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If a VPS works locally but not from the internet, troubleshoot the path in layers: verify the hostname and public address, confirm the service is listening on the intended interface, inspect the VPS firewall, inspect the provider firewall, then verify routes and test the exact protocol and port from an external network. A successful check at one layer does not prove that the other layers will pass traffic.
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What “unreachable” actually means
An outside connection can fail before it reaches the application, at the application itself, or on the return path. The most useful distinction is between these independent controls:
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- Endpoint: the hostname must resolve to the VPS’s current public IPv4 or IPv6 address.
- Listener: the application must be running, using the expected TCP or UDP port, and bound to an address reachable from the client.
- Guest firewall: UFW, firewalld, nftables, or iptables on the VPS can allow or discard packets.
- Provider firewall: cloud security controls can block traffic before it enters the virtual machine.
- Routing and return traffic: the VPS needs a valid public route and must send replies through the correct interface.
These layers are cumulative. An open provider rule cannot make a loopback-only service public, and a listening service cannot receive packets that the provider network drops.
1. Confirm the public endpoint first
Resolve the hostname
Resolve the name from the client network and compare every returned address with the VPS’s current public addresses. A stale DNS record can send you to an old machine, while an AAAA record can direct IPv6-capable clients onto a path that is not configured.
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Use the exact hostname that users enter, not an internal name shown only inside the VPS provider’s network. If both A and AAAA records exist, test each address family separately so that a broken IPv6 path is not mistaken for a general outage.
Test from outside the provider network
A test made on the VPS itself, or from another resource inside the same private network, can bypass an internet-facing control. Use a workstation on a different network, a mobile hotspot, or another independent host. Record the destination address, protocol, port, source network, and result for each test.
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2. Verify that the service is listening correctly
Check that the process is running
Confirm that the application started successfully and is not repeatedly crashing. Then check its configured port and protocol. A web server may use TCP while a game, voice, or telemetry service may require UDP; an allow rule for one does not permit the other.
Check the bind address
The service must bind to the VPS address on the intended interface or to 0.0.0.0 for IPv4. Google Cloud’s guidance states: “The server must be bound to the IP address of a particular interface or to 0.0.0.0, and it must accept connections from the correct client IP prefix or 0.0.0.0.” A process bound only to 127.0.0.1 accepts connections from the VPS itself but is invisible to public clients.
For IPv6, the application also needs an IPv6-capable bind address, commonly the VPS’s IPv6 address or the IPv6 wildcard. An IPv4 listener does not automatically accept IPv6 connections.
Match the client source range
Some services or host-based access controls accept only a defined client prefix. If the service is listening but rejects the source address, adjust that application policy or test from an address inside the permitted range. Do not broaden access to every source unless the service genuinely requires it.
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3. Inspect the VPS guest firewall
Review the rule dimensions
Inspect the active rules in UFW, firewalld, nftables, or iptables. For the failing connection, verify all of these dimensions:
- Protocol: TCP or UDP.
- Destination port: the port configured in the application.
- Source range: the public client address or CIDR that should be allowed.
- Interface: the network device receiving the packet.
- Address family: IPv4 and IPv6 rules are separate unless your firewall explicitly combines them.
Look for an earlier deny rule, a default-deny policy, or a rule that applies only to a different interface. A rule that appears to allow the port can still be ineffective if a higher-priority or earlier rule drops the packet.
Use packet counters to prove a drop
With iptables or nftables, inspect packet and byte counters on matching rules while making one controlled external test. Google Cloud’s documentation gives an illustrative iptables result of 10,342 packets and 2,078,513 bytes; the important point is interpretation, not the number itself. If the counter on a matching DROP rule rises during your test, the VPS is discarding the traffic at that rule. If no relevant counter changes, the packet may be blocked upstream, aimed at the wrong address, or using a different protocol or interface.
Change the narrowest rule
Permit only the required port and protocol, ideally from a restricted source CIDR. Avoid an “allow all ports” rule while diagnosing the problem. After each change, retest from the external network and record whether the symptom changed before making another modification.
4. Check the provider-side firewall separately
Why a cloud rule can override a correct VPS rule
Provider controls operate outside the guest operating system. Google Cloud VPC firewall rules can allow or deny traffic by direction, protocol, destination port, and source range. AWS identifies security groups, network ACLs, route tables, Network Firewall, and the operating system as separate failure points. Azure documents network security groups (NSGs), guest firewalls, and routes as independent causes; default rules can block inbound internet traffic until a higher-priority allow rule exists.
Compare effective rules, not just the rule you edited
Check that the rule is attached to the correct virtual machine, network, interface, tag, or security group. Confirm its direction is inbound, its protocol and port match the test, and its source range includes the actual client address. Review priority and the default action: an allow rule below a higher-priority deny will never be reached.
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Use provider diagnostics
Google Cloud offers connectivity tests that can identify dropped traffic. Equivalent effective-rule views, flow logs, or network diagnostics in your provider can show whether a packet reached the virtual network and which policy decided its fate. These results are more useful than repeatedly changing guest rules when the packet never enters the VPS.
5. Verify routes and return traffic
Confirm the expected public route
Ensure the VPS has the provider’s expected route for internet traffic and that the public address is actually assigned to the active interface. A missing route, an incorrect next hop, or a detached interface can prevent replies even when inbound packets arrive.
Look for forced tunnels and appliances
Network appliances, inspection gateways, VPNs, and forced-tunneling policies can redirect traffic away from the normal internet path. Azure specifically lists route problems and forced tunneling as possible causes. Verify that the return route does not send replies through an interface or tunnel that cannot reach the original client.
Recognize asymmetric paths
If a packet enters through one interface but the reply leaves through another, stateful firewalls may discard it. Compare the interface receiving the connection with the interface selected for the response. Fix the route or policy that creates the asymmetry rather than weakening every firewall rule.
6. Test the exact port, not just ping
TCP probe
For a TCP service, run:
nc -vz HOST PORTA successful connection indicates that a TCP handshake reached a listener. “Connection refused” generally means the host responded but no permitted process accepted that port; a timeout commonly indicates filtering, routing, or a host that never replied. Treat those as clues, not as proof of one specific cause.
HTTP test
For an HTTP endpoint, use:
curl -v http://HOST:PORTThe verbose output separates name resolution, connection establishment, and the HTTP response. For an IPv6 literal, use bracket notation such as http://[2001:db8::10]:8080 with the VPS’s real address.
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Windows TCP test
On Windows, Microsoft’s TCPing utility tests the specified TCP port. If TCPing fails, inspect the guest firewall and the provider firewall rather than disabling the Windows firewall; Microsoft explicitly warns that turning it off is not a fix.
Why ping misleads
ICMP echo and application ports are different traffic types. A VPS can answer ping while dropping TCP 22, TCP 443, or a UDP game port. It can also block ICMP while allowing the application. Use ping only as a limited reachability signal, never as a verdict on a service port.
7. Keep protocol and address family aligned
TCP versus UDP
Provider and guest rules are protocol-specific. A TCP allow rule does not permit UDP on the same number, and a UDP rule does not make a TCP service reachable. Confirm the application’s documented transport and test with a tool that speaks that transport.
IPv4 versus IPv6
IPv4 and IPv6 follow separate addressing and filtering paths. An IPv4 rule does not automatically solve an IPv6 connection, and an AAAA record can cause clients to choose IPv6 first. Create matching IPv6 listener and firewall rules when IPv6 is intended, or remove an unintended AAAA record until that path is ready.
8. A controlled troubleshooting workflow
- Write down the target: hostname, resolved A and AAAA addresses, protocol, port, and the external source network.
- Test each address family: use the exact public IPv4 and IPv6 addresses where possible, then test the hostname again.
- Confirm the listener: verify the process, transport, port, bind address, and any application-level source restriction.
- Inspect guest rules: check protocol, port, source, interface, address family, default policy, and packet counters.
- Inspect provider rules: verify attachment, direction, priority, source range, tags or security-group membership, and default-deny behavior.
- Validate routing: check the public route, active interface, return path, and any VPN, appliance, or forced tunnel.
- Run a direct probe: use
nc,curl, or TCPing from outside the provider network. - Make one narrow change: alter only the rule or route implicated by the evidence.
- Retest and record: note whether the result became success, refusal, or timeout, then continue to the next layer only if necessary.
This sequence prevents a common diagnostic mistake: changing several firewalls at once and losing the evidence that identifies the blocking layer.
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| Control | Where it operates | What it matches | Priority and defaults | Diagnostics |
|---|---|---|---|---|
| Guest firewall | Inside the VPS operating system | Protocol, destination port, source range, interface, and address family | UFW, firewalld, nftables, or iptables policy and rule order | Rule packet and byte counters; local logs |
| Provider firewall | Virtual network or cloud edge before the guest | Direction, protocol, destination port, source range, and resource attachment such as tags or security groups | Rule priority and provider default-deny behavior | Effective-rule views, flow logs, and provider connectivity tests |
Both controls must permit the same flow. The provider rule cannot repair a service bound to loopback, and the guest rule cannot admit traffic discarded at the cloud edge.
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Common symptoms and the next check
Local connection works; outside connection times out
Check the bind address first, then guest and provider firewall drops, followed by the public route. A loopback-only listener or an upstream default deny is more likely than an application protocol error.
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The host answered but no usable listener accepted the connection on that protocol and port. Verify that the process is running, listening on the public interface, and using the port you tested. A reject rule can produce a similar result, so inspect firewall counters as well.
Ping succeeds; the website or SSH port fails
Ping proves only that ICMP reached a responding host. Test the application port directly, then inspect protocol-specific rules and the listener.
IPv4 works; IPv6 fails
Check the AAAA record, IPv6 bind address, IPv6 guest rules, provider IPv6 rules, and the IPv6 return route. Do not assume an IPv4 allow rule covers IPv6.
The rule looks correct, but counters stay at zero
Confirm that the test is using the expected address, protocol, interface, and source range. Zero counters often means the packet is blocked upstream or is arriving on a different path than the rule covers.
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Safe fixes and changes to avoid
- Allow only the required protocol and destination port.
- Restrict source addresses to a known CIDR when the service permits it.
- Apply equivalent IPv4 and IPv6 rules only when both address families are intended.
- Change one layer at a time and keep a record of the previous rule or route.
- Do not open every port as a diagnostic shortcut.
- Do not disable the guest firewall; use counters, logs, and targeted temporary rules to isolate the fault.
- After recovery, remove temporary broad rules and confirm that the intended default-deny posture remains.
What to collect before escalating
A concise evidence bundle lets a provider or administrator identify the failing layer quickly:
- Hostname and the A and AAAA answers observed from the client network.
- VPS public addresses, active interface, and expected default route.
- Application name, transport, port, bind address, and process status.
- Guest firewall configuration, relevant rule order, and packet counters before and after a test.
- Provider firewall rule, priority, attachment, source range, and any effective-rule or connectivity-test result.
- Exact external test command and its output, including whether the result was success, refusal, or timeout.
- Any VPN, forced tunnel, network appliance, or recent route change.
With those details, “the VPS is down” becomes a specific question such as “TCP 443 reaches the interface but matches this guest DROP rule” or “IPv6 is selected by DNS but has no provider allow rule.”
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