Nginx · The edge of your stack

What is a server block and how does Nginx choose one?

Easyserver blocksvirtual hostsserver_nameUpdated Oct 2026

Short answer

A server block is a virtual host defined by listen and server_name. Nginx picks the listener, then matches server_name exactly, by wildcard or regex, and falls back to the default server.

One Nginx instance can host many domains. The listen directive decides the address and port, and server_name decides which host gets the request. A default server handles requests whose Host header matches nothing.

nginx

server {
    listen 80 default_server;
    server_name _;
    return 444;
}

server {
    listen 80;
    server_name example.com www.example.com;
    root /var/www/example;
    index index.html;
}

The first block rejects unmatched hosts; the second serves the real site.

server_name matching order

Exact names match first
Longest wildcard starting with a star, such as *.example.com
Longest wildcard ending with a star, such as www.example.*
First matching regular expression in config order
Otherwise the default_server for that listen address is used

The name underscore is not a special wildcard. It is just an invalid hostname that will not match real traffic, which makes it useful in a default reject block.

Follow-ups you should expect

Interviewers use these to check whether the first answer was understanding or memorisation.

1What happens when no server_name matches the Host header?
2Can two server blocks listen on the same address and port?
3What is the difference between default_server and server_name _?
4How do SNI and server_name interact for HTTPS?

Say it in the interview

A server block is a virtual host. I use listen to choose the address and port and server_name to match the Host header. Nginx tries exact names, then wildcards, then regexes, and finally the default server for that listener. I usually add a default server that rejects unknown hosts, and for HTTPS I remember that certificate selection uses SNI before the HTTP request is parsed.

Use this as a model, not a script. Saying it in your own words is what makes it survive follow-up pressure.

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server_name matching order

Follow-ups you should expect

Related questions

Now say it out loud.