Why is pcap only capturing PTP messages in live capture mode? - pcap

Question

I am using a Intel i210-T1 Network Interface Card.
I am running the avnu gptp client (https://github.com/Avnu/gptp) with:
sudo ./daemon_cl -S -V
The other side is a gPTP Master.
I want to live capture incoming UDP packets on an network interface with hardware timestamps.
I can see the UDP Packets with wireshark, so the packets are actually on the wire.
My problem is that pcap doesn't return any packets other than PTP (ethertype 0x88f7) at all.
Is this a bug or am i using pcap the wrong way?
I wrote a minimal example to show my problem.
The code prints:
enp1s0
returnvalue pcap_set_tstamp_type: 0
returnvalue pcap_set_tstamp_precision: 0
returnvalue pcap_activate: 0
and afterwards only:
packet received with ethertype:88f7
#include <iostream>
#include <netinet/in.h>
#include <netinet/if_ether.h>
#include <pcap/pcap.h>
int main(int argc, char **argv)
{
char errbuf[PCAP_ERRBUF_SIZE];
std::string dev = "enp1s0";
pcap_t* pcap_dev;
int i = 0;
printf("%s\n", dev.c_str());
pcap_dev = pcap_create(dev.c_str(), errbuf);
if(pcap_dev == NULL)
{
printf("pcap_create(): %s\n", errbuf);
exit(1);
}
i = pcap_set_tstamp_type(pcap_dev, PCAP_TSTAMP_ADAPTER_UNSYNCED);
printf("returnvalue pcap_set_tstamp_type: %i\n", i);
i = pcap_set_tstamp_precision(pcap_dev, PCAP_TSTAMP_PRECISION_NANO);
printf("returnvalue pcap_set_tstamp_precision: %i\n", i);
i = pcap_activate(pcap_dev);
printf("returnvalue pcap_activate: %i\n", i);
struct pcap_pkthdr* pkthdr;
const u_char* bytes;
while (pcap_next_ex(pcap_dev, &pkthdr, &bytes))
{
struct ether_header* ethhdr = (struct ether_header*) bytes;
std::cout << "packet received with ethertype:" << std::hex << ntohs(ethhdr->ether_type) << std::endl;
}
}

Answer 1

The solution is to enable promiscuous mode by using function:
https://linux.die.net/man/3/pcap_set_promisc
promiscuous mode disables any filtering by lower layers so you get every message arriving on the interface.
int pcap_set_promisc(pcap_t *p, int promisc);
pcap_set_promisc() sets whether promiscuous mode should be set on a capture handle when the handle is activated. If promisc is non-zero, promiscuous mode will be set, otherwise it will not be set.
Return Value
pcap_set_promisc() returns 0 on success or PCAP_ERROR_ACTIVATED if called on a capture handle that has been activated.