pcap_setdirection.3pcap man page
pcap_setdirection — set the direction for which packets will be captured
#include <pcap/pcap.h> int pcap_setdirection(pcap_t *p, pcap_direction_t d);
pcap_setdirection() is used to specify a direction that packets will be captured. d is one of the constants PCAP_D_IN, PCAP_D_OUT or PCAP_D_INOUT. PCAP_D_IN will only capture packets received by the device, PCAP_D_OUT will only capture packets sent by the device and PCAP_D_INOUT will capture packets received by or sent by the device. PCAP_D_INOUT is the default setting if this function is not called.
pcap_setdirection() isn't necessarily fully supported on all platforms; some platforms might return an error for all values, and some other platforms might not support PCAP_D_OUT.
This operation is not supported if a “savefile” is being read.
pcap_setdirection() returns 0 on success and -1 on failure. If -1 is returned, pcap_geterr() or pcap_perror() may be called with p as an argument to fetch or display the error text.