1

I'm familiar with subdomain takeover when the following is the situation:

a.site.com CNAME site.mktoweb.com

If site.mktoweb.com isn't registered then you can create an account on Heroku and try to register the subdomain for yourself.

I'm confused on what to do when the following is the scenario:

b.site.com A 123.456.789.0

I think you need to own site.com in order to register a subdomain like b.site.com, so is subdomain takeover only possible when the subdomain is a CNAME for a site hosted on another service like Heroku?

4

Subdomains cannot be registered.

Domains are registered, and then the designated Nameservers (DNS Servers) are able to control the subdomains.

In many cases, the Registrar is also handling Nameservers, but sometimes the Registrar is instructed to use 3rd party Nameservers.

The Nameserver is able to serve specific DNS records for each subdomain.

In this example:

a.site.com CNAME site.mktoweb.com

The browser will look up site.mktoweb.com; first by looking up the Registration of mktoweb.com in order to determine which Nameservers to communicate with; then that Nameserver will return the IP Address for site.mktoweb.com. (and then the browser will connect to that IP Address to load the site)

In this example:

b.site.com A 123.456.789.0

The browser will simply connect to that IP Address.

I get that you need to own site.com in order to register a subdomain like b.site.com, so is subdomain takeover only possible when the subdomain is a CNAME for a site hosted on another service like Heroku?

  • site.com is registered with a Registar.
  • b.site.com is listed as a DNS entry on the Nameservers.

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