What is the difference between Nginx variables $host, $http_host, and $server_name?

What is the difference between the three Nginx variables $host, $http_host, and $server_name?

I have a rewrite rule where I'm not sure which one I should be using:

location = /vb/showthread.php {
    # /vb/showthread.php?50271-What-s-happening&p=846039
    if ($arg_p) {
        return 301 $scheme://$host/forum/index.php?posts/$arg_p/;
        }

I'm looking for an answer that doesn't just say 'use ___ variable in your rewrite rule' but also explains the theoretical differences between them.

You should almost always use $host, as it's the only one guaranteed to have something sensible regardless of how the user-agent behaves, unless you specifically need the semantics of one of the other variables.

The difference is explained in the nginx documentation:

• $host contains "in this order of precedence: host name from the request line, or host name from the 'Host' request header field, or the server name matching a request"
• $http_host contains the content of the HTTP "Host" header field, if it was present in the request
• $server_name contains the server_name of the virtual host which processed the request, as it was defined in the nginx configuration. If a server contains multiple server_names, only the first one will be present in this variable.

Since it is legal for user-agents to send the hostname in the request line rather than in a Host: header, though it is rarely done except when connecting to proxies, you have to account for this.

You also have to account for the case where the user-agent doesn't send a hostname at all, e.g. ancient HTTP/1.0 requests and modern badly-written software. You might do so by diverting them to a catch-all virtual host which doesn't serve anything, if you are serving multiple web sites, or if you only have a single web site on your server you might process everything through a single virtual host. In the latter case you have to account for this as well.

Only the $host variable accounts for all the possible things that a user-agent may do when forming an HTTP request.

I would like to add another important point not mentioned in the accepted answer.

$host do NOT have port number, while $http_host does include the port number.

edit: not always.

I set up a header "add_header Y-blog-http_host "$http_host" always;"

Then curl -I -L domain.com:80 (or 443) and the header doesn't show a port number at all. Verified with nginx-extra 1.10.3. Is is because it's common http(s) ports or nginx configuration? This comment just to say things do not always behave the way you think.

I also struggled with this for a while. It became clear when I understood that $http_XXXXX refer to all declared header variables.

So $http_user_agent, $http_referer are "USER AGENT", "REFERER" referenced in lower case and underslash. This explained me where the hell $http_upgrade was coming from in many NGINX configuration samples.

Read it from https://stackoverflow.com/questions/15414810/whats-the-difference-of-host-and-http-host-in-nginx