$TTL 300
@ IN SOA serverb.backend.lab.example.com. root.serverb.backend.lab.example.com. (
                    2020041805    ;serial number
                    1H            ;refresh secondary
                    5M            ;retry refresh
                    1W            ;expire zone
                    1M )          ;cache time-to-live for negative answers

; owner                   TTL   CL  type    RDATA
                          600   IN  NS      serverb.backend.lab.example.com.

10.0.168.192.IN-ADDR.ARPA.      IN  PTR     servera.backend.lab.example.com.
11                              IN  PTR     serverb.backend.lab.example.com.
12                              IN  PTR     serverc.backend.lab.example.com.
13                              IN  PTR     serverd.backend.lab.example.com.