| Policy regarding LAN access switchport mode has been established in the Network Infrastructure STIG by NET1416 which states “ensure trunking is disabled on all access ports (do not configure trunk on, desirable, non-negotiate, or auto—only off).” The reason for this is that a malicious user could affect a VLAN hopping attack. VLAN “hopping” occurs when a tagged frame destined for one VLAN is redirected to a different VLAN, threatening network security. The redirection can be initiated using two methods: “tagging attack” and “double encapsulation.” Frame tagging attacks allow a malicious user on a VLAN to get unauthorized access to another VLAN. For example, if a switch port’s trunk mode were configured as auto (enables a port to become a trunk if the connected switch it is negotiating trunking with has its state set to on or desirable) and were to receive a fake DTP packet specifying trunk on or desirable, it would become a trunk port and it could then start accepting traffic destined for all VLANs that belong to that trunk group. The attacker could start communicating with other VLANs through that compromised port—including the management VLAN. Insuring that trunk mode for any non-trunking port is configured as off can prevent this type of attack. Double encapsulation can be initiated by an attacker who has access to a switch port belonging to the native VLAN of the trunk port. Knowing the victim’s MAC address and with the victim attached to a different switch belonging to the same trunk group, thereby requiring the trunk link and frame tagging, the malicious user can begin the attack by sending frames with two sets of tags. The outer tag that is the attacker’s VLAN ID (probably the well known and omnipresent VLAN 1) is stripped off by the switch, and the inner tag that will have the victim’s VLAN ID is used by the switch as the next hop and sent out the trunk port. To ensure the integrity of the trunk link and prevent unauthorized access, the native VLAN of the trunk port should be changed from the default VLAN 1 to its own unique VLAN. NOTE: Trunk mode is typically used between LAN switches to extend defined VLANs across a network. This mode is used to interconnect LAN switches and routers with other LAN switches and routers to create a network across multiple NEs. Trunk mode generally requires each packet to be tagged with the VLAN ID that it belongs to. This tagging can follow the 802.1Q standard format or can use a vendor proprietary protocol such as Cisco’s ISL. Access mode on the other hand is used on a switchport that supports a connection to a LAN endpoint device. Typically single endpoint devices connected to a switchport send traffic that does not contain a VLAN tag. In this case, if a VLAN is defined for the endpoint, the switchport is statically assigned to a VLAN. As such, when a packet is received and sent out the “Trunk” the packet is tagged with the VLAN ID. Best practices dictate that the implementation of VVoIP on a network requires one or more VVoIP VLANs be established within the network. Therefore LAN access switchports that support VVoIP and VTC endpoints that do not tag or are capable but not configured to add a VLAN tag to their traffic must be statically assigned to the local VVoIP VLAN. VVoIP and VTC endpoints that do have the capability of adding the VLAN tag to their traffic typically use 802.1Q format and also typically support a PC port on the device. The PC port is added to the VVoIP or VTC endpoints since it reduces cabling and LAN infrastructure cost. Typically, VVoIP or VTC endpoints that support a PC port pass traffic from this port unchanged whether the traffic is tagged or not, while adding the VVoIP VLAN tag for the locally defined VVoIP VLAN to its VVoIP traffic. As such, a LAN access switchport must now support tagged and untagged traffic and keep the traffic separated. This is done by defining a default “data” VLAN (that is not the default VLAN on the NE such as VLAN 0 or 1) for the switchport in which untagged traffic is placed and defining a secondary VVoIP VLAN that matches the 802.1Q tag used for the VVoIP traffic. This method maintains the LAN access nature of the port even though it supports multiple VLANs while not requiring it to be configured as a trunk. |
