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If I register with a dyn DNS service and am allowed to select my own prefix, how secret is the URL?

For example: I register with a dyn DNS service and ask for my prefix to be 204e9800998ecf8427e and the service gives me the URL 204e9800998ecf8427e.some-dyn-dns.org. I configure my home router to tell the service its IP and from then on the URL points to my router.

Given I don't tell anyone about the URL, who will be able to find my router (without using brute force on either an IP or DNS level)? Can the name be looked up in a registry? Can the DNS server of the service be probed to reveal the URL? Does it depend on the service?

  • No DNS entry anywhere is secret and is meant to be public. – schroeder Sep 29 '18 at 19:57
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It's not designed to be secure. It's designed to be convenient. A well configured DNS server should not reveal all names it controls, but all it takes is a minor misconfiguration somewhere.

Anyway, the entire IPv4 space is scanned regularly by various actors, including shodan.io. Do not assume any privacy for anything that is visible at a public IPv4 address. There's tools that will map out the entire IPv4 space in hours. Some people scan for their own purposes, benign or not - and others like shodan.io put it online in a searchable form for anyone (willing to pay).

In short; if you rely on your host name to be secret, you are doing something wrong. It should not be a concern.

  • That's a good answer. Since this was a thought experiment and I'm not trying to do anything with it, the "a well configured DNS server should not reveal" part was the part I'm after. The stuff about the whole IPV4 internet being scannable within minutes is interesting too, though. +1 – felher Sep 30 '18 at 14:59

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