317 lines
13 KiB
Plaintext
317 lines
13 KiB
Plaintext
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<sect1 id="ntsec"><title>NTSEC Documentation</title>
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<para>The design goal of the ntsec patch was to get a more UNIX like
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permission structure based upon the security features of Windows NT.
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To describe the changes, I will give a short overview of NT security
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in chapter one.</para>
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<para>Chapter two discusses the changes in ntsec related to privileges on
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processes.</para>
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<para>Chapter three shows the UNIX like setting of file permissions.</para>
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<para>The setting of UNIX like object permissions is controlled by the new
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<EnVar>CYGWIN</EnVar> variable setting <literal>(no)ntsec</literal>.</para>
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<para>On NT ntsec is now turned on by default.</para>
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<sect2 id="ntsec-common"><title>NT security</title>
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<para>The NT security allows a process to allow or deny access of
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different kind to `objects'. `Objects' are files, processes,
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threads, semaphores, etc.</para>
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<para>The main data structure of NT security is the `security descriptor'
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(SD) structure. It explains the permissions, that are granted (or denied)
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to an object and contains information, that is related to so called
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`security identifiers' (SID).</para>
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<para>An SID is a unique identifier for users, groups and domains.
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SIDs are comparable to UNIX UIDs and GIDs, but are more complicated
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because they are unique across networks. Example:</para>
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<example>
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<screen>
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SID of a system `foo':
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S-1-5-21-165875785-1005667432-441284377
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SID of a user `johndoe' of the system `foo':
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S-1-5-21-165875785-1005667432-441284377-1023
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</screen>
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</example>
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<para>The above example shows the convention for printing SIDs. The leading
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`S' should show that it is a SID. The next number is a version number which
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is always 1. The next number is the so called `top-level authority' that
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identifies the source that issued the SID.</para>
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<para>While each system in a NT network has it's own SID, the situation
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is modified in NT domains: The SID of the domain controller is the
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base SID for each domain user. If an NT user has one account as domain
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user and another account on his local machine, this accounts are under
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any circumstances DIFFERENT, regardless of the usage of the same user
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name and password!</para>
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<example>
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<screen>
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SID of a domain `bar':
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S-1-5-21-186985262-1144665072-740312968
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SID of a user `johndoe' in the domain `bar':
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S-1-5-21-186985262-1144665072-740312968-1207
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</screen>
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</example>
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<para>The last part of the SID, the so called `relative identifier' (RID),
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is used as UID and/or GID under cygwin. As the name and the above example
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implies, this id is unique only relative to one system or domain.</para>
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<para>Note, that it's possible, that an user has the same RID on two
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different systems. The resulting SIDs are nevertheless different, so
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the SIDs are representing different users in an NT network.</para>
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<para>There is a big difference between UNIX IDs and NT SIDs, the existence of
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the so called `well known groups'. For example UNIX has no GID for the
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group of `all users'. NT has an SID for them, called `Everyone' in the
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English versions. The SIDs of well-known groups are not unique across
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an NT network but their meanings are unmistakable.
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Examples of well-known groups:</para>
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<screen>
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<example>
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everyone S-1-1-0
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creator/owner S-1-3-0
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batch process (via `at') S-1-5-3
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authenticated users S-1-5-11
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system S-1-5-18
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</screen>
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</example>
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<para>The last important group of SIDs are the `predefined groups'. This
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groups are used mainly on systems outside of domains to simplify the
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administration of user permissions. The corresponding SIDs are not unique
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across the network so they are interpreted only locally:</para>
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<screen>
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<example>
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administrators S-1-5-32-544
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users S-1-5-32-545
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guests S-1-5-32-546
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...
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</screen>
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</example>
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<para>Now, how are permissions given to objects? A process may assign an SD
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to the object. The SD of an object consists of three parts:</para>
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<itemizedlist spacing="compact">
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<listitem><para>- the SID of the owner </para></listitem>
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<listitem><para>- the SID of the group </para></listitem>
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<listitem><para>- a list of SIDs with their permissions, called
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`access control list' (ACL) </para></listitem>
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</itemizedlist>
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<para>UNIX is able to create three different permissions, the permissions
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for the owner, for the group and for the world. In contrast the ACL
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has a potentially infinite number of members. Every member is a so called
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`access control element' (ACE). An ACE contains three parts:</para>
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<itemizedlist spacing="compact">
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<listitem><para>- the type of the ACE </para></listitem>
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<listitem><para>- permissions, described with a DWORD </para></listitem>
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<listitem><para>- the SID, for which the above mentioned permissions are
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set </para></listitem>
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</itemizedlist>
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<para>The two important types of ACEs are the `access allowed ACE' and the
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`access denied ACE'. The ntsec patch only uses `access allowed ACEs'.</para>
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<para>The possible permissions on objects are more complicated than in
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UNIX. For example, the permission to delete an object is different
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from the write permission.</para>
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<para>With the aforementioned method NT is able to grant or revoke permissions
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to objects in a far more specific way. But what about cygwin? In a POSIX
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environment it would be fine to have the security behavior of a POSIX
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system. The NT security model is able to reproduce the POSIX model.
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The ntsec patch tries to do this in cygwin.</para>
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<para>The creation of explicit object security is a bit complicated, so
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typically only two simple variations are used:</para>
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<itemizedlist spacing="compact">
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<listitem><para>- default permissions, computed by the operating system </para></listitem>
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<listitem><para>- each permission to everyone </para></listitem>
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</itemizedlist>
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<para>For parameters to functions that create or open securable objects another
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data structure is used, the `security attributes' (SA). This structure
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contains an SD and a flag, that specifies whether the returned handle
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to the created or opened object is inherited to child processes or not.
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This property is not important for the ntsec patch description, so in
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this document SDs and SAs are more or less identical.</para>
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</sect2>
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<sect2 id="ntsec-processes"><title>Process privileges</title>
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<para>Any process started under control of cygwin has a semaphore attached
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to it, that is used for signaling purposes. The creation of this semaphore
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can be found in sigproc.cc, function `getsem'. The first parameter to the
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function call `CreateSemaphore' is an SA. Without ntsec patch this SA
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assigns default security to the semaphore. There is a simple disadvantage:
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Only the owner of the process may send signals to it. Or, in other words,
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if the owner of the process is not a member of the administrators' group,
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no administrator may kill the process! This is especially annoying, if
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processes are started via service manager.</para>
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<para>The ntsec patch now assigns an SA to the process control semaphore, that
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has each permission set for the user of the process, for the
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administrators' group and for `system', which is a synonym for the
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operating system itself. The creation of this SA is done by the function
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`sec_user', that can be found in `shared.cc'. Each member of the
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administrators' group is now allowed to send signals to any process
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created in cygwin, regardless of the process owner.</para>
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<para>Moreover, each process now has the appropriate security settings, when
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it is started via `CreateProcess'. You will find this in function
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`spawn_guts' in module `spawn.cc'. The security settings for starting a
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process in another user context have to add the sid of the new user, too.
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In the case of the `CreateProcessAsUser' call, sec_user creates an SA with
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an additional entry for the sid of the new user.</para>
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</sect2>
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<sect2 id="ntsec-files"><title>File permissions</title>
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<para>If ntsec is turned on, file permissions are set as in UNIX. An SD is
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assigned to the file containing the owner and group and ACEs for the
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owner, the group and `Everyone'. If the group of the file is not the
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administrators' group, the administrators' group gets the permissions
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to read the permissions (yes, this is an own permission flag
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<literal>:-)</literal>) and to take the ownership on this file.
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If the file's group is the administrators group itself, this behaviour
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is modified to support the typical behaviour of NT better:
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As you know, if one is member of admin group, all her files are owned
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by the group instead of by her. This is not the case with ntsec but the
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other admins should have easier access to the administrative files.
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So in this case the admin group gets additionally the permissions to
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write permissions and to write extended attributes, also in the case
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where group permissions are set to 0.</para>
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<para>The complete settings of UNIX like permissions can be found in the file
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`security.cc'. The two functions `get_nt_attribute' and `set_nt_attribute'
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are the main code. The reading and writing of the SDs is done by the
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functions `ReadSD' and `WriteSD'. They are using the Backup API functions
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`BackupRead' and `BackupWrite', that have the advantage not to crash,
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if they are used on non NTFS file systems! These crashes are the default
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behavior of the security API, if it's used on, e.g., FAT or SAMBA
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file systems <literal>:-(</literal></para>
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<para>Unfortunately, the settings of NT file security are only available
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on NTFS. SAMBA doesn't support them.</para>
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<para>If you are creating a file `foo' outside of cygwin, you will see something
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like the following on <command>ls -ln</command>:</para>
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<para>If your login is member of the administrators' group:</para>
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<screen>
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rwxrwxrwx 1 544 513 ... foo
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</screen>
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<para>if not:</para>
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<screen>
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rwxrwxrwx 1 1000 513 ... foo
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</screen>
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<para>Note the user and group IDs. 544 is the UID of the administrators' group.
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This is a `feature' <literal>:-P</literal> of WinNT. If one is a member of
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the administrators' group, every file, that he has created is owned by the
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administrators' group, instead by him.</para>
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<para>The second example shows the UID of the first user, that has been
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created with NT's the user administration tool. The users and groups are
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sequentially numbered, starting with 1000. Users and groups are using the
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same numbering scheme, so a user and a group don't share the same ID.</para>
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<para>In both examples the GID 513 is of special interest. This GID is a
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well known group with different naming in local systems and domains.
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Outside of domains the group is named 'None' (`Kein' in German, `Aucun'
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in French, etc.), in domains it is named 'Domain Users'. Unfortunately,
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the group `None' is never shown in the user admin tool outside of domains!
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This is very confusing but it seems that this has no negativ influences.</para>
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<para>To work correctly the ntsec patch depends on reasoned files
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<filename>/etc/passwd/</filename> and <filename>/etc/group</filename>.
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The names and the IDs must correspond to the appropriate
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NT IDs! The IDs used in cygwin are the RID of the NT SID, as aforementioned.
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An SID of e.g. the user `corinna' on my NT workstation:</para>
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<example>
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<screen>
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S-1-5-21-165875785-1005667432-441284377-1000
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</screen>
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</example>
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<para>Note the last number: It's the RID 1000, the cygwin's UID.</para>
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<para>Unfortunately, workstations and servers outside of domains are not
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able to set primary groups! In these cases, where there is no correlation
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of users to primary groups, NT returns 513 (None) as primary group,
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regardless of the membership to regular groups of these users.</para>
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<para>when using <command>mkpasswd -l -g</command> on such systems, you
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have to change the primary group by hand if `None' as primary group is
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not what you want (and I'm sure, it's not what you want!)</para>
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<para>To get help in creating correct passwd and group files, look at
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the following examples, that are part of my files. With the exception
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of my personal user entry, all entries are well known entries. For a
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better understanding, the names are translated to the equivalents of the
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English NT version:</para>
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<example>
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<title>/etc/passwd:</title>
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<screen>
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everyone:*:0:0:::
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system:*:18:18:::
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administrator::500:544::/home/root:/bin/bash
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guest:*:501:546:::
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administrators:*:544:544::/home/root:
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corinna::1000:547:Corinna Vinschen:/home/corinna:/bin/tcsh
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</screen>
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</example>
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<example>
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<title>/etc/group:</title>
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<screen>
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everyone::0:
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system::18:
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none::513:
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administrators::544:
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users::545:
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guests::546:
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powerusers::547:
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</screen>
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</example>
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<para>Groups may be mentioned in the passwd file, too. This has two
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advantages:</para>
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<itemizedlist spacing="compact">
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<listitem><para>- Because NT assigns them to files as owners, a
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<command>ls -l</command> is often better readable. </para></listitem>
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<listitem><para>- Moreover it's possible to assigned them to files as
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owners with cygwin's <command>chown</command>. </para></listitem>
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</itemizedlist>
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<para>The group `system' is the aforementioned synonym for the operating system
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itself and is normally the owner of processes, that are started through
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service manager. The same is true for files, that are created by
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processes, which are started through service manager.</para>
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</sect2>
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</sect1>
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