability of collisions increases with each additional component.

The host network mode experiment is a microcosm of a larger truth in distributed systems development: optimizations that improve performance often reduce isolation, and reduced isolation means that assumptions about resource availability (like "port 8080 is free") become global constraints rather than local ones. The assistant's journey through this debugging session — from performance optimization to port conflict to configuration audit to architectural re-evaluation — is a classic arc in infrastructure engineering.

In the messages that follow, the assistant will ultimately decide to revert to bridge networking, acknowledging that the port conflict management overhead of host network mode outweighs the performance benefits for a development test cluster. But message 1312 is the moment where that realization begins — the point where a specific failure is recognized as a systemic problem, and the search for a solution starts in earnest.