Understanding Linux Network Internals

IP Configuration

The Linux IP protocol can be tuned and configured manually by a system administrator in different ways. This tuning includes both changes to the protocol itself and to device configuration . The four main interfaces are:

ioctl calls made via ifconfig

ifconfig is the older Unix-legacy tool for configuring IP on network devices.

RTNetlink via ip

ip, which is part of the IPROUTE2 package, is the newer tool that Linux offers for configuring IP on network devices.

/proc filesystem

Protocol behavior can be tuned via a collection of files in the directory /proc/sys/net/ipv4.

RARP/BOOTP/DHCP

These three protocols can be used to dynamically assign an IP configuration to a host and its interfaces.

The last set of protocols in the preceding list have an interesting twist. They are normally implemented in user space, but Linux also has a simple kernel-space implementation that is useful when used together with the nfsroot boot option. The latter allows the kernel to mount the root directory (/) via NFS. To do that, it needs an IP configuration at boot time before the system is able to initialize the IP configuration from user space (which, by the way, could be stored in a remote partition and not even be available to the system when it mounts the root directory). Via kernel boot options, it is possible to give nfsroot a static configuration, or specify what protocols (yes, more than one can be used concurrently) to use to obtain the configuration. The IP configuration code is ...