Linux Kernel Architecture

Chapter 12: Networks

the protocol family. Netlink usesnetlink_createfor this purpose.^36 The function allocates an instance

ofstruct sockthat is connected with the socket viasocket->sk. However, space is not only reserved

forstruct sockbut for a larger structure that is(simplified) defined as follows:

net/netlink/af_netlink.c

struct netlink_sock {

/* struct sock has to be the first member of netlink_sock */

struct sock sk;

u32 pid;

u32 dst_pid;

...

void (*netlink_rcv)(struct sk_buff *skb);

...

};

In reality, there are many more netlink-specific elements, and the above code is a selection of the most

essential ones.

Thesockinstance is directly embedded intonetlink_sock. Given an instance ofstruct sockfor netlink

sockets, the associated netlink-specific structurenetlink_socketcan be obtained using the auxiliary

functionnlk_sk. The port IDs of both ends of the connection are kept inpidanddst_pid.netlink_rcv

points to a function that is called to receive data.

Message Format

Netlink messages need to obey a certain format as depicted in Figure 12-35.

Message 1 Message 2

struct nlmsg_hdr

Header Payload Header

aligned on

NLMSG_ALIGNTO

Padding

Figure 12-35: Format of a netlink message.

Each message consists of two components: the header and the payload. While the header is required to

be represented bystruct nlmsghdr, the payload can be arbitrary.^37 The required contents of the header

are given by the following data structure:

<netlink.h>

struct nlmsghdr

{

__u32 nlmsg_len; /* Length of message including header */

__u16 nlmsg_type; /* Message content */

__u16 nlmsg_flags; /* Additional flags */

(^36) The protocol family operationsnetlink_family_opspoint to this function. Recall from Section 12.10.4 that the creation func-
tion is automatically called when a new socket is created.
(^37) The kernel offers the standard data structurestruct nlattrif netlink is used to transport attributes. This possibility is
not discussed in detail, but note that all attribute definitions and a useful set of auxiliary helper functions can be found in
include/net/netlink.h.