linux - How to know the IP address for interfaces in C using IPv6

Question

Suppose I'd like a list of all IP addresses on my Linux machine by their interface names, using both IPv6 and IPv4.

The best advice I could find is to use getifaddrs() that should support IPv6, similarly to a post from here. However, the getifaddrs() uses struct ifaddrs which uses struct sockaddr which is incompatible with IPv6. Instead, it should be a pointer to a union with struct in6_addr as well.

How is this handled? How does getifaddrs() support IPv6? Is the documentation obsolete?

Answer 1

My C is terribly rusty, and structs with members containing unions (like struct sockaddr_in6) no longer fit in my brain, so in the best cut and paste traditions I adapted chrisaycock's answer (which enumerates the system's IPv4 addresses) to use getnameinfo() instead, with some help from the getifaddrs() man page (which has a better example):

#define _GNU_SOURCE    # required for NI_NUMERICHOST
#include <arpa/inet.h>
#include <sys/socket.h>
#include <ifaddrs.h>
#include <stdio.h>
#include <stdlib.h>
#include <netdb.h>

int main ()
{
  struct ifaddrs *ifap, *ifa;
  struct sockaddr_in6 *sa;
  char addr[INET6_ADDRSTRLEN];

  if (getifaddrs(&ifap) == -1) {
      perror("getifaddrs");
      exit(1);
  }

  for (ifa = ifap; ifa; ifa = ifa->ifa_next) {
    if (ifa->ifa_addr && ifa->ifa_addr->sa_family==AF_INET6) {
      sa = (struct sockaddr_in6 *) ifa->ifa_addr;
      getnameinfo(ifa->ifa_addr, sizeof(struct sockaddr_in6), addr,
                  sizeof(addr), NULL, 0, NI_NUMERICHOST);
      printf("Interface: %sAddress: %s
", ifa->ifa_name, addr);
    }
  }

  freeifaddrs(ifap);
  return 0;
}

The output on my system:

Interface: lo   Address: ::1
Interface: br0  Address: fdbf:e684:d5fb:6:6e62:6dff:fed1:dfad
Interface: br0  Address: 2001:db8:1f80:81c6:6e62:6dff:fed1:dfad
Interface: br0  Address: fe80::6e62:6dff:fed1:dfad%br0
Interface: virbr1       Address: fe80::5054:ff:fece:bfec%virbr1
Interface: virbr0       Address: fe80::5054:ff:fef9:c92e%virbr0
Interface: virbr2       Address: fe80::5054:ff:fedd:ea18%virbr2
Interface: vnet0        Address: fe80::fc54:ff:fe90:de19%vnet0
Interface: vnet1        Address: fe80::fc54:ff:fede:b69c%vnet1

Remember to add error checking to everything; some error checks have been omitted from this example.