pcap_lookupnet returns incorrect IP address

Question

Following example code from the libpcap documentation yields the following code which should report the IP address of the given interface (eth0 in this case) [Error checking omitted for brevity]

#include <stdio.h>
#include <pcap.h>
#include <arpa/inet.h>

int main(int argc, char *argv[])
{
  char errbuf[PCAP_ERRBUF_SIZE]; 
  bpf_u_int32 mask;   
  bpf_u_int32 ip;    
  struct in_addr ip_addr;

  /* Find the properties for the device */
  pcap_lookupnet("eth0", &ip, &mask, errbuf);

  ip_addr.s_addr = ip;

  printf("IP Address: %s\n", inet_ntoa(ip_addr));

  return 0;
}

However, this results in 192.168.1.0, rather than the correct 192.168.1.100. Running this on a different machine on a different subnet yields 10.0.0.0 rather than the correct 10.0.0.107 which leads me to believe libpcap is not copying the last octet correctly. I've manually converted the integer returned by pcap_lookupnet to ensure it's not an issue with the use of inet_ntoa (I've also tried inet_ntop, with identical results). Following the code from this question: Get IP address of an interface on Linux reports the correct IP address. Is this a bug in libpcap or am I doing something wrong?

Answer 1

Your statement "which should report the IP address of the given interface" is incorrect.

From the manpage:

pcap_lookupnet() is used to determine the IPv4 network number and mask associated with the network device device. Both netp and maskp are bpf_u_int32 pointers.

are you sure you have a network number of 10.0.0.107 or 192.168.1.100 respectively? Sounds rather unusual.