Network Security1 – Chapter 5 – Secure LAN Switching Layer 2 security –Port security –IP permit lists –Protocol filtering –Controlling LAN floods (using.

Network Security 1

– Chapter 5 – Secure LAN Switching

• Layer 2 security– Port security– IP permit lists– Protocol filtering– Controlling LAN floods (using port filtering, protocol

filtering, etc.)– Private VLANs– Using IEEE 802.1x for port authentication and access

control

Network Security 2

Switch 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

• More example attacks: http://www.cisco.com/ca/events/pdfs/L2-security-Bootcamp-final.pdf

• Focus of the chapter: Cisco Catalyst 5000 series switches

Network Security 3

Switch and Layer 2 security (cont.)

• Rules of thumb: 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. 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.

- 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.

“VLAN Insecurity” (http://www.spirit.com/Network/net0103.html)

Network Security 4

Switch and Layer 2 security (cont.)

• 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)

Network Security 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)

Network Security 6

MAC Address Flooding

Network Security 7

MAC Address Flooding (cont.)

• Counter-measures:

1. Hard-coding the MAC addresses that are allowed to connect on a port, or

2. Limiting the number of hosts that are allowed to connect on a port

Example 5-1: approach 1 + timed suspension Example 5-2: approach 2

Network Security 8

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

• Example 5-3

Network Security 9

Protocol Filtering• Purpose: To limit broadcast/multicasts for certain protocols

• With Cisco Catalyst 5000 series of switches, packets are classified into protocol groups:

1. IP 2. IPX2. 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 protocolOff no broadcast/multicast traffic for that protocolAuto 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

Network Security 10

Controlling LAN floods• Attackers may cause frame flood (e.g., CAM flooding),

or send broadcast/multicast messages to flood the LAN.

• Counter-measures:1. Protocol filtering2. 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%.

Network Security 11

Private VLANs• An enhancement to Catalyst 6000 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.

• Three types of private VLAN ports:1. Promiscuous ports: communicate with all other private VLAN

ports2. Isolated ports: have complete Layer 2 isolation from other

ports within the same private VLAN3. Community ports: communicate among themselves and with

their promiscuous ports

Network Security 12

Using IEEE 802.1x• Purpose: (a) port authentication; (b) access control• Other usage: used in 802.11i for WLAN security

Network Security 13

Summary

• Next: NAT and security