One way to identify a LAN is to say that all the devices in the same LAN have a common Layer 3 IP network address and that they also are all located in the same Layer 2 broadcast domain. A virtual LAN (VLAN) is another name for a Layer 2 broadcast domain. A VLAN is controlled by a switch. The switch also controls which ports are associated with which VLANs. If the switches are in their default configuration, all ports by default are assigned to VLAN 1, which means all the devices, including the two users and the router, are in the same broadcast domain, or VLAN.
As you start adding hundreds of users, you might want to separate groups of users into individual subnets and associated individual VLANs. To do this, you assign the switch ports to the VLAN, and then any device that connects to that specific switch port is a member of that VLAN. Hopefully, all the devices that connect to switch ports that are assigned to a given VLAN also have a common IP network address configured so that they can communicate with other devices in the same VLAN. Often, Dynamic Host Configuration Protocol (DHCP) is used to assign IP addresses from a common subnet range to the devices in a given VLAN.
One problem with having two users in the same VLAN but not on the same physical switch is that Switch 1 tells Switch 2 that a broadcast or unicast frame is supposed to be for VLAN 10. The solution is simple: For connections between two switches that contain ports in VLANs that exist in both switches, you configure specific trunk ports instead of configuring access ports. If the two switch ports are configured as trunks, they include additional information called a tag that identifies which VLAN each frame belongs to. 802.1Q is the standard protocol for this tagging. The most critical piece of information (for this discussion) in this tag is the VLAN ID.
Currently, the two hosts in Figure (Host A and Host B) cannot communicate because they are in separate VLANs (VLAN 10 and VLAN 20, respectively). The inter-switch links (between the two switches) are configured as trunks. A broadcast frame sent from Host A and received by Switch 1 would forward the frame over the trunk tagged as belonging to VLAN 10 to Switch 2. Switch 2 would see the tag, know it was a broadcast associated with VLAN 10, remove the tag, and forward the broadcast to all other interfaces associated with VLAN 10, including the switch port that is connected to Host B. These two core components (access ports being assigned to a single VLAN and trunk ports that tag the traffic so that a receiving switch knows which VLAN a frame belongs to) are the core building blocks for Layer 2 switching, where a VLAN can extend beyond a single switch.
Host A and Host B communicate with each other, and they can communicate with other devices in the same VLAN (which is also the same IP subnet), but they cannot communicate with devices outside their local VLAN without the assistance of a default gateway. A router could be implemented with two physical interfaces: one connecting to an access port on the switch that is been assigned to VLAN 10 and another physical interface connected to a different access port that has been configured for a different VLAN. With two physical interfaces and a different IP address on each, the router could perform routing between the two VLANs.
Now that you are familiar with VLANs and their purpose, let’s look at VLAN-related attacks.
Virtual local area network (VLAN) hopping is a method of gaining access to traffic on other VLANs that would normally not be accessible.
There are two primary methods of VLAN hopping:
Switch spoofing#
When you perform a switch spoofing attack, you imitate a trunking switch by sending the respective VLAN tag and the specific trunking protocols. Several best practices can help mitigate VLAN hopping and other Layer 2 attacks. Earlier in the module you learned about different best practices for securing your infrastructure (including Layer 2). You should always avoid using VLAN 1 anywhere because it is a default. Do not use this native VLAN for any of your enabled access ports. On a new switch, shut down all ports and assign them to a VLAN that is not used for anything else other than a parking lot. Then bring up the ports and assign correct VLANs as the ports are allocated and needed. Following these best practices can help prevent a user from maliciously negotiating a trunk with a switch and then having full access to each of the VLANs by using custom software on the computer that can both send and receive dot1q-tagged frames. A user with a trunk established could perform VLAN hopping to any VLAN desired by just tagging frames with the VLAN of choice. Other malicious tricks could be used as well, but forcing the port to an access port with no negotiation removes this risk.
Double tagging.#
Another 802.1Q VLAN hopping attack is a double-tagging VLAN hopping attack. Most switches configured for 802.1Q remove only one 802.1Q tag. An attacker could change the original 802.1Q frame to add two VLAN tags: an outer tag with his or her own VLAN and an inner hidden tag of the victim’s VLAN. When the double-tagged frame reaches the switch, it only processes the outer tag of the VLAN that the ingress interface belongs to. The switch removes the outer VLAN tag and forwards the frame to all the ports belong to native VLAN. A copy of the frame is forwarded to the trunk link to reach the next switch.
A double tagging VLAN hopping attack is made by exploiting the native VLAN. Since VLAN 1 is the default VLAN for access ports and the default native VLAN on trunks, it’s an easy target. The first countermeasure is to remove access ports from the default VLAN 1 since the attacked port must match the native VLAN of the switch.