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If your Java or Spring Boot application responds to curl http://127.0.0.1:8080 on the VPS but times out when you use the server’s public IP, the JVM is not the whole story. Check the effective application port, the socket’s bind address, the VPS firewall, the cloud provider’s ingress firewall, and any reverse proxy—in that order. A process can be running while no HTTP listener exists on the expected port, or while the listener accepts loopback traffic only.
The fastest method is to test from inside out: loopback, the VPS interface, then a genuinely external network. The first test that fails identifies the likely fault domain.
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Understand the four things that must line up
An IP-address failure is usually caused by treating several independent settings as one. Verify each layer separately:
- Process: the Java process started successfully and remains running.
- Port: an HTTP or HTTPS listener exists on the port you are testing.
- Bind address: the listener is attached to an interface that can receive the connection, not only to loopback.
- Network policy: the VPS firewall and the provider’s ingress/security-group firewall allow the traffic.
A healthy JVM proves only the first item. Likewise, a successful local request proves that something answered locally; it does not prove that packets arriving on the public interface can reach that listener.
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1. Confirm the application’s effective settings
Do not assume that the value in a checked-in application.properties or application.yml is the value used in production. Spring Boot can receive configuration from environment variables and command-line properties, and those values can override file-based settings. Read the startup log and the deployment environment, then record the effective port, context path, TLS mode, and bind address.
Check the port
For a standalone Spring Boot application, the documented default HTTP port is 8080, and the setting is controlled by server.port. A different value may be supplied by an environment variable, a service unit, a container definition, or a command-line argument.
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Check the bind address
For direct IPv4 exposure, a typical configuration is:
server.address=0.0.0.0
server.port=80800.0.0.0 means the process listens on all IPv4 interfaces. It is useful when clients should connect directly to the VPS through any of its IPv4 addresses. Use the narrowest address that matches the design instead: if a reverse proxy is the only intended caller, bind Java to the private interface or loopback and publish only the proxy.
A value of 127.0.0.1 deliberately limits the listener to the VPS itself. That configuration explains the common pattern in which a localhost curl succeeds while a curl to the VPS private or public address fails.
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Record path and protocol details
A correct TCP connection can still produce an apparently broken application when the URL is wrong. Record the context path, whether the connector expects HTTP or HTTPS, and whether a proxy will supply the public host and scheme. A request to / can return 404 when the application is mounted under a context path; that is an HTTP routing issue, not proof that the port is closed. Sending plain HTTP to a TLS listener produces a protocol error even though the socket is reachable.
2. Prove that a listener exists on the VPS
Inspect listening sockets rather than inferring them from the Java process list. On the VPS, use the platform’s socket utility; on Linux, a common command is:
ss -lntpFind the expected port and correlate it with the Java PID. Pay attention to the local-address column:
127.0.0.1:8080accepts loopback connections only.0.0.0.0:8080accepts IPv4 connections on the VPS interfaces, subject to routing and firewall policy.- A different port means the application and your test are out of sync.
- No matching listener means the process may have failed during startup, may be serving HTTPS on another port, or may not be the service you think it is.
Correlating the socket with the PID also catches a surprisingly common error: another service owns the port while the intended Java process is listening elsewhere or has stopped.
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3. Test connectivity from inside out
Run the tests in order and keep the exact status, error text, and destination address. Use the effective port discovered in the previous steps.
- Test loopback on the VPS.
curl -v http://127.0.0.1:8080/For an HTTPS connector, use the corresponding HTTPS URL and port. A response confirms that a local TCP path and an HTTP handler exist, but it does not test the public interface.
- Test the VPS interface.
curl -v http://<vps-private-address>:8080/Use the VPS’s private address when one is available. If this fails while loopback works, inspect the bind address, local routing, and the host firewall. If it succeeds, the listener is reachable beyond loopback.
- Test the public address from outside. From a separate network, request
http://<vps-public-address>:8080/only when port 8080 is intentionally public. A mobile data connection or an external probe is preferable to another machine on the same private network. - Test the intended public endpoint. If a reverse proxy fronts the application, test the public IP on port 80 or 443, usually without appending
:8080. The Java backend port may be private by design.
Interpret the result, not just the command exit code
| First failing test | Likely fault domain | Next checks |
|---|---|---|
| Loopback | Application, port, protocol, or startup | Startup log, effective server.port, context path, TLS setting, and listening sockets |
| VPS interface after loopback succeeds | Bind address, local routing, or host firewall | Socket local address, interface address, and operating-system firewall policy |
| External request after VPS-interface access succeeds | Provider ingress, public-IP attachment, upstream routing, or external policy | Cloud firewall rule, public address assignment, source restrictions, and route configuration |
| TCP connects but the HTTP result is wrong | Path, proxy routing, virtual-host rule, TLS scheme, authentication, or application error | Verbose curl output, proxy configuration, forwarded headers, and application logs |
A 404 response is useful evidence: TCP reached a web server and the server generated an HTTP response. Investigate the path or context path rather than opening more firewall ports. A timeout and a connection refusal provide different clues, but neither should be treated as conclusive until you compare them with the socket and firewall state.
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4. Fix a loopback-only bind
If the socket inspection shows 127.0.0.1:8080, change the effective server.address to the interface required by the deployment. For direct public IPv4 access, that is commonly:
server.address=0.0.0.0
server.port=8080Restart the service after changing configuration, then run ss -lntp again. Confirm that the new listener is present before testing from outside.
Do not choose 0.0.0.0 automatically. It exposes the process to every IPv4 interface on the VPS, including interfaces that may be intended only for internal traffic. If Nginx, Apache, or a load balancer is the only caller, binding Java to a private interface or loopback reduces the public attack surface. The proxy can still accept public traffic on 80/443 and forward requests internally.
5. Open both firewall layers
Cloud VPS deployments commonly have two independent firewalls:
- Operating-system firewall: runs inside the VPS and controls packets reaching the instance.
- Provider ingress or security-group firewall: runs at the cloud network edge and controls which traffic is delivered to the instance.
Allow the exact TCP port required by the architecture at both layers. For a reverse-proxy design, that normally means public 80 and 443, while the Java backend port remains restricted to the proxy or private network. For a temporary direct test, you may allow 8080, but restrict the source range whenever the use case permits.
Google Cloud’s guidance emphasizes that when a VM has a public IP address, only the services and traffic you intend to expose should be reachable. Apply that principle to every provider: open the smallest port set, restrict source addresses for administrative or internal endpoints, and remove temporary rules after testing.
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Firewall checks that prevent false conclusions
- Verify that the rule applies to the correct VM, network, tag, security group, or interface.
- Confirm the protocol is TCP and that the destination port matches the listener.
- Check source restrictions; a rule allowing only an office CIDR will still block a mobile-network test.
- Check rule priority where the provider evaluates deny rules before or alongside allow rules.
- Keep management, actuator, and diagnostic endpoints off the public interface unless they are separately protected.
6. Account for a reverse proxy or load balancer
A reverse proxy changes the public endpoint. Clients connect to the proxy’s public port—usually 80 for HTTP or 443 for HTTPS—while the Java process can remain on a private backend port such as 8080. In that arrangement, http://public-ip:8080 is expected to fail if the backend is intentionally not exposed.
Verify the proxy path
- Confirm that the proxy service is running and owns the expected public port.
- Check that its upstream points to the Java listener’s actual address and port.
- Ensure the VPS firewall permits proxy-to-backend traffic.
- Test the proxy’s public URL, including the correct scheme and hostname.
- Read proxy error logs together with the Java application log; a 502 or 504 usually indicates an upstream connection or response problem rather than a missing public listener.
Proxies and load balancers can change the apparent host, port, and scheme. Configure forwarded or X-Forwarded-* headers consistently so redirects and generated links use the public URL instead of an internal address. If the application redirects from HTTPS to an internal HTTP URL, investigate proxy scheme forwarding and Spring’s forwarded-header handling before changing the firewall.
7. Distinguish direct exposure from a reverse-proxy design
| Decision point | Direct Java port | Reverse proxy in front |
|---|---|---|
| Public endpoint | Java listener, often 8080 during a test | Proxy on 80/443; Java backend can stay private |
| Firewall surface | Must permit the Java port to intended clients | Permit public 80/443 and restrict backend traffic |
| TLS termination | Handled by the Java connector if enabled | Usually handled at the proxy, with controlled forwarding to Java |
| Binding requirement | Java must bind an address reachable by clients | Java needs to bind only an address reachable by the proxy |
| Operational fit | Simple for a short-lived test or internal service | Better suited to public TLS, request filtering, and a constrained port surface |
Choose one design deliberately. Many “port 8080 is unreachable” reports are actually a correct reverse-proxy deployment being tested at the wrong public endpoint.
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8. Check address, DNS, and network-path mistakes
Use the literal public IPv4 address first
Test the public IPv4 address before introducing DNS. A hostname may still resolve to an old address, a different address family, or a proxy service. Once the literal address works, verify that the hostname resolves to the same current address and that any virtual-host rules match the requested host.
Do not rely on a hairpin test
A request made from inside the same private network can be affected by NAT loopback, also called hairpin behavior. It may fail even though Internet clients can connect, or succeed through a path that does not represent an external user. Repeat the test from a genuinely separate network after local and VPS-interface checks pass.
Verify public-IP attachment and routing
An address shown in a control panel is not necessarily attached to the instance or routed to the interface where Java listens. Confirm the public IP assignment, the instance’s private address, and the provider’s route and ingress policy. A correctly bound socket cannot receive packets that never reach the VPS.
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9. Work through common failure patterns
“The Java process is running, but localhost fails”
Inspect startup errors and the effective port. The process may have started a non-HTTP worker, failed to initialize its web server, or be listening with TLS while you send plain HTTP. Check the context path and repeat the request with the correct scheme and port.
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Look for a loopback-only bind first. If the socket is on 0.0.0.0 or the private interface, inspect local routing and the host firewall. Do not jump to provider rules yet; the request has not demonstrated that it can leave the local interface successfully.
“The private address works, but the public IP times out”
Focus on provider ingress, public-IP attachment, upstream routing, and source restrictions. Confirm that the test originates from an allowed network and that the provider rule targets this exact VM.
“The port connects, but I receive 404”
Keep the network configuration intact. A 404 means an HTTP server answered. Check the application context path, route, host header, and proxy location rules.
“The proxy returns 502 or 504”
Verify the upstream address and port, confirm Java is listening on that address, and ensure the host firewall allows the proxy’s connection. Compare proxy and application timestamps to identify whether the request reached Java.
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Check DNS resolution, the current public address, virtual-host configuration, and TLS certificate or SNI expectations. Keep the IP test as a control while correcting name-based routing.
Quick Recap
10. A safe repair sequence
- Read the startup log and deployment environment; write down the effective port, context path, TLS mode, and bind address.
- Run
ss -lntpand correlate the listener with the Java PID. - Run a verbose loopback curl using the correct scheme and port.
- Run the same request against the VPS private interface.
- Correct
server.addressonly if the listener is unnecessarily limited to loopback. - Decide whether the public endpoint is Java itself or a proxy on 80/443.
- Allow only the required TCP ports in the VPS firewall and provider ingress firewall.
- Test the literal public IPv4 address from a separate network.
- Test the hostname and inspect proxy, application, and firewall logs together.
- Remove temporary direct-port rules and keep backend and management endpoints private wherever possible.
11. Final exposure checklist
- The Java process is running and its PID owns the expected listener.
- The listener port matches the value actually supplied at startup, not merely a value in a source file.
- The bind address matches the architecture: public interface for direct exposure, private or loopback for a proxy-only backend.
- The URL uses the correct HTTP or HTTPS scheme and context path.
- The VPS firewall permits the required TCP port.
- The provider’s ingress or security-group firewall permits the same port and the intended source ranges.
- The public IP is attached to the correct VPS and routes to it.
- External testing comes from a separate network, not only from inside the same private network.
- Proxy upstream, public ports, and forwarded headers are consistent when a proxy is present.
- Only intended services are reachable; temporary diagnostic openings are removed.
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