There are two ways to ensure accessibility of a .de domain (i.e. to "connect" the domain):

1. If the domain is delegated, the applicable Resource Records (RRs) are stored on two or more of the name servers of the domain concerned (strictly speaking one should already use the term "zone" here). The .de domain servers refer requests for a delegated domain to those servers. The name servers and zones must meet a variety of technical requirements. In the .de zone, only the referring NS records and DNSSEC key material, if any, are available for delegated zones
2. Alternatively, the data can be administered directly in the .de zone. When this option is chosen, it is not necessary to provide own name servers. However, some restrictions will apply to the publishable Resource Records:

• The maximum number of Resource Records allowed to be entered directly in the .de zone for each .de domain is limited to five. (Minimum: one RR)
• Only type A (IPv4 addresses), AAAA (IPv6 addresses), and type MX (Mailserver / Mail eXchange) records are supported.
• These records are subjected to further technical checks.

The following prerequisites must be met by NSentry records

The syntax of each NSentry must be structured as follows:

de-example.de. IN A <IPv4 address>

subdomain.de-example.de. IN A <IPv4 address>

de-example.de. IN AAAA <IPv6 address>

subdomain.de-example.de. IN AAAA <IPv6 address>

de-example.de. IN  MX <preference> <host name of mail server>

*de-example.de. IN MX <preference> <host name of mail server>

For address records (IN A and IN AAAA) the following rules applies: You can use any sub-domain at any depth, provided it meets the requirements of a host name (as defined in RFC 952 / RFC 1123). You are not allowed to use any wildcards indicated by "*". The IP addresses reserved according to RFC 6890 are not permitted. However, for the purpose of load balancing, you can state the same NSentries with several different IP addresses.

For mail exchange records (IN MX) the following rules applies: You can use any number between "0" and "999" as preference value; "0" will denominate the highest and "999" the lowest preference. However, you are not allowed to use wildcards for MX records. The name of the mail server stated in the MX record must meet the requirements for valid host names (as defined in RFC 952 / RFC 1123). Please note, that resolution of the mail server must be possible in the public DNS.

An appropriate address record must be on hand for the mail server. If the mail server is recorded in the name of the requested domain, the corresponding address record must be included in the domain request.

Example:
de-example.de. IN MX 10 mailer.de-example.de.

mailer.de-example.de. IN A 192.0.2.42

You will find more detailed information in the documentation DENIC-23p.