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Repair common Windows errors and clear accumulated junk for a smoother, more stable PC - no reinstall needed.Free scan · no reinstallDNS cannot point a subdomain to port 3000, 8080, or any other application port. DNS records resolve a name to an IP address or another hostname. Nginx receives the request on a public listener—normally HTTP port 80 or HTTPS port 443—chooses a virtual server with server_name, and sends the request to the required internal service with proxy_pass.
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For example, app1.example.com can reach an application listening on 127.0.0.1:3000, while app2.example.com reaches another service on 127.0.0.1:8080. Visitors use clean URLs without exposing either backend port.
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The three layers that make subdomain-to-port routing work
A reliable setup separates name resolution, public request handling, and application delivery.
1. DNS resolves the hostname
At your authoritative DNS provider, create a record for each subdomain. An A record maps a name to an IPv4 address; an AAAA record maps it to an IPv6 address; and a CNAME aliases the name to another fully qualified domain name. None of these records contains an application port.
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For a server with IPv4 address 203.0.113.10, the records might look like this:
| Name | Type | Value | Purpose |
|---|---|---|---|
| app1 | A | 203.0.113.10 | Resolves app1.example.com to the Nginx host |
| app2 | A | 203.0.113.10 | Resolves app2.example.com to the same Nginx host |
| app1 | AAAA | Your Nginx host’s IPv6 address | Provides IPv6 resolution when the server supports it |
| app2 | CNAME | edge.example.net | Aliases app2.example.com to another hostname when that is your DNS provider’s recommended model |
Use the subdomain label, such as app1, in the DNS name field if the provider automatically appends example.com. Some interfaces require the complete hostname instead. Check the result with dig app1.example.com and dig app2.example.com; each answer should lead to the machine running Nginx, not to an inaccessible private address.
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2. Nginx listens publicly and selects a server block
Nginx normally accepts HTTP on port 80 and HTTPS on port 443. The hostname in the request is compared with each block’s server_name. An exact name is the clearest choice for separate applications. If no name matches, Nginx uses the default server for that listening address and port.
3. proxy_pass selects the application port
Inside a matching location, proxy_pass specifies the upstream protocol, address, and port. Nginx sends the request to that upstream, receives the response, and returns it to the browser. The applications can remain bound to loopback or a private network while only ports 80 and 443 are exposed publicly.
Prerequisites and a safe network layout
- A domain whose DNS you can edit, with each intended subdomain identified.
- A server reachable from the Internet where Nginx is installed and running.
- Applications listening on known addresses and ports, such as
127.0.0.1:3000and127.0.0.1:8080. - Firewall and cloud security-group rules allowing public TCP 80 and, for HTTPS, TCP 443.
- Permission to edit Nginx configuration and reload its service.
- For HTTPS, a certificate and private key covering every hostname you will serve.
Do not open backend ports merely to make proxying work. If Nginx and the applications share a machine, bind the applications to 127.0.0.1. If they run on separate machines, use private addresses and restrict access so only the Nginx host can reach the upstream ports.
Configure DNS before debugging Nginx
- Open the authoritative DNS zone for
example.com. - Create an A record named
app1whose value is the Nginx server’s public IPv4 address. - Create an A record named
app2with the same address when both names use that Nginx host. - Add AAAA records only when the Nginx host has a working IPv6 address and firewall path. An AAAA record pointing at an unusable IPv6 endpoint can make some clients fail even when IPv4 works.
- Use a CNAME instead of an A or AAAA record when the subdomain must alias another hostname and your DNS provider supports that arrangement.
- Wait for the record’s TTL and query each name with
dig. Confirm the returned address is the intended public endpoint.
DNS propagation does not configure Nginx and does not select port 3000 or 8080. It only determines where the client’s connection begins.
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Basic HTTP configuration for two subdomains
Create one server block per hostname. The following example exposes both services on port 80 and keeps their application ports private:
server {
listen 80;
server_name app1.example.com;
location / {
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;
proxy_pass http://127.0.0.1:3000;
}
}
server {
listen 80;
server_name app2.example.com;
location / {
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;
proxy_pass http://127.0.0.1:8080;
}
}Replace the names, addresses, and ports with your own values. The two blocks may share listen 80; Nginx distinguishes them by server_name. Keep the names exact unless you intentionally need a wildcard such as *.example.com.
Why the proxy headers are included
Nginx changes the proxied Host header to $proxy_host by default and sets Connection to close. Explicitly sending Host $host lets an upstream application see the hostname the visitor requested. X-Real-IP carries the client address, while X-Forwarded-For preserves the proxy chain. X-Forwarded-Proto tells an application whether the original request used HTTP or HTTPS.
Configure the application or framework to trust these headers only from your Nginx proxy. Blindly trusting client-supplied forwarding headers can allow incorrect addresses or protocol information to reach access-control, logging, and redirect logic.
Understand proxy_pass path and trailing-slash behavior
The difference between a URI-bearing and URI-less proxy_pass matters when an application is mounted below a path.
| Directive | Request example | Upstream URI | Use when |
|---|---|---|---|
proxy_pass http://127.0.0.1:3000; |
/users?id=4 |
/users?id=4 |
The upstream should receive the complete original path |
proxy_pass http://127.0.0.1:3000/; inside location /app/ |
/app/users |
/users |
The matching /app/ prefix should be replaced |
proxy_pass http://127.0.0.1:3000/api/; inside location /service/ |
/service/users |
/api/users |
The location prefix should become another upstream prefix |
Nginx normalizes the request URI first. When proxy_pass includes a URI, it replaces the part that matched the location; without a URI, it forwards the full normalized request URI. Decide this deliberately, especially for single-page games, APIs, and asset paths where an unexpected prefix produces 404 responses.
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Add HTTPS with certificates and SNI
For production traffic, terminate TLS at Nginx. A basic HTTPS block for the first application is:
server {
listen 443 ssl;
server_name app1.example.com;
ssl_certificate /etc/letsencrypt/live/app1.example.com/fullchain.pem;
ssl_certificate_key /etc/letsencrypt/live/app1.example.com/privkey.pem;
location / {
proxy_set_header Host $host;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header X-Forwarded-Proto https;
proxy_pass http://127.0.0.1:3000;
}
}The listen 443 ssl directive enables TLS for this virtual server, and the certificate paths must point to readable files containing the certificate chain and private key. Create a corresponding 443 block for every other hostname, or use a certificate whose subject alternative names (SANs) cover all names. A wildcard certificate can cover an appropriate subdomain level when its scope matches your names.
Multiple HTTPS virtual servers can share one IP because TLS Server Name Indication (SNI) carries the requested hostname during the handshake. Nginx still needs the correct server_name and a certificate covering that name; otherwise clients receive a certificate warning or the default certificate.
Redirect HTTP to HTTPS
After HTTPS works, use a separate port-80 block to redirect the hostname:
server {
listen 80;
server_name app1.example.com;
return 301 https://$host$request_uri;
}Repeat the redirect for each hostname, or place several names in one block when they share identical redirect behavior. Keep the actual proxying configuration on port 443. Test the certificate and redirect before enforcing policies such as strict transport security.
Deploy the configuration step by step
- Verify the upstream first. On the Nginx host, confirm the service is listening with
ss -ltnpor its own status command. A process that is stopped, bound to a different address, or listening on another port cannot be reached by Nginx. - Create the DNS records. Point every subdomain at the public Nginx address and verify with
dig. - Write the server blocks. Use one exact
server_nameper application, a publiclistenport, the required proxy headers, and the correctproxy_passtarget. - Check syntax. Run
nginx -t. Fix every reported error before reloading; a failed syntax check leaves the old configuration active. - Reload Nginx. Use the host’s service manager, for example
systemctl reload nginx, so existing connections can finish while the new configuration is read. - Test locally with the Host header. Run
curl -H 'Host: app1.example.com' http://127.0.0.1/. This isolates Nginx name matching from public DNS and external firewalls. Test the second name the same way. - Test publicly. Request each HTTP and HTTPS URL from a client outside the server’s network. Confirm the response, redirects, certificate name, and application-generated links.
- Watch logs while testing. Nginx access and error logs show which block handled the request and whether the upstream connection failed.
Troubleshoot by symptom
| Symptom | Likely cause | Checks and fixes |
|---|---|---|
| DNS returns no address | Missing, incorrect, or not-yet-published record | Query with dig; verify the authoritative zone, record name, value, and TTL. |
| The browser reaches another site | Hostname does not match a block, or the default server is handling it | Check the exact server_name, the requested Host header, and which block is marked default. |
| Connection refused or timed out | Port 80/443 is blocked, Nginx is not listening, or the public address is wrong | Check Nginx status, listening sockets, host firewall, cloud firewall, and DNS results. |
502 Bad Gateway |
Nginx cannot connect to the configured upstream | Confirm the process is running, the address and port match, loopback/private routing works, and the upstream firewall permits Nginx. |
| 404 from the application | URI prefix was rewritten unexpectedly | Compare the location and proxy_pass forms; remove or add the trailing URI deliberately. |
| Certificate warning | Certificate does not cover the hostname, wrong certificate block selected, or incomplete chain | Check SANs, certificate paths, exact server_name, SNI behavior, and the certificate chain. |
| Redirect loop or wrong absolute URLs | Application does not know the original protocol | Send X-Forwarded-Proto and configure the application to trust Nginx’s forwarding headers. |
| Correct service works locally but not publicly | DNS or perimeter firewall issue | Test locally with a Host header, then verify public DNS and that only the intended external ports are allowed. |
Choose the right routing architecture
| Architecture | Public ports and IPs | Backend exposure | Certificate model | Best fit |
|---|---|---|---|---|
| One Nginx host, many subdomains | One IP; 80 and 443 | Ports remain private on the same host | Per-host, SAN, or wildcard certificate | Several applications on one server |
| Nginx with private upstream machines | One public IP; 80 and 443 | Private network ports between Nginx and services | Terminate TLS at Nginx or pass it upstream when required | Separating frontend proxy from game, API, or admin servers |
| Separate public port per application | May use one IP but exposes 3000, 8080, and others | Backend ports are Internet-reachable unless filtered | Each service may need its own TLS setup | Special cases where the client must explicitly include a port |
| Path-based routing on one hostname | One IP; 80 and 443 | Private upstream ports | One certificate for the hostname | Applications that can tolerate URI prefix rewriting |
Subdomain routing is usually easier to reason about than path routing because each application receives its own hostname and can keep its native root paths. Path routing can reduce the number of certificates and names, but it requires careful prefix handling and application awareness.
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Operational and security details
Expose only what must be public
Permit inbound TCP 80 and 443 at the host and cloud firewall when those are the public listeners. Keep 3000, 8080, and similar service ports closed to the Internet unless a specific design requires direct access. Restrict private upstream traffic to Nginx and monitor for accidental listeners.
Keep names and defaults unambiguous
Use exact hostnames for production services. A wildcard can catch names you did not intend to publish, while an unmatched request goes to the default server. Create an intentional default server that rejects unknown hosts if your deployment needs strict isolation.
Plan certificate coverage before adding names
Each HTTPS hostname must be present in the certificate’s names. Decide whether separate certificates, one SAN certificate, or a wildcard certificate gives the right renewal and scope trade-off. Adding a DNS record alone does not make HTTPS valid.
Preserve the original request context
Host and forwarding headers affect redirects, cookies, logging, rate limits, and security checks. Pass them consistently and configure each application with the correct trusted-proxy setting. If an upstream does not need a header, do not invent a substitute value.
Reload safely
Always run nginx -t before a reload and retain a known-good configuration. A syntax failure prevents the new file from loading; an upstream failure passes the syntax check but appears as a 502 during requests, so application health checks remain necessary.
Common mistakes to avoid
- Trying to put
:3000or:8080into an A, AAAA, or CNAME record. DNS records do not carry ports. - Pointing DNS directly at a private address such as
192.168.x.xor10.x.x.xfor an Internet-facing service. - Opening every backend port publicly instead of allowing Nginx to proxy privately.
- Copying a 443 block without changing its
server_nameor certificate paths. - Omitting the trailing-slash decision when an application lives under a path prefix.
- Assuming a successful
nginx -tproves that the application is reachable; syntax validation does not test upstream connectivity. - Trusting
X-Forwarded-Forfrom arbitrary clients instead of only from the known proxy. - Testing only by IP address. Name-based routing depends on the Host header, so test the real hostname or send an explicit Host header.
A compact verification checklist
- Each subdomain resolves with A, AAAA, or CNAME to the intended Nginx endpoint.
- The application is listening on the exact address and port in
proxy_pass. - Nginx has one correct server block for each hostname and listener.
- Port 80 and, when used, port 443 are permitted through host and cloud firewalls.
- Backend ports remain private unless direct public access is deliberate.
nginx -tsucceeds before every reload.- Local Host-header tests reach the expected service.
- Public HTTP and HTTPS tests show the expected content and certificate.
- Logs distinguish name-selection problems from upstream 502 errors.
- URI and trailing-slash behavior matches what the application expects.
Frequently Asked Questions
Can an A record point app.example.com directly to port 3000?
No. A and AAAA records contain addresses, and CNAME records contain hostnames. Put the port mapping in Nginx with a matching server block and a proxy_pass target.
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Do all subdomains need separate IP addresses?
No. Multiple subdomains can resolve to one Nginx IP. Nginx selects the destination by server_name and can proxy each name to a different private port.
Why does Nginx return 502 when the application works in a browser on the server?
The proxy target may use the wrong address or port, the service may listen only on a different interface, or a private firewall may block Nginx. Verify the listening socket and the exact proxy_pass address.
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Use it according to the desired URI mapping. Without a URI, Nginx forwards the full normalized path; with a URI, it replaces the part matched by location. Test a representative path before deploying.
Can several HTTPS subdomains share one IP?
Yes. TLS SNI lets Nginx choose among HTTPS virtual servers on one address, provided each server_name is correct and the selected certificate covers the requested hostname.
Which ports need to be open on the firewall?
For the normal reverse-proxy design, expose TCP 80 and 443 publicly as needed. Keep application ports such as 3000 and 8080 private when Nginx can reach them locally or over a protected network.
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