Permissions
This chapter contains details about basic file security through
file ownership and file permissions.
file ownership
==============
user owner and group owner
The users and groups of a system can be locally managed in
/etc/passwd/etc/passwd and /etc/group/etc/group, or they can be in a
NIS, LDAP, or Samba domain. These users and groups can own files.
Actually, every file has a user ownerfile ownership and a
group owner, as can be seen in the following screenshot.
paul@rhel65:~/owners$ ls -lh
total 636K
-rw-r--r--. 1 paul snooker 1.1K Apr 8 18:47 data.odt
-rw-r--r--. 1 paul paul 626K Apr 8 18:46 file1
-rw-r--r--. 1 root tennis 185 Apr 8 18:46 file2
-rw-rw-r--. 1 root root 0 Apr 8 18:47 stuff.txt
paul@rhel65:~/owners$
User paul owns three files; file1 has paul as user owner and has the
group paul as group owner, data.odt is group owned by the group
snooker, file2 by the group tennis.
The last file is called stuff.txt and is owned by the root user and the root group.
listing user accounts
You can use the following command to list all local user accounts.
paul@debian7~$ cut -d: -f1 /etc/passwd | column
root ntp sam bert naomi
daemon mysql tom rino matthias2
bin paul wouter antonio bram
sys maarten robrecht simon fabrice
sync kevin bilal sven chimene
games yuri dimitri wouter2 messagebus
man william ahmed tarik roger
lp yves dylan jan frank
mail kris robin ian toon
news hamid matthias ivan rinus
uucp vladimir ben azeddine eddy
proxy abiy mike eric bram2
www-data david kevin2 kamel keith
backup chahid kenzo ischa jesse
list stef aaron bart frederick
irc joeri lorenzo omer hans
gnats glenn jens kurt dries
nobody yannick ruben steve steve2
libuuid christof jelle constantin tomas
Debian-exim george stefaan sam2 johan
statd joost marc bjorn tom2
sshd arno thomas ronald
chgrp
You can change the group owner of a file using the chgrpchgrp(1)
command.
root@rhel65:/home/paul/owners# ls -l file2
-rw-r--r--. 1 root tennis 185 Apr 8 18:46 file2
root@rhel65:/home/paul/owners# chgrp snooker file2
root@rhel65:/home/paul/owners# ls -l file2
-rw-r--r--. 1 root snooker 185 Apr 8 18:46 file2
root@rhel65:/home/paul/owners#
chown
The user owner of a file can be changed with chownchown(1) command.
root@laika:/home/paul# ls -l FileForPaul
-rw-r--r-- 1 root paul 0 2008-08-06 14:11 FileForPaul
root@laika:/home/paul# chown paul FileForPaul
root@laika:/home/paul# ls -l FileForPaul
-rw-r--r-- 1 paul paul 0 2008-08-06 14:11 FileForPaul
You can also use chown to change both the user owner and the group
owner.
root@laika:/home/paul# ls -l FileForPaul
-rw-r--r-- 1 paul paul 0 2008-08-06 14:11 FileForPaul
root@laika:/home/paul# chown root:project42 FileForPaul
root@laika:/home/paul# ls -l FileForPaul
-rw-r--r-- 1 root project42 0 2008-08-06 14:11 FileForPaul
list of special files
When you use ls -lls -l, for each file you can see ten characters
before the user and group owner. The first character tells us the type
of file. Regular files get a -, directories get a d, symbolic links
are shown with an l, pipes get a p, character devices a c, block
devices a b, and sockets an s.
first character file type
\- normal file
d directory
l symbolic link
p named pipe
b block device
c character device
s socket
: Unix special files
Below a screenshot of a character device (the console) and a block device (the hard disk).
paul@debian6lt~$ ls -ld /dev/console /dev/sda
crw------- 1 root root 5, 1 Mar 15 12:45 /dev/console
brw-rw---- 1 root disk 8, 0 Mar 15 12:45 /dev/sda
And here you can see a directory, a regular file and a symbolic link.
paul@debian6lt~$ ls -ld /etc /etc/hosts /etc/motd
drwxr-xr-x 128 root root 12288 Mar 15 18:34 /etc
-rw-r--r-- 1 root root 372 Dec 10 17:36 /etc/hosts
lrwxrwxrwx 1 root root 13 Dec 5 10:36 /etc/motd -> /var/run/motd
permissions
rwx
The nine characters following the file type denote the permissions in
three triplets. A permission can be r for read access, w for write
access, and x for execute. You need the r permission to list (ls)
the contents of a directory. You need the x permission to enter (cd) a
directory. You need the w permission to create files in or remove
files from a directory.
permission on a file on a directory
r (read) read file contents (cat) read directory contents (ls)
w (write) change file contents (vi) create files in (touch)
x (execute) execute the file enter the directory (cd)
: standard Unix file permissions
three sets of rwx
We already know that the output of ls -lls starts with ten characters
for each file. This screenshot shows a regular file (because the first
character is a - ).
paul@RHELv4u4:~/test$ ls -l proc42.bash
-rwxr-xr-- 1 paul proj 984 Feb 6 12:01 proc42.bash
Below is a table describing the function of all ten characters.
position characters function
1 \- this is a regular file
2-4 rwx permissions for the `user owner`
5-7 r-x permissions for the `group owner`
8-10 r\-- permissions for `others`
: Unix file permissions position
When you are the user ownerowner of a file, then the
user owner permissions apply to you. The rest of the permissions have
no influence on your access to the file.
When you belong to the group that is the group owner of a file, then
the group owner permissions apply to you. The rest of the permissions
have no influence on your access to the file.
When you are not the user owner of a file and you do not belong to the
group owner, then the others permissions apply to you. The rest of
the permissions have no influence on your access to the file.
permission examples
Some example combinations on files and directories are seen in this screenshot. The name of the file explains the permissions.
paul@laika:~/perms$ ls -lh
total 12K
drwxr-xr-x 2 paul paul 4.0K 2007-02-07 22:26 AllEnter_UserCreateDelete
-rwxrwxrwx 1 paul paul 0 2007-02-07 22:21 EveryoneFullControl.txt
-r--r----- 1 paul paul 0 2007-02-07 22:21 OnlyOwnersRead.txt
-rwxrwx--- 1 paul paul 0 2007-02-07 22:21 OwnersAll_RestNothing.txt
dr-xr-x--- 2 paul paul 4.0K 2007-02-07 22:25 UserAndGroupEnter
dr-x------ 2 paul paul 4.0K 2007-02-07 22:25 OnlyUserEnter
paul@laika:~/perms$
To summarise, the first rwx triplet represents the permissions for the
user owner. The second triplet corresponds to the group owner; it
specifies permissions for all members of that group. The third triplet
defines permissions for all other users that are not the user owner
and are not a member of the group owner.
setting permissions (chmod)
Permissions can be changed with chmodchmod(1). The first example gives
the user owner execute permissions.
paul@laika:~/perms$ ls -l permissions.txt
-rw-r--r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt
paul@laika:~/perms$ chmod u+x permissions.txt
paul@laika:~/perms$ ls -l permissions.txt
-rwxr--r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt
This example removes the group owners read permission.
paul@laika:~/perms$ chmod g-r permissions.txt
paul@laika:~/perms$ ls -l permissions.txt
-rwx---r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt
This example removes the others read permission.
paul@laika:~/perms$ chmod o-r permissions.txt
paul@laika:~/perms$ ls -l permissions.txt
-rwx------ 1 paul paul 0 2007-02-07 22:34 permissions.txt
This example gives all of them the write permission.
paul@laika:~/perms$ chmod a+w permissions.txt
paul@laika:~/perms$ ls -l permissions.txt
-rwx-w--w- 1 paul paul 0 2007-02-07 22:34 permissions.txt
You don\'t even have to type the a.
paul@laika:~/perms$ chmod +x permissions.txt
paul@laika:~/perms$ ls -l permissions.txt
-rwx-wx-wx 1 paul paul 0 2007-02-07 22:34 permissions.txt
You can also set explicit permissions.
paul@laika:~/perms$ chmod u=rw permissions.txt
paul@laika:~/perms$ ls -l permissions.txt
-rw--wx-wx 1 paul paul 0 2007-02-07 22:34 permissions.txt
Feel free to make any kind of combination.
paul@laika:~/perms$ chmod u=rw,g=rw,o=r permissions.txt
paul@laika:~/perms$ ls -l permissions.txt
-rw-rw-r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt
Even fishy combinations are accepted by chmod.
paul@laika:~/perms$ chmod u=rwx,ug+rw,o=r permissions.txt
paul@laika:~/perms$ ls -l permissions.txt
-rwxrw-r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt
setting octal permissions
Most Unix administrators will use the old schooloctal permissions
octal system to talk about and set permissions. Look at the triplet
bitwise, equating r to 4, w to 2, and x to 1.
binary octal permission
000 0 \-\--
001 1 \--x
010 2 -w-
011 3 -wx
100 4 r\--
101 5 r-x
110 6 rw-
111 7 rwx
: Octal permissions
This makes 777777 equal to rwxrwxrwx and by the same logic, 654 mean
rw-r-xr-- . The chmodchmod command will accept these numbers.
paul@laika:~/perms$ chmod 777 permissions.txt
paul@laika:~/perms$ ls -l permissions.txt
-rwxrwxrwx 1 paul paul 0 2007-02-07 22:34 permissions.txt
paul@laika:~/perms$ chmod 664 permissions.txt
paul@laika:~/perms$ ls -l permissions.txt
-rw-rw-r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt
paul@laika:~/perms$ chmod 750 permissions.txt
paul@laika:~/perms$ ls -l permissions.txt
-rwxr-x--- 1 paul paul 0 2007-02-07 22:34 permissions.txt
umask
When creating a file or directory, a set of default permissions are
applied. These default permissions are determined by the
umaskumask(1). The umask specifies permissions that you do not want
set on by default. You can display the umask with the umask command.
[Harry@RHEL4b ~]$ umask
0002
[Harry@RHEL4b ~]$ touch test
[Harry@RHEL4b ~]$ ls -l test
-rw-rw-r-- 1 Harry Harry 0 Jul 24 06:03 test
[Harry@RHEL4b ~]$
As you can also see, the file is also not executable by default. This is
a general security feature among Unixes; newly created files are never
executable by default. You have to explicitly do a chmod +xchmod +x to
make a file executable. This also means that the 1 bit in the umask
has no meaning--a umask of 0022 is the same as 0033.
mkdir -m
When creating directories with mkdirmkdir(1) you can use the -m
option to set the mode. This screenshot explains.
paul@debian5~$ mkdir -m 700 MyDir
paul@debian5~$ mkdir -m 777 Public
paul@debian5~$ ls -dl MyDir/ Public/
drwx------ 2 paul paul 4096 2011-10-16 19:16 MyDir/
drwxrwxrwx 2 paul paul 4096 2011-10-16 19:16 Public/
cp -p
To preserve permissions and time stamps from source files, use cp -p.
paul@laika:~/perms$ cp file* cp
paul@laika:~/perms$ cp -p file* cpp
paul@laika:~/perms$ ll *
-rwx------ 1 paul paul 0 2008-08-25 13:26 file33
-rwxr-x--- 1 paul paul 0 2008-08-25 13:26 file42
cp:
total 0
-rwx------ 1 paul paul 0 2008-08-25 13:34 file33
-rwxr-x--- 1 paul paul 0 2008-08-25 13:34 file42
cpp:
total 0
-rwx------ 1 paul paul 0 2008-08-25 13:26 file33
-rwxr-x--- 1 paul paul 0 2008-08-25 13:26 file42
practice: standard file permissions
1. As normal user, create a directory ~/permissions. Create a file owned by yourself in there.
2. Copy a file owned by root from /etc/ to your permissions dir, who owns this file now ?
3. As root, create a file in the users ~/permissions directory.
4. As normal user, look at who owns this file created by root.
5. Change the ownership of all files in ~/permissions to yourself.
6. Make sure you have all rights to these files, and others can only read.
7. With chmod, is 770 the same as rwxrwx--- ?
8. With chmod, is 664 the same as r-xr-xr-- ?
9. With chmod, is 400 the same as r-------- ?
10. With chmod, is 734 the same as rwxr-xr-- ?
11a. Display the umask in octal and in symbolic form.
11b. Set the umask to 077, but use the symbolic format to set it. Verify that this works.
12. Create a file as root, give only read to others. Can a normal user read this file ? Test writing to this file with vi.
13a. Create a file as normal user, give only read to others. Can another normal user read this file ? Test writing to this file with vi.
13b. Can root read this file ? Can root write to this file with vi ?
14. Create a directory that belongs to a group, where every member of that group can read and write to files, and create files. Make sure that people can only delete their own files. solution: standard file permissions ===================================
1. As normal user, create a directory ~/permissions. Create a file owned by yourself in there.
mkdir ~/permissions ; touch ~/permissions/myfile.txt
2. Copy a file owned by root from /etc/ to your permissions dir, who owns this file now ?
cp /etc/hosts ~/permissions/
The copy is owned by you.
3. As root, create a file in the users ~/permissions directory.
(become root)# touch /home/username/permissions/rootfile
4. As normal user, look at who owns this file created by root.
ls -l ~/permissions
The file created by root is owned by root.
5. Change the ownership of all files in ~/permissions to yourself.
chown user ~/permissions/*
You cannot become owner of the file that belongs to root.
6. Make sure you have all rights to these files, and others can only read.
chmod 644 (on files)
chmod 755 (on directories)
7. With chmod, is 770 the same as rwxrwx--- ?
yes
8. With chmod, is 664 the same as r-xr-xr-- ?
No
9. With chmod, is 400 the same as r-------- ?
yes
10. With chmod, is 734 the same as rwxr-xr-- ?
no
11a. Display the umask in octal and in symbolic form.
umask ; umask -S
11b. Set the umask to 077, but use the symbolic format to set it. Verify that this works.
umask -S u=rwx,go=
12. Create a file as root, give only read to others. Can a normal user read this file ? Test writing to this file with vi.
(become root)
# echo hello > /home/username/root.txt
# chmod 744 /home/username/root.txt
(become user)
vi ~/root.txt
13a. Create a file as normal user, give only read to others. Can another normal user read this file ? Test writing to this file with vi.
echo hello > file ; chmod 744 file
Yes, others can read this file
13b. Can root read this file ? Can root write to this file with vi ?
Yes, root can read and write to this file. Permissions do not apply to root.
14. Create a directory that belongs to a group, where every member of that group can read and write to files, and create files. Make sure that people can only delete their own files.
mkdir /home/project42 ; groupadd project42
chgrp project42 /home/project42 ; chmod 775 /home/project42
You can not yet do the last part of this exercise...