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A Dokku 502 Bad Gateway usually means nginx cannot reach a healthy web process for your app. Start with process health and the app’s nginx error log, then verify the host-to-container port mapping, the listener’s bind address, nginx configuration, and Ubuntu firewall rules. Do not begin by tuning nginx: Dokku intentionally serves 502 when an app has not been deployed, has no web process type, or has no running web processes.
Replace APP in the commands below with the Dokku app name. The exact final fix depends on your framework, Dockerfile or buildpack, Dokku release, internal listen port, and the error-log message.
What a 502 means in Dokku
Dokku places nginx in front of each application. Nginx accepts the browser request and proxies it to the web process inside the app container. A 502 is therefore a failure between nginx and that upstream process, not proof that the deployment command itself failed.
Dokku documents an intentional 502 configuration for three states: an app exists but has not been deployed, the app has no web process type, or it has no running web processes. In those cases nginx generates a minimal configuration that returns 502 Bad Gateway. Starting or repairing the web process is required; reloading nginx alone cannot create one.
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The five layers to check
| Layer | Healthy condition | Typical evidence | Corrective action |
|---|---|---|---|
| Application deployment | The app has a successful release and a formed web process. |
dokku ps:report APP and deployment logs |
Redeploy or correct the Procfile/process formation. |
| Process runtime | The web process stays running instead of exiting. | dokku logs APP --tail |
Fix the start command, environment variables, migrations, or crash shown in the log. |
| Port routing | Nginx’s mapped internal port matches the port the server listens on. | dokku ports:report APP |
Set the correct mapping with dokku ports:set. |
| Container listener | The server listens on the container interface at the mapped port. | Application startup output and container inspection | Bind to 0.0.0.0, not only 127.0.0.1, and use the intended port. |
| Nginx and host networking | Generated configuration is valid and traffic is not blocked. | Nginx error log, config validation, UFW or other firewall rules | Repair stale configuration, reload after validation, and allow the Docker/Dokku path. |
1. Confirm the app and web process are healthy
Run the process report and tail the application log before changing configuration:
dokku ps:report APP
dokku logs APP --tailA healthy deployment must show a running web process. If no web process exists, inspect how the app is formed. A Procfile or equivalent process definition must declare the command that serves HTTP. Correct that definition and redeploy rather than trying to repair nginx.
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Read the last startup and crash messages in dokku logs APP --tail. Common causes include an incorrect start command, a required environment variable that is missing, a migration or initialization step that fails, and a server that cannot bind to its configured address or port. Fix the cause shown by the application, then redeploy or start the corrected release. A restart is useful only after the underlying process problem is fixed; it cannot supply a missing process type or repair a bad port map.
If the app was never deployed
Dokku’s deliberate minimal nginx configuration returns 502 until a release with a web process is deployed. Complete a normal deployment and verify the process report again. Do not edit the generated nginx file to bypass this state.
2. Read nginx’s per-app evidence while reproducing the error
Open the app-specific logs and configuration:
dokku nginx:error-logs APP -t
dokku nginx:access-logs APP
dokku nginx:show-config APPThe default files are /var/log/nginx/APP-error.log and /var/log/nginx/APP-access.log. Keep the error log following while you make one request in another terminal or browser. The newest line usually identifies which layer failed.
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Connection refused
A refusal generally means nginx reached the container address but nothing is accepting connections on the mapped port. Return to dokku ps:report APP and the application log. Check that the process is still running, that the server started successfully, and that it listens on the same port reported by Dokku.
Upstream timeout
A timeout indicates that the upstream did not answer in time. Investigate a wedged or overloaded process, a startup operation that never completes, or an application endpoint that blocks. The nginx log tells you that the proxy waited; the application log and runtime state identify why.
Upstream address or port mismatch
If the error refers to an unreachable upstream address or port, compare the generated configuration with the port report and the application’s actual listen setting. Correct the mapping before reloading nginx.
3. Verify and correct Dokku’s port mapping
Ask Dokku what external-to-internal mapping it is using:
dokku ports:report APPCompare the reported internal port with the port used by the web server in the container. For example, if the application genuinely listens on port 5000, the documented form of a correction is:
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How to determine the real internal port
- Read the framework start command and its explicit port option.
- Check startup output in
dokku logs APP --tail; many servers print the bound address and port. - Inspect the Dockerfile’s
EXPOSEdeclaration. Dokku documents that exposed ports can influence proxy configuration, butEXPOSEis not proof that a process is listening. - Confirm the runtime setting used by the image or buildpack rather than relying on a local-development default.
A correct mapping with no listener still produces 502, so always pair ports:report with process and listener checks.
4. Make sure the container listener is reachable
The web server must bind to 0.0.0.0:<port> inside the container. Binding only to 127.0.0.1 makes the service reachable from its own process namespace but not through Docker networking, so nginx receives a refusal or timeout even when the process appears “up.”
Typical binding corrections
- Set the framework’s host or bind option to
0.0.0.0. - Set the application port to the same value used by
dokku ports:report APP. - Ensure the process command does not start a development server that binds only to localhost.
- After changing the image, Procfile, or environment, redeploy and check the new process state.
Dockerfile EXPOSE values and Dokku port settings both affect what nginx is configured to proxy, but neither one starts a web server. The listening process remains the source of truth.
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Once process and port issues are corrected, validate the configuration before activating it:
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sudo nginx -t
dokku nginx:validate-config APP
dokku nginx:reloadUse the commands supported by your Dokku release. A successful validation should report a syntactically valid configuration. If validation fails, do not reload. Read the error, inspect the generated app configuration, and correct the source of the bad directive or mapping.
When generated configuration is stale
Display the current generated file with:
dokku nginx:show-config APPCompare its upstream address and port with dokku ports:report APP and the application’s actual listener. If the file is out of date, rebuild the proxy configuration using the configuration-build command documented for your installed Dokku release, then run validation again. The exact rebuild command can vary by Dokku version, so do not substitute an arbitrary nginx command.
Why validation and reload are separate
dokku nginx:validate-config APP or sudo nginx -t checks the proposed configuration without activating it. dokku nginx:reload applies a valid configuration to running nginx. Keeping these operations separate prevents a syntax error or incomplete generated file from taking down otherwise healthy applications.
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Dokku’s troubleshooting documentation identifies a firewall such as UFW as a possible cause of Bad Gateway on Ubuntu. This is especially relevant when deployment reports success, the web process is running, and the mapping appears correct.
What to inspect
- Review UFW status and rules on the Ubuntu host.
- Confirm that public HTTP and HTTPS traffic can reach nginx.
- Check that host firewall policy does not block the Docker or Dokku forwarding path.
- Reproduce the request while following
dokku nginx:error-logs APP -t; a new log line helps distinguish a firewall path from an application refusal.
Do not broadly disable the firewall as a first step. Add the narrow rules required by your host policy, then test again. Firewall behavior can differ with Ubuntu version, Docker networking mode, and existing administrator rules.
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7. Re-test from the outside and isolate the repaired layer
After each targeted change, make a fresh request:
curl -I http://APP.example.comUse the hostname that resolves to this Dokku host. Interpret the result together with the logs:
- If the status changes after the web process is repaired, process availability was the failing layer.
- If it changes after
dokku ports:set, nginx had been sending traffic to the wrong internal port. - If nginx validation fails, the generated configuration or its source settings still need repair.
- If the map is correct but the log still shows refusal, check the bind address, process crash output, and current runtime state.
- If the app works locally but external requests fail and the error log remains quiet, verify DNS, the public listener, and host firewall policy before changing application code.
Keep the error log open during the test so you correlate one request with one upstream event instead of guessing from an old 502 response.
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| Observed situation | Most likely layer | First check | Remedy that persists |
|---|---|---|---|
| New app immediately shows 502 | Deployment or process formation | dokku ps:report APP |
Deploy a release with a declared, running web process. |
| Deploy succeeds, then 502 appears | Runtime crash or missing environment | dokku logs APP --tail |
Fix the startup failure and redeploy. |
| Error log says connection refused | No listener or wrong port | dokku ports:report APP plus app logs |
Start the server, bind it to 0.0.0.0, or correct the mapping. |
| Error log says upstream timeout | Hung or slow upstream | Application logs and process state | Resolve the blocking startup or request path; do not mask it with a blind reload. |
| Generated config points to an old port | Stale nginx configuration | dokku nginx:show-config APP |
Regenerate with the command for your Dokku release, validate, then reload. |
| Process and map are correct but external requests fail | Host firewall or forwarding | UFW rules and live nginx log | Allow the required HTTP/HTTPS and Docker/Dokku path under your security policy. |
What not to do
- Do not reload repeatedly. Reloading cannot create a missing web process, change a wrong port, or fix a crash loop.
- Do not edit generated nginx files as a permanent fix. Dokku can regenerate them during deployment or configuration changes.
- Do not assume
EXPOSEproves availability. It describes intended container networking; the process must still listen. - Do not force port 5000. That value is only an example; use the port your image actually serves.
- Do not treat a successful deploy message as an end-to-end test. Verify a running process, a reachable listener, a valid proxy configuration, and an external request.
Preventing the next Dokku 502
Before deploying a change, keep the web process definition, bind address, and internal port consistent. Record the intended port in the image or application configuration and make the Dokku mapping match it. After deployment, check dokku ps:report APP, watch dokku logs APP --tail during startup, and make one real HTTP request.
When changing ports, update the application and Dokku mapping as one change, then inspect dokku nginx:show-config APP and validate before reloading. When changing firewall rules, test both public HTTP and HTTPS paths. These checks catch the five failure layers before users encounter a generic 502 page.
A compact recovery sequence
- Identify the app and run
dokku ps:report APP. - Follow
dokku logs APP --tailand repair any crash, missing variable, migration failure, or bad start command. - Confirm a running
webprocess; redeploy if the process definition is missing. - Read
dokku nginx:error-logs APP -twhile reproducing the request. - Run
dokku ports:report APPand compare the internal port with the actual listener. - Correct a mismatch with
dokku ports:set APP http:80:INTERNAL_PORT, substituting the real port. - Ensure the server binds to
0.0.0.0and that anyEXPOSEvalue names the intended web port. - Inspect
dokku nginx:show-config APP; rebuild stale generated configuration using your Dokku release’s documented command. - Run
sudo nginx -tordokku nginx:validate-config APP. - Only after validation succeeds, run
dokku nginx:reload, check UFW and forwarding rules, and verify withcurl -I http://APP.example.com.
The Bottom Line
Fix the layer that the evidence identifies: restore a running web process, map nginx to the port it really serves, bind that server to 0.0.0.0, validate and reload generated nginx configuration, then check Ubuntu firewall rules. A Dokku 502 is usually a precise upstream-health or routing signal, not a reason to restart blindly.
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