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
$116.47
Apple Inc.
+0.16
MSFT
$47.98
Microsoft Corpora
-0.72
GOOG
$537.50
Google Inc.
+2.67

MacTech Search:
Community Search:

Software Updates via MacUpdate

Cobook 3.0.7 - Intelligent address book....
Cobook Contacts is an intuitive, engaging address book. Solve the problem of contact management with Cobook Contacts and its simple interface and powerful syncing and integration possibilities.... Read more
StatsBar 1.9 - Monitor system processes...
StatsBar gives you a comprehensive and detailed analysis of the following areas of your Mac: CPU usage Memory usage Disk usage Network and bandwidth usage Battery power and health (MacBooks only)... Read more
Cyberduck 4.6 - FTP and SFTP browser. (F...
Cyberduck is a robust FTP/FTP-TLS/SFTP browser for the Mac whose lack of visual clutter and cleverly intuitive features make it easy to use. Support for external editors and system technologies such... Read more
Maya 2015 - Professional 3D modeling and...
Maya is an award-winning software and powerful, integrated 3D modeling, animation, visual effects, and rendering solution. Because Maya is based on an open architecture, all your work can be scripted... Read more
Evernote 6.0.1 - Create searchable notes...
Evernote allows you to easily capture information in any environment using whatever device or platform you find most convenient, and makes this information accessible and searchable at anytime, from... Read more
calibre 2.11 - Complete e-library manage...
Calibre is a complete e-book library manager. Organize your collection, convert your books to multiple formats, and sync with all of your devices. Let Calibre be your multi-tasking digital... Read more
Herald 5.0.1 - Notification plugin for M...
Note: Versions 2.1.3 (for OS X 10.7), 3.0.6 (for OS X 10.8), and 4.0.8 (for OS X 10.9) are no longer supported by the developer. Herald is a notification plugin for Mail.app, Apple's Mac OS X email... Read more
Firetask 3.7 - Innovative task managemen...
Firetask uniquely combines the advantages of classical priority-and-due-date-based task management with GTD. Stay focused and on top of your commitments - Firetask's "Today" view shows all relevant... Read more
TechTool Pro 7.0.6 - Hard drive and syst...
TechTool Pro is now 7, and this is the most advanced version of the acclaimed Macintosh troubleshooting utility created in its 20-year history. Micromat has redeveloped TechTool Pro 7 to be fully 64... Read more
PhotoDesk 3.0.1 - Instagram client for p...
PhotoDesk lets you view, like, comment, and download Instagram pictures/videos! (NO Uploads! / Image Posting! Instagram forbids that! AND you *need* an *existing* Instagram account). But you can do... Read more

Latest Forum Discussions

See All

Ubisoft Gives Everyone Two New Ways to E...
Ubisoft Gives Everyone Two New Ways to Earn In-Game Stuff for Far Cry 4 Posted by Jessica Fisher on November 21st, 2014 [ permalink ] | Read more »
Golfinity – Tips, Tricks, Strategies, an...
Dig this: Would you like to know what we thought of being an infinite golfer? Check out our Golfinity review! Golfinity offers unlimited ways to test your skills at golf. Here are a few ways to make sure your score doesn’t get too high and your... | Read more »
Dark Hearts, The Sequel to Haunting Meli...
Dark Hearts, The Sequel to Haunting Melissa, is Available Now Posted by Jessica Fisher on November 21st, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Meowza! Toyze Brings Talking Tom to Life...
Meowza! | Read more »
Square Enix Announces New Tactical RPG f...
Square Enix Announces New Tactical RPG for Mobile, Heavenstrike Rivals. Posted by Jessica Fisher on November 21st, 2014 [ permalink ] With their epic stories and gorgeous graphics, | Read more »
Quest for Revenge (Games)
Quest for Revenge 1.0.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0.0 (iTunes) Description: The great Kingdom of the west has fallen. The gods ignore the prayers of the desperate. A dark warlord has extinguished... | Read more »
Threadz is a New Writing Adventure for Y...
Threadz is a New Writing Adventure for You and Your Friends Posted by Jessica Fisher on November 21st, 2014 [ permalink ] In the tradition of round-robin storytelling, | Read more »
SteelSeries Stratus XL Hardware Review
Made by: SteelSeries Price: $59.99 Hardware/iOS Integration Rating: 4 out of 5 stars Usability Rating: 4.5 out of 5 stars Reuse Value Rating: 4.25 out of 5 stars Build Quality Rating: 4.5 out of 5 stars Overall Rating: 4.31 out of 5 stars | Read more »
ACDSee (Photography)
ACDSee 1.0.0 Device: iOS iPhone Category: Photography Price: $1.99, Version: 1.0.0 (iTunes) Description: Capture, perfect, and share your photos with ACDSee. The ACDSee iPhone app combines an innovative camera, a powerful photo... | Read more »
ProTube for YouTube (Entertainment)
ProTube for YouTube 2.0.2 Device: iOS Universal Category: Entertainment Price: $1.99, Version: 2.0.2 (iTunes) Description: ProTube is the ultimate, fully featured YouTube app. With it's highly polished design, ProTube offers ad-free... | Read more »

Price Scanner via MacPrices.net

Save up to $400 with Apple refurbished 2014 1...
The Apple Store has restocked 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 shipping... Read more
New 13-inch 1.4GHz MacBook Air on sale for $8...
 Adorama has the 2014 13″ 1.4GHz/128GB MacBook Air on sale for $899.99 including free shipping plus NY & NJ tax only. Their price is $100 off MSRP. B&H Photo has the 13″ 1.4GHz/128GB MacBook... Read more
Apple Expected to Reverse Nine-Month Tablet S...
Apple and Samsung combined accounted for 62 percent of the nearly 36 million branded tablets shipped in 3Q 2014, according to early vendor shipment share estimates from market intelligence firm ABI... Read more
Stratos: 30 Percent of US Smartphone Owners t...
Stratos, Inc., creator of the Bluetooth Connected Card Platform, has announced results from its 2014 Holiday Mobile Payments Survey. The consumer survey found that nearly one out of three (30 percent... Read more
2014 1.4GHz Mac mini on sale for $449, save $...
 B&H Photo has lowered their price on the new 1.4GHz Mac mini to $449.99 including free shipping plus NY tax only. Their price is $50 off MSRP, and it’s the lowest price available for this new... Read more
Check Apple prices on any device with the iTr...
MacPrices is proud to offer readers a free iOS app (iPhones, iPads, & iPod touch) and Android app (Google Play and Amazon App Store) called iTracx, which allows you to glance at today’s lowest... Read more
64GB iPod touch on sale for $249, save $50
Best Buy has the 64GB iPod touch on sale for $249 on their online store for a limited time. Their price is $50 off MSRP. Choose free shipping or free local store pickup (if available). Sale price for... Read more
15″ 2.2GHz Retina MacBook Pro on sale for $17...
 B&H Photo has the 2014 15″ 2.2GHz Retina MacBook Pro on sale for $1799.99 for a limited time. Shipping is free, and B&H charges NY sales tax only. B&H will also include free copies of... Read more
New Logitech AnyAngle Case/Stand Brings Flexi...
Logitec has announced the newest addition to its suite of tablet products — the Logitech AnyAngle. A protective case with an any-angle stand for iPad Air 2 and all iPad mini models, AnyAngle is the... Read more
Notebook PC Shipments Rise Year-Over-Year as...
According to preliminary results from the upcoming DisplaySearch Quarterly Mobile PC Shipment and Forecast Report, the global notebook PC market grew 10 percent year-over-year in Q3’14 to 49.4... Read more

Jobs Board

*Apple* Solutions Consultant (ASC)- Retail S...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
Project Manager, *Apple* Financial Services...
**Job Summary** Apple Financial Services (AFS) offers consumers, businesses and educational institutions ways to finance Apple purchases. We work with national and Read more
*Apple* Store Leader Program - College Gradu...
Job Description: Job Summary As an Apple Store Leader Program agent, you can continue your education as you major in the art of leadership at the Apple Store. You'll Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
Senior Event Manager, *Apple* Retail Market...
…This senior level position is responsible for leading and imagining the Apple Retail Team's global event strategy. Delivering an overarching brand story; in-store, Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.