The DNS SOA Minimum TTL value has had a lot of meanings. The old meaning was that the value was used as a default TTL value when the TTL value associated with a record was absent. The new and current meaning (see RFC 2308) is that the value is used as the duration for negative caching. So, if your name server does not answer authoritatively for a domain name, this is the duration, in seconds, for which the recursor (the ISP’s name server) will cache this information before asking your name server again for the DNS records of the domain name.

In this post I will analyze the SOA minimum ttl values of over 2 million domain names, comment on the distribution and then end with some values used by top domain names.

The following density plot shows the distribution of the SOA minimum TTL value across over 2 million domain names.

• The mean of the distribution is 70,000 seconds (20 hours)
• The mode (most common value) is 86400 seconds (24 hours)
• The median (middle value) is 7200 seconds (2 hours)
• 0.07% of domain names use a value of 0 (which is probably not a very bad idea)

The five most common values are as follows:

The following chart shows the distribution of the SOA minimum TTL values used across over 2 million domain names. The five most commonly used values are shown as red dots.

The following table shows the value as used by the top 10 domain names:

As this shows, the maximum value is 7200 seconds (2 hours) used by baidu.com. Most of the top domain names use a very low value which is probably the right thing to do, considering that the value is used for negative caching. As compared to the A record TTL values, it does not hurt to have a very low SOA minimum TTL value. I recommend that you ignore the fact that 86400 seconds is the most commonly used value and use a much lower value. If you have a very high value and due to some technical error, your name server does not respond properly, your site could be down for a long period of time!