nginx first decides which server should process the request. Let’s start with a simple configuration where all three virtual servers listen on port *:80:

server {
    listen       80;
    server_name  nginx.org  www.nginx.org;
    ...
}

server {
    listen       80;
    server_name  nginx.net  www.nginx.net;
    ...
}

server {
    listen       80;
    server_name  nginx.com  www.nginx.com;
    ...
}

In this configuration nginx tests only the request’s header line “Host” to determine which server the request should be routed to. If the “Host” header line does not match any server name, or the request does not contain this line at all, then nginx will route the request to the default server. In the configuration above, the default server is the first one — which is nginx’s standard default behaviour. If you do not want the first server listed to be the default server, you may set it explicitly with the default_server parameter in the listen directive:

server {
    listen       80  default_server;
    server_name  nginx.net  www.nginx.net;
    ...
}

The default_server parameter has been available since version 0.8.21. In earlier versions the default parameter should be used instead.

Note that the default server is a property of the listen port and not of the server name. More about this later.

If you do not want to process requests without the “Host” header line, you may define a server that just drops the requests:

server {
    listen       80;
    server_name  "";
    return       444;
}

Here, the server name is set to an empty string which will match requests without the “Host” header line, and a special nginx’s non-standard code 444 is returned that closes the connection. Since version 0.8.48, this is the default setting for the server name, so the server_name "" can be omitted. In earlier versions, the machine's hostname was used as a default server name.

Let’s look at a more complex configuration where some virtual servers listen on different addresses:

server {
    listen       192.168.1.1:80;
    server_name  nginx.org  www.nginx.org;
    ...
}

server {
    listen       192.168.1.1:80;
    server_name  nginx.net  www.nginx.net;
    ...
}

server {
    listen       192.168.1.2:80;
    server_name  nginx.com  www.nginx.com;
    ...
}

In this configuration, nginx first tests the IP address and port of the request against the listen directives of the server blocks. It then tests the “Host” header line of the request against the server_name entries of the server blocks that matched the IP address and port. If the server name is not found, the request will be processed by the default server. For example, a request for www.nginx.com received on the 192.168.1.1:80 port will be handled by the default server of the 192.168.1.1:80 port, i.e., by the first server, since there is no www.nginx.com defined for this port.

As already stated, a default server is a property of the listen port and different default servers may be defined for different listen ports:

server {
    listen        192.168.1.1:80;
    server_name   nginx.org  www.nginx.org;
    ...
}

server {
    listen        192.168.1.1:80  default_server;
    server_name   nginx.net  www.nginx.net;
    ...
}

server {
    listen        192.168.1.2:80  default_server;
    server_name   nginx.com  www.nginx.com;
    ...
}

Now let’s look at how nginx chooses a location to process a request for a typical, simple PHP site:

server {
    listen        80;
    server_name   nginx.org  www.nginx.org;
    root          /data/www;

    location / {
        index     index.html  index.php;
    }

    location ~* \.(gif|jpg|png)$ {
        expires   30d;
    }

    location ~ \.php$ {
        fastcgi_pass   localhost:9000;
        fastcgi_param  SCRIPT_FILENAME
                       $document_root$fastcgi_script_name;
        include        fastcgi_params;
    }
}

nginx first searches for the most specific location given by literal strings regardless of the listed order. In the configuration above the only literal location is “/” and since it matches any request it will be used as a last resort. Then nginx checks locations given by regular expression in the order listed in the configuration file. The first matching expression stops the search and nginx will use this location. If no regular expression matches a request, then nginx uses the most specific literal location found earlier.

Note that locations of all types test only a request URI part without a query string. This is done because arguments in the query string may be given in several ways, for example:

/index.php?user=john&page=1
/index.php?page=1&user=john

Besides, anyone may request anything in the query string:

/index.php?page=1&something+else&user=john

Now let’s look at how requests would be processed in the configuration above:

• A request “/logo.gif” is matched by the literal location “/” first and then by the regular expression “\.(gif|jpg|png)$”, therefore, it is handled by the latter location. Using the directive “root /data/www” the request is mapped to a file /data/www/logo.gif, and the file is sent to the client.

• A request “/index.php” is also matched by the literal location “/” first and then by the regular expression “\.(php)$”. Therefore, it is handled by the latter location and the request is passed to a FastCGI server listening on localhost:9000. The fastcgi_param directive sets the FastCGI parameter SCRIPT_FILENAME to “/data/www/index.php”, and the FastCGI server executes the file. The variable $document_root is equal to the value of the root directive and the variable $fastcgi_script_name is equal to the request URI, i.e. “/index.php”.

• A request “/about.html” is matched by the literal location “/” only, therefore, it is handled in this location. Using the directive “root /data/www” the request is mapped to the file /data/www/about.html, and the file is sent to the client.

• Handling a request “/” is more complex. It is matched by the literal location “/” only, therefore, it is handled by this location. Then the index directive tests for the existence of an index file according to its parameters and the “root /data/www” directive. If a file /data/www/index.php exists, then the directive does an internal redirect to “/index.php”, and nginx searches the locations again as if the request had been sent by a client. As we saw before, the redirected request will eventually be handled by the FastCGI server.