rtnetlink

rtnetlink - macros to manipulate rtnetlink messages

#include <asm/types.h>
#include <linux/netlink.h>
#include <linux/rtnetlink.h>
#include <sys/socket.h> I rtnetlink_socket = socket(PF_NETLINK, int socket_type \ ", NETLINK_ROUTE);" I int RTA_OK(struct rtattr * rta , int rtabuflen ); I void *RTA_DATA(struct rtattr * rta ); I unsigned int RTA_PAYLOAD(struct rtattr * rta ); I struct rtattr *RTA_NEXT(struct rtattr * rta \ ", unsigned int " rtabuflen ); I unsigned int RTA_LENGTH(unsigned int length ); I 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 only manipulated using the macros provided here.

I 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 non-zero.

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

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

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

I RTA_LENGTH( len ) returns the length which is required for len bytes of data plus the header.

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

These macros are non-standard Linux extensions.

This manual page is incomplete.

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

    struct {
        struct nlmsghdr  nh;
        struct ifinfomsg if;
        char             attrbuf[512];
    } req;

    struct rtattr *rta;
    unsigned int mtu = 1000;

    int rtnetlink_sk = socket(PF_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 = RTML_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(n->nlmsg_len));
    rta->rta_type = IFLA_MTU;
    rta->rta_len = sizeof(unsigned int);
    req.n.nlmsg_len = NLMSG_ALIGN(req.n.nlmsg_len) +
                                  RTA_LENGTH(sizeof(mtu));
    memcpy(RTA_DATA(rta), &mtu, sizeof(mtu));
    send(rtnetlink_sk, &req, req.n.nlmsg_len);

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