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Ask HN: How did the internet discover my subdomain?

287 points govideo | 1 comments | 06 Mar 25 22:34 UTC | HN request time: 0.21s | source

I have a domain that is not live. As expected, loading the domain returns: Error 1016.

However...I have a subdomain with a not obvious name, like: userfileupload.sampledomain.com

This subdomain IS LIVE but has NOT been publicized/posted anywhere. It's a custom URL for authenticated users to upload media with presigned url to my Cloudflare r2 bucket.

I am using CloudFlare for my DNS.

How did the internet find my subdomain? Some sample user agents are: "Expanse, a Palo Alto Networks company, searches across the global IPv4 space multiple times per day to identify customers' presences on the Internet. If you would like to be excluded from our scans, please send IP addresses/domains to: scaninfo@paloaltonetworks.com", "Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10_7; en-us) AppleWebKit/534.20.8 (KHTML, like Gecko) Version/5.1 Safari/534.20.8", "Mozilla/5.0 (Linux; Android 9; Redmi Note 5 Pro) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/76.0.3809.89 Mobile Safari/537.36",

The bots are GET requests which are failing, as designed, but I'm wondering how the bots even knew the subdomain existed?!

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BLKNSLVR ◴[07 Mar 25 10:26 UTC] No.43288985[source]▶
There are a number of companies, not just Palo Alto Networks, that perform various different scales of scans of the entire IPv4 space, some of them perform these scans multiple times per day.

I setup a set of scripts to log all "uninvited activity" to a couple of my systems, from which I discovered a whole bunch of these scanner "security" companies. Personally, I treat them all as malicious.

There are also services that track Newly Registered Domains (NRDs).

Tangentially:

NRD lists are useful for DNS block lists since a large number of NRDs are used for short term scam sites.

My little, very amateur, project to block them can be found here: https://github.com/UninvitedActivity/UninvitedActivity

Edited to add: Direct link to the list of scanner IP addresses (although hasn't been updated in 8 months - crikey, I've been busy longer than I thought): https://github.com/UninvitedActivity/UninvitedActivity/blob/...

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mr_mitm ◴[07 Mar 25 10:49 UTC] No.43289105[source]▶
Getting the domain name from the IP address is not trivial, though. In fact, it should be impossible, if the name really hasn't been published (barring guessing attempts), so OP's question stands.
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okasaki ◴[07 Mar 25 11:44 UTC] No.43289396[source]▶

    $ host 209.216.230.207
    207.230.216.209.in-addr.arpa domain name pointer news.ycombinator.com.
replies(3): >>43289544 #>>43289769 #>>43290839 #
1. dspillett ◴[07 Mar 25 12:53 UTC] No.43289769[source]▶
That is when there is an explicit PTR record, for instance one of my assigned addresses can be named that way due to:

    74.231.187.81.in-addr.arpa. 3600 IN PTR ns2.nogoodnamesareleft.com.
in the zone file for that IPv4, but unless they've explicitly configured, or are using a hosting service that does it without asking, this it won't be what is happening.

It isn't practical to do a reverse lookup from “normal” name-to-address records like

    ns2.nogoodnamesareleft.com. IN A 81.187.231.74
(it is possible to build a partial reverse mapping by collecting a huge number of DNS query results, but not really practical unless you are someone like Google or Cloudflare running a popular resolution service)