IT Learning | Server | SOA record

Everything might be not exactly correct in this article but it’s very useful for beginners to understand IT terms. If you want to learn IT but you don’t have any experience to work in the IT industry, I wish it helps you to understand IT and you like to study IT more.

Let’s start our three minutes lesson!

What is SOA record?

SOA record is written in a zone file.
SOA record shows management information about Zone.
SOA record is Start of Authority record abbreviated as SOA record.
This article is not about “Service-Oriented Architecture”.

SOA is a type of resource record in the Domain Name System(DNS). It contains administrative information about the zone, especially regarding zone transfers. If you don’t know the following words, you better check the following articles before this one.

DNS: IT Learning | Network | DNS | Japan Teams

Secondary name server(DNS): IT Learning | Server | Secondary DNS | Japan Teams

Authoritative name server: IT Learning | Server | Authoritative name server | Japan Teams

Zone file: IT Learning | Server | Zone file | Japan Teams

1. SOA record is one of information in a zone file

There are two DNS servers. One is Caching DNS and another is an Authoritative name server. Usually, computers(such as your laptops, servers) ask Caching DNS to find IP addresses of domain names. If Mr Caching DNS doesn’t know the answer, he asks Ms Authoritative DNS the question.

**Mr Caching asks Ms Authoritative the question.**

When Ms Authoritative receives the question, she checks her zone file at first.

**Ms Authoritative checks her zone file at first.**

There is a lot of information in her zone file, and the SOA record is one of that information. And the SOA record tells her which zone she manages. In short, the SOA shows her Zone information.

2. How to write SOA record?

I think I better show you how to write an SOA record.

Let’s check items one by one!

TTL

TTL is the number of seconds for the period of caching domain information. It’s usually 86,400 seconds(One day).

TTLs also occur in the Domain Name System (DNS), where they are set by an authoritative name server for a particular resource record. When a caching (recursive) nameserver queries the authoritative nameserver for a resource record, it will cache that record for the time (in seconds) specified by the TTL.
Time to live – Wikipedia

@

This is a written domain name, but it’s usually written only @. This is kind of a rule, so just remember that you write @ when you make a zone file.

IN SOA

They are called class. After “IN”, it is written, “what kind of DNS record”. It’s a SOA record this time, so SOA (Start Of Authority) is written.

MNAME

MNAME is the Primary master name server for this zone. It means a kind of introduction like “My name is XXXX, and I use this zone file.”.

RNAME

RNAME is the Email address of the administrator responsible for this zone. This is a kind of emergency contact. When any troubles occur, this email address is used. It’s “japan.teams.meat.org.” in this sample. You think this is not email address format, No! Because the email address is encoded as a name. @ becomes “.” and “.” will be “\” when “.” is already used. For example,

big.meat@example.com → big\.meat.example.com

SERIAL

Serial is the Serial number for this zone. This is a kind of version number. The biggest number is usually the latest. The format is free, but it should be clear. So YYYYMMDDnn is usually used in Japan. I’m explaining the detail later.

If a secondary name server slaved to this one observes an increase in this number, the slave will assume that the zone has been updated and initiate a zone transfer.
SOA record – Wikipedia

Retry

Retry is the number of seconds for which the secondary name server refreshes zone information. After secondary name servers transfer zone and this refresh time passed, secondary name servers query the master for the SOA record to check if zone information is updated.

Expire

Expire is the number of seconds after which secondary name servers should stop answering requests for this zone if the master does not respond. After this expire time, secondary name servers give up querying the primary server.

TTL, a.k.a. MINIMUM

TTL, a.k.a. MINIMUM is time to live, and it’s for purposes of negative caching. DNS servers use the cheat sheet to answer the question quickly. That cheat sheet is called caching, and “Negative caching” is ” I don’t know.” answer in a cheat sheet.

3. More explanation about Serial number

Here are more details about the Serial number. Usually, people prepare the secondary DNS to design DNS. That secondary DNS server is a substitute for the primary DNS server. When some troubles occur to the primary server, the secondary DNS works instead of the primary DNS.

Mr Secondary should provide the same answers to Ms. primary’s ones, so they should have the same zone file.

**Contents of their zone files should be the same.**

Sometimes a zone file is updated by humans, but Mr. secondary is afraid to ask humans to update his zone file. Because he is just the second. So Mr Secondary regularly asks Ms. primary if she has a new zone file which humans updated. That timing is defined by “Refresh”. And if there are any updates, Mr Secondary updates his zone file copied Ms primary’s one. But he doesn’t update his file if there are no updates. Yes, he uses “Serial Number” to check if there are any updates on Ms Primary’s zone file.

**Mr Secondary copies Ms Primary’s zone file if there are updates.**

So humans must update “Serial Number” on Ms Primary’s zone file when they update her file. Otherwise, Mr Secondary will not update his file. So there are no formats on “Serial Number” but it should be simple and it should be clear not to forget to update “Serial Number”. Thus date(YYYY-MM-DD) and numbers(NN) are used. If you update 25th of July, 2021 and this is the second time,