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CHAPTER 5 CONFIGURING YOUR SERVER FOR SECURITY
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3. Next, use su to become root. Then use groupadd to create a new group (for example, use groupadd -g 901 sales to create a group with the name sales and group ID 901). Next, as root, use usermod -g 901 youruser to make youruser (the user you used in step 1) a member of that group. After changing this group information, use exit to close the su session and become the normal user account again. 4. As the normal user, use groups again to get an overview of all groups you are currently a member of. The new group should appear now, probably as the last group in the list. 5. Now, as the normal user, use newgrp yournewgroup to set the primary group to your new group on a temporary basis. You can use the groups command to check this; the new group should now be listed first. You ll also see that, if you create a new file (use touch somenewfile), the new group will be group owner of the new file. This ensures that all users who are members of the same group can do the same thing to this file.
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Until now, I ve covered just the three basic Linux permissions. But there are more. To start with, Linux has a set of advanced permissions, and this section describes the working of these permissions. Before diving into detail, the following list provides a short overview of the advanced permissions and the way they re used: SUID: If this permission is applied to an executable file, (also known as Set User ID and setuid ), the user who executes that file will have the permissions of the owner of the file while he is executing it. You can see, then, that SUID is a very dangerous permission that, if wrongly applied, creates serious backdoors on your server. On the other hand, some applications /usr/bin/passwd, for example can t function without it because these applications need the permissions of their owner root to be able to do their job. SGID: This permission is also known as the Set Group ID (also commonly known as setgid ) permission. It is applied in two ways. First, if applied to executable files, the user who executes the file will get the permissions of the group who is owner of the file upon execution. Next, the permission can be used to set the default group owner of files created in a given directory. If applied on a directory, all files and directories created in this directory and even in its subdirectories, will get the group owner of the directory as its group owner. Imagine all members of the group sales normally save the files they create in /data/salesfiles. In that case, you would want all files created in that directory to be owned by the group sales as well. This goal can be accomplished when setting sales as the group owner for salesfiles and next applying the SGID permission bit to this directory. Sticky bit: If the sticky bit is used, users can remove files only if one of the following conditions is met: The user has write permissions on the file. The file is in a directory of which the user is owner. The user is owner of the file.
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CHAPTER 5 CONFIGURING YOUR SERVER FOR SECURITY
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The sticky bit permission is especially useful in a shared data directory. Imagine user linda creates a file in the directory /data/sales. She wouldn t want her coworkers from the group sales who also have write permissions in that directory to remove her file by accident (normally they d be able to because they have the write permission on the directory). If the sticky bit is applied, however, other users can remove the file only if one of those listed conditions has been met. Some comments on these permissions may be helpful. First, you should realize the dangers of the SUID and SGID permissions if they are applied improperly. Imagine, for example, that a given application has a security issue that allows users with the right knowledge to access a shell environment and, at the same time, the user root owns this application. This would make the user misusing the security issue root and give him permissions on your entire system! So you should be extremely careful when applying SUID or SGID to files. On the other hand, you may notice that some files have SUID set by default. For example, the program file /usr/bin/passwd cannot work without it. This is because a user who changes his password needs to write information to the /etc/shadow file. Only the user root can write data to this file, and normal users cannot even read its contents. The operating system solves this problem by applying the SUID permission, which temporarily grants users root permissions to change their passwords.
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Someone using a back door to get access to your server may use SUID on some obscure file to get Tip
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access the next time as well. As an administrator, you should regularly check your server for any occurrence of the SUID permission on unexpected files. You can do this by running find / -perm +4000, which will display all files that have the SUID permissions set.
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The Set Group ID (SGID) permission has a dangerous side because it gives the user who runs a command that has this permission the same permissions as the group owner of the command. However, the SGID permission can be very useful. You may apply it on directories where members of some user group need to share data with each other. The advantage of the SGID permission, if it is applied to a directory, is that all files created in that directory will get the same group owner. This allows all members of that group to read the file. Even if the group just has read rights on files that are created in this directory, the SGID permission may create a workable situation by allowing a user who is a member of the group to read the original file. Without the write permission she cannot change its contents, but she can save the file with a new name in the same directory. With the SGID permission applied to the directory, all files created in the complete tree structure under this directory will get the same group as owner, so the file will always be accessible for all members of the group. Thus, all users can work together in a shared group-data directory in an efficient way. However, in the scenario I ve just described, there is still the problem that one user can remove the files created by another user who is a member of the same group: both have write permissions in the directory, and that s enough to remove files. This can be prevented by applying the sticky bit as well. When this permission is set, a user cannot remove a file if he has only write permissions to the directory the file is in, without being the owner of the file.
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