Microsoft network server: Digitally sign communications (always)

This security setting determines whether packet signing is required by the SMB server component.

The server message block (SMB) protocol provides the basis for Microsoft file and print sharing and many other networking operations, such as remote Windows administration. To prevent "man-in-the-middle" attacks that modify SMB packets in transit, the SMB protocol supports the digital signing of SMB packets. This policy setting determines whether SMB packet signing must be negotiated before further communication with an SMB client is permitted.

If this setting is enabled, the Microsoft network server will not communicate with a Microsoft network client unless that client agrees to perform SMB packet signing. If this setting is disabled, SMB packet signing is negotiated between the client and server.

Default:

Disabled for member servers.
Enabled for domain controllers.

Notes

All Windows operating systems support both a client-side SMB component and a server-side SMB component. To take advantage of SMB packet signing, both the client-side SMB component and server-side SMB component that are involved in a communication must have SMB packet signing either enabled or required. On Windows 2000 and later, enabling or requiring packet signing for client and server-side SMB components is controlled by the following four policy settings:
Microsoft network client: Digitally sign communications (always) - Controls whether or not the client-side SMB component requires packet signing.
Microsoft network client: Digitally sign communications (if server agrees) - Controls whether or not the client-side SMB component has packet signing enabled.
Microsoft network server: Digitally sign communications (always) - Controls whether or not the server-side SMB component requires packet signing.
Microsoft network server: Digitally sign communications (if client agrees) - Controls whether or not the server-side SMB component has packet signing enabled.
If server-side SMB signing is required, a client will not be able to establish a session with that server unless it has client-side SMB signing enabled. By default, client-side SMB signing is enabled on workstations, servers, and domain controllers.
Similarly, if client-side SMB signing is required, that client will not be able to establish a session with servers that do not have packet signing enabled. By default, server-side SMB signing is enabled only on domain controllers.
If server-side SMB signing is enabled, SMB packet signing will be negotiated with clients that have client-side SMB signing enabled.
Using SMB packet signing can degrade performance up to 15 percent on file service transactions.

Important

For this policy to take effect on computers running Windows 2000, server-side packet signing must also be enabled. To enable server-side SMB packet signing, set the following policy:
Microsoft network server: Digitally sign communications (if server agrees)

For Windows 2000 servers to negotiate signing with Windows NT 4.0 clients, the following registry value must be set to 1 on the Windows 2000 server:
HKLM\System\CurrentControlSet\Services\lanmanserver\parameters\enableW9xsecuritysignature

Computers that have this policy set will not communicate with computers that do not have client-side packet signing enabled. Client-side packet signing can be enabled on computers running Windows 2000 and later by setting the following policy: