pcap_set_protocol_linux(3PCAP) man page
#include <pcap/pcap.h> int pcap_set_protocol_linux(pcap_t *p, int protocol);
<linux/if_ether.h>header file, specified in host byte order. If protocol is non-zero, packets of that protocol will be captured when the handle is activated, otherwise, all packets will be captured. This function is only provided on Linux, and, if it is used on any device other than a network interface, it will have no effect.
It should not be used in portable code; instead, a filter should be specified with pcap_setfilter(3PCAP).
If a given network interface provides a standard link-layer header, with a standard packet type, but provides some packet types with a different socket-layer protocol type from the one in the link-layer header, that packet type cannot be filtered with a filter specified with pcap_setfilter() but can be filtered by specifying the socket-layer protocol type using pcap_set_protocol_linux().
PCAP_ERROR_ACTIVATEDif called on a capture handle that has been activated.