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Host header

As explained in Learn Docker in a Month of Lunches, Second Edition

A host header is an HTTP request header that identifies the destination domain or host requested by a client.

A host header identifies the domain requested by an HTTP client. A reverse proxy such as Nginx uses it to choose the site configuration when multiple applications share one port. section 20.1

A local routing workflow uses the host header like this:

  • Send the request to Nginx; its matching server configuration proxies the request to the application container. section 20.1

Routing whoami.local

After adding whoami.local to the hosts file, starting the who-am-I container, and reloading Nginx, curl http://whoami.local reaches the application through the proxy.

How Learn Docker in a Month of Lunches, Second Edition covers it

How deep it goes
Explains domain matching and header forwarding through Nginx, without detailing broader HTTP-header behavior.
Examples
Uses whoami.local, api.numbers.local, curl, and Nginx proxy_set_header configuration.
What it assumes
Uses HTTP requests, a hosts file, Docker containers, and basic Nginx configuration without detailing their internals.
What you can do after
Operate/configure Nginx to preserve the requested host while proxying an application container.
How it connects
Connects HTTP requests with reverse proxies, DNS, Docker networks, and REST APIs.
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Related concepts

This explainer was generated by Manning's knowledge graph from the text of Learn Docker in a Month of Lunches, Second Edition. The section links point to the parts of the book it draws on.