Thread: Get local IP address

How can I find the IP that's binded to a socket? When doing this:

Code:

struct sockaddr_in local;
local.sin_family      = AF_INET;
local.sin_port        = htons(0);
local.sin_addr.s_addr = INADDR_ANY;
memset(&local.sin_zero,0,8);

// Bind stuff

int saSize            = sizeof(struct sockaddr);
getsockname(sock,(struct sockaddr *)&local,&saSize);

printf("%s:%d\n",inet_ntoa(local.sin_addr),ntohs(local.sin_port));

It gives me "0.0.0.0:<port>", where <port> is the binded port. So I can get the port number, but now I also need the IP address...

Ok nevermind, I found the answer. For those of you who're interested, it's (obviously) quite logical:

INADDR_ANY means it will listen to all addresses that the current host owns, so it's impossible to do getsockname() on the listening socket (because it would have to store more than one sockaddr, for example one for 127.0.0.1 and one for your real IP). But when calling getsockname() on a new socket created by accept(), you will get the right IP address. Here's a little code to test it:

Code:

#include <stdio.h>
#include <winsock2.h>

int main() {
  WSADATA wsaData;
  if(WSAStartup(MAKEWORD(2,0),&wsaData) == -1)
    return 0;

  int sock;
  if((sock = socket(AF_INET,SOCK_STREAM,IPPROTO_TCP)) == -1)
    return 0;

  struct sockaddr_in local;
  local.sin_family      = AF_INET;
  local.sin_port        = htons(1032);
  local.sin_addr.s_addr = INADDR_ANY;
  memset(&local.sin_zero,0,8);

  if(bind(sock,(struct sockaddr *)&local,sizeof(struct sockaddr)) == -1)
    return 0;

  if(listen(sock,0) == -1)
    return 0;

  int saSize            = sizeof(struct sockaddr);
  getsockname(sock,(struct sockaddr *)&local,&saSize);

  struct sockaddr_in remote;

  int client;
  while(client = accept(sock,(struct sockaddr *)&remote,&saSize)) {
    getsockname(client,(struct sockaddr *)&local,&saSize);
    printf("%s:%d\n",inet_ntoa(local.sin_addr),ntohs(local.sin_port));
    close(client);
  }

  return 0;
}

When telnetting to localhost port 1032, it will give "127.0.0.1:1032". When telnetting to <my ip> port 1032, it will give "<my ip>:1032".

Does it show up as 0.0.0.0 because it isnt binded yet?

In routing, we use 0.0.0.0 to define a gateway of last resort.

To you the local machines ip address, call gethostname(), then gethostbyname().

In routing, we use 0.0.0.0 to define a gateway of last resort.

No, you dont. A gateway of last resort is just an ip address to send packets to that are destined to a subnet that the router doesn't know about. Setting the gateway to 0.0.0.0 wouldn't be very helpful at all.

Actually the only way to be sure to bind to all interfaces is to bind to INADDR_ANY. Binding to 0.0.0.0 may work depending upon the underlying implementation.

by default, every pc has a "route all to default gateway" line in it's routing table, as a last resolve.

This is only true for windows machines. On my linux machine, I have to set the gateway of last resort manually.

Yes, it can if you have a DHCP client installed, and your router is running a DHCP server.

I'm not sure what point you're trying to make here...

Oh, and my ISP certanly is not running a DHCP server. Neither is my router. In fact, my computer doesn't even have a DHCP client installed. What brought up DCHP anyways? It really has nothing at all to do with gateways which is what I thought this conversation was about. All DHCP does is dynamically set a computer/routers network settings.

DSL. It uses PPPoE instead of DHCP. In fact, the only ISP I can think of that uses DHCP is cable providers.