Netlinken, in the context of computing, refers to the Netlink protocol that facilitates communication between the kernel and user space processes in the Linux operating system. It replaces the ioctl system call interfaces that were previously used and offers a simpler and easier-to-use alternative.
Netlink is a part of the Linux kernel interface and uses the socket abstraction to facilitate advanced inter-process communication (IPC) between user and kernel space. It essentially provides a full-duplex connection which is both reliable and ordered. It allows userspace applications to subscribe to notifications from the kernel, await data notifications from different kernel modules, or send commands to kernel space.
Netlink socket family is defined by AF_NETLINK, similar to how TCP and UDP have AF_INET and AF_INET6. Various types of services are defined using netlink families like NETLINK_ROUTE for routing and link updates, NETLINK\_NETFILTER for networking and firewalling, etc. To each of these families, multiple groups can be defined to which the processes can subscribe to. Any message sent to a group will be multicast to all the users subscribed to that group.
The structure of a Netlink message includes a header followed by payload. The header contains length of the message, message type, unique sequence number and Process ID (PID) of the sending process. The payload is followed by optional padding bytes used to ensure that subsequent messages start on 4Byte boundary.
Netlink is useful for system programming and networking applications necessitating deep integration with the kernel. For example, the ‘ip’ utility by ‘iproute2’ package uses Netlink protocol for system configuration. In the same way, Netfilter subsystem uses Netlink to send messages from kernel to user-space processes, the subsystem specifically uses NETLINK\_NETFILTER family.
For more technical details, you can refer to the man pages (‘man 7 netlink’), that are included in the Linux manual, for a comprehensive overview of the Netlink protocol. Linux source code is also a primary source for understanding the technical implementation of Netlink. Papers and articles on Linux kernel development (like “Linux Kernel Development” by Robert Love) also often cover various facets of Netlink.
To summarise, Netlink offers a bi-directional conduit allowing both kernel and user-space processes to communicate in a much cleaner and simpler fashion compared to older methods like ioctl. It’s widely used for tasks ranging from system configuration to networking tasks, making it a vital component of the Linux operating system.
Sources:
1. Linux Manual Pages – ‘man 7 netlink’.
2. Linux Kernel Source Code
3. “Linux Kernel Development” by Robert Love.
