[LPIC-2]Domain Name Server 14

Performing a DNS lookup with dig results in this answer:

;; QUESTION SECTION:
:5.123.168.192.in-addr.arpa.          IN    PTR

;; ANSWER SECTION:
5.123.168.192.in-addr.arpa.    600    IN    PTR    linuserv.example.net.123.168.192.in-addr.arpa.

;; AUTHORITY SECTION:
123.168.192.in-addr.arpa.      600    IN    NS     linuserv.example.net.

;; ADDITIONAL SECTION:
linuserv.example.net.          600    IN    A      192.168.123.5

What might be wrong in the zone definition?

A. There is no . after linuserv.example.net in the PTR record in the forward lookup zone file
B. There is no . after linuserv in the PTR record in the forward lookup zone file
C. There is no . after linuserv.example.net in the PTR record in the reverse lookup zone file
D. The . in the NS definition in the reverse lookup zone has to be removed

題解

PTR類型的DNS資源記錄，可以指定IP所對應的網域名稱，用於反解(reverse lookup)。

DNS資源記錄的值，如果要填入網域名稱的話，最好都使用完整網域名稱(FQDN, Fully Qualified Domain Name)，也就是將一般的網域名稱在結尾處，加上「.」，以示結尾，否則就會造成題目所說的結果。原本PTR記錄查出來應是「linuserv.example.net.」，但因為在「zone」區塊中的DNS資源記錄設定沒有用完整網域名稱填寫好，而是寫成了「linuserv.example.net」，所以就會被DNS當成是「linuserv.example.net.@」。

這題答案是選項「C」。