Help!! How to steal UDP traffic?

[Guilherme Cesar Soares Ruppert]

  Hi everyone, please I need some help!!

  I'm developing a program that needs to intercept an UDP traffic generated
  locally by another application. It's like a tunnel. My program will steal the
  outgoing packets from the application (in the same machine) and will send to
  a tcp tunnel.

  I am using Libipq to do that, but I have a BIG problem. After send the UDP
  packets to my tunnel, I need to DROP the packets because I don't want them to
  be sent over the network. So I am doing:
  ipq_set_verdict(handle, m->packet_id, NF_DROP, 0, NULL);

  But the problem is that when I drop the packet, the application that
  generated the packet remains blocked in sendto() until the packet is
  accepted. When I change to NF_ACCEPT, the application doesn't block but the
  packets are sent to the network and I don't want it.

  How could I drop a packet silently, without blocking the application? Is
  there any way to intercept outgoing packets locally without let the
  applications notice that their packet were stolen?

  Here is the same example using netcat:
  $ iptables -A OUTPUT -p udp -j QUEUE
  $ echo "Test" | nc -n -u 10.1.1.23 800
  And this last remains blocked, not returning to shell.

  I tested will ping (icmp echo) also and the same happened, but ping didn't
  block. Instead, it said "operation not permitted".

  Please give me some hope!!! :-)

  Thanks

  Guilherme Ruppert