通过UDP套接字手动发送到iperf？ (C++)

Question

I have a program that needs to measure total loss after routing through a machine.

Essentially, I generate UDP traffic with iperf on machine A that is destined for machine C. However, I first route this traffic through machine B**, then send it to machine C via a raw socket to port 5001 (the default port that iperf listens to) with sendto() . While running a tcpdump shows that the packets are being received on machine C, the iperf server doesn't see these connections or the packet.

I've hunted around the iperf source code a bit to see how it works, and I see that packets are accepted with the function

rc = recvfrom( mSettings->mSock, mBuf, mSettings->mBufLen, 0, 
                       (struct sockaddr*) &server->peer, &server->size_peer );. 

Basically, because its simply a recvfrom , I don't see why there's a problem if I'm very sure I'm not modifying the packets at any time, and am sending them with the function sendto(s, buf, len, 0, (struct sockaddr*) &si_other, slen) and a socket(AF_INET, SOCK_RAW, IPPROTO_UDP) .

Anyone have any ideas? Why isn't iperf noticing this connection?

**I actually route the packets to a TUN device on machine A, then read them from a userspace program on A, send them to B with a UDP socket, read them on a userspace program on B, then send them via a raw socket with IP_HDRINCL disabled. I print out the headers when received on machine A and B, and I don't see anything weird.

Answer 1

Interesting. Turned out the problem was fixed by assigning the TUN device on machine A to eth0 's IP address. Before, I assigned the TUN device to it's own IP address (I did attempt to remove an IP address from the TUN device, but then it defaulted to eth1 . That didn't work either.).

I guess some modifications were occurring because of this, or that some sort of tunneling/connection wasn't established by iperf because the packets weren't being sent out of eth0 before.

If anyone has any further insights into how/why this works, I'd be very interested.