A Windows user account can be renamed. You almost certainly never want to do it, and changing your Yale Netids is almost impossible because of all the systems that use it, but Windows does allow it. What doesn’t change is a unique binary identifier created when the account was defined: the Security ID or SID. When you login, Windows identifies you through your SID. Access to files and directories are through the SID. Each group you are a member of also has its own SID.
There are three sources for SIDs:
Microsoft generates some built in “factory installed” SIDs for standard Windows components, and many of these built in accounts also have a dummy User name.
As an administrator, you create Local User accounts on your own machine. The SID for a Local User is unique and exists only on the one computer, even if you create an account with the same User name on another computer.
Microsoft’s Active Directory creates SIDs that can be shared across machines. All computers that are members of the same AD, including all the Managed Workstations at Yale, share the SIDs in the yu.yale.edu domain also known by the nickname YALE. This is where your Netid and its password are saved.
Factory Installed SIDs
The Microsoft TrustedInstaller identity is built into Windows and has permission to update the most sensitive files in the operating system. It installs the monthly maintenance and optional components of Windows. You do not have and do not want to have direct access to it.
SYSTEM is an anonymous identity for most of the programs that run as background Services. There are some special versions of SYSTEM for SQL Server and Hyper-V. SYSTEM identities can access any file on a local hard disk but this SID is meaningless to other computers, so programs that run under this SID have no access to anything on another computer. There are two practical consequences of this:
SQL Server can only backup and restore databases to files on a local disk.
Hyper-V can only start a VM if all its VM VHDX disk files and ISO DVD image files are on a local disk, and it can only export/import a VM to local directories.
In some cases, the utility that you are running under your userid may have access to a file through your logon that the background service cannot access. You may be able to build a SQL Server backup request or configure a VM through the Hyper-V manager, but when you try to run the backup or start the VM, that will fail. Typically, an administrator learns to copy files to local disk before an operation and then move or copy files to a network disk when they are no longer needed.
Local User SIDs created by You
Yale Security Policy expects users to login with their Yale Netid to Yale Managed Workstations and other computers that are Yale owned and on the Yale network. This section describes the security procedures for Hyper-V VMs that run inside your laptop or personally owned computers on a home network.
A Local User is an account you create when you are acting as an administrator of your Windows 11 or Windows Server VM. You assign a name and a password to the account and then add it to local groups. A local user can login to the machine, own files, and can be given various permissions.
The SID for a Local User account is unique to the one machine where it was created and is meaningless on other machines. You can see this by logging in as a non-admin Local User, creating a file on an NTFS formatted thumb drive, and then move the drive to another computer. The other computer has no idea who owns or created the file because the Local SID generated by that computer is meaningless to other machines.
When you login to a computer, it runs everything in your session under your SID.
Workgroup (Local User) Authentication
Yale owned machines running Windows must be members of the YALE AD Domain. Yale Policy requires you to use your Netid Domain account to login to such machines.
Personally owned machines or VMs are typically not a member of the YALE Domain or any other AD. Microsoft created AD in 2000, so from around 1994 to 2000 Windows machines shared files with another protocol named “Workgroups” (from the “Windows for Workgroups 3.11” product release of 1994).
Any Windows 10/11 or Windows Server can create Local Users and Local Groups. Local Users are assigned a userid and password by an administrator of the machine. Someone knowing the password can login to that machine as that user.
When you have more than one computer (in a home, department, lab, or small company) then the same userid and password can be defined as a Local User on more than one computer.
On each computer, each Local User is assigned a large generated binary ID number called the “SID”. Local User objects on different machines have different SID values on each machine. Although you may think of yourself by your “userid” and that name will show up in every tool and command that presents information about files and programs to a person, in reality it is the SID and not the userid that is stored in memory or written to disk as the owner of a file or someone permitted to use the file.
Since the SID of user “johndoe” is different on each machine, if you mount a USB disk formatted with NTFS and create a file as user johndoe on one machine, then eject the disk and move it to another machine, the owner of the file will not be displayed as “johndoe” but rather as some very long number that is the SID generated on the other machine. No Windows system recognizes the SID of a Local User from another Windows system.
However, if two Windows computers are on the same network, and the same userid with the same password is defined on both machines, then “johndoe” on one computer can transparently through the network access files owned by or permitted to “johndoe” on the other computer. The trick here is Workgroup Authentication. If the two machines have Local Users with the same userid and same password on both machines, then when the machines talk to each other over the network they correlate the two Local Users accounts and “map” the SID on the client machine to the corresponding SID for the same userid on the server machine.
Until Windows 11 24H2 this authentication was done with a Microsoft protocol named “NTLMv2”. Starting sometime after the introduction of 24H2, a new protocol based on Kerberos called LocalKDC will be phased in and NTLM will be phased out. This will happen under the covers and will be transparent to Windows users, although it may be more visible to Linux users who access Windows file shares, where the same support will have to be added.
Although Workgroup Authentication is normally used for sharing files over the network, it also applies to commands and utilities that control, configure, administer, or query information between two Windows computers.
Since Yale policy prohibits creation of Local User accounts on Yale Managed Workstations, you may have to access files on Workgroup machines by interactively presenting the userid and password as alternate credentials in the dialog boxes or commands used to access the Workgroup Machine. If you install OpenSSH Server and add your public key to the authorized_keys file in the .ssh subdirectory of your home directory, then on a Workgroup computer from an account with your matching private key you can SSH to yale\netid@mwhostname and enter commands or scp files. You can also execute PowerShell command using PowerShell remoting with “-hostname mwhostname -username yale\netid”.
Fully Qualified Usernames
While two userids with the same password match each other and allow access over the network, they still have different SIDs, and this may be important when you move disks from one computer to another. There is a Windows notation to explicitly prefix a username with the machine or AD domain where it was defined.
Assume there are two computers named SNARK and BOOJUM and each has an account named “bellman” with the same password on both machines. The notation “SNARK\bellman” means the account (and SID) for bellman on SNARK and “BOOJUM\bellman” means the account on BOOJUM.
If bellman logged into SNARK contacts BOOJUM through the network without providing explicit alternate credentials, then BOOJUM defaults to Workgroup Authentication and verifies that it has a local user also named “bellman” and that the password for “bellman” is the same on both BOOJUM and SNARK. It then assigns the SID of BOOJUM\bellman to the network session.
This works for access to files and directories owned by or permitted to BOOJUM\bellman. It also works for SQL Server sessions using Windows authentication, which are then associated with the SQL Server login identified as BOOJUM\bellman (even though the remote client is really SNARK\bellman). It also works for Windows admin command that can be directed over the network to another computer.
There is, however, no way to generate a permission on a network session for a specific user on a remote computer. The user SNARK\baker could not connect to BOOJUM at all unless there is a BOOJUM\baker with the same password, and SQL Server can only generate Logins for Local Users (and Domain Users if the machine is also a member of a Domain).
Active Directory Domain User SIDs
An Active Directory is a central network service that creates SIDs for user accounts, and Domain SIDs can be shared with all the computers that are members of the Domain. Any user logging onto a machine with a Domain account gets the SID from the Domain. When that user moves to another computer in the same Domain and logs in, he gets the same SID on that machine too.
If you are logged into a computer with a Domain account, and you create a file on a USB drive formatted with NTFS, and you then move that drive to another computer in the same Domain, that computer recognizes the SID on the file and displays your Domain userid as the owner instead of the SID number.
So, if “bellman” is a Yale Netid, and SNARK and BOOJUM are Yale machines, then there would be only one userid named YALE\bellman, it would be the same on both machines, it would have the same SID on both machines, and disks could be freely moved from one machine to the other.
While it is against Yale policy to do this on a Yale owned machine, Windows allows a computer in a Domain to have both Domain Users and Local Users. In this case, when a user is logged in with his Domain account, then the machine uses Kerberos authentication to access files and services on other computers in the same Domain. Logoff and log back in as a Local User account, and now the computer uses Workgroup Authentication to access files and services on other computers connected to the same LAN.
A Local User account can have the same userid as a Domain account. The Domain account is referred to by the fully qualified name of domainname\userid. In the YALE Domain, YALE\bellman is a Domain account, and it is entirely different and completely unrelated to the Local Users BOOJUM\bellman and SNARK\bellman. They have different home directories, own different files, and have different permissions (because they have different SIDs).
Every Yale Managed Workstation is a member of the YALE Domain. Other computers and VMs can be added to the YALE Domain, though it is best if you administer a departmental OU and can create a “new computer” object with that hostname in that OU before you join them to the Domain. When you have two systems in the Yale OU, you can use and test most Domain identity features.
Your Own Domain
You can only create an AD on a computer or VM running Windows Server. It takes about 10 minutes to install the VM from an ISO image file (or you can download a VHDX file where it is already installed). You can then install your own AD with two PowerShell commands. Unless you really want to learn about ADs, or you need to test a program that manages them, having your own AD is probably more trouble than it is worth.
Yale’s site license allows us to run as many copies of Windows Server as we want, so there is no cost to installing it in a VM. It is not an attractive choice as the primary system on the first two or three personal computers that you own because it lacks a few consumer-oriented features. Windows Server 2025 will be better, but traditionally Server expected to run unattended in a machine room and lacked good WiFi, audio, webcam, and drivers for some devices. Don’t plan on using it for Zoom meetings.
Windows Server 2022 looks like Windows 10. Server 2025 looks like Windows 11 and can, for the most part, be configured and managed the same way that you manage a desktop.
The instructions for adding AD to a Windows Server VM are covered in detail in Howto Create Standalone AD Server - Identity and Access Management - Confluence (atlassian.net).
A Domain needs a DNS name and a nickname. We will call this one yu.yale.sandbox with nickname HUNTING.
When the DNS Server Role was added, Windows tells it to forward all requests it cannot resolve internally to the same external DNS server that
That is because when you create the AD, they also become Domain Admins. This initial group of admins becomes especially useful because they are both Local Users and Domain Users at the same time. This will allow you to use either Workgroup Authentication or Domain Authentication to connect to this Server and access files or services (including AD Admin services). We create a user named “bellman”, or to be precise “FIT\bellman”.
For testing and software development, it is only necessary for this test Domain to have one computer (FIT) which is its Domain Controller and, because it is required, is also a DNS Server.
There are two PowerShell commands to install the Active Directory support and to create the new Domain. You may have to reboot between the two commands. The AD needs a full DNS name and a nickname. For this example, we make the nickname HUNTING.
The DNS name will only be known to the DNS server that you created while the second PowerShell command generated the Domain. You are free to define this name in other computers. The HUNTING nickname is, at this point, only known on the Server itself.
Fortunately, FIT is still a computer on your network, and you can reference it from other machines using standard tricks like typing in “\\fit.local”. Remember that part about how you can use either Workgroup or Domain authentication to talk to a machine in your network and authenticate as a Local/Domain user like “bellman”.
Here is the Big Idea. The Local User FIT\bellman is the same SID and therefore the same account as the Domain user HUNTING\bellman. Generally speaking, anywhere you could enter the DNS name of a Domain, you can instead enter the name of a Server that is a Domain Controller of the Domain. For example, in any AD PowerShell command, the -Server parameter can specify the Domain Name, but it can also be
get-ADxxxx -Server fit.local
Even though “fit.local” is the name of a computer and not the name of a Domain, it doesn’t matter inside your personal development network. The computer name designates a Domain controller, so AD stuff goes to the Domain even though it wasn’t explicitly named in the command.
At Yale, the Domain Controllers are secured and nobody can login to them except AD administrators. However, when you create an AD in your own VM, you are in full control. Of course, nothing you put in it will control any other machines. You can use it to understand and test various forms of authentication or learn more about AD in your personal Sandbox LAN.
Impersonate
All programs run under a SID. Some programs run in the background under Microsoft built in SIDs like SYSTEM. Ordinary users login with a userid and password, get a session with their corresponding SID, and then all the programs they run in that session use that SID.
Anyone can run a new program under a different userid and SID by triggering an internal Re-Login where they provide or prompt for a new userid and password. This is called “RunAs”.
Components that are part of the Windows System are authorized to temporarily change their SID to any other SID that they can find by searching for the Users on the computer.
When a System Program simply changes the SID to some other SID, this is called “impersonation”.
Impersonation affects the permissions the program has to access Local files or call Local services. It works only on the local computer.
Changing the current SID did not create a password that can be used for Workgroup Authentication, nor did it obtain a Kerberos “TGT” object needed to do Domain Authentication, so there is no access to files or services on any other computer on the Local LAN. Even if the SID is a Domain SID that came from the AD, you still need the Kerberos TGT to communicate with any other Domain computers on the network.
Impersonation is used by SSH when you use public key authentication.
SSH Server
Every new Windows system gets the SSH client. On Windows 10/11 or Windows Server 2022, the SSH Server is an optional Feature that can be installed. The OpenSSH server (sshd.exe) runs as a background service under a SYSTEM userid. It is authorized to use impersonation.
On Windows Server 2025 OpenSSH server will be pre-installed but will have to be started manually or reconfigured to start automatically.
The client is ssh.exe on Windows and just “ssh” on Linux.
You can run ssh.exe to prompt for a password. When SSH is called with a userid and password, it does a internal Re-Login and then the password is available to do Workgroup Authentication, and if the userid was a Domain userid (HUNTING\bellman) then Kerberos was used and leaves behind the TGT needed to talk to other machines.
Normally, people put a secret key file in their Home directory on the SSH client and a pubic key in their Home directory on the SSH Server and do “public key” SSH authentication. The SSH server verifies that the client knows the secret key, but now with no password to do a Login, all it can do is impersonate the user by starting cmd.exe under that user’s SID. This session will then have access only to local files and services.
SSH uses the secret key in the Home directory of the currently logged in use on the client machine. The ssh command contains the userid and computer name of the SSH Server. The userid in the command defaults to a Local User unless you explicitly qualify it with a Domain name. The userid you are logging in as does not have to in any way match the userid you are logged into on the client machine. SSH is the only form of programmed session that works between a Domain user account on one machine and a Local user or even a user in a different Domain on another machine.
When the OpenSSH Server starts cmd.exe, whether through a userid/password or public key impersonation, if the user specified in the command is a member of the Administrators group on the machine, sshd.exe has to decide whether cmd.exe will run in user or administrator mode. It has to be one or the other, and there is no way in cmd.exe to elevate from user to administrator mode through SSH. They decided that most people using SSH want to do admin stuff, so they opted for cmd.exe to run elevated from the start.
However, they wanted an extra level of permission to do this. Therefore, to login as any user in the Administrators group of the remote machine, the SSH public key has to be a line in the Windows file location of C:\ProgramFiles\ssh\administrators_authorized_keys (instead of the authorized_keys file in the individual user’s home directory).
PowerShell Remote using SSH
There are some standard Windows Management tools that allow you to administer other machines over the network. The Windows Management Instrumentation (WMI) protocol authenticates between two machines that are members of Domains using the Kerberos protocol, and it authenticates between two machines that are not members of any domain using the workgroup protocol. If one machine is in a domain and the other is either not in any domain or is a member of a different untrusted domain, then WMI doesn’t work.
A developers Sandbox consists of a laptop that must be a member of the YALE Domain, plus VMs that may be workgroup machines or may be members of the internal test SANDBOX domain. WMI doesn’t work across all of them, and yet the developer wants to be able to administer the machines centrally without have to login to each of them one at a time.
PowerShell Remoting can now use SSH between computers. To set this up, you need your private key on the system from which you will be doing administration (typically your native laptop in the YALE Domain) and you need a userid in all the other machines that is a member of the Administrators group, and you need your public key to be in the C:\ProgramData\ssh\administrators_authorized_keys file.
Now domain membership doesn’t matter for any of your VMs. You can run PowerShell commands that do machine administration by adding, for example:
-UserName bellman -HostName SNARK
on PowerShell commands such as Invoke-Command or New-PSSession that operate across computers. Note that you replace the WMI parameter -Computer netname with the SSH parameter -HostName netname.