How to send packet through a particular interface using scapy, while you are connected to 2 networks?

Question

I am having two Ethernet card installed on my PC, thus having two interfaces. These two interfaces are connected to Router's two interfaces. Such that:

PC_INT_A --- Network1 ---> ROUTER_INT_1

PC_INT_B --- Network2 ---> ROUTER_INT_2

All the interfaces are fully configured for IPv6 communication. Ping6 is working well through command line.

But when I am trying to send through scapy, while both the networks are connected. I am able to send only first interface in the code. Or I have to disconnect one particular network.

See Below code:

def ns_with_ll(src, dst):
       base = IPv6(src=src, dst=dst, nh=58, hlim=255)
       ns = ICMPv6ND_NS(tgt=dst)
       ll = ICMPv6NDOptSrcLLAddr()
       pkt = base / ns / ll
       return pkt

if __name__ == '__main__':
    SRC_A = 'IPV6_ADDR_OF_ETH0'
    SRC_B = 'IPV6_ADDR_OF_ETH1'
    DST_1 = 'IPV6_ADDR_ROUTER_INT_1'
    DST_2 = 'IPV6_ADDR_ROUTER_INT_2'

    ns_a = ns_with_ll(SRC_A, DST_1)
    ns_b = ns_with_ll(SRC_B, DST_2)

    sr(ns_a, iface='eth0')
    sr(ns_b, iface='eth1')

The above code gives me output as follows:

Begin emission:
*Finished sending 1 packets.

Received 1 packets, got 1 answers, remaining 0 packets
Begin emission:
Finished sending 1 packets.
..........................................................................

It keeps waiting for the answer from eth1 interface.

If i change the order of packet sending in code like I am sending packet to eth1 first. I receive answer from eth1 and it keeps waiting for answer from eth0.

I also check in wireshark, but I do not receive any packet for the second interface.

I have also tried same with sending ICMPv6EchoRequest, it behaves in the same way.

Can somebody tell me, how to simultaneously work with multiple networks using scapy. I am using scapy version 2.4.2 with python 3.4?

Answer 1

Hi!

In Scapy, you have two ways of sending packets. The "layer 3 way", which uses the internal routing table (conf.route), and the "layer 2 way", which sends a packet on a given interface (conf.iface being the default).

send() is a "layer 3" function (so it relies on Scapy's routing table), while sendp() is a "layer 2" function (it has an iface= optional parameter and will use conf.iface by default). That's the same for sr() and srp().

So in your case, using srp() instead of sr() and providing the Ether() layer should work:

if __name__ == '__main__':
[...]
    srp(Ether() / ns_a, iface='eth0')
    srp(Ether() / ns_b, iface='eth1')