Why does su fail with "authentication error"?

Question

I tried to su to root so I could install lights, but I get an authentication error when I try:

user@host:~$ su
Password: 
su: Authentication failure

Are you following a guide? In Ubuntu the root account is disabled and you may [need sudo instead](http://askubuntu.com/q/6676/169736) — Braiam, Apr 11 '14 at 18:22
using su as mentioed by bogdan in the above comment worked for me.(I upvoted the comment) — ambassallo, Jan 04 '21 at 15:32

Seth · Answer 1 · 2014-04-13T02:19:05.293

241

The root account is disabled by default in Ubuntu, so there is no root password, that's why su fails with an authentication error.

Use sudo to become root:

sudo -i

edited Apr 13 '14 at 02:19

answered Apr 11 '14 at 18:18

Seth

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thanks. I thoguht this error message is caused by wrong password – verystrongjoe Feb 08 '17 at 00:56
2

in my case, setting the SUID bit solved the issue: `sudo chmod +s /bin/su` – CybeX May 08 '17 at 01:19
What is `-i` and what's the difference with `-s`? @Seth – Shayan Jun 18 '19 at 16:40
not working for me. – Sarfaraj Sipai Oct 24 '19 at 05:39
@SarfarajSipai Can you elaborate more on what your situation is? What are you trying to do and why are you trying to do it? Thanks :) – Seth Oct 24 '19 at 06:22
@Seth while print this "sudo -i" i got this "sudo: error in /etc/sudo.conf, line 0 while loading plugin "sudoers_policy" sudo: /usr/lib/sudo/sudoers.so must be only be writable by owner sudo: fatal error, unable to load plugins" – Sarfaraj Sipai Oct 24 '19 at 06:32
@SarfarajSipai That's a much bigger problem than we can solve here. Please [ask] a new question providing all the details you can. Thanks! – Seth Oct 26 '19 at 18:13
This is really helpful, thanks – ndotie Feb 18 '22 at 06:23

e.thompsy · Answer 2 · 2014-04-29T16:59:32.570

22

If su doesn't work, I do this (in bash):

user@host:~$ sudo bash
root@host:~# su
root@host:/home/user#

Voila! You are now root!

A shortcut for this would be sudo su. In this case given that you are a member of /etc/sudoers with all privileges, then you would only need your user's password.

edited Apr 29 '14 at 16:59

answered Apr 11 '14 at 19:19

e.thompsy

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After `sudo bash` you are already root... – edwin Apr 11 '14 at 19:43
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@edwin After `sudo bash` you are running bash as root, but '~' still points to /home/user or wherever your user's home directory is. So you are not *quite* root. A shortcut for this would be `sudo su` – e.thompsy Apr 21 '14 at 12:59
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For all intends and purposes, you are already root... What's happening is that `sudo` is preserving some environment variables. Instead of `sudo bash` it's better to just use `sudo -i`. – edwin Apr 21 '14 at 16:50
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@edwin In some cases these preserved environment variables matter a lot. So I would argue for *most* intents and purposes you are absolutely right. However, the OP has asked specifically about su to root. I was assuming they knew why they want to do that and that they have a good reason to do it. So I was adding an alternate path to becoming root to the discussion. And here is yet another way: `sudo -i` then `su`. But I would *totally* agree that in most cases just using sudo should be fine. That is what I usually do. Unless I **need** root. Then I use `su`. – e.thompsy Apr 23 '14 at 16:31
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`sudo -i` already is enough. Seriously, you just `sudo su` or `sudo -i`, this is enough to become root (no need to "su" again)... – edwin Apr 23 '14 at 17:00
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@edwin: Ok man. You are right. Have an awesome day! – e.thompsy Apr 29 '14 at 16:56
up voted for maintaining a positive attitude – Freedom_Ben Aug 04 '16 at 01:19
sudo -i is correct, and use ctrl+d if you need to logout of the root account. – john Jan 17 '20 at 13:27

score 12 · Answer 3 · edited Dec 09 '17 at 13:32

You are getting Authentication failure because you are trying to become root which is disabled by default in all versions of Ubuntu. This can be easily circumvented in two ways:

Enabling the root account. This can be achieved by setting up a password.
Instead of su use sudo -i or better yet, append to any command sudo in the way of:
```
sudo apt-get update
[sudo] password for braiam:
```

I wouldn't recommend enabling root, since it could raise a security concern, for example, if you use any service exposed to the web.

score 11 · Answer 4 · answered Apr 29 '14 at 17:04

11

Open root with sudo -s and when it's in this mode type:

passwd

Then, choose password. This password will be for su command.

answered Apr 29 '14 at 17:04

aastefanov

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score 4 · Answer 5 · answered Apr 11 '14 at 18:18

4

Use sudo your_command in place of su.
ie

sudo apt-get install "program to install"

answered Apr 11 '14 at 18:18

girardengo

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Why does su fail with "authentication error"?

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