What are ICMP Echo Requests in host discovery

Question

ICMP Echo Requests are used to check if a host is active on a network. How do these packets help identify live systems during a host discovery scan?

Answer 1

​ICMP Echo Requests, commonly known as "ping" requests, are fundamental tools in network diagnostics and host discovery. They operate using the Internet Control Message Protocol (ICMP), which is utilized by network devices to send error messages and operational information, indicating success or failure when communicating with another IP address.

How ICMP Echo Requests Facilitate Host Discovery

1. Mechanism of Operation

   • An ICMP Echo Request (Type 8) is sent to a target IP address.​

   • If the target is active and reachable, it responds with an ICMP Echo Reply (Type 0).

   • This exchange confirms the presence of a live host at the specified IP address.​

2. Implementation in Network Scanning Tools

   • Tools like Nmap utilize ICMP Echo Requests during the host discovery phase to identify active devices on a network. Nmap sends an ICMP Echo Request to target IP addresses and anticipates an Echo Reply from responsive hosts.

3. Advantages

   • Efficiency: ICMP Echo Requests are lightweight and impose minimal bandwidth usage, making them suitable for quickly scanning large networks.​

   • Simplicity: The straightforward request-reply mechanism simplifies the process of detecting active hosts.​

Limitations and Considerations

• Firewall and Security Configurations:

  • Many modern firewalls and security settings are configured to block ICMP traffic, including Echo Requests, to prevent potential reconnaissance by malicious actors. ​

  • As a result, the absence of an Echo Reply does not definitively indicate that a host is inactive; it may simply be unresponsive to ICMP requests.​

• Alternative Host Discovery Methods:

  • Due to the potential blocking of ICMP traffic, additional techniques are often employed:​

    • TCP SYN Scans: Sending TCP SYN packets to common ports to elicit responses from active hosts.

    • ARP Requests: On local networks, Address Resolution Protocol (ARP) requests can identify active devices regardless of their ICMP configurations.

Practical Example

Using Nmap to perform a ping scan on a subnet:​

nmap -sn 192.168.1.0/24

In this command:

• -sn instructs Nmap to perform a ping scan (host discovery) without port scanning.​

• 192.168.1.0/24 specifies the target subnet.​

Nmap will send ICMP Echo Requests to all addresses in the specified range and list the responsive (live) hosts.