The way to get local IP address in Linux

Kernel Routing tables

open "/proc/net/route" then parse content

$ cat /proc/net/routeIface   Destination     Gateway         Flags   RefCnt  Use     Metric  Mask            MTU     Window  IRTTeth1    0000A8C0        00000000        0001    0       0       0       00FFFFFF        0       0       0eth0    005111AC        00000000        0001    0       0       1       00FFFFFF        0       0       0eth1    0000FEA9        00000000        0001    0       0       1000    0000FFFF        0       0       0eth0    00000000        FE5111AC        0003    0       0       0       00000000        0       0       0

00000000   is default interface

Then get this interface ip address.

Sample code http://man7.org/linux/man-pages/man3/getifaddrs.3.html

Using getsockname with an external connection

new connection outside then use this socket value to check own ip address.

/*

 * Find local ip used as source ip in ip packets.

 * Use getsockname and a udp connection

 */

 

#include<stdio.h> //printf

#include<string.h>    //memset

#include<errno.h> //errno

#include<sys/socket.h>    //socket

#include<netinet/in.h> //sockaddr_in

#include<arpa/inet.h> //getsockname

#include<unistd.h>    //close

 

int main ( int argc , char *argv[] )

{

    const char* google_dns_server = "8.8.8.8";

    int dns_port = 53;

     

    struct sockaddr_in serv;

     

    int sock = socket ( AF_INET, SOCK_DGRAM, 0);

     

    //Socket could not be created

    if(sock < 0)

    {

        perror("Socket error");

    }

     

    memset( &serv, 0, sizeof(serv) );

    serv.sin_family = AF_INET;

    serv.sin_addr.s_addr = inet_addr( google_dns_server );

    serv.sin_port = htons( dns_port );

 

    int err = connect( sock , (const struct sockaddr*) &serv , sizeof(serv) );

     

    struct sockaddr_in name;

    socklen_t namelen = sizeof(name);

    err = getsockname(sock, (struct sockaddr*) &name, &namelen);

         

    char buffer[100];

    const char* p = inet_ntop(AF_INET, &name.sin_addr, buffer, 100);

         

    if(p != NULL)

    {

        printf("Local ip is : %s \n" , buffer);

    }

    else

    {

        //Some error

        printf ("Error number : %d . Error message : %s \n" , errno , strerror(errno));

    }

 

    close(sock);

     

    return 0;

}