Linux Device Drivers, Second Edition

The net directory

The net directory in the Linux file hierarchy is the repository for the socket abstraction and the network protocols; these features account for a lot of code, since Linux supports several different network protocols. Each protocol (IP, IPX, and so on) lives in its own subdirectory; the directory for IP is called ipv4 because it represents version 4 of the protocol. The new standard (not yet in wide use as we write this) is called ipv6 and is implemented in Linux as well. Unix-domain sockets are treated as just another network protocol; their implementation can be found in the unix subdirectory.

The network implementation in Linux is based on the same file operations that act on device files. This is natural, because network connections (sockets) are described by normal file descriptors. The file socket.c is the locus of the socket file operations. It dispatches the system calls to one of the network protocols via a struct proto_ops structure. This structure is defined by each network protocol to map system calls to its specific, low-level data handling operations.

Not every subdirectory of net is used to define a protocol family. There are a few notable exceptions: core, bridge, ethernet, sunrpc, and khttpd.

Files in core implement generic network features such as device handling, firewalls, multicasting, and aliases; this includes the handling of socket buffers (core/skbuff.c) and socket operations that remain independent of the underlying protocol (core/sock.c). The device-independent data management that sits near device-specific code is defined in core/dev.c.

The ethernet and bridge directories are used to implement specific low-level functionalities, specifically, the Ethernet-related helper functions described in Chapter 14, and bridging functionality.

sunrpc and khttpd are peculiar because they include kernel-level implementations of tasks that are usually carried out in user space.

In sunrpc you can find support functions for the kernel-level NFS server (which is an RPC-based service), while khttpd implements a kernel-space web server. Those services have been brought to kernel space to avoid the overhead of system calls and context switches during time-critical tasks. Both have demonstrated good performance in this mode. The khttpd subsystem, however, has already been rendered obsolete by TUX, which, as of this writing, holds the record for the world’s fastest web server. TUX will likely be integrated into the 2.5 kernel series.

The two remaining source files within net are sysctl_net.c and netsyms.c. The former is the back end of the sysctl mechanism,^[66] and the latter is just a list of EXPORT_SYMBOL declarations. There are several such files all over the kernel, usually one in each major directory.

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^[66] sysctl has not been described in this book; interested readers can have a look at Alessandro’s description of this mechanism at http://www.linux.it/kerneldocs/sysctl.