Sniffing is the process of monitoring and capturing all the packets passing through a given network using sniffing tools. It is a form of “tapping phone wires” and get to know about the conversation.
It is also called WIRETAPPING applied to computer networks. There is so much possibility that if a set of enterprise switch ports are open, then one of their employees can sniff the whole traffic of the network.
Anyone in the same physical location can plug into the network using Ethernet cable or connect
wirelessly to that network and sniff the total traffic.
In other words, Sniffing allows you to see all sorts of traffic, both protected and unprotected. In the
right conditions and with the right protocols in place, an attacking party may be able to gather
information that can be used for further attacks or to cause other issues for the network or system owner.

WHAT CAN BE SNIFFED

One can sniff the following sensitive information from a network:
  • Email traffic
  • FTP passwords
  • Web traffics
  • Telnet passwords
  • Router configuration
  • Chat sessions
  • DNS traffic
HOW IT WORKS

A sniffer normally turns the NIC of the system to the promiscuous mode so that it listens
to all the data transmitted on its segment.
Promiscuous mode refers to the unique way of Ethernet hardware, in particular, network interface
cards (NICs), that allows a NIC to receive all traffic on the network, even if it is not addressed to this NIC.
By default, a NIC ignores all traffic that is not addressed to it, which is done by comparing the
destination address of the Ethernet packet with the hardware address (a.k.a. MAC) of the device. While this makes perfect sense for networking, non-promiscuous mode makes it difficult to use network monitoring and analysis software for diagnosing connectivity issues or traffic accounting.
A sniffer can continuously monitor all the traffic to a computer through the NIC by decoding the
information encapsulated in the data packets.

TYPES OF SNIFFING

Sniffing can be either Active or Passive in nature.

PASSIVE SNIFFING: In passive sniffing, the traffic is locked but it is not altered in any way. Passive sniffing allows listening only. It works with Hub devices. On a hub device, the traffic is sent to all the ports. In a network that uses hubs to connect systems, all hosts on the network can see the traffic. Therefore, an attacker can easily capture traffic going through. The good news is that hubs are
almost obsolete nowadays. Most modern networks use switches. Hence, passive sniffing is no more
effective.

ACTIVE SNIFFING: In active sniffing, the traffic is not only locked and monitored, but it may also be altered in some way as determined by the attack. Active sniffing is used to sniff a switch-based network. It involves injecting address resolution packets (ARP) into a target network to flood on the switch content addressable memory (CAM) table. CAM keeps track of which host is connected to which port. Following are the Active Sniffing Techniques:
  • MAC Flooding
  • DHCP Attacks
  • DNS Poisoning
  • Spoofing Attacks
  • ARP Poisoning
image source: https://banknxt.com/51147/criminal-eavesdroppers/