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If a local code-analysis server will not open at localhost:9000, first determine whether anything is listening on port 9000. Test IPv4, IPv6, and proxy bypass separately; then check the listener, startup logs, effective port configuration, and—if applicable—container port mapping. A refusal, timeout, or HTTP error points to a different part of the problem.
What a connection error on port 9000 means
Port 9000 is not reserved for one particular application. The service’s own configuration determines whether it uses that port. A failed request is initially a TCP reachability problem; an HTTP response means the client reached a server, even if the requested page or API call failed.
- Connection refused: Nothing is listening at that address and port, or local policy actively rejected the connection.
- Timeout: Traffic may be dropped, the address may be wrong, or the service may be hung.
- Connection reset: A process accepted the connection and then closed it. A wrong protocol, TLS setting, host header, or incomplete startup can cause this.
- HTTP 401, 404, or 503: TCP connectivity works. Investigate authentication, URL path, or application readiness rather than treating it as a port connection failure.
Record the exact error and command, operating system, how the server is running (native process, service manager, Docker/Compose, WSL, or VM), configured host and port, HTTP or HTTPS scheme, context path, and which client fails. The browser, CLI, IDE, CI job, and another container may have different network and proxy settings.
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Run these from the same environment as the failing client. The --noproxy '*' option bypasses configured proxies for this test:
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curl -v --noproxy '*' http://127.0.0.1:9000/
curl -v --noproxy '*' http://localhost:9000/
curl -g -v --noproxy '*' 'http://[::1]:9000/'On Unix-like shells, curl reports exit code 7 when it cannot connect to the host. See the curl manual. localhost can resolve to both 127.0.0.1 and ::1; testing each explicitly can reveal an IPv4/IPv6 binding mismatch. See curl’s URL syntax documentation.
If 127.0.0.1 works but localhost does not, inspect name resolution and which address the server listens on. If the proxy-bypassed command works while the browser, IDE, or build tool does not, look at that client’s proxy settings. Use https:// only if the service expects TLS on this port; sending HTTPS to a plain HTTP socket, or HTTP to a TLS socket, can produce protocol errors or resets.
Check whether a process is listening on port 9000
Linux and macOS
Use either command to inspect TCP listeners:
ss -ltnp | grep ':9000'lsof -nP -iTCP:9000 -sTCP:LISTENss and netstat report listening sockets; see the socket-listing reference. lsof can filter for TCP sockets in LISTEN state; see its network-filtering documentation. Depending on the operating system and permissions, identifying the owning process may require elevated privileges; do not elevate unnecessarily.
Windows
In Command Prompt, find the port entry and use its PID to identify the process:
netstat -aon | findstr :9000
tasklist /FI "PID eq <PID>"Replace <PID> with the number shown by netstat. Microsoft documents this port-to-process procedure and the tasklist command. In PowerShell, you can also run:
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Test-NetConnection 127.0.0.1 -Port 9000
Get-NetTCPConnection -LocalPort 9000 -State ListenMatch the listener address to the address you tested
127.0.0.1:9000indicates an IPv4 loopback listener.[::1]:9000indicates an IPv6 loopback listener.0.0.0.0:9000or[::]:9000generally indicates listening on available interfaces, subject to operating-system and application settings.- No listener may mean the service is stopped, still starting, crashed, configured for another port, or unable to bind.
To compare addresses directly, test http://127.0.0.1:9000/, http://[::1]:9000/, and, when appropriate, http://<host-LAN-IP>:9000/. A service listening only on loopback cannot be reached from another machine or container. Binding to a LAN address or all interfaces increases reachability and may expose a development service; do so only when required and with appropriate firewall controls. Docker documents the access implications of port publishing and localhost binding.
Inspect startup status and logs
A visible process does not establish that its HTTP service is healthy. Check startup output for address-in-use or permission errors, missing configuration or environment variables, unsupported runtime versions, dependency or database failures, out-of-memory or disk-space errors, initialization or migration failures, and crash-and-restart loops. If the application has a documented foreground start command, run it directly so errors remain visible. Change one setting at a time and preserve the original configuration while diagnosing.
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systemctl status <service-name>
journalctl -u <service-name> -b --no-pager
journalctl -fu <service-name>Replace <service-name> with the unit name. The first journal command filters entries for that unit during the current boot; the last follows new entries. See the journalctl reference and systemd debugging guidance.
For a container
Check its state and recent logs before changing ports:
docker ps -a
docker logs --tail=200 <container>
docker inspect <container> --format '{{.State.Status}} {{.State.ExitCode}}'A container can exist but have exited during startup, or its main process can be running while the application is not ready. Use the application logs and container state together to distinguish these cases.
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Verify the effective port and find collisions
Check every configuration layer that could override the port: command-line arguments, environment variables, configuration files, service-manager unit files, Docker or Compose settings, and reverse-proxy or IDE configuration. The port in the client URL must match the host-side port that actually reaches the service.
If startup says the address is already in use, identify the process or container that owns the port before taking action. It may be an old instance, another development tool, or a system service. Prefer stopping a duplicate cleanly or changing the intended service’s port. Avoid killing an unknown PID. For a container, changing only its host-side port can avoid a collision—for example, -p 9001:9000 makes host port 9001 forward to container port 9000.
Docker’s -p HOST_PORT:CONTAINER_PORT convention maps the host port on the left to the container port on the right. Thus -p 8080:80 forwards host port 8080 to container port 80. See Docker’s published-ports overview. Inspect a running container’s mapping with:
docker ps
docker port <container>
docker inspect <container> --format '{{json .NetworkSettings.Ports}}'A mapping such as 127.0.0.1:9000->9000/tcp publishes the container port on host loopback. A container that exposes port 9000 internally but has no published mapping is not thereby reachable at the host’s localhost:9000.
Resolve Docker and Compose address confusion
- Host to container: Confirm the container is running, the application listens on the expected container port, and that port is published to the host port you test.
- Container to container: A container’s
localhostrefers to itself, not another container. On a shared Compose network, use the service name and container port, such ashttp://analysis-server:9000, rather thanlocalhost. - Container to host: Use the host-gateway mechanism documented for the platform. The exact hostname and behavior vary across Linux, Docker Desktop, WSL, and remote Docker engines.
- Host port occupied: Change the host side of the mapping, such as
-p 9001:9000, and update the client URL to use host port 9001.
Docker’s network and firewall behavior can depend on the host platform and publishing configuration; consult its port-publishing documentation when changing exposure.
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Check proxy, firewall, VPN, and security settings
Command-line tools and IDEs may send loopback requests through a proxy even when a direct curl request succeeds. Inspect proxy variables in Unix-like shells with:
env | grep -i proxyIn PowerShell:
Get-ChildItem Env:*proxy*Ensure the client’s NO_PROXY setting bypasses the proxy for localhost, 127.0.0.1, and any relevant local hostname. A successful direct request proves the service is reachable from that environment; it does not prove an IDE or build tool uses the same proxy configuration.
Loopback traffic is usually allowed, but local firewall policy, VPN software, endpoint security, or sandbox isolation can interfere. Check the relevant firewall or security logs and, only when policy permits, allow the specific executable or local port. Do not disable the firewall globally to test a local connection.
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When curl receives an HTTP response, verify the scheme, path, trailing slash, authentication, redirects, host-header requirements, and whether the application is still initializing. A reverse proxy may terminate TLS on a different port, so do not assume port 9000 accepts HTTPS. Test the root path and the service’s documented health or status path:
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curl -i http://127.0.0.1:9000/<known-health-or-status-path>Replace the placeholder with a path documented by the service. A 401 can indicate that authentication is required; a 404 can indicate the wrong path; a 503 can indicate that the application or a dependency is not ready. None of those responses means that TCP could not reach the port.
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Some services bind their port before the HTTP API is ready. Slow dependency initialization, database migrations, first-run indexing, failing health checks, or restart loops can cause temporary failures. If a startup delay is expected, bounded retries can help:
curl --retry 10 --retry-delay 2 --retry-connrefused --fail http://127.0.0.1:9000/The retry options are documented in the curl manual. Retries do not fix a wrong port, a permanent bind failure, or a crashed process.
Isolate the operating system and loopback path
If you are unsure whether the problem is in the application or the local networking path, temporarily run Python’s built-in HTTP server on port 9000, provided the port is free:
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python -m http.server 9000 --bind 127.0.0.1Then request http://127.0.0.1:9000/ from the same environment. Python documents the HTTP server command and bind option. If the temporary server also cannot be reached, investigate port ownership, loopback, proxy configuration, or security software. If it works, the code-analysis server’s startup or configuration is more likely to be the cause. Stop the temporary server before restarting the real service.
Use the symptom to choose the next check
| Symptom | Likely cause | Next check |
|---|---|---|
| No listener on port 9000 | Stopped or crashed service, startup failure, or different configured port | Start command, effective configuration, and logs |
| Listener exists on another port | Client and server configuration differ | Configured port at each layer and client URL |
127.0.0.1 works but localhost fails |
IPv4/IPv6 binding or name-resolution mismatch | Test ::1 and inspect the listener address |
| Host works but a container does not | Container network namespace, wrong hostname, or proxy | Use the service name and container port; inspect proxy settings |
| Container runs but host port is absent | Missing or incorrect published-port mapping | docker port and Compose port settings |
| Startup reports address already in use | Another process or container owns the port | Identify the PID or container before stopping anything |
| TCP connects but the request fails | Wrong scheme or path, authentication, or incomplete readiness | curl -i and application logs |
| Browser fails but direct curl works | Browser proxy, cached HTTPS behavior, or extension | Try the explicit HTTP URL and inspect browser proxy settings |
| Refusal is intermittent | Restart loop or readiness race | Follow service or container status and live logs |
| Another machine times out | Loopback-only binding, firewall, or wrong host address | Listener address, target IP, and permitted firewall rules |
What to include when escalating
Share the exact failing command or URL and full error, operating system, runtime or service version, how the service is launched, configured host and port, and whether the request comes from the host or a container. Include listener output, relevant startup logs, and—if Docker is involved—the container state and port mapping. Redact credentials, tokens, and sensitive host details.
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