www.LinuxHowtos.org

rtnetlink

Section: C Library Functions (3)
Updated: 202-0-08
Index Return to Main Contents

NAME

rtnetlink - macros to manipulate rtnetlink messages

LIBRARY

Standard C library (libc,~-lc)

SYNOPSIS

#include <asm/types.h>
#include <linux/netlink.h>
#include <linux/rtnetlink.h>
#include <sys/socket.h>
rtnetlink_socket = socket(AF_NETLINK, int socket_type, NETLINK_ROUTE);
int RTA_OK(struct rtattr *rta, int size);
void *RTA_DATA(struct rtattr *rta);
unsigned int RTA_PAYLOAD(struct rtattr *rta);
struct rtattr *RTA_NEXT(struct rtattr *rta, unsigned int size);
unsigned int RTA_LENGTH(unsigned int size);
unsigned int RTA_SPACE(unsigned int size);

DESCRIPTION

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.

STANDARDS

Linux.

BUGS

This manual page is incomplete.

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);