How does Nginx choose a server block for a domain?

Question

IT Interview Practice · Accepted Answer

A server block groups the configuration for a website. Nginx uses the receiving address and port, then the requested host name, to choose the appropriate HTTP website.

## Two domains, one server

Place these blocks inside `http`. Assume both domains point to the same machine:

```nginx
server {
    listen 80 default_server;
    server_name shop.example.com;
    return 200 "shop";
}
server {
    listen 80;
    server_name docs.example.com;
    return 200 "docs";
}
```

A request with `Host: docs.example.com` receives `docs`. One with `Host: shop.example.com` receives `shop`. The Host header tells Nginx which domain the visitor requested.

## What if the name matches neither?

For this example, an unknown host receives `shop` because its block has `default_server`. That default belongs to the listening address and port. In production, a separate default block can reject unknown hosts instead of showing a real website.

## HTTPS adds an earlier step

The TLS handshake selects a certificate before the HTTP request arrives. SNI is the domain name the client sends during that handshake. When diagnosing HTTPS, check the certificate and SNI as well as the later Host header.

[Official documentation](https://nginx.org/en/docs/http/request_processing.html).

Two domains, one server

What if the name matches neither?

HTTPS adds an earlier step

Related questions