Microsoft network client: Digitally sign communications (if server agrees)

This security setting determines whether the SMB client attempts to negotiate SMB packet signing.

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 the SMB client component attempts to negotiate SMB packet signing when it connects to an SMB server.

If this setting is enabled, the Microsoft network client will ask the server to perform SMB packet signing upon session setup. If packet signing has been enabled on the server, packet signing will be negotiated. If this policy is disabled, the SMB client will never negotiate SMB packet signing.

Default: Enabled.

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.