The /etc/hosts Headache If you are building a SaaS that uses subdomains (like app.mysite.com or tenant.mysite.com), testing them locally is usually a pain.

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lvh.me is a free DNS service that resolves all subdomains (e.g., app.lvh.me, tenant.lvh.me) to 127.0.0.1, eliminating the need to manually edit /etc/hosts when testing subdomain-based Rails apps locally. The guide covers whitelisting lvh.me in Rails 6+ host protection config, accessing subdomains in the browser, reading request.subdomain in controllers, and configuring session/cookie sharing across subdomains using tld_length: 2. For HTTPS requirements, puma-dev is recommended as an alternative.

The Magic of lvh.me: Testing Rails Subdomains in Development