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Reading and understanding file permissions

Limiting file permissions to block access to sensitive information

Setting user permissions with a browser

Keeping private documents out of the hands of snoops is easy with well-assigned permissions and good system administration practices.

In this technique, we explain how to use file permissions to limit access to sensitive documents and dangerous programs on a Linux machine. You can set the permissions either at the command line or with a graphical interface. If you have access to a graphical environment (either Konqueror or Nautilus will work), it’s definitely the friendlier way to go.

Reading and Understanding File Permissions

Knowing how to read and use file permissions is important for maintaining privacy in your file system. Limiting a user’s access is simple when you understand the permissions.

Every file and directory in a Linux computer is owned by one specific user and one specific group. When you try to access a file, you’re classified into one of three categories:

Owner: You’re the owner if your effective user ID is the same as the file’s owner.

Group member: You’re a member of the group if your effective group ID is the same as the file’s group.

Other: You’re an “other” if you’re not the owner and not a member of the group.

After you’ve been categorized, Linux looks at the permissions assigned to your category. For example, if you’re accessing a file owned by user freddie and you’re logged in as user freddie, Linux examines the owner permissions for that file (because you are the owner). If you’re not logged in as user freddie but you are a member of the group that owns the file,

Linux examines the group permissions for the file. If you’re not the owner and you’re not a member of the file’s group, Linux examines the other permissions.

Next, Linux compares the permissions (owner permissions, group permissions, or other permissions) with the type of access you’re attempting. If you are the file’s owner and are trying to read the file, Linux checks the read bit in the owner permissions. If you’re trying to modify the file, Linux checks the write bit on the owner permissions.

You can control three types of access for each user category:

Read permissions grant (or deny) the right to read a file or to list the contents of a directory.

Write permissions control the right to change the contents of the file or directory.

Execute permissions control the right to run a program or script. If you’re accessing a directory, the execute permissions control whether you can access the files within the directory.

To display the permissions for an entire directory, enter ls -l at the command line. The -l flag forces ls to display permissions (and the file’s owner and group) along with the name of each file (see Figure 27-1).

The listing shows seven groups of information. From right to left, they are as follows:

File permissions

Link count

Name of the user that owns the file

Name of the group that owns the file

File size (in bytes)

Date and time the file was last modified

File (or directory) name

The file permissions are displayed as an odd collection of ten characters, with dashes replacing the privileges that are denied to that user category.

For the sake of dissection, we’ve chosen the following listing as an example:

drwxr-x--- 4 freddie acctg 65595 Jan 20 14:14 tables

The first letter in the permissions column doesn’t have anything to do with permissions. Instead, it tells you what kind of file you’re dealing with. The most common entries are d for directory or - for file. In our example, the d at the beginning of the listing indicates that it is a directory.

The next three-letter grouping tells you the access permissions for the file’s owner (user freddie, in this example). The letters rwx stand for read, write, and execute. freddie has full permissions to this directory.

The next three-letter grouping tells you the group privileges. In our example, r-x stands for read and execute. The acctg group isn’t allowed to change files in the directory.

The last three-letter group tells you the file permissions for any other user (not the owner and not a

member of the file’s group). In our example, the ---

means that anyone who isn’t a member of the acctg group or isn’t the file owner is denied all access to the file.

Controlling Permissions at

the Command Line

If you have access to a desktop environment, maintaining permissions is easy and quick with a browser. If you need to set permissions over an

SSH connection or for a dedicated server (without a desktop environment), you can use the command line to change ownership and file permissions.

See Table 27-1 for details on changing ownership permissions.

Use the chmod command to change the permissions for users, groups, or others. Table 27-2 explains how. The chmod command also works with the + (plus) and - (minus) signs to turn permissions on and off. Substitute the plus or minus sign for the equal sign in the command, and the permissions listed in the argument are turned on or off. A plus sign turns the listed permission on, and a minus sign turns it off.

TABLE 27-1: CHANGING OWNERSHIP PERMISSIONS

For example, the following command turns on read permissions for the user without changing any of the other permissions:

$ chmod u+r filename

This command turns off write permissions for a group:

$chmod g-w filename

This command turns off all permissions for nongroup users:

$ chmod o-rwx filename

You can see a pattern starting to form.

If you need to, you can use the command line to set privileges, but there is a friendlier way, as described in the next section.

If you really want to test the spin in your propeller, you can set the permissions with octal numbers. The process is a little cryptic, but if you’re inclined to try it, check out the info chmod page and follow the links to Note File Permissions Numeric Modes.

Changing File Permissions from a Desktop

Sometimes the command line is your only choice, but if you have access to a graphical interface, there’s a friendlier way to manage file permissions: the Konqueror browser.

Nautilus offers the same functionality to GNOME users. Double-click the Start Here icon on the desktop and surf along.

To view and modify file permissions with the Konqueror browser, follow these steps:

1. Click the Start Here or Home icon on your KDE desktop or taskbar.

2. When Konqueror opens, locate the file you want to modify.

You can navigate to the file you want to modify in one quick step by entering file:/pathname in the Location field to step directly into the directory that contains your file.

3. After you’ve located the file you want to work with, right-click the file and choose Properties from the pop-up menu.

The Properties dialog opens, as shown in Figure 27-2.

•Figure 27-2: The Properties dialog.

4.Click the Permissions tab.

The Permissions dialog opens showing the current permissions for the file (see Figure 27-3). The permissions dialog is easy to interpret and change.

If the check boxes and fields displayed on the window are disabled, you don’t have enough privileges to edit the permissions for that file. If you need superuser privileges to change permissions for a file or directory, open a terminal window and use the su command to gain

superuser privileges. Then type konqueror in the command line and press Enter to start a copy of the browser with your newly enhanced privileges.

•Figure 27-3: The Permissions dialog.

5.To change read, write, or execute privileges for the file or directory, simply check the boxes next to User, Group, or Others.

If a box is checked, the user (or group) has that permission. If the box is empty, that permission is denied.

Before you modify the check boxes in the Special column, make sure that you know what you’re doing; read the sidebar, “What makes a check box Special?” for details.

6. To change ownership, enter the new user or group owner in the appropriate field.

Nautilus users: Use the drop-down list boxes to choose from existing users and groups when you change the ownership of a file or directory.

7. After you make the necessary changes to the

Permissions dialog, click OK to save your changes and close the dialog.

What makes a check box Special?

Under the column header labeled Special, you see check boxes next to the labels Set UID, Set GID, and Sticky. These check boxes merit special consideration.

Set UID: When a Set UID bit is turned on for a program, your EUID (effective user ID) becomes the same as the program’s owner when you run the program. You also gain all the privileges of the program’s owner. In other words, if user freddie runs a Set UID program owned by user root, freddie has superuser privileges while that program is running.

Set UID affects programs only; it’s really not important for data files and directories. If you find a Set UID program, read the WARNING!

Warning: Set UID programs are potentially dangerous. Users can gain extra privileges by using files that grant them superuser privileges. See Technique 57 for information about preventing security breaches uncovered by the setuid and setgid bits.

Set GID: The Set GID bit works similarly to Set UID — anyone running the program gains all the privileges of the group.

When the Set GID box is checked for a directory, the files in that directory belong to that group regardless of who puts the files in the directory.

Sticky: Checking the Sticky box affects directories only. If this box is checked, you can’t remove (or rename) a file in that directory unless you’re the file owner.

Save Time By

Using kgpg in your desktop environment

Using gpg at the command line

Encrypting and signing e-mail with Evolution

Adding Enigmail security to Mozilla

This technique is all about privacy. Keeping private data private can save you a lot of headaches. Public-key cryptography provides a quick and easy way to safeguard e-mail messages and sensitive files.

Public-key cryptography involves two big numbers: a public key and a private key. When you create a key pair, you keep the private key to yourself and share the public key with anyone you wish. The numbers in a key pair are related in a fiendishly clever way: Data that you encrypt with the public key can be decrypted only with the private key, and data encrypted with the private key can be decrypted only with the public key (private decrypts public, public decrypts private).

If you encrypt a message with your friend’s public key, only your friend can decrypt it (because your friend has never shared his private key with anyone, even with you). Encrypt a message with your private key and send it to your friend: If he uses your public key to decrypt the message (and the result looks meaningful), he’ll know the message came from you. (Nobody else could have sent the message because it’s encrypted with your private key, and no one else knows your private key.) You can combine these techniques to encrypt and sign a message.

In this technique, we show you how to use gpg (the GNU Privacy Guard) at the command line for those cases where you need security but don’t have access to a desktop environment. You can use gpg to encrypt, decrypt, and sign e-mail messages. gpg can also encrypt documents that you don’t intend to share with others, so snoops won’t be able to read anything you need to keep private. Encrypt the original document, delete the unencrypted version, and only you can decrypt it again to read it.

kgpg (KGpg) is a graphical interface for gpg that runs in the KDE Desktop environment. It’s a great tool that packs lots of functionality into a userfriendly package. In this technique, we show you how to download, install, and use kgpg. We also show you how to create a gpg key pair (from the command line and with kgpg). After you’ve created a key pair, you can distribute your public key to your friends, or you can upload the public key to a server — a number of public-key registries are available on the Web.