user & group of a file changed after rebooting and login as another user

Question

I am using ubuntu 12.04 in VirtualBox, and I created two accounts with usernames lyg and ygl.

I found a directory VBOXADDITIONS_4.1.18_78361/ in /media, questions got into my mind when I type "ls -l /media/" in my terminal. Below are two outputs:

1. When I boot and login as lyg,

-xr-xr-x 4 lyg lyg 2048 Jun 5 22:33 VBOXADDITIONS_4.1.18_78361

1. Reboot and login as ygl,

-xr-xr-x 4 ygl ygl 2048 Jun 5 22:33 VBOXADDITIONS_4.1.18_78361

So, why do the user and group change?

Answer 1

/media/VBOXADDITIONS_4.1.18_78361 is automatically mounted by your system upon login from a user using udev rules.

What you are seeing is just the system applying those rules. These rules are set by default so that any user can have full permissions to the mounted volumes.