1. Secure LAN Switching Layer 2 security Introduction Port-level controls
Storm control
Protected ports
Private VLAN
Port blocking
Port security
ACLs on switches
STP features for layer-2 security
DHCP features against snooping
IP source guard
Dynamic ARP Inspection (DAI)
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2. Switches and Layer 2 security
Security of lower layer devices is important, because some threats are initiated on Layer 2 rather than Layer 3 and above.
Example: A firewall or a router cannot block a compromised server on a DMZ LAN from connecting to another server on the same segment.  because the connection occurs at Layer 2
Focus of the chapter: Cisco Catalyst series switches (principles applicable to other types of switches)
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3. Source: http://www. cisco
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4. Switch and Layer 2 security
Sample L2 attacks and their mitigations
See
(local copy)
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5. Port security
A mechanism to restrict the MAC addresses that can connect via a particular port of the switch
Allows a range of MAC addresses to be specified for a particular port
Only frames with a right MAC address can go through the switch.
Useful for preventing MAC address flooding attacks
CAM overflow: Content-Addressable Memory (aka. associated memory)
CAM table stores information such as MAC addresses available on physical ports, with their associated VLAN params.
CAM table has fixed size.
When a CAM table is full, the switch is unable to create a new entry.
 It forwards a received frame to all ports, resulting in increased traffic and allowing the attacker to examine all frames.
So, CAM overflow attacks may lead to subsequent DoS and traffic analysis attacks (next slide)
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6. MAC Address Flooding
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7. MAC Address Flooding (cont.)
4/21/2017
MAC Address Flooding (cont.)
Counter-measures:
Hard-coding the MAC addresses that are allowed to connect on a port, or
Limiting the number of hosts that are allowed to connect on a port
Example 5-1: approach 1 + timed suspension
Console> (enable) set port security 2/1 enable
Console> (enable) set port security 2/1 enable b
Console> (enable) set port security 2/1 shutdown 600
Example 5-2: approach 2
Console> (enable) set port security 3/2 maximum 20
set port security
To configure port security and unicast flood on a port or range of ports, use the set port security command.
set port security mod/port... [enable | disable] [mac_addr] [age {age_time}] [maximum {num_ of_mac}] [shutdown {shutdown_time}] [unicast-flood {enable | disable}] [violation {shutdown | restrict}]
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8. 4/21/2017
IP permit lists
Purpose: To restrict higher layer traffic, such asTelnet, SSH, HTTP, and SNMP, from entering a switch
Allows IP addresses to be specified that are allowed to send these kinds of traffic through the switch
Command: set ip permit enable
Example 5-3
set ip permit enable [telnet | snmp | ssh]
Enable the IP permit list.
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9. Protocol Filtering
Purpose: To limit broadcast/multicasts for certain protocols
With Cisco Catalyst 5000 series of switches, packets are classified into protocol groups:
IP 2. IPX
AppleTalk, DECnet, Banyan VINES 4. Other protocols
A port is configured to belong to one or more of these groups.
For each of the groups a port belong to, the port is in one of the following states (for that group):
On  Receive all broadcast/multicast traffic for that protocol
Off  no broadcast/multicast traffic for that protocol
Auto  auto-configured port
The port becomes a member of the protocol group only after the device connected to the port transmits packets of that specific protocol group.
Once the attached device stops transmitting packets for that protocol for 60 minutes, the port is removed form that protocol group.
Example 5-4
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10. Controlling LAN floods
Attackers may cause frame flood (e.g., CAM flooding), or send broadcast/multicast messages to flood the LAN.
Counter-measures:
Protocol filtering
Setting up threshold limits for broadcast/multicast traffic on ports
Catalyst switches allow thresholds for broadcast traffic to be set up on a per-port basis.
The thresholds can depend on either the bandwidth consumed by broadcasts or the number of broadcast packets being sent across a port.
‘Bandwidth consumed’ is a preferred measure. (Why?)
Example: Console> (enable) set port broadcast 2/1-6 75%
Other broadcast/multicast traffic is dropped when the bandwidth consumed by broadcast/multicast traffic reaches 75%.
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11. VLAN
Virtual LAN
Source:
Group of devices on one or more LANs that are configured (using management software) so that they can communicate as if they were attached to the same wire, when in fact they are located on a number of different LAN segments.
based on logical instead of physical connections
extremely flexible
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12. Sample VLAN configurations
Source:
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13. Multiple-VLAN Network
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14. VLAN & Trunking
A trunk is an interface on a switch that can carry packets for any VLAN. When packets get sent between switches, each packet gets tagged, based on the IEEE standard for passing VLAN packets between bridges, 802.1Q. The receiving switch removes the tag and forwards the packet to the correct port or VLAN in the case of a broadcast packet.
Be aware! “VLAN Insecurity” (by Rik Farrow):
“VLANs make it possible to share a switch among many LANs, by filtering and limiting broadcast traffic.”
“But this form of isolation relies on software and configuration, not the physical isolation that security people like myself really like to see.”
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15. Private VLANs An enhancement to some Cisco switches
Traditional VLAN: no layer 2 segregation of devices of the same VLAN
 So when one of the devices in a VLAN is compromised, other devices on the same VLAN may be compromised as well.
Purpose of private VLANs: To allow restrictions to be placed on the Layer 2 traffic of a VLAN.
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16. Private VLANs
Provides layer 2 isolation to quarantine hosts from one another among ports within the same PVLAN
Three types of private VLAN ports:
Promiscuous ports: communicate with all other private VLAN ports
Community ports: communicate among themselves and with their promiscuous ports
Isolated ports: have complete Layer 2 isolation from other ports within the same private VLAN (e.g., Ethernet ports in hotel rooms)
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18. Other security features in VLAN
Port blocking
When a packet’s MAC address can not be found in a switch’s MAC address table, the switch will broadcast (flood) unknown unicast and multicast traffic to all the ports in the same VLAN
This flooding can be blocked using port blocking feature
Port filtering based on MAC address
A set of MAC addresses are allowed to access a port.
Packets with source MAC addresses outside the set are dropped.
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19. ACLs on Switches, 1/4 Router ACL MAC ACL
Switched virtual interfaces (SVI) are virtual layer 3 interfaces.
Both standard and extended ACLs are supported on SVI.
MAC ACL
Aka Ethernet ACL
Filters non-IP traffic using MAC address filtering
Supports only inbound traffic
mac access-list extended
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20. ACLs on Switches, 2/4 Port ACL VLAN ACL (VACL)
Supported on physical layer 2 interfaces
Supports only inbound traffic filtering
Supports standard, extended, and MAC-extended ACLs
Can filter both IP and non-IP traffic
VLAN ACL (VACL)
Aka VLAN map
Filters all types of traffic bridged within a VLAN or routed into or out of the VLAN
Examples next page
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21. ACLs on Switches, 3/4 VLAN ACL (VACL)
Filters all types of traffic bridged within a VLAN
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22. ACLs on Switches, 4/4 VLAN ACL (VACL)
Filters all types of traffic routed into or out of the VLAN
Order of processing: VACL for input VLAN, input IOS ACL, Outbound IOS ACL, VACL for output VLAN
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23. L2 security using STP features
Spanning Tree Protocol (STP)
Source:
prevents loops from being formed when switches or bridges are interconnected via multiple paths.
implements the IEEE 802.1D algorithm by exchanging BPDU messages with other switches to detect loops, and then removes the loop by shutting down selected bridge interfaces.
guarantees that there is one and only one active path between two network devices.
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24. Illustration of the STP
Root ports vs Designated ports
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25. The STP PortFast feature
Source:
STP configures meshed topology into a loop-free, tree-like topology.
When the link on a bridge port goes up, STP calculation occurs on that port. The result of the calculation is the transition of the port into forwarding or blocking state. The result depends on the position of the port in the network and the STP parameters.
This calculation and transition period usually takes about 30 to 50 seconds. At that time, no user data pass via the port. Some user applications can time out during the period.
In order to allow immediate transition of the port into forwarding state, enable the STP PortFast feature. PortFast immediately transitions the port into STP forwarding mode upon linkup. The port still participates in STP. So if the port is to be a part of the loop, the port eventually transitions into STP blocking mode.
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26. The STP PortFast feature: vulnerability
Source:
As long as the port participates in STP, some device can assume the root bridge function and affect active STP topology.
To assume the root bridge function, the device would be attached to the port and would run STP with a lower bridge priority than that of the current root bridge.
This is a simple form of a denial of service (DoS) attack on the network. The temporary introduction and subsequent removal of STP devices with low (0) bridge priority cause a permanent STP recalculation.
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27. The STP PortFast feature: vulnerability & mitigation
Source:
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.
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28. L2 security using STP features
BPDU Guard
Bridge protocol data units (BPDU) are messages exchanged btwn bridges using STP to detect loops in a network.
BPDU Guard feature keeps the active topology predictable and enhances switch network reliability by enforcing the STP domain borders.
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29. L2 security using STP features
Root Guard
A feature preventing a bridge in a particular network from becoming the root bridge of that network
e.g., enable Root Guard on service provider (SP) side switch interfaces that connect to a customer-side switch
 If a switch outside the SP network becomes the root switch, the interface is put in a blocked state, and STP will select a new root switch.
spanning tree guard root
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30. L2 security using STP features
EtherChannel Guard
“Cisco EtherChannel technology is a trunking technology based on grouping several full-duplex Ethernet links to provide fault-tolerant, high-speed links between switches, routers, and servers.” (source:
EtherChannel parameters must be the same on both sides (e.g., btwn a switch and a connected device)
When the switch detects an EtherChannel misconfiguration, the EtherChannel Guard places the switch interface in the error-disabled state
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31. L2 security using STP features
Loop Guard prevents alternative or root ports from becoming designated ports, thus avoiding loops
An STP loop is created when an STP blocking port in a redundant topology erroneously transitions to the forwarding state.
This usually happens because one of the ports of a physically redundant topology (not necessarily the STP blocking port) no longer receives STP BPDUs. In its operation, STP relies on continuous reception or 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 designated and moves to a forwarding state. This situation creates a loop.
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32. DHCP Snooping feature Protection from rogue DHCP servers
Ports on a switch are divided into trusted and untrusted ports.
DHCP clients and other hosts are connected to the untrusted ports.
DHCP servers are connected to trusted ports.
The switch maintains a DHCP binding database (aka DHCP snooping table).
Incoming packets received on untrusted ports are dropped if the source MAC address does not match the MAC in the binding table.
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33. IP Source Guard
Protection from IP spoofing attacks when a host tries to spoof and use the IP address of another host
Any IP traffic coming into the interface with a source IP address other than that assigned will be filtered out.
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34. Dynamic ARP Inspection (DAI)
ARP (address resolution protocol) resolves a 32-bit IP address into a 48-bit Ethernet address, using an ARP Table.
The ARP caches can be “poisoned” (an invalid ARP entry is created in the table).
Result: packet redirection to the attacker
DAI locks down the IP-MAC mapping for hosts so that the attacking ARP is denied and logged.
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35. Setting up a secure Layer 2 switching environment
Overview of Counter-measures:
Use VLANs to create logical groupings of devices  Each of the groups may have different security levels.
Disable unused ports, and place them in a VLAN with no Layer 3 access.
Besides VLANs, other mechanisms must be used (e.g., port security)
Separate devices should be used for zones at different security levels.
Disable Layer 3 connection (e.g., Telnet, HTTP) to the switch.
Disable trunking on ports that do not require it (and place the trunk port in its own VLAN).
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36. Need for other counter-measures
4/21/2017
Need for other counter-measures
How about attacks launched from hosts sitting on a LAN?
In general, those hosts are considered as trusted entities.
So it is difficult to stop a host when it becomes an attacker.
Solution: Make sure access to the LAN is secured.
 MAC address filtering (e.g., Cisco’s port security, DHCP)
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