DNS records consist primarily of several units of information placed in different fields. In addition to information such as the name of a specific domain’s mail server, other details can also be transferred in this way:

  • <name>: The first field contains the domain name.
  • <ttl>: Time to live specifies how long the information is guaranteed to be valid.
  • <class>: The class field specifies the type of network.
  • <type>: The DNS record type in this case is MX.
  • <priority>: The smaller the value, the higher the mail server’s priority.
  • <rdata>: Resource data defines the mail server name.

Domain names are always specified in the Fully Qualified Domain Names (FQDNs) format in all record types. This means that the content of each field is ostensibly completed with a period. Actually, the FQDN maps the complete path to the root server – from the smallest unit located to the far left, to the largest situated to the far right. The highest level is unnamed however, which is why there is nothing after the preceding period.

The time to live specifies how long the requestor can hold and use the information in their cache. This is intended to ensure that no outdated records are used. Should the time lapse, the client must again request the record from the name server. The TTL is specified in seconds. This field frequently does not appear in the actual record. Instead, the time for the complete zone is defined at the beginning of the zone file.

The class field is optional, as nowadays only the internet is possible. When resource records were first introduced however, both the Hesiod (HS) and Chaos (CH) classes were possible. However, both networks are today no longer in operation. For this reason, the field remains either empty or invariably contains the IN abbreviation.

The field relating to priority is a special feature of MX records. Many administrators – in particular those of larger projects – have several mail servers running in parallel. As a result, failures and load peaks can be compensated and email communication doesn’t run the risk of sudden failure. In doing so, however, distribution is not necessarily supposed to occur evenly. For this reason, priorities can be assigned. The server with the lowest value has the highest priority and is selected first. If it isn’t available, then the server with the next-highest value is chosen.

The data field finally contains the mail server name. This is also specified in the FQDN format. In the record, all fields follow one another within a line. The record isn’t ended using a specific symbol; a line break is sufficient.

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