Please test AD integration changes, documentation attached (was Re: timeout in LDAP access)
Tue Jul 29 09:07:00 GMT 2014
On Jul 28 20:51, Denis Excoffier wrote:
> On 2014-07-28 11:21, Corinna Vinschen wrote:
> > Ping?
> > On Jul 18 21:18, Corinna Vinschen wrote:
> >> We really should do that to avoid collisions with system accounts, IMHO.
> >> But maybe we should handle it as a border case of a border case, and
> >> reliably. Rather than using the default fake mechanism, what if
> >> we use default offsets for the two cases:
> >> Case 1: posix offset is < 0x100000 ==> Enforce posix 0ffset 0xfe80000
> >> Case 2: posix offset can't be fetched (this points to a local user
> >> having no access to this kind of domain information)
> >> ==> Enforce posix offset 0xfe000000.
> >> This would result in potential collisions in very rare border cases,
> >> but it would result in reliable mappings throught all processes.
> >> And, the complexity would be quite small.
> > any feedback on this one? Shall I create a snapshot with a matching
> > patch?
> I have nothing to add except that i am a great fan of cygwin snapshots in
> general, and i suppose that if several posix offsets are set to 0, it is
> a minor problem if all of them get replaced by the same 0xfe80000.
I chose even more unlikely offsets 0xfea00000 and 0xfe500000, but
otherwise, the 2014-07-29 snapshot I just uploaded implements the above
method, see http://cygwin.com/snapshots/
I also described this briefly in my preliminary documentation (attached),
which, I fear, it's time to merge into the official docs.
I'm inclined to create a new official Cygwin version with the AD
integration changes pretty soon. There seem to be no other way to get
more feedback on these changes :|
Corinna Vinschen Please, send mails regarding Cygwin to
Cygwin Maintainer cygwin AT cygwin DOT com
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For as long as Cygwin has existed, it has stored user and group
information in /etc/passwd and /etc/group files. Under the assumption
that these files would never be too large, the first process in a
process tree, as well as every execing process within the tree would
parse them into structures in memory. Thus every Cygwin process would
contain an expanded copy of the full information from /etc/passwd and
This approach has a few downsides. One of them is that the idea to have
always small files is flawed. Another one is that reading the entire
file is most of the time entirely useless, since most processes only
need information on their own user and the primary group. Last but not
least, the passwd and group files have to be maintained separately from
the already existing Windows user databases, the local SAM and Active
On the other hand, we have to have a mapping between Windows SIDs and
POSIX uid/gid values (see ), so we rely on some mechanism to convert
SIDs to uid/gid values and vice versa.
Microsoft "Services for UNIX" (SFU) (which are unfortunately deprecated
since Windows 8/Server 2012) never used passwd/group files. Rather, SFU
used a fixed, computational mapping between SIDs and POSIX uid/gid. It
allows to generate uid/gid values from SIDs and vice versa. The
mechanism is documented, albeit in a confusing way and spread over
multiple MSDN articles. The Cygwin approach clones the mapping, with
just tiny differences for backward compatibility.
How does it work?
The following description assumes you're comfortable with the concept of
Windows SIDs and RIDs. For a brief introduction, please read .
Cygwin's new mapping between SIDs and uid/gid values works in two ways.
- Read /etc/passwd and /etc/group files, like before, mainly for
- If no files are present, or if an entry is missing in the files, ask
At least, that's the default behaviour now. It will be configurable
using a file /etc/nsswitch.conf, which is discussed in a later section.
Let's explore the default for now.
If files are present, they will be scanned on demand as soon as a
mapping from SIDs to uid/gid or account names is required. The new
mechanism will never read the entire file into memory, but only scan for
the requested entry and cache this one in memory.
If no entry is found, or no passwd or group file was present, Cygwin
will ask the OS.
Note: If the first process in a Cygwin process tree determines that no
/etc/passwd or /etc/group file is present, no other process in
the entire process tree will try to read the files later on.
This is done for self-preservation. It's rather bad if the uid
or gid of a user changes during the lifetime of a process tree.
For the same reason, if you delete the /etc/passwd or /etc/group
file, this will be ignored. The passwd and group records read
from the files will persist in memory until either a new
/etc/passwd or /etc/group files is created, or you exit all
processes in the current process tree.
See the note in the section on /etc/nsswitch.conf for some
So if we've drawn a blank reading the files, we're going to ask the OS.
First thing, we ask the local machine for the SID or the username. The
OS functions LookupAccountSid and LookupAccountName are pretty
intelligent. They have all the stuff built in to ask for any account of
the local machine, the Active Directory domain of the machine, the
Global Catalog of the forest of the domain, as well as any trusted
domain of our forest for the information. One OS call and we're
Except, the calls only return the mapping between SID, account name and
the account's domain. We don't have a mapping to POSIX uid/gid and
we're missing information on the user's home dir and login shell.
Let's discuss the SID<=>uid/gid mapping first. Here's how it works.
- Well-known SIDs in the NT_AUTHORITY domain of the S-1-5-RID type, or
aliases of the S-1-5-32-RID type are mapped to the uid/gid value
RID. For an overview of well-known SIDs, see .
"SYSTEM" S-1-5-18 <=> uid/gid: 18
"Users" S-1-5-32-545 <=> uid/gid: 545
- Other well-known SIDs in the NT_AUTHORITY domain (S-1-5-X-RID):
S-1-5-X-RID <=> uid/gid: 0x1000 * X + RID
S-1-5-64-10 <=> uid/gid: 0x4000A == 262154
- Other well-known SIDs:
S-1-X-Y <=> uid/gid: 0x10000 + 0x100 * X + Y
"LOCAL" S-1-2-0 <=> uid/gid: 0x10200 == 66048
S-1-3-1 <=> uid/gid: 0x10301 == 66305
- Logon SIDs:
The own LogonSid is converted to the fixed uid 0xfff == 4095 and
named "CurrentSession". Any other LogonSid is converted to the
fixed uid 0xffe == 4094 and named "OtherSession".
- Mandatory Labels:
S-1-16-RID <=> uid/gid: 0x60000 + RID
"Medium Mandatory Level"
S-1-16-8192 <=> uid/gid: 0x62000 == 401408
- Accounts from the local machine's user DB (SAM):
S-1-5-21-X-Y-Z-RID <=> 0x30000 + RID
S-1-5-21-X-Y-Z-500 <=> uid/gid: 0x301f4 == 197108
- Accounts from the machine's primary domain:
S-1-5-21-X-Y-Z-RID <=> 0x100000 + RID
S-1-5-21-X-Y-Z-513 <=> 0x100201 == 1049089
- Accounts from a trusted domain of the machine's primary domain:
S-1-5-21-X-Y-Z-RID <=> trustPosixOffset(domain) + RID
"trustPosixOffset"? This needs a bit of explanation. This value
exists in Windows domains already since before Active Directory days.
What happens is this. If you create a domain trust between two
domains, a trustedDomain entry will be added to both databases. It
describes how *this* domain trusts the *other* domain. One attribute
of a trust is a 32 bit value called "trustPosixOffset" For each new
trust, trustPosixOffset will get some automatic value. In recent AD
domain implementations, the first trusted domain will get
trustPosixOffset set to 0x80000000. Following domains will get lower
values. Unfortunately the domain admins are allowed to set the
trustPosixOffset value for each trusted domain to some arbitrary 32
bit value, no matter what the other trustPosixOffsets are seet to,
thus allowing any kind of collisions between the trustPosixOffsets of
domains. That's not exactly helpful, but as the user of this value,
we have to *trust* the domain admins to set trustPosixOffset to
sensible values, or to keep it at the system chosen values.
So, for the first (or only) trusted domain of your domain, the
automatic offset is 0x80000000. An example for a user of that trusted
S-1-5-21-X-Y-Z-1234 <=> uid/gid 0x800004d2 == 2147484882
There's one problem with this approach. Assuming you're running in
the context of a local SAM user on a domain member machine. Local
users don't have the right to fetch this kind of domain information
from the DC, they'll get permission denied. In this case Cygwin will
fake a sensible trustPosixOffset value.
Another problem is if the AD administrators chose an unreasonable small
POSIX offset value. Anything below the hexadecimal value 0x100000 (the
POSIX offset of the primary domain) is bound to produce collisions with
system accounts as well as local accounts.
The right thing to do in this case is to notify your administrator of
the problem and to ask for moving the offset to a more reasonable
However, to reduce the probability for collisions, Cygwin overrides
this offset with a sensible fixed replacement offset.
- Local accounts from another machine in the network:
There's no SID<=>uid/gid mapping implemented for this case. The
problem is, there's no way to generate a bijective mapping. There's
no central place which keeps an analogue value of the
trustPosixOffset, and there's the additional problem that the
LookupAccountSid and LookupAccountName functions cannnot resolve the
SIDs, unless they know the name of the machine this SID comes from.
And even then it will probably suffer a "Permission denied" when
trying to ask the machine for its local account.
SFU just prints the account RID in this case, Cygwin maps the account
to the fake accounts "Unknown+User"/"Unknown+Group" with uid/gid -1.
Now we have a semi-bijective mapping between SIDs and POSIX uid/gid
values, but, given that we have potentially users and groups in
different domains having the same name, how do we uniquely differ
between them by name? Well, we can do that by making their names unique
in a per-machine way. Dependent on the domain membership of the
account, and dependent of the machine being a domain member or not, the
user and group names will be generated using a domain prefix and a
separator character between domain and account name. The default
separator character is the plus sign, '+', as in SFU.
- Well-known SIDs will have the separator character prepended:
"+SYSTEM", "+LOCAL", "+Medium Mandatory Level", ...
- If the machine is no domain member machine, only local accounts can be
resolved into names, so for ease of use, just the account names are
used as Cygwin user/group names:
"corinna", "bigfoot", "None", ...
- If the machine is a domain member machine, all accounts from the
primary domain of the machine are mapped to Cygwin names without
"corinna", "bigfoot", "Domain Users", ...
while accounts from other domains are prepended by their domain:
"DOMAIN1+corinna", "DOMAIN2+bigfoot", "DOMAIN3+Domain Users", ...
- Local machine accounts of a domain member machine get a Cygwin user
name the same way as accounts from another domain: The local machine
name gets prepended:
"MYMACHINE+corinna", "MYMACHINE+bigfoot", "MYMACHINE+None", ...
- If LookupAccountSid fails, Cygwin checks the accounts against the
known trusted domains. If the account is from one of the trusted
domains, an artificial account name is created. It consists of the
domain name, and a special name created from the account RID:
Otherwise we know nothing about this SID, so it will be mapped to the
fake accounts "Unknown+User"/"Unknown+Group" with uid/gid -1.
The information fetched from file or the Windows account database is cached
by the process. The cached information is inherited by child processes.
While usually working fine, this has some drawbacks. Consider a shell
calling `id'. `id' fetches all group information from the current token
and caches them. Unfortunately `id' doesn't start any child processes,
so the information is lost as soon as `id' exits.
But there's another caching mechanism available. If cygserver is
running it will provide passwd and group entry caching for all processes
in a Cygwin process tree, which first process has been started after
cygserver. So, if you start a Cygwin Terminal and cygserver is
running at the time, mintty, the shell, and all child processes will
use cygserver caching. If you start a Cygwin Terminal and cygserver is
not running a the time, none of the processes started inside this
terminal window will use cygserver caching.
The advantage of cygserver caching is that it's system-wide and, as long
as cygserver is running, unforgetful. Every Cygwin process on the system
will have the cygserver cache at its service. Additionally, all information
requested from cygserver once, will be cached inside the process itself
and, again, propagated to child processes.
Cygwin user names, home dirs, login shells
Obviously, if you don't maintain passwd and group files, you need to
have a way to maintain the other fields of a passwd entry as well.
Three things come to mind:
- You want to use a Cygwin username different from your Windows username.
Note: This is only supported via /etc/passwd and /etc/group files.
A Cygwin username maintained in the Windows user databases would
require very costly (read: slow) seach operations.
- You want a home dir different from the default /home/$USER.
- You want to specify a different login shell than /bin/bash.
How this is done depends on your account being a domain account or a
local account. Let's start with the default. Assuming your Windows
account name is "bigfoot" and your domain is "MY_DOM". Your default
passwd entry in absence of anything I'll describe below looks like this:
or, if your account is from a different domain than the primary domain of
Yes, the default homedir is still /home/bigfoot.
If your account is a domain account:
Either create an /etc/passwd and/or /etc/group file with entries for
your account and use that, just as before.
Or, Cygwin will utilize the posixAccount/posixGroup attributes per RFC
2307. These attributes are by default available in Active Directory
since Windows Server 2003 R2. They are "not set", unless utilized by
the (deprecated since Server 2012 R2) Active Directory "Server for
NIS" feature. The user attributes utilized by Cygwin are:
unixHomeDirectory If set, will be used as Cygwin home directory.
loginShell If set, will be used as Cygwin login shell.
gecos Content will be added to the pw_gecos field.
uidNumber See next section.
The group attributes utilized by Cygwin are:
gidNumber See next section.
Apart from power shell scripting or inventing new CLI tools, these
attributes can be changed using the "Attribute Editor" tab in the user
properties dialog of the "Active Directory Users and Computers"
MMC snap-in. Alternatively, if the "Server for NIS" administration
feature has been installed, there will be a "UNIX Attributes" tab which
contains the required fields, except for the gecos field, which isn't
really important anyway. Last resort is "ADSI Edit".
The primary group of a user is always the Windows primary group set in
Active Directory and can't be changed.
If your machine is not a domain member machine or your account is a
local account for some reason:
Either create an /etc/passwd and/or /etc/group file with entries for
your account and use that, just as before.
Or enter the information into the "Comment" field of your local user
entry. In the "Local Users and Groups" MMC snap-in it's called
You can utilze this field even if you're running a "home edition" of
Windows, using the command line. The "net user" command allows to set
all values in the SAM, even if the GUI is crippled.
A Cygwin SAM comment entry looks like this:
<cygwin key="value" key="value" [...] />
The supported keys are
home="value" Sets the Cygwin home dir to value.
shell="value" Sets the Cygwin login shell to value.
group="value" Sets the Cygwin primary group of the account
to value, provided that the user *is* already
a member of that group. This allows to
override the default "None" primary group for
One nice idea here is, for instance group="Users".
unix="value" Sets the NFS/Samba uid of the user to the decimal
value. See the next chapter.
The <cygwin .../> string can start at any point in the comment, but
you have to follow the rules:
- It starts with "<cygwin " and ends with "/>".
- The "cygwin" string and the key names have to be lowercase.
- No spaces between key and "value", just the equal sign.
- The value must be placed within double quotes and it must not
contain a double quote itself. The double quotes are required
for the decimal values as well!
net user corinna /comment:"<cygwin home=\"/home/foo\"/>"
Bash example (use single quotes):
net user corinna /comment:'<cygwin home="/home/foo"/>'
For changing group comments, use the `net localgroup' command. The
supported key/value pair for groups are
unix="value" Sets the NFS/Samba gid of the group to the
decimal value. See the next chapter.
NFS account mapping
Microsoft's NFS client does not map the uid/gid values on the NFS shares
to SIDs. There's no such thing as a (fake) security descriptor returned
to the application. Rather, via an undocumented API an applications can
fetch RFC 1813 compatible NFSv3 stat information from the share.
This is what Cygwin is using to show stat information for files on NFS
The problem is, while all other information in this stat record, like
timestamps, file size etc., can be used by Cygwin, Cygwin had no way to
map the values of the st_uid and st_gid members to a Windows SID for a
long time. So it just faked the file owner info and claimed that it's
However, SFU has, over time, developed multiple methods to map UNIX
uid/gid values on NFS shares to Windows SIDs. You'll find the full
documentation of the mapping methods in .
Cygwin now utilizes the RFC 2307 mapping for this purpose. This is most
of the time provided by an AD domain, but it could also be a standalone
LDAP mapping server. Per RFC 2307, the uid is in the attribute
uidNumber. For groups, the gid is in the gidNumber attribute.
When Cygwin stat's files on an NFS share, it asks the mapping server via
LDAP in two different ways, depending on the role of the mapping server.
- If the server is an AD domain controller, it asks for an account with
uidNumber attribute == st_uid field of the stat record returned by
NFS. If an account matches, AD returns the Windows SID, so we have an
immediate mapping from UNIX uid to a Windows SID, if the user account
has a valid uidNumber attribute. For groups, the method is the same,
just that Cygwin asks for a group with gidNumber attribute == st_gid
field of the stat record.
- If the server is a standalone LDAP mapping server Cygwin asks for the
same uidNumber/gidNumber attributes, but it can't expect that the LDAP
server knows anything about Windows SIDs. Rather, the mapping server
returns the account name. Cygwin then asks the DC for an account with
this name, and if that succeeds, we have a mapping between UNIX
uid/gid and Windows SIDs.
The mapping will be cached for the lifetime of the process, and inherited
by child processes.
Samba account mapping
A fully set up Samba with domain integration is running winbindd to
map Window SIDs to artificially created UNIX uids and gids, and this
mapping is transparent within the domain, so Cygwin doesn't have to do
However, setting up winbindd isn't for everybody, and it fails to map
Windows accounts to already existing UNIX users or groups. In contrast
to NFS, Samba returns security descriptors, but unmapped UNIX accounts
get special SIDs:
- A UNIX user account with uid X is mapped to the Windows SID S-1-22-1-X.
- A UNIX group account with gid X is mapped to SID S-1-22-2-X.
As you can see, even though we have SIDs, they just reflect the actual
uid/gid values on the UNIX box in the RID value. It's only marginally
different from the NFS method, so why not just use the same method as
That's what Cygwin will do. If it encounters a S-1-22-x-y SID, it
will perform the same RFC 2307 mapping as for NFS shares.
For home users without any Windows domain or LDAP server per RFC 2307,
but with a Linux machine running Samba, just add this information to
your SAM account. Assuming the uid of your Linux user account is 505
and the gid of your primary group is, say, 100, just add the values to
your SAM user and group accounts. The following example assumes you
didn't already add something else to the comment field.
To your user's SAM comment (remember: called "Description" in the GUI),
<cygwin group="Users" unix="505"/>
To the user's group SAM comment add:
This should be sufficient to work on your Samba share and to see
all files owned by your Linux user account as your files.
The /etc/nsswitch.conf file
Last, but not least, let's talk about the way to configure how the
mapping works on your machine. On Linux and some other UNIXy OSes, we
have a file called /etc/nsswitch.conf. One part of it is to specify
how the passwd and group entries are generated. That's what Cygwin now
provides as well.
The /etc/nsswitch.conf file is optional. If you don't have one, Cygwin
uses sensible defaults.
Note: The /etc/nsswitch.conf file is read exactly once by the first
process of a Cygwin process tree. If there was no /etc/nsswitch.conf
file when this first process started, then no other process in
the running Cygwin process trees will try to read the file.
If you create or change /etc/nsswitch.conf, you need to restart
all Cygwin processes that need to see the change. If the process
you want to see the change is a child of another process, you
need to restart all of that process's parents, too.
For example, if you run Vim inside the default Cygwin Terminal,
Vim is a child of your shell, which is a child of mintty.exe. If
you edit /etc/nsswitch.conf in that Vim instance, your shell
won't immediately see the change, nor will Vim if you restart it
from that same shell instance. This is because both are getting
their nsswitch information from their ancestor, mintty.exe. You
need to start a fresh terminal window for the change to take
By contrast, if you leave that Cygwin Terminal window open after
making the change to /etc/nsswitch.conf, then restart a Cygwin
service like cron, cron will see the change, because it is not a
child of mintty.exe or any other Cygwin process. (Technically, it
is a child of cygrunsrv, but that instance also restarts when you
restart the service.)
The reason we point all this out is that the requirements for
restarting things are not quite as stringent as when you replace
cygwin1.dll. If you have three process trees, you have three
independent copies of the nsswitch information. If you start a
fresh process tree, it will see the changes. As long as any
process in an existing process tree remains running, all
processes in that tree will continue to use the old information.
So, what mischief can we perform with /etc/nsswitch.conf? To explain,
lets have a look into an /etc/nsswitch.conf file set up to all default
passwd: files db
group: files db
db_enum: cache builtin
The first line, starting with a hash '#' is a comment. The hash
character starts a comment, just as in shell scripts. Everything up to
the end of the line is ignored. So this:
foo: bar # baz
means, for the entry "foo", do "bar", ignore everything after the hash
sign. "baz" is only a comment.
The other lines define the available settings. The first word up to a
colon is a keyword. Note that the colon *must* follow immediately after
the keyword. This is a valid line:
This is not valid:
foo : bar
Apart from this restriction, the reminder of the line can have as
may spaces and TABs as you like. This is a valid line:
foo: bar baz
Now let's have a look at the available keywords and settings.
The two lines starting with the keywords "passwd" and "group" define
where Cygwin gets its passwd and group information from. "files" means,
fetch the information from the corresponding file in the /etc directory.
"db" means, fetch the information from the Windows account databases,
the SAM for local accounts, Active Directory for domain account.
Read passwd entries only from /etc/passwd.
Read group entries only from SAM/AD.
group: files # db
Read group entries only from /etc/group ("db" is ignored due to the
preceding hash sign).
passwd: files db
Read passwd entries from /etc/passwd. If a user account isn't found,
try to find it in SAM or AD. This is the default for both, passwd
and group information.
group: db files
This is a valid entry, but the order will be ignored by Cygwin. If
both, files and db are specified, Cygwin will always try the files
first, then the db.
The remaining entries define certain aspects of the Windows account
database search. "db_prefix" determines how the Cygwin user or group
name is created:
This is the default. If your account is from the primary domain of
your machine, or if your machine is a standalone machine (not a domain
member), your Cygwin account name is just the Windows account
If your account is from another domain, or if you're logged in as
local user on a domain machine, the Cygwin username will be the
combination of Windows domainname and username, with the separator
char in between:
MY_DOM+username (foreign domain)
MACHINE+username (local account)
Builtin accounts have just the separator char prepended:
Unknown accounts on NFS or Samba shares (that is, accounts which
cannot be mapped to Windows user accounts via RFC 2307) get a
Cygwin account name consisting of the artificial domains "Unix_User"
or "Unix_Group" and the uid/gid value, for instance:
Like "auto", but primary domain accounts will be prepended by
the domainname as well.
All accounts, even the builtin accounts, will have the domain
"db_separator" defines the spearator char used to prepend the domain
name to the user or group name. The default is '+':
With "db_separator", you can define any ASCII char except space,
tab, carriage return, line feed, and the colon, as separator char.
"db_enum" defines the depth of a database search, if an application
calls one of the enumeration functions getpwent or getgrent.
The problem with these functions is, they neither allow to define how many
entries will be enumerated when calling them in a loop, nor do they
allow to add some filter criteria. They were designed back in the days,
when only /etc/passwd and /etc/group files existed and the number of
user accounts on a typical UNIX system was seldomly a three-digit
These days, with user and group databases sometimes going in the
six-digit range, they are a potential burden. For that reason, Cygwin
does not enumerate all user or group accounts by default, but rather
just a very small list, consisting only of the accounts cached in memory
by the current process, as well as a handful of predefined builtin
"db_enum" allows to specify the accounts to enumerate in a fine-grained
way. It takes a list of sources as argument:
db_enum: source1 source2 ...
The recognized sources are the following:
none No output from getpwent/getgrent at all.
all The opposite. Enumerates accounts from all known
sources, including all trusted domains.
cache Enumerate all accounts currently cached in memory.
builtin Enumerate the predefined builtin accounts for backward
compatibility. These are five passwd accounts
(SYSTEM, LocalService, NetworkService, Administrators,
TrustedInstaller) and two group accounts (SYSTEM and
files Enumerate the accounts from /etc/passwd or /etc/group.
local Enumerate all accounts from the local SAM.
primary Enumerate all accounts from the primary domain.
alltrusted Enumerate all accounts from all trusted domains.
some.domain Enumerate all accounts from the trusted domain some.domain.
The trusted domain can be given as Netbios flat name
(MY_DOMAIN) or as dns domain name (my_domain.corp).
In contrast to the aforementioned fixed source keywords,
distinct domain names are caseinsensitive. Only domains
which are actually trusted domains are enumerated.
Unknown domains are simply ignored.
Please note that getpwent/getgrent do *not* test if an account was
already listed from another source, so an account can easily show up
twice or three times. Such a test would be rather tricky, nor does the
Linux implementation perform such test. Here are a few examples for
No output from getpwent/getgrent at all. The first call to the
function immediately returns a NULL pointer.
db_enum: cache files
Enumerate all accounts cached by the current process, plus all entries
from either the /etc/passwd or /etc/group file.
db_enum: cache local primary
Enumerate all accounts cached by the current process, all accounts
from the SAM of the local machine, and all accounts from the
primary domain of the machine.
db_enum: local primary alltrusted
Enumerate the accounts from the machine's SAM, from the primary domain
of the machine, and from all trusted domains.
db_enum: primary domain1.corp sub.domain.corp domain2.net
Enumerate the accounts from the primary domain and from the domains
domain1.corp, sub.domain.corp and domain2.net.
Enumerate everything and the kitchen sink.
 This may change. Right now the file is read in 32K chunks, but
we could easily read the file in 64K chunks and, if we find the
file is < 64K anyway, just cache the entire bunch, like before.
Not implemented yet, but something to keep in mind.
 This is where Cygwin differs from SFU. The reason is that we need
the old uid/gid values for backward compatibility. There are Cygwin
packages (cron, for instance) who rely on the fact that the uid of
SYSTEM is 18. In SFU, these accounts get mapped like the other
built in SIDs.
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