netlink

netlink - a simple Golang netlink library

what's netlink ?

The netlink package provides a communication protocol which allows process exchanging information no matter ther are in user or kernel space.

The advantage of netlink

Most kernal's process need communicate with user's process in Linux, but traditional Unix's IPC (pipe, message queue, shared memory and singal) can not offer a strong support for the communication between user's process and kernel. Linux provides a lot other methods which allow user's process can communicate with kernel, but they are very hard to use. To make these method easier to user for user, especially for Operational Engineer is the reason why we develop netlink.

Compared with native method, netlink has following advantages:

• The prerequistie of using netlink is just adding a new type in include/linux/netlink.h
• nektlink is asynchronous. An instance will not be blocked after sending messages. The messages is actually saved in socket's messages buffer.
• netlink is modularizeed. netlink'application and kernel parts are independent. They do not have any compile dependency. But system call does. Whenever a new system all want to be used. It has to be compiled statically into kernel.
• netlink supports multicast. Kernel modules or process can pass messages into multiple netlink group.
• netlink can be called from kernel, unlike ioctl.
• netlink uses standard socket API, hence it is pretty simple to use unlike system call or iotcl. But these traditional protocol can not give enough support of the communication between process(user space) and kernel. Linux provides a lot of methods to fix such problem, but they are hard to learn. A new protocol netlink is easy to learn.

netlink feature

netlink only provides basic communication protocol between user's process and kernel's process. These specific tasks are based on the subprotocol in netlink. Additionally, inside Linux's kernel, it already has a protocol which is:

#define NETLINK_ROUTE       0   /* Routing/device hook              */
#define NETLINK_UNUSED      1   /* Unused number                */
#define NETLINK_USERSOCK    2   /* Reserved for user mode socket protocols  */
#define NETLINK_FIREWALL    3   /* Unused number, formerly ip_queue     */
#define NETLINK_SOCK_DIAG   4   /* socket monitoring                */
#define NETLINK_NFLOG       5   /* netfilter/iptables ULOG */
#define NETLINK_XFRM        6   /* ipsec */
#define NETLINK_SELINUX     7   /* SELinux event notifications */
#define NETLINK_ISCSI       8   /* Open-iSCSI */
#define NETLINK_AUDIT       9   /* auditing */
#define NETLINK_FIB_LOOKUP  10  
#define NETLINK_CONNECTOR   11
#define NETLINK_NETFILTER   12  /* netfilter subsystem */
#define NETLINK_IP6_FW      13
#define NETLINK_DNRTMSG     14  /* DECnet routing messages */
#define NETLINK_KOBJECT_UEVENT  15  /* Kernel messages to userspace */
#define NETLINK_GENERIC     16
/* leave room for NETLINK_DM (DM Events) */
#define NETLINK_SCSITRANSPORT   18  /* SCSI Transports */
#define NETLINK_ECRYPTFS    19
#define NETLINK_RDMA        20
#define NETLINK_CRYPTO      21  /* Crypto layer */
#define NETLINK_INET_DIAG   NETLINK_SOCK_DIAG

How to use netlink?

user space

If you plan use netlink in user space, then you should consider standard socker API. For creating a new netlink sockert, user need use the following arguments to call socker():

socket(AF_NETLINK, SOCK_RAW, netlink_type)

You can find more details here

kernel space

Any kernel's module wants to use netlink(in Linux, not this project) need to include header file linux/netlink.h. Compared with the usage of netlink, kernel has to use particular API defined in netlink. For now, netlink alraedy implemented a generic protocol NETLINK_GENERIC which reduces the extra work.

example

func (c *ExecCollector) prepareSocket() (*netlink.NetlinkSocket, error) {
	ns, err := netlink.NewNetlinkSocket(netlink.CONNECTOR, netlink.CN_IDX_PROC)
	if err != nil {
		return nil, fmt.Errorf("create netlink socket: %v", err)
	}
	req := netlink.NewNetlinkRequest()
	{
		msg := netlink.NewCnMsg()
		req.AddData(msg)
		op := netlink.PROC_CN_MCAST_LISTEN
		req.AddData(&op)
	}
	err = ns.Send(req)
	if err != nil {
		return nil, fmt.Errorf("Exec: %v", err)
	}
	go func() {
		<-c.shutdown
		ns.Close()
	}()
	return ns, nil
}

future work

• need add all test for this project

miscellaneous

more information here