6.7

Table Of Contents
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.
Forged Transmits
The Forged transmits option affects traffic that is transmitted from a virtual machine.
When the Forged transmits option is set to Accept, ESXi does not compare source and effective MAC
addresses.
To protect against MAC impersonation, you can set the Forged transmits option to Reject. If you do, the
host compares the source MAC address being transmitted by the guest operating system with the
effective MAC address for its virtual machine adapter to see if they match. If the addresses do not match,
the ESXi host drops the packet.
The guest operating system does not detect that its virtual machine adapter cannot send packets by
using the impersonated MAC address. The ESXi host intercepts any packets with impersonated
addresses before they are delivered, and the guest operating system might assume that the packets are
dropped.
Promiscuous Mode Operation
Promiscuous mode eliminates any reception filtering that the virtual machine adapter performs so that the
guest operating system receives all traffic observed on the wire. By default, the virtual machine adapter
cannot operate in promiscuous mode.
vSphere Security
VMware, Inc. 199