Domain name of additional RR in a response to apex NS query is wrong

[ntoofu]

Description

When I query NS records at the apex of the zone, a RR in the additional section of the response has wrong domain name as follows:

$ dig +norec example.com. ns -p10053 @127.0.0.1

; <<>> DiG 9.16.22 <<>> +norec example.com. ns -p10053 @127.0.0.1
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 61786
;; flags: qr; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 2

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
; COOKIE: 85b08ee266e90b20 (echoed)
;; QUESTION SECTION:
;example.com.           IN  NS

;; ANSWER SECTION:
example.com.        60  IN  NS  ns.test.example.com.

;; ADDITIONAL SECTION:
example.com.        60  IN  A   10.20.30.40

;; Query time: 1 msec
;; SERVER: 127.0.0.1#10053(127.0.0.1)
;; WHEN: Wed Aug 03 22:54:41 JST 2022
;; MSG SIZE  rcvd: 123

The domain name of the RR in the additional section should be ns.test.example.com., but that returned by k8s_gateway plugin is example.com..

Reproduction

1. Create Loadbalancer type service in kubernetes cluster
   • I prepare kubernetes 1.23.4 cluster by using kind
   • Create namespace: kubectl create namespace test
   • Create dummy service: kubectl create service -n test loadbalancer ns --tcp 53:53
   • Add external IP address: kubectl edit service -n test ns --subresource=status
     • Patch status field as follows:

       status: 
       loadBalancer: 
         ingress: 
         - ip: 10.20.30.40

2. Run CoreDNS 1.9.3 built with k8s_gateway plugin
   • CoreDNS config file:

     .:10053 {
     log
     k8s_gateway example.com {
       apex ns.test
       kubeconfig /home/ntoofu/.kube/config
     }
     }

3. Test: dig +norec example.com. ns -p10053 @127.0.0.1