Layer 2 Switch Security Requirements Guide

A compromised switch introduces risk to the entire network infrastructure as well as data resources that are accessible via the network. The perimeter defense has no oversight or control of attacks by malicious users within the network. Preventing network breaches from within is dependent on implementing a comprehensive defense-in-depth strategy, including securing each device connected to the network. This is accomplished by following and implementing all security guidance applicable for each node type. A fundamental step in securing each switch is to enable only the capabilities required for operation.

VLAN Trunk Protocol (VTP) provides central management of VLAN domains, thus reducing administration in a switched network. When configuring a new VLAN on a VTP server, the VLAN is distributed through all switches in the domain. This reduces the need to configure the same VLAN everywhere. VTP pruning preserves bandwidth by preventing VLAN traffic (unknown MAC, broadcast, multicast) from being sent down trunk links when not needed, that is, there are no access switch ports in neighboring switches belonging to such VLANs. An attack can force a digest change for the VTP domain enabling a rogue device to become the VTP server, which could allow unauthorized access to previously blocked VLANs or allow the addition of unauthorized switches into the domain. Authenticating VTP messages with a cryptographic hash function can reduce the risk of the VTP domain's being compromised.

Denial of service is a condition when a resource is not available for legitimate users. Packet flooding DDoS attacks are referred to as volumetric attacks and have the objective of overloading a network or circuit to deny or seriously degrade performance, which denies access to the services that normally traverse the network or circuit. Volumetric attacks have become relatively easy to launch by using readily available tools such as Low Orbit Ion Cannon or by using botnets. Measures to mitigate the effects of a successful volumetric attack must be taken to ensure that sufficient capacity is available for mission-critical traffic. Managing capacity may include, for example, establishing selected network usage priorities or quotas and enforcing them using rate limiting, Quality of Service (QoS), or other resource reservation control methods. These measures may also mitigate the effects of sudden decreases in network capacity that are the result of accidental or intentional physical damage to telecommunications facilities (such as cable cuts or weather-related outages).

If the switch fails in an unsecure manner (open), unauthorized traffic originating externally to the enclave may enter or the device may permit unauthorized information release. Fail secure is a condition achieved by employing information system mechanisms to ensure, in the event of an operational failure of the switch, that it does not enter into an unsecure state where intended security properties no longer hold. If the device fails, it must not fail in a manner that will allow unauthorized access. If the switch fails for any reason, it must stop forwarding traffic altogether or maintain the configured security policies. If the device stops forwarding traffic, maintaining network availability would be achieved through device redundancy. An example is a firewall that blocks all traffic rather than allowing all traffic when a firewall component fails (e.g., fail closed and do not forward traffic). This prevents an attacker from forcing a failure of the system in order to obtain access. Abort refers to stopping a program or function before it has finished naturally. The term abort refers to both requested and unexpected terminations.

Without authenticating devices, unidentified or unknown devices may be introduced, thereby facilitating malicious activity. For distributed architectures (e.g., service-oriented architectures), the decisions regarding the validation of authentication claims may be made by services separate from the services acting on those decisions. In such situations, it is necessary to provide authentication decisions (as opposed to the actual authenticators) to the services that need to act on those decisions. This requirement applies to applications that connect either locally, remotely, or through a network to an endpoint device (including, but not limited to, workstations, printers, servers (outside a datacenter), VoIP Phones, and VTC CODECs). Gateways and SOA applications are examples of where this requirement would apply. Device authentication is a solution enabling an organization to manage devices. It is an additional layer of authentication ensuring only specific pre-authorized devices can access the system.

If a rogue switch is introduced into the topology and transmits a Bridge Protocol Data Unit (BPDU) with a lower bridge priority than the existing root bridge, it will become the new root bridge and cause a topology change, rendering the network in a suboptimal state. The STP PortFast BPDU guard enhancement allows network designers to enforce the STP domain borders and keep the active topology predictable. The devices behind the ports that have STP PortFast enabled are not able to influence the STP topology. At the reception of BPDUs, the BPDU guard operation disables the port that has PortFast configured. The BPDU guard transitions the port into errdisable state and sends a log message.

The Spanning Tree Protocol (STP) loop guard feature provides additional protection against STP loops. An STP loop is created when an STP blocking port in a redundant topology erroneously transitions to the forwarding state. In its operation, STP relies on continuous reception and transmission of BPDUs based on the port role. The designated port transmits BPDUs, and the non-designated port receives BPDUs. When one of the ports in a physically redundant topology no longer receives BPDUs, the STP conceives that the topology is loop free. Eventually, the blocking port from the alternate or backup port becomes a designated port and moves to a forwarding state. This situation creates a loop. The loop guard feature makes additional checks. If BPDUs are not received on a non-designated port and loop guard is enabled, that port is moved into the STP loop-inconsistent blocking state.

Access layer switches use the Content Addressable Memory (CAM) table to direct traffic to specific ports based on the VLAN number and the destination MAC address of the frame. When a router has an Address Resolution Protocol (ARP) entry for a destination host and forwards it to the access layer switch and there is no entry corresponding to the frame's destination MAC address in the incoming VLAN, the frame will be sent to all forwarding ports within the respective VLAN, which causes flooding. Large amounts of flooded traffic can saturate low-bandwidth links, causing network performance issues or complete connectivity outage to the connected devices. Unknown unicast flooding has been a nagging problem in networks that have asymmetric routing and default timers. To mitigate the risk of a connectivity outage, the Unknown Unicast Flood Blocking (UUFB) feature must be implemented on all access layer switches. The UUFB feature will block unknown unicast traffic flooding and only permit egress traffic with MAC addresses that are known to exit on the port.

In an enterprise network, devices under administrative control are trusted sources. These devices include the switches, routers, and servers in the network. Host ports and unknown DHCP servers are considered untrusted sources. An unknown DHCP server on the network on an untrusted port is called a spurious DHCP server, any device (PC, Wireless Access Point) that is loaded with DHCP server enabled. The DHCP snooping feature determines whether traffic sources are trusted or untrusted. The potential exists for a spurious DHCP server to respond to DHCPDISCOVER messages before the real server has time to respond. DHCP snooping allows switches on the network to trust the port a DHCP server is connected to and not trust the other ports. The DHCP snooping feature validates DHCP messages received from untrusted sources and filters out invalid messages as well as rate-limits DHCP traffic from trusted and untrusted sources. DHCP snooping feature builds and maintains a binding database, which contains information about untrusted hosts with leased IP addresses, and it utilizes the database to validate subsequent requests from untrusted hosts. Other security features, such as IP Source Guard and Dynamic Address Resolution Protocol (ARP) Inspection (DAI), also use information stored in the DHCP snooping binding database. Hence, it is imperative that the DHCP snooping feature is enabled on all VLANs.

IP Source Guard provides source IP address filtering on a Layer 2 port to prevent a malicious host from impersonating a legitimate host by assuming the legitimate host's IP address. The feature uses dynamic DHCP snooping and static IP source binding to match IP addresses to hosts on untrusted Layer 2 access ports. Initially, all IP traffic on the protected port is blocked except for DHCP packets. After a client receives an IP address from the DHCP server, or after static IP source binding is configured by the administrator, all traffic with that IP source address is permitted from that client. Traffic from other hosts is denied. This filtering limits a host's ability to attack the network by claiming a neighbor host's IP address.

DAI intercepts Address Resolution Protocol (ARP) requests and verifies that each of these packets has a valid IP-to-MAC address binding before updating the local ARP cache and before forwarding the packet to the appropriate destination. Invalid ARP packets are dropped and logged. DAI determines the validity of an ARP packet based on valid IP-to-MAC address bindings stored in the DHCP snooping binding database. If the ARP packet is received on a trusted interface, the switch forwards the packet without any checks. On untrusted interfaces, the switch forwards the packet only if it is valid.

Spanning Tree Protocol (STP) is implemented on bridges and switches to prevent layer 2 loops when a broadcast domain spans multiple bridges and switches and when redundant links are provisioned to provide high availability in case of link failures. Convergence time can be significantly reduced using Rapid STP (802.1w) instead of STP (802.1d), resulting in improved availability. Rapid STP should be deployed by implementing either Rapid Per-VLAN-Spanning-Tree (Rapid-PVST) or Multiple Spanning-Tree Protocol (MSTP), the latter scales much better when there are many VLANs.

In topologies where fiber optic interconnections are used, physical misconnections can occur that allow a link to appear to be up when there is a mismatched set of transmit/receive pairs. When such a physical misconfiguration occurs, protocols such as STP can cause network instability. UDLD is a layer 2 protocol that can detect these physical misconfigurations by verifying that traffic is flowing bidirectionally between neighbors. Ports with UDLD enabled periodically transmit packets to neighbor devices. If the packets are not echoed back within a specific time frame, the link is flagged as unidirectional and the interface is shut down.

When trunk negotiation is enabled via Dynamic Trunk Protocol (DTP), considerable time can be spent negotiating trunk settings (802.1q or ISL) when a node or interface is restored. While this negotiation is happening, traffic is dropped because the link is up from a layer 2 perspective. Packet loss can be eliminated by setting the interface statically to trunk mode, thereby avoiding dynamic trunk protocol negotiation and significantly reducing any outage when restoring a failed link or switch.

It is possible that a disabled port that is assigned to a user or management VLAN becomes enabled by accident or by an attacker and as a result gains access to that VLAN as a member.

In a VLAN-based network, switches use the default VLAN (i.e., VLAN 1) for in-band management and to communicate with other networking devices using Spanning-Tree Protocol (STP), Dynamic Trunking Protocol (DTP), VLAN Trunking Protocol (VTP), and Port Aggregation Protocol (PAgP)—all untagged traffic. As a consequence, the default VLAN may unwisely span the entire network if not appropriately pruned. If its scope is large enough, the risk of compromise can increase significantly.

The default VLAN (i.e., VLAN 1) is a special VLAN used for control plane traffic such as Spanning-Tree Protocol (STP), Dynamic Trunking Protocol (DTP), VLAN Trunking Protocol (VTP), and Port Aggregation Protocol (PAgP). VLAN 1 is enabled on all trunks and ports by default. With larger campus networks, care needs to be taken about the diameter of the STP domain for the default VLAN. Instability in one part of the network could affect the default VLAN, thereby influencing control-plane stability and therefore STP stability for all other VLANs.

Switches use the default VLAN (i.e., VLAN 1) for in-band management and to communicate with directly connected switches using Spanning-Tree Protocol (STP), Dynamic Trunking Protocol (DTP), VLAN Trunking Protocol (VTP), and Port Aggregation Protocol (PAgP)—all untagged traffic. As a consequence, the default VLAN may unwisely span the entire network if not appropriately pruned. If its scope is large enough, the risk of compromise can increase significantly.

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 will have the attacker's VLAN ID (probably the well-known and omnipresent default VLAN) 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.

VLAN hopping can be initiated by an attacker who has access to a switch port belonging to the same VLAN as the native VLAN of the trunk link connecting to another switch that the victim is connected to. If the attacker knows the victim’s MAC address, it can forge a frame with two 802.1q tags and a layer 2 header with the destination address of the victim. Since the frame will ingress the switch from a port belonging to its native VLAN, the trunk port connecting to the victim’s switch will simply remove the outer tag because native VLAN traffic is to be untagged. The switch will forward the frame on to the trunk link unaware of the inner tag with a VLAN ID of which the victim’s switch port is a member.

Configuring the network device to implement organization-wide security implementation guides and security checklists ensures compliance with federal standards and establishes a common security baseline across DoD that reflects the most restrictive security posture consistent with operational requirements. Configuration settings are the set of parameters that can be changed that affect the security posture and/or functionality of the network device. Security-related parameters are those parameters impacting the security state of the network device, including the parameters required to satisfy other security control requirements.

Separating critical system components and functions from other noncritical system components and functions through separate subnetworks may be necessary to reduce susceptibility to a catastrophic or debilitating breach or compromise that results in system failure. For example, physically separating the command and control function from the in-flight entertainment function through separate subnetworks in a commercial aircraft provides an increased level of assurance in the trustworthiness of critical system functions.

An incident, whether adversarial- or nonadversarial-based, can disrupt established communications paths used for system operations and organizational command and control. Alternate communications paths reduce the risk of all communications paths being affected by the same incident. To compound the problem, the inability of organizational officials to obtain timely information about disruptions or to provide timely direction to operational elements after a communications path incident, can impact the ability of the organization to respond to such incidents in a timely manner. Establishing alternate communications paths for command and control purposes, including designating alternative decision makers if primary decision makers are unavailable and establishing the extent and limitations of their actions, can greatly facilitate the organization's ability to continue to operate and take appropriate actions during an incident.