5-12 MTUの設定

#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <net/if.h>
#include <net/if_arp.h>
#include <linux/netlink.h>
#include <linux/rtnetlink.h>

int
main()
{
  int soc;
  struct sockaddr_nl sa;
  char buf[4096];
  int n;
  int seq=100;
  struct nlmsghdr *nlhdr;
  struct ifinfomsg *ifihdr;
  struct rtattr *rta_mtu;

  soc = socket(AF_NETLINK, SOCK_DGRAM, NETLINK_ROUTE);

  memset(&sa, 0, sizeof(sa));
  sa.nl_family = AF_NETLINK;
  sa.nl_pid = 0; /* kernel */
  sa.nl_groups = 0;

  memset(buf, 0, sizeof(buf));
  nlhdr = (struct nlmsghdr *)buf;
  ifihdr = NLMSG_DATA(nlhdr);
  nlhdr->nlmsg_type = RTM_SETLINK;
  nlhdr->nlmsg_flags = NLM_F_REQUEST | NLM_F_ACK;
  nlhdr->nlmsg_pid = 0;
  seq++;
  nlhdr->nlmsg_seq = seq;

  ifihdr->ifi_family = ARPHRD_ETHER;
  ifihdr->ifi_type = ARPHRD_LOOPBACK;
  ifihdr->ifi_index = if_nametoindex("lo");
  ifihdr->ifi_change = 0;

  nlhdr->nlmsg_len = NLMSG_LENGTH(sizeof(struct ifinfomsg));

  rta_mtu = (void *)((char *)nlhdr + nlhdr->nlmsg_len);
  rta_mtu->rta_type = IFLA_MTU;
  *(int *)RTA_DATA(rta_mtu) = 17000;
  rta_mtu->rta_len = RTA_LENGTH(sizeof(int));

  nlhdr->nlmsg_len += rta_mtu->rta_len;

  n = sendto(soc, (void *)nlhdr, nlhdr->nlmsg_len, 0,
             (struct sockaddr *)&sa, sizeof(sa));
  if (n < 0) {
    perror("sendto");
    return 1;
  }

  printf("recv\n");
  n = recv(soc, buf, sizeof(buf), 0);
  if (n < 0) {
    perror("recvmsg");
    return 1;
  }

  for (nlhdr = (struct nlmsghdr *)buf; NLMSG_OK(nlhdr, n);
       nlhdr = NLMSG_NEXT(nlhdr, n)) {
    printf("len : %d\n", nlhdr->nlmsg_len);
    printf("type : %d\n", nlhdr->nlmsg_type);

    if (nlhdr->nlmsg_type == NLMSG_ERROR) {
      struct nlmsgerr *errmsg;
      errmsg = NLMSG_DATA(nlhdr);

      printf("%d, %s\n", errmsg->error, strerror(-errmsg->error));
    }
  }

  close(soc);

  return 0;
}

拙著「Linuxネットワークプログラミング」の紹介