Send raw IP packet with tun device - linux

Question

I'm trying to programmatically construct and send IP packet through TUN device.
I've setup the TUN device and proper routes:
# ip tuntap add mode tun tun0
# ip link set tun0 up
# ip addr add 10.0.0.2/24 dev tun0
which results in:
$ route -n
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use
Iface
0.0.0.0 192.168.0.1 0.0.0.0 UG 600 0 0 wlp3s0
10.0.0.0 0.0.0.0 255.255.255.0 U 0 0 0 tun0
192.168.0.0 0.0.0.0 255.255.255.0 U 600 0 0 wlp3s0
$ ifconfig tun0
tun0: flags=4241<UP,POINTOPOINT,NOARP,MULTICAST> mtu 1500
inet 10.0.0.2 netmask 255.255.255.0 destination 10.0.0.2
inet6 fe80::f834:5267:3a1:5d1d prefixlen 64 scopeid 0x20<link>
unspec 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00 txqueuelen 500 (UNSPEC)
IP forwarding is ON: # echo 1 > /proc/sys/net/ipv4/ip_forward
I've setup NAT for tun0 packets:
# iptables -t nat -A POSTROUTING -s 10.0.0.0/24 -o wlp3s0 -j MASQUERADE
# iptables -t nat -L -v
Chain POSTROUTING (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
0 0 MASQUERADE all -- any wlp3s0 10.0.0.0/24 anywhere
Then I have python script to produce ICMP packets:
import os
from fcntl import ioctl
import struct
import time
import random
# pip install pypacker==4.0
from pypacker.layer3.ip import IP
from pypacker.layer3.icmp import ICMP
TUNSETIFF = 0x400454ca
IFF_TUN = 0x0001
IFF_NO_PI = 0x1000
ftun = os.open("/dev/net/tun", os.O_RDWR)
ioctl(ftun, TUNSETIFF, struct.pack("16sH", b"tun0", IFF_TUN | IFF_NO_PI))
req_nr = 1
req_id = random.randint(1, 65000)
while True:
icmp_req = IP(src_s="10.0.0.2", dst_s="8.8.8.8", p=1) +\
ICMP(type=8) +\
ICMP.Echo(id=req_id, seq=req_nr, body_bytes=b"povilas-test")
os.write(ftun, icmp_req.bin())
time.sleep(1)
req_nr += 1
I can see packets originating from tun0 interface:
# tshark -i tun0
1 0.000000000 10.0.0.2 → 8.8.8.8 ICMP 48 Echo (ping) request id=0xb673, seq=1/256, ttl=64
2 1.001695939 10.0.0.2 → 8.8.8.8 ICMP 48 Echo (ping) request id=0xb673, seq=2/512, ttl=64
3 2.003375319 10.0.0.2 → 8.8.8.8 ICMP 48 Echo (ping) request id=0xb673, seq=3/768, ttl=6
But wlp3s0 interface is silent, thus it seems that packets don't get NAT'ed and routed to wlp3s0 interface, which is my WLAN card.
Any ideas what I am missing?

Answer 1

I'm running Debian 9.
And turns out that packet forwarding was disabled - default policy for FORWARD chain is DROP:
# iptables -L -v
Chain FORWARD (policy DROP 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
So I changed the policy:
# iptables -P FORWARD ACCEPT
Chain FORWARD (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
Also, I had to change the IP packet source address to something other than 10.0.0.2 which is the preferred source address for tun0 interface:
$ ip route
10.0.0.0/24 dev tun0 proto kernel scope link src 10.0.0.2
So I changed the packet source address to 10.0.0.4:
icmp_req = IP(src_s="10.0.0.4", dst_s="8.8.8.8", p=1) +\
ICMP(type=8) +\
ICMP.Echo(id=req_id, seq=req_nr, body_bytes=b"povilas-test")
Then kernel started forwarding packets coming from tun0 interface to the gateway interface:
# tshark -i wlp3s0
5 0.008428567 192.168.0.103 → 8.8.8.8 ICMP 62 Echo (ping) request id=0xb5c7, seq=9/2304, ttl=63
6 0.041114028 8.8.8.8 → 192.168.0.103 ICMP 62 Echo (ping) reply id=0xb5c7, seq=9/2304, ttl=48 (request in 5)
Also ping responses were sent back to tun0:
# tshark -i tun0
1 0.000000000 10.0.0.4 → 8.8.8.8 ICMP 48 Echo (ping) request id=0xb5c7, seq=113/28928, ttl=64
2 0.035470191 8.8.8.8 → 10.0.0.4 ICMP 48 Echo (ping) reply id=0xb5c7, seq=113/28928, ttl=47 (request in 1)