There was some specific cyber security challenge (Cyber Cube 2018, GE). In one of the tasks, objective was to gain the access to some specific file that server included.

After successfully solving some steps, there was a base64 encoded string:


Decoding this string gave following output:

'name: eternalromance, protocol: dns(53/tcp)\n'

I figured that the only way to access the server was to perform a DNS attack.

Creators of this challenge gave a hint that choosing TCP port over UDP for DNS may cause certain vulnerabilities.

I'm not experienced in domain name systems, but I know that generally DNS uses UDP port 53 to serve requests - DNS queries require single UDP datagram request and require single UDP datagram response. Main reasons to choose TCP protocol is to respond with data that is larger than 512 bytes (although some DNS implementations can allow UDP datagram responses up to 4096 bytes by my knowledge), and for DNS zone transfers.

But what vulnerabilities may DNS have when TCP is used over UDP?

Thank you.

  • TCP is not used "over" UDP, but "instead of" UDP in some cases. Is this what you meant?
    – schroeder
    Commented May 8, 2018 at 13:23
  • You are making a lot of assumptions. Did you perform a DNS zone transfer to see what you would get? Why do you assume that there must be vulnerabilities? Why could it not have been more clues?
    – schroeder
    Commented May 8, 2018 at 13:44
  • @schroeder Yes, I meant instead of UDP (apologies for my English). No I didn't perform DNS zone transfer but I've read that information from wikipedia (I'm assuming that TCP is used for DNS zone transfer in most of the cases). Also, I didn't understand that either, creator of this challenge told me that if DNS exists on TCP port, there are some vulnerabilities, there were no other clues.
    – ShellRox
    Commented May 8, 2018 at 14:40

1 Answer 1


Although it's impossible to know for sure, one option would have been AFXR.


What does it do?

AFXR would give you a list of all of the records on the server.

Why does TCP affect it?

RFC 5936, section 4 says:

AXFR sessions are currently restricted to TCP by Section 4.3.5 of RFC 1034, which states:

Because accuracy is essential, TCP or some other reliable protocol
must be used for AXFR requests.

The restriction to TCP is also mentioned in Section of "Requirements for Internet Hosts - Application and Support" [RFC1123].


How could it have been used?

These records could have contained either a flag, or clues to the next step.

  • The OP mentions zone transfers as a use for TCP.
    – schroeder
    Commented May 8, 2018 at 13:46
  • I think it is the most likely way that the challenge was meant to be completed
    – jrtapsell
    Commented May 8, 2018 at 14:34
  • but then this answer adds nothing to what is not already in the question
    – schroeder
    Commented May 8, 2018 at 14:36
  • I think you are right, another hint was to view DNS records, although I was unable to do that in proper time.
    – ShellRox
    Commented May 8, 2018 at 14:41

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