How to Programmatically Retrieve a Linux Machine's IP Addresses in C ?

How to Retrieve IP Addresses of a Linux Machine in C

Problem:

Determine the IP addresses of a Linux machine programmatically using C , specifically targeting the public IP address among multiple potential addresses.

Solution:

Using getifaddrs()

The getifaddrs() function provides a more reliable method for retrieving IP addresses on Linux and POSIX-compliant operating systems. It returns a linked list of ifaddrs structures, each representing a network interface and its associated addresses. Here's a code example demonstrating its usage:

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

int main (int argc, const char * argv[]) {
    struct ifaddrs * ifAddrStruct=NULL;
    struct ifaddrs * ifa=NULL;
    void * tmpAddrPtr=NULL;

    getifaddrs(&amp;ifAddrStruct);

    for (ifa = ifAddrStruct; ifa != NULL; ifa = ifa->ifa_next) {
        if (!ifa->ifa_addr) {
            continue;
        }
        if (ifa->ifa_addr->sa_family == AF_INET) { // check it is IP4
            // is a valid IP4 Address
            tmpAddrPtr=&amp;((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) { // check it is IP6
            // is a valid IP6 Address
            tmpAddrPtr=&amp;((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;
}

In the code, getifaddrs() fills the ifAddrStruct linked list with network interface information. Each ifaddrs structure contains an ifa_addr member, which is a pointer to the socket address structure representing the IP address. AF_INET is for IPv4 addresses, and AF_INET6 is for IPv6 addresses.

The code iterates through the linked list, checks if the ifa_addr is valid, and then retrieves and prints the IP address string by converting it to a human-readable format using inet_ntop().

To identify the public IP address, you can check the network interface name (e.g., "eth0") or compare the IP addresses to determine which one matches the external network.

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