IPv4 rDNS

Note: This example assumes your address is 192.168.1.1 which isn't a valid internet address. Your rDNS should use your actual internet address, so replace 1.1.168.192 with the reverse of your internet address.

In /var/nsd/etc/nsd.conf:

zone:
        name: "1.1.168.192.in-addr.arpa"
        zonefile: "master/1.1.168.192.in-addr.arpa"

(:ifend:)

In /var/nsd/zones/master/1.1.168.192.in-addr.arpa:

$ORIGIN 1.1.168.192.in-addr.arpa.
@              3600          IN         SOA          ns1.username.example.com. admin.username.example.com. (
                                        2021072300   ; serial YYYYMMDDnn
                                        3600        ; refresh
                                        3600         ; retry
                                        86400       ; expire
                                        3600 )      ; minimum TTL
               3600          IN          NS       ns1.username.example.com.
               3600          IN          NS       ns2.username.example.com.
@              3600          IN          PTR      username.example.com.