TL;DR

It is important to understand that Nginx will only evaluate the server_name directive when it needs to distinguish between server blocks that match to the same level of specificity in the listen directive.

While configuring a machine with a fresh Nginx installation, I found that the server_name setting did not work:

  • In a server block, server_name and location blocks were configured, and an application was running on local port 3000. *.internal and *.localhost already pointed to 127.0.0.1 in DNS records:
server {
    listen 80;
    server_name sub.internal;

    location / {
        proxy_pass http://127.0.0.1:3000;
    }
}

  • Under this configuration, the expected behavior is that accessing sub.internal is redirected to 127.0.0.1:3000, while accessing other domains such as whatever.internal or whatever.localhost does nothing.

  • But the actual behavior was: not only the specified domain gets redirected, but the domains mentioned in the two examples above, and indeed any *.internal, are redirected to 127.0.0.1:3000.

The problem occurred because this machine’s nginx was freshly configured and had only this one server block. According to this article, server_name is only used to determine which server block is selected when the listen directive resolves more than one matching server block. Therefore the solution is to add another server block.

server {
    listen 80;
    server_name sub.internal;

    location / {
        proxy_pass http://127.0.0.1:3000;
    }
}

server {
    listen 80;
    server_name *.internal;

    return 404;
}