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What a 502 means in Dokku
Dokku deliberately generates a minimal nginx configuration that returns 502 Bad Gateway when an app has been created but not deployed, has no web process type, or has no running web processes. A stopped app therefore needs process or application repair before proxy tuning.
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“when an app is created but not yet deployed, has no
webprocess type, or has no running web processes, Dokku generates a minimal nginx configuration that returns502 Bad Gatewayresponses.” — Dokku documentation, Nginx Proxy page.
This makes the status code useful: it identifies a failed connection between nginx and the app, not necessarily a problem with the browser or DNS. The exact cause can be a crashed process, a wrong port, a listener bound only to loopback, invalid generated nginx configuration, or a host firewall rule.
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A successful build or deployment message is not the same as a successful HTTP service. Dokku can finish building an image while the resulting web process exits immediately, listens on a different port, or never becomes reachable from the proxy. Treat the 502 as a layer-identification problem and collect evidence before changing several settings at once.
Start with process health
Replace APP in every command with the Dokku app name. Check both the process report and recent application output:
dokku ps:report APP
dokku logs APP --tailThe process report establishes whether Dokku has formed a web process and whether a container is currently running. The log stream shows why a container might stop between two checks. Run both checks close together, because a process that appears briefly in a report may exit before nginx receives the next request.
When no web process exists
A deployment needs a process type named web for HTTP traffic. If the report shows no running web process, inspect the application’s Procfile or process formation and redeploy after correcting it. A worker, release, or scheduled process cannot satisfy nginx’s web proxy. Adding or keeping a worker does not make it an HTTP upstream; the application must explicitly form a continuously running web process.
When the web process repeatedly exits
Read the application log for the first startup error rather than repeatedly restarting it. Common evidence includes a bad start command, a missing environment variable, a migration failure, or a bind error. Correct that application-level fault and deploy again. A restart is useful only after the process can start successfully; it cannot create a missing process type or repair a failing command.
Look for the earliest error in a restart loop. A later “connection refused” entry may only be a consequence of the process already having crashed. Preserve the original command, environment, and exit message while correcting the application, then verify that the process remains alive for more than one request.
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Continue while reproducing the request. A process can be listed briefly and still exit before nginx connects, so compare the process report with the live logs and the nginx error log. Health at the moment of deployment does not prove that the process is healthy at the moment of the 502.
Read nginx’s evidence while reproducing the 502
Dokku exposes separate commands for the app’s nginx error log, access log, and generated configuration:
dokku nginx:error-logs APP -t
dokku nginx:access-logs APP
dokku nginx:show-config APPThe default per-app files are /var/log/nginx/APP-error.log and /var/log/nginx/APP-access.log. Run the error-log command, make one request that produces the 502, and read the newest entry. The message usually tells you which layer to inspect next. The access log confirms that the request reached nginx, while the generated configuration shows the upstream address and port that nginx is actually using.
Keep the request pattern consistent while testing. Make one request, note its time, then correlate that time with the newest access and error entries. This avoids attributing an older timeout or a different virtual host’s failure to the current change.
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Connection refused
A refusal normally means nginx reached the container address but nothing is accepting connections on the configured port. Check whether the web process has exited, whether it is listening on a different port, and whether it bound to the correct interface. If the process report is healthy, verify the listener and port map before changing nginx timeouts.
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Upstream timeout
A timeout indicates that the upstream did not answer within the proxy’s wait period. Check for a wedged or overloaded application, a startup operation that never completes, or a process that is alive but unable to serve requests. The nginx log is the evidence for choosing this branch; do not change the port map solely because the response is slow. A timeout and a refusal are different symptoms: one suggests a nonresponsive upstream, while the other suggests that no listener accepted the connection.
Address or port mismatch
An upstream address or port that does not match the container’s listener points to routing or image configuration. Compare the generated config with the port reported by Dokku and the port used by the application. If those values disagree, repair the mapping or server configuration first, then regenerate or reload only as required.
Verify and repair the Dokku port map
Display the mapping that nginx and Docker are using:
dokku ports:report APPCompare the reported internal port with the port on which the web server actually listens. The public side in a typical HTTP mapping is port 80; the internal side must be the application’s real listening port. For an app that genuinely listens on port 5000, the documented repair pattern is:
dokku ports:set APP http:80:5000Use the application’s real port instead of copying 5000 as a default. If the server listens on 8080, map http:80:8080; if it listens on another port, substitute that value. After changing the mapping, reproduce the request and watch the error log again.
A successful deploy does not prove that the mapping is correct. The image may have changed its listen port, a previous manual mapping may remain, or generated proxy settings may be stale. Treat ports:report as the authoritative starting point for the current Dokku mapping. Do not infer the internal port from the public URL: the public port and container port are separate values.
The mapping is a persistent deployment setting, whereas a one-off container restart is temporary. If a later deploy regenerates proxy configuration from image metadata or Dokku settings, a manual correction that was never recorded in the app’s port configuration can disappear. Recheck the report after redeploying.
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The mapped port must have a running listener inside the container. Verify the application’s server configuration, startup output, and container state using the diagnostic facilities appropriate to your image. The important questions are:
- Is the web process still running when the request arrives?
- Does it listen on the same internal port shown by
dokku ports:report APP? - Does it bind to
0.0.0.0:<port>rather than only127.0.0.1:<port>?
Binding only to loopback can make an application appear healthy from inside its own process while remaining unreachable through Docker networking. Configure the framework or server to listen on the container’s external interface, then redeploy or restart the web process. The address and port must be correct at the same time; fixing only one still leaves nginx unable to connect.
Do not treat an open application socket inside the process as proof that the proxy can use it. A loopback-only listener accepts connections from the same container namespace but not from the Docker path nginx uses. Conversely, binding to the correct interface on an unlisted port still produces a 502.
Check Dockerfile exposure
If the image uses a Dockerfile, inspect its EXPOSE declaration and ensure it names the intended web port. Dokku documents that exposed ports can influence proxy configuration, but EXPOSE is not a substitute for a running listener. An image can advertise a port while the process listens elsewhere, and nginx will still return 502.
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Keep three values aligned: the port advertised by the image, the port used by the application server, and the internal port in Dokku’s mapping. If the image metadata and the actual server disagree, use the logs and listener behavior to determine the real port, then make the deployment configuration consistent rather than relying on an advertisement alone.
Port, process, and bind failures are different
A process crash requires an application fix. A listener on the wrong port requires a mapping or server-port fix. A listener bound to loopback requires a bind-address fix. These faults can produce similar 502 responses, so use the process report, application log, port report, and nginx error text together instead of applying all changes at once.
Validate nginx before reloading
Do not reload a generated configuration that has not passed validation. Use the system-level check or Dokku’s per-app validator:
sudo nginx -t
dokku nginx:validate-config APPOnly after validation succeeds should you reload Dokku’s nginx configuration:
dokku nginx:reloadValidation and reload answer different questions. Validation checks whether nginx can parse and accept the configuration; reload activates the accepted configuration. Keeping those steps separate prevents an invalid generated file from replacing the last working configuration.
If dokku nginx:show-config APP reveals an obsolete upstream, missing server block, or other drift, inspect how your Dokku release rebuilds generated app configuration and run that release-specific rebuild command. The exact rebuild command varies by Dokku version, so do not substitute a command from an unrelated release. Validate again after rebuilding, then reload.
Interpret validation failures
An nginx syntax or include error is a configuration problem, not proof that the application is down. Repair the template, include, or generated app configuration identified by the validator. Keep the last known-good configuration active until the new one passes; an invalid reload must not become the next outage.
If validation succeeds but the error continues, return to the upstream evidence. A syntactically valid configuration can still point to the wrong internal port or to a process that has exited. Syntax validation is necessary, but it does not test application readiness.
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Dokku’s troubleshooting guidance specifically identifies a host firewall such as UFW as a possible cause of Bad Gateway on Ubuntu. This branch is especially important when deployment reports success, the web process remains running, and the port map and listener agree.
Inspect the host’s UFW policy and any other firewall layer controlling Docker traffic. Public HTTP and HTTPS must be allowed to reach nginx, and host rules must not block the Docker/Dokku path between nginx and the app container. Change only the rule that explains the observed failure, then reproduce the request while following dokku nginx:error-logs APP -t.
A firewall adjustment should not replace application checks. If nginx logs show connection refusal and the container is not listening, opening another host port will not fix the process. Conversely, if the container is healthy but packets are blocked by host policy, restarting the app will not change the result.
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Retest and isolate the failing layer
After one corrective change, make a direct HTTP request to the app’s hostname:
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curl -I http://APP.example.comUse the result together with the newest nginx log entry. A status change after starting the web process identifies process availability as the original fault. A change after ports:set identifies routing. A failed nginx validation identifies configuration. A refusal with a correct map sends you back to the container’s bind address, crash output, and health state.
Use one change per test
Changing the process command, port map, firewall, and nginx configuration simultaneously removes the evidence needed to know what worked. Record the command and result for each test, and keep the request pattern the same. This is faster than repeatedly redeploying without a hypothesis.
Prefer the smallest reversible change that tests the suspected layer. A process-definition correction belongs in the application and should be redeployed. A port correction belongs in Dokku’s port settings. A generated-config correction should be rebuilt with the command for the installed Dokku release. A firewall change should be limited to the policy that blocks the demonstrated HTTP, HTTPS, or Docker path.
Check after a redeploy
Some settings are regenerated during deployment. After a successful redeploy, confirm that the web process is still formed, the internal listen port has not changed, and the generated configuration still points to that port. A temporary manual correction can disappear when Dokku rebuilds the app’s proxy configuration.
Also repeat the error-log check after the redeploy. A new 502 may have a different cause than the original one: correcting a missing process can expose a port mismatch, and correcting a port can expose a loopback bind. Treat each new log entry as fresh evidence.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Choose the remedy by the failing layer
| Evidence | Layer | Corrective action | What makes it durable |
|---|---|---|---|
No web process or the app is stopped |
Process formation | Fix the Procfile or process definition and redeploy | A valid, continuously running web command |
| Web process exits with startup errors | Application startup | Fix the reported command, environment, migration, or bind failure | The application starts cleanly with required settings |
| Refusal on the mapped upstream | Listener | Start the server on the mapped port and bind to 0.0.0.0 |
Server and Dokku use the same internal port |
| Reported internal port differs from the server port | Port mapping | Set http:80:INTERNAL_PORT with the actual port |
Keep the mapping aligned with the image and deployment settings |
| Generated config is stale or fails validation | Nginx configuration | Rebuild using the command for your Dokku release, validate, then reload | Configuration generation remains correct after deployment |
| Healthy app and map, but host traffic is blocked | Ubuntu firewall | Adjust UFW or host policy so the nginx/Docker path is permitted | A persistent rule that matches the required HTTP/HTTPS path |
This comparison also indicates when a remedy must be repeated. Starting a stopped process may restore service immediately, but it is not durable if the Procfile still forms the wrong process. Setting the correct port is durable when it matches the image and deployment configuration, but it should be checked after a redeploy. Rebuilding and reloading nginx is appropriate only after the generated configuration reflects the corrected process and port state.
A complete repair sequence
- Run
dokku ps:report APPanddokku logs APP --tail. Establish whether a runningwebprocess exists and remains alive. - While making a failing request, run
dokku nginx:error-logs APP -t. Use refusal, timeout, or mismatch evidence to select the next check. - Run
dokku ports:report APP. Compare the internal mapping with the server’s actual listen port. - Correct a mismatch with
dokku ports:set APP http:80:INTERNAL_PORT, replacingINTERNAL_PORTwith the real value. - Confirm that the server binds to
0.0.0.0on that port and that any DockerfileEXPOSEdeclaration names the intended web port. - Inspect
dokku nginx:show-config APP. If it is stale, rebuild it with the command documented for your Dokku release. - Run
sudo nginx -tordokku nginx:validate-config APP. Reload only after validation succeeds withdokku nginx:reload. - If the deployment is healthy but requests still fail, inspect UFW and other host firewall rules for blocked HTTP, HTTPS, or Docker traffic.
- Run
curl -I http://APP.example.comand correlate the response with a fresh nginx error-log entry.
The final command and setting depend on your framework, Dokku version, Dockerfile or buildpack, internal listen port, and the exact nginx error text. When those values are unknown, branch on the observed evidence rather than applying an app-specific fix by guesswork.
Frequently asked questions
Why does Dokku return 502 immediately after an apparently successful deploy?
A build can succeed while the app has no running web process, exits during startup, listens on the wrong port, or binds only to loopback. Check dokku ps:report APP, dokku logs APP --tail, and the per-app nginx error log before restarting.
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Only when the underlying process was temporarily stopped and can now start correctly. A restart cannot create a missing web process, correct a wrong port map, change a loopback bind, repair stale nginx configuration, or alter a blocking firewall rule.
Which log should I read first?
Start with dokku nginx:error-logs APP -t while reproducing one failed request. Use dokku logs APP --tail for application startup and crash details, dokku nginx:access-logs APP to confirm the request reached nginx, and dokku nginx:show-config APP to inspect the generated upstream.
How do I correct a wrong internal port?
Run dokku ports:report APP, identify the port the server actually listens on, and set the mapping with dokku ports:set APP http:80:INTERNAL_PORT. Replace the placeholder with the real port; the documented 5000 example is illustrative, not a universal default.
Does Dockerfile EXPOSE guarantee that nginx can connect?
No. EXPOSE can influence Dokku proxy configuration, but the application must still be running, listening on that same port, and bound to 0.0.0.0. An exposed but unused port still results in a 502.
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Loopback accepts traffic only from within the container’s own network namespace. nginx reaches the app through Docker networking, so configure the server to bind to 0.0.0.0:<port> on the mapped internal port.
When should I reload nginx?
Inspect or rebuild the generated configuration first, then run sudo nginx -t or dokku nginx:validate-config APP. Reload with dokku nginx:reload only after validation succeeds.
Can UFW cause a Dokku 502 even when deployment succeeds?
Yes. Dokku’s Ubuntu troubleshooting guidance identifies UFW or another host firewall as a possible cause. Check that HTTP and HTTPS can reach nginx and that host policy does not block the Docker/Dokku path, then reproduce the request while watching the nginx error log.
What should I do if the fix disappears after redeployment?
Recheck process formation, dokku ports:report APP, and dokku nginx:show-config APP. Deployment can regenerate proxy settings, so a durable fix must be represented in the process definition, image/listener configuration, Dokku port settings, or persistent host firewall policy.
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Frequently Asked Questions
Why does Dokku return 502 immediately after an apparently successful deploy?
A build can succeed while the app has no running web process, exits during startup, listens on the wrong port, or binds only to loopback. Check the process report, app logs, and nginx error log.
Is restarting the app enough to clear a Dokku 502?
Only if the process was temporarily stopped and can now start correctly. Restarting cannot fix a missing web process, wrong port map, loopback bind, stale nginx configuration, or firewall block.
How do I correct a wrong internal port?
Run dokku ports:report APP, identify the application’s actual listening port, then run dokku ports:set APP http:80:INTERNAL_PORT with that value.
Does Dockerfile EXPOSE guarantee that nginx can connect?
No. The application must also be running, listening on that port, and bound to 0.0.0.0.
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Can UFW cause a Dokku 502 on Ubuntu?
Yes. Check that HTTP/HTTPS can reach nginx and that host firewall policy does not block Docker/Dokku traffic.
The Bottom Line
A Dokku 502 is an evidence-driven troubleshooting problem. Confirm a continuously running web process, read the nginx error log during a failed request, align Dokku’s internal port with the application’s listener on 0.0.0.0, validate and reload generated nginx configuration, and then check UFW or other host firewall rules. Test one layer at a time and recheck after redeployments so the correction remains effective.
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