Man page - rtnetlink(3)

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Manual

rtnetlink

NAME
LIBRARY
SYNOPSIS
DESCRIPTION
STANDARDS
BUGS
EXAMPLES
SEE ALSO

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 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 manipulated only 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 nonzero.

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 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 a message with len 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);