I'm working on a simple project that listens on a tun interface and modified the packets then re-sends them to the real interface.
I have tried songgao/water, pkg/tuntap and even writing my own based on some C code floating around but no matter what I tried, I can't receive TCP packets (ICMP/UDP works fine).
I feel like i'm missing something extremely obvious but I can't figure it for the life of me...
The code:
package main
import (
"log"
"os"
"os/exec"
"golang.org/x/net/ipv4"
"github.com/songgao/water"
)
const (
// I use TUN interface, so only plain IP packet, no ethernet header + mtu is set to 1300
BUFFERSIZE = 1600
MTU = "1300"
)
func main() {
iface, err := water.New(water.Config{})
fatalIf(err)
log.Printf("tun interface: %s", iface.Name())
runBin("/bin/ip", "link", "set", "dev", iface.Name(), "mtu", MTU)
runBin("/bin/ip", "addr", "add", "10.2.0.10/24", "dev", iface.Name())
runBin("/bin/ip", "link", "set", "dev", iface.Name(), "up")
buf := make([]byte, BUFFERSIZE)
for {
n, err := iface.Read(buf)
if err != nil {
log.Fatal(err)
}
header, _ := ipv4.ParseHeader(buf[:n])
log.Printf("isTCP: %v, header: %s", header.Protocol == 6, header)
}
}
func fatalIf(err error) {
if err != nil {
log.Fatal(err)
}
}
func runBin(bin string, args ...string) {
cmd := exec.Command(bin, args...)
cmd.Stderr = os.Stderr
cmd.Stdout = os.Stdout
cmd.Stdin = os.Stdin
fatalIf(cmd.Run())
}
Your code works for me. If I run it, then I can see TCP packets after I initiate some TCP-related activity. For this instance, I used "ssh 10.2.0.11". Here is what I got:
2020/03/07 15:52:23 isTCP: false, header: ver=6 hdrlen=0 tos=0x0 totallen=0 id=0x8 flags=0x1 fragoff=0x1aff ttl=254 proto=128 cksum=0x0 src=0.0.0.0 dst=126.87.58.227
2020/03/07 15:52:27 isTCP: false, header: ver=6 hdrlen=0 tos=0x0 totallen=0 id=0x8 flags=0x1 fragoff=0x1aff ttl=254 proto=128 cksum=0x0 src=0.0.0.0 dst=126.87.58.227
2020/03/07 15:52:34 isTCP: false, header: ver=6 hdrlen=0 tos=0x0 totallen=0 id=0x8 flags=0x1 fragoff=0x1aff ttl=254 proto=128 cksum=0x0 src=0.0.0.0 dst=126.87.58.227
2020/03/07 15:52:36 isTCP: true, header: ver=4 hdrlen=20 tos=0x0 totallen=60 id=0x9cec flags=0x2 fragoff=0x0 ttl=64 proto=6 cksum=0x89b7 src=10.2.0.10 dst=10.2.0.11
2020/03/07 15:52:37 isTCP: true, header: ver=4 hdrlen=20 tos=0x0 totallen=60 id=0x9ced flags=0x2 fragoff=0x0 ttl=64 proto=6 cksum=0x89b6 src=10.2.0.10 dst=10.2.0.11
2020/03/07 15:52:39 isTCP: true, header: ver=4 hdrlen=20 tos=0x0 totallen=60 id=0x9cee flags=0x2 fragoff=0x0 ttl=64 proto=6 cksum=0x89b5 src=10.2.0.10 dst=10.2.0.11
2020/03/07 15:52:43 isTCP: true, header: ver=4 hdrlen=20 tos=0x0 totallen=60 id=0x9cef flags=0x2 fragoff=0x0 ttl=64 proto=6 cksum=0x89b4 src=10.2.0.10 dst=10.2.0.11
As you can see, a TCP client at 10.2.0.10 is trying to establish (TCP-SYN) a connection to 10.2.0.11.