Another week has passed and I’ve used it to concentrate on layer 2 security: DHCP Snooping, Dynamic ARP Inspection and IP Source Guard. I had trouble getting the latter two to work until I realised they work together with DHCP Snooping, and static entries you have to define. But it works now, another thing learned, another check box for the Cisco SWITCH exam.

Time to list some common layer 2 security methods. I will briefly discuss each one here. Note that I use example values, and where I use a VLAN, you can also use a VLAN list most of the time. The interface range command works for these commands as well, allowing for faster configuration.

Port security
The easiest one I think: binds a MAC address to a switchport, so only that host can connect to only that switchport. By default, only one MAC address is allowed, but you can set it to more. Recommended in case of IP Phones, or a hypervisor with multiple virtual machines. Commands:

Switch(config-if)#switchport port-security
Switch(config-if)#switchport port-security mac-address sticky
Switch(config-if)#switchport port-security violation restrict

The second command uses the ‘sticky’ keyword. This means the first MAC address to be detected will be used and added to the running-config. Saves you the time of typing a lot of MAC addresses. The violation mode is restrict here, which drops frames from other MAC addresses and logs an SNMP trap. ‘protected’ would do the same without SNMP trap, and ‘shutdown’ would shutdown the port so nothing can be received on it anymore.

DHCP Snooping
Prevents a rogue DHCP server from handing out IP addresses in the network. The point is not that IP addresses are handed out, but the DHCP server determines the default router, allowing it to influence routes. Personally, I don’t think it’s an often used attack, but it also builds a DHCP binding table on the switch that can be used to prevent other attacks. Configuration:

Switch(config)#ip dhcp snooping
Switch(config)#ip dhcp snooping vlan 1

This activates it for VLAN 1. Of course, the link going to the right DHCP server should be trusted:

Switch(config-if)#ip dhcp snooping trust

IP Source Guard
This function makes sure that a frame sent by a host really came from that host. It does so by comparing the source IP with the source switchport. For the source IP, it either needs the DHCP snooping database (which you can see with the ‘show ip dhcp snooping’), or statically defined entries. An example of a statically defined entry:

Switch(config)#ip source binding 0010.72ab.07f5 vlan 1 192.168.0.5 interface FastEthernet0/3

To enable it, the command must be done on the interface(s):

Switch(config-if)#ip verify source

You can also add the keyword ‘port-security’ after this to check the right MAC address too, but port-security has to be enabled.

Dynamic ARP Inspection
DAI makes sure no false ARP replies or gratuitous ARPs (GARP) are sent. Like IP Source Guard, it uses either the DHCP snooping database or static entries. The static entries are defined with an arp access-list:

Switch(config)#arp access-list ExampleARPlist
Switch(config-acl)#permit ip host 192.168.0.5 mac host 0010.72ab.07f5

To apply it on a VLAN:

Switch(config)#ip arp inspection vlan 1
Switch(config)#ip arp inspection filter ExampleARPlist vlan 1

Private VLANs
I’m mentioning them here because it adds to the security. Very useful in environments where you want hosts to communicate with the gateway, but not each other. For a configuration example, see one of my first blog posts.

802.1x
This makes sure only hosts with the right credentials can connect. A client device must have support for this, but every modern operating system has this. A client device provides a username and password (usually to be filled in somewhere in the network interface options). This password is then checked against a database, usually by RADIUS. If the right credentials are provided, the client device gains access to the network. Activating it requires several commands, as authentication has to be set up. I’m not going to explain how to set up the RADIUS server here.

Switch(config)#aaa new-model
Switch(config)#radius-server host 192.168.1.1
Switch(config)#aaa authentication dot1x default group radius
Switch(config)#dot1x system-auth-control

Switch(config-if)#dot1x port-control auto

Disable Dynamic Trunking Protocol
To prevent a device from forming a trunk link with a switch, and thus gain access to all VLANs, always disable DTP on end-node links:

Switch(config-if)#switchport nonegotiate

BPDU Guard
To prevent any of the end-nodes from taking over as a spanning-tree root bridge, it’s best to configure BPDU Guard. Since these switchports can also be configured with portfast for faster convergence, you can enable BPDU Guard by default too:

Switch(config)#spanning-tree portfast bpduguard default

Switch(config-if)#spanning-tree bpduguard enable

Alternatively, you can configure ‘bpdufilter’ instead of ‘bpduguard’. The difference is that with the first command BPDUs will be filtered, whereas the second command will disable the port.

Preventing double tagging
And finally, this method has no real commands but is just a best-practice: always try to set the native VLAN on trunks to an unused VLAN, and choose another VLAN than VLAN 1 for management. This makes it harder to find the management VLAN, and prevents VLAN hopping attacks. VLAN hopping is done when a frame is sent with two 802.1q tags, of which the first one belongs to a native VLAN. At a trunk link, the first tag will be stripped off the frame, and when received on the next switch, the second VLAN tag will be used. This way, the frame ‘hops’ between VLANs, making attacks possible that are hard to trace.

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