Can't obtain local IP using gethostbyname()

Question

A friend used the following snippet of code to retrieve the local IP address of the host in his LAN.

int buffersize = 512;
char name[buffersize];

if(gethostname         


        

Answer 1

The method you suggest relies upon a "correct" local host name, and a "correctly" configured DNS system. I put "correct" in quotes, because having a local host name that is not registered with DNS is perfectly valid for every purpose, except your program.

If you have a connected socket (or can generate a connected socket), I suggest you use getsockname() to find a local IP address. (Note: not the local IP address -- you could have several.)

Here is a sample program that does that. Obviously, most of it is connecting to google.com and printing the result. Only the call to getsockname() is germane to your question.

#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>

#include <stdio.h>
#include <errno.h>
#include <unistd.h>


int main(int ac, char **av) {
    addrinfo *res;
    if(getaddrinfo("google.com", "80", 0, &res)) {
        fprintf(stderr, "Can't lookup google.com\n");
        return 1;
    }

    bool connected = false;
    for(; res; res=res->ai_next) {
        int fd = socket(res->ai_family, res->ai_socktype, res->ai_protocol);
        if(fd < 0) {
            perror("socket");
            continue;
        }

       if( connect(fd, res->ai_addr, res->ai_addrlen) ) {
           perror("connect");
           close(fd);
           continue;
       }

       sockaddr_in me;
       socklen_t socklen = sizeof(me);
       if( getsockname(fd, (sockaddr*)&me, &socklen) ) {
           perror("getsockname");
           close(fd);
           continue;
       }

       if(socklen > sizeof(me)) {
           close(fd);
           continue;
       }

       char name[64];
       if(getnameinfo((sockaddr*)&me, socklen, name, sizeof name, 0, 0, NI_NUMERICHOST)) {
           fprintf(stderr, "getnameinfo failed\n");
           close(fd);
           continue;
        }
        printf("%s ->", name);

       if(getnameinfo(res->ai_addr, res->ai_addrlen, name, sizeof name, 0, 0, NI_NUMERICHOST)) {
           fprintf(stderr, "getnameinfo failed\n");
           close(fd);
           continue;
        }
        printf("%s\n", name);
        close(fd);
    }
}

Answer 2

If you really use Qt, you can use this code

foreach (const QHostAddress& address, QNetworkInterface::allAddresses() ) {
        qDebug() << address.toString();
    }

If you are not using Qt, you can study, how it is made there.