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diff --git a/blog/2020-08-22-redirect-github-pages.org b/blog/2020-08-22-redirect-github-pages.org new file mode 100644 index 0000000..bd86a26 --- /dev/null +++ b/blog/2020-08-22-redirect-github-pages.org @@ -0,0 +1,120 @@ +#+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)?]] |