System, Local User, Domain, and SSH IDs

In all computer systems, there is some mechanism to define Accounts for users and services. It may be as simple as the Linux /etc/passwd file where each account has an integer (the UID) with lower numbers for system services, and numbers starting at 1000 for Users. Since Windows NT in 1994, each Windows machine has its own account database called the SAM, and each account has a much larger binary string called the Security ID or SID.

Unix has a separate set of numbers for groups, and Windows also assigns SIDs to groups. By default a file is owned by the User who creates it, unless that user is in the Administrators group. When an administrator creates a file, ownership defaults to the group. This is a special rule that applies only to Administrators.

Just as Linux stores the UID of the owner in directories and files, Windows stores the SID of the owner of directories and files of an NTFS file system. It also grants access permissions to SIDs. When displaying any system information that contains a SID, Windows typically replaces it with the username or groupname of the account associated with the SID.

During the 1990’s, a Windows NT Domain was simply one NT computer on the network that essentially “shared” its SAM with other computers that were members of the Domain. In Windows 2000, Microsoft created a more flexible, reliable, and secure type of Domain called Active Directory, but it kept the SAM account in individual Windows machines that were not members of a Domain and included the same SID structure in AD.

Every Windows 11 desktop system still has its own SAM. While Yale policy prohibits creating Local Users on a Yale Managed Workstation, inside the system Microsoft has already created Built-In Local Users such as “LocalSystem” (a.k.a. SYSTEM), Administrator, and Guest, and the SAM also defines the Local Groups even when all the members of the Group are Domain accounts.

A given computer can only be a member of one Domain, so by default it will only have SIDs from its SAM and the one Domain to which it is joined, but disks can move around from machine to machine. If you format a USB drive with NTFS instead of one of the FAT filesystems, then it can pick up directories and files with ownership and permissions that include SIDs from the SAM of other computers or even other Domains. When Windows encounters a SID that is not Local or from the Domain this computer is currently joined to, it cannot display a user or group name and will either ignore it or display the SID as a string of numbers.

A large corporation or university system with multiple campuses may have an AD with more than one Domain. If a User is moved from one Domain to another, it gets a new SID from the new Domain. The previous SID is stored in a history list and may still be used to grant access permissions.

AD uses Kerberos, which regards each computer as a type of User. When a computer joins a Domain, it gets a computer account record and a SID. Every time the computer starts up, it logs into the Domain like any other User.

A User who logs in with Kerberos gets a token called the “TGT”. Windows stores this token in a secure part of the OS and uses it to gain access to files and services on other computers. The SID stored by the Windows system on this computer is sufficient to prove the user’s identity to services running on the same system, but the SID itself is public information and therefore by itself proves nothing to other computers.

When you start up a brand-new copy of Windows, it generates a mostly random 12-byte binary string that is unique to this computer. That string becomes part of every SID generated by this computer. When a new AD Domain is created, a similar unique 12-byte string is generated and becomes part of the SIDs that belong to that Domain. Individual User, Group, or Computer accounts are then just represented by an administratively managed sequence of numbers managed by the computer or the Domain.

When you enter a User or Group name and when Windows displays the User or Group name corresponding to a SID it finds in a file or permission, the convention is to prefix the name of the individual object with the Hostname of the computer for Local Users and Groups from the SAM of that computer, or else the nickname of the Domain for AD accounts. Thus YALE\netid is your User object in the Yale Domain, YALE\IAMDEV is the “iamdev” group in the Yale AD, while on a computer named CORNMUFFIN there could be a User named CORNMUFFIN\FRED and a Local Group named CORNMUFFIN\DEVELOPERS.

Built in SID Permissions

The Microsoft TrustedInstaller identity owns subdirectories of C:\Windows that contain Windows components. Even administrators do not have permission to change these files. The system itself modifies these directories when you install monthly patches or perform an annual upgrade of the system.

SYSTEM (also called LocalSystem) is the built-in identity used by default to run ordinary background services. When you add features and products to the computer, additional versions of the “system” user can be created for components such as SQL Server or Hyper-V. SYSTEM is a member of the Administrators group, has read/write access to all files on local disk not owned by TrustedInstaller, but has access on the network only to files that have been granted anonymous access (where Everyone can read them).

As a result:

• 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 local disk, and it can only export/import a VM to local directories.

Utilities like SSMS for SQL Server and Hyper-V Manager run under your userid and have access to your files. Sometimes they will let you browse to a directory that is on a network disk to which you have access. However, when you try to run the Backup or start the VM, an error will be reported because the actual SQL Server database or Hyper-V supervisor cannot access the files that you can access. So before or after one of these operations, you may have to move database backup or virtual disk files between local disk and network disk.

Local User SIDs (in VMs or on Personally Owned Machines)

Yale Security Policy prohibits the use of Local User accounts in the native Windows system of Yale Managed Workstations. However, you are allowed to install Hyper-V and create a separate Windows VM. Since such a VM cannot join the YALE Domain, you must use only Local User accounts in the VM. You will also have Local User accounts on personally owned machines in your home network.

When a new Windows VM starts up, you may be prompted for a password for the “administrator” account and a username and password for the first Local User. After that, you create additional Local Users as an administrator of the machine.

It is possible that you will have a personally owned home computer with several users who are family members who share the machine. If you are a developer creating a test environment, then you will typically define just your own username and perhaps a handful of test case users.

When you have a group of VMs running different parts of your development environment, it is important to define the same username and password for yourself and any other administrator accounts you choose to use. As will be explained below, this allows you to share files and issue commands from one VM to another in what is called a Workgroup.

Local SIDs are meaningful only on one computer. If you move a disk from one computer to another, Local SIDs from other machines display as unknown numbers.

However, you can subvert this if you have a utility to clone (make a complete copy) of a physical disk or if you copy virtual disk files or even entire VMs. In this case you may have more than one computer or VM running a copy of the same SAM with the same SIDs, and then if a USB disk is moved from one system to a clone of the same system, it may match the SID to a User object in its SAM and display the Local username for that SID.

The YALE Domain, Netids, and Managed Workstations

Every person at Yale is given a Netid. The Netid is also the unique username of a Domain User object in the YALE Domain.

Yale Faculty and appropriate Staff members are assigned one personal Yale Managed Workstation. When you get it, it will have been joined to the YALE Domain and your Netid can login to it.

The Windows system that was preinstalled and starts up when you power on the machine is the “native” or “host” operating system. Yale policy applies to it. That system will be locked down by Yale policy to prevent access from other machines in the network.

However, modern Microsoft system design encourages you to put you work files on your OneDrive, and to define the same Office 365 account on all your computers. So, locking down the host operating system on the laptop may not really secure information.

Modern Microsoft systems include features based on Virtual Machine technology. Windows Subsystem for Linux creates a VM which is also used by Docker Containers. Microsoft ensures that data is isolated and can only be accessed by the logged-on user.

However, once Windows requires support for VMs, it makes sense to use that feature. VMs that you create running inside your Yale laptop are completely separate from the host Windows operating system. They appear as entirely separate machine connected on a network, and are indistinguishable from personally owned computers on your home network.

The more technically trained a user is, the more likely that they will use VMs. In particular, anyone developing and testing computer programs will want to create an environment where the program can in isolation, safely separated from the real Yale network. To do this, they create VMs running SQL Server with test versions of databases. An application that modifies Users and Groups in the YALE AD may need a VM with an internal isolated dummy AD to do unit testing.

A group of VMs that simulates a real system and can be used to safely test software is sometimes called the Developer’s Sandbox.

So a software developer will have a Yale laptop that is a member of the YALE Domain, has no local users, and will not share files with non-Yale computers. Internally it can have VMs that are not members of the Domain, have only local users, and can share their own files to the laptop OS, but cannot access directories on the laptop. The developer can use Remote Desktop to login to the VMs. If the VMs are connected to the physical LAN they can appear as additional personally owned computers on the home network.

A developer who needs to create an Active Directory Domain in the Sandbox further complicates things, because now in addition to Local User accounts there will be Domain accounts belonging to a different Domain (which we will name SANDBOX). Now there are three different types of accounts YALE, Local, and SANDBOX, that are specifically designed not to talk to each other.

Workgroup (Local User) Authentication

When you start up a personally owned computer, you give it a name and define your own account. You end up with a “standalone” Windows system that is not a member of any Domain.

When you then bring into your house your new Yale Managed Workstation, it is a member of the Yale Domain and only has Domain accounts.

You can associate the same OneDrive account on each computer. This causes a separate copy of the shared files to be stored on each real or virtual machine. Your OneDrive account can hold a Terabyte of space, but that is a lot of data to duplicate and a lot of network activity to upload and download.

Before there was Office 365 and even before there was Active Directory, Microsoft allowed files to be shared directly between two computers connected to the same network without a common Domain. This type of communication is called a Workgroup, and it has been supported since Windows NT 3.1 in 1994. Today Microsoft would like to sell you an Office 365 account, but they still support Workgroups even if they don’t encourage them.

Workgroups are based on Local Users and Groups. This means that the native operating system on your Yale laptop cannot be a member of a Workgroup, but it can access files shared by computers in the Workgroup if when you select the option to “Map network drive”, you click the checkbox for “Connect with different credentials” and in the popup enter the username and password for the Local User in the Workgroup.

As you start to add VMs, all the physical or virtual Windows computers on your network except for the Yale laptop can combine to form the Workgroup. Sharing is possible by default when you create a Local User account on each member of the Workgroup with the same username and password.

On each computer, each Local User is internally assigned a different SID. Files created by that user are owned by that machine specific SID, and permissions are granted to that SID. If you have a SQL Server on that system, then Windows Logins are really associated with the local SID.

Although the SIDs are different on each machine and on disks attached to each machine, over the network the Workgroup protocols verify when the same username exists on two systems and has the same password on both systems. Then even though the SID is different on each computer in the Workgroup, access to files, services, and databases from a program run under the username on one computer are translated during the network communication between the two computers so that the operations on the other computer run under the SID assigned to Local account with the matching username.

Domains provide better security, which is why they are used by companies and universities. Workgroups are acceptable when they are physically isolated from the outside world. VMs running inside your laptop are isolated. Computers on your home network are isolated (provided that you do not share your WiFi with someone else).

From 1994 to Windows 11 24H2, Workgroup authentication was done with a Microsoft protocols named some version of “NTLM”. Starting sometime after the introduction of 24H2, a new protocol based on Kerberos called LocalKDC will be phased in, and NTLM will slowly be phased out. This will happen under the covers and will be transparent to Windows users, although it may be more visible to people running obsolete versions of Windows on some personally owned computers, or Linux computers that will require an upgrade of their support for Windows shared files.

Workgroup Authentication is also used for commands that can be issued on one computer but which run on another computer. Some Windows management tools can display activity running on several machines of the same Workgroup. Some commands can display data from another machine. Blocks of PowerShell code can be directed to run on another Workgroup computer.

However, the Yale Managed Workstation native Windows OS is a member of the YALE Domain and not a member of the Workgroup, so it cannot participate in all this automatic cross-computer sharing.

Your own Test AD in a VM

Workgroups provide all the services you need. Nobody will get a benefit from creating an AD in their home that outweighs the headaches of learning how to administer it.

However, this assumes that you are really using an AD to control machines, users, services, and to enforce policy.

You can download an ISO image of Windows Server and use it to create a new VM. It behaves almost the same as desktop Windows. Then two PowerShell commands and two reboots create an AD on that VM. Only that one VM is a member of the AD, or even knows it exists.

You can then create any number of Users, Groups, OUs, or any other data structures you need to use to test applications. The dummy Users you create could have the same names as real people, or you can make names up. Even if you create a User object that contains the same information as your YALE Netid account, it controls no machines, owns no resources, and has no real permissions. You don’t need to control any other computers or set any policy with this AD. Its only use is to test applications that read, modify, or create information in an AD, but to test changes in a safe environment where mistakes do not affect real users or even other developers.

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).

Up to this point, we were dealing with YALE Domain users and groups that were used only by the native Windows system of the Yale owned laptop, and Local users and groups defined on VMs created with Hyper-V in the laptop and perhaps also on personally owned computers. When the same Local username and password is defined on each machine, they form a Workgroup where you can share files and issue administrative commands across systems.

A test Domain creates a third set of identities. Windows systems in one Domain cannot interoperate with Windows systems in a different Domain, and with real Domains (like YALE) identities in a Workgroup cannot interoperate with computers in the Domain.

However, the article referenced above explains that if before you create the AD on the Windows Server VM you first add all the usual usernames and passwords that you have been defining on all the other Windows VMs in the Workgroup, then this group of Local Users (who are members of the Administrators group) are given special treatment when the Domain is created. The resulting VM has an AD Domain, but it also continues to behave as if it were a member of the Workgroup. You can use Workgroup authentication to access shared directories across systems. You can even use PowerShell commands on a Workgroup computer to Get, Set, or Add ADUsers and ADGroups in the test domain even though the computer on which the PowerShell script is running is not itself a member of that Domain and is running while you are logged on as a Local User instead of a Domain User.

You would never do this in a real Domain. Microsoft warns everyone that this creates a security exposure. However, you do not really need an AD to be secure if it is hidden away in a VM running on a virtual network inside your laptop that no other computer can get to, and the AD has no real users, no real passwords or other sensitive data, and it controls no real resources.

When you create an AD on a Windows Server, you also have to create a DNS Server on the VM. This server will resolve DNS names that end in the Domain name. For example, if you created a Domain named yu.yale.sandbox, then this DNS server will resolve all names ending in yu.yale.sandbox. However, none of the other VMs in the workgroup need to use this name. They can continue to reference the VM by any of its Workgroup names (typically hostname.local) and in any place where they have to configure the name of the AD (for example, in the -Server parameter of a PowerShell command) they can substitute a network name of the VM in place of the official name of the Domain. If testing requires that you add a Windows VM as a member of the Domain, then change the network configuration on that VM to use the IP address of the Domain Controller as the DNS Server IP address of the Windows system you want to join to the Domain before you perform the change to add it to the Domain.

Impersonate

Programs running as SYSTEM (also called LocalSystem) have permission to launch new programs under an arbitrary SID that they choose from the local account database.

Any program that knows a username and password of another account can pass that to an internal “re-Login” service and run a program under that account, but services running as SYSTEM do not need to know the password. A system component just changing the SID like this is called “impersonation”.

Changing the current SID provides access to files and services on the current computer, but it 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. Therefore, the program has 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, that Domain SID only provides access to files and services permitted to that Domain user on the local computer. To access anything on other computers in the Domain, you need to use Kerberos and you cannot do that without a TGT object.

Impersonation is used by SSH when you use public key authentication.

SSH Server

SSH is a protocol developed for Linux and then adapted by Microsoft to Windows. It is important for interoperability between Linux and Windows, but as a result it does not precisely align with the other Microsoft strategic decisions for Windows communication and security.

The SSH client comes preinstalled on modern Windows systems. Add the usual files to your home directory and you can use it to access Linux computers.

On Windows 10/11 and Windows Server 2022, the SSH Server is an Optional Feature that you can install from a Settings panel. It becomes a service and runs a program called sshd.exe in the system background. You have to administratively start the service and configure it to start automatically if that is what you want.

On Windows Server 2025 OpenSSH Server will be pre-installed, but the service will still be stopped and has to be started.

The Windows version of the SSH client program is compatible with the Linux command, but tolerates the Windows “\” file separator. If you want to login as a Domain username, the command form is:

ssh YALE\netid@computername

If you leave off the Domain prefix, it searches for a Local User account.

You can login with a password. SSH will prompt for the password if needed. If you are connecting to sshd.exe on a Windows computer, it does a re-Login (RunAs) to generate new local credentials to run commands on cmd.exe on that machine. If the account is a Local User then the saved password allows Workgroup authentication. If this is a Domain User, then a login was done to the Domain and a TGT object was generated. Either way, the commands running on the Server machine have full access to services in the Workgroup or Domain.

However, Linux programmers normally use “public key” authentication. They put a text file with their secret key in their Home directory on the client machine, and a text file containing their public key in the home directory of the account to which they want to login (the account name before the “@” in the ssh command). If the public key verifies that the client has the secret key, then sshd.exe will use Impersonation to run cmd.exe under the SID associated with the account named in the command.

As with all impersonation, without a password there is no access on the server machine to Workgroup files or services, and without a Kerberos TGT there is no access to Domain files and services. However, the cmd.exe session has access to file, services, and even SQL Server databases that are local to that one computer.

If the user you are logging in as is an Administrator of the machine, ssh.exe starts cmd.exe in “elevated” mode so it can run admin commands. For whatever reason, the programmers who wrote sshd.exe decided that they wanted an extra level of permission to run administrators. 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 C:\ProgramFiles\ssh\administrators_authorized_keys instead of the normal $HOME/.ssh/authorized_keys file.

PowerShell Remote using SSH

PowerShell has powerful command to administer all aspects of a computer. By default, it uses Windows Management Instrumentation (WMI) to communicate between machines. WMI works very well between two machines in the same Domain, and it works with a bit more effort between two machines in the same Workgroup. It does not work at all between two machines one of which is a member of a Domain and the other is not a member of the same Domain.

PowerShell added the ability to send commands or whole blocks of code between computers using SSH instead of WMI. All you do is use the -Hostname parameter instead of the -Computer parameter to specify the target system (the string after the “@” in an ssh command), and you must add -Username to specify the account name (the string before the “@”). Using PowerShell over SSH means that every VM or computer in your Sandbox, including Linux machines and the Yale Managed Workstation can participate and all interoperate freely.