Introduction linux under the netlink

Address reprint: https://www.jianshu.com/p/6810f42b9f8f

First, what is netlink

Netlink socket is used to achieve the user process and the kernel process between a communication is a special process communication (IPC), the most common network interface communication with the core application.

In the Linux kernel, used by applications to communicate with the kernel netlink for many applications, such as

• Routing daemon (NETLINK_ROUTE)
• User mode protocol socket (NETLINK_USERSOCK)
• Firewall (NETLINK_FIREWALL)
• netfilter subsystem (NETLINK_NETFILTER)
• Kernel event notification to the user mode (NETLINK_KOBJECT_UEVENT)
• General netlink (NETLINK_GENERIC)

Netlink is a bi-directional data transfer between a very good way of kernel and user application, user mode application using the standard socket API you can use the power of netlink provided kernel mode requires the use of special kernel API to use netlink.

Generally communication between user space and kernel space in three ways: /proc、ioctl、Netlink. The first two are unidirectional, and Netlink duplex communication can be realized.

Netlink with respect to the system call, and the ioctl / proc file system has the following advantages:

• netlink simple use, only need include/linux/netlink.hto add a new type of protocol definition to the netlink, (e.g., #define NETLINK_TEST 20then the kernel and user mode applications can immediately by using the socket API netlink data exchange protocol type)
• netlink is an asynchronous communication, message passing between the kernel and user mode application stored in the socket buffer queue, the message is simply transmitted message stored in the recipient socket receive queue, without waiting for the recipient received message
• Netlink core portion may use a modular way to achieve, without reliance compiled using netlink application part and the core part
• netlink support multicast, kernel module or application can put a message to a netlink multicast group belong to any application or kernel module that neilink group can receive the message, the kernel event notification mechanism to user mode to use this feature
• You can use the kernel netlink first session is initiated

Netlink BSD socket based protocol and AF_NETLINKaddress family, the use of 32-bit address port numbers, each with a typically Netlink protocol or a set of kernel services / associated components, such as NETLINK_ROUTEfor getting and setting routing and link information NETLINK_KOBJECT_UEVENTto the kernel udev to process user space to send notifications.

Second, the user-mode data structure

User mode applications using the standard socket API with a sendto (), recvfrom (), sendmsg (), recvmsg ().

Netlink Communication with common UDP Socket Communications Similarly, struct sockaddr_nla netlink mailing address, with ordinary socket struct sockaddr_insimilar.

1. struct sockaddr_nl structure:

struct sockaddr_nl {
     __kernel_sa_family_t    nl_family;  /* AF_NETLINK （跟AF_INET对应）*/
     unsigned short  nl_pad;     /* zero */
     __u32       nl_pid;     /* port ID  （通信端口号）*/
     __u32       nl_groups;  /* multicast groups mask */
};

2. struct nlmsghd structure:

/* struct nlmsghd 是netlink消息头*/
struct nlmsghdr {   
    __u32       nlmsg_len;  /* Length of message including header */
    __u16       nlmsg_type; /* Message content */
    __u16       nlmsg_flags;    /* Additional flags */ 
    __u32       nlmsg_seq;  /* Sequence number */
    __u32       nlmsg_pid;  /* Sending process port ID */
};

• nlmsg_type: Message state, the kernel include/uapi/linux/netlink.his defined in the following 4 general message types, they are:

#define NLMSG_NOOP      0x1 /* Nothing.     */
#define NLMSG_ERROR     0x2 /* Error        */
#define NLMSG_DONE      0x3 /* End of a dump    */
#define NLMSG_OVERRUN       0x4 /* Data lost        */
#define NLMSG_MIN_TYPE      0x10    /* < 0x10: reserved control messages */

• nlmsg_flags: message marking, to indicate which type of message, as follows

/* Flags values */
#define NLM_F_REQUEST       1   /* It is request message.   */
#define NLM_F_MULTI     2   /* Multipart message, terminated by NLMSG_DONE */
#define NLM_F_ACK       4   /* Reply with ack, with zero or error code */
#define NLM_F_ECHO      8   /* Echo this request        */
#define NLM_F_DUMP_INTR     16  /* Dump was inconsistent due to sequence change */

/* Modifiers to GET request */
#define NLM_F_ROOT  0x100   /* specify tree root    */
#define NLM_F_MATCH 0x200   /* return all matching  */
#define NLM_F_ATOMIC    0x400   /* atomic GET       */
#define NLM_F_DUMP  (NLM_F_ROOT|NLM_F_MATCH)

/* Modifiers to NEW request */
#define NLM_F_REPLACE   0x100   /* Override existing        */
#define NLM_F_EXCL  0x200   /* Do not touch, if it exists   */
#define NLM_F_CREATE    0x400   /* Create, if it does not exist */
#define NLM_F_APPEND    0x800   /* Add to end of list       */

3. struct msghdr structure

struct iovec {                    /* Scatter/gather array items */
     void  *iov_base;              /* Starting address */
     size_t iov_len;               /* Number of bytes to transfer */
 };
  /* iov_base: iov_base指向数据包缓冲区，即参数buff，iov_len是buff的长度。msghdr中允许一次传递多个buff，以数组的形式组织在 msg_iov中，msg_iovlen就记录数组的长度 （即有多少个buff）  */
 struct msghdr {
     void         *msg_name;       /* optional address */
     socklen_t     msg_namelen;    /* size of address */
     struct iovec *msg_iov;        /* scatter/gather array */
     size_t        msg_iovlen;     /* # elements in msg_iov */
     void         *msg_control;    /* ancillary data, see below */
     size_t        msg_controllen; /* ancillary data buffer len */
     int           msg_flags;      /* flags on received message */
 };

---

Three, netlink kernel data structures

1. netlink message types:

#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

#define MAX_LINKS 32

2. netlink commonly used macros:

#define NLMSG_ALIGNTO   4U
/* 宏NLMSG_ALIGN(len)用于得到不小于len且字节对齐的最小数值 */
#define NLMSG_ALIGN(len) ( ((len)+NLMSG_ALIGNTO-1) & ~(NLMSG_ALIGNTO-1) )

/* Netlink 头部长度 */
#define NLMSG_HDRLEN     ((int) NLMSG_ALIGN(sizeof(struct nlmsghdr)))

/* 计算消息数据len的真实消息长度（消息体 +　消息头）*/
#define NLMSG_LENGTH(len) ((len) + NLMSG_HDRLEN)

/* 宏NLMSG_SPACE(len)返回不小于NLMSG_LENGTH(len)且字节对齐的最小数值 */
#define NLMSG_SPACE(len) NLMSG_ALIGN(NLMSG_LENGTH(len))

/* 宏NLMSG_DATA(nlh)用于取得消息的数据部分的首地址，设置和读取消息数据部分时需要使用该宏 */
#define NLMSG_DATA(nlh)  ((void*)(((char*)nlh) + NLMSG_LENGTH(0)))

/* 宏NLMSG_NEXT(nlh,len)用于得到下一个消息的首地址, 同时len 变为剩余消息的长度 */
#define NLMSG_NEXT(nlh,len)  ((len) -= NLMSG_ALIGN((nlh)->nlmsg_len), \
                  (struct nlmsghdr*)(((char*)(nlh)) + NLMSG_ALIGN((nlh)->nlmsg_len)))

/* 判断消息是否 >len */
#define NLMSG_OK(nlh,len) ((len) >= (int)sizeof(struct nlmsghdr) && \
               (nlh)->nlmsg_len >= sizeof(struct nlmsghdr) && \
               (nlh)->nlmsg_len <= (len))

/* NLMSG_PAYLOAD(nlh,len) 用于返回payload的长度*/
#define NLMSG_PAYLOAD(nlh,len) ((nlh)->nlmsg_len - NLMSG_SPACE((len)))

3. netlink common kernel function

netlink_kernel_create kernel function to create a user to communicate with the kernel mode socket

static inline struct sock *
netlink_kernel_create(struct net *net, int unit, struct netlink_kernel_cfg *cfg)
/* net: net指向所在的网络命名空间, 一般默认传入的是&init_net(不需要定义);  定义在net_namespace.c(extern struct net init_net);
   unit：netlink协议类型
   cfg： cfg存放的是netlink内核配置参数（如下）
*/

/* optional Netlink kernel configuration parameters */
struct netlink_kernel_cfg {
    unsigned int    groups;  
    unsigned int    flags;  
    void        (*input)(struct sk_buff *skb); /* input 回调函数 */
    struct mutex    *cb_mutex; 
    void        (*bind)(int group); 
    bool        (*compare)(struct net *net, struct sock *sk);
};

4. unicast netlink_unicast () and multicast netlink_broadcast ()

/* 发送单播消息 */
extern int netlink_unicast(struct sock *ssk, struct sk_buff *skb, __u32 portid, int nonblock);
/*
 ssk: netlink socket 
 skb: skb buff 指针
 portid： 通信的端口号
 nonblock：表示该函数是否为非阻塞，如果为1，该函数将在没有接收缓存可利用时立即返回，而如果为0，该函数在没有接收缓存可利用定时睡眠
*/

/* 发送多播消息 */
extern int netlink_broadcast(struct sock *ssk, struct sk_buff *skb, __u32 portid,
                 __u32 group, gfp_t allocation);
/* 
   ssk: 同上（对应netlink_kernel_create 返回值）、
   skb: 内核skb buff
   portid： 端口id
   group: 是所有目标多播组对应掩码的"OR"操作的合值。
   allocation: 指定内核内存分配方式，通常GFP_ATOMIC用于中断上下文，而GFP_KERNEL用于其他场合。这个参数的存在是因为该API可能需要分配一个或多个缓冲区来对多播消息进行clone
*/

---

Four, netlink examples

1. User mode program (sendto (), recvfrom ())

#include <stdio.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <string.h>
#include <linux/netlink.h>
#include <stdint.h>
#include <unistd.h>
#include <errno.h>

#define NETLINK_TEST    30
#define MSG_LEN            125
#define MAX_PLOAD        125

typedef struct _user_msg_info
{
    struct nlmsghdr hdr;
    char  msg[MSG_LEN];
} user_msg_info;

int main(int argc, char **argv)
{
    int skfd;
    int ret;
    user_msg_info u_info;
    socklen_t len;
    struct nlmsghdr *nlh = NULL;
    struct sockaddr_nl saddr, daddr;
    char *umsg = "hello netlink!!";

    /* 创建NETLINK socket */
    skfd = socket(AF_NETLINK, SOCK_RAW, NETLINK_TEST);
    if(skfd == -1)
    {
        perror("create socket error\n");
        return -1;
    }

    memset(&saddr, 0, sizeof(saddr));
    saddr.nl_family = AF_NETLINK; //AF_NETLINK
    saddr.nl_pid = 100;  //端口号(port ID) 
    saddr.nl_groups = 0;
    if(bind(skfd, (struct sockaddr *)&saddr, sizeof(saddr)) != 0)
    {
        perror("bind() error\n");
        close(skfd);
        return -1;
    }

    memset(&daddr, 0, sizeof(daddr));
    daddr.nl_family = AF_NETLINK;
    daddr.nl_pid = 0; // to kernel 
    daddr.nl_groups = 0;

    nlh = (struct nlmsghdr *)malloc(NLMSG_SPACE(MAX_PLOAD));
    memset(nlh, 0, sizeof(struct nlmsghdr));
    nlh->nlmsg_len = NLMSG_SPACE(MAX_PLOAD);
    nlh->nlmsg_flags = 0;
    nlh->nlmsg_type = 0;
    nlh->nlmsg_seq = 0;
    nlh->nlmsg_pid = saddr.nl_pid; //self port

    memcpy(NLMSG_DATA(nlh), umsg, strlen(umsg));
    ret = sendto(skfd, nlh, nlh->nlmsg_len, 0, (struct sockaddr *)&daddr, sizeof(struct sockaddr_nl));
    if(!ret)
    {
        perror("sendto error\n");
        close(skfd);
        exit(-1);
    }
    printf("send kernel:%s\n", umsg);

    memset(&u_info, 0, sizeof(u_info));
    len = sizeof(struct sockaddr_nl);
    ret = recvfrom(skfd, &u_info, sizeof(user_msg_info), 0, (struct sockaddr *)&daddr, &len);
    if(!ret)
    {
        perror("recv form kernel error\n");
        close(skfd);
        exit(-1);
    }

    printf("from kernel:%s\n", u_info.msg);
    close(skfd);

    free((void *)nlh);
    return 0;
}

2. Netlink kernel module code

#include <linux/init.h>
#include <linux/module.h>
#include <linux/types.h>
#include <net/sock.h>
#include <linux/netlink.h>

#define NETLINK_TEST     30
#define MSG_LEN            125
#define USER_PORT        100

MODULE_LICENSE("GPL");
MODULE_AUTHOR("zhangwj");
MODULE_DESCRIPTION("netlink example");

struct sock *nlsk = NULL;
extern struct net init_net;

int send_usrmsg(char *pbuf, uint16_t len)
{
    struct sk_buff *nl_skb;
    struct nlmsghdr *nlh;

    int ret;

    /* 创建sk_buff 空间 */
    nl_skb = nlmsg_new(len, GFP_ATOMIC);
    if(!nl_skb)
    {
        printk("netlink alloc failure\n");
        return -1;
    }

    /* 设置netlink消息头部 */
    nlh = nlmsg_put(nl_skb, 0, 0, NETLINK_TEST, len, 0);
    if(nlh == NULL)
    {
        printk("nlmsg_put failaure \n");
        nlmsg_free(nl_skb);
        return -1;
    }

    /* 拷贝数据发送 */
    memcpy(nlmsg_data(nlh), pbuf, len);
    ret = netlink_unicast(nlsk, nl_skb, USER_PORT, MSG_DONTWAIT);

    return ret;
}

static void netlink_rcv_msg(struct sk_buff *skb)
{
    struct nlmsghdr *nlh = NULL;
    char *umsg = NULL;
    char *kmsg = "hello users!!!";

    if(skb->len >= nlmsg_total_size(0))
    {
        nlh = nlmsg_hdr(skb);
        umsg = NLMSG_DATA(nlh);
        if(umsg)
        {
            printk("kernel recv from user: %s\n", umsg);
            send_usrmsg(kmsg, strlen(kmsg));
        }
    }
}

struct netlink_kernel_cfg cfg = { 
        .input  = netlink_rcv_msg, /* set recv callback */
};  

int test_netlink_init(void)
{
    /* create netlink socket */
    nlsk = (struct sock *)netlink_kernel_create(&init_net, NETLINK_TEST, &cfg);
    if(nlsk == NULL)
    {   
        printk("netlink_kernel_create error !\n");
        return -1; 
    }   
    printk("test_netlink_init\n");
    
    return 0;
}

void test_netlink_exit(void)
{
    if (nlsk){
        netlink_kernel_release(nlsk); /* release ..*/
        nlsk = NULL;
    }   
    printk("test_netlink_exit!\n");
}

module_init(test_netlink_init);
module_exit(test_netlink_exit);

3..Makeflie

#
#Desgin of Netlink
#

MODULE_NAME :=netlink_test
obj-m :=$(MODULE_NAME).o

KERNELDIR ?= /lib/modules/$(shell uname -r)/build
PWD := $(shell pwd)

all:
    $(MAKE) -C $(KERNELDIR) M=$(PWD)

clean:
    $(MAKE) -C $(KERNELDIR) M=$(PWD) clean

4. Run results

First Netlink compiled kernel module is inserted into the system (insmod netlink_test.ko), and then run the application, see the following output:

# 应用程序打印
send kernel:hello netlink!!
from kernel:hello users!!!
# 内核打印 
[25024.276345] test_netlink_init
[25117.548350] kernel recv from user: hello netlink!!