NTFS links are theabstraction used in theNTFS file system—the default file system for allMicrosoft Windows versions belonging to theWindows NT family—to associatepathnames and certain kinds ofmetadata, with entries in the NTFSMaster File Table (MFT). NTFS broadly adopts a pattern akin to typicalUnix file systems in the way it stores and references file data and metadata; the most significant difference is that in NTFS, the MFT "takes the place of"inodes, fulfilling most of the functions which inodes fulfill in a typical Unix filesystem.
In NTFS, an entity in the filesystem fundamentally exists as: a record stored in the MFT of an NTFS volume, the MFT being the coredatabase of the NTFS filesystem; and, any attributes andNTFS streams associated with said record. A link in NTFS isitself a record, stored in the MFT, which "points" toanother MFT record: thetarget of the link. Links are the file "entries" in the volume's hierarchicalfile tree: an NTFSpathname such as\foo.exe or\foobar\baz.txtis a link. If the volume containing said pathnames were mapped toD: in a Windows system, these could be referenced asD:\foo.exe andD:\foobar\baz.txt. (Compare and contrast with typical Unix file systems, where a link is an entry in a directory—directories themselves being just a type of file stored in the filesystem—pointing either to another link, or to an inode.)
NTFS has four types of links. These map relatively closely to the generichard link andsoft link concepts which modern file systems tend to follow.
Hard links are typical in behavior. A hard link "points" to an MFT record. Thattarget record will be the record for a "regular" file, such as atext file orexecutable (assuming the NTFS volume is in a normal "healthy" state). Compare with a typicalUnix file system, where a hard link points to aninode. As in such file systems, an NTFS hard link cannot point to a directory.
A typical new file creation event on an NTFS volume, then, simply involves NTFS allocating and creating one new MFT record, for storing the new file entity's filemetadata—including, about any of the data clusters assigned to the file, and the file's data streams; one MFT record for a hard link which points to the first newly-created MFT record as itstarget; storing a reference to the hard link in a directory file; and setting thereference count of both these MFT records to 1. Anyfile name provided as part of the file creation event is stored in the hard link. An MFT record can be the target of up to 1024 hard links; each time a new hard link is successfully created, targeting a previously extant MFT record, the target's reference count is incremented.
Symmetrically, the immediate tasks performed by NTFS in a typicalfile deletion event, when deleting a hard link, are simply: removing the reference to the link from the directory file containing it (the root directory, if applicable); and decrementing by 1 the reference counts of the MFT record targeted by the link, and, of the entry containing the hard linkitself. Any MFT record which now has a refcount of 0, is now in the "deleted" state: all its associated resources are considered "free" by NTFS, to be freely overwritten and used as needed.
Junction points areNTFS reparse points and operate similarly tosymbolic links in Unix or Linux, but are only defined for directories, and may only be absolute paths on local filesystems (as opposed to remote filesystems being accessed). They are created and behave in a similar way to hard links, except that if the target directory is renamed, moved, or deleted, the link will no longer be valid.[1][2]
Symbolic links arereparse points which operate similarly to Junction Points, or symbolic links in Unix or Linux, and accept relative paths and paths to files as well as directories. Support for directory and UNC paths were added in NTFS 3.1.
All NTFS links are intended to be transparent to applications. This means that the application accessing a link will be seamlessly redirected by the file system driver, and no special handling is needed. To users, they appear as normal directories or files. This also leads to an aliasing effect: writes to a link will pass the write to the underlying, linked file orMFT entry.
Symbolic links and junction points contain the path to the linked file, and a tag identifying the driver which implements the behaviour. Because they record the path, they can link to files on other volumes or even remote files. However this also means that if the referenced file is deleted or renamed, the link becomes invalid, and if the referenced file or directory is replaced with another, the link will now refer to the new file or directory.
An NTFS symbolic link is not the same as aWindows shortcut file, which is a regular file. The latter may be created on any filesystem (such as the earlierFAT32), may contain metadata (such as an icon to display when the shortcut is viewed in Remove links), and is not transparent to applications.
Implementations of unix-like environments for Windows such asCygwin andMingw can use shortcut files to emulate symbolic links where the host operating system does not support them, if configured to do so.
C:\Documents and Settings pointing toC:\Users%USERPROFILE%\Application Data pointing to%USERPROFILE%\AppData\Roaming%USERPROFILE%\My Documents\My Pictures pointing to%USERPROFILE%\PicturesBy setting a junction point that points to a directory containing a particular version of a piece of software, it may be possible to add another version of the software and redirect the junction point to point to the version desired.
The contents of a junction use almost no storage space (they simply point to the original directory). If an administrator needs to have multiple points of entry to a large directory, junction points can be an effective solution. Junction points should not be confused with acopy of something as junctions simply point to the original. If directories need to be modified separately a junction cannot be used as it does not provide a distinct copy of the directory or files within.
Likewise, symbolic links and hard links are useful for merging the contents of individual files.
Since reinstalling Windows (or installing a new version) often requires deleting the contents of theC: drive, it is advantageous to create multiplepartitions so only one partition needs to be deleted during the installation. However, some programs don't let the user choose the installation directory, or install some of their files to theC: drive even when they are installed to a different drive. By creating a junction point, the program can be tricked into installing to a different directory.
Windows comes with several tools capable of creating and manipulating NTFS links.
New-Item cmdlet ofWindows PowerShell that can create empty files, folders, junctions, and hard links.[3] In PowerShell 5.0 and later, it can create symbolic links as well.[4] TheGet-Item andGet-ChildItem cmdlets can be used to interrogate file system objects, and if they are NTFS links, find information about them. TheRemove-Item cmdlet can remove said items, although there has been a record of a bug preventing this cmdlet from working properly.[5]mklink internal command can create junctions, hard links, and symbolic links.[6] This command is also available inReactOS.[7] In addition, the venerabledir command can display and filter junction points via the/aL switch.[8] Finally, therd command (also known asrmdir) can delete junction points.hardlink sub-command can make hard links or list hard links associated with a file.[9] Another sub-command,reparsepoint, can query or deletereparse points, the file system objects that make up junction points, hard links, and symbolic links.[10]In addition, the following utilities can create NTFS links, even though they don't come with Windows.
To create hard links, apps may use theCreateHardLink() function ofWindows API. All versions of theWindows NT family can useGetFileInformationByHandle() to determine the number of hard links associated with a file. There can be up to 1024 links associated with an MFT entry. Similarly, theCreateSymbolicLink() function can create symbolic links. Junctions are more complex to create. They require manualreparse point information filling.[15] A code example is found inlibuv.[16] Junctions are defined for directories only: although the API does not fail when one creates a junction pointing to a file, the junction will not be interpreted successfully when used later.
Junctions and symbolic links, even those pointing to directories, can be removed withpNtSetInformationFile. Libuv's implementation ofunlink on Windows demonstrates this use.[17] Alternatively, the .NETSystem.IO.Directory.Delete() method works on them as well.[18]
Symbolic links and NTFS junctions can point to non-existent targets because the operating system does not continuously ensure that the target exists.[19]
Additional hazards lurk in the use of NTFS directory junctions that:
X:\path\to\parent which points to eitherX:\path\ orX:\path\to\, orX:, inX:\some\path\.The problem in the first case is that it creates recursive paths, which further implies infinite recursion in the directory structure. By introducing reentrancy, the presence of one or more directory junctions changes the structure of the file system from a simpleproper tree into adirected graph, but recursive linking further complicates the graph-theoretical character fromacyclic to cyclic. Since the same files and directories can now be encountered through multiple paths, applications which traverse reentrant or recursive structures naively may give incorrect or incoherent results, or may never terminate. Worse, if recursivelydeleting, such programs may attempt to delete a parent of the directory it is currently traversing.
Note that both of the conditions listed above exist in the system of hard links established on theC: drive in the default Windows setup. For example, everyWindows 10 installation defines the recursive path:
C:\ProgramData\C:\ProgramData\Application Data\C:\ProgramData\Application Data\Application Data\C:\ProgramData\Application Data\Application Data\Application Data\C:\ProgramData\Application Data\Application Data\Application Data\Application Data\C:\ProgramData\Application Data\Application Data\Application Data\Application Data\Application Data\ ...
Each additional path name in this seemingly infinite set is an actual valid Windows path which refers to the same location. In practice, path names are limited by the 260-character DOS path limit (or newer 32,767 character limit), but truncation may result in incomplete or invalid path and file names. Whenever a copy of a Windows installation is archived, with directory junctions intact, to another volume on the same—or worse—another computer, the archived copy may still incorporate active folders from the running installation. For example, depending on the method used for copying, a backup copy of a Windows driveX:\archive\... will include a hard link calledX:\archive\Users\USERNAME\My Documents which still points to folderC:\Users\USERNAME\Documents\ in the current, active installation.
The second form of deferred target mis-referral, while conceptually simpler, can have more severe consequences. When a self-consistent volume or directory structure containing hard links which use volume drive-letter path names is copied or moved to another volume (or when the drive letter of a volume is reassigned by some other means), such links may no longer point to the corresponding targetin the copied structure. Again the results depend on the software that was used for copying; while some programs may intercede by modifying any fully subsumed hard links in the copy in order to preserve structural consistency, others may ignore, copy exactly, or even traverse into hard links, copying their contents.
The serious problems occur if hard links are copied exactly such that they become, in the new copy, cross-volume hard links which still point to original files and folders on the source volume. Unintentional cross-volume hard links, such as hard links in an "archive" folder which still point to locations on the original volume (according to drive letter), are catastrophes waiting to happen. For example, deleting what is much later presumed to be an unused archive directory on a disused backup volume may result in deleting current, active user data or system files.
A preventative measure for the drive-letter hazard is to use volume GUID path syntax,[20] rather than paths containing volume drive letters, when specifying the target path for a directory junction. For example, consider creating an alias forX:\Some\Other\Path atX:\Some\Path\Foo:
X:\Some\Path> linkd Foo X:\Some\Other\Path
As described above, if the folder structure that contains the resulting link is moved to a disk with a drive letter other thanX:, or if the letter is changed on driveX: itself, the data content at the target location is vulnerable to accidental corruption or malicious abuse. A more resilient version of this link can partially mitigate this risk by referencing the target volume by its GUID identifier value (which can be discovered by running thefsutil volume list command).
X:\Some\Path> linkd Foo \\?\Volume{12345678-abcd-1234-abcd-1234567890ab}\Some\Other\Path
Doing so ensures that the junction will remain valid if drive letterX: is changed by any means.
As for a proactive means of avoiding directory junction disasters, the commanddir /AL /S /B "X:\Some\Path" can be used to obtain, for careful analysis prior to committing any irreversible file system alterations, a list of all hard links "below" a certain file system location. While by definition every link in the resulting list has a path name that starts withX:\Some\Path\, if any of those hard links contains a target which is not subsumed byX:\Some\Path, then the specified scope has been escaped, and the starting directory you specified is not fully-subsuming. Extra caution may be indicated in this case, since the specified directory includes files and directories which reside on other physical volumes, or whose own parent-traversal-to-root does not include the specified directory.
The default security settings in Windows disallow non-elevated administrators and all non-administrators from creating symbolic links but not junctions. This behavior can be changed running "secpol.msc", the Local Security Policy management console (under: Security Settings\Local Policies\User Rights Assignment\Create symbolic links). It can be worked around by startingcmd.exe withRun as administrator option or therunas command. Starting with Windows 10 Insiders build 14972 the requirement for elevated administrator privileges was removed in Windows "Developer Mode", allowing symlinks to be created without needing to elevate the console as administrator. At the API level, aSYMBOLIC_LINK_FLAG_ALLOW_UNPRIVILEGED_CREATE flag is supplied for this purpose.[21]
TheWindows startup process does not support junction points, so it is impossible to redirect certain system folders:
Other critical system boot files, such as The sleep image filehiberfil.sys, also do not support redirection.
It is technically possible to redirect the following non-critical system folder locations:
Doing this may lead to long-term Windows reliability or compatibility issues. Creating junctions for\Users and\ProgramData pointing to another drive is not recommended as it breaks updates and Windows Store Apps.[22]
Creating junctions for \Users, \ProgramData,\Program Files or\Program Files (x86) pointing to other locations breaks installation or upgrade of Windows.[23]
Creating junctions for\Program Files or\Program Files (x86) pointing to another drive breaks Windows'Component Based Servicing which hardlinks files from its repository \Windows\SxS to their installation directory.[citation needed]
Windows Installer does not fully support symbolic links. Redirecting\Windows\Installer will cause most .msi-based Windows installers to fail with error 2755 and/or error 1632.
Since Windows XP uses the same NTFS format version as later releases, it's feasible to enable symbolic links support in it. For using NTFS symbolic links under Windows 2000 and XP, a third-party driver exists that does it by installing itself as a file system filter.[24][25]
Symbolic links to directories or volumes, calledjunction points and mount points, were introduced with NTFS 3.0 that shipped with Windows 2000. From NTFS 3.1 onwards, symbolic links can be created for any kind of file system object. NTFS 3.1 was introduced together withWindows XP, but the functionality was not made available (through ntfs.sys) to user mode applications. Third-party filter drivers – such as Masatoshi Kimura's opensourcesenable driver – could however be installed to make the feature available in user mode as well. The ntfs.sys released withWindows Vista made the functionality available to user mode applications by default.
Since NTFS 3.1, a symbolic link can also point to a file or remoteSMB network path. While NTFS junction points support only absolute paths on local drives, the NTFS symbolic links allow linking using relative paths. Additionally, the NTFS symbolic link implementation provides full support for cross-filesystem links. However, the functionality enabling cross-host symbolic links requires that the remote system also support them, which effectively limits their support to Windows Vista and later Windows operating systems.
If your location is in a FileSystem drive, the following values are allowed: If your location is in a FileSystem drive, the following values are allowed: File[,] Directory[,] Junction[,] HardLink
