Understanding Linux Network Internals

Chapter 25. Internet Control Message Protocol (ICMPv4)

The Internet Control Message Protocol (ICMP) is a transport protocol used by Internet hosts to exchange control messages, notably error notifications and information requests. In this chapter, we will look at ICMPv4, the version used by IPv4. IPv6 uses the ICMPv6 protocol, a protocol that includes other functionalities besides the ones in ICMPv4.

Over the years, the ICMP protocol has increasingly been used as the basis for the development of monitoring and measurement applications. Unfortunately, the ICMP protocol is also often used as the basis for security attacks, such as DoS or remote fingerprint collection. For this reason, network administrators often configure routers and firewalls to filter out most ICMP message types. Sometimes they filter too much, going against the RFC recommendations. Regardless of whether messages are filtered, they are often rate limited. It follows that any application built on top of ICMP is not always reliable for measurement or monitoring purposes. However, because measurements were not in its original design goal, ICMP often does not allow monitoring applications to collect all the information they need. Instead, dedicated applications have been written for that purpose, often based on TCP or UDP.

For readers interested in the security aspects of ICMP, I recommend the paper "ICMP Usage in Scanning" from the Israeli security consultant Ofir Arkin (http://www.sys-security.com/archive/papers/ICMP_Scanning_v3.0.zip ...