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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.

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.

returns a pointer to the start of this attribute's data.

returns the length of this attribute's data.

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.

returns the length which is required for len bytes of data plus the header.

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);

See Also

  1. netlink(3),
  2. netlink(7),
  3. rtnetlink(7)