tap and tun

tun in the network layer
tap in the second layer
lsmod|grep tun

the use of TUN/TAP virtual network card under linux
http://www.tuicool.com/articles/mu6vY3
ibm article
http://www.ibm.com/developerworks/cn /linux/l-tuntap/
skb_buffer
http://blog.chinaunix.net/uid-21768364-id-209652.html

modinfo tun
do modprobe
lsmod | grep tun
yum install tunctl -y

centos7 is gone
http://heanet.dl.sourceforge.net/project/tunctl/tunctl/1.5/tunctl-1.5.tar.gz
http://netassist.dl.sourceforge.net/project/tunctl/tunctl/1.5/ tunctl-1.5.tar.gz

yum install docbook* -y
tar zxvf tunctl-1.5.tar.gz
cd tunctl-1.5
make
make install

http://www.blogjava.net/kuuyee/archive/2010/12/07/339987.html
http://www.tuicool.com/articles/mu6vY3

Establish
tunctl -t tap0 -u root
remove use
tunctl -d tap0

Set up bridging,
connect the local network eth0 to the tap device through the bridge, so that the tap device can communicate with the outside world

ip link
ifconfig eth0 0.0.0.0 promisc
brctl addif br0 eth0
ip link set br0 up
dhclient br0
brctl addif br0 tap0

Set up the network for the tap

ifconfig tap0 10.0.2.2  netmask 255.255.255.0 promisc

http://blog.csdn.net/ixidof/article/details/10148899

The code of tunctl is very simple:
tunctl.c:

/ * Copyright 2002 Jeff Dike
 * Licensed under the GPL
 */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <fcntl.h>
#include <unistd.h>
#include <pwd.h>
#include <grp.h>
#include <net/if.h>
#include <sys/ioctl.h>
#include <linux/if_tun.h>

/* TUNSETGROUP appeared in 2.6.23 */
#ifndef TUNSETGROUP
#define TUNSETGROUP   _IOW('T', 206, int)
#endif

static void Usage(char *name)
{
  fprintf(stderr, "Create: %s [-b] [-u owner] [-g group] [-t device-name] "
          "[-p|-n] [-f tun-clone-device]\n", name);
  fprintf(stderr, "Delete: %s -d device-name [-f tun-clone-device]\n\n",
          name);
  fprintf(stderr, "The default tun clone device is /dev/net/tun - some systems"
          " use\n/dev/misc/net/tun instead\n\n");
  fprintf(stderr, "-b will result in brief output (just the device name)\n");
  fprintf(stderr, "-n will result in a point-to-point tun device,\n");
  fprintf(stderr, "-p in an ethernet tap device. Default is a tap,\n");
  fprintf(stderr, "   except the device contains \"tun\" in the name.\n");
  exit(1);
}

int main(int argc, char **argv)
{
  struct ifreq ifr;
  struct passwd *pw;
  struct group *gr;
  uid_t owner = -1;
  gid_t group = -1;
  int tap_fd, opt, delete = 0, brief = 0, type = 0;
  char *tun = "", *file = "/dev/net/tun", *name = argv[0], *end;

  while((opt = getopt(argc, argv, "bd:f:npt:u:g:h")) > 0){
    switch(opt) {
      case 'b':
        brief = 1;
        break;
      case 'd':
        delete = 1;
        do = optarg;
        break;
      case 'f':
        file = optarg;
        break;
      case 'p':
        if(type != 0)
          Usage(name);
        type = IFF_TAP;
        break;
      case 'n':
        if(type != 0)
          Usage(name);
        type = IFF_TUN;
        break;
      case 'u':
        pw = getpwnam (optarg);
        if(pw != NULL){
          owner = pw->pw_uid;
          break;
        }
        owner = strtol(optarg, &end, 0);
        if(*end != '\0'){
          fprintf(stderr, "'%s' is neither a username nor a numeric uid.\n",
                  optarg);
          Usage(name);
        }
        break;
      case 'g':
        gr = getgrnam(optarg);
        if(gr != NULL){
          group = gr->gr_gid;
          break;
        }
        group = strtol(optarg, &end, 0);
        if(*end != '\0'){
          fprintf(stderr, "'%s' is neither a groupname nor a numeric group.\n",
                  optarg);
          Usage(name);
        }
        break;

      case 't':
        do = optarg;
        break;
      case '?':
      case 'h':
      default:
        Usage(name);
    }
  }

  argv += optind;
  argc -= optind;

  if(argc > 0)
    Usage(name);

  if((tap_fd = open(file, O_RDWR)) < 0){
    fprintf(stderr, "Failed to open '%s' : ", file);
    perror("");
    exit(1);
  }

  if(type == 0) {
    type = strstr(tun, "tun") ? IFF_TUN : IFF_TAP;
  }

  memset(&ifr, 0, sizeof(ifr));

  ifr.ifr_flags = type | IFF_NO_PI;
  strncpy(ifr.ifr_name, tun, sizeof(ifr.ifr_name) - 1);
  if(ioctl(tap_fd, TUNSETIFF, (void *) &ifr) < 0){
    perror("TUNSETIFF");
    exit(1);
  }

  if(delete){
    if(ioctl(tap_fd, TUNSETPERSIST, 0) < 0){
      perror("disabling TUNSETPERSIST");
      exit(1);
    }
    printf("Set '%s' nonpersistent\n", ifr.ifr_name);
  }
  else {
    /* emulate behaviour prior to TUNSETGROUP */
    if(owner == -1 && group == -1) {
      owner = geteuid();
    }

    if(owner != -1) {
      if(ioctl(tap_fd, TUNSETOWNER, owner) < 0){
        perror("TUNSETOWNER");
        exit(1);
      }
    }
    if(group != -1) {
      if(ioctl(tap_fd, TUNSETGROUP, group) < 0){
        perror("TUNSETGROUP");
        exit(1);
      }
    }

    if(ioctl(tap_fd, TUNSETPERSIST, 1) < 0){
      perror("enabling TUNSETPERSIST");
      exit(1);
    }

    if(brief)
      printf("%s\n", ifr.ifr_name);
    else {
      printf("Set '%s' persistent and owned by", ifr.ifr_name);
      if(owner != -1)
          printf(" uid %d", owner);
      if(group != -1)
          printf(" gid %d", group);
      printf("\n");
    }
  }
  return(0);
}

Execute the compilation:

gcc -g -Wall -o tunctl tunctl.c