||This article may contain original research. (January 2012)|
Netlink is a socket family used for IPC between the kernel and user space processes, as well as between user processes (e.g. Unix domain sockets) or a mixture of both types. However, unlike INET sockets, it cannot traverse host boundaries, as it addresses processes by their (inherently local) PIDs.
Netlink was designed for and is used to transfer miscellaneous networking information between the Linux kernel space and user space processes. Networking utilities such as iproute2 use Netlink to communicate with the Linux kernel from user space. Netlink consists of a standard socket-based interface for user space processes and an internal kernel API for kernel modules. It is designed to be a more flexible successor to ioctl. Originally, Netlink used the AF_NETLINK socket family.
RFC 3549 describes this protocol in detail.
Netlink was created[by whom?] as a more flexible alternative to the sophisticated but awkward
ioctl communication method which software used for setting and getting external socket options. The Linux kernel continues to support
ioctl for backward compatibility. It can be used as follows:
error = ioctl(ip_socket, ioctl_type, &value_result);
Netlink was first provided in Linux kernel 2.0 as a character device. As of 2013[update] this interface is obsolete, but as an ioctl communication method, it can still be used. (Compare the use of
rtnetlink.) The Netlink socket interface appeared in the 2.2 Linux kernel.
Packet structure 
Unlike the BSD socket access to Internet protocols such as TCP where the headers specifying flags and destination are autogenerated, the Netlink message header (available as struct nlmsghdr) must be prepared by the caller, as the socket generally works in a SOCK_RAW-like mode, even if SOCK_DGRAM was used to create it.
The data portion then contains a subsystem-specific message that may be further nested.
The AF_NETLINK family offers multiple protocol subsets. Each interfaces to a different kernel component and has a different messaging subset. The following protocol is referenced in the field below:
int socket(AF_NETLINK, SOCK_DGRAM or SOCK_RAW, protocol)
Lacking a standard, SOCK_DGRAM and 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 SOCK_RAW is always the parameter, however iproute2 uses both interchangeably.
A non-exhaustive list of the supported protocol entries follows:
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 provides an interface for a user-space app to receive packets from the firewall.
NETLINK_ARPD provides an interface to manage the ARP table from user space.
NETLINK_AUDIT provides an interface to the audit subsystem found in Linux kernel versions 2.6.6 and later.
NETLINK_IPV6_FW provides an interface to transport packets from netfilter to userspace.
NETLINK_XFRM provides an interface to manage the IPsec security association and security policy databases. It is mostly used by Key Manager daemons when they are used in Internet Key Exchange protocol.
The user can add a netlink handler in their own kernel routines. This allows additional Netlink protocols to be developed to address new kernel modules.
- Crowcroft, Jon; Phillips, Iain, eds. (2002). TCP/IP and Linux protocol implementation: systems code for the Linux Internet. Wiley Networking Council series. Wiley. p. 624. ISBN 9780471408826. Retrieved 2013-05-21. "All rtnetlink messages consist of a netlink message header and appended attributes."
- Why and How to Use Netlink Sockets
- 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
- Why and How to Use Netlink Sockets
- RFC 3549
- http://netfilter.org/projects/libmnl — “Minimalist Library for Netlink” — userspace library for construction and parsing of Netlink messages
- http://www.infradead.org/~tgr/libnl = "Netlink Protocol Library Suite" - full functional library covering almost all aspects of working with netlink sockets
- Manipulating the Networking Environment Using RTNETLINK
- Netlink Sockets - Overview