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Virtual switches, such as those created by VMware platforms, provide a number of useful network security features.
Create and enforce policies
One of the network security features virtualization admins might not know about is that it's possible to employ policies with virtual switch ports. Where physical switch ports have no insight into the configuration of the physical network interface card ports attached to them, virtual switches can detect the configuration of virtual network ports connected to them. This makes it possible for administrators to create and enforce policies that help maintain a secure posture.
For example, a virtual switch can prevent a guest VM from changing its media access control (MAC) address -- a common sign of malicious activity.
Promiscuous mode for VMs is disabled by default. When enabled, promiscuous mode enables VMs to see all unicast network traffic traversing a virtual switch. Since this isn't desirable behavior from a security standpoint, promiscuous mode is disabled, so a VM only sees the data it is intended to see. The security policy for promiscuous mode is set at the virtual switch or the port group level.
Lock down MAC addresses
Another one of the valuable network security features associated with virtual switches is that MAC addresses are locked down. A MAC address represents the permanent physical identifier for every network device -- it's a bit like a physical home address.
VM networking explained
The difference between physical, virtual and virtual distributed switches; the difference between physical, virtual, uplink and group ports; how NIC teaming works in a VM network environment; and the best practices for configuring a VM network.
VMs are assigned MAC addresses as part of their network configuration, but MAC addresses can be changed fairly easily in VMs. Unfortunately, this is undesirable from a security perspective and can be a sign of malicious activity. Locking down the MAC address prevents this vulnerability.
Block forged traffic from VMs
Finally, virtual switches block forged traffic from VMs. Normally, a network device -- such as a virtual switch -- doesn't compare MAC addresses in IP packets with the MAC address of the sending device to make sure they match. This could enable malicious traffic to be sent using tactics such as MAC spoofing. When the virtual switch compares MAC addresses, it's able to block forged -- or spoofed -- traffic.
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