[Fwd: Re: Testing Dual Ethernet via Loopback]

[Ben Greear]

Looks like this message failed to go through the first time...


-------- Original Message --------
Subject: Re: Testing Dual Ethernet via Loopback
Date: Mon, 19 Apr 2004 18:44:27 -0700
From: Ben Greear <greearb@xxxxxxxxxxxxxxx>
Organization: Candela Technologies
To: Nick Popoff <cryptic-lkml@xxxxxxxxxxxxxxxx>
CC: linux-kernel@xxxxxxxxxxxxxxx
References: <200404190614.21764.cryptic-lkml@xxxxxxxxxxxxxxxx>

Nick Popoff wrote:
> Greetings,
>
> I am trying to write some software to test a dual port ethernet card.  I
> was hoping to be able to use an ethernet cable to just connect the
> ethernet board to itself and then write a program that talks to itself to
> make sure that both ports are working.  However, I've noticed that Linux
> is smart enough to realize it is talking to its own IP address, and it
> just delivers the data internally rather than use the network hardware at
> all.
>
> So what I'm wondering is if there is a way to force Linux to actually
> utilize its network hardware in sending these packets to itself?  In other
> words, a ping or file transfer from an IP assigned to eth0 to another IP
> assigned to eth1 should fail if I unplug the network cable connecting the
> two.  Any advice on this would be much appreciated.  I'm not afraid of
> reading kernel source but have no idea where to start on this one.
>
> I'm using 2.4.22 but would use any 2.4 or 2.6 kernel that supported this
> behavior. The National Semiconductor DP83815 (natsemi.o) is the 
> ethernet chipset.

You can apply the candela* patch from http://www.candelatech.com/~greear/vlan.html
and then use SO_BINDTODEVICE with something similar to this, where dev_to_bind_to
is a local interface, like "eth2":

int createTcpSocket(unsigned int ip_addr, int ip_port, const char* dev_to_bind_to) {
   LF_TRC_IN_NT;
   VLOG_DBG(VLOG << "ip_addr -:" << ip_addr << ": ip_port: " << ip_port << endl);
   char *opt;

   int s = socket(AF_INET, SOCK_STREAM, 0);

   VLOG_INF(VLOG << "createTcpSocket:  ip_addr -:" << ip_addr << ":  " << toStringIP(ip_addr)
            << ": ip_port: " << ip_port << " socket: " << s << endl);

   if (s < 0) {
      cerr << "ERROR: tcp socket:  " << strerror(errno) << endl;
      VLOG << "ERROR: tcp socket:  " << strerror(errno) << endl;
      return s;
   }

   if (setsockopt(s, SOL_SOCKET, SO_REUSEADDR, (char*)&opt,
                  sizeof(opt)) < 0) {
      cerr << "ERROR:  setsockopt:  " << strerror(errno) << endl;
      LFEXIT(100);
   }//if

   if (dev_to_bind_to) {
      // Bind to specific device.
      if (setsockopt(s, SOL_SOCKET, SO_BINDTODEVICE,
                     dev_to_bind_to, DEV_NAME_LEN + 1)) {
         VLOG_ERR(VLOG << "ERROR:  tcp-connect, setsockopt (BINDTODEVICE):  "
                  << strerror(errno) << "  Not fatal in most cases..continuing...\n");
      }
   }//if

   struct sockaddr_in my_ip_addr;
   memset(&my_ip_addr, 0, sizeof(my_ip_addr));

   my_ip_addr.sin_family = AF_INET;
   my_ip_addr.sin_addr.s_addr = htonl(ip_addr);
   my_ip_addr.sin_port = htons(ip_port);

   int r; //retval
   r = bind(s, (struct sockaddr*)(&my_ip_addr), sizeof(my_ip_addr));
   if (r < 0) {
      //system("netstat -an");
      cerr << "ERROR: tcp bind:  " << strerror(errno) << endl;
      VLOG_ERR(VLOG << "ERROR: tcp bind:  " << strerror(errno) << "  IP: "
               << toStringIP(ip_addr) << " ipPort: " << ip_port << endl);
      close(s);
      return r;
   }
   else {
      VLOG_INF(VLOG << "Successfully bound to IP: " << toStringIP(ip_addr) << " port: "
               << ip_port << endl);
   }

   nonblock(s);
   return s;
}


You will need to enable the send-to-self flag with something like this:

            // Set us up to accept local-generate packets if the kernel can support
            // it... (send-to-self)
            struct ifreq ifr;
            int fd = socket(PF_INET, SOCK_DGRAM, 0);
            if (fd < 0) {
               mudlog << "ERROR: socket: " << strerror(errno) << endl;
            }
            else {
               memset(&ifr, 0, sizeof(struct ifreq));

               // query to see if we are set up for send-to-self
               strcpy(ifr.ifr_name, dev_name);
               ifr.ifr_addr.sa_family = AF_INET;
               if (ioctl(fd, 0x89a1, &ifr) < 0) {
                  VLOG_ERR(VLOG << "ERROR: send-to-self ioctl(0x89a1): " << strerror(errno)
                           << " ifr.ifr_name -:" << ifr.ifr_name << ":-" << endl);
                  setSendToSelf(false);
               }
               else {
                  if (ifr.ifr_flags) {
                     setSendToSelf(true);
                  }
                  else {
                     // Try to make send_to_self true
                     memset(&ifr, 0, sizeof(struct ifreq));

                     strcpy(ifr.ifr_name, dev_name);
                     ifr.ifr_addr.sa_family = AF_INET;
                     ifr.ifr_flags = 1;
                     if (ioctl(fd, 0x89a0, &ifr) < 0) {
                        VLOG_ERR(VLOG << "ERROR: send-to-self ioctl(0x89a0): "
                                 << strerror(errno) << " ifr.ifr_name -:"
                                 << ifr.ifr_name << ":-" << endl);
                        setSendToSelf(false);
                     }
                     else {
                        setSendToSelf(true);
                        VLOG_ERR(VLOG << "Set send-to-self bit to true for interface: "
                                 << ifr.ifr_name << endl);
                     }
                  }//else, try to enable send-to-self
               }//else, we could read send-to-self IOCTL
               close(fd);
            }//else, could open socket


Hope that helps,
Ben

--
Ben Greear <greearb@xxxxxxxxxxxxxxx>
Candela Technologies Inc  http://www.candelatech.com

-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/