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

       These macros are nonstandard Linux extensions.

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

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