Add canonical URL support for custom domains

[Copilot]

Sites hosted on GitHub Pages with custom domains have duplicate content at both username.github.io and customdomain.com, causing SEO issues. Adding canonical URLs tells search engines which URL is authoritative.

Changes

• layouts/partials/header.html: Add canonical <link> tag using Hugo's .Permalink (which derives from baseURL). Supports existing relcanonical front matter for page-specific overrides.
• layouts/_default/single.html: Remove duplicate canonical handling (now centralized in header.html).

Usage

Set your custom domain in hugo.yaml:

baseURL: https://customdomain.com/

All pages automatically get canonical URLs pointing to your custom domain. Individual pages can override via front matter:

---
relcanonical: /some/other/path
---

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• publish.twitter.com
  • Triggering command: /tmp/hugo hugo --themesDir ../.. (dns block)
  • Triggering command: /tmp/hugo hugo (dns block)
  • Triggering command: /tmp/hugo hugo --renderToMemory (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Feature Request: support custom domains</issue_title>
<issue_description>I have tried to set up a custom domain for a personal GitHub page. It works smoothly but I noticed some SEO problems because of the duplicate content at username.github.io and customdomain.com

The internet suggests the following solution:
add to the head

<meta http-equiv="refresh" content="0; URL=https://customdomain.com/">
<link rel="canonical" href="https://customdomain.com/">

I considered modifying just my fork of the theme, but I am now wondering if there are other places where
the custom domain should be added.

It is also unclear why there are two different parameters for the site URL: baseURL in hugo.yaml and openGraph.url.
Is there any case where they should differ from each other?

Since this may be a common situation, adding an option to the theme to support custom domains may be worth adding.
</issue_description>

Comments on the Issue (you are @copilot in this section)

@BernatBC Hi @maiani, sorry for the late response. I also have a custom domain, and I don't have any problems. I just added my custom domain into `hugo.yaml`, and followed the guide on github pages.

However, if you really wanted to add the html metadata, you can create a file into your site's layout/partials/header.html, paste the content of this file: https://github.com/hugo-toha/toha/blob/main/layouts/partials/header.html, and add your lines there.

Hope this solves the issue</comment_new>

• Fixes Feature Request: support custom domains #953

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[hossainemruz]

@copilot Do we need <meta http-equiv="refresh" content="0; URL=https://customdomain.com/">?

[Copilot]

@copilot Do we need <meta http-equiv="refresh" content="0; URL=https://customdomain.com/">?

No, the <meta http-equiv="refresh"> tag is not needed. The <link rel="canonical"> tag is the correct solution for the SEO duplicate content issue - it tells search engines which URL is authoritative without redirecting users. A meta refresh would forcibly redirect visitors, which isn't necessary since GitHub Pages serves the same content on both domains. The canonical tag lets search engines know which URL to index while users can still access the site from either URL.