TAP client supporting specifying TAP ifname and TAP client network namespace

tapclient.c

/* modified from: https://github.com/a34729t/exp/blob/master/tunclient.c */
#define _GNU_SOURCE
#include <sched.h>

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <net/if.h>
#include <linux/if_tun.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/ioctl.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <arpa/inet.h>
#include <sys/select.h>
#include <sys/time.h>
#include <errno.h>
#include <stdarg.h>


int tun_alloc(char *dev, int flags) {

  struct ifreq ifr;
  int fd, err;
  char *clonedev = "/dev/net/tun";

  /* Arguments taken by the function:
   *
   * char *dev: the name of an interface (or '\0'). MUST have enough
   *   space to hold the interface name if '\0' is passed
   * int flags: interface flags (eg, IFF_TUN etc.)
   */

   /* open the clone device */
   if( (fd = open(clonedev, O_RDWR)) < 0 ) {
     return fd;
   }

   /* preparation of the struct ifr, of type "struct ifreq" */
   memset(&ifr, 0, sizeof(ifr));

   ifr.ifr_flags = flags;   /* IFF_TUN or IFF_TAP, plus maybe IFF_NO_PI */

   if (*dev) {
     /* if a device name was specified, put it in the structure; otherwise,
      * the kernel will try to allocate the "next" device of the
      * specified type */
     strncpy(ifr.ifr_name, dev, IFNAMSIZ);
   }

   /* try to create the device */
   if( (err = ioctl(fd, TUNSETIFF, (void *) &ifr)) < 0 ) {
     close(fd);
     return err;
   }

  /* if the operation was successful, write back the name of the
   * interface to the variable "dev", so the caller can know
   * it. Note that the caller MUST reserve space in *dev (see calling
   * code below) */
  strcpy(dev, ifr.ifr_name);

  /* this is the special file descriptor that the caller will use to talk
   * with the virtual interface */
  return fd;
}

int main(int argc, char *argv[]) {
  char tun_name[IFNAMSIZ];
  int tun_fd, nread;
  char buffer[2000];
  /* Connect to the device */
  if (argc < 2) {
    strcpy(tun_name, "");
  } else {
    strcpy(tun_name, argv[1]);
  }
  tun_fd = tun_alloc(tun_name, IFF_TAP | IFF_NO_PI);  /* tun interface */

  if(argc >= 3) {
    int netnsfd;
    netnsfd = open(argv[2], O_RDONLY);
    if(netnsfd < 0){
      perror("Network namespace reference");
      exit(1);
    }
    if(setns(netnsfd, CLONE_NEWNET) < 0){
      perror("Switching network namespace");
      exit(1);
    }
    printf("Switched into network namespace %s\n", argv[2]);
  }

  if(tun_fd < 0){
    perror("Allocating interface");
    exit(1);
  }

  printf("Reading from interface %s\n", tun_name);

  /* Now read data coming from the kernel */
  while(1) {
    /* Note that "buffer" should be at least the MTU size of the interface, eg 1500 bytes */
    nread = read(tun_fd,buffer,sizeof(buffer));
    if(nread < 0) {
      perror("Reading from interface");
      close(tun_fd);
      exit(1);
    }

    /* Do whatever with the data */
    printf("Read %d bytes from device %s\n", nread, tun_name);
  }
}

test-fdinfo.sh

#!/bin/bash
# remove stale tapclient(s)
for PID in $(ps a|grep "tapclient"|grep -v "grep"|awk '{print $1}'); do
        echo "Killing tapclient $PID"
        kill $PID
done
# remove stale netns
for NETNS in $(findmnt -n -t nsfs|awk '/\/run\/netns\//{n=split($1,P,"/");print P[n]}'|sort -u); do
        echo "Removing old netns $NETNS"
        ip netns del $NETNS
done
# set up netns
ip netns add ahh
ip netns add beh
# start tapclient, create TAP, move TAP to ahh, switch tapclient to beh
./tapclient tap123 /run/netns/beh >/dev/null &
export TAPPID=$!
ip link set tap123 netns ahh
# show results
echo "----"
echo host: $(readlink /proc/self/ns/net)
for NETNS in $(findmnt -n -t nsfs|awk '/\/run\/netns\//{n=split($1,P,"/");print P[n]}'|sort -u); do
        echo $NETNS: $(findmnt -n /run/netns/$NETNS|sort -u|grep -o "net:\[[0-9]*\]")
done
echo tapclient: $(readlink /proc/$TAPPID/ns/net)
echo
grep -e "iff:" -e "netns:" /proc/$TAPPID/fdinfo/3