TweetFollow Us on Twitter

Access Control Lists

Volume Number: 20 (2004)
Issue Number: 7
Column Tag: Programming

Patch Panel

by John C. Welch

Access Control Lists

A look at one of the features in Tiger that's going to change how Mac Networks are run

Welcome

With all the hype that Tiger is getting for such "ooh..aah" features like multipoint iChat, Dashboard, etc, I thought today we should take a look at one new feature of Tiger that, while not as obvious as such GUI-goodness, will have a near - instant and far - reaching effect on all who use the new version of OS X: The implementation of Access Control Lists, aka, ACLs in Tiger.

Access Con-who Huh?

ACLs are a new way of controlling, well, access to objects such as files and folders on a computer. Mac users currently don't have a traditional ACL implementation that ships with the OS. The closest thing we have now, is the login controls. That's a list of users who can log in to a Mac or Macs, and controls what they can do on a given Mac. So, in a sense, that's an ACL. It's a login ACL.

Unix Permission Basics

However, what most people use ACLs with are file system objects. Note: ACLs are by no means restricted to the file system. They can exist at any level where you have things you wish to control access to. But for this article, we're going to limit our discussion to the file system.

With current versions of Mac OS X and Mac OS X Server, you have traditional three-level Unix permissions. With any object on the file system, there are three levels of access:

  • Owner, the user that owns the object

  • Group, the group that has explicit access to the object. The Owner does not have to be in the Group

  • Everyone else, or every user who's not the Owner, or in the Group, or every group who is not the Group.

There's a fourth access here: root. Root's the super user, it has full access to everything so there's no need to explicitly include root. The Owner is always a user; it can never be a group. (This is a step backwards in some ways from the older AppleShare permissions, which did allow for group ownership of a file.) The Group, (obviously) must always be a group, not a user. Everyone else is just that.

Within this access structure, there are three things you can do to an object:

  • Read, that is, use the contents of the object in a non-modifying manner, i.e. viewing and printing

  • Write, which lets you modify the object and/or its contents, so you can erase the contents of the object, delete the object, etc.

  • Execute, which is what allows you to run a program, list the contents of a directory.

Everything you do with a File System (FS) object in Mac OS X is based on those nine bullets. It's fairly simple, although there are things that will catch you off guard. For example, if you own a file, but that file is in a directory that you don't have write access for, you can erase the contents of that file, but you can't delete the file itself.

Sometimes simple is bad

Now, for a long time, this was all you needed. Access was simple, and could be simply controlled. But in a modern computing environment, these simple permissions fall down. For example, you can have a situation with different groups needing different access to a single directory and its files. The obvious solution is to put each group's files in a different directory, but then you could have some files needing to be seen by both groups. You can put people into different groups, but then you have to create another group with more restrictive permissions. In a large company, you can easily hit the Mac OS X limit of 16 groups per user in this kind of situation.

For example: You have an accounting group with its own directory on a server.

  • The head of accounting needs unrestricted access to every object in that directory, and unrestricted control over the directory.

  • The accounting managers need to be able to modify the contents of a directory, but not change permissions

  • The IS group needs full control

  • The accountants need to be able to modify files, add files, but not delete

  • Non - accounting department heads need to be able to read and list specific files, but not modify them in any way

  • Everyone else has no access at all to the directory

Now, you could make the head accountant the owner, he's set. IS can always use root (if all of them have that) or sudo to bypass permissions. You can set everyone to 000. So far we have rwx???---, because we haven't decided groups. Here's where Unix permissions fail. We have three groups now that we have to deal with:

  • Accountants

  • Accounting managers

  • Department heads

We could make the group set for accounting managers, and now the directory is rwxrwx---, and put all three groups into a new group. Wait. We don't want department heads to have write/delete. Okay, put them into everyone. Wait, we don't want everyone to see inside the directory. The problem is, you have two groups that need rwx, and one that needs r-x. But within the first two groups, you don't want one to be able to delete files. Okay, set the sticky bit. but now, the group that DOES have to be able to delete can't, because the sticky bit means that only the head of accounting can delete files. So now he's doing the job of the accounting managers. Unix permissions are too simplistic to handle this, and this is not a terribly complex situation. If you are talking about a very large company, this scenario can get much worse.

Sometimes, complex is good

However, a decent set of ACLs can make this really simple. For our example, since we don't know how Apple is implementing ACLs in Tiger, and even if we did, we couldn't talk about it, we'll use the ACL implementation from the OpenAFS project, (http://www.openafs.org/). AFS is the acronym for the Andrew File System, an open - source distributed file system that can run on Mac OS X along with almost every other modern OS. While AFS can easily emulate Unix file permissions, it has a much richer native set of permissions:

  • r read the contents of files in the directory

  • l list the names of files in the directory

  • i insert files into the directory

  • d delete files from the directory

  • w write (or modify) files into the directory

  • k lock (or modify the write-mode bit) of files in the directory

  • a administer or change the acl of the directory

Note how the permissions, while more complex than standard Unix permissions, give you finer control. For example, adding a file to a directory is not the same as having write access to a directory. Deleting is its own permission, as is administering the ACLs of a directory. This is not the richest model of ACLs in use by any means. Both Novell and Windows have far richer ACL models, but this one is good as an example, and runs on many more environments than Novell or Windows do.

So let's revisit our accounting example, but with our new ACL goodness at our disposal:

  • The head accountant is the owner still, and as such, he gets: rlidwka He can do anything, including modify the rights of others needing access. Cool.

  • The IS group gets: rlidwka. Cool, they have the access they need.

  • The accounting managers get: rlidwk. They can modify files as needed, but they can't change anyone's permissions in the directory. Cool.

  • The accountants get: rliw. They can do their work, and add new files as needed, but they cannot delete files, lock files, or change permissions. Cool

  • The department heads get: rl. They can see and view the files, but not modify them or the directory.

No one else has any rights; the directory is a black box to them. IS has the access it needs without needing sudo, or root access. If we had very strict security needs, (Sarb-Ox or HIPAA anyone?), we could set it up so that IS could manage ACLs for a directory full of files they could see, but not read. You could even set it up so they could run a directory they couldn't read at all. (Yes, obviously if they can manage ACLs, there's a fast bypass of ACL limitations, but the point is, you have more flexibility with ACLs.)

Another advantage that ACLs give us is individual user permissions outside of groups. So you can have multiple users with different access to a file system object. So you can easily assign new groups or new users permissions without having to modify the user and group organization of your network setup. You do have to be more careful with ACLs than Unix permissions, because if you aren't, it's really easy to give someone, or a group a lot more access than you thought you were.

We also don't know how various commands are going to be changed, or what new ones will be introduced to deal with the new permissions structures. But in the end, we're going to get a lot more benefit from ACLs than pain.

Conclusion

This is a bit of a quickie look at ACLs, but it should give you an idea of the kind of power that ACLs give you. Since ACLs are going to be implemented throughout Mac OS X, they should also finally give you the same permissions in the Finder as you have at the command line, unlike the current situation. With the way modern networking environments are becoming more complex, even k-12, Mac OS X needed a more flexible permissions structure, and ACLs are the best way to get there.

Bibliography and References

http://web.mit.edu/answers/unix/unix_chmod.html

http://www.apple.com/macosx/tiger/unix.html


John Welch <jwelch@provar.com> is an IT Staff Member for Kansas City Life Insurance, a Technical Strategist for Provar, (http://www.provar.com/) and the Chief Know-It-All for TackyShirt, (http://www.tackyshirt.com/. He has over fifteen years of experience at making Macs work with other computer systems. John specializes in figuring out ways in which to make the Mac do what nobody thinks it can, showing that the Mac is a superior administrative platform, and teaching others how to use it in interesting, if sometimes frightening ways. He also does things that don't involve computertry on occasion, or at least that's the rumor.

 
AAPL
$107.80
Apple Inc.
+0.82
MSFT
$46.82
Microsoft Corpora
+0.77
GOOG
$558.34
Google Inc.
+8.03

MacTech Search:
Community Search:

Software Updates via MacUpdate

Vitamin-R 2.20b1 - Personal productivity...
Vitamin-R creates the optimal conditions for your brain to work at its best by structuring your work into short bursts of distraction-free, highly focused activity alternating with opportunities for... Read more
Dropbox 2.10.44 - Cloud synchronization...
Dropbox is an application that creates a special Finder folder that automatically syncs online and between your computers. It allows you to both backup files and keep them up-to-date between systems... Read more
Sandvox 2.9.2 - Easily build eye-catchin...
Sandvox is for Mac users who want to create a professional looking website quickly and easily. With Sandvox, you don't need to be a Web genius to build a stylish, feature-rich, standards-compliant... Read more
Cocktail 8.0.1 - General maintenance and...
Cocktail is a general purpose utility for OS X that lets you clean, repair and optimize your Mac. It is a powerful digital toolset that helps hundreds of thousands of Mac users around the world get... Read more
LibreOffice 4.3.3.2 - Free Open Source o...
LibreOffice is an office suite (word processor, spreadsheet, presentations, drawing tool) compatible with other major office suites. The Document Foundation is coordinating development and... Read more
VMware Fusion 7.0.1 - Run Windows apps a...
VMware Fusion allows you to create a Virtual Machine on your Mac and run Windows (including Windows 8.1) and Windows software on your Mac. Run your favorite Windows applications alongside Mac... Read more
OneNote 15.3.2 - Free digital notebook f...
OneNote is your very own digital notebook. With OneNote, you can capture that flash of genius, that moment of inspiration, or that list of errands that's too important to forget. Whether you're at... Read more
Audio Hijack Pro 2.11.4 - Record and enh...
Audio Hijack Pro drastically changes the way you use audio on your computer, giving you the freedom to listen to audio when you want and how you want. Record and enhance any audio with Audio Hijack... Read more
Iridient Developer 3.0.0 beta 3 - Powerf...
Iridient Developer (was RAW Developer) is a powerful image conversion application designed specifically for OS X. Iridient Developer gives advanced photographers total control over every aspect of... Read more
TextWrangler 4.5.11 - Free general purpo...
TextWrangler is the powerful general purpose text editor, and Unix and server administrator's tool. Oh, and also, like the best things in life, it's free. TextWrangler is the "little brother" to... Read more

Latest Forum Discussions

See All

Monster Flash Review
Monster Flash Review By Jordan Minor on October 31st, 2014 Our Rating: :: ALONE IN THE DARKUniversal App - Designed for iPhone and iPad Solid shooting and a surprising amount of spooky tension make Monster Flash a great portable... | Read more »
Retry Review
Retry Review By Rob Thomas on October 31st, 2014 Our Rating: :: SOARING HIGHUniversal App - Designed for iPhone and iPad Flappy who? Let Retry wash all those bad bird-related memories away on a cool retro-flavored flight… right... | Read more »
Dementia: Book of the Dead Review
Dementia: Book of the Dead Review By Lee Hamlet on October 31st, 2014 Our Rating: :: A TOUGH READUniversal App - Designed for iPhone and iPad A witch hunter is sent after a demonic book in the spooky but short-lived Dementia: Book... | Read more »
Card Dungeon, the Semi-Board Game Roguel...
Card Dungeon, the Semi-Board Game Roguelike, Has Been Renovated Posted by Jessica Fisher on October 31st, 2014 [ permalink ] | Read more »
Logitech Protection + Power iPhone5/5S C...
Made by: Logitech Price: $99.99 Hardware/iOS Integration Rating: 3 out of 5 stars Usability Rating: 0.5 out of 5 stars Reuse Value Rating: 0.75 out of 5 stars Build Quality Rating: 0.75 out of 5 stars Overall Rating: 1.25 out of 5 stars | Read more »
This Is Not a Test Goes Free, Permanentl...
This Is Not a Test Goes Free, Permanently Posted by Jessica Fisher on October 31st, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Swap Heroes Review
Swap Heroes Review By Campbell Bird on October 31st, 2014 Our Rating: :: STRATEGIC SWAPPINGUniversal App - Designed for iPhone and iPad Rotate a cast of heroes to fend of waves of monsters in this difficult, puzzle rpg.   | Read more »
Night Sky Pro™ (Reference)
Night Sky Pro™ 3.0.1 Device: iOS Universal Category: Reference Price: $2.99, Version: 3.0.1 (iTunes) Description: Night Sky Pro™Wonder No More™ Night Sky Pro™ is the ultimate stargazing experience. From the creators of the original... | Read more »
Audio Defence : Zombie Arena (Games)
Audio Defence : Zombie Arena 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: A zombie shooter audio game. Made from gut-wrenching 3D binaural sound, for a new kind of weird immersion. You... | Read more »
RPG Asdivine Hearts (Games)
RPG Asdivine Hearts 1.1.0 Device: iOS Universal Category: Games Price: $3.99, Version: 1.1.0 (iTunes) Description: SPECIAL PRICE50% OFF (USD 7.99 -> USD 3.99)!!! Travel alongside four companions and a cat in the adventure of a... | Read more »

Price Scanner via MacPrices.net

Apple now offering refurbished 2014 13-inch R...
The Apple Store is now offering Apple Certified Refurbished 2014 13″ Retina MacBook Pros for up to $270 off the cost of new models. An Apple one-year warranty is included with each model, and... Read more
Apple Regains Momentum As Windows Stutters An...
The latest smartphone sales data from Kantar Worldpanel ComTech, for the three months to March 2014, shows Apple performing strongly in the first quarter of the year, with sales bouncing back in... Read more
Worldwide Smartphone Shipments Increase 25.2%...
New smartphone releases and an increased emphasis on emerging markets drove global smartphone shipments above 300 million units for the second consecutive quarter, according to preliminary data from... Read more
Apple now offering refurbished 2014 15-inch M...
The Apple Store is now offering Apple Certified Refurbished 2014 15″ Retina MacBook Pros for up to $400 off the cost of new models. An Apple one-year warranty is included with each model, and... Read more
Apple drops prices on refurbished 2013 Retina...
The Apple Store has dropped prices on 2013 Apple Certified Refurbished 13″ and 15″ Retina MacBook Pros, with Retina models now available starting at $999. Apple’s one-year warranty is standard, and... Read more
New 2.8GHz Mac mini on sale for $949, save $5...
Abt Electronics has the new 2.8GHz Mac mini in stock and on sale for $949.05 including free shipping. Their price is $50 off MSRP, and it’s the lowest price available for this model from any reseller... Read more
Sale! 3.7GHz Quad Core Mac Pro available for...
 B&H Photo has the 3.7GHz Quad Core Mac Pro on sale for $2649 including free shipping plus NY sales tax only. Their price is $350 off MSRP, and it’s the lowest price for this model from any... Read more
Mujjo Steps Up The Game With Refined Touchscr...
Netherlands based Mujjo have just launched their Refined Touchscreen Gloves, stepping up their game. The gloves feature a updated elegant design that takes these knitted gloves to the next level. A... Read more
Sale! Preorder the new 27-inch 5K iMac for $2...
 Abt Electronics has the new 27″ 3.5GHz 5K iMac on sale and available for preorder for $2374.05 including free shipping. Their price is $125 off MSRP, and it’s the lowest price available for this... Read more
Simplex Solutions Inc. Brings Secure Web Surf...
New York based Simplex Solutions Inc. has announced the release and immediate availability of Private Browser 1.0, its revolutionary new secure web browser developed for iPhone, iPad and iPod touch... Read more

Jobs Board

Position Opening at *Apple* - Apple (United...
**Job Summary** Every day, business customers come to the Apple Store to discover what powerful, easy-to-use Apple products can do for them. As a Business Leader, Read more
Sr. Manager, *Apple* Deployment Programs fo...
**Job Summary** Apple is seeking candidates for a new position on the Education Content and Technology team. iPad and Mac is in the hands of millions of teachers and Read more
*Apple* Solutions Consultant (ASC) - Apple I...
…important role that the ASC serves is that of providing an excellent Apple Customer Experience. Responsibilities include: * Promoting Apple products and solutions Read more
*Apple* Solutions Consultant (ASC) - Apple I...
…important role that the ASC serves is that of providing an excellent Apple Customer Experience. Responsibilities include: * Promoting Apple products and solutions Read more
*Apple* Solutions Consultant (ASC) - Apple I...
…important role that the ASC serves is that of providing an excellent Apple Customer Experience. Responsibilities include: * Promoting Apple products and solutions Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.