I have installed npcap driver which supports loopback interface. I installed it because I need to inyect packets to loopback interface and read them from it. I can easily read packets in the loopback with "pcap_next_ex" as I can do in winpcap in ethernet interfaces, but when I want to inyect a packet to the loopback with "pcap_sendpacket" dosent work, and the function returns 0 (successfull).
I verified this by opening wireshark and watching the packets incoming to the interface, when I use pcap_sendpacket on ethernet interface I can watch the packets, but in the loopback they dont appear when I tried to inject them. Why?
//The array which contains the data of the test package
UCHAR packet[] = "\x..";
//loopback adapter is already opened here
for (int i = 0; i < 100; i++)
printf("%d ", pcap_sendpacket(loopbackAdapter, packet, sizeof(packet)));
I use this code on loopback interface and didnt work (but pcap_sendpacket returned always success), because in wireshark the packets didnt appear, but in ethernet interfaces the injection was successfull.
Does npcap support loopback packet inyection?
Thank you and regards!.
You need to understand that every network packet (even a loopback one) has a direction: on send path (Tx) or on receive path (Rx). Usually, when you send a packet to Npcap Loopback Adapter, you are actually sending it to the Tx path of the Windows TCP/IP stack. When you are sending Tx packets, you usually wait for a localhost application (or a protocol, a driver, etc) responding to them.
I don't know why you want to inject packets to loopback interface and read them from it. Npcap's low-level logic for this part is just not letting the current session (a pcap_t) receive the packets that this session injects. But other sessions can see them, this is why Wireshark can see the packets that you inject.
I just don't know why you are doing this. You seem not to want any other applications respond to these packets. But as a workaround, I think you can get what you want by injecting the packets to the Rx path. By sending packets to Rx, it means to fool Windows to believe that these packets are received from outside. I don't remember how I implement it, but the same session should be able to see these packets in Rx.
We didn't document the Rx feature very clearly. There's only some description in releases: v0.05-r6 and v0.05-r7, and an example here: https://github.com/hsluoyz/UserBridge. They should work even in latest Npcap.