linux - [Solved-5 Solutions] How to get the ip address of the machine in Linux ? - ubuntu - red hat - debian - linux server - linux pc




Linux - Problem :

How to get the ip address of the machine ?

Linux - Solution 1:

#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(&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=&((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=&((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;
}
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Linux - Solution 2:

  • Create a socket.
  • Perform ioctl(, SIOCGIFCONF, (struct ifconf)&buffer);

Read /usr/include/linux/if.h for information on the ifconf and ifreq structures. This should give you the IP address of each interface on the system. Also read /usr/include/linux/sockios.h for additional ioctls.

Linux - Solution 3:

  • It uses the local routing table to find the IP address of the ethernet interface that would be used for a connection to a specific external host.
  • By using a connected UDP socket, you can get the information without actually sending any packets.
  • The approach requires that you choose a specific external host.
void GetPrimaryIp(char* buffer, size_t buflen) 
{
    assert(buflen >= 16);

    int sock = socket(AF_INET, SOCK_DGRAM, 0);
    assert(sock != -1);

    const char* kGoogleDnsIp = "8.8.8.8";
    uint16_t kDnsPort = 53;
    struct sockaddr_in serv;
    memset(&serv, 0, sizeof(serv));
    serv.sin_family = AF_INET;
    serv.sin_addr.s_addr = inet_addr(kGoogleDnsIp);
    serv.sin_port = htons(kDnsPort);

    int err = connect(sock, (const sockaddr*) &serv, sizeof(serv));
    assert(err != -1);

    sockaddr_in name;
    socklen_t namelen = sizeof(name);
    err = getsockname(sock, (sockaddr*) &name, &namelen);
    assert(err != -1);

    const char* p = inet_ntop(AF_INET, &name.sin_addr, buffer, buflen);
    assert(p);

    close(sock);
}
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Linux - Solution 4:

As you have found out there is no such thing as a single "local IP address". Here's how to find out the local address that can be sent out to a specific host.

  • Create a UDP socket
  • Connect the socket to an outside address (the host that will eventually receive the local address)
  • Use getsockname to get the local address

Linux - Solution 5:

You can try this:

// ifconfig | perl -ne 'print "$1\n" if /inet addr:([\d.]+)/'

#include <stdlib.h>

int main() {
        setenv("LANG","C",1);
        FILE * fp = popen("ifconfig", "r");
        if (fp) {
                char *p=NULL, *e; size_t n;
                while ((getline(&p, &n, fp) > 0) && p) {
                   if (p = strstr(p, "inet ")) {
                        p+=5;
                        if (p = strchr(p, ':')) {
                            ++p;
                            if (e = strchr(p, ' ')) {
                                 *e='\0';
                                 printf("%s\n", p);
                            }
                        }
                   }
              }
        }
        pclose(fp);
        return 0;
}
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