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A non-working VPS subdomain is usually failing at one layer: DNS, network access, web-server host matching, the upstream application, or TLS. Check those layers in that order. Use dig to verify the public DNS answer, confirm ports 80 and 443 are open and listening, match the exact hostname in Nginx or Apache, test the application locally, and issue a certificate only after HTTP resolves correctly.
Use this three-minute diagnostic order
Do not start by changing Nginx or Apache when the name does not resolve. A browser cannot reach a web-server configuration until DNS has returned an address and the network allows a connection.
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- Verify the authoritative DNS record. At the DNS provider that is authoritative for your domain, check that the exact label exists as an A record (IPv4), AAAA record (IPv6), or CNAME (an alias to another hostname). Cloudflare’s documentation notes that your hosting provider must be able to serve the hostname before you create the corresponding “A, AAAA, or CNAME” record: Cloudflare’s subdomain DNS instructions.
- Resolve the name from outside the VPS. Query both public and authoritative DNS. If there is no answer or an old address appears, correct the DNS zone or delegation before touching the web server.
- Check reachability. Confirm the VPS is listening on TCP 80 and/or 443, and that both the operating-system firewall and your VPS provider’s firewall allow those ports.
- Match the hostname in the web server. Add the exact fully qualified domain name (FQDN) to the right Nginx
server_nameor ApacheServerNameblock. - Test the application separately. If the web server is a reverse proxy, test the local application port or socket. A proxy error and a DNS error are different failures.
- Configure HTTPS last. Once HTTP works, obtain a certificate, then verify the certificate names and the TLS virtual host selected by SNI.
1. Confirm the subdomain’s DNS record
A subdomain is only a label in DNS until it has a record. For a VPS with a fixed public IPv4 address, an A record such as app.example.com A 203.0.113.10 points directly to that address. An AAAA record performs the same job for IPv6. A CNAME, such as app.example.com CNAME edge.example.net, aliases the subdomain to another hostname; the alias’s eventual A or AAAA answer determines where clients connect.
Check the record at the authoritative provider
- Identify the domain’s authoritative nameservers with
dig +short NS example.com. - In that provider’s zone, verify the label is exactly the intended one. A record for
www.app.example.comdoes not answer forapp.example.com. - Check for conflicting records. An obsolete A record, an unexpected AAAA record, or a CNAME left beside another record can send different clients to different destinations.
- If you recently changed nameservers, update the zone at the provider named by the parent domain, not at an old DNS dashboard. Delegation errors make an apparently correct record invisible to the Internet.
Query public and authoritative answers
Run these commands from a workstation or another network, replacing the hostname:
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dig +short sub.example.com A
dig +short sub.example.com AAAA
dig +trace sub.example.comThe first two commands show the addresses returned to a normal client. dig +trace follows the delegation chain and helps reveal which nameserver is authoritative. Compare the public resolver’s result with a direct query to an authoritative nameserver. A missing answer, the wrong IP, or an old CNAME means the problem is still in DNS. Local resolver caches can retain an earlier value until its TTL expires, so test from more than one network before assuming the VPS configuration is at fault.
Choose A/AAAA or CNAME deliberately
| Record | Use it when | What must remain correct |
|---|---|---|
| A | The subdomain should point directly to the VPS’s IPv4 address. | The address must remain current if the VPS is replaced or renumbered. |
| AAAA | The VPS accepts IPv6 connections and has a stable IPv6 address. | IPv6 routing, firewall rules, and the web server must work; a broken AAAA record can make some clients fail even when IPv4 works. |
| CNAME | You want the subdomain to follow another hostname managed elsewhere. | The target hostname must resolve, and DNS providers generally do not allow a CNAME alongside other data at the same label. |
2. Make sure the VPS can accept the connection
Correct DNS only gets a client to an IP address. The VPS must have a process listening on the requested port, and every firewall between the client and that process must allow it.
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On a Linux VPS, inspect listening sockets with a command such as:
sudo ss -ltnp | grep -E ':(80|443)b'An Nginx or Apache listener should normally be bound to 0.0.0.0:80/0.0.0.0:443 and, when IPv6 is enabled, [::]:80/[::]:443. A listener bound only to 127.0.0.1 cannot accept Internet traffic directly; that may be intentional for an application behind a reverse proxy, but not for the public web-server socket.
Review the local firewall (for example, the active nftables, iptables, or distribution firewall rules) and the hosting provider’s security-group or network-firewall panel. Allow inbound TCP 80 for HTTP and TCP 443 for HTTPS from the Internet when the site is meant to be public. A timeout commonly indicates a dropped packet by a firewall or an unreachable address; “connection refused” generally means the host was reached but no process accepted the port.
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Test without relying on browser messages
curl -I http://sub.example.com
curl -4 -I http://sub.example.com
curl -6 -I http://sub.example.comThe protocol-specific tests distinguish IPv4 and IPv6 problems. If IPv4 succeeds but IPv6 fails, inspect the AAAA record, IPv6 route, and IPv6 firewall rules. If both fail before an HTTP status appears, return to DNS and network checks.
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3. Configure Nginx for the exact hostname
Nginx chooses a server {} block using the request’s host name and port. Its documentation explains that, when no Host value matches a server_name, Nginx sends the request to the default server for that port: Nginx server-name selection and Nginx’s web-server administration guide.
HTTP virtual host
Create or edit a site configuration with the exact FQDN:
server {
listen 80;
listen [::]:80;
server_name sub.example.com;
root /var/www/sub.example.com;
index index.html;
location / {
try_files $uri $uri/ =404;
}
}For a reverse proxy, replace the static-file location with the local service, for example:
location / {
proxy_pass http://127.0.0.1:3000;
proxy_set_header Host $host;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header X-Forwarded-Proto $scheme;
}Enable the file according to your distribution, test the complete configuration, and reload only after the test passes:
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sudo nginx -t
sudo systemctl reload nginxRecognize a default-site symptom
If the subdomain displays your main site, Nginx probably handled the request with a default block because the subdomain is absent, misspelled, or placed in the wrong block. Check every HTTPS block as well as HTTP; adding the name to port 80 does not automatically add it to port 443. A request sent by IP address also may select the default site because its Host header is not the subdomain.
4. Configure Apache name-based virtual hosts
Apache uses ServerName as the canonical hostname and ServerAlias for additional names. Define the subdomain in the relevant name-based virtual host, enable the site on systems that use separate site files, and inspect Apache’s parsed result with apachectl -S. The Apache virtual-host documentation covers these directives and the diagnostic command: Apache virtual hosts.
HTTP example
<VirtualHost *:80>
ServerName sub.example.com
DocumentRoot /var/www/sub.example.com
<Directory /var/www/sub.example.com>
Require all granted
AllowOverride None
</Directory>
</VirtualHost>Add a ServerAlias only for names that should serve the same site. For HTTPS, create a separate <VirtualHost *:443> with the same ServerName, the certificate and key paths, and the required SSL module settings. Then run:
sudo apachectl configtest
sudo apachectl -S
sudo systemctl reload apache2The apachectl -S output shows which virtual host Apache considers the default and which file owns each name. If the subdomain is mapped to the main site’s block, correct the name or virtual-host ordering rather than changing DNS.
5. Test a reverse-proxy application independently
When Nginx or Apache forwards requests to an application, debug the two hops separately. First test the application on its local bind address and port:
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curl -i http://127.0.0.1:3000Use the actual port or Unix socket configured for your service. A successful local response but a failed public request points to proxy routing, headers, permissions, or the public listener. A failed local request means the application process, bind address, socket, environment, or service manager needs attention. Review service logs and confirm the process is running after reboot.
| Observed result | Most likely layer | Next check |
|---|---|---|
| DNS error before an HTTP status | DNS or delegation | Authoritative A/AAAA/CNAME answer and nameservers |
| Timeout or refused connection | Network, firewall, or listener | Provider firewall, local firewall, ss, and the destination IP |
| Main site appears | Virtual-host matching | Nginx server_name or Apache ServerName and default-host order |
| 502 Bad Gateway | Reverse-proxy upstream | Local application port/socket, process status, and proxy target |
| Application response locally but not publicly | Proxy or public web-server configuration | Correct site block, reload, and access/error logs |
6. Enable HTTPS only after HTTP works
Certificate issuance cannot repair DNS or a closed port. Certbot’s documentation requires the machine to remain reachable from the Internet on the validation port for each requested name: Certbot usage documentation.
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HTTP-01 places a challenge under the requested hostname and validates it over TCP port 80. The A/AAAA record must lead to the server handling the challenge, and port 80 must be allowed through every firewall. After those conditions are true, the Nginx or Apache integration can be used:
sudo certbot --nginx -d sub.example.com
sudo certbot --apache -d sub.example.comRun the command appropriate to the web server, not both. If the challenge fails, test http://sub.example.com/.well-known/acme-challenge/ and inspect the web-server logs.
DNS-01 validation
DNS-01 proves control by requiring a TXT record beneath _acme-challenge.sub.example.com. It does not require port 80 to be reachable, but the TXT record must be published at the authoritative DNS provider and visible to the certificate authority. This method is useful when HTTP is intentionally unavailable or when issuing a wildcard certificate.
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Match certificate names and SNI
The certificate must contain sub.example.com explicitly or cover it with an appropriate wildcard. Nginx explains that TLS is established before the browser sends an HTTP request, so the server may select a default certificate when several HTTPS sites share one IP and the requested name is not selected correctly: Nginx HTTPS configuration.
Ensure the 443 virtual host has all of these items together:
server_name sub.example.com;(or the matching ApacheServerName).- The certificate file whose names include the subdomain.
- The corresponding private-key file and correct file permissions.
- A listener on 443, including IPv6 when an AAAA record is published.
- The same proxy or document-root configuration intended for the subdomain.
After reloading, inspect the served certificate and negotiated address with curl -Iv https://sub.example.com. A certificate-name warning means the wrong certificate or HTTPS virtual host was selected, not that the HTTP application is necessarily down.
Match the symptom to the failing layer
| Browser or command symptom | Meaning | Fix |
|---|---|---|
DNS_PROBE_FINISHED_NXDOMAIN or “server not found” |
No usable DNS answer, wrong authoritative nameservers, or an incorrect target | Create or correct the record at the authoritative provider and wait for caches to expire |
| Connection timeout | Traffic is being dropped, the address is wrong, or the service is unreachable | Check A/AAAA results, provider and OS firewalls, routes, and listeners |
| Connection refused | The destination answered but no process accepted that port | Start the web server, bind it to the public interface, or correct the port |
| Main website on the subdomain | The default virtual host handled an unmatched name | Correct Nginx server_name or Apache ServerName and reload |
| 502 Bad Gateway | The reverse proxy cannot reach or use its upstream | Test the local upstream, process status, socket permissions, and proxy address |
| Certificate name mismatch or unexpected certificate | The certificate lacks the subdomain or SNI selected another HTTPS host | Issue/install a certificate with the right name and match the 443 host block |
| Certbot challenge failure | DNS points elsewhere, port 80 is blocked for HTTP-01, or the DNS-01 TXT record is missing | Correct the validation method’s DNS and network prerequisite, then retry |
Nginx versus Apache: which configuration should you inspect?
| Area | Nginx | Apache |
|---|---|---|
| Hostname directive | server_name inside a server {} block |
ServerName and optional ServerAlias inside <VirtualHost> |
| Configuration check | nginx -t |
apachectl configtest |
| Parsed-host diagnostic | Review matching server blocks and the default server for each listen port |
apachectl -S prints parsed virtual hosts and defaults |
| Reverse proxy | Usually configured with proxy_pass |
Typically uses the proxy modules and forwarding directives |
| Common wrong-site symptom | Request falls through to the default server block |
Request matches the first/default virtual host for the address and port |
The diagnostic sequence is the same regardless of the server: prove DNS, prove connectivity, prove host matching, then prove the upstream and TLS.
Quick Recap
HTTP-01 versus DNS-01 certificate validation
| Method | Required proof | Network prerequisite | Typical failure |
|---|---|---|---|
| HTTP-01 | A challenge file is reachable at the requested hostname | Public DNS must reach the validation server and TCP 80 must be open | Port 80 blocked, wrong A/AAAA target, redirect or proxy prevents the challenge path |
| DNS-01 | A TXT value under _acme-challenge |
Control of the authoritative DNS zone; the web server need not expose port 80 | TXT record published at the wrong provider, wrong name, or not yet visible |
Final verification checklist
- The authoritative zone contains the intended A, AAAA, or CNAME record, with no stale conflicting data.
digfrom an external network returns the VPS address you expect for both IPv4 and any advertised IPv6.- The provider firewall and operating-system firewall permit TCP 80 and 443 as required.
- Nginx or Apache is listening on the required interfaces and ports.
- The exact subdomain appears in both the HTTP and HTTPS host configuration where applicable.
- The web-server configuration test passes before reload.
- A local
curlrequest reaches the upstream application and the proxy target matches its actual bind address and port. - The certificate contains the exact hostname (or a valid wildcard), and the 443 host serves the matching key and certificate.
- Logs are checked after one fresh request, so the timestamped entry corresponds to the failing hostname.
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