Thread: connect two interfaces of the same host

Hi,

I have following situation:
my Linux host has two interfaces eth0 (192.168.254.254) and eth1 (192.168.253.254). For the test purpose I want connect these two port with a cable and make some data exchange. For both the server and client socket I make bind(). The IP addresses seem to be assigned properly. When I start the test and lo interface is up everything runs successfully but not over the cable. When I make lo interface down I don't get any connection. I also tried to set route for these hosts but with the same result. What am I doing wrong?

Code:

// server init code
int open_server(char *interface, char *server_ip, struct _tester *tester)
{
  int sockfd, i;
  struct sockaddr_in *server;
  struct ifreq ifr;

  printf("Server:\nInterface: %s\nIP: %s\n", interface, server_ip);
  memset(&ifr, 0, sizeof(struct ifreq));

  if((sockfd = socket(AF_INET,SOCK_STREAM, 0)) < 0)
  {
    perror("Socket");
    return TST_ERROR;
  }

  i = 1;
  if(setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &i, sizeof(i)) < 0)
  {
    perror("setsockopt");
    return TST_ERROR;
  }

  server = (struct sockaddr_in *)&(ifr.ifr_addr);
  server->sin_family = AF_INET;
  server->sin_port = htons(SERVER_PORT);
  inet_aton(server_ip, &server->sin_addr);

  strncpy(ifr.ifr_name, interface, IFNAMSIZ);
  if(ioctl(sockfd, SIOCSIFADDR, &ifr) < 0)
  {
    perror("IOCTL (SIOCSIFADDR)");
    return TST_ERROR;
  }

  // bring interface up
  ifr.ifr_flags = IFF_UP;

  if(ioctl(sockfd, SIOCSIFFLAGS, &ifr) < 0)
  {
    perror("IOCTL (SIOCSIFADDR)");
    return TST_ERROR;
  }

  if(bind(sockfd, (struct sockaddr *)server, sizeof(struct sockaddr_in)) < 0)
  {
    perror("Bind");
    return TST_ERROR;
  }

  if(listen(sockfd, 5) < 0)
  {
    perror("Listen");
    return TST_ERROR;
  }

  printf("IP assigned %s\n", inet_ntoa(server->sin_addr));
  return sockfd;
}

Code:

// client init code
int open_client(char *interface, char *client_ip, struct _tester *tester)
{
  int sockfd, i;
  struct sockaddr_in *client;
  struct ifreq ifr;

  printf("Client:\nInterface: %s\nIP: %s\n", interface, client_ip);
  memset(&ifr, 0, sizeof(struct ifreq));

  if((sockfd = socket(AF_INET,SOCK_STREAM, 0)) < 0)
    perror("Socket");

  i = 1;
  setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &i, sizeof(i));

  
  client = (struct sockaddr_in *)&(ifr.ifr_addr);
  client->sin_family = AF_INET;
  client->sin_port = htons(SERVER_PORT + 1);
  inet_aton(client_ip, &client->sin_addr);

  strncpy(ifr.ifr_name, interface, IFNAMSIZ);
  if(ioctl(sockfd, SIOCSIFADDR, &ifr) < 0)
    perror("IOCTL (SIOCSIFADDR)");

  // bring interface up
  ifr.ifr_flags = IFF_UP;

  if(ioctl(sockfd, SIOCSIFFLAGS, &ifr) < 0)
    perror("IOCTL (SIOCSIFADDR)");

  if(bind(sockfd, (struct sockaddr *)client, sizeof(struct sockaddr_in)) < 0)
  {
    perror("Bind");
    return TST_ERROR;
  }

  return sockfd;
}

Thanks in advance

to make a cross cable , you want to swap the twisted pair going to pin 1/2 with the one going to 3/5 one one end of the cable only. This way 1/2 on end A goes to 3/5 on end B and vice versa. either that or you could just get 2 cables and plug them both into a router

Originally Posted by dwks

Or you can just buy a crossover cable . . .

oh sure, throw money at the problem

I have two interfaces with Auto-MDI (When switch UTP port or LAN card supports auto MDI/MDI-X function, the connection is independent of cable’s crossover or non-crossover type and allows using a straight cable to make a switch-to-switch connection.)

I have solved the problem by putting the interfaces in promiscuous mode and sending raw ethernet broadcast packet. If I unplug the cable I still can receive the packet but the RX bytes on the interface won't be incremented. So checking RX bytes (read /proc/net/dev) I can recognize if the communication was successfully gone over the cable.

How new is your system? People are talking about crossover cables -- on newer network cards, the device is able to throw a switch and "cross over" by itself, using a regular cable -- this is autodetected and handled automatically. But if your system is pretty old, you may in fact need a crossover cable.

It could be that your routing table is not set up right. Simply configuring the IP addresses isn't quite enough to get communication going. And it would probably help if you configured the interfaces in "point-to-point" mode.

Is this Linux? Can you post the ifconfig and route information?

EDIT: I see your post saying that it works in promiscuous mode. That proves the problem isn't crossover. You probably just don't have your routes set up properly.

Originally Posted by vart

And to put the connector manually to this custom build cable - you need a tool - that will cost you even more than the cable itself

but then you will have the tool and you can never have enough tools, unless they are the kind that walk around and say things like 'we need to collateralize the synergy into a proactive mission oriented solution'