10 ms·

The root cause of all this is a relatively obscure NTFS feature called alternate data streams. Obscure indeed, I've never seen them used for anything other tha

The root cause of all this is a relatively obscure NTFS feature called alternate data streams. Obscure indeed, I've never seen them used for anything other than hiding malicious content. Curious, I read about them on Wikipedia[1] and it turns out they were originally created to support resource forks in Services for Macintosh. Browsers also use them to flag files downloaded from the internet. [1] https://en.wikipedia.org/wiki/NTFS#Alternate_data_streams_.28ADS.29 https://en.wikipedia.org/wiki/NTFS#Alternate_data_streams_.2...

J_Darnley 10y ago
> Browsers also use them to flag files downloaded from the internet. Is that where that annoying shit comes from? Good to know. When firefox kills off DownThemAll I will then use a FAT partition to store downloaded files (and see if I can force the temporary files to go there too).
- icebraining 10y ago
  Unless it has changed in newer Windows versions, you can simply disable that warning in the Internet Settings, no need to keep files in an outdated filesystem.
- anonymfus 10y ago
  >use a FAT partition to store downloaded files Do you never download anything bigger than or equal to 4 GiB?
  - J_Darnley 10y ago
    Not from a browser and, AFAIK, only a browser idiotically marks files as coming from the internet.
- mfenniak 10y ago
  I laugh these days when OSX warns me, "This application was downloaded from the internet." when I first access an app. Every application on my machine was downloaded from the internet. Even the OS, after the first upgrade. That's not what is dangerous.
- ChoGGi 10y ago
  stick this in a reg file REGEDIT4 ;https://support.microsoft.com/en-us/kb/889815 [HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Environment] "SEE_MASK_NOZONECHECKS"="1" ;https://technet.microsoft.com/en-us/library/cc783259 [HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\Download] "CheckExeSignatures"="no" "RunInvalidSignatures"=dword:00000001 ;https://support.microsoft.com/kb/883260 [HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\Associations] "LowRiskFileTypes"=".zip;.rar;.nfo;.txt;.exe;.bat;.com;.cmd;.reg;.msi;.htm;.html;.gif;.bmp;.jpg;.avi;.mpg;.mpeg;.mov;.mp3;.m3u;.wav;" "DefaultFileTypeRisk"=dword:00001808 [HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\Attachments] "SaveZoneInformation"=dword:00000001
2close4comfort 10y ago
they should market this as a feature! alternate streams for people who think it is "an obscure feature" I mean that many people using alternate streams would be interesting for anyone forensicating systems for malware or as protection from...
CoolGuySteve 10y ago
iTunes for Windows uses them to store how much of a streaming file it has already downloaded. I wrote it (but I won't take credit for most things in iTunes for Windows) It's a nifty feature but I'll admit NTFS is really obscure at times.
- enjoy-your-stay 10y ago
  Great place to store meta data about a file, never thought about that before. I guess if the download stream is interrupted it reads that to know where to pick up again if resumed? Another obscure feature of NTFS is Transactional NTFS which I'd never heard of until recently. https://msdn.microsoft.com/en-us/library/windows/desktop/aa363859(v=vs.85).aspx https://msdn.microsoft.com/en-us/library/windows/desktop/aa3...
  - ygra 10y ago
    Windows even includes mechanisms to perform transactions over different things like file system, registry, and even multiple machines. Back when SVN was horribly slow and implemented transactions by actually touching thousands of small files in the .svn directories, I actually wanted to implement its file system layer on Windows with NTFS transactions, figuring that a native solution would probably be better. But by now they completely changed their working copy format so I don't think it's necessary anymore.
    - int_19h 10y ago
      Unfortunately, transactional NTFS is being deprecated. MSDN says: "Microsoft strongly recommends developers utilize alternative means to achieve your application’s needs. Many scenarios that TxF was developed for can be achieved through simpler and more readily available techniques. Furthermore, TxF may not be available in future versions of Microsoft Windows." Which is a shame, because, conceptually speaking, a true transactional filesystem with snapshot semantics makes some things so much easier.
    - flukus 10y ago
      Do you mean DTC or something else?
deleted 10y ago
[deleted]
wslh 10y ago
This is used in specific sectors like data loss prevention. For example, you can tag files based on the security sensitiveness and if the file is copied it retains the tags.
- __jal 10y ago
  > if the file is copied it retains the tags. ...if your miscreant is technically illiterate and only uses NTFS.
tamana 10y ago
It's metadata. It's as obscure as file permissions bits.
- OJFord 10y ago
  Except that millions of developers routinely make use of file permissions; as evidenced by this discussion, many - perhaps even a majority - haven't heard of alternate data streams.
  - OJFord 10y ago
    Actually, I'm not going to shy away from it. I'd be willing to bet that a clear majority haven't heard of alternate data streams. There are too many developers who care not for NTFS at all, never mind some little-used feature, for that not to be true.
    - tracker1 10y ago
      I really like NTFS as a file system... seems to offer a lot more than many other file systems, and pretty interestingly so for as old as it is now. That said, hopefully broader adoption can happen when the patents expire (ugh, in 7 years). Maybe the "new" MS could be convinced to create a royalty-free spec release/promise. Would love for NTFS to become default for external storage, I already use it, but getting it on macOS and Linux isn't always as straight forward as it could be. NTFS-3G ftw.
  - viraptor 10y ago
    But millions of developers also go "I don't get how it works, just make it world-writable". Not sure they understand either.
_wmd 10y ago
Hardly obscure, every modern OS has an equivalent feature, but only OSX and Windows unify it with the regular filesystem API. Streams and resource forks are a play on a now-standard UNIX feature that almost nobody uses because it has a shitty non-file based API that also breaks most tools unless they are specifically aware of them: extended attributes. Resource forks and extended attributes are almost equivalent in every single way, except that extended attributes can only be read/written atomically (limiting their size to strings that will fit in RAM), whereas a fork or stream can be opened like a regular file. Stick that in your pipe and smoke it, UNIX sycophants, another case where Windows is more UNIX than UNIX ;) The file-or-directory vagueness created by the hierarchy of resources buried within a file also more closely maps how the most popular path naming scheme on the planet (URLs) work: an URL can always represent both a file and a collection simultaneously, so I see this as closer to an ideal than the alternative where files can have no children at all. Sadly nobody actually uses these APIs like that, because all our tooling sucks so bad at coping with it. I sometimes wonder what the world would look like if directories on popular operating systems had simply been made 0 byte files
- ksherlock 10y ago
  Solaris unfies it too. You can even use the runat command to open a shell where extended attributes are exposed as and can be manipulated as regular files. http://docs.oracle.com/cd/E23824_01/html/821-1474/fsattr-5.html http://docs.oracle.com/cd/E23824_01/html/821-1474/fsattr-5.h...
- amyjess 10y ago
  > feature that almost nobody uses because it has a shitty non-file based API that also breaks most tools unless they are specifically aware of them: extended attributes Mind you, OS X makes extensive use of extended attributes in addition to resource forks (and it's largely deprecated resource forks in favor of app folders). Spend some time poking around Siracusa's reviews (since Tiger); he loves to go into detail about every new way Apple makes use of extended attributes. Also, it's not fair to say that almost nobody uses them. Chrome makes use of extended attributes, as does KDE's metadata system and a few other things. > (limiting their size to strings that will fit in RAM) That's an understatement. The Linux kernel API limits the size of all extended attributes to 64KB, and the most popular filesystems limit them further to 4KB. That's not really comparable to a true fork. ZFS is the exception: its extended attributes are implemented as forks, and the maximum size of an extended attribute is the same as that of a file. Unfortunately, those aren't accessible on ZOL because the kernel won't support it, so you can really only take advantage of it on Solaris/Illumos (and maybe FreeBSD?).
banana_giraffe 10y ago
It's used by all the browsers on Windows these days. They all create a 'Zone.Identifier' stream when a file is downloaded to mark is downloaded. It's content's is what triggers the "You downloaded this file! It's Evil!' warning in Windows. To be fair, it's not used by a ton of things, since it requires NTFS, disappears when files are moved to different filesystems, and various things that read and write files destroy them if they're not careful, not to mention actually enumerating the streams is tricky, last I checked.
- ryanburk 10y ago
  some history: this was introduced with XP SP2 as part of the windows security push. was a clever way to track the information without touching the binary data directly and supporting it in IE meant the majority of customers saw the benefit right away. and since most people (in windows) don't move files across file systems.
  - ams6110 10y ago
    People in windows often move files across file systems: between the internal hard drive (generally NTFS) to external USB drives (often FAT32, or exFAT)
    - GirlsCanCode 10y ago
      Exactly! It's amazing how unaware Mac zealots are of other operating systems. They may learn something if they'd shut up and listen.
    - Mikhail_Edoshin 10y ago
      When this happens and the file has alternate streams you get a warning that some of the file's attributes cannot be copied.
      - banana_giraffe 10y ago
        I've not seen that warning before. I just tested on a file I downloaded, which had the 'Zone.Identifier' stream. Using Explorer, I copied it to a FAT32 volume, then back to my NTFS drive. Sure enough, it lost the 'Zone.Identifier' stream, and there was no warning when I opened it. This is on a fairly normal Windows 10 installation. YMMV on different versions, of course.
donatj 10y ago
I used them for a VCS thought experiment I was playing with a while ago.
sixothree 10y ago
Just pretend they're "resource forks".
jahewson 10y ago
There were once plans to store the individual streams which make up Microsoft Office files (OLE2) as alternate data streams, which would have been... interesting.
Animats 10y ago
The original idea on the Macintosh was to have some place to put non-code assets - icons, images, etc - that came with an application. So MacOS files had a "data fork" and a "resource fork". The "resource fork" was a tree structure managed by the Resource Manager. The problem was that the original Macintosh had limited memory and only a floppy disk, and the implementation of writing to the resource fork wasn't very good. Many programs wrote to their own resource fork for preferences and such. The tree structure wasn't updated fully until the program was closed, because writing to the floppy was so slow. If the program exited abnormally, the resource fork's links were broken. This gave the resource fork approach a bad reputation. Since Windows programs had to run on DOS, which didn't have resource forks, Windows never used this much. Windows put non-code assets in the executable as read-only objects. NT, which was supposed to do everything (originally it had POSIX and OS/2 compatibility, and ran on MIPS, Alpha, and x86) added generalized support for resource forks, just in case. But since most applications were written for Windows 3.1/95/ME, they didn't use those facilities. So that's how we got here.
- zwieback 10y ago
  In the early 90's I worked at a company that made server software that allowed Mac AppleTalk (AFP) clients to connect to a PC network. Eventually IBM had us write a custom version for OS/2 called LAN Server for Macintosh. We were really excited about using the streams/resource forks feature but had to give up eventually. We used a separate database to store what's in the resource forks instead.
- kalleboo 10y ago
  > The tree structure wasn't updated fully until the program was closed, because writing to the floppy was so slow Not to mention in many cases on the original Macs, you probably didn't even have the program floppy in the drive when you were working, because with only 400K on a disk you had to swap to the disk with your document on it. I recall Inside Macintosh had a big disclaimer at the top that warned "The Resource Manager IS NOT A DATABASE". It was originally just meant to handle localizable resources, but since it was already there it was handy for developers (including Apple themselves) to use to load any kind of structured data. And who didn't love going messing around in system and application files with ResEdit?
jobigoud 10y ago
I have used them. We had a system that generated millions of images and needed to be sure that from one version to the next the images produced by a given request were the same, and also have some diagnostic data in case of problematic images. The images could be either JPG or PNG and we needed a unified way to associate arbitrary metadata with them. We had a special mode that would store an equivalent of the request in an alternate data stream of the image. When a problem was detected we would open the alternate data stream and test the request manually.
- vocatus_gate 10y ago
  Very cool niche case. Thanks for sharing.
andrewaylett 10y ago
I've worked on Windows-only software that used resource forks. It stored mail messages, one per file, with the message metadata in a resource fork so we didn't have to modify the file containing the actual mail when the metadata changed.
Ecio78 10y ago
SQLServer uses it from version 2005 til 2012 to create databases snapshots in order to run DBCC CHECKDB (consistency check). So for actually a critical feature of MSSQL. I suppose this was the reason why ReFS was not supported for SQL data disks. It seems they are not used anymore since sql 2014. See for example http://www.sqlskills.com/blogs/paul/issues-around-dbcc-checkdb-and-the-use-of-hidden-database-snapshots/ http://www.sqlskills.com/blogs/paul/issues-around-dbcc-check...

J_Darnley 10y ago

> Browsers also use them to flag files downloaded from the internet. Is that where that annoying shit comes from? Good to know. When firefox kills off DownThemAll I will then use a FAT partition to store downloaded files (and see if I can force the temporary files to go there too).

icebraining 10y ago
Unless it has changed in newer Windows versions, you can simply disable that warning in the Internet Settings, no need to keep files in an outdated filesystem.
anonymfus 10y ago
>use a FAT partition to store downloaded files Do you never download anything bigger than or equal to 4 GiB?
- J_Darnley 10y ago
  Not from a browser and, AFAIK, only a browser idiotically marks files as coming from the internet.
mfenniak 10y ago
I laugh these days when OSX warns me, "This application was downloaded from the internet." when I first access an app. Every application on my machine was downloaded from the internet. Even the OS, after the first upgrade. That's not what is dangerous.
ChoGGi 10y ago
stick this in a reg file REGEDIT4 ;https://support.microsoft.com/en-us/kb/889815 [HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Environment] "SEE_MASK_NOZONECHECKS"="1" ;https://technet.microsoft.com/en-us/library/cc783259 [HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\Download] "CheckExeSignatures"="no" "RunInvalidSignatures"=dword:00000001 ;https://support.microsoft.com/kb/883260 [HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\Associations] "LowRiskFileTypes"=".zip;.rar;.nfo;.txt;.exe;.bat;.com;.cmd;.reg;.msi;.htm;.html;.gif;.bmp;.jpg;.avi;.mpg;.mpeg;.mov;.mp3;.m3u;.wav;" "DefaultFileTypeRisk"=dword:00001808 [HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\Attachments] "SaveZoneInformation"=dword:00000001

2close4comfort 10y ago

they should market this as a feature! alternate streams for people who think it is "an obscure feature" I mean that many people using alternate streams would be interesting for anyone forensicating systems for malware or as protection from...

CoolGuySteve 10y ago

iTunes for Windows uses them to store how much of a streaming file it has already downloaded. I wrote it (but I won't take credit for most things in iTunes for Windows) It's a nifty feature but I'll admit NTFS is really obscure at times.

enjoy-your-stay 10y ago
Great place to store meta data about a file, never thought about that before. I guess if the download stream is interrupted it reads that to know where to pick up again if resumed? Another obscure feature of NTFS is Transactional NTFS which I'd never heard of until recently. https://msdn.microsoft.com/en-us/library/windows/desktop/aa363859(v=vs.85).aspx https://msdn.microsoft.com/en-us/library/windows/desktop/aa3...
- ygra 10y ago
  Windows even includes mechanisms to perform transactions over different things like file system, registry, and even multiple machines. Back when SVN was horribly slow and implemented transactions by actually touching thousands of small files in the .svn directories, I actually wanted to implement its file system layer on Windows with NTFS transactions, figuring that a native solution would probably be better. But by now they completely changed their working copy format so I don't think it's necessary anymore.
  - int_19h 10y ago
    Unfortunately, transactional NTFS is being deprecated. MSDN says: "Microsoft strongly recommends developers utilize alternative means to achieve your application’s needs. Many scenarios that TxF was developed for can be achieved through simpler and more readily available techniques. Furthermore, TxF may not be available in future versions of Microsoft Windows." Which is a shame, because, conceptually speaking, a true transactional filesystem with snapshot semantics makes some things so much easier.
  - flukus 10y ago
    Do you mean DTC or something else?

deleted 10y ago

[deleted]

wslh 10y ago

This is used in specific sectors like data loss prevention. For example, you can tag files based on the security sensitiveness and if the file is copied it retains the tags.

__jal 10y ago
> if the file is copied it retains the tags. ...if your miscreant is technically illiterate and only uses NTFS.

tamana 10y ago

It's metadata. It's as obscure as file permissions bits.

OJFord 10y ago
Except that millions of developers routinely make use of file permissions; as evidenced by this discussion, many - perhaps even a majority - haven't heard of alternate data streams.
- OJFord 10y ago
  Actually, I'm not going to shy away from it. I'd be willing to bet that a clear majority haven't heard of alternate data streams. There are too many developers who care not for NTFS at all, never mind some little-used feature, for that not to be true.
  - tracker1 10y ago
    I really like NTFS as a file system... seems to offer a lot more than many other file systems, and pretty interestingly so for as old as it is now. That said, hopefully broader adoption can happen when the patents expire (ugh, in 7 years). Maybe the "new" MS could be convinced to create a royalty-free spec release/promise. Would love for NTFS to become default for external storage, I already use it, but getting it on macOS and Linux isn't always as straight forward as it could be. NTFS-3G ftw.
- viraptor 10y ago
  But millions of developers also go "I don't get how it works, just make it world-writable". Not sure they understand either.

_wmd 10y ago

Hardly obscure, every modern OS has an equivalent feature, but only OSX and Windows unify it with the regular filesystem API. Streams and resource forks are a play on a now-standard UNIX feature that almost nobody uses because it has a shitty non-file based API that also breaks most tools unless they are specifically aware of them: extended attributes. Resource forks and extended attributes are almost equivalent in every single way, except that extended attributes can only be read/written atomically (limiting their size to strings that will fit in RAM), whereas a fork or stream can be opened like a regular file. Stick that in your pipe and smoke it, UNIX sycophants, another case where Windows is more UNIX than UNIX ;) The file-or-directory vagueness created by the hierarchy of resources buried within a file also more closely maps how the most popular path naming scheme on the planet (URLs) work: an URL can always represent both a file and a collection simultaneously, so I see this as closer to an ideal than the alternative where files can have no children at all. Sadly nobody actually uses these APIs like that, because all our tooling sucks so bad at coping with it. I sometimes wonder what the world would look like if directories on popular operating systems had simply been made 0 byte files

ksherlock 10y ago
Solaris unfies it too. You can even use the runat command to open a shell where extended attributes are exposed as and can be manipulated as regular files. http://docs.oracle.com/cd/E23824_01/html/821-1474/fsattr-5.html http://docs.oracle.com/cd/E23824_01/html/821-1474/fsattr-5.h...
amyjess 10y ago
> feature that almost nobody uses because it has a shitty non-file based API that also breaks most tools unless they are specifically aware of them: extended attributes Mind you, OS X makes extensive use of extended attributes in addition to resource forks (and it's largely deprecated resource forks in favor of app folders). Spend some time poking around Siracusa's reviews (since Tiger); he loves to go into detail about every new way Apple makes use of extended attributes. Also, it's not fair to say that almost nobody uses them. Chrome makes use of extended attributes, as does KDE's metadata system and a few other things. > (limiting their size to strings that will fit in RAM) That's an understatement. The Linux kernel API limits the size of all extended attributes to 64KB, and the most popular filesystems limit them further to 4KB. That's not really comparable to a true fork. ZFS is the exception: its extended attributes are implemented as forks, and the maximum size of an extended attribute is the same as that of a file. Unfortunately, those aren't accessible on ZOL because the kernel won't support it, so you can really only take advantage of it on Solaris/Illumos (and maybe FreeBSD?).

banana_giraffe 10y ago

It's used by all the browsers on Windows these days. They all create a 'Zone.Identifier' stream when a file is downloaded to mark is downloaded. It's content's is what triggers the "You downloaded this file! It's Evil!' warning in Windows. To be fair, it's not used by a ton of things, since it requires NTFS, disappears when files are moved to different filesystems, and various things that read and write files destroy them if they're not careful, not to mention actually enumerating the streams is tricky, last I checked.

ryanburk 10y ago
some history: this was introduced with XP SP2 as part of the windows security push. was a clever way to track the information without touching the binary data directly and supporting it in IE meant the majority of customers saw the benefit right away. and since most people (in windows) don't move files across file systems.
- ams6110 10y ago
  People in windows often move files across file systems: between the internal hard drive (generally NTFS) to external USB drives (often FAT32, or exFAT)
  - GirlsCanCode 10y ago
    Exactly! It's amazing how unaware Mac zealots are of other operating systems. They may learn something if they'd shut up and listen.
  - Mikhail_Edoshin 10y ago
    When this happens and the file has alternate streams you get a warning that some of the file's attributes cannot be copied.
    - banana_giraffe 10y ago
      I've not seen that warning before. I just tested on a file I downloaded, which had the 'Zone.Identifier' stream. Using Explorer, I copied it to a FAT32 volume, then back to my NTFS drive. Sure enough, it lost the 'Zone.Identifier' stream, and there was no warning when I opened it. This is on a fairly normal Windows 10 installation. YMMV on different versions, of course.

donatj 10y ago

I used them for a VCS thought experiment I was playing with a while ago.

sixothree 10y ago

Just pretend they're "resource forks".

jahewson 10y ago

There were once plans to store the individual streams which make up Microsoft Office files (OLE2) as alternate data streams, which would have been... interesting.

Animats 10y ago

The original idea on the Macintosh was to have some place to put non-code assets - icons, images, etc - that came with an application. So MacOS files had a "data fork" and a "resource fork". The "resource fork" was a tree structure managed by the Resource Manager. The problem was that the original Macintosh had limited memory and only a floppy disk, and the implementation of writing to the resource fork wasn't very good. Many programs wrote to their own resource fork for preferences and such. The tree structure wasn't updated fully until the program was closed, because writing to the floppy was so slow. If the program exited abnormally, the resource fork's links were broken. This gave the resource fork approach a bad reputation. Since Windows programs had to run on DOS, which didn't have resource forks, Windows never used this much. Windows put non-code assets in the executable as read-only objects. NT, which was supposed to do everything (originally it had POSIX and OS/2 compatibility, and ran on MIPS, Alpha, and x86) added generalized support for resource forks, just in case. But since most applications were written for Windows 3.1/95/ME, they didn't use those facilities. So that's how we got here.

zwieback 10y ago
In the early 90's I worked at a company that made server software that allowed Mac AppleTalk (AFP) clients to connect to a PC network. Eventually IBM had us write a custom version for OS/2 called LAN Server for Macintosh. We were really excited about using the streams/resource forks feature but had to give up eventually. We used a separate database to store what's in the resource forks instead.
kalleboo 10y ago
> The tree structure wasn't updated fully until the program was closed, because writing to the floppy was so slow Not to mention in many cases on the original Macs, you probably didn't even have the program floppy in the drive when you were working, because with only 400K on a disk you had to swap to the disk with your document on it. I recall Inside Macintosh had a big disclaimer at the top that warned "The Resource Manager IS NOT A DATABASE". It was originally just meant to handle localizable resources, but since it was already there it was handy for developers (including Apple themselves) to use to load any kind of structured data. And who didn't love going messing around in system and application files with ResEdit?

jobigoud 10y ago

I have used them. We had a system that generated millions of images and needed to be sure that from one version to the next the images produced by a given request were the same, and also have some diagnostic data in case of problematic images. The images could be either JPG or PNG and we needed a unified way to associate arbitrary metadata with them. We had a special mode that would store an equivalent of the request in an alternate data stream of the image. When a problem was detected we would open the alternate data stream and test the request manually.

vocatus_gate 10y ago
Very cool niche case. Thanks for sharing.

andrewaylett 10y ago

I've worked on Windows-only software that used resource forks. It stored mail messages, one per file, with the message metadata in a resource fork so we didn't have to modify the file containing the actual mail when the metadata changed.

Ecio78 10y ago

SQLServer uses it from version 2005 til 2012 to create databases snapshots in order to run DBCC CHECKDB (consistency check). So for actually a critical feature of MSSQL. I suppose this was the reason why ReFS was not supported for SQL data disks. It seems they are not used anymore since sql 2014. See for example http://www.sqlskills.com/blogs/paul/issues-around-dbcc-checkdb-and-the-use-of-hidden-database-snapshots/ http://www.sqlskills.com/blogs/paul/issues-around-dbcc-check...