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Packet sniffers and how to protect yourself from them


This post explores packet sniffers, which are a useful tool and a potential threat. It provides detailed information about packet sniffers, software used as packet sniffers, how sniffers work, types of sniffing, protocols vulnerable to sniffing, Wireshark® filters, threats of Address Resolution Protocol (ARP) poisoning, span port, and how to defend against packet sniffing.

What are packet sniffers?

A sniffer is a piece of software that captures network traffic and performs network analysis, traffic analysis, protocol analysis, sniffing, packet analysis, and so on. A packet sniffer is essentially a tool that aids in monitoring network traffic and troubleshooting a network. It works by capturing and analyzing packets of data that flow through a network. In some cases, a packet sniffer might be a dedicated hardware device.

Sniffers capture, decode, and analyze network traffic and answer questions such as the following:

  • Why is the network slow?
  • What is the network traffic pattern?
  • How is the traffic being shared between nodes?

Network analyzer

A network analyzer is a combination of hardware and software tools that can detect, decode, and manipulate traffic on the network. Network administrators use network analyzers to troubleshoot networking issues, but many hackers use them to gather vital information.

Uses include:

  • Passive sniffing (detection) - Difficult to detect
  • Active sniffing (attack)

Common network analyzers

The following list contains some common analyzers:

  • Wireshark
  • Ettercap®
  • Dsniff®
  • Tcpdump
  • Etherpeak®
  • Cain and Abel®

How a sniffer works

Data travels through a network in the form of packets. Packet-switched networks break down the data to transmit into several packets. They then reassemble these packets after all the data packets reach their intended destination.

When you install a packet sniffer in the network, the sniffer intercepts the network traffic and captures the raw data packets. Subsequently, the packet sniffing software analyzes the captured data packet and presents the results to the network administrators in a user-friendly format so that they can interpret the data.

Image source: https://www.eccouncil.org/

Types of sniffing

There are two types of sniffing: passive and active.

Network administrators use passive sniffing on the hub used by the network to send traffic to all the ports. It only monitors packets sent by others and does not insert any additional data packets into the network traffic.

Attackers use active sniffing to steal data from others. In a network that uses hubs to connect systems, all hosts on the network can see all the traffic, and therefore, an attacker can easily capture data packets.

What is Wireshark?

Formerly known as Ethereal, Wireshark is an open-source program with many free features that provides the following functionality:

  • Helps you to decode over 750 protocols.
  • Is compatible with many other sniffers.
  • Has plenty of online resources available.
  • Supports the command-line and GUI interfaces.
  • Offers the TShark command-line interface that has the following three components:
  • Editcap: Reads the captured packets from the infile and reads and writes the same capture files that are supported by Wireshark.
  • Mergecap: Combines multiple saved capture files into a single output file.
  • Text2pcap: Reads in an ASCII hex dump and writes the data described into a pcap or pcapng capture file. Text2pcap can read hex dumps with multiple packets in them and build a capture file of multiple packets.

Protocols vulnerable to sniffing

The following protocols are vulnerable to sniffing:

  • HTTP
  • Telnet
  • rlogin
  • POP
  • IMAP
  • SMTP and NNTP
  • FTP

Users of network analyzers

The following roles use network analyzers:

  • System administrators
  • Understand system problems and performance
  • Malicious individuals (intruders)
  • Capture cleartext data
  • Passively collect data on the following vulnerable protocols: FTP, POP3, IMAP, SMATP, rlogin, HTTP, and so on.
  • Capture VoIP data
  • Map the target network
  • Discover traffic patterns
  • Actively break into the network (backdoor techniques)

Filters

You can use filters to analyze captured data.

Image source: https://www.wireshark.org

Sometimes, you can observe and record traffic traveling on a network, which might contain valuable information such as the following:

– Usernames and passwords

  • Encrypted
  • Unencrypted – Email, web requests (and replies), data files, and so on.

Wireshark

Wireshark a is very popular network analyzer tool, which is used by network administrators to capture packets traversing through a network. Administrators mostly use it to identify network problems, but hackers also use it to decode secure information.

The following image shows a Wireshark screen:

Image source: https://www.wireshark.org

Example – A nmap port scan:

  • Target host: 10.0.0.1 – Start Wireshark
  • Source host: 10.0.0.2 or the attacker’s computer – Perform a TCP-connect scan: nmap –sT <target host>
  • View results

Example – A web connection:

  • Target host: 10.0.0.1 – Start Wireshark
  • Source host: 10.0.0.2 – Open web browser: Type any website name
  • View results

Example – An FTP connection:

  • Target host: 10.0.0.1 – Start Wireshark
  • Source host: 10.0.0.2 – Use the ftp client: ftp <target host>
  • View results

Man-in-the-middle

The man-in-the-middle is a common attack tactic.

In a switched environment, a host receives only the following:

  • Traffic destined for itself
  • Broadcast traffic

The host cannot see traffic between other hosts. The man-in-the-middle attack enables you to insert yourself as an (undetected) intermediary between communicating hosts.

What is ARP poisoning?

Address Resolution Protocol (ARP) poisoning is when an attacker sends falsified ARP messages over a local area network (LAN) to link an attacker’s MAC address with an IP address of a legitimate computer or a server on the network. After the attacker’s media access control (MAC) address is linked to an authentic IP address, the attacker can receive any messages directed to the legitimate MAC address. As a result, the attacker can intercept, modify, or block communications to the legitimate MAC address.

Attackers look for the following opportunities to use ARP poisoning:

– Sensitive, unencrypted communications

  • Web requests or replies, email, FTP, or HTTP – Weakly-encrypted communications
  • Old versions of SSH or RDC

ARP poisoning countermeasures

Dynamic ARP inspection in Cisco® systems helps to prevent man-in-the-middle attacks by not relaying invalid or gratuitous ARP replies to other ports in the same VLAN. Dynamic ARP inspection intercepts all ARP requests and all replies on untrusted ports. Each intercepted packet is verified for valid IP-to-MAC bindings via DHCP snooping. Denied ARP packets are either dropped or logged by the switch for auditing when ARP poisoning attacks are stopped. Incoming ARP packets on the trusted ports are not inspected. Dynamic ARP inspection can also rate-limit ARP requests from client ports to minimize port scanning mechanisms.

How to defend against sniffing

Use the following techniques and best practices to protect yourself from sniffing attacks:

  • Restrict the physical access to the network media to ensure that a packet sniffer cannot be installed.
  • Use encryption to protect confidential information.
  • Permanently add the MAC address of the gateway to the ARP cache.
  • Use static IP addresses and static ARP tables to prevent attackers from adding spoofed ARP entries for their machines to the network.
  • Turn off network identification broadcasts, and if possible, restrict the network to authorized users in order to protect the network from being discovered with sniffing tools.
  • Use the IPv6 instead of the IPv4 protocol.
  • Use encrypted sessions such as Secure Shell (ssh) instead of Telnet.
  • Use Secure Copy (scp) instead of a file transfer protocol (ftp).
  • Use Secure Socket Layer (SSL) for email connections.

Conclusion

These days, many attacks happen through packet sniffing. Packet sniffers are placed in cyber cafes and on open wifi in restaurants, hotels, and public places. You can protect your data with a little caution. You should never use open wifi and should stop using open text protocols like ftp, http, IMAP, Telnet, and SNMP V1 and V2. You must install SSL certificates in your websites, use Secure File Transfer Protocol (sftp) instead of ftp, and use SSH instead of telnet. You should use SNMP V3 and opt for the strongest encryption.

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Prasuram Jaishwal

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