RTNETLINK(3)

NAME
rtnetlink - Macros to manipulate rtnetlink messages

SYNOPSIS
#include <asm/types.h>
#include <linux/netlink.h>
#include <linux/rtnetlink.h>
#include <sys/socket.h> rtnetlink_socket = socket(PF_NETLINK, int socket_type, NETLINK_ROUTE);

int RTA_OK(struct rtattr *rta, int rtabuflen);

void *RTA_DATA(struct rtattr *rta);

unsigned int RTA_PAYLOAD(struct rtattr *rta);

struct rtattr *RTA_NEXT(struct rtattr *rta, unsigned int rtabuflen);

unsigned int RTA_LENGTH(unsigned int length);

unsigned int RTA_SPACE(unsigned int length);

DESCRIPTION
All rtnetlink(7) messages consist of a netlink(7) message header and appended attributes. The attributes should be only manipulated using the macros provided here.

RTA_OK(rta, attrlen) returns true if rta points to a valid routing attribute; attrlen is the running length of the attribute buffer. When not true then you must assume there are no more attributes in the message, even if attrlen is non-zero.
RTA_DATA(rta) returns a pointer to the start of this attribute's data.
RTA_PAYLOAD(rta) returns the length of this attribute's data.
RTA_NEXT(rta, attrlen) gets the next attribute after rta. Calling this macro will update attrlen. You should use RTA_OK to check for the validity of the returned pointer.
RTA_LENGTH(len) returns the length which is required for len bytes of data plus the header.
RTA_SPACE(len) returns the amount of space which will be needed in the message with len bytes of data.

BUGS
This manual page is lacking and incomplete.

EXAMPLE
Creating a rtnetlink message to set a MTU of a device.
    struct {
        struct nlmsghdr  nh;
        struct ifinfomsg if;
        char             attrbuf[512];
    } req;
    struct rtattr *rta;
    unsigned int mtu = 1000;
    int rtnetlink_sk = socket(PF_NETLINK, SOCK_DGRAM, NETLINK_ROUTE);
    memset(&req, 0, sizeof(req));
    req.nh.nlmsg_len = NLMSG_LENGTH(sizeof(struct ifinfomsg));
    req.nh.nlmsg_flags = NLM_F_REQUEST;
    req.nh.nlmsg_type = RTML_NEWLINK;
    req.if.ifi_family = AF_UNSPEC;
    req.if.ifi_index = INTERFACE_INDEX;
    req.if.ifi_change = 0xffffffff; /* ???*/
    rta = (struct rtattr *)(((char *) &req) +
                                  NLMSG_ALIGN(n->nlmsg_len));
    rta->rta_type = IFLA_MTU;
    rta->rta_len = sizeof(unsigned int);
    req.n.nlmsg_len = NLMSG_ALIGN(req.n.nlmsg_len) +
                                  RTA_LENGTH(sizeof(mtu));
    memcpy(RTA_DATA(rta), &mtu, sizeof(mtu));
    send(rtnetlink_sk, &req, req.n.nlmsg_len);

SEE ALSO
netlink(3), netlink(7), rtnetlink(7)