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 PCAP_ERROR on
  failure. If PCAP_ERROR is returned, pcap_geterr(3) or
  pcap_perror(3) may be called with p as an argument to fetch or
  display the error text.