Manpages - rtnetlink.3

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",unsignedint"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(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);

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

This page is part of release 5.13 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 https://www.kernel.org/doc/man-pages/.