Where did Wireshark/tcpdump/libpcap intercept packet inside Linux kernel?

Question

According to this, wireshark is able to get the packet before it is dropped (therefore I cannot get such packets by myself). And I'm still wondering the exact location in linux kernel for wireshark to fetch the packets.

The answer goes as "On UN*Xes, it uses libpcap, which, on Linux, uses AF_PACKET sockets." Does anyone have more concrete example to use "AF_PACKET sockets"? If I understand wireshark correctly, the network interface card (NIC) will make a copy of all incoming packets and send it to a filter (berkeley packet filter) defined by the user. But where does this happen? Or am I wrong with that understanding and do I miss anything here?

Thanks in advance!

Answer 1

But where does this happen?

If I understood you correctly - you want to know, where is initialized such socket.
There is pcap_create function, that tries to determine type of source interface, creates duplicate of it and activates it.
For network see pcap_create_interface function => pcap_create_common function => pcap_activate_linux function.
All initialization happens in pcap_activate_linux => activate_new function => iface_bind function
( copy descriptor of device with handlep->device = strdup(device);,
create socket with socket(PF_PACKET, SOCK_DGRAM, htons(ETH_P_ALL)),
bind socket to device with bind(fd, (struct sockaddr *) &sll, sizeof(sll)) ).
For more detailed information read comments in source files of mentioned functions - they are very detailed.
After initialization all work happens in a group of functions such as pcap_read_linux, etc.

Answer 2

On Linux, you should be able to simply use tcpdump (which leverages the libpcap library) to do this. This can be done with a file or to STDOUT and you specify the filter at the end of the tcpdump command..