6.5.1

Table Of Contents
When sending packets through a network adapter, the guest operating system typically places its own
adapter effective MAC address in the source MAC address field of the Ethernet frames. It places the MAC
address for the receiving network adapter in the destination MAC address field. The receiving adapter
accepts packets only if the destination MAC address in the packet matches its own effective MAC
address.
An operating system can send frames with an impersonated source MAC address. An operating system
can therefore impersonate a network adapter that the receiving network authorizes, and stage malicious
attacks on the devices in a network.
Protect virtual traffic against impersonation and interception Layer 2 attacks by configuring a security
policy on port groups or ports.
The security policy on distributed port groups and ports includes the following options:
n
MAC address changes (see MAC Address Changes)
n
Promiscuous mode (see Promiscuous Mode Operation)
n
Forged transmits (see Forged Transmits)
You can view and change the default settings by selecting the virtual switch associated with the host from
the vSphere Web Client. See the vSphere Networking documentation.
MAC Address Changes
The security policy of a virtual switch includes a MAC address changes option. This option affects traffic
that a virtual machine receives.
When the Mac address changes option is set to Accept, ESXi accepts requests to change the effective
MAC address to a different address than the initial MAC address.
When the Mac address changes option is set to Reject, ESXi does not honor requests to change the
effective MAC address to a different address than the initial MAC address. This setting protects the host
against MAC impersonation. The port that the virtual machine adapter used to send the request is
disabled and the virtual machine adapter does not receive any more frames until the effective MAC
address matches the initial MAC address. The guest operating system does not detect that the MAC
address change request was not honored.
Note The iSCSI initiator relies on being able to get MAC address changes from certain types of storage.
If you are using ESXi iSCSI with iSCSI storage, set the MAC address changes option to Accept.
In some situations, you might have a legitimate need for more than one adapter to have the same MAC
address on a network—for example, if you are using Microsoft Network Load Balancing in unicast mode.
When Microsoft Network Load Balancing is used in the standard multicast mode, adapters do not share
MAC addresses.
vSphere Security
VMware, Inc. 182