DNS Entries

DNS Syntax Types Explained

An “A” record, which stands for “address” is the most basic type of syntax used in DNS records, indicating the actual IP address of the domain.

The “AAAA” record is an IPV6 address record that maps a hostname to a 128-bit Ipv6 address.  Regular DNS addresses are mapped for 32-bit IPv4 addresses.

The “CNAME” record stands for “canonical name” and serves to make one domain an alias of another domain. CNAME is often used to associate new subdomains with an existing domain’s DNS records.

The “MX” record stands for “mail exchange” and is basically a list of mail exchange servers that are to be used for the domain.

The “PTR” record stands for “pointer record” and maps an Ipv4 address to the CNAME on the host.

The “NS” record stands for “name server” and indicates which Name Server is authoritative for the domain.

An “SOA” record stands for “State of Authority” and is easily one of the most essential DSN records because it stores important information like when the domain was last updated and much more.

An “SRV” record stands for “service” and is used to define a TCP service on which the domain operates.

A “TXT” record lets the administrator insert any text they’d like into the DNS record, and it is often used for denoting facts about the domain.

 

The SOA resource record contains the following information:

Source host – The host where the file was created.

Contact e-mail – The e-mail address of the person responsible for administering the domain’s zone file. Note that a “.” is used instead of an “@” in the e-mail name.

Serial number – The revision number of this zone file. Increment this number each time the zone file is changed. It is important to increment this value each time a change is made, so that the changes will be distributed to any secondary DNS servers.

Refresh Time – The time, in seconds, a secondary DNS server waits before querying the primary DNS server’s SOA record to check for changes. When the refresh time expires, the secondary DNS server requests a copy of the current SOA record from the primary. The primary DNS server complies with this request. The secondary DNS server compares the serial number of the primary DNS server’s current SOA record and the serial number in it’s own SOA record. If they are different, the secondary DNS server will request a zone transfer from the primary DNS server. The default value is 3,600.

Retry time – The time, in seconds, a secondary server waits before retrying a failed zone transfer. Normally, the retry time is less than the refresh time. The default value is 600.

Expire time – The time, in seconds, that a secondary server will keep trying to complete a zone transfer. If this time expires prior to a successful zone transfer, the secondary server will expire its zone file. This means the secondary will stop answering queries, as it considers its data too old to be reliable. The default value is 86,400.

Minimum TTL – The minimum time-to-live value applies to all resource records in the zone file. This value is supplied in query responses to inform other servers how long they should keep the data in cache. The default value is 3,600.

The following is an example of a Microsoft DNS server generated default SOA resource record:

@   IN  SOA     nameserver.place.dom.  postmaster.place.dom. (
                               1            ; serial number
                               3600         ; refresh   [1h]
                               600          ; retry     [10m]
                               86400        ; expire    [1d]
                               3600 )       ; min TTL   [1h]

Parentheses allow the SOA record to wrap to multiple lines.

In the above example:

Source Host = nameserver.place.dom.
Contact Email = postmaster.place.dom.

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