afpacket does not get GRE tunneled packets?

[Chun Zhang]

Hi, friends, 

I am trying to create a very simple network sniffer. Initially I implemented it with libpcap, but I got not only performance issue, but also timing issue.  Now I am trying to replace libpcap with afpacket. However, I am not able to get any packet from the GRE tunneled terminated at the listening NIC.

I know I have quite a few packets terminated at this NIC. If using pcap as below, then I see all those packets, decoded up to the GRE layer, and its payload. 

But switching to afpacket, I can only get non-GRE packet, such as IGMP, ICMP, etc. (These packets are also seen in the above pcap version). I can see normal TCP packets as well, but no GRE packets, they are not even read from wire as far as I can see, since there is no decoding error at all. 

So, my dear gophers, what have I done wrong? Any help is appreciated. I have googled quite a bit, but does not really get any useful answer.

Do I need to somehow magically enable the promiscuous mode? I don't see an option in afpacket. 

Here is my code snippet

//pcap version
handle, err = pcap.OpenLive(device, snapshot_len, promiscuous, timeout)
if err == nil {
       Info.Println("Open interface ", device, "successfully")
}
defer handle.Close()

//afpacket version 
handle, err := afpacket.NewTPacket(afpacket.OptInterface(device))
  
if err != nil {
       Error.Printf("Error opening afpacket interface: %s", err)
}

defer handle.Close()

//create a source and listen to it
packetSource := gopacket.NewPacketSource(handle, layers.LinkTypeEthernet)

for {
     packet, err := packetSource.NextPacket()
     // Iterate over all layers, printing out each layer type
     for _, layer := range packet.Layers() {
       fmt.Println("PACKET LAYER:", layer.LayerType())
}


Here is my code snippet,

[Chun Zhang]

I just figured this out.  I need to manually set the NIC to promiscuous mode.

Now the question is, is there a way to set the NIC to promiscuous mode in code, like in pcap.openlive?? I don't see that option with AF_packet. 

Thanks,

Chun

[Juliusz Chroboczek]

This is not Go-specific, I hope I'm not breaking some unwritten rule by
replying.

> I just figured this out. I need to manually set the NIC to promiscuous
> mode.

> Now the question is, is there a way to set the NIC to promiscuous mode
> in code, like in pcap.openlive?

You want to call sysctl(SIOCGIFFLAGS), set the IFF_PROMISC flag in
ifr.ifr_flags, and pass the results back to sysctl(SIOCSIFFLAGS).
I guess one could also use rtnetlink, if one were so inclined.

If I were you, I'd cheat:

ifname := "eth0"
cmd := exec.Command("ip", "link", "set", "dev", ifname, "promisc", "on")
err := cmd.Run()

-- Juliusz