linux kernel 2.6.26

I found that there are good samples in kernel document folder.
e.g.
NETLINK_CONNECTOR:
	linux/Documentation/connector/
NETLINK_GENERIC:
	linux/Documentation/accounting/

and one from linux foundation
http://www.linuxfoundation.org/en/Net:Generic_Netlink_HOWTO

libnl
http://people.suug.ch/~tgr/libnl/

In general we should use NETLINK_CONNECTOR or NETLINK_GENERIC.
But here I just want to touch the 'raw' netlink.



/*
 * Multicast send/recv in user app
 */
Multicast in user app is done by setting sockaddr_nl::nl_groups, which one bit
represents one group. hence the most we can have 32 groups.

Someone might send one msg to all the groups you bind() with sockaddr_nl::
nl_groups. You still can only receive one copy of that msg.

/*
 * Multicast recv in kernel
 */
Kernel can join multiple groups by:
struct sock * netlink_kernel_create(struct net *net, int unit,
	unsigned int groups,
	void (*input)(struct sk_buff *skb),
	struct mutex *cb_mutex, struct module *module)
The @groups parameter has the same meaning as sockaddr_nl::nl_groups in
userland.

/*
 * Multicast send in kernel......?
 */
extern int netlink_broadcast(struct sock *ssk, struct sk_buff *skb, u32 pid,
	u32 group, gfp_t allocation);
The @group is bit indicator, which only choose one of 32 bits. That is, only
one of the 32 groups.

As its name suggests, it does *not* multicast, but does broadcast on *one*
group.

Multicast in kernel space is a bit troublesome. You have to call
netlink_broadcast many times for every group.


// test
# Kernel join 2 groups(3: 0011). broadcast to 1 group(2: 0010)
bash> insmod ./dummy_netlink r_grps=3 b_grp=2

# rr_nlk join 2 group(10: 1010) 
bash> ./rr_nlk -g 10 
# then rr_nlk should receiv msgs from kernel periodically

# ww_nlk multicast to 1 group(1: 0001)
bash> ./ww_nlk -g 1
# kernel receives from ww_nlk and print

# ww_nlk multicast to 2 group(9: 1001)
bash> ./ww_nlk -g 9
# kernel and rr_nlk receive from ww_nlk and print

bash> ./ww_nlk -i ${rr_nlk_pid}
# kernel receives msg from ww_nlk and unicast to rr_nlk

// build
bash> cd /path/to/module-source/
bash> echo "obj-m += dummy_netlink.ko" > Makefile
bash> make -C /usr/src/linux/ M=/root/src/dummy_netlink modules