rtnetlink - macros to manipulate rtnetlink messages

       This manual page is incomplete.

       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.

       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);

       Standard C library (libc, -lc)

       rtnetlink - macros to manipulate rtnetlink messages

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

Linux man-pages 6.9.1                              2024-05-02                                       rtnetlink(3)

       Linux.

#include<asm/types.h>#include<linux/netlink.h>#include<linux/rtnetlink.h>#include<sys/socket.h>rtnetlink_socket=socket(AF_NETLINK,intsocket_type,NETLINK_ROUTE);intRTA_OK(structrtattr*rta,intrtabuflen);void*RTA_DATA(structrtattr*rta);unsignedintRTA_PAYLOAD(structrtattr*rta);structrtattr*RTA_NEXT(structrtattr*rta,unsignedintrtabuflen);unsignedintRTA_LENGTH(unsignedintlength);unsignedintRTA_SPACE(unsignedintlength);