Layer two, in accordance to the OSI model, is an innately insecure layer. We have to understand that within our networks, security is only as strong as our weakest link and if a layer is compromised since it is at the lower end, it may never be detectable, making layer two security crucial.  Popular attacks that can arise on this layer are, VLAN Hopping, ARP spoofing, STP attacks, CAM table overflows, DHCP spoofing, and CDP/LLDP Reconnaissance. Here, we are going to examine what each attack is and certain counter measures you can take to prevent them.

1. VLAN Hopping:
      Now, the goal of VLAN Hopping is to send traffic to a different VLAN from your own and we can accomplish this by one of two methods, switch spoofing or by double tagging. Switch spoofing is the idea of connecting to a switch within the network and setting up a trunk. Thus allowing the hacker to analyze the traffic and possibly alter it if the switch receives traffic from multiple VLANs. This attack takes advantage of the default enabled dynamic trunking. To prevent this, disabling dynamic trunking on all non-trunk ports should do the trick.
Double tagging however is a bit different. VLANs offer security by isolating traffic, this is done through tagging. However since tags can be nested, meaning that you can have a tag on-top of another tag, if the traffic being received by a switch has the tag of the native VLAN for that switch, then the switch will remove the outer tag and forward the traffic to the VLAN of the second tag, thus hopping a VLAN.

2. ARP Spoofing:
        The goal of ARP Spoofing is to attempt to impersonate a computer within the same network. This can be done because ARP is an innately insecure protocol because it does not have a check operation. Therefore, if Bob's computer wants to communicate with Alice's and asks for her MAC address, but instead Susan replies, then all of the traffic sent from Bob that is supposed to go to Alice will instead be sent to Susan. You can counteract this by allowing Dynamic ARP Inspection, which verifies all ARP requests and responses sent across the network, segmenting out LANs to be as small as possible, and always keeping 802.1x in mind.

​3. STP Attacks:
      Spanning Tree Protocol is used to prevent loops within a network, thus making it an extremely crucial protocol. Because STP attempts to always find the best paths throughout the network, if an attacker, connects a device that triggers the optimization operation it can affect how traffic flows and can possibly re-route traffic through the newly connected switch. Due to the pause of STP, this can create a Denial of Service, greatly reducing the flow of traffic if repeated. You can counteract this by enabling Root Guard on the switches ports that should not become root ports and using BPDU Guard on all ports that have PortFast enabled.

4. CAM Table Overflows:
     The CAM table is responsible for learning which MAC addresses are connected to which interfaces on the switch and sending the traffic in accordance to this table. This is a highly effective mechanism as the more addresses are learned the less broadcasts are needed, thus saving bandwidth. However each table can only be so large, therefore if this limit is reached, all traffic from unknown MAC addressed gets flooded throughout every interface. This leads to a Denial of Service as the switch, along with the network, eventually crawls to a halt. However, this can be counteracted by enabling Port Security to limit the number of MAC addresses that can be learned per-interface. 

5. DHCP spoofing:
     Similar to ARP spoofing, the goal of DHCP spoofing is to attempt to impersonate a computer. However in this case the computer is the DHCP server within the network. If the attacker connects a malicious DHCP server to the network, whenever a client requests an IP address the server that responds the fastest gets to assign the client an address and can control the gateway seen by that device. If the attack works perfectly, the hacker can re-route the traffic to his or herself and then send all of the traffic to the correct gateway, thus creating a MITM attack. The best way to counteract this attack is to use DHCP snooping, where you can configure ports to be either trusted on un-trusted.

6. CDP/LLDP Reconnaissance:
   Link Layer Discovery Protocol and the Cisco propriety, Cisco Discovery Protocol, do essentially the same thing, they attempt to discover what type of device is connected to a particular port mainly for troubleshooting purposes. The issue however is that this information is available for anyone on the network, so if an attacker is able to listen in, he or she can gain this information as well. The best countermeasure is to simply disable the default enabled Cisco Discovery Protocol on your routers.

Nmap is one of the most important and versatile tools in any hacker's toolbox. It's powerful engine along with it's ease of use allows people who have no prior command line experience to pick it up fairly quickly. Personally, I have used nmap to help exploit systems but also to tell me what ports are open on a printer so I can access the web portal. Overall it is an extremely useful tool and below I am going to explain the 10 most useful Nmap commands you should learn.

At number 10: Traditional Scan (mostly checks to see if host is alive)
# nmap [IP address]

Example 9: A ping scan of the network:
# nmap -sP [network ID/subnet CIDR] 

Number 8: SYN TCP port scan from ports 1 to 65535
# nmap -sS -p1-65535 [IP address]
 
Number 7: UDP port scan from ports 1 to 65535
# nmap -sU -p1-65535 [IP address]

Number 6: Skip the ping, scan specific ports for activity
# nmap -sn -p22,80,443 [IP address]
 
Number 5: OS detection with an aggressive scan
# nmap -O -A [IP address]

Number 4: Conducts an ACK TCP scan and attempts to find the versions of what is running on the ports
# nmap -sV -sA -p22,80,8080 [IP address]

Number 3: Spoofs the IP address on interface eth0, while fragmenting the packets, and conducting an ACK scan
# nmap -S [Spoof source IP address] -e eth0 -f -p20,21,22,2380,143,443,589,8080 -sA [IP address]

Number 2: Incorporating Nikto into your Nmap Scan
# nmap -p80,443 [IP address] -oG - | nikto -h -

Number 1: Nmap Scripting engine
# nmap --script-help=
Example script: http-brute, sshv1, smb-vuln-ms10-054,...

The nmap scripting engine is an incredibly useful tool, that I recommend everyone to learn.