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a simple python script that sends a dhcp discover packet and recieves the dhcp offer that contains a suggested ip address, gateway, dns servers and displays them.

Python, 128 lines
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'''
Created on Mar 27, 2011

@author: hassane
'''
import socket
import struct
from uuid import getnode as get_mac
from random import randint

def getMacInBytes():
    mac = str(hex(get_mac()))
    mac = mac[2:]
    while len(mac) < 12 :
        mac = '0' + mac
    macb = b''
    for i in range(0, 12, 2) :
        m = int(mac[i:i + 2], 16)
        macb += struct.pack('!B', m)
    return macb

class DHCPDiscover:
    def __init__(self):
        self.transactionID = b''
        for i in range(4):
            t = randint(0, 255)
            self.transactionID += struct.pack('!B', t) 

    def buildPacket(self):
        macb = getMacInBytes()
        packet = b''
        packet += b'\x01'   #Message type: Boot Request (1)
        packet += b'\x01'   #Hardware type: Ethernet
        packet += b'\x06'   #Hardware address length: 6
        packet += b'\x00'   #Hops: 0 
        packet += self.transactionID       #Transaction ID
        packet += b'\x00\x00'    #Seconds elapsed: 0
        packet += b'\x80\x00'   #Bootp flags: 0x8000 (Broadcast) + reserved flags
        packet += b'\x00\x00\x00\x00'   #Client IP address: 0.0.0.0
        packet += b'\x00\x00\x00\x00'   #Your (client) IP address: 0.0.0.0
        packet += b'\x00\x00\x00\x00'   #Next server IP address: 0.0.0.0
        packet += b'\x00\x00\x00\x00'   #Relay agent IP address: 0.0.0.0
        #packet += b'\x00\x26\x9e\x04\x1e\x9b'   #Client MAC address: 00:26:9e:04:1e:9b
        packet += macb
        packet += b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'   #Client hardware address padding: 00000000000000000000
        packet += b'\x00' * 67  #Server host name not given
        packet += b'\x00' * 125 #Boot file name not given
        packet += b'\x63\x82\x53\x63'   #Magic cookie: DHCP
        packet += b'\x35\x01\x01'   #Option: (t=53,l=1) DHCP Message Type = DHCP Discover
        #packet += b'\x3d\x06\x00\x26\x9e\x04\x1e\x9b'   #Option: (t=61,l=6) Client identifier
        packet += b'\x3d\x06' + macb
        packet += b'\x37\x03\x03\x01\x06'   #Option: (t=55,l=3) Parameter Request List
        packet += b'\xff'   #End Option
        return packet

class DHCPOffer:
    def __init__(self, data, transID):
        self.data = data
        self.transID = transID
        self.offerIP = ''
        self.nextServerIP = ''
        self.DHCPServerIdentifier = ''
        self.leaseTime = ''
        self.router = ''
        self.subnetMask = ''
        self.DNS = []
        self.unpack()
    
    def unpack(self):
        if self.data[4:8] == self.transID :
            self.offerIP = '.'.join(map(lambda x:str(x), data[16:20]))
            self.nextServerIP = '.'.join(map(lambda x:str(x), data[20:24]))  #c'est une option
            self.DHCPServerIdentifier = '.'.join(map(lambda x:str(x), data[245:249]))
            self.leaseTime = str(struct.unpack('!L', data[251:255])[0])
            self.router = '.'.join(map(lambda x:str(x), data[257:261]))
            self.subnetMask = '.'.join(map(lambda x:str(x), data[263:267]))
            dnsNB = int(data[268]/4)
            for i in range(0, 4 * dnsNB, 4):
                self.DNS.append('.'.join(map(lambda x:str(x), data[269 + i :269 + i + 4])))
                
    def printOffer(self):
        key = ['DHCP Server', 'Offered IP address', 'subnet mask', 'lease time (s)' , 'default gateway']
        val = [self.DHCPServerIdentifier, self.offerIP, self.subnetMask, self.leaseTime, self.router]
        for i in range(4):
            print('{0:20s} : {1:15s}'.format(key[i], val[i]))
        
        print('{0:20s}'.format('DNS Servers') + ' : ', end='')
        if self.DNS:
            print('{0:15s}'.format(self.DNS[0]))
        if len(self.DNS) > 1:
            for i in range(1, len(self.DNS)): 
                print('{0:22s} {1:15s}'.format(' ', self.DNS[i])) 

if __name__ == '__main__':
    #defining the socket
    dhcps = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)    #internet, UDP
    dhcps.setsockopt(socket.SOL_SOCKET, socket.SO_BROADCAST, 1) #broadcast
    
    try:
        dhcps.bind(('', 68))    #we want to send from port 68
    except Exception as e:
        print('port 68 in use...')
        dhcps.close()
        input('press any key to quit...')
        exit()
 
    #buiding and sending the DHCPDiscover packet
    discoverPacket = DHCPDiscover()
    dhcps.sendto(discoverPacket.buildPacket(), ('<broadcast>', 67))
    
    print('DHCP Discover sent waiting for reply...\n')
    
    #receiving DHCPOffer packet  
    dhcps.settimeout(3)
    try:
        while True:
            data = dhcps.recv(1024)
            offer = DHCPOffer(data, discoverPacket.transactionID)
            if offer.offerIP:
                offer.printOffer()
                break
    except socket.timeout as e:
        print(e)
    
    dhcps.close()   #we close the socket
    
    input('press any key to quit...')
    exit()
    

2 comments

zvika 9 years, 1 month ago  # | flag

Thanks for the code !! with Python 2.6 i had to update the code and things looked great. However, it seems that if you do not actually own an IP , you cannot send the broadcast message For example: run "ifconfig XXXXX 0.0.0.0" and then execute this script, on my system it didn't work due to a "network is unreachable". Unless i'm missing something here (new Python/kernel?) i guess that the correct implementation should use raw-sockets instead of standard UDP sockets.

hassane (author) 9 years, 1 month ago  # | flag

Hello, you're welcome! yes you're right, a better implementation would be with raw sockets. on Windows it works most of the time since Windows assigns an automatic private IP address for the machine.

Created by hassane on Sun, 10 Apr 2011 (MIT)
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