Microsoft network client: Digitally sign communications (always)

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

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

Default: Disabled.

Important

For this policy to take effect on computers running Windows 2000, client-side packet signing must also be enabled. To enable client-side SMB packet signing, set Microsoft network client: Digitally sign communications (if server agrees).
Computers that have this policy set will not be able to communicate with computers that do not have server-side packet signing enabled. By default, server-side packet signing is enabled only on domain controllers running Windows 2000 and later.
Server-side packet signing can be enabled on computers running Windows 2000 and later by setting Microsoft network server: Digitally sign communications (if client agrees)
Server-side packet signing can be enabled on computers running Windows NT 4.0 Service Pack 3 and later by setting the following registry value to 1:
HKLM\System\CurrentControlSet\Services\LanManServer\Parameters\EnableSecuritySignature

Server-side packet signing cannot be enabled on computers running Windows 95 or Windows 98.

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 operating systems, 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 impose up to a 15 percent performance hit on file service transactions.