pmtu_want.c

#include <assert.h>

#include <sys/ioctl.h>
#include <errno.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <sys/socket.h>
#include <sys/types.h>

#include <linux/errqueue.h>

#define max_mtu_size_bytes (1500)
#define header_length_bytes (28)
#define max_payload_size_bytes (max_mtu_size_bytes - header_length_bytes)


int main(int argc, char* argv[])
{
  assert(argc == 2);

  int s = socket(AF_INET, SOCK_DGRAM, 0);
  assert(s > 0);

  const int pmtu_option = IP_PMTUDISC_WANT;
  assert(setsockopt(s, IPPROTO_IP, IP_MTU_DISCOVER, &pmtu_option, sizeof pmtu_option) == 0);
  const int one = 1;
  assert(ioctl(s, FIONBIO, &one) != -1);

  struct in_addr ip_addr;
  assert(inet_pton(AF_INET, argv[1], (void*)&ip_addr) == 1);

  const unsigned short echo_port = 7;

  struct sockaddr_in addr = {
    .sin_family = AF_INET,
    .sin_port = htons(echo_port),
    .sin_addr = ip_addr,
  };

  assert(connect(s, (const struct sockaddr *)&addr, sizeof addr) == 0);

  int i = 0;
  for (i = 0; i < 100; i++)
  {
    size_t payload_size_bytes;
    socklen_t option_len = sizeof payload_size_bytes;
    assert(getsockopt(s, IPPROTO_IP, IP_MTU, &payload_size_bytes, &option_len) == 0);
    payload_size_bytes -= header_length_bytes;
    const char* payload[max_payload_size_bytes] = {0};

    assert(send(s, payload, payload_size_bytes, 0) == payload_size_bytes);
  }

  return i;
}
~