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#+date: <2020-08-22 Sat 00:00:00>
#+title: Redirect GitHub Pages from Subdomain to the Top-Level Domain
#+description:
#+slug: redirect-github-pages
* 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)?]]
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