6.5.1

Table Of Contents
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.
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 traffic destined for other guest or host operating systems.
Note In some situations, you might have a legitimate reason to configure 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 sniffer.
Standard Switch Protection and VLANs
VMware standard switches provide safeguards against certain threats to VLAN security. Because of the
way that standard switches are designed, they protect VLANs against a variety of attacks, many of which
involve VLAN hopping.
Having this protection does not guarantee that your virtual machine configuration is invulnerable to other
types of attacks. For example, standard switches do not protect the physical network against these
attacks; they protect only the virtual network.
vSphere Security
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