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Start with the fault domain, not the Java process
“The service is running” and “the service is reachable by IP” are different statements. A JVM can remain active after the web server failed to start, listen on a different port, or bind only to loopback. A firewall can then discard traffic even when the listener is correct. Treat the path as separate checkpoints:
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- The application starts without a web-server error.
- An HTTP listener exists on the port you expect.
- The listener is bound to an interface that can receive the request.
- The VPS operating-system firewall permits the port.
- The cloud provider’s ingress or security-group firewall permits the source.
- A proxy or load balancer, if present, forwards the public request to the Java listener.
Test from inside out. The first checkpoint that fails narrows the repair; changing several settings at once makes the original fault harder to identify.
How the three core checks differ
Process
A Java PID only shows that a process exists. Startup may have selected a different profile, failed to create an embedded server, or started a non-HTTP worker. Read the current startup log and inspect the running deployment rather than relying on a checked-in configuration file.
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Port
The port is the TCP number on which the HTTP server accepts connections. In a standalone Spring Boot application, the documented default main HTTP port is 8080; the value can be changed with server.port. A JVM listening on 9090 will not answer requests sent to 8080, even if every firewall rule is correct.
Bind address
The bind address determines which local interfaces accept packets. A listener on 127.0.0.1:8080 accepts loopback traffic only. That explains a successful curl http://127.0.0.1:8080/ alongside a failure against the VPS address. Binding to 0.0.0.0 asks the process to listen on all IPv4 interfaces; binding to a specific private interface is narrower and is preferable when only a reverse proxy should call the app.
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Begin with the values the running instance actually selected. Spring Boot can receive properties from configuration files, environment variables, and command-line arguments; deployment-time values can override what is committed in the project. Record the active HTTP port, bind address, context path, and whether TLS is enabled.
For a direct HTTP test on every IPv4 interface, the relevant properties are:
server.address=0.0.0.0
server.port=8080Use the narrowest address that matches the design. If Nginx, Apache, or a load balancer is the only caller, bind Java to the private interface or loopback and expose only the proxy. Use 0.0.0.0 when the application itself must accept connections arriving on any IPv4 interface.
Read startup evidence
Search the startup output for the embedded server’s port and for binding or startup errors. A message indicating that the application started on a port other than 8080 is decisive. Errors about an address already being in use, an unavailable certificate, or a failed context can leave a Java process present without a usable HTTP endpoint.
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Check deployment overrides
For a systemd service, container, or launch script, inspect its environment and command line. A command-line property such as --server.port=9090 takes effect even when application.properties still says 8080. After changing a property, restart the service and verify the new listener; editing a file without restarting does not change an already running JVM.
Step 2: Prove that a listener exists
Run the socket inspection command on the VPS:
sudo ss -lntpLocate the expected port and correlate the owning PID with the Java process. The address and port shown by ss are more reliable than assumptions from source files.
127.0.0.1:8080: the web server is loopback-only. Local curl can succeed while packets addressed to the VPS interface are rejected before HTTP processing.0.0.0.0:8080: the process is listening on all IPv4 interfaces, so proceed to routing and firewall checks.- A private-interface address only: requests must arrive through that interface; a public-IP request may not be accepted directly.
- A different port: use that port for the diagnostic request or correct
server.port. - No matching listener: investigate startup failure, a wrong profile, a stopped service, or a non-HTTP application.
Do not infer that a port is open merely because the Java PID is present. The socket must be in a listening state and owned by the expected process.
Step 3: Test locally, then test the VPS interface
Start with loopback:
curl -v http://127.0.0.1:8080/If the application uses a different port, substitute the effective value. A response status of 200, 401, 403, or 404 still proves that TCP reached an HTTP server. A 404 means the requested path or context path is not mapped; it does not mean that port 8080 is closed.
Next, request the VPS private address from the VPS itself:
curl -v http://<vps-private-address>:8080/This distinguishes loopback binding from a listener reachable on the server interface. If the private-address test fails while loopback works, inspect ss, the bind setting, local routing, and the host firewall before changing cloud rules.
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Read connection symptoms carefully
- Connection refused: the destination was reachable but no service accepted the connection, or an active rule rejected it. Confirm the listener and port rather than treating refusal as proof of a single cause.
- Timeout: packets may be filtered, routed incorrectly, or sent to an address with no responding service. Check both firewall layers and the public-IP attachment.
- HTTP response with an unexpected status: networking and TCP are working; investigate path, context path, authentication, virtual-host rules, TLS scheme, or application behavior.
Step 4: Correct a loopback-only bind
If ss shows 127.0.0.1, set the address required by your architecture. For direct exposure, use:
server.address=0.0.0.0
server.port=8080Restart the application, run sudo ss -lntp again, and repeat both curl tests. If a reverse proxy is the sole public entry point, do not broaden the Java bind unnecessarily; configure the proxy’s upstream to the private address and keep the backend inaccessible from the internet.
Remember that 0.0.0.0 is an IPv4 wildcard. It does not by itself make an IPv6 listener available, and it does not bypass firewalls. If clients use an IPv6 address, verify that the application, host firewall, provider firewall, and DNS all support IPv6 separately.
Step 5: Open the correct port in both firewall layers
After the listener and bind address are correct, permit the exact TCP port at the VPS operating-system firewall and at the cloud provider’s ingress, security-group, or network-firewall layer. A rule in only one place is insufficient: the host can drop traffic that the provider allows, and the provider can drop traffic before it reaches a permissive host firewall.
Choose the smallest exposure that meets the use case:
- For a reverse-proxy deployment, expose public TCP 80 and 443 and keep the Java backend port private.
- For a temporary direct test, expose the actual backend port, such as 8080, only for the required source ranges where possible.
- For an administrative or internal service, restrict ingress to the known office, VPN, or proxy source addresses instead of the whole internet.
Google Cloud’s guidance for VMs with external IP addresses emphasizes that only intended services and traffic should be reachable and that firewall rules can restrict selected ports and source IPs. The same least-exposure principle applies to other VPS platforms.
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Verify the rule matches reality
Confirm protocol (TCP rather than UDP), destination port, source range, network or security-group attachment, and rule priority. Check that the public IP is attached to the intended VM and that the firewall policy applies to its network interface. If a provider offers separate IPv4 and IPv6 rules, configure the family you actually test.
Step 6: Account for a reverse proxy or load balancer
A proxy changes the public endpoint. If Nginx, Apache, or a load balancer owns ports 80 or 443, users should connect to the public IP or hostname on those ports, not automatically to :8080. The Java process can remain on a private backend port while the proxy handles TLS, filtering, and forwarding.
Trace the proxy path
- Confirm the proxy service is running and listening on the public port with
ss -lntp. - Verify its upstream target matches the Java listener’s actual address and port.
- From the VPS, curl the upstream address directly to prove that proxy-to-Java traffic works.
- Ensure the host firewall permits the proxy’s connection to the backend port.
- From an external network, request the public IP or hostname on 80 or 443 and inspect the proxy logs.
A proxy can return its own 404, 502, or TLS error. A 502 commonly means the proxy cannot connect to the configured upstream; compare the upstream setting with the listener shown by ss.
Preserve the public URL
Spring Boot documents that a proxy or load balancer can change the apparent host, port, and scheme. Configure forwarded or X-Forwarded-* headers consistently so redirects and generated links use the public HTTPS URL rather than an internal address or port. Header handling must match the proxy configuration; enabling it on one side only can produce incorrect redirects even when the initial request succeeds.
Step 7: Test from a genuinely external network
Once loopback and VPS-interface requests work, test from a separate network, such as a mobile connection or an external probe. A request from the same private network can be affected by NAT loopback (hairpin) behavior and may not represent what an internet client sees.
Use the literal public IPv4 address first. This removes DNS as a variable. After the address works, test the hostname and confirm it resolves to the current VPS address rather than an old VM, proxy, or load balancer. If HTTPS is configured, test the correct scheme and public port; sending plain HTTP to a TLS listener or HTTPS to a plain HTTP port produces misleading errors.
Interpret the first failed layer
| Observed result | Most useful next checks |
|---|---|
| Localhost fails | Application startup errors, effective port, context path, TLS mode, and whether an HTTP listener exists |
| Localhost works; VPS interface fails | Bind address, local routing, and VPS host firewall |
| VPS interface works; external access fails | Provider ingress rules, public-IP attachment, upstream routing, and external firewall policy |
| TCP connects; HTTP result is wrong | Path, context path, proxy routing, virtual-host rules, TLS scheme, authentication, and application logs |
This sequence prevents a common mistake: opening increasingly broad firewall rules when the application is still listening only on loopback or on the wrong port.
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| Design | Public endpoint | Java bind | TLS and filtering | Firewall surface | Forwarded-header concern | Best fit |
|---|---|---|---|---|---|---|
| Direct Java port | Public IP with the application port, commonly 8080 | Public interface or 0.0.0.0 |
Handled by the Java application | Backend port must be exposed | Usually absent unless another proxy exists | Temporary tests or a simple controlled deployment |
| Reverse proxy | Public IP or hostname on 80/443 | Private interface or loopback | Proxy can terminate TLS and apply request filtering | Expose 80/443; restrict backend traffic | Must be handled consistently for host, port, and scheme | Public services that need a constrained edge and centralized TLS |
A direct port is quicker for isolating application connectivity. A reverse proxy generally reduces the publicly exposed surface and gives one place for certificates, request limits, access logs, and routing, but adds another service whose status and upstream configuration must be checked.
Common cases and targeted repairs
The properties file says 8080, but ss shows 9090
A deployment override is active. Inspect environment variables, launch arguments, service definitions, and selected Spring profiles. Use the effective 9090 value for a diagnostic curl, then remove or correct the override and restart if 8080 is the intended endpoint.
Port 8080 is listening on 0.0.0.0, but the public IP times out
The application bind is no longer the likely fault. Check the VPS firewall, provider ingress rule, source range, network attachment, route, and whether the public IP belongs to this VM. Confirm the test is sent to the correct protocol and port from an external network.
The proxy returns 502 while localhost:8080 works
Test the proxy’s configured upstream address from the VPS. A proxy pointed at 127.0.0.1:8080 cannot work if Java moved to another port; a proxy pointed at a private address cannot work if the application binds only to loopback. Correct the upstream, restart or reload the proxy, and verify backend firewall permissions.
The IP works, but the hostname redirects to an internal port
The request reached the application, so inspect forwarded-header handling and the proxy’s public scheme and host settings. Ensure the application trusts the headers supplied by the intended proxy and that redirects are generated for the public URL.
A 404 appears during the test
Keep the successful TCP evidence. Check the application context path and route, then request the correct path. If a proxy is involved, determine whether the 404 came from the proxy’s virtual-host or location rule rather than from Spring.
Security checks before leaving the port public
- Expose only the ports required by the chosen architecture.
- Keep management and actuator endpoints off the public interface unless they are explicitly protected and needed.
- Prefer TLS at the public edge for an internet-facing service.
- Restrict backend ports to the proxy, VPN, or administrative source ranges.
- Review both host and provider firewall rules after temporary troubleshooting access is removed.
- Monitor proxy and application logs so connection failures can be separated from authentication or route errors.
Changing the bind address to make a test pass also changes who can reach the process. Treat that setting as a security decision, not merely a connectivity toggle.
A repeatable verification checklist
- Read the current startup log and deployment settings.
- Write down the effective port, bind address, context path, and TLS mode.
- Run
sudo ss -lntpand match the listener to the Java PID. - Run
curl -vagainst loopback. - Run
curl -vagainst the VPS private interface. - Correct
server.addressorserver.portif the socket is wrong, then restart. - Check the VPS host firewall for the exact TCP port.
- Check provider ingress or security-group rules, source ranges, and public-IP attachment.
- If a proxy exists, test its public listener, upstream target, and forwarded headers.
- Repeat the request from an external network using the literal public IP, then verify DNS and HTTPS.
Following these steps turns “localhost works, IP fails” into a bounded investigation: application, socket, interface, host firewall, provider firewall, proxy, or external routing. Fix the first failed layer and retest before moving outward.
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