Add canonical host redirect feature

[JangoCG]

Summary

Adds host canonicalization to redirect all requests to a single canonical host, e.g., force root (domain.com) or www (www.domain.com).
Supports both directions:

• www.domain.com → domain.com
• domain.com → www.domain.com

Redirects happen in one hop even with TLS (e.g., http://www → https://root).

Changes

• Added CanonicalHost to ServiceOptions
• Centralized redirect logic to handle both scheme and host together
• Added --canonical-host flag to deploy command
• Added full test coverage for canonical host redirects (including TLS cases)

Motivation

Kamal currently has no built-in way to handle www ↔ root redirects, forcing users to rely on Cloudflare or custom reverse proxy rules.
This is a basic feature in most reverse proxies, and this PR makes it available natively in Kamal.

Compatibility

Backwards compatible, only active when explicitly configured.

[Talha345]

Would love to get this merged!

[AnyCPU]

@JangoCG @Talha345 why not to register a one service for two hosts, for instance: --host=example.com, www.example.com?

[Talha345]

@JangoCG @Talha345 why not to register a one service for two hosts, for instance: --host=example.com, www.example.com?

Having 2 hosts is fine but there should be a way to configure redirections from one to the other so a typical use case is from www to non-www domain or vice versa, which is required for SEO and its always a good idea to have your site running on just one domain usually.

[AnyCPU]

@Talha345 i see your point.
however, it would be good point if we were discussing it 20 years ago.

these days if you want to have a one website just use a one host, keep your website name as short as possible for humans.
the www part is not required at all. Besides a lot of websites may use something like www1, ww5 ... for some reasons.

p.s.: create websites for humans, not for SEO.

[Talha345]

@Talha345 i see your point.
however, it would be good point if we were discussing it 20 years ago.

these days if you want to have a one website just use a one host, keep your website name as short as possible for humans.
the www part is not required at all. Besides a lot of websites may use something like www1, ww5 ... for some reasons.

p.s.: create websites for humans, not for SEO.

There could be various other use cases as well and that is out of the scope of this discussion.

In general, a proxy should support this kind of basic redirection.

I still have a couple of sites which are using Kamal 1 with Traefik and I simply have not upgraded to Kamal 2 for this exact reason.

[AnyCPU]

@JangoCG

1. do we still need following methods: shouldRedirectToHTTPS & redirectToHTTPS?
2. do we have to check whether a canonical host is also presented in values for a --host option?

[kevinmcconnell]

Thanks @JangoCG. This feature seems like a good idea to me!

I've left some small suggestions about the details. Let me know what you think. I'm also happy to address those myself if you don't have time.

I also like @AnyCPU's suggestion that if both --canonical-host and --host are set, we should validate that there is a --host that matches the --canonical-host (because otherwise, that deployment would be invalid). We can validate that is the command before attempting the deployment.

[JangoCG]

Thanks @JangoCG. This feature seems like a good idea to me!

I've left some small suggestions about the details. Let me know what you think. I'm also happy to address those myself if you don't have time.

I also like @AnyCPU's suggestion that if both --canonical-host and --host are set, we should validate that there is a --host that matches the --canonical-host (because otherwise, that deployment would be invalid). We can validate that is the command before attempting the deployment.

Awesome, I’m glad you like it @kevinmcconnell thank you for the review! And thanks as well @AnyCPU for your input.
I recently started a new job so I didn’t have much time to address your suggestions earlier, but I’ve now implemented the changes you mentioned.