Let’s start with the propagation term. Think about your business. How long does it take to spread a new policy, philosophy, process’ change, etc., among all your employees? The process to communicate it from executives until the last employee in the chain takes some time. Right?
What is DNS?
Domain name system (DNS) is the infrastructure that makes it possible for humans to surf the Internet easily. Several processes take place through this system. DNS resolution is a big one. Every time you query a domain by typing its name, the browser requests it to a server. DNS translates that domain name into a numerical language (IP address) for machines (computers, authoritative servers, recursive servers…) to understand it, look for it, and make a match between the name and that IP address. If the match is successful, the domain is loaded for you.
What is DNS propagation?
Maintaining and updating your online business or network will involve regular changes on the DNS. Adding a new record or editing another in order to change IP addresses, modifying time-to-live (TTL) values, redirecting users to specific subdomains, enabling SSL certificate, routing your e-mail, etc.
DNS propagation is exactly the process to update, to spread all the modifications you make in the DNS, all across the network.
Changes will be made and saved on one authoritative server, but there are more DNS servers on the network like recursive ones located at different points of the planet. All of them have to be updated for the modifications to operate fully. Remember, all those servers will be part of the DNS resolution process.
Factors that affect DNS propagation time
It takes from hours to days to fully propagate DNS modifications. Maybe you have experienced that after a change on your DNS, users in a specific country still get the previous (not updated yet) information or are still directed to an old or wrong page.
This means propagation is not complete yet. Different factors can affect DNS propagation time.
DNS record’s TTL. Servers are set for holding DNS records for a specific time before looking for an update. Changes you made will be completely propagated until those TTLs expire.
User’s computer DNS cache. When users access websites, DNS records of those sites are stored on their devices. How long? Exactly what it is set on the TTL value. Before that value expires, users will try to reach an old IP address before the change you made. Propagation will be made as soon as the TTL of the cache expires or if you directly clear the DNS cache.
Internet service providers (ISPs) servers’ TTLs. TTLs values on your ISP servers are set differently from yours. ISPs regularly set servers’ TTL to last longer to control DNS traffic and administrate their servers’ resources. They also cache DNS records of domains for using them in as many users’ queries of the same domain, as possible, for speeding the response. If ISP ignores your TTLs, your DNS modifications will be propagated until the ISP server’s TTL expires.
Changes at DNS higher hierarchy. Root servers’ TTLs are set for longer periods, like 2 days or more, in order to prevent stress due to the hard use.
There are ways to accelerate DNS propagation. But to program critical modifications, calculating time enough for propagating them completely is a good practice to avoid stress.