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

In Nginx, the variables `$host`, `$http_host`, and `$server_name` serve different purposes and hold distinct values within the context of an HTTP request. Here's a breakdown of their differences:

## $host:

- The `$host` variable in Nginx represents the value of the Host header in the HTTP request. It contains the hostname provided by the client in the HTTP request.
- It is derived from the `Host` header sent by the client and may not always represent the server's hostname. This header can be set by the client or by intermediary systems.

Example:

```
server {
    listen 80;
    server_name example.com;

    location / {
        echo $host;  # Will output the value of the Host header in the client's request
    }
}
```

## $http_host:

- The `$http_host` variable holds the value of the Host header sent by the client in an HTTP request.
- It's specifically a part of the `$http_` variable family in Nginx. It's the same as `$host` and holds the value of the Host header.

Example:

```
server {
    listen 80;
    server_name example.com;

    location / {
        echo $http_host;  # Will output the value of the Host header in the client's request
    }
}
```

## $server_name:

- The `$server_name` variable in Nginx holds the server name that matches the current request.
- It's typically used to match the server block in the Nginx configuration based on the incoming request.

Example:

```
server {
    listen 80;
    server_name example.com;

    location / {
        echo $server_name;  # Will output the server name defined in the current server block
    }
}
```

Each of these variables holds the hostname or server name but may have different sources and contexts. While `$host` and `$http_host` typically represent the value of the Host header sent by the client, `$server_name` refers to the server name defined in the Nginx configuration for the current request. Understanding their differences can help in making proper use of these variables in your Nginx configuration.