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To put app.example.com on an NGINX server, do four things in order: create DNS that points the hostname to the server, add an NGINX server block with an exact server_name, test and reload NGINX, and install a certificate for HTTPS. DNS only gets the request to your infrastructure; NGINX uses the request’s Host header to select the subdomain’s virtual server.
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The procedure below covers a static site and an application running on a local port, including DNS checks, configuration files, certificate issuance, and the failure modes that cause a subdomain to show the wrong site.
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Have these details ready:
- Administrative access to the DNS zone for
example.com. - The public IPv4 address of the NGINX server, and its IPv6 address if you will publish an AAAA record.
- Shell access with permission to read and edit NGINX configuration and reload the service.
- The content directory for a static site, or the local address and port of the application you will proxy.
- Inbound firewall access for port 80 and, for HTTPS, port 443.
Replace app.example.com, the IP addresses, paths, and upstream port in the examples with your own values. A hostname must resolve correctly before NGINX or a certificate authority can serve it reliably.
1. Create DNS for the subdomain
Use an A, AAAA, or CNAME record
At your DNS provider, add an A record named app (which creates app.example.com) and set its value to the server’s public IPv4 address. If the server is reachable over IPv6, add an AAAA record with the IPv6 address. When the application is hosted on another platform, use that platform’s documented target; a CNAME is appropriate when the provider instructs you to alias the hostname.
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Do not point the record at a private address such as 192.168.x.x for a public site. If your DNS provider uses a different field layout, the important result is the same: a lookup for the complete hostname must return the address or target that can serve the request.
Verify the record from the deployment environment
Check both the name and the record type from a machine that can reach the server:
dig +short app.example.com A
dig +short app.example.com AAAA
nslookup app.example.comThe answer may differ while nameservers update. Verify that the authoritative nameservers contain the new record and that the hostname is spelled exactly as it will appear in NGINX. A stale resolver cache can make one network see the old answer, so test from the server and from an external connection before diagnosing NGINX.
2. Create an NGINX server block
NGINX defines virtual servers with a server directive inside the http context. The server_name value determines which block handles a request after NGINX examines the listening address, port, and Host header. Use an exact name when this block is for one subdomain.
Static files
Create a document root and place the site files there:
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sudo mkdir -p /var/www/app
sudo chown -R www-data:www-data /var/www/appThe service account differs by distribution, so use the account your NGINX worker processes actually run as. A minimal HTTP block for static content is:
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server {
listen 80;
listen [::]:80;
server_name app.example.com;
root /var/www/app;
index index.html;
}The root directive maps a request path to a file below /var/www/app. The index directive tells NGINX which file to return for a directory request. Confirm that the index file exists and that NGINX can traverse every parent directory.
Reverse proxy to an application
If the site is generated by a process listening on a local port, keep the public hostname in NGINX and proxy requests to that process:
server {
listen 80;
listen [::]:80;
server_name app.example.com;
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;
}
}Change 127.0.0.1:3000 to the address and port used by your application. The application must be running and listening there; NGINX cannot proxy to a process that has stopped or is bound to an inaccessible socket. Header requirements vary by framework, so preserve the headers your application needs for original host, client IP, and scheme handling.
3. Put the file in the active configuration
Save the block in the configuration directory used by your operating system. On distributions that use the sites-available/sites-enabled layout, a typical sequence is:
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sudo nano /etc/nginx/sites-available/app.example.com
sudo ln -s /etc/nginx/sites-available/app.example.com /etc/nginx/sites-enabled/app.example.comIf your distribution loads files from a different directory, follow that layout instead of creating a link that NGINX never reads. Check the main configuration’s include directives to see which files are active. Remove or disable duplicate blocks for the same hostname; two definitions can make troubleshooting misleading.
4. Test the configuration and reload NGINX
Always test before reloading. The test must finish successfully; a reload should not be treated as proof that the new host works.
- Check syntax and referenced files:
sudo nginx -tLook for the explicit successful-test message. Errors identify the file and line that must be corrected.
- Inspect the effective configuration when necessary:
sudo nginx -TThis prints the loaded configuration, which helps find an unexpected wildcard or duplicate
server_name. - Reload through the operating system’s service manager:
sudo systemctl reload nginxUse the equivalent command when your system uses another service manager.
- Check service state and logs:
sudo systemctl status nginx sudo journalctl -u nginx --since "10 minutes ago"Confirm that the reload completed and that no permission, bind, or upstream errors appeared.
Test the hostname itself rather than only the server’s IP address. A request made to the IP can select a different default server because it carries a different Host header.
How NGINX chooses the subdomain’s server block
For an incoming request, NGINX first considers the listening address and port. It then compares the Host header with the server_name values on the matching listeners. An exact name such as app.example.com is the clearest choice for one host. Wildcard names such as *.example.com cover multiple names, while regular expressions handle advanced patterns.
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Matching priority matters when several blocks could accept a request. An exact name should not be accidentally shadowed by a broad wildcard or by an older default block. Review all enabled files if the main site appears at the new hostname. DNS wildcard records and TLS wildcard certificates are separate features: one controls name resolution and the other controls which names a certificate covers.
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Static hosting versus reverse proxying
| Approach | NGINX directives | What must be available | Typical failure |
|---|---|---|---|
| Static files | root and index |
Readable files and executable (traversable) parent directories | 404 when the path or index file is wrong |
| Reverse proxy | location / and proxy_pass |
A running application listening at the configured address | 502 when NGINX cannot connect upstream |
Do not configure both models unintentionally. A static root does not make a separate application reachable, and a proxy block does not serve files from that directory unless you add a separate location with a deliberate design.
5. Add HTTPS for the exact hostname
Prepare DNS and port 80 for validation
Obtain a certificate that covers app.example.com. For an HTTP-01 challenge, the hostname must resolve to the validation server, port 80 must be reachable, and the challenge path under /.well-known/acme-challenge/ must be served rather than blocked or redirected incorrectly. Keep a port-80 listener available for this purpose even after enabling HTTPS.
Before changing the configuration with an automated tool, back up the NGINX files. Certificate clients can install a certificate and modify supported web-server configuration, but you remain responsible for checking the resulting files and testing the service.
Use Certbot’s NGINX integration
Run Certbot for the exact hostname using the package and command appropriate to your operating system. The integration can request a certificate and update a supported NGINX configuration. After it finishes, inspect the certificate paths it reports, run nginx -t, reload NGINX, and open the HTTPS URL. Do not claim that renewal works until you have run a renewal test and reviewed its result.
Configure a 443 server block manually
A TLS virtual server includes the certificate and private-key files:
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server {
listen 443 ssl;
listen [::]:443 ssl;
server_name app.example.com;
ssl_certificate /etc/letsencrypt/live/app.example.com/fullchain.pem;
ssl_certificate_key /etc/letsencrypt/live/app.example.com/privkey.pem;
root /var/www/app;
index index.html;
}For a proxied application, retain the location / and its upstream settings in this 443 block instead of the static root. The certificate’s subject alternative names must include the exact hostname. Install the full chain in ssl_certificate and protect the private key from unauthorized reads.
NGINX’s HTTPS guidance uses TLS 1.2 and TLS 1.3 in its example defaults. Avoid disabling protocols or changing cipher settings without a compatibility and security reason. After editing the TLS block, run sudo nginx -t and reload before testing https://app.example.com.
Redirect HTTP only after validation is working
Once the certificate and HTTPS endpoint work, you can make the port-80 block redirect ordinary requests:
server {
listen 80;
listen [::]:80;
server_name app.example.com;
location / {
return 301 https://$host$request_uri;
}
}When using HTTP-01 renewal, make sure the challenge location remains reachable. A blanket redirect is often followed correctly, but a custom rule that blocks or rewrites the challenge path can still cause issuance or renewal failures.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Exact names, wildcards, and certificates
| Decision | Best fit | Important consequence |
|---|---|---|
Exact server_name |
One known hostname such as app.example.com |
Least ambiguous routing and the easiest configuration to audit |
Wildcard server_name |
Several subdomains sharing behavior | Every covered name still needs DNS that reaches the server or service |
| Individual certificate | A small, explicit set of hostnames | Request and renew each covered hostname as required |
| Wildcard certificate | Many names under one DNS suffix | Certificate coverage does not create DNS records or NGINX routing |
Use a wildcard only when the shared routing and certificate policy are intentional. A wildcard DNS record and wildcard certificate do not automatically configure one another.
Troubleshooting by symptom
The DNS lookup fails
- Confirm the A, AAAA, or CNAME value and the authoritative nameservers for the zone.
- Check that the hostname is spelled exactly as in
server_name. - Query from the deployment server and an external network to distinguish propagation or resolver caching from an incorrect record.
The main site appears instead
- Confirm the browser request is to
app.example.com, not the server IP or the parent domain. - Inspect enabled files for duplicate, wildcard, or default
server_nameentries. - Verify that the block listens on the address and port receiving the request.
- Run
nginx -Tto confirm the intended block is actually loaded, then reload after corrections.
The new host returns 404
- For static content, check the
rootpath, the requested filename, and theindexfile. - Check file ownership and directory traversal permissions for the NGINX worker account.
- For a proxy, make sure the request reaches the intended
locationand is not being handled by a static block or another server.
The new host returns 502 Bad Gateway
- Confirm that the upstream process is running and listening at the exact address and port in
proxy_pass. - Test the upstream locally from the server with an appropriate HTTP client.
- Check local firewall rules, Unix-socket permissions, and the NGINX error log.
- Review whether the application binds only to a different interface or uses a different port after restart.
The browser reports a certificate warning
- Inspect the certificate’s subject alternative names and verify that
app.example.comis covered. - Check that the 443 block uses the matching
ssl_certificateandssl_certificate_keyfiles. - Use the full-chain file where required, not only the leaf certificate.
- Confirm that the request is reaching the intended 443 server block rather than a default certificate.
ACME HTTP-01 validation fails
- Resolve the hostname and confirm it points to the server that will answer the challenge.
- Allow inbound port 80 and verify that the challenge path is not denied by NGINX, a redirect rule, or another proxy.
- Check the port-80 access and error logs while repeating the validation.
- After fixing DNS or routing, rerun the certificate command and then test NGINX before reloading.
Operational checklist
- Create the correct DNS record and verify it resolves from the server and an external network.
- Choose static serving or reverse proxying; do not leave an obsolete example block enabled.
- Set an exact
server_namefor the hostname unless a deliberate wildcard design is required. - Enable the configuration using the layout your operating system actually loads.
- Run
nginx -t, inspect errors, and reload only after a successful test. - Confirm HTTP routing with the hostname and inspect logs if the wrong site appears.
- Issue a certificate for the exact hostname, keeping port 80 reachable for HTTP-01 validation when used.
- Configure the 443 block, test the certificate and chain, and reload NGINX.
- Run a renewal test before relying on automated renewal.
With DNS resolving to the right infrastructure and an active NGINX block whose server_name matches the Host header, the subdomain has independent content or proxy behavior. HTTPS then adds a certificate and key that explicitly cover that hostname; it does not replace the DNS or virtual-server configuration.
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