spf.gwdg.de has no NS records.
NS records for this zone are defined at gwdg.de.
NS records for this zone are defined at gwdg.de.
Question and response
Question
dig spf.gwdg.de. NS
Answer
The response has no answer records.
Authority
gwdg.de. 60 SOA dns1.gwdg.de. hostmaster.gwdg.de. 679234888 21600 3600 2419200 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0