17.1 Introduction

The ioctl function has traditionally been the system interface used for everything that didn't fit into some other nicely defined category. POSIX is getting rid of ioctl for certain functionality by creating specific wrapper functions to replace ioctls whose functionality is being standardized by POSIX. For example, the Unix terminal interface was traditionally accessed using ioctl, but POSIX created 12 new functions for terminals: tcgetattr to get the terminal attributes, tcflush to flush pending input or output, and so on. In a similar vein, POSIX has replaced one network ioctl: the new sockatmark function (Section 24.3) replaces the SIOCATMARK ioctl. Nevertheless, numerous ioctls remain for implementation-dependent features related to network programming: obtaining interface information and accessing the routing table and ARP cache, for example.

This chapter provides an overview of the ioctl requests related to network programming, but many of these are implementation-dependent. Additionally, some implementations, including 4.4BSD-derived systems and Solaris 2.6 and later, use sockets in the AF_ROUTE domain (routing sockets) to accomplish many of these operations. We will cover routing sockets in Chapter 18.

A common use of ioctl by network programs (typically servers) is to obtain information on all the host's interfaces when the program starts: the interface addresses, whether the interface supports broadcasting, whether the interface supports multicasting, and so on. We will develop our own function to return this information and provide an implementation using ioctl in this chapter, and examine another implementation using routing sockets in Chapter 18.