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Old 06-17-2009, 05:52 AM   #1
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Question User permissions on removable media

Hello. I have been wondering about the issues of user permissions on removable media.

Let's say I have an external HD (ext3 formatted, let's assume) and I create some files/folders with read/write permissions just for myself. Later, I plug that drive in another computer, for example, a lab PC at the university. With no root access, would I be able to read/write those files/folders? The user (id, name...) is not the same.

And another question related to this. Let's say that I have the same drive. If I give it to a friend, can he simply create a user with the same name in his PC and be able to access those files/folders? What I mean is, what is exactly taken into account when checking for permissions in Linux? Just the username? The UID?

I know that in case I don't want anyone to peek into my folders I should take more advanced measures such as encryption, but these questions make me wonder if the whole users/permissions "thing" (forgive me linux wise men makes sense when using removable media.

Thank you in advance.
Old 06-17-2009, 06:14 AM   #2
Registered: Oct 2007
Location: Norway
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The file system keep track of file permissions with the UID. So in order for that to work, I guess you would need to use the same UID at home and at your school. But, if that's a problem, you can always override with permissions. You can set 'others' to rw(x) to give your account at school/work the permission to do what you like. Or you could add a group on your home computer with the same GID as your primary group at school.

So, to summarize;the file system use UID, and not names to identify users. But of course your friend with a GNU/Linux box can make a new user with what ever UID he likes..
Old 06-17-2009, 07:29 AM   #3
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Thanks Dinithion. That is what I thought. As a security measure it is not very effective, permissions can be easily overriden. If I need to use the drive in another computer with a different user I'd simply give read/write permissions to the files. I was simply unsure about how Linux handled the permissions.


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