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Domain TTL and DNS Filtering

The Time-to-Live (TTL) of a domain is an important parameter that can significantly affect the behavior of Domain Name System (DNS) resolvers. DNS resolvers are the components of the DNS infrastructure that respond to DNS queries from clients and provide them with the IP addresses associated with domain names.

TTL is a value specified in seconds that is set by the domain owner or the DNS administrator and is stored in the DNS record for that domain. It indicates how long a DNS resolver should cache the DNS information before it expires and needs to be refreshed. When a DNS resolver receives a query for a domain name, it first checks its cache to see if it has a valid (non-expired) copy of the DNS information for that domain. If it does, it can quickly provide the IP address associated with the domain name from its cache, without having to perform a full DNS resolution process. However, if the TTL of the domain is expired, the resolver needs to perform a fresh DNS query to the authoritative DNS server for that domain to obtain the updated DNS information.

Here are three ways that TTL can effect DNS resolution:

CachingDNS resolvers heavily rely on caching to improve performance and reduce the load on authoritative DNS servers. When a DNS resolver receives a DNS response with a TTL value, it caches the response and associates it with the domain name. The resolver will then refer to this cached response for subsequent queries for the same domain until the TTL expires. A shorter TTL means that the DNS information will be cached for a shorter duration, and the resolver will need to refresh its cache more frequently, resulting in increased DNS queries to the authoritative DNS server. On the other hand, a longer TTL means that the DNS information will be cached for a longer duration, reducing the frequency of queries to the authoritative DNS server.
DNS Resolution SpeedThe TTL value also affects the speed of DNS resolution. If the TTL is set to a very low value, DNS resolvers will need to refresh their cache frequently, resulting in increased DNS queries and potentially longer resolution times. This can happen especially during periods of high traffic or when there are issues with the authoritative DNS server. Conversely, if the TTL is set to a high value, DNS resolvers can rely on their cache for a longer duration, resulting in faster resolution times as they do not need to frequently query the authoritative DNS server.
Flexibility and AgilityTTL provides flexibility and agility for domain owners to quickly update their DNS records and propagate the changes across the internet. For example, if a domain owner needs to switch their website to a new IP address or change their email provider, they can update the DNS records and set a low TTL value to ensure that the changes propagate quickly to DNS resolvers. Once the changes have propagated, the domain owner can increase the TTL value to reduce the load on authoritative DNS servers and improve performance.

The effects of a domain with a high TTL can materialize into a prolonged delay for local devices and resolvers to update after making changes within a DNS-based filtering platform like CleanBrowsing.

How to Check the TTL of Domains

dig is a popular command-line tool used to perform DNS queries in a Unix/Linux environment. It can also be used to find the Time-to-Live (TTL) value of a domain. Here’s how you can use dig to find the TTL of a domain:

  1. Open a terminal window on your Unix/Linux system.
  2. Type the following command, replacing “example.com” with the domain name for which you want to find the TTL:
dig example.com

This will send a DNS query to the default DNS resolver configured on your system for the domain “example.com”.

  1. Look for the “TTL” value in the output. The TTL is usually displayed in the “ANSWER SECTION” of the dig output, next to the DNS resource record (RR) type that you queried for. For example, if you queried for the A record (IPv4 address) of “example.com”, the TTL value will be displayed next to the “A” record in the “ANSWER SECTION” of the output, like this:
example.com.        86400   IN      A

In this example, the TTL value is “86400” seconds, which is equivalent to 24 hours.

Note that the TTL is displayed in seconds, so if you see a value of “3600”, it means a TTL of 1 hour, “1800” means 30 minutes, and so on.

It’s important to note that the TTL value displayed by dig is the TTL specified in the authoritative DNS server for the domain. However, the TTL may also be cached by intermediate DNS resolvers, so the actual TTL observed by DNS resolvers may be different due to caching. Additionally, some DNS servers may enforce a minimum TTL, so even if a domain’s authoritative DNS server specifies a lower TTL, the actual TTL observed by DNS resolvers may be higher.

Updated on April 11, 2023

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