How to display server's IP on its own terminal?

yingjusuen · 05-30-2013, 03:29 PM

Hi,

I am writing a server and client program using TCP/IP. The client works with the server fine. But what I'd like to do is: when I run the server, I'd like it to display its IP and port number on its terminal. Below is what I did, but the part for printing out server IP doesn't work:

struct sockaddr_in serv_addr;

serv_addr.sin_family = AF_INET;
serv_addr.sin_addr.s_addr = htonl(INADDR_ANY);
serv_addr.sin_port = htons(5001); //specify server port number

printf("IP Address: %s\n", inet_ntoa(serv_addr.sin_addr)); //OK
printf("Port Number: %d\n", ntohs(serv_addr.sin_port)); //gets 0.0.0.0

Any ideas what went wrong? Thanks!

jason_m · 05-30-2013, 10:42 PM

struct sockaddr_in is only a data structure for holding address information, and nothing more. You're probably going to need to associate it with a socket descriptor and make some other system calls before you can get your hands on an ip address. It has been a long time since I've played around with this api, but I do remember this being an invaluable reference:
http://www.beej.us/guide/bgnet/outpu...age/index.html

Edit: To elaborate a little further:

Quote:

serv_addr.sin_addr.s_addr = htonl(INADDR_ANY);

This is specifically the reason you are getting 0.0.0.0 back. That is the value for INADDR_ANY. What this is saying is allow connections from any network interface. Your computer might only have one network interface. But some computers have multiple (consider a personal computer with a wired ethernet device and a wireless device, or think of a commercial router with multiple network interfaces). An IP address is associated with a network interface / device.

Continuing with the example of the personal computer with a wired and wireless device: say the wired device has address 192.168.1.1 and the wireless device has address 192.168.1.2. If you only wanted your program to accept connections from the wired device, you could assign that 192.168.1.1 to serv_addr.sin_addr.s_addr. By assigning INADDR_ANY (which is represented internally by 0.0.0.0), you are saying incoming connections from either network device are allowed.

So, in that case, it doesn't make sense to talk about the IP address of the server. At least, not until a connection is made - because we don't know which interface that connection will come in on. If you want to see which interface an incoming connection came in on, you'll have to accept() an incoming connection and investigate from there.

NevemTeve · 05-30-2013, 11:17 PM

> printf("Port Number: %d\n", ntohs(serv_addr.sin_port)); //gets 0.0.0.0

Not possible. Try again.

PS: functions getsockname and getpeername might be your friends

ta0kira · 05-31-2013, 03:01 PM

You can use getifaddrs, which I think is a BSD extension that's also available on Linux. Keep in mind that there might be multiple IP addresses for one interface (at least on FreeBSD, maybe not on Linux.)

Kevin Barry

yrs7 · 06-06-2013, 10:28 AM

Thanks, jason_m, for your detailed explaination. It really solved my doubts of struct sockaddr_in. As for what I did to get my IP, I used "ifconfig" cmd to list all my IP interfaces and manually entered the IP to my code. Not sure if it's the best way, but that's all I can get so far.

However, I'd really like to know a way for my server program to get it's local IP address without my hard-coding it into my code. Any idea? Thanks.

ta0kira · 06-06-2013, 02:45 PM

Quote:

Originally Posted by yrs7

However, I'd really like to know a way for my server program to get it's local IP address without my hard-coding it into my code. Any idea? Thanks.

Code:

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


int main(int argc, char *argv[])
{
  //get a linked list containing the interface addresses

  struct ifaddrs *addrs = NULL;
  if (getifaddrs(&addrs) != 0) {
    fprintf(stderr, "%s: error reading interface addresses: %s\n", argv[0], strerror(errno));
    return 1;
  }

  struct ifaddrs *current = addrs;

  //iterate through the linked list

  while (current) {
    //skip over anything with a NULL or zero netmask
    if (current->ifa_netmask && ((struct sockaddr_in*) current->ifa_netmask)->sin_addr.s_addr)
      fprintf(stderr, "%s\t%s\n", current->ifa_name,
              current->ifa_addr? inet_ntoa(((struct sockaddr_in*) current->ifa_addr)->sin_addr) : NULL);
    current = current->ifa_next;
  }

  freeifaddrs(addrs);
}

Tested on both Linux and FreeBSD.

Kevin Barry