Hi there!
I'm new at LQ, and I asking for help already ....

I have installed and configured Debian 8.5 at a VPS (DigitalOcean). Then my 1st task was install and configure PostFix. Once I have finished to do this, I was analysing an smtp service error, when I suddenly lost root access to the server, receiving the following message: "authentication failed. remaining authentication methods 'publickey password'".
Although I have many years administering Unix/Linux servers, this is the first time I lose root access to a Linux platform (and my 1st time also using a third party VPS).

What am I supposed to do in order to recover root access...? (from outside the OS!!!)

Thanks in advance for you help!
Best Regards...

Lets clarify. You logged in as user, did su - and failed? Or you set up key-based root login and it is failing now?

Hi, Emerson.
Not using other user than root, and I didn't set up key-based root login (that I'm aware of ... )
In this scenario, I start my next working day not being able to login as root, so I have no control over the OS ... ;(
Thanks for your quick replay!

You should not log in as root with password, it is possible the safe configuration was activated for you, which is key-based login for root.

Ok, I will leave the investigation about why this could happen for later, and I will focus on the key-based login method so I can recover access to the OS. So... is it this possible from outside the OS?

No. If password-based authentication is not allowed for root and you did not create a user then you are locked out.

1 members found this post helpful.

Emerson, is what I was affraid of... never happened to me to lose a Server in more than 20 years!
It's my first experience with Debian, but I don't really know what happened so I cannot blame the distribution and I can't even blame myself since the only configuration I was working on was the PostFix.
Time to start the deployment of Debian all over again from scratch.

Thanks for your time and help.
Long life to Linux! (and the Unix Spirit!)
Alec.

By the way... your Mark Twain's cite is just great!

In your situation, does rescue mode do anything?

I am also working on something similar.

--added on edit.
Expart install??

Hi Kaz!
I'm brand new to VPS services... this was my first experience in deploying a Linux Server there, so the root login issue took me by surprise, not having a bkp image yet and without knowing how to replace the physical procedures we have when the servers are right with us... like the rescue mode.
I have to ask DigitalOcean if they have anticipated options like a "virtual rescue mode" or something like that... :{
So, long story short... I couldn't go for the rescue mode and, as far as I know, I had no option ... I did a fresh deploying of the OS (the server is not in production yet and the only lost job was the PostFix configuration).

Kaz, about Expert Install, not for the recovering need but for the deploying, I'm using the One-Click Linux Images offered by DigitalOcean, Debian 8.5 in this case, and then I customize as needed.

I am not familiar with VPS, nor with DigitalOcean. Thus I may be wrong.

With keyboard-based login on VPS, try send "shift" key at boot to guest debian virtual machine. It may invoke grub2 screen. On grub2 screen, you may be able to select and boot recovery (rescue) mode.