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+#+date: 2020-08-22
+#+title: Redirect GitHub Pages from Subdomain to the Top-Level Domain
+
+* Short answer
+
+** Step 1
+
+Add a new file CNAME to your GitHub Pages repository containing only one line:
+your top-level domain name. E.g.: =example.com=
+
+** Step 2
+
+[Optional] but highly recommended
+
+2.1: Remove all other top-level records (prefixed with @) of type A from your
+DNS configuration.
+
+2.2: Remove a CNAME record for the second-level domain www if it is present.
+
+** Step 3
+
+Add these 5 entries to the very top of your DNS configuration:
+
+#+BEGIN_SRC txt
+@        A        185.199.108.153
+@        A        185.199.109.153
+@        A        185.199.110.153
+@        A        185.199.111.153
+www      CNAME    your_github_username.github.io.
+#+END_SRC
+
+Replace =your_github_username= with your actual GitHub username.
+
+** Step 4
+
+Wait for your DNS changes to propagate. DNS changes aren't effective
+immediately. They can take up to a full day to propagate.
+
+* Long answer
+
+This issue has two sides. One is the DNS configuration itself. Another one is
+the way GitHub Pages will forward HTTP requests.
+
+We need to know a few things to understand what GitHub is trying to say in their
+documentation.
+
+** DNS Entry Types
+
+There are two types of DNS records which interest us: CNAME and A.
+
+=A= is also known as =Apex= or sometimes as =root entry=. It forwards requests
+to a specified fixed IP address. =CNAME= entry forwards requests to a specified
+URL (actual valid plain text URL, not an IP address).
+
+** DNS Load balancing
+
+GitHub has one central URL address which accepts all DNS requests for GitHub
+Pages: =http://username.github.io=. That URL is resolved to different IP
+addresses based on your geographical location. Website hosted on GitHub Pages is
+a simple collection of =HTML=, =CSS= and =JS= files. GitHub distributes these
+files to different servers across the globe. So that when your browser sends a
+request from Europe, it receives data from a server in Europe. The same is valid
+for the requests from Asia and the USA.
+
+** What GitHub is trying to say
+
+Since =A= records in DNS must contain IP addresses, and they must be either
+=185.199.108.153= or =185.199.109.153= or =185.199.110.153= or
+=185.199.111.153=, there is no way to forward requests to a server located
+somewhere in Europe or Asia. Your website hosted at GitHub Pages will be
+downloaded from a central GitHub Pages server. There is a minor risk that if
+GitHub Pages DNS servers (=x.x.x.153=) are down for some reason, all custom
+domains which use fixed GitHub Pages IP addresses will not be accessible (their
+DNS requests will not be resolvable).
+
+That is why GitHub strongly suggests to either use a second-level domain for
+your GitHub Pages (e.g. =blog.example.com=) or use a DNS service provider that
+supports a record type =ALIAS= that acts as =A= record but forwards request to a
+URL address (e.g. =username.github.io=) instead of a fixed IP address.
+
+** How GitHub Pages treats HTTP requests
+
+After a DNS request for =your_github_username.github.io= is resolved into an IP
+address, e.g. =185.199.108.153= your browser sends an HTTP request to that
+server with an HTTP header =Host=. Below are =curl= examples that load the same
+website (these examples might not work if you are behind a proxy server):
+
+#+BEGIN_SRC sh
+curl --header "Host: your_github_username.github.io" http://185.199.108.153/
+curl --header "Host: www.example.com" http://185.199.108.153/
+curl --header "Host: example.com" http://185.199.108.153/
+#+END_SRC
+
+This way GitHub Pages servers know which user website to serve.
+
+#+BEGIN_QUOTE
+GitHub Pages server will automatically redirect HTTP requests to the top-level
+domain if your =CNAME= file contains =example.com= but =www.example.com= is
+requested.
+
+The same is valid if your =CNAME= file contains =www.example.com= but the
+header =Host= in the =HTTP= request contains =example.com=.
+#+END_QUOTE
+
+** Why can't I add a =CNAME= record entry that accepts a top-level request (=@=) to my DNS configuration?
+
+Quote from the GitHub Pages documentation:
+
+#+BEGIN_QUOTE
+Warning: Do not create a CNAME record for your custom apex domain! Doing so
+may cause issues with other services, such as email, on that domain.
+#+END_QUOTE
+
+* References:
+
+1. [[https://docs.github.com/en/github/working-with-github-pages/configuring-a-custom-domain-for-your-github-pages-site][Setting up a custom domain with GitHub Pages]]
+2. [[https://docs.github.com/en/github/working-with-github-pages/troubleshooting-custom-domains-and-github-pages][My custom domain isn't working]]
+3. [[https://serverfault.com/questions/589370/cannot-access-my-github-pages-website-by-ip-address][Cannot access my GitHub Pages website by IP Address]]
+4. [[https://stackoverflow.com/questions/23375422/how-do-i-set-up-github-pages-to-redirect-dns-requests-from-a-subdomain-e-g-www][How do I set up GitHub Pages to redirect DNS requests from a subdomain
+   (e.g. www) to the top-level domain (TLD, Apex record)?]]