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Let s examine the results, after creating a file called data.txt, after setting the umask to 000:
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$ touch data.txt $ ls -l total 4 -rw-rw-rw1 root
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Everyone now has full access permissions. However, you are more likely to set a umask such as 022, which would give new files the permissions 755 (777 022 = 755). This would give the file owner read, write, and execute access, but only read permissions for group members and other users:
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If you now create a new file called newdata.txt with the new umask, you should see that the default permissions have changed:
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File System Access Control
$ touch newtest.txt $ ls -l total 4 -rw-r--r-1 root -rwxrwxrwx 1 root
root users
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If you re more conservative and don t want to grant any access permissions to other users (including group members), you can set the umask to 077, which still gives the file owner full access permissions:
bash-2.03$ umask 077
Let s see what happens when you create a new file called lastminute.txt:
bash-2.03$ touch lastminute.txt bash-2.03$ ls -l total 4 -rw-r--r-1 root root -rw------1 root root -rwxrwxrwx 1 root users
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The new file has full access permissions for the owner, but no access permissions for other users. Resetting the umask does not affect the permissions of other files that have already been created.
setUID and setGID Permissions
The file permissions we ve covered so far are used by users in their day-to-day filemanagement strategies. However, administrators can use a different set of file permissions that allows files to be executed as a particular user (setUID) and/or as a member of a particular group (setGID). These facilities are very powerful, because they allow unprivileged users to gain access to limited superuser privileges in many cases, without requiring superuser authentication. For example, the volume daemon (vold) allows unprivileged users logged into the console to mount and unmount CD-ROMs and floppy disks, an operation that required superuser privileges in previous Solaris releases. Here, the effective user ID is set to 0, meaning that unprivileged users can effectively run processes as root. The downside to this is obvious: setGID and setUID permissions open a Pandora s box in terms of security, because normal authentication procedures are bypassed. For example, imagine a device management tool that needed to run as setUID 0 in order to read and write device files. If the tool had a standard feature of many UNIX programs, the ability to spawn a shell, the shell spawned would have full root privileges, rather than the privileges of the original user. For this reason, some administrators refuse to
Part III:
Security
allow setGID and setUID permissions to be set. The find command, for example, can be used to scan all local file systems and show files with setUID or setGID privileges:
# find / -local -type f \( -perm -4000 -o -perm -2000 \) -print
You can determine whether a file is setUID by root by first checking for files that are owned by root and then checking whether those files have the s flag assigned to the user s permissions. For example, if a file-management tool called filetool were setUID root, the following directory listing would clearly indicate this property:
-r-sr-sr-x 3 root sys 1220334 Jul 18 11:01 /usr/local/bin/filetool
The first s in the permissions table refers to setUID root. In addition, this file is also setGID for the sys group, which is indicated by the second s in the permissions table. The setUID bit can be set by using a command like this
# chmod u+s file.txt
where file.txt is the file that requires setUID to be set. The setGID bit can be set by using a command like this
# chmod g+s file.txt
where file.txt is the file that requires setGID to be set. Setting chmod o+s has no impact on the file.
Sticky Bit Permissions
A network administrator once explained to me that sticky bits were those bits that slowed down network transmission rates, because they were highly attracted to magnetic qualities of the Ethernet. This is not true! A sticky bit is a special permission that prevents files in common file areas from being deleted by other users. For example, a download area consisting of a large, 10GB partition may be set aside for user downloads, which are not counted against individual user quotas. This means that users could download up to 10GB of data without infringing on their allocated directory space. However, although a shared public file area sounds like a great idea, it would be unwise to allow users to overwrite one another s files. In this case, the sticky bit can be set on the top-level directory of the public file area, allowing only users who created individual files to delete them. You can set the sticky bit by using a command like this
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