Thursday, March 12, 2015

Hacking the network with Scapy and Python

I've been learning Python for infosec work. Scapy is built on Python and allows you to interact with the network at a much lower level than the Python sockets library. If I were to say it allows you to build your own packets/frames that would be an understatement.

From the Scapy home page:

What is Scapy

Scapy is a powerful interactive packet manipulation program. It is able to forge or decode packets of a wide number of protocols, send them on the wire, capture them, match requests and replies, and much more. It can easily handle most classical tasks like scanning, tracerouting, probing, unit tests, attacks or network discovery (it can replace hping, 85% of nmap, arpspoof, arp-sk, arping, tcpdump, tethereal, p0f, etc.). It also performs very well at a lot of other specific tasks that most other tools can't handle, like sending invalid frames, injecting your own 802.11 frames, combining technics (VLAN hopping+ARP cache poisoning, VOIP decoding on WEP encrypted channel, ...), etc. See interactive tutorial and the quick demo: an interactive session (some examples may be outdated).

What makes scapy different from most other networking tools

First, with most other tools, you won't build someting the author did not imagine. These tools have been built for a specific goal and can't deviate much from it. For example, an ARP cache poisoning program won't let you use double 802.1q encapsulation. Or try to find a program that can send, say, an ICMP packet with padding (I said padding, not payload, see?). In fact, each time you have a new need, you have to build a new tool.
Second, they usually confuse decoding and interpreting. Machines are good at decoding and can help human beings with that. Interpretation is reserved to human beings. Some programs try to mimic this behaviour. For instance they say "this port is open" instead of "I received a SYN-ACK". Sometimes they are right. Sometimes not. It's easier for beginners, but when you know what you're doing, you keep on trying to deduce what really happened from the program's interpretation to make your own, which is hard because you lost a big amount of information. And you often end up using tcpdump -xX to decode and interpret what the tool missed.
Third, even programs which only decode do not give you all the information they received. The network's vision they give you is the one their author thought was sufficient. But it is not complete, and you have a bias. For instance, do you know a tool that reports the padding ?
Scapy tries to overcome those problems. It enables you to build exactly the packets you want. Even if I think stacking a 802.1q layer on top of TCP has no sense, it may have some for somebody else working on some product I don't know. Scapy has a flexible model that tries to avoid such arbitrary limits. You're free to put any value you want in any field you want, and stack them like you want. You're an adult after all.
In fact, it's like building a new tool each time, but instead of dealing with a hundred line C program, you only write 2 lines of Scapy.
After a probe (scan, traceroute, etc.) Scapy always gives you the full decoded packets from the probe, before any interpretation. That means that you can probe once and interpret many times, ask for a traceroute and look at the padding for instance.

Here are my notes on Scapy. For detailed usage examples see the link below.

  • The ls() command shows a list of all available protocols.
    • For a listing of individual protocol options and defaults, use ls(protocol). For example ls(TCP)
  • To see a list of scapy commands: lsc()
  • Packets need to be created from a header perspective:
    • Ethernet | IP | TCP/UDP | Application
    • Ether()/IP()/TCP()/Data
  • Send a layer 3 packet ICMP example: (scapy handles the ethernet frame for you)
    • pkt = IP(dst="")/ICMP()/"data")
    • send(pkt)
  • To send a layer 3 TCP packet, you must add a port.
    • pkt = IP(dst="")/TCP()/(dport=23))
  • For an easier to read format of your sent or received packet, use:
  • To add a layer 2 frame you must add the ethernet header and include the interface. Note that we are now using sendp vs send.
    • example: sendp(Ether()/IP(dst="")/ICMP()/"data", iface="eth0")
  • Sending a packet repeatedly:
    • sendp(Ether()/IP(dst="")/ICMP()/"data", iface="eth0", loop=1)
  • To add a sending interval: (Interval is seconds)
    • sendp(Ether()/IP(dst="")/ICMP()/"data", iface="eth0", inter=1)
  • So far we have only seen the sent packets. To send and receive:
    • Layer 3:
      • sr() returns answers and unanswered packets
        • sr(IP(dst="")/ICMP()/"data")
      • To see the response (or lack of)
        • response, no_response = _
        • response[0]
        • no_response[0]
        • In Python,  the "_" variable is used to store the result of the last evaluation.
      • sr1() returns only answer or sent packets (1 packet)
    • Layer 2:
      • srp()
      • srp1()
  • You can manipulate the routing table in scapy without affecting the global routing table which is useful when you have a multihomed host.
    • Show the routing table: conf.route
    • add a host route: conf.route.add(host="", gw="")
    • add a network route: conf.route.add(net="", gw="")
    • To reset scapy's routing table: conf.route.resync()
  • Packet sniffing: pkts = sniff(iface="eth0", filter="arp", count=3)
    • Allows the use of bpf (Berkely packet filters)
    • Save sniffed packets to a file: pkts = sniff(offline="offline.pcap")
    • Print packets live while sniffing:
      • pkts=sniff(iface="eth0", filter="arp", count=20, prn=lambda x: x.summary())
  • Write packets to a pcap file: wrpcap("demo.pcap", pkts)

  • Read packets from a pcap file: rdpcap("demo.pcap")