netlink

Netlink was created by Alexey Kuznetsov{{cite web|url=https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/net/core/rtnetlink.c |title=kernel/git/torvalds/linux.git: root/net/core/rtnetlink.c | work=Linux kernel source tree |publisher=kernel.org |access-date=2014-05-27}} as a more flexible alternative to the sophisticated but awkward ioctl communication method used for setting and getting external socket options. The Linux kernel continues to support {{code|ioctl}} for backward compatibility.

Netlink was first provided in the 2.0 series of the Linux kernel, implemented as a character device. By 2013, this interface is obsolete, but still forms an ioctl communication method; compare the use of {{code|rtnetlink}}.{{cite book

| editor1-last = Crowcroft

| editor1-first = Jon

| editor1-link = Jon Crowcroft

| editor2-last = Phillips

| editor2-first = Iain

| title = TCP/IP and Linux protocol implementation: systems code for the Linux Internet

| url = https://books.google.com/books?id=qOdSAAAAMAAJ

| access-date = 2013-05-21

| series = Wiley Networking Council series

| year = 2002

| publisher = Wiley

| isbn = 9780471408826

| page = 624

| quote = All rtnetlink messages consist of a netlink message header and appended attributes.}} The Netlink socket interface appeared in 2.2 series of the Linux kernel.

In 2022, experimental support for the Netlink protocol was added to FreeBSD. Initially, only a subset of the NETLINK_ROUTE family and NETLINK_GENERIC is supported.{{r|freebsd}}

Unlike BSD sockets using Internet protocols such as TCP, where the message headers are autogenerated, the Netlink message header (available as {{code|struct nlmsghdr}}) must be prepared by the caller. The Netlink socket generally works in a {{code|SOCK_RAW}}-like mode, even if {{code|SOCK_DGRAM}} was used to create it.

The data portion then contains a subsystem-specific message that may be further nested.

The {{code|AF_NETLINK}} family offers multiple protocol subsets. Each interfaces to a different kernel component and has a different messaging subset. The subset is referenced by the protocol field in the socket call:

int socket(AF_NETLINK, SOCK_DGRAM or SOCK_RAW, protocol)

Lacking a standard, {{code|SOCK_DGRAM}} and {{code|SOCK_RAW}} are not guaranteed to be implemented in a given Linux (or other OS) release. Some sources state that both options are legitimate, and the reference below from Red Hat states that {{code|SOCK_RAW}} is always the parameter. However, iproute2 uses both interchangeably.

A non-exhaustive list of the supported protocol entries follows:

; NETLINK_ROUTE

{{code|NETLINK_ROUTE}} provides routing and link information. This information is used primarily for user-space routing daemons. Linux implements a large subset of messages:

• Link layer: RTM_NEWLINK, RTM_DELLINK, RTM_GETLINK, RTM_SETLINK
• Address settings: RTM_NEWADDR, RTM_DELADDR, RTM_GETADDR
• Routing tables: RTM_NEWROUTE, RTM_DELROUTE, RTM_GETROUTE
• Neighbor cache: RTM_NEWNEIGH, RTM_DELNEIGH, RTM_GETNEIGH
• Routing rules: RTM_NEWRULE, RTM_DELRULE, RTM_GETRULE
• Queuing discipline settings: RTM_NEWQDISC, RTM_DELQDISC, RTM_GETQDISC
• Traffic classes used with queues: RTM_NEWTCLASS, RTM_DELTCLASS, RTM_GETTCLASS
• Traffic filters: RTM_NEWTFILTER, RTM_DELTFILTER, RTM_GETTFILTER
• Others: RTM_NEWACTION, RTM_DELACTION, RTM_GETACTION, RTM_NEWPREFIX, RTM_GETPREFIX, RTM_GETMULTICAST, RTM_GETANYCAST, RTM_NEWNEIGHTBL, RTM_GETNEIGHTBL, RTM_SETNEIGHTBL

; NETLINK_FIREWALL

{{code|NETLINK_FIREWALL}} provides an interface for a user-space app to receive packets from the firewall.

; NETLINK_NFLOG

{{code|NETLINK_NFLOG}} provides an interface used to communicate between Netfilter and iptables.

; NETLINK_ARPD

{{code|NETLINK_ARPD}} provides an interface to manage the ARP table from user-space.

; NETLINK_AUDIT

{{code|NETLINK_AUDIT}} provides an interface to the audit subsystem found in Linux kernel versions 2.6.6 and later.

; NETLINK_IP6_FW

{{code|NETLINK_IP6_FW}} provides an interface to transport packets from netfilter to user-space.

; NETLINK_ROUTE6

; NETLINK_TAPBASE

; NETLINK_NETFILTER

; NETLINK_TCPDIAG

; NETLINK_XFRM

{{code|NETLINK_XFRM}} provides an interface to manage the IPsec security association and security policy databases - mostly used by key-manager daemons using the Internet Key Exchange protocol.

; NETLINK_KOBJECT_UEVENT

{{code|NETLINK_KOBJECT_UEVENT}} provides the interface in which the kernel broadcasts uevents, typically consumed by udev.

; NETLINK_GENERIC

One of the drawbacks of the Netlink protocol is that the number of protocol families is limited to 32 ({{code|MAX_LINKS}}).This is one of the main reasons that the generic Netlink family was created—to provide support for adding a higher number of families. It acts as a Netlink multiplexer and works with a single Netlink family {{code|NETLINK_GENERIC}}. The generic Netlink protocol is based on the Netlink protocol and uses its API.

Users can add a Netlink handler in their own kernel routines. This allows the development of additional Netlink protocols to address new kernel modules.{{Cite web|url=https://www.linuxjournal.com/article/7356|title=Kernel Korner - Why and How to Use Netlink Socket | Linux Journal|website=www.linuxjournal.com}}

{{Portal|Computer programming|Free and open-source software|Linux}}

• Comparison of open-source wireless drivers – [https://web.archive.org/web/20150117035401/http://wireless.kernel.org/en/developers/Documentation/mac80211/ mac80211]-based drivers rely on Netlink as the API to user space
• POSIX

{{Reflist}}

• [https://web.archive.org/web/20110902031641/http://1984.lsi.us.es/~pablo/docs/spae.pdf Pablo Neira Ayuso, Rafael M. Gasca, Laurent Lefèvre. Communicating between the kernel and user-space in Linux using Netlink sockets. Software: Practice and Experience, 40(9):797-810, August 2010]
• [https://www.linuxjournal.com/article/7356 Why and How to Use Netlink Sockets]
• [https://www.ietf.org/rfc/rfc3549.txt RFC 3549]
• [https://netfilter.org/projects/libmnl/ netfilter/iptables project homepage - The netfilter.org "libmnl" project] – Minimalist Library for Netlink – userspace library for construction and parsing of Netlink messages
• [https://www.infradead.org/~tgr/libnl libnl - Netlink Protocol Library Suite] – Netlink Protocol Library Suite – full functional library covering almost all aspects of working with Netlink sockets
• [https://www.linuxjournal.com/article/8498 Manipulating the Networking Environment Using RTNETLINK]
• [https://web.archive.org/web/20170428010447/https://qos.ittc.ku.edu/netlink/html/index.html Netlink Sockets – An Overview]
• [http://www.carisma.slowglass.com/~tgr/libnl/ Netlink Protocol Library Suite]
• [https://www.apress.com/gp/book/9781430261964 "Linux Kernel Networking" by Rami Rosen, Apress 2013: Chapter 2, Netlink Sockets]

{{Inter-process communication}}

{{Linux kernel}}

Category:Interfaces of the Linux kernel

Category:Linux kernel features