map $request_uri $sub {
    default       front;
    ~^/api/.*     api;
    ~^/rest/.*    rest;
}

map $host $sub {
    hostnames;

    #default    $sub; # Tried, no success

    ping.*     ping;
    admin.*    admin;
}

server {
    listen 80;
    server_name _;

    root /usr/share/nginx/html;

    location / {
        add_header X-Sub $sub;
        try_files $uri $uri/ /index.html;
    }
}