This guide describes the meaning of TTL, an abbreviation for «Time To Live» as well as the principle of DNS propagation delays, important concepts in the operation of domain name-related services.

Preamble

• The TTL determines how long DNS information is cached before being updated.
• This directly affects the speed at which DNS changes propagate across servers worldwide.
• While reducing the TTL can speed up propagation, the behaviors of ISPs and their caches can introduce additional delays.

The TTL (Time to Live)

The TTL (Time to Live) is a key parameter in managing DNS records. It represents the duration, expressed in seconds, during which DNS information can be stored in cache before being updated. When a DNS server or Internet Service Provider (ISP) queries a DNS zone, it temporarily keeps the responses obtained to reduce the load on servers and speed up browsing for users.

For example, if the TTL is set to 24 hours (86,400 seconds), the DNS information related to a domain will be cached and will not be updated for the entire duration, even if you have modified parameters such as the domain's IP address. This means that DNS changes will only take effect once this delay has elapsed.

It is possible to speed up the application of changes by reducing the TTL before proceeding with updates. For example, lowering the TTL to one hour (3600 seconds) allows DNS servers to check the new information more quickly after a modification.

DNS propagation

The TTL is directly linked to the DNS propagation process. DNS propagation refers to the time required for changes made to a DNS zone to be disseminated across all DNS servers worldwide. This delay depends not only on the configured TTL, but also on how Internet Service Providers (ISPs) and other DNS servers manage their caches.

When you modify DNS records, DNS servers and ISPs must wait for the TTL period to expire before fetching the new information. Thus, the shorter the TTL, the faster the changes propagate through the network. However, some ISPs sometimes ignore the TTL duration and keep the information cached for a longer period (for example, 24 hours or more), which can slow down the propagation of DNS changes, even if you have configured a reduced TTL.