Not able to set the MTU size of the ethernet interfaces using NETLINK sockets via NETLINK_ROUTE option

[sekhar417]

I have written a program to set the MTU size of the particular interface(say eth0 or eth1) to 1100. And the Request message is send from user space using Netlink sockets via NETLINK_ROUTE option.

The message is sent successfully from user space, but when i verified the ifconfig eth0, the MTU size still shows the old value (1500). Am I verifying correctly? And how do I know the kernel is setting the MTU size correctly or not? Please find my program below and correct me if i am wrong.

#include <stdio.h>
#include <stdlib.h>
#include <net/if.h>
#include <string.h>
#include <sys/socket.h>
#include <linux/netlink.h>
#include <linux/rtnetlink.h>
#include <stdbool.h>

struct {
struct nlmsghdr nh;
struct ifinfomsg ifinfo;
char data[100];
}req;

int ret;

struct rtattr *rta;

/* MTU Set */
unsigned int mtu = 1100;

/* rtNetlinkSockFd */
int rtNetlinkSockFd = -1;

int main()
{

int ret;

rtNetlinkSockFd = socket(AF_NETLINK, SOCK_DGRAM, NETLINK_ROUTE);

if(rtNetlinkSockFd < 0)
{
printf("Creation of NetLink Socket is failed \n");
return -1;
}

/* Memset the Requested Structure */
memset( &req, 0x00, sizeof(req));

/* Populate the Netlink Header Fields */
req.nh.nlmsg_len = NLMSG_LENGTH(sizeof(struct ifinfomsg));


/* Link Layer: RTM_NEWLINK, RTM_DELLINK, RTM_GETLINK, RTM_SETLINK */
req.nh.nlmsg_type = RTM_SETLINK;

/* NLM_F_REQUEST Must be set on all request messages. */
req.nh.nlmsg_flags = NLM_F_REQUEST;

req.nh.nlmsg_seq = 0;
req.nh.nlmsg_pid = 0; //getpid();

/* Populate the Ifinfo Structure Attributes */
req.ifinfo.ifi_family = AF_UNSPEC;

/* Give the Interface Name and get the Index */
req.ifinfo.ifi_index = if_nametoindex("eth0");

printf(" The NetLink Ifi_index :%d\n", req.ifinfo.ifi_index);

/* ifi_change is reserved for future use and
* should be always set to 0xFFFFFFFF. */
req.ifinfo.ifi_change = 0xFFFFFFFF;
req.ifinfo.ifi_type = 0;
req.ifinfo.ifi_flags = 0;

/* RTA is Pointed to (req+32) it means points to the DATA */
rta = (struct rtattr *)(((char *) &req) + NLMSG_ALIGN(req.nh.nlmsg_len));

/* IFLA_MTU unsigned int MTU of the device. */
rta->rta_type = IFLA_MTU;

/* Len Attribute */
rta->rta_len = sizeof(unsigned int);

req.nh.nlmsg_len = NLMSG_ALIGN(req.nh.nlmsg_len) + RTA_LENGTH(sizeof(mtu));

memcpy(RTA_DATA(rta), &mtu, sizeof(mtu));

ret = send(rtNetlinkSockFd, &req, req.nh.nlmsg_len, 0);

if (ret < 0)
{
printf( "netlink: Sending failed: (assume operstate is not supported)");
}
else
{
printf( "netlink: Sent successfully");
}
return 0;
}