“su: Authentication failure” – in Docker

The Problem

In some situations, a normal user within a Docker container cannot run ‘su’ command to switch user. When ‘su’ command is issued, the following error returns.

$ su -
Password: [entering correct password]
su: Authentication failure

The Solution

The sticky permission may be missing in /usr/bin/su within the container. With root privilege, you may fix as follows:

1. First check the current permissions for the /usr/bin/su binary file.

# ls -l /usr/bin/su
-rwxr-xr-x 1 root root 32208 Mar 14 01:39 /usr/bin/su

As we can see in the output above, the sticky bit permissions are missing.

2. Add the sticky bit permissions the the /usr/bin/su file as follows:

# chmod u+s /usr/bin/su

3. Verify the permissions again and check for “x” flag at the end of permission field.

# ls -l /usr/bin/su
-rwsr-xr-x 1 root root 32208 Mar 14 01:39 /usr/bin/su

4. Try doing su again inside docker container.

$ su - postgres
Password:
Last login: Tue Aug 6 12:13:57 JST 2019 on pts/1
postgres@[hostname] $