Is PermitRootLogin based on UID or user name?

Question

The man page states that PermitRootLogin

Specifies whether root can log in using ssh(1).

However, it is not clear if this check is based on the user name ("root") or the UID (0).

What happens if the root account is renamed to "admin"? Will "admin" be able to log in when PermitRootLogin=no?

What happens if there are two accounts with UID=0, i.e. "root" and "admin"? Will either of them be able to login?

Answer 1

It seems the check is done on UID (tested on OpenSSH_6.7p1 Debian-5+deb8u3, OpenSSL 1.0.1t 3 May 2016):

Set PermitRootLogin off:

mtak@pdv1:~$ grep PermitRootLogin /etc/ssh/sshd_config
PermitRootLogin no

Make sure a user named admin is created with UID 0:

mtak@pdv1:~$ sudo grep admin /etc/passwd
admin:x:0:0:Root User:/root:/bin/bash

Make sure the user can be used to log on to the system:

mtak@pdv1:~$ su - admin
Password: 
root@pdv1:~# 

Check if we can log on to the system using SSH:

mtak@rubiks:~$ ssh admin@pdv1
admin@pdv1's password: 
Permission denied, please try again.

---

If we turn PermitRootLogin on:

mtak@pdv1:~$ grep PermitRootLogin /etc/ssh/sshd_config
PermitRootLogin yes

And try to log on:

mtak@rubiks:~$ ssh admin@pdv1
admin@pdv1's password: 
Linux pdv1 4.4.8-1-pve #1 SMP Tue May 17 16:14:08 CEST 2016 x86_64
Last login: Wed Aug 24 12:05:28 2016 from xxx
root@pdv1:~# 

Answer 2

I appreciate the approach of @mtak in the other answer, but the answer is obvious even without this trials.

It is based on the UID, as you can see in the source code of openssh:

if (authctxt->pw->pw_uid == 0 &&
            !auth_root_allowed(auth_method))
authenticated = 0;

Also every authentication method shows something like

if (pw->pw_uid == 0 && options.permit_root_login != PERMIT_YES)
    ok = 0;

grep-ing further in the code, you may notice, there is no strcmp('root', pw->pw_name) or some alternative, if it will be enough for you.