Chapter 13 -- Raw Sockets

Chapter 13

A raw socket is a socket that allows access to the underlying transport protocol. This chapter is dedicated to illustrating how raw sockets can be used to simulate IP utilities, such as Traceroute and Ping. Raw sockets can also be used to actually manipulate the IP header information. This chapter is concerned only with the IP protocol; we will not address raw sockets with any other protocol, as most protocols (except ATM) do not support raw sockets at all. All raw sockets are created using the SOCK_RAW socket type and are currently supported only under Winsock 2. Therefore, neither Microsoft Windows CE nor Windows 95 (without the Winsock 2 update) can utilize raw sockets.

Additionally, using raw sockets requires substantial knowledge of the underlying protocol structure, which is generally not the focus of this book. In this chapter, we will discuss the Internet Control Message Protocol (ICMP), the Internet Group Management Protocol (IGMP), and the User Datagram Protocol (UDP). ICMP is used by the Ping utility, which can detect whether a route to a host is valid and whether the host machine is responding. Developers often need a programmatic method of determining whether a machine is alive and reachable. IGMP is used by IP multicasting to advertise multicast group membership to routers. Recently support was added to most Win32 platforms to support IGMP version 2. However, in some cases you might want to send your own IGMP packets to drop group membership. We will examine the UDP protocol in conjunction with the IP_HDRINCL socket option as an example of how to send your own IGMP packets. For all three of these protocols, we will cover only the aspects necessary to fully explain the code in this chapter and in the example programs. For more detailed information, consult W. Richard Stevens's book on IP, TCP/IP Illustrated Vol. 1 (Addison-Wesley, 1994).