What's the Difference of $Host and $Http_host in Nginx

In Nginx, $host and $http_host are variables used to represent different aspects of the HTTP request, specifically related to the Host header. While they may seem similar, they serve slightly different purposes and can be used in different contexts.

1. $host Variable

Definition:

• The $host variable represents the value of the Host header from the client request, or, if the Host header is not present, it will default to the server name specified in the Nginx configuration.

Behavior:

• If the Host header is present: $host will contain the value of this header.
• If the Host header is not present: $host will fall back to the server's server_name or listen directive’s IP address or domain.

Use Case:

• $host is often used to construct URLs or to handle requests based on the requested host name. It's useful in situations where you want to ensure that your server handles requests based on the Host header, including cases where the header might be missing or malformed.

Example:

server {
    listen 80;
    server_name example.com;

    location / {
        # Logs the value of the Host header or the default server_name
        access_log /var/log/nginx/access.log '$host';
    }
}

2. $http_host Variable

Definition:

• The $http_host variable specifically represents the value of the Host header as it was received from the client request. It directly corresponds to the Host header value in the HTTP request.

Behavior:

• Always contains the raw Host header value from the client request.
• No fallback: Unlike $host, $http_host does not have a fallback mechanism. It is strictly the header value sent by the client.

Use Case:

• $http_host is useful when you need to work with the exact Host header value as sent by the client, without any modification or fallback. This is particularly relevant when dealing with multiple virtual hosts or when debugging issues related to header values.

Example:

server {
    listen 80;

    location / {
        # Logs the exact value of the Host header sent by the client
        access_log /var/log/nginx/access.log '$http_host';
    }
}

Key Differences

• Fallback Mechanism:
  • $host: Will fall back to the server name or IP if the Host header is not present.
  • $http_host: Directly reflects the value of the Host header; no fallback.
• Use Cases:
  • $host: General usage when you want to handle requests based on the Host header or default server name.
  • $http_host: When you need to log or use the exact value of the Host header sent by the client.

Practical Example

Consider a request where the Host header is set to example.com:

• With $host:
  • If the Host header is example.com, $host will be example.com.
  • If the Host header is missing, $host will be the server's server_name (e.g., localhost or default_server).
• With $http_host:
  • If the Host header is example.com, $http_host will be example.com.
  • If the Host header is missing or empty, $http_host will be empty.

Configuration Example:

server {
    listen 80;
    server_name example.com;

    location / {
        # Example of using $host and $http_host
        add_header X-Host '$host';
        add_header X-Http-Host '$http_host';
    }
}

In this configuration:

• X-Host will be example.com or the fallback server name.
• X-Http-Host will be example.com or empty if the Host header is missing.

Summary

• $host: Represents the Host header value or server name if the header is missing. Useful for general server configuration and fallback scenarios.
• $http_host: Represents the exact Host header value received from the client. Useful for precise logging or when you need the raw header value.

By understanding the differences between $host and $http_host, you can more accurately configure and debug Nginx to handle HTTP requests as per your requirements.

Got an article suggestion? Let us know

This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.