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ACL HOWTO


Copyright


Copyright (c)  2004  Michael Roessler.
Permission is granted to copy, distribute and/or modify this document under
the terms of the GNU Free Documentation License, Version 1.2
or any later version published by the Free Software Foundation;
with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.
A copy of the license is included in the section entitled 
"GNU Free Documentation License".

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THIS DOCUMENT IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS "AS IS" AND ANY
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Introduction


Using Access Control Lists (ACL) enables the systemadministrator to set up a fine
grained security policy, with possibilities far beyond the traditional permission
model. This helps to avoid nonobvious group setups to grant access to files and
directiories.

Requisites


  • OS supports ACL
    • Red Hat Enterprise Linux 3 (RHEL 3) or higher required
  • Filesystem supports ACL
    • EXT3
    • XFS
  • ACL support is enabled
    • Replace defaults with acl in /etc/fstab for each partition that should be used with ACL

Background information


 

 

The traditional permission model consists of the classes: owner, group and other. Each class may have read (r), write (w) and execute (x) permissions for a file or directory. For example:

-rw-r--r--    1 peter  users           0 Jul  6 16:38 example1

indicates that the owner of the file is peter with read+write permissions, the owning group is users with read permissions and that other users have read permissions.

 

The equivalent ACL is called minimal ACL, this ACL represents the standard file permissions. ACLs with additional permission, regardsless if these permissions belong to Named Users or Named Groups are called extended ACL. An ACL consists of a set of entries shown in the table below:

 

 

Entry

used in

shown as

Owner

minimal + extended

user::rwx

Named user

extended

user:name:rwx

Owning Group

minimal+extended

group::rwx

Named Group

extended

group:name:rwx

Mask

extended

mask::rwx

Other

minimal+extended

other::rwx

 

An extended ACL may contain any number of Named Users and Named Group entries.
Lets have a look at some examples:

In all examples below we are using the following users and group

User

Primary group

Secondary group

peter

users

none

paul

users

none

mary

sales

none

 

staff

 

 

 A standard file permission without ACL settings

rw-r--r-- 1 peter users 0 Jul 6 16:38 example1

The minimal ACL of this file

# file: example1
# owner: peter
# group: users
user::rw-
group::r--
other::r--

A file with extended ACL as shown by ls -l

rw-r--r--+ 1 peter users 0 Jul 6 16:38 example2

The extended ACL of this file

# file: example2
# owner: peter
# group: users
user::rw-
group::r--
group:staff:r--
mask::r--
other::r--

All entries for Named Users and Named Groups are assigned to the group class. The group class already holds the permissions of the Owning Group entry, with this additional entries which may contain different permissions, the meaning of the group class permissions is redefined. The group class shows the upper bound of permissions that any entry in the group class will grant now.
If we grant rw access to the file example2 for user mary, this leads into the following output:

-rw-rw-r--+   1 peter  users           0 Jul  6 16:38 example2

Compared to the example above we come to a conclusion: In minimal ACLs the group class permissions are identical to the Owning Group permissions. In extended ACLs the group class may contain entries for additional Named Users or Named Groups. A Mask entry is used to translate the extended ACL entries to standard file permissions for programs that are unaware of ACL.

The ACL has an additional entry for user mary now and the mask entry has changed to rw-.

# file: example2 
# owner: peter
# group: users
user::rw-
user:mary:rw-
group::r--
group:staff:r--
mask::rw-
other::r--

The mask is used in the same way as a subnet mask in ip adresses. Setting the mask to r-- results in a loss of write access for user mary.

# file: example2 
# owner: peter
# group: users
user::rw-
user:mary:rw- #effective:r--
group::r--
group:staff:r--
mask::r--
other::---

If an application changes the permissions of owner, group or other e.g. by using chmod the ACL entries will be changed as well. For operations on the group class the mask entry will be changed in a way that it reflects the desired filemode. E.g. running chmod g-rw example2 results in:

-rw-------+   1 peter  users           0 Jul  6 16:38 example2 
# file: example2
# owner: peter
# group: users
user::rw-
user:mary:rw- #effective:---
group::r-- #effective:---
group:staff:r-- #effective:---
mask::---
other::---

 

All ACLs shown so far are so called access ACL, beneath the access ACLs there are default ACLs which can only be associated to directories. The default ACLs will be inherited by subdirectories and files below this directory. This is quite similar to the SGID bit for directories. Default ACLs for files doesn't make any sense, as files can't bequest anything.

 

ACL commands


The system administrator only needs two commands to set ACLs or get information about the settings of a file or directory.

  • setfacl
    • set file access control lists
    • call it with
      • setfacl [-bkndRLP] [{-m|-x} acl_spec] [{-M|-X} acl_file] file ...
      • setfacl --restore=file
  • getfacl
    • get file access control lists
    • call it with
      • getfacl [-dRLP] file ...
      • getfacl [-dRLP] -

The getfacl command

The command getfacl is used to gather information about the ACLs of a file or directory. There are several options that can be provided on commandline. The most important options are:

    -d Display the default access control list
    -R List the ACLs of all files and directories recursively
    -L Logical walk, follow symbolic links.
    -P Physical walk, skip all symbolic links.

For more options have a look at the manpage.

 

Examples

getfacl example2

# file: example2 
# owner: peter
# group: users
user::rw-
user:mary:rw- #effective:---
group::r-- #effective:---
group:staff:r-- #effective:---
mask::---
other::---

Some may prefer another form of output by using getfacl --tab --abs example2

# file: example2 
USER peter rw-
user mary RW-
GROUP users R--
group staff R--
mask ---
other ---

In this output uppercase letters represent permissions set but not granted because of the mask setting. The uppercase USER and GROUP are the owner and owning group.
Lets have a look at the default ACLs of a directory and a file created below this directory:

getfacl exampledir/ 
# file: exampledir
# owner: peter
# group: users
user::rwx
group::r-x
other::---
default:user::rwx
default:user:mary:rwx
default:group::r-x
default:group:staff:r-x
default:mask::rwx
default:other::---

A shorter form is:

getfacl -d exampledir 
# file: exampledir
# owner: peter
# group: users user::rwx user:mary:rwx group::r-x group:staff:r-x mask::rwx other::---

A file created below exampledir with a simple touch example3 shows this ACLs:

# file: exampledir/example3 
# owner: peter
# group: users
user::rw-
user:mary:rwx #effective:rw-
group::r-x #effective:r--
group:staff:r-x #effective:r--
mask::rw-
other::---

As expected the user mary has read and write access and the group staff has read access only.

 

Attention: adding a named user with execute rights will manipulate the mask entry to rwx. This will enable all named users and named groups to execute the file.

The setfacl command

The command setfacl is used to manipulate the ACLs of a file or directory. There are several options that can be provided on commandline. The most important options are:

    -d All modifications reference to the default ACLs
    -m modify the ACLs of a file or directory
    -R List the ACLs of all files and directories recursively
    -L Logical walk, follow symbolic links.
    -P Physical walk, skip all symbolic links.
    -k Delete the default ACLs
    -b Delete all ACLs (extended and default).
    -x Remove ACLs
    --restore Read ACLs out of a file and restore them.

For more options have a look at the manpage.

 

Examples

Setting up a default ACL

mkdir newdir 
setfacl -d -m user:mary:rwx newdir
setfacl -d -m group:staff:r-x newdir

Or shorter as setfacl -d -m user:mary:rwx,group:staff:r-x newdir sets the default ACLs for the directory newdir:

getfacl newdir 
# file: newdir
# owner: peter
# group: users
user::rwx
group::r-x
other::---
default:user::rwx
default:user:mary:rwx
default:group::r-x
default:group:staff:r-x
default:mask::rwx
default:other::---

At this point newly created subdirectories or files below newdir will inherit the permissions. But they won't be accesseable until the ACLs allow access to newdir for the specific users. The next step will set this ACLs:

setfacl -m user:mary:rwx,group:staff:r-x newdir 
getfacl newdir
# file: newdir
# owner: peter
# group: users
user::rwx
user:mary:rwx
group::r-x
group:staff:r-x
mask::rwx
other::---
default:user::rwx
default:user:mary:rwx
default:group::r-x
default:group:staff:r-x
default:mask::rwx
default:other::---

To remove the access for a user who is member of the owning group users, e.g. paul, simply use:

setfacl -m user:paul:--- newdir/ 
getfacl --tab --abs newdir
# file: newdir
USER peter rwx rwx
user paul ---
user mary rwx rwx
GROUP users r-x r-x
group staff r-x r-x
mask rwx rwx
other --- ---

Now the user paul get a permission denied error when trying to access the directory newdir. Without ACLs this a a very tricky situation, where you have to create a new group that holds everybody from the groups users except of the users paul, staff and  the user mary.
The directory must be owned by this group and every user needs to relogin after being added to the new group.
The next scenario will make it nearly unable to handle without ACLs: chmod o+rx newdir

getfacl --tab --abs newdir/ 
# file: newdir
USER peter rwx rwx
user paul ---
user mary rwx rwx
GROUP users r-x r-x
group staff r-x r-x
mask rwx rwx
other r-x ---

The user paul still has no access to the directory. Have a lot of fun setting up this scenario without ACLs.

 

Lets give the access rights back to the user paul: setfacl -x user:paul: newdir will do the job. Now all rights of the user paul are restored.

Restrictions


Currently there are some limitations with ACLs. This limitations are not Linux specific, indeed every Posix ACL implementation on every other Unix platform may suffer from them.

    Not all applications are aware of ACLs
      This problem is partly solved by the owning group mapping described above
      The tar command doesn't save the ACLs
      The sshd has problems with some extended ACLs in combination with public key access
    Older versions of NFS are unable to handle ACLs correctly
      Due to caching NFSv2 clients may get unauthorized read access to files. The option nfs_permission_mode=1 for the kernel nfsd solves this problem.
    NFS doesn't support getfacl and setfacl
      getfacl only gives back the minimal ACLs
      setfacl fails with an error message "Operation not supported"

Workarounds


To solve some of the restrictions mentioned above workarounds are already in place.

    tar problem:
      Use star instead of tar if possible. The available options are nearly the same and ACLs as well as Extended Attributes (EA) are saved and restored.
        The option -acl must be specified to backup ACLs
        The header type -Hexustar must be specified to backup ACLs
      star -cz -Hexustar -acl -f=acltest.tar.gz exampledir 
      star -xz -acl -f=acltest.tar.gz
      Save the ACLs into a plain text file that can be saved together with the data using every backup program you prefer.
      getfacl -R --skip-base . > backup.acl 
      setfacl --restore=backup.acl
      After the restore process all ACLs can be restored with setfacl.
      Use a script to set the desired ACLs. This script can be executed after each restore.
    ssh problem:
      Either set StrictModes no in /etc/ssh/sshd_config (not recommended)
      Or use a transfer dir like /var/spool/transfer/<USER> instead of delivering data into the home directory of the user.

Further information


You can find further information about ACL in the arcticle published by Andreas Gr├╝nbacher: POSIX Access Control Lists on Linux

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License will not have their licenses terminated so long as such
parties remain in full compliance.


10. FUTURE REVISIONS OF THIS LICENSE

The Free Software Foundation may publish new, revised versions
of the GNU Free Documentation License from time to time. Such new
versions will be similar in spirit to the present version, but may
differ in detail to address new problems or concerns. See
www.gnu.org/copyleft/.

Each version of the License is given a distinguishing version number.
If the Document specifies that a particular numbered version of this
License "or any later version" applies to it, you have the option of
following the terms and conditions either of that specified version or
of any later version that has been published (not as a draft) by the
Free Software Foundation. If the Document does not specify a version
number of this License, you may choose any version ever published (not
as a draft) by the Free Software Foundation.


ADDENDUM: How to use this License for your documents

To use this License in a document you have written, include a copy of
the License in the document and put the following copyright and
license notices just after the title page:

Copyright (c) YEAR YOUR NAME.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.2
or any later version published by the Free Software Foundation;
with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.
A copy of the license is included in the section entitled "GNU
Free Documentation License".

If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts,
replace the "with...Texts." line with this:

with the Invariant Sections being LIST THEIR TITLES, with the
Front-Cover Texts being LIST, and with the Back-Cover Texts being LIST.

If you have Invariant Sections without Cover Texts, or some other
combination of the three, merge those two alternatives to suit the
situation.

If your document contains nontrivial examples of program code, we
recommend releasing these examples in parallel under your choice of
free software license, such as the GNU General Public License,
to permit their use in free software.
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Last update:25.03.2009 11:22