6.0.3

Table Of Contents
n
MAC address changes (see “MAC Address Changes,” on page 233)
n
Forged transmits (see “Forged Transmits,” on page 233)
You can view and change the default seings 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 aects trac
that a virtual machine receives.
When the Mac address changes option is set to Accept, ESXi accepts requests to change the eective MAC
address to a dierent address than the initial MAC address.
When the Mac address changes option is set to Reject, ESXi does not honor requests to change the eective
MAC address to a dierent address than the initial MAC address. This seing 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 eective MAC address matches the
initial MAC address. The guest operating system does not detect that the MAC address change request was
not honored.
N 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 aects trac that is transmied from a virtual machine.
When the Forged transmits option is set to Accept, ESXi does not compare source and eective 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 transmied by the guest operating system with the eective
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 ltering that the virtual machine adapter performs so that the
guest operating system receives all trac observed on the wire. By default, the virtual machine adapter
cannot operate in promiscuous mode.
Although promiscuous mode can be useful for tracking network activity, it is an insecure mode of operation,
because any adapter in promiscuous mode has access to the packets even if some of the packets are received
only by a particular network adapter. This means that an administrator or root user within a virtual machine
can potentially view trac destined for other guest or host operating systems.
N In some situations, you might have a legitimate reason to congure a standard or a distributed
virtual switch to operate in promiscuous mode, for example, if you are running network intrusion detection
software or a packet snier.
Chapter 8 Securing vSphere Networking
VMware, Inc. 233