title: Netlink socket show ip
keywords: kernel,linux,netlink,socket,ip

# Netlink show network interface ip's

This example based on reading netlink socket and using kernel builtin netlink protocols.

As base is used netlink example.


__Makefile__
```Makefile
make:
	gcc netlink_show_ip.c -o netlink_show_ip
```


__netlink_show_ip.c__
```c
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

#include <sys/socket.h>
#include <linux/netlink.h>

#define PAYLOAD_SIZE 1024  /* maximum payload size*/
#define NETLINK_PROTO 23

int main(int argc, char **argv) {

    struct sockaddr_nl src_addr;
    struct nlmsghdr *nlh;
    struct msghdr msg;
    struct iovec iov;
    int sock_fd;
    int ret;


    sock_fd = socket(AF_NETLINK, SOCK_RAW, NETLINK_ROUTE);
	if (sock_fd < 0)
	{
		printf("socket creation for NETLINK failed...\n");
		return -1;
	}

    close(sock_fd);

    return 0;
}
```

## Add socket configuration 

Here we configure netlink socket and send message to request ip addresses from NETLINK_ROUTE

```c
...
int main(int argc, char **argv) {
...
	memset(&src_addr, 0, sizeof(src_addr));
	src_addr.nl_family = AF_NETLINK;

	//bind(sock_fd, (struct sockaddr*)&src_addr, sizeof(src_addr));//???


	// assemble the message according to the netlink protocol
	nlh = (struct nlmsghdr *)malloc(NLMSG_SPACE(PAYLOAD_SIZE));
	memset(nlh, 0, NLMSG_SPACE(PAYLOAD_SIZE));
	nlh->nlmsg_len = NLMSG_SPACE(PAYLOAD_SIZE);
	nlh->nlmsg_type = RTM_GETADDR;
	nlh->nlmsg_flags = NLM_F_REQUEST | NLM_F_ROOT;

	struct ifaddrmsg *ifa;
	ifa = (struct ifaddrmsg*)NLMSG_DATA(nlh);
	ifa->ifa_family = AF_INET; // we only get ipv4 address here

	// prepare struct msghdr for sending.
	memset(&iov, 0, sizeof(iov));
	iov.iov_base = (void *)nlh;
	iov.iov_len = nlh->nlmsg_len;

	memset(&msg, 0, sizeof(msg));
	msg.msg_name = (void *)&src_addr;
	msg.msg_namelen = sizeof(src_addr);
	msg.msg_iov = &iov;
	msg.msg_iovlen = 1;


	// send netlink message to kernel.
	ret = sendmsg(sock_fd, &msg, 0);
	if (ret < 0) {
		printf("Can't send message to netlink socket\n");
	}
...
}

```

## Receive and process netlink replies

```c
#define NLMSG_NEXT(nlh,len)	 ((len) -= NLMSG_ALIGN((nlh)->nlmsg_len), \
				  (struct nlmsghdr*)(((char*)(nlh)) + NLMSG_ALIGN((nlh)->nlmsg_len)))
#define NLMSG_OK(nlh,len) ((len) >= (int)sizeof(struct nlmsghdr) && \
			   (nlh)->nlmsg_len >= sizeof(struct nlmsghdr) && \
			   (nlh)->nlmsg_len <= (len))
```


Message types

```c
#define NLMSG_NOOP		0x1	/* Nothing.		*/
#define NLMSG_ERROR		0x2	/* Error		*/
#define NLMSG_DONE		0x3	/* End of a dump	*/
#define NLMSG_OVERRUN		0x4	/* Data lost		*/
```

Definition of message rtnetlink types

```c
	RTM_NEWADDR	= 20,
#define RTM_NEWADDR	RTM_NEWADDR
```

List of type of RTA types 

```c
enum {
	IFA_UNSPEC,
	IFA_ADDRESS,
	IFA_LOCAL,
	IFA_LABEL,
	IFA_BROADCAST,
	IFA_ANYCAST,
	IFA_CACHEINFO,
	IFA_MULTICAST,
	IFA_FLAGS,
	IFA_RT_PRIORITY,	/* u32, priority/metric for prefix route */
	IFA_TARGET_NETNSID,
	IFA_PROTO,		/* u8, address protocol */
	__IFA_MAX,
};
```

Receiving netlink message from a 

```c
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <string.h>
#include <unistd.h>

#include <net/if.h>
#include <arpa/inet.h>
#include <sys/socket.h>
#include <linux/netlink.h>
#include <linux/rtnetlink.h>
...
...

int main(){
...
do {
        ret = recvmsg(sock_fd, &msg_recv, 0);
        if (ret < 0) {
            printf("Can't receive reply message\n");
            break;
        }
        
        if (ret==0) {
            printf("received 0 byte reply\n");
            break;
        }
        
        ret_len = ret;
        nlh_recv = (struct nlmsghdr*)recv_buf;
        while (NLMSG_OK(nlh_recv, (uint32_t)ret_len) && (nlh_recv->nlmsg_type != NLMSG_DONE))
        {
            printf("received message type %d\n", nlh_recv->nlmsg_type);
            if (nlh_recv->nlmsg_type == NLMSG_ERROR) {
                printf("Message error\n");
                break;
            }

            if (nlh_recv->nlmsg_type == RTM_NEWADDR) {
            	...
            }
            nlh_recv = NLMSG_NEXT(nlh_recv, ret);
        }  
    } while ((nl_msg_type != NLMSG_DONE) && (nl_msg_type != NLMSG_ERROR));
...
}
```

After message type is detected parse the RTNETLINK message

```c
int main() {
...
              ifa_recv = (struct ifaddrmsg*)NLMSG_DATA(nlh_recv);
                int ifa_len = IFA_PAYLOAD(nlh_recv);
                rta = IFA_RTA(ifa_recv);
                printf("interface %s\n", if_indextoname(ifa_recv->ifa_index, ifname));
                printf("prefix length %d\n", ifa_recv->ifa_prefixlen);
                while (RTA_OK(rta, ret_len)) {
                    rta = RTA_NEXT(rta, ifa_len);

                    char ip[INET6_ADDRSTRLEN];

                    //parse the message
                    if (rta->rta_type == IFA_ADDRESS) {
                        inet_ntop(ifa_recv->ifa_family, RTA_DATA(rta), ip, INET6_ADDRSTRLEN);
                        printf("IFA_ADDRESS %s\n", ip);
                    }

                    if (rta->rta_type == IFA_LOCAL) {
                        inet_ntop(ifa_recv->ifa_family, RTA_DATA(rta), ip, INET6_ADDRSTRLEN);
                        printf("IFA_LOCAL %s\n", ip);
                    }

                    if (rta->rta_type == IFA_BROADCAST) {
                        inet_ntop(ifa_recv->ifa_family, RTA_DATA(rta), ip, INET6_ADDRSTRLEN);
                        printf("IFA_BROADCAST %s\n", ip);
                    }
                }
...
}
```



## Build

```sh
make
```

## Run 

```sh
./netlink_show_ip
```

Output looks like

```sh
received message type 20
interface lo
prefix length 8
IFA_LOCAL 127.0.0.1
received message type 20
interface enp0
prefix length 22
IFA_LOCAL 192.168.1.149
IFA_BROADCAST 192.168.1.255
```

## Final result


__netlink_show_ip.c__
```c
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <string.h>
#include <unistd.h>

#include <net/if.h>
#include <arpa/inet.h>
#include <sys/socket.h>
#include <linux/netlink.h>
#include <linux/rtnetlink.h>

#define PAYLOAD_SIZE 4096  /* maximum payload size*/

char recv_buf[4096];

int main(int argc, char **argv) {

    struct sockaddr_nl src_addr;
    struct nlmsghdr *nlh;
    struct nlmsghdr *nlh_recv;
    struct msghdr msg;
    struct msghdr msg_recv;
    struct iovec iov;
    struct iovec iov_recv;
    struct ifaddrmsg *ifa;
    struct ifaddrmsg *ifa_recv;
    struct rtattr *rta = NULL;
    int sock_fd;
    int ret;
    int ret_len;
    uint32_t nl_msg_type;
    char ifname[IF_NAMESIZE];
    
    sock_fd = socket(AF_NETLINK, SOCK_RAW, NETLINK_ROUTE);
	if (sock_fd < 0)
	{
		printf("socket creation for NETLINK failed...\n");
		return -1;
	}

    memset(&src_addr, 0, sizeof(src_addr));
    src_addr.nl_family = AF_NETLINK;

    bind(sock_fd, (struct sockaddr*)&src_addr, sizeof(src_addr));//???


    // assemble the message according to the netlink protocol
	nlh = (struct nlmsghdr *)malloc(NLMSG_SPACE(PAYLOAD_SIZE));
    memset(nlh, 0, NLMSG_SPACE(PAYLOAD_SIZE));
	nlh->nlmsg_len = NLMSG_SPACE(PAYLOAD_SIZE);
	nlh->nlmsg_type = RTM_GETADDR;
	nlh->nlmsg_flags = NLM_F_REQUEST | NLM_F_ROOT;

    
	ifa = (struct ifaddrmsg*)NLMSG_DATA(nlh);
	ifa->ifa_family = AF_INET; // we only get ipv4 address here

	// prepare struct msghdr for sending.
    memset(&iov, 0, sizeof(iov));
    iov.iov_base = (void *)nlh;
    iov.iov_len = nlh->nlmsg_len;

	//struct msghdr msg = { src_addr, sizeof(*src_addr), &iov, 1, NULL, 0, 0 };
    memset(&msg, 0, sizeof(msg));
    msg.msg_name = (void *)&src_addr;
    msg.msg_namelen = sizeof(src_addr);
    msg.msg_iov = &iov;
    msg.msg_iovlen = 1;

	// send netlink message to kernel.
	ret = sendmsg(sock_fd, &msg, 0);
	if (ret < 0) {
        printf("Can't send message to netlink socket\n");
    }

    //prepare to receive reply
    memset(&iov_recv, 0, sizeof(iov_recv));
    iov_recv.iov_base = recv_buf;
    iov_recv.iov_len = PAYLOAD_SIZE;

    memset(&msg_recv, 0, sizeof(msg_recv));
	msg_recv.msg_name = (void *)&src_addr;
	msg_recv.msg_namelen = sizeof(src_addr);
	msg_recv.msg_iov = &iov_recv;
	msg_recv.msg_iovlen = 1;


    do {
        ret = recvmsg(sock_fd, &msg_recv, 0);
        if (ret < 0) {
            printf("Can't receive reply message\n");
            break;
        }
        
        if (ret==0) {
            printf("received 0 byte reply\n");
            break;
        }
        
        ret_len = ret;
        nlh_recv = (struct nlmsghdr*)recv_buf;
        while (NLMSG_OK(nlh_recv, (uint32_t)ret_len) && (nlh_recv->nlmsg_type != NLMSG_DONE))
        {
            printf("received message type %d\n", nlh_recv->nlmsg_type);
            if (nlh_recv->nlmsg_type == NLMSG_ERROR) {
                printf("Message error\n");
                break;
            }

            if (nlh_recv->nlmsg_type == RTM_NEWADDR) {
                ifa_recv = (struct ifaddrmsg*)NLMSG_DATA(nlh_recv);
                int ifa_len = IFA_PAYLOAD(nlh_recv);
                rta = IFA_RTA(ifa_recv);
                printf("interface %s\n", if_indextoname(ifa_recv->ifa_index, ifname));
                printf("prefix length %d\n", ifa_recv->ifa_prefixlen);
                while (RTA_OK(rta, ret_len)) {
                    rta = RTA_NEXT(rta, ifa_len);

                    char ip[INET6_ADDRSTRLEN];

                    //parse the message
                    if (rta->rta_type == IFA_ADDRESS) {
                        inet_ntop(ifa_recv->ifa_family, RTA_DATA(rta), ip, INET6_ADDRSTRLEN);
                        printf("IFA_ADDRESS %s\n", ip);
                    }

                    if (rta->rta_type == IFA_LOCAL) {
                        inet_ntop(ifa_recv->ifa_family, RTA_DATA(rta), ip, INET6_ADDRSTRLEN);
                        printf("IFA_LOCAL %s\n", ip);
                    }

                    if (rta->rta_type == IFA_BROADCAST) {
                        inet_ntop(ifa_recv->ifa_family, RTA_DATA(rta), ip, INET6_ADDRSTRLEN);
                        printf("IFA_BROADCAST %s\n", ip);
                    }
                }
            }
            nlh_recv = NLMSG_NEXT(nlh_recv, ret);
        }  
    } while ((nl_msg_type != NLMSG_DONE) && (nl_msg_type != NLMSG_ERROR));

    close(sock_fd);

    return 0;
}
```

## Links

1. [/notes/kernel/netlink_socket.md](/notes/kernel/netlink_socket.md)
1. https://github.com/d0u9/examples/blob/master/C/netlink/ip_show.c
2. https://gist.github.com/luohao-brian/7535fa346f37450137e5db45cd793ace  
3. https://github.com/mwarning/netlink-examples/tree/master  
4. https://elixir.bootlin.com/linux/v6.6.20/source/include/uapi/linux/rtnetlink.h#L42
5. https://elixir.bootlin.com/linux/v6.6.20/source/include/uapi/linux/netlink.h#L100