rtnetlink — macros to manipulate rtnetlink messages


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


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.


These macros are nonstandard Linux extensions.


This manual page is incomplete.


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(struct ifinfomsg));
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) +
rta−>rta_type = IFLA_MTU;
rta−>rta_len = RTA_LENGTH(sizeof(unsigned int));
req.nh.nlmsg_len = NLMSG_ALIGN(req.nh.nlmsg_len) +
memcpy(RTA_DATA(rta), &mtu, sizeof(mtu));
send(rtnetlink_sk, &req, req.nh.nlmsg_len, 0);


netlink(3), netlink(7), rtnetlink(7)


This page is part of release 4.16 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at−pages/.

  This man page is Copyright (C) 1999 Andi Kleen <>.

Permission is granted to distribute possibly modified copies
of this page provided the header is included verbatim,
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$Id: rtnetlink.3,v 1.2 1999/05/18 10:35:10 freitag Exp $