How to read/write from existing TAP interface in C - c

Question

I have an existing tap device (tap0) that I created on command line.
# ip tuntap add dev tap0 mode tap
I want to read any data coming in on that interface using a C program. I checked other SO questions, but found code that create an interface by opening /dev/net/tun.
Can anyone provide some direction on how to open and read existing interface ? I'm not sure which file I should open for tap0 ?

Answer 1

Open the existing tun/tap interface similar way than creating a new one.
Just give name for the interface when ioctl(TUNSETIFF) is used:
const int fd = open("/dev/net/tun", O_RDWR);
if (fd != -1)
{
struct ifreq ifr;
memset(&ifr, 0, sizeof(ifr));
ifr.ifr_flags = IFF_TUN | IFF_NO_PI;
strncpy(ifr.ifr_name, "tun0", IFNAMSIZ); // <<<<=== THIS WAY
if (ioctl(fd, TUNSETIFF, &ifr) != -1)
{
Without the strncpy line, the code creates a new interface with some free number.
With the line, it tries to open tun0. Note: it fails if the tun0 is already opened by some other process.
Above is tested with IFF_TUN. I haven't tried IFF_TAP.
You can use the file descriptor (fd) for reading and writing:
Example for tun:
char buffer[0x1000];
const int len = read(fd, buffer, sizeof(buffer));
if (len > 0)
{
static const char IPV6_VER_MASK = 0x60;
if ((buffer[0] & IPV6_VER_MASK) == IPV6_VER_MASK)
{
handle_ipv6_packet((const struct ip6_hdr*)buffer, len);
}
}

Answer 2

You can implement like a tcpdump to capture packets, can use libpcap or
use socket RAW_SOCKET
sock = socket(AF_PACKET, SOCK_RAW, htons(ETH_P_ALL)
Of course you can also add the bpf filter
setsockopt(sock, SOL_SOCKET, SO_ATTACH_FILTER, ...
simple sample:
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>
#include <arpa/inet.h>
#include <net/if.h>
#include <netinet/ether.h>
#include <linux/if_packet.h>
#include <sys/ioctl.h>
int main(int argc, char **argv)
{
int n;
int ret = 0;
int sock;
char buf[2048];
struct ifreq ifreq;
struct sockaddr_ll saddr;
// create socket
if((sock = socket(AF_PACKET, SOCK_RAW, htons(ETH_P_ALL))) == -1) {
ret = errno;
goto error_exit;
}
// bind tap0
snprintf(ifreq.ifr_name, sizeof(ifreq.ifr_name), "tap0");
if (ioctl(sock, SIOCGIFINDEX, &ifreq)) {
ret = errno;
goto error_exit;
}
memset(&saddr, 0, sizeof(saddr));
saddr.sll_family = AF_PACKET;
saddr.sll_protocol = htons(ETH_P_ALL);
saddr.sll_ifindex = ifreq.ifr_ifindex;
saddr.sll_pkttype = PACKET_HOST;
if(bind(sock, (struct sockaddr *)&saddr, sizeof(saddr)) == -1) {
ret = errno;
goto error_exit;
}
// recv data
while(1) {
n = recvfrom(sock, buf, sizeof(buf), 0, NULL, NULL);
printf("%d bytes recieved\n", n);
}
error_exit:
if (ret) {
printf("error: %s (%d)\n", strerror(ret), ret);
}
close(sock);
return ret;
}