To put app.example.com on one VPS, complete two separate configurations: create a DNS record that sends the hostname to the VPS, then add an Nginx server block that decides which site or application answers. Open TCP ports 80 and 443, keep application ports private, issue a certificate for the exact hostname, and test both HTTP redirection and HTTPS. One public IP can serve many subdomains because Nginx uses the requested hostname and TLS SNI to select the right configuration.
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How the pieces fit together
A subdomain is not created by Nginx alone. DNS and web-server routing perform different jobs:
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- DNS: translates
app.example.cominto the VPS’s public IPv4 or IPv6 address, or aliases it to another hostname. - Nginx: receives the connection on the VPS and matches the hostname against
server_name. It can serve files directly or proxy the request to an application listening on a private port. - TLS: supplies a certificate for the exact hostname and encrypts traffic on port 443. SNI lets several HTTPS names share one IP address.
Cloudflare’s subdomain documentation, updated April 16, 2026, states that a hosting provider must first be able to serve the requested hostname, such as <subdomain>.example.com. A DNS record by itself does not create a website or make an application listen for that name.
Plan the hostname and application target
Decide the names and local destinations before changing DNS. For example, you might use:
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| Hostname | Purpose | Nginx destination |
|---|---|---|
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 | A local document root such as /var/www/blog |
Write down the exact spelling of every hostname. Certificates, Nginx matching, browser requests, and DNS records all depend on that spelling. Decide whether each service is static or proxied, and verify that a proxied application will listen on the address and port you intend to use.
Create the DNS record
Use the DNS control panel at the authoritative provider for example.com. The record name is normally only the label, not the complete hostname.
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|---|---|---|---|
| A | app |
203.0.113.10 |
The VPS has this public IPv4 address. |
| AAAA | app |
Your VPS’s IPv6 address | IPv6 is configured end to end and reaches the same Nginx service. |
| CNAME | app |
A target hostname | The subdomain should alias another hostname rather than point directly to an IP. |
For the example, an A record may appear as A app 203.0.113.10. Replace the documentation address with the VPS address assigned to you. Add an AAAA record only after testing that IPv6 traffic reaches this VPS, its firewall, and the intended Nginx listener. An unreachable or misrouted AAAA record can make some clients and certificate validators choose a broken IPv6 path even though IPv4 works.
Do not create several conflicting A or AAAA records unless you deliberately operate several working endpoints. During setup, check for old records at the authoritative nameservers; a stale address is a common reason a browser reaches the wrong server.
Open the public network path
Permit inbound TCP 80 and TCP 443 in both layers that can filter traffic: the VPS operating-system firewall and any firewall supplied by the VPS provider. Port 80 is needed for ordinary HTTP and for the HTTP-01 certificate challenge; port 443 carries HTTPS.
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Keep application ports such as 3000 and 8080 private when Nginx is the public entry point. Bind applications to loopback (127.0.0.1) where possible, or restrict their firewall rules to the VPS itself. Exposing every development port creates an avoidable attack surface and bypasses the hostname routing, logging, and TLS policy in Nginx.
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Install an Nginx server block for a subdomain
Nginx chooses a server block by comparing the requested hostname with server_name. Each subdomain should have its own block, a distinct name, and either a document root or an upstream destination.
Static content
Create the document root and place the site’s files there. A minimal block for blog.example.com is:
server {
listen 80;
listen [::]:80;
server_name blog.example.com;
root /var/www/blog;
index index.html;
location / {
try_files $uri $uri/ =404;
}
}The try_files directive checks for a matching file or directory and returns a 404 when neither exists. Ensure the Nginx worker can read the document root and that the path contains the expected index.html or other entry file.
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Reverse proxy to an application
For app.example.com forwarding to a service on loopback port 3000, use:
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;
}
}NGINX’s web-server guide documents proxy_pass for forwarding requests. The application must actually be running and listening on the address and port in that directive. A process listening only on a different interface, a stopped service, or a typo in the port will produce a 502 response.
Enable and validate the configuration
- Save the block in the distribution’s Nginx configuration directory, commonly under
/etc/nginx/sites-available/or an equivalent included directory. - Enable it using the mechanism used by your operating system, often a symbolic link into
sites-enabled. - Run
sudo nginx -t. Do not reload while this test reports a syntax or include-file error. - After a successful test, reload Nginx with your system’s service manager, for example
sudo systemctl reload nginx. - Check the active configuration if the wrong site appears. A different server block may be selected first, or the requested hostname may be absent from
server_name.
Keep a copy of the last known-good configuration. A failed syntax test should be corrected and retested before any reload; a successful test confirms syntax and file inclusion, not that the upstream application is healthy.
Confirm DNS before requesting a certificate
Query the public DNS view for the exact name:
dig +short app.example.comThe result should contain the VPS’s intended IPv4 address. If you published IPv6, query it as well and verify that it is the address on this VPS. DNS changes can be cached, so a local resolver may temporarily return an older value; authoritative nameservers and multiple external resolvers are useful when diagnosing propagation.
When the name resolves correctly, request http://app.example.com and confirm that Nginx answers. If a certificate challenge later fails, you can inspect the HTTP-01 path, including a temporary request to /.well-known/, while troubleshooting. The challenge must be able to make a direct connection to port 80; a DNS record that points somewhere else, a closed firewall, or a proxy mode that does not pass the request can prevent validation.
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Issue and install HTTPS
Use an ACME client such as Certbot or another client supported by your operating system. Request a certificate for the exact hostname, for example app.example.com, after DNS and port 80 are working. The client proves control of the name, obtains the certificate, and should be configured to renew it automatically.
Certbot can edit an Nginx configuration, add an HTTPS server block and HTTP-to-HTTPS redirect, then reload Nginx. Review the changes rather than assuming every existing rule is appropriate. If you manage the block yourself, the HTTPS portion follows this pattern:
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;
}Use the certificate and key paths supplied by your ACME client, not the illustrative paths above. Protect the private key, keep renewal enabled, and reload Nginx after a renewed certificate is installed. Port 443 must be reachable from the public internet for browsers to establish HTTPS.
For several names on one address, obtain coverage for each hostname or use a certificate that explicitly includes the names. NGINX describes TLS Server Name Indication (SNI) as a generic solution that lets a browser pass the requested server name during the SSL handshake, allowing several HTTPS servers to share one IP.
Verify the finished subdomain
- Run
dig +short app.example.comand confirm the expected address or addresses. - Open
http://app.example.com. It should either show the intended HTTP response during setup or return a 301/308 redirect to HTTPS after the redirect is enabled. - Open
https://app.example.comand inspect the certificate hostname, expiration, and issuing chain in the browser. - Confirm that the response comes from the correct static root or application. A 200 response from the wrong site still indicates a hostname-matching problem.
- Review Nginx access and error logs while making a request. Logs reveal the requested host, status code, upstream failure, and the file or proxy path Nginx selected.
- Test from an IPv6-capable network if an AAAA record exists. Remove or correct AAAA before relying on the name if IPv6 fails while IPv4 succeeds.
Repeat these checks after adding every new subdomain. Each name needs a DNS record, a matching Nginx block, certificate coverage, and a reachable application or document root.
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Add more subdomains to the same VPS
There is no requirement for one VPS or one public IP per subdomain. Repeat the same four-part pattern:
| Layer | app.example.com |
api.example.com |
|---|---|---|
| DNS | A record to the VPS IPv4 address | A record to the same VPS IPv4 address |
| Hostname match | server_name app.example.com |
server_name api.example.com |
| Local target | 127.0.0.1:3000 |
127.0.0.1:8080 |
| TLS | Certificate containing app.example.com |
Certificate containing api.example.com |
Use a separate server block and upstream port where the applications are different. A request for api.example.com reaches the same IP as app.example.com, but Nginx’s hostname match sends it to the API block. Keep names explicit instead of relying on a broad default block that could accidentally serve the wrong application.
Diagnose common failures
DNS returns the wrong address
- Query the authoritative nameservers and compare the result with the VPS address.
- Check that the record name is
app, not a duplicated full name such asapp.example.com.example.comin providers that automatically append the zone. - Remove stale or conflicting A and AAAA records.
- Confirm that the domain’s delegated nameservers are the ones where you made the change.
The certificate challenge fails
- Verify that TCP 80 is open in both VPS and provider firewalls.
- Request the hostname directly over HTTP and check that Nginx answers it.
- Check whether a proxy or CDN mode is intercepting the challenge instead of forwarding it to this VPS.
- Test the IPv6 address if an AAAA record exists. An unreachable IPv6 path can cause validation trouble even when IPv4 is healthy.
- Confirm that the certificate request uses the exact hostname in
server_nameand DNS.
Browser shows 502 Bad Gateway
A 502 usually means Nginx could not obtain a valid response from the configured upstream. Confirm that the application is running, listening on 127.0.0.1:3000 (or the chosen port), and not bound only to an unrelated address. Compare the live configuration with proxy_pass, then inspect the Nginx error log for connection-refused, timeout, or upstream-reset details.
The default site appears
The request may be matching another server block first, or the hostname may be missing or misspelled in server_name. Check that the intended file is enabled, run nginx -t, reload Nginx, and remove or disable an unwanted default block where appropriate. Test with the real hostname rather than an IP address; an IP-only request does not supply the same host match.
HTTPS shows a warning
Inspect the certificate presented by the server. The warning can mean the certificate does not contain the exact hostname, has expired, or the browser reached a different IP than the server where you installed the certificate. Compare DNS A and AAAA results with the VPS addresses, verify the 443 block’s certificate paths, and reload Nginx after correcting them.
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- Separate public and private ports: expose 80 and 443 through Nginx, while keeping application listeners private.
- Validate before reload: always run
nginx -t; a syntax error should never replace a working configuration. - Keep certificate renewal observable: ensure the ACME client’s scheduled renewal runs and that Nginx reloads after a successful renewal.
- Monitor both address families: an AAAA record is a commitment to serve IPv6, not merely an additional DNS value.
- Use logs during changes: access logs confirm hostname and status; error logs distinguish routing, file, and upstream failures.
- Back up configuration: retain known-good Nginx blocks and record which local port belongs to each subdomain.
When a different implementation may fit
Nginx is the implementation shown here because it makes DNS-to-hostname-to-upstream routing explicit. If you evaluate a different web server or a control panel, compare the same operational questions rather than choosing by branding:
| Question | What to verify |
|---|---|
| Certificates | Does it issue and renew ACME certificates automatically, and can you inspect the resulting configuration? |
| Routing | Can it match hostnames and reverse-proxy each subdomain to a separate local port? |
| Transparency | Can you review, back up, test, and roll back generated configuration? |
| Networking | Does it clearly handle firewall rules, port 80 validation, HTTPS on 443, and IPv6? |
| Operations | What additional services, updates, and monitoring will it require on the VPS? |
Regardless of the tool, the underlying sequence remains the same: publish a correct hostname, make the network path reachable, route that hostname to the intended content, issue a matching certificate, and verify every address family you advertise.
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