The default libpcap buffer size on Linux is 2MB:

https://github.com/the-tcpdump-group/libpcap/blob/151fe13a136db5a4e5486c4f788868d6d414cb51/pcap-linux.c#L2674-L2676

Depending on the volume of network traffic, this may not be enough to receive all applicable packets.

To check whether packets are being dropped, there's (*pcap.Handle).Stats(). To increase the buffer size, there's (*pcap.InactiveHandle).SetBufferSize().

Thanks for your time.