There is confusion around VMware ESX with regards to security zones. On one hand VMware ESX is a single multi-homed physical machine. On the other hand, it contains multiple security zones. We need to look within the physical to properly understand security zones within VMware ESX and ESXi.
Security zones within VMware ESX are either based on networks employed (Management, VMotion, Storage, DMZ, Production, etc.) or it can be based on functionality such as hypervisor, management tool, backup systems, or virtual machine.
Each of these zones reflects a configuration for the VMware ESX host. It also reflects what is being run within the virtual machines as well as what network upon which the VMs and hosts resides.
Network security zones are relatively easy to understand. Each network would be isolated from one another except through well defined gateways and firewalls. The confusion among some security administrators is that they believe since the VMware ESX or ESXi host has multiple adapters that it would therefore act as an undefined gateway between the disparate networks.
This is simply not the case. Unlike other VMware products where networking is done using bridging technology, the virtual switch within the ESX host is similar to any other layer 2 physical switch. The physical NICs within the VMware ESX or ESXi host act as uplink ports between the physical switch and the virtual switch. Whether a virtual NIC used by virtual machines (VMs), or a vmkernel NIC used by the hypervisor, each virtual device connects to a portgroup within a virtual switch then out the physical NICs to the physical switches unless the communication is VM to VM on the same portgroup.
Since there is no way to layer virtual switches or portgroups, the only way to speak across them is by the use of well-defined gateways or firewalls. VMs can act as firewalls and gateways but only if they have more than one virtual NIC. So the rule of no multi-homed VMs is the best place to start unless it is a well defined virtual firewall or gateway.
The other definition of security zones is based on the roles used within the virtual infrastructure as well. There are three well-defined roles within the virtual infrastructure: hypervisor, virtual machine, and management tool.
The hypervisor controls everything so access to this must be limited to only the service console or management appliance, data stores used as VM storage, or VMware VMotion interfaces. Two of these are through vmkernel NIC used by the hypervisor as described previously: data stores for VM Storage and VMware VMotion Interfaces. Anything that accesses the hypervisor must be protected from those items that should not be able to access the hypervisor.
A virtualization management tool is another role within the system, these tools interact with the service console or management appliance for the virtualization host which will indirectly interact with the hypervisor. This indirect interaction is the key to handling this role. Virtualization management tools such as VMware VirtualCenter, HP SIM VMM plugin, VMware Lab Manager, etc. should be firewalled from other networks as they have indirect access to the hypervisor.
Backup systems act as another role within a virtual environment. These systems can interact directly with the storage devices attached to virtualization hosts through VMware VCB, or through the service console or management appliance just like virtualization management tools. This can also lead to indirect interaction with the hypervisor. Due to the direct and indirect access these systems should be firewalled as well from other networks.
The last role is that of a standard virtual machine. These virtual machines could live on any network but they should not be able to directly access the service console or management appliance of thse virtualization host in addition they should not be able to directly attach to the storage device ued by the virtualization hosts. Virtual machines are considered to be hostile to the virtualization host and should be treated as such.
Security zones depend on the security roles each component of your virtual environment directly or indirectly interacts. These few tips will aid in designing a secure environment.