rtnetlink3

Usage

#include <asm/types.h>
#include <linux/netlink.h>
#include <linux/rtnetlink.h>
#include <sys/socket.h>

Library

Standard C library (libc, -lc)

Overview

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

RTA_OK(rta, size) returns true if rta points to a valid routing attribute; size is the running size of the attribute buffer. When not true then you must assume there are no more attributes in the message, even if size is nonzero.

RTA_DATA(rta) returns a pointer to the start of this attribute's data.

RTA_PAYLOAD(rta) returns the size of this attribute's data.

RTA_NEXT(rta, size) gets the next attribute after rta. Calling this macro will update size. You should use RTA_OK to check the validity of the returned pointer.

RTA_LENGTH(size) returns the size which is required for size bytes of data plus the header.

RTA_SPACE(size) returns the amount of space which will be needed in a message with size bytes of data.

Examples

Creating a rtnetlink message to set the MTU of a device:

#include <linux/rtnetlink.h>
...
struct {
    struct nlmsghdr  nh;
    struct ifinfomsg if;
    char             attrbuf[512];
} req;
struct rtattr *rta;
unsigned int mtu = 1000;
int rtnetlink_sk = socket(AF_NETLINK, SOCK_DGRAM, NETLINK_ROUTE);
memset(&req, 0, sizeof(req));
req.nh.nlmsg_len = NLMSG_LENGTH(sizeof(req.if));
req.nh.nlmsg_flags = NLM_F_REQUEST;
req.nh.nlmsg_type = RTM_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(req.nh.nlmsg_len));
rta->rta_type = IFLA_MTU;
rta->rta_len = RTA_LENGTH(sizeof(mtu));
req.nh.nlmsg_len = NLMSG_ALIGN(req.nh.nlmsg_len) +
                              RTA_LENGTH(sizeof(mtu));
memcpy(RTA_DATA(rta), &mtu, sizeof(mtu));
send(rtnetlink_sk, &req, req.nh.nlmsg_len, 0);