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The architecture: three separate jobs
A subdomain does not become available merely because a DNS record exists. A complete deployment needs all of the following:
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- DNS resolution: the authoritative DNS provider returns the VPS address for the hostname.
- Network access: the VPS and any upstream provider firewall allow TCP 80 for HTTP and TCP 443 for HTTPS.
- Web-server routing: Nginx matches the requested hostname with
server_nameand either serves files or forwards the request to an application.
For example, app.example.com can proxy to 127.0.0.1:3000 while api.example.com proxies to 127.0.0.1:8080. Both names can use the same public IPv4 address. Nginx distinguishes them by the hostname sent in the request, and TLS Server Name Indication (SNI) lets several HTTPS names share one IP address.
Plan the hostname and its application target
Choose each hostname and decide what should answer it before changing DNS. Record the local listener for every application; the examples below use reserved documentation address 203.0.113.10, so replace it with your VPS’s real public address.
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| Hostname | Purpose | Nginx target |
|---|---|---|
app.example.com |
Web application | 127.0.0.1:3000 |
api.example.com |
API service | 127.0.0.1:8080 |
blog.example.com |
Static files | /var/www/blog |
Using loopback addresses keeps the application ports off the public Internet. Nginx is the public entry point; clients connect to 80 or 443, and Nginx connects locally to the selected upstream.
Create the DNS record
Sign in to the provider that is authoritative for example.com and add a record whose name is normally just the label, such as app or api. Cloudflare’s subdomain documentation, updated April 16, 2026, states: “To host content on a subdomain of your domain, first ensure that your hosting provider can serve content for the given hostname (<subdomain>.example.com).”
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| Record | Name | Value in this example | Use it when |
|---|---|---|---|
| A | app |
203.0.113.10 |
The VPS accepts IPv4 traffic. |
| AAAA | app |
Your VPS’s configured IPv6 address | IPv6 reaches the same Nginx service and firewall path. |
| CNAME | app |
Another hostname, such as a provider endpoint | The alias should follow a hostname rather than point directly to an address. |
Use an A record for the VPS’s IPv4 address. Add an AAAA record only after testing that IPv6 traffic reaches this VPS, its firewall, and Nginx; an unreachable AAAA path can make browsers or certificate validation choose a broken route. A CNAME is for an alias to another hostname, not for entering an IPv4 address.
After saving the record, query the authoritative result from a shell:
dig +short app.example.comThe expected IPv4 result is the VPS address. Query IPv6 separately if you published AAAA:
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dig +short AAAA app.example.comIf the answer is wrong, check which nameservers are authoritative, whether the record name was entered as app rather than an unintended full name, and whether conflicting A or AAAA records remain.
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Allow inbound TCP 80 and 443 in both layers that can filter traffic: the VPS’s local firewall and the hosting provider’s network firewall. Port 80 is needed for ordinary HTTP and for the HTTP-01 ACME challenge; port 443 carries HTTPS. Do not expose ports 3000 or 8080 when Nginx is the public reverse proxy. Bind those applications to loopback where possible and allow only local connections.
Before requesting a certificate, make sure a request to the hostname can reach Nginx on port 80. If a provider proxy or CDN sits in front of the VPS, its mode and origin firewall rules must still permit the ACME challenge to reach the correct web server.
Configure an Nginx server block
Static subdomain
Create a separate server block for a site that consists of files. A typical file is /etc/nginx/sites-available/blog.example.com:
server {
listen 80;
listen [::]:80;
server_name blog.example.com;
root /var/www/blog;
index index.html;
location / {
try_files $uri $uri/ =404;
}
}Create the document root and place an index.html there. Enable the site using the distribution’s normal sites-enabled mechanism, for example:
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sudo ln -s /etc/nginx/sites-available/blog.example.com /etc/nginx/sites-enabled/blog.example.comEnsure the Nginx worker can read the files and that the path does not contain a permissions barrier. The try_files rule serves an existing file or directory and returns 404 instead of passing an unknown path elsewhere.
Application subdomain
For an application listening on 127.0.0.1:3000, use a reverse-proxy block:
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;
}
}The server_name value must exactly match the hostname users request. Nginx uses hostname matching to choose among server blocks listening on the same address and port. The proxy_pass directive forwards the request to the local application; the forwarded headers preserve the original host, client address and scheme for applications that use them.
For api.example.com, create another block with its own server_name and change only the upstream:
server {
listen 80;
listen [::]:80;
server_name api.example.com;
location / {
proxy_pass http://127.0.0.1:8080;
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;
}
}Keep each hostname in its own block while you are setting up the system. That makes the target, logs and later HTTPS changes easy to audit and roll back.
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Validate the configuration before reloading
Never reload an untested configuration. Run:
sudo nginx -tA successful test reports that the syntax is valid and the test is successful. Only then reload:
sudo systemctl reload nginxIf the test identifies a file, directive or permission error, fix that error and run nginx -t again. A reload applies the new configuration without unnecessarily stopping existing workers.
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curl -I http://app.example.comFor a proxy site, the response should come from the application. For a static site, it should be the Nginx-served file. If DNS is not yet returning the VPS address but you need to test the web server, use a temporary hosts-file entry on your workstation or query the VPS by address with an explicit host header; remove that temporary override after DNS is correct.
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Use an ACME client, such as Certbot, for a trusted certificate. The HTTP-01 challenge requires the certificate authority to connect directly to the hostname on port 80, so confirm DNS, firewall rules and the port-80 server block first. If IPv6 is published, the validation service must also be able to reach the same Nginx service over IPv6.
- Verify that
dig +short app.example.comreturns the VPS IPv4 address and that any AAAA result is reachable. - Confirm that TCP 80 is open in both firewalls and that Nginx answers the hostname.
- Run your ACME client for the exact hostname, for example
app.example.com. Follow the client package’s installation instructions for your operating system. - Install the issued certificate and private key in a 443 server block, or allow the client to edit the Nginx configuration when that integration is supported.
- Configure the HTTP server block to redirect to HTTPS and reload Nginx after another
nginx -tcheck. - Enable and verify the client’s renewal timer or scheduled job. A certificate that works today still needs an automatic renewal path.
A resulting HTTPS block for an application has this shape; the exact certificate paths depend on the ACME client:
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server {
listen 443 ssl;
listen [::]:443 ssl;
server_name app.example.com;
ssl_certificate /path/to/fullchain.pem;
ssl_certificate_key /path/to/privkey.pem;
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 $scheme;
}
}
server {
listen 80;
listen [::]:80;
server_name app.example.com;
return 301 https://$host$request_uri;
}Replace the certificate paths with the paths created by your client. The HTTP block remains available so that clients and the ACME HTTP-01 challenge can connect; normal requests receive a permanent redirect to HTTPS.
Verify the finished subdomain
Check each layer independently instead of treating a browser error as a single problem.
- DNS: run
dig +short app.example.comand, when applicable,dig +short AAAA app.example.com. Confirm every returned address belongs to the intended service. - HTTP: run
curl -I http://app.example.comand verify the redirect target is the same hostname over HTTPS. - HTTPS: open
https://app.example.com, inspect the certificate hostname and expiration, and confirm the page or API response is from the intended upstream. - Routing: review Nginx access and error logs while making a request. The access log shows which hostname and status were served; the error log often identifies a failed upstream connection or permission problem.
- Application: test a health endpoint or normal request and verify that the application sees the forwarded host and scheme it expects.
During certificate troubleshooting, requesting a path under /.well-known/ can help establish whether the challenge request reaches the correct server block. Do not permanently expose debugging content.
Add more subdomains on the same VPS
Repeat the process for each name:
- Add an A record for the new label, and an AAAA record only when IPv6 is ready.
- Choose a distinct local upstream port or a separate document root.
- Create an Nginx server block with the new
server_name. - Run
sudo nginx -t, reload Nginx, and test HTTP. - Obtain a certificate containing that exact hostname, then configure the 443 block and redirect.
Several HTTPS names can share one IPv4 address because the browser sends the requested hostname during the TLS handshake through SNI. NGINX describes SNI as “a more generic solution for running several HTTPS servers on a single IP address,” allowing the browser to pass the requested server name during the SSL handshake. The certificate still must contain each hostname; sharing an IP does not make a certificate valid for unrelated names.
Quick Recap
Troubleshoot by symptom
| Symptom | Likely cause | Checks and fix |
|---|---|---|
| DNS returns the wrong address | Incorrect record, wrong authoritative nameservers, or stale/conflicting A or AAAA records | Check the authoritative zone, record label and every address returned by dig. Remove conflicting records and correct the target. |
| ACME certificate challenge fails | Port 80 blocked, proxy/CDN mode prevents direct validation, or an unreachable IPv6 path | Permit TCP 80, make the hostname reach the intended Nginx block, review proxy settings, and remove or repair AAAA until IPv6 reaches the service. |
502 Bad Gateway |
The application is stopped or is not listening at the address and port in proxy_pass |
Check the application’s service status and listening socket, confirm 127.0.0.1:3000 or :8080 is correct, then inspect the Nginx error log. |
| The default site appears | Another server block is selected first or the requested hostname is absent from server_name |
Check the exact hostname in the request and configuration, disable or adjust the default block if appropriate, test with nginx -t, and reload. |
| Browser reports an HTTPS warning | The certificate lacks the exact hostname, has expired, or the browser reached a different IP | Inspect the served certificate, renew it if necessary, compare DNS A/AAAA results with the VPS, and verify the 443 block for that hostname. |
Operational safeguards
- Keep a copy of each working Nginx configuration before editing so a failed change can be rolled back quickly.
- Use one clear server block per hostname and one documented upstream per application.
- Retest with
nginx -tbefore every reload. - Monitor both Nginx logs and the application logs; a successful TCP connection does not prove that the upstream is healthy.
- Renew certificates automatically and test renewal before the expiration date.
- Do not publish an AAAA record as a placeholder. It must lead to a functioning IPv6 route, firewall rule and Nginx listener.
- Leave application ports private unless a separate design specifically requires public access.
Final setup checklist
- The chosen subdomain resolves to the VPS’s intended IPv4 address.
- Any published IPv6 address reaches the same Nginx service.
- Inbound TCP 80 and 443 are allowed at both firewall layers.
- The Nginx block contains the exact
server_nameand correct static root orproxy_passtarget. sudo nginx -tsucceeds before reload.- The ACME certificate includes the exact hostname and has an enabled renewal mechanism.
- HTTP redirects to HTTPS, and HTTPS serves the intended site or application rather than the default block.
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