How to Programmatically Get a Linux Server's Public IP Address in C ?

Determining the IP Address of a Linux Machine in C

Problem

How do we programmatically obtain the IP addresses of a Linux server running our application, particularly the external (public) one?

Linux Network Interfaces

A Linux server typically has multiple network interfaces, each with its own IP address. It has:

• Loopback interface: 127.0.0.1
• Internal management interface: 172.16.x.x
• External public interface: 80.190.x.x

Solution

The getifaddrs function can be used to retrieve all the IP addresses associated with a machine. It provides a linked list of ifaddrs structures. Here's a C example:

#include <stdio.h>
#include <sys/types.h>
#include <ifaddrs.h>
#include <netinet/in.h>
#include <string.h>
#include <arpa/inet.h>

using namespace std;

int main() {
  struct ifaddrs *ifAddrStruct, *ifa;
  void *tmpAddrPtr;

  getifaddrs(&ifAddrStruct);

  for (ifa = ifAddrStruct; ifa != NULL; ifa = ifa->ifa_next) {
    if (!ifa->ifa_addr) {
      continue;
    }
    if (ifa->ifa_addr->sa_family == AF_INET) {
      tmpAddrPtr = &((struct sockaddr_in *)ifa->ifa_addr)->sin_addr;
      char addressBuffer[INET_ADDRSTRLEN];
      inet_ntop(AF_INET, tmpAddrPtr, addressBuffer, INET_ADDRSTRLEN);
      printf("%s IP Address: %s\n", ifa->ifa_name, addressBuffer);
    } else if (ifa->ifa_addr->sa_family == AF_INET6) {
      tmpAddrPtr = &((struct sockaddr_in6 *)ifa->ifa_addr)->sin6_addr;
      char addressBuffer[INET6_ADDRSTRLEN];
      inet_ntop(AF_INET6, tmpAddrPtr, addressBuffer, INET6_ADDRSTRLEN);
      printf("%s IP Address: %s\n", ifa->ifa_name, addressBuffer);
    }
  }
  if (ifAddrStruct != NULL) freeifaddrs(ifAddrStruct);
  return 0;
}

This code iterates through all the network interfaces and prints the IP addresses for both IPv4 and IPv6 addresses.

By inspecting the network interface names (e.g., eth0, eth1, etc.), you can identify the external (public) IP address and use it to bind your application.

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