How to bind the second-level domains to subdomains (e.g. site2.ru to sub1.site.ru)?

You make such entry:
site2.ru. IN CNAME sub1.site.ru.
And after that, everyone went to site2.ru will get on sub1.site.ru where already you will need to configure a web server or anything else on the reception of these clients.

In order to prescribe something in site.ru you need some authority.
In your case, hostmaster habrahabr.ru may (but need not) make a record of the form
nllm IN A 111.222.333.444
or you delegate a subdomain nllm.habrahabr.ru where you will have to steer yourself.
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Domain nllm.ru you have to register yourself.
And manage it accordingly.
Further, for example, in Apache you need to do is rewrite requests to nllm.habrahabr.ru to queries nllm.ru for example.
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In any case, the subdomains in the zone habrahabr.ru managed the hostmaster for this domain, or are delegated and arbitrariness there can not be.
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Somewhere so...

Or I the question is not correctly understood, or dosage of soda is broken...
A domain name domain.com.
He has a NS (Name Server) which answers all queries for domains and subdomains domain.com if sub.domain.com not installed your NS.
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In order to add a record to the zone domain.com you need to have administrative privileges.
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I do not know exactly how and what users do blogspot.com but I suspect that through some kind of interface they are given the authority to change the a record or CNAME for your user.blogspot.com.
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If you are owner of site.ru then the powers you have.
And you can steer the subdomains sub.site.ru in full.
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How to give the user the opportunity to change their (and only their) binding, you decide.
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But entry site2.ru. IN CNAME sub1.site.ru.
only possible in the area site2.ru,
but no it is not possible in the area site.ru.
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Now you understand why the question of "will Take nllm.habrahabr.ru I want to tie your domain (for example, nllm.ru)" got the answer?

Faced yesterday with the similar problem.
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I have 2 domain site.ru and site.com
I want to request a.site.ru opened what lies on a.site.com
site.com located on a virtual server.
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I prescribe a.site.ru IN CNAME a.site.com
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logically it should work, BUT since site.com located on virtual hosting with recognition at the name and he receives the request show "a.site.ru" — and he does not know this, then the whole scheme does not work.
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not enough entries for the vhost a.site.ru on my virtual hosting. that's the problem, solutions I personally do not see but to ask the provider to prescribe it, but most likely they won't bother with that.