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Introduction

What are ACLs?

Advanced permissions beyond the traditional Unix permissions. It has powerful features such as being able to give permissions to more than one user and more than one group.

ACLs versus Traditional Permissions

ACLs though powerful add additional complexity to the system and do have some limitations discussed further down this articlebelow. You will notice throughout the Bonsai Framework we add ACLs only when absolutely necessary.

Limitations

Support in Utilities - For example, the version of GNU tar packaged with the OS may not back up or restore ACLs.

Standardizing Across Operating Systems - Moving files with ACLs between operating systems that both support ACLs may not workthat I tried and then pretty much gave up on using ACLs. Instead, I am looking at application based virtualization solutions to segregate control.

Limitations

umask - ACLs are only applied generally during create. More specifically, create(), mkdir(), mknod(), mkfifo(), or open(). Other operations will be limited by what the umask of the user performing the operation such as copy or move. (I need to go into more detail here but this is very very limiting and intuitively not the behaviour most people expect). In other words, you apply an ACL to a directory and subdirectory expecting files copied in would inherit those ACLs, well they don't because of the default umask.

Copying Files - This is no-longer an issue with modern (2012 is when I checked) versions of Ubuntu and I would guess other *nix systems. If you want to preserve specific ACL permissions and not inherit, use -p. With an older system, check that when setting default ACLs on a directory, the following commands will inherit permissions properly:  local copy, sftp remote create and sftp remote copy.

Move - Moving a file(s) or folder(s) created outside of an ACL directory into an ACL directory will not result in inheritance of permissions. (even with defaults setif you have specified inheritance in the ACL directory)

Backup - The most used backup command in *nix, tar, does not support ACLs unless modified. Some distributions like redhat have this built into tar. Otherwise your options are to use star or manually backup and restore the ACLs. Options are covered in this article.

Copying Files - This is no-longer an issue with modern (2012 is when I checked) versions of Ubuntu and I would guess other *nix systems. If you want to preserve specific ACL permissions and not inherit, use -p. With an older system, check that when setting default ACLs on a directory, the following commands will inherit permissions properly:  local copy, sftp remote create and sftp remote copy.

SFTP - copy/create will inherit ACLs, but move from outside needs to be tested.

Creating Users and Groups

sudo --gid 3100 wgdailyplanet
sudo --gid 3101 wglexcorp
sudo useradd -d /opt/web/php/ckent -m -g wgdailyplanet -u 4001 -c "Clark Kent" -s /bin/bash ckent
sudo useradd -d /opt/web/php/lluthor -m -g wglexcorp -u 4006 -c "Lex Luthor" -s /bin/bash lluthor

Support in Utilities - For example, the version of GNU tar packaged with the OS may not back up or restore ACLs.

Standardizing Across Operating Systems - Moving files with ACLs between operating systems that both support ACLs may not work.

ACLs and Groups

The most scalable way to use ACLs is to apply groups. A tutorial approach will be used to illustrate the commands.

The scenario is we want to provide website hosting for two different wgdailyplanets who we will start by categorizing into two different groups,

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clients, The Daily Planet and LexCorp. Employees from the respective companies will kept in the system under the following groups, wgdailyplanet and wglexcorp. The web server process also plays a factor and uses the group www-data.

We do not want employees from different companies access or even have awareness of each others web directory. At the same time, the Apache Server running as user www-data belonging to group www-data also needs access to all the directories. We also want to grant users of the staff group read access for support purposes.

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Finally, we want all subsequent directories and files under the respective Web Root Directories to inherit the same permissions.

This is just not possible using standard Unix groups.

Directory Structure and Permissions

These directories start at /opt/

All directories must will be owned by serveradmin:staff

Creating Users and Groups

First create the groups following the standards of the Bonsai Framework,

sudo --gid 4000 wgdailyplanet
sudo --gid 4010 wglexcorp
sudo useradd -d /opt/web/php/dailyplanet01 -m -g wgdailyplanet -u 4000 -c "clark.kent@dailyplanet.com" -s /bin/bash dailyplanet01
sudo useradd -d /opt/web/php/lexcorp01 -m -g wglexcorp -u 4010 -c "lex.luthor@lexcorp.com" -s /bin/bash lexcorp01

Create Directories

web Folder

Start by creating the web folder. We do this in your home directory so you do not have to keep using the sudo command,

sudo mkdir web
#next set the permissions
sudo chmod u+rwX,g+r-w+X,o-rw+Xrwx web
sudo chown -R serveradmin:staff web
#now set www-data for the acl
sudo setfacl -Rm g:www-data:rX ./web/
sudo setfacl -Rm g:staff:rX ./web/

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getfacl web
# file: web
# owner: serveradminrfongyee
# group: staff
user::rwx
group::r-x
group:www-data:r-x
group:staff:r-x
mask::r-x
other::--x

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getfacl --default web
# file: web
# owner: serveradminrfongyee
# group: staff

To apply the permissions to defaults use

getfacl --access ./web/ | sudo setfacl -d -RM - ./web/
chmod o-rw+X web

getfacl --access =  retrieves the ACL the permissions applied to the directory only (default permissions are not returned). The details are then piped to setfacl and the parameters read,

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getfacl --default ./web/
# file: web
# owner: serveradmin
# group: staff
user::rwx
group::r-x
group:www-data:r-x
group:staff:r-x
mask::rwx
other::---x

If you want to see what the applied and default look like dont specify

getfacl ./web/
# file: web
# owner: serveradmin
# group: staff
user::rwx
group::r-x
group:www-data:r-x
group:staff:r-x
mask::rwx
other::---x
default:user::rwx
default:group::r-x
default:group:www-data:r-x
default:group:staff:r-x
default:mask::rwx
default:other::---

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cd web
sudo mkdir php
sudo chown -R serveradmin:staff php
getfacl php
# file: php
# owner: serveradmin
# group: staff
user::rwx
group::r-x
group:www-data:r-x
group:staff:r-x
mask::r-x
other::--x
default:user::rwx
default:group::r-x
default:group:www-data:r-x
default:group:staff:r-x
default:mask::r-x
default:other::--x

The php folder has inherited the permissions from web. If you notice the next set of folders do not use execute for others so rather than setting others permission for each folder simply change the php others permission.

sudo chmod o-rwxrw+X php
#next apply this change to default
getfacl --access ./php/ | sudo setfacl -d -RM - ./php/
#check the permissions
getfacl php
# file: php
# owner: serveradmin
# group: staff
user::rwx
group::r-x
group:www-data:r-x
group:staff:r-x
mask::r-x
other::---x
default:user::rwx
default:group::r-x
default:group:www-data:r-x
default:group:staff:r-x
default:mask::r-x
default:other::---

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cd php
sudo mkdir dailyplanet.com sudo mkdir lexcorp.com sudo mkdir tmp
sudo mkdir logs
#remember serveradmin:staff must own the directory
cd ..
sudo chown -R serveradmin:staff php
#change the php folder permissions back to the way it was
sudo chmod o-rw+X php
tmp logs

Now to add the necessary groups to their respective virtual hosts

cd php
setfacl -Rm g:wgdailyplanet:rwX dailyplanet.com
getfacl --access ./dailyplanet.com/ | sudo setfacl -d -RM - ./dailyplanet.com/
cd dailyplanet.com
mkdir www

Now do the same to lexcorp.com

setfacl -Rm g:wglexcorp:rwX lexcorp.com
getfacl --access ./phplexcorp.com/ | sudo setfacl -d -RM - ./phplexcorp.com/
#checkcd thelexcorp.com
permissions 
getfacl php

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mkdir www

Setting Permissions for tmp and logs

cd php
sudo setfacl -Rm g:wgdailyplanetwww-data:rwX dailyplanettmp
getfacl --access ./dailyplanettmp/ | sudo setfacl -d -RM - ./dailyplanettmp/
cdsetfacl dailyplanet
sudo mkdir www
sudo mkdir blog
cd ..-Rm g:www-data:rwX logs
getfacl --access ./logs/ | sudo setfacl -d -RM - ./logs/

Next move the web folder to the opt directory and make serveradmin:staff own it

cd ~
sudo mv web /opt/
cd /opt/
sudo chown -R serveradmin:staff dailyplanetweb

Repeat the same steps for lexcorp and change the group accordingly,

Change the group ownership and apply acls for lexcorp.com,

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Testing Restrictions

User Lex Luthor has been given access to his directory "lexcorp.com" but learns of the "dailyplanet.com" directory by using his robots to spy on Clark Kent's computer. So Lex terminals in...

cd /opt/web/php/
cd dailyplanet.com
-su: cd: dailyplanet.com: Permission denied

Backup and Restore

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sudo getfacl -R dailyplanet.com/ > ~/dailyplanet.com.acl.bck.txt

It is important to run getfacl with sudo so that getfacl can properly transverse the directories and owner comments or group comments will be retained.

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cd /opt/web/php/
sudo setfacl --restore ./bck.dailyplanet.com.acl.bck.txt

References

Good introduction from the Ubuntu docs - https://help.ubuntu/community/FilePermissionsACLs

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Notes on backup and restoring ACLs using dump file - http://projectenvision/blog/4/Enable-Support-for-ACL-in-Debian-Ubuntu

Good article on masks but I think it might be wrong - http://novell/documentation/suse91/suselinux-adminguide/html/apbs03.html

Looks like another good read on masks in ACLS - http://users.suse.com/~agruen/acl/linux-acls/online/