Ask HN: How did the internet discover my subdomain?

> If it is on DNS, it is discoverable.

In the context of what OP is asking this is not true. DNS zones aren't enumerable - the only way to reliably get the complete contents of the zone is to have the SOA server approve a zone transfer and send the zone file to you. You can ask if a record in that zone exists but as a random user you can't say "hand over all records in this zone". I'd imagine that tools like Cloudflare that need this kind of functionality perform a dictionary search since they get 90% of records when importing a domain but always seem to miss inconspicuously-named ones.

> Even if it were not, the message you pasted says outright that they scan the entire IP space, so they could be hitting your server's IP without having a clue there is a subdomain serving your stuff from it.

This is likely what's happening. If the bot isn't using SNI or sending a host header then they probably found the server by IP. The fact that there's a heretofore unknown DNS record pointing to it is of no consequence. *EDIT: Or the Cert Transparency log as others have mentioned, though this isn't DNS per se. I learn something new every day :o)

In practice it's not so far fetched: A zone transfer is just another dns query at the protocol level, i suppose you can conceptually view it as sending a file if you consider the dns response a file. Something like "host -t axfr my.domain ns1.my.domain" will show the zone depending on how a domain's name server is configured (eg in bind, allow-transfer directive can be used to make it public, require ip acl to match the query source, etc).

No sensible DNS provider has zone transfers enabled by default. OP mentioned using CloudFlare, and they certainly don't.

> In the context of what OP is asking this is not true. DNS zones aren't enumerable - the only way to reliably get the complete contents of the zone is to have the SOA server approve a zone transfer and send the zone file to you.

This is generally true but also if you watch authoritative-only dns server logs for text strings matching ACL rejections, there's plenty of things out there which are fully automated crawlers attempting to do entire zone transfers.

There are a non zero number of improperly configured authoritative dns servers out there on the internet which will happily give away a zone transfer to anyone who asks for it, at least, apparently enough to be useful that somebody wrote crawlers for it. I would guess it's only a few percent of servers that host zonefiles but given the total size of the public Internet, that's still a lot.

In the context of DNSSEC dns zones are very much enumerable. Cloudflare does amazing tricks to avoid this https://blog.cloudflare.com/black-lies/

> in bind, allow-transfer directive

Configuring BIND as an authoritative server for a corporate domain when I was a wee lad is how I learned DNS. It was and still is bad practice to allow zone transfers without auth. If memory serves I locked it down between servers via key pairs.

Cloudflare themselves gives more information here:

> NSEC3 was a “close but no cigar” solution to the problem. While it’s true that it made zone walking harder, it did not make it impossible. Zone walking with NSEC3 is still possible with a dictionary attack.

So, hardening it against enumerability is a question of inserting non-dictionary names.

Zone transfers are super interesting topic. Thanks for mentioning that.

It's basically the way how to get all DNS records a DNS server has. Interestingly in some countries this is illegal and in some this is considered best practice.

Generally, enabled zone transfers is considered as misconfiguration and should be disabled.

We did research on that few months back and found out that 8% of all global name servers have it enabled.[0]

[0] - https://reconwave.com/blog/post/alarming-prevalence-of-zone-...

That's concerning. I thought everyone knows that zone transfers should be generally disallowed, especially when coming from random hosts.