One nginx can answer for many domains at once. Two directives decide which requests a server block picks up: listen sets the port, and server_name matches the domain. This lesson explains both, plus the default_server that catches everything else.
listen: the port
listen tells the server block which port to answer on.
server {
listen 80;
server_name example.com;
root /var/www/example;
}
Port 80 is plain HTTP, the default for http:// addresses. Port 443 is HTTPS, but that needs a certificate, which we cover later in the HTTPS chapter. For now, port 80 is all you need.
You can also bind to a specific address:
listen 127.0.0.1:80;
That means "only answer requests arriving on the local loopback address." Without an address, nginx listens on every network interface, which is what you usually want for a public site.
server_name: the domain
Many server blocks can share port 80. So how does nginx know which one should answer? It reads the Host header the browser sends (the domain in the address bar) and matches it against server_name.
server {
listen 80;
server_name example.com;
root /var/www/example;
}
server {
listen 80;
server_name blog.example.com;
root /var/www/blog;
}
A request for example.com lands in the first block. A request for blog.example.com lands in the second. Same port, different site.
server_name accepts more than one name:
server_name example.com www.example.com;
Now both the bare domain and the www version hit this block.
Hosting several domains
Put the two ideas together and you can host as many sites as you like from one nginx:
server {
listen 80;
server_name shop.example.com;
root /var/www/shop;
}
server {
listen 80;
server_name docs.example.com;
root /var/www/docs;
}
Each domain points at its own folder. This is the everyday way to run many sites on one machine.
What handles a request that matches no server_name?
Sooner or later a request arrives with a Host that matches no server_name. Maybe someone typed your server's raw IP address, or pointed a stray domain at it.
Nginx still has to answer with something, so it falls back to a default server. If you do not mark one, nginx quietly uses the first server block listening on that port. You can pick it on purpose instead:
server {
listen 80 default_server;
server_name _;
root /var/www/default;
}
The default_server flag is what actually says "send unmatched requests here." The _ does not do that job; it is just a name that matches no real domain, a common habit for a catch-all block. Many people point the default at a plain page or a 404 so stray traffic never leaks into a real site.
That split trips people up: they write server_name _; expecting it to catch everything, leave off the flag, and wonder why a different block answers. The name is decoration. The flag decides.
Common mistake
Setting default_server on more than one block for the same port. Nginx allows only one default per port and fails nginx -t if you add a second. Pick one block to be the default and drop the flag from the rest.
FAQ
Can two server blocks use the same port?
Yes. That is the whole point. They share the port and nginx tells them apart by server_name.
What if I leave server_name out?
The block still loads, but it can only ever be reached as the default server, because it has no name to match.
Do I need the www and non-www versions both?
Only if you want both to work. List every name you want this block to answer to in server_name.