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       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, 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);


       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.

       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 = sizeof(unsigned int);
           req.n.nlmsg_len = NLMSG_ALIGN(req.nh.nlmsg_len) +
           memcpy(RTA_DATA(rta), &mtu, sizeof(mtu));
           send(rtnetlink_sk, &req, req.nh.nlmsg_len);


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


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