Assume that "host1.mydomain.dom" has an IP address of 145.128.16.8. Which of…

2005

Assume that "host1.mydomain.dom" has an IP address of 145.128.16.8. Which of the following options would be most appropriate as a subsequence of steps in performing the reverse lookup of 145.128.16.8? In the following options "NS" is an abbreviation of "nameserver".

  1. A.

    Query a NS for the root domain and then NS for the "dom" domains

  2. B.

    Directly query a NS for "dom" and then a NS for "mydomain.dom" domains

  3. C.

    Query a NS for in-addr.arpa and then a NS for 128.145.in-addr.arpa domains

  4. D.

    Directly query a NS for 145.in-addr.arpa and then a NS for 128.145.in-addr.arpa domains

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Correct answer: C

Answer: Perform the reverse lookup in the in-addr.arpa namespace and follow delegations for the reversed octets.

Key idea: Reverse DNS for an IPv4 address uses the in-addr.arpa domain with the IP octets reversed. For 145.128.16.8 the full PTR name is 8.16.128.145.in-addr.arpa.

  1. Step 1: Query the nameserver(s) for in-addr.arpa to find the delegation for 145.in-addr.arpa.

  2. Step 2: Query the nameserver(s) for 145.in-addr.arpa to find the delegation for 128.145.in-addr.arpa (or to find the authoritative server if delegated further).

  3. Step 3: Query the nameserver(s) for 128.145.in-addr.arpa (and if necessary for 16.128.145.in-addr.arpa) until you reach the zone that contains the PTR record for 8.16.128.145.in-addr.arpa.

  4. Step 4: Request the PTR record for 8.16.128.145.in-addr.arpa from the authoritative server; the PTR should map to host1.mydomain.dom.

Why other approaches are wrong: Querying forward domains like "dom" or "mydomain.dom" targets the forward DNS namespace and will not locate PTR records. Similarly, skipping the in-addr.arpa start and not following the reversed-octet delegation path may miss the authoritative server that holds the PTR.

Conclusion: Begin at in-addr.arpa and follow delegations for the reversed octets (145, then 128, etc.) until the PTR for 8.16.128.145.in-addr.arpa is obtained.

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