This script listens for DHCP packets using scapy to learn when LAN hosts request IP addresses from DHCP Servers. Learn more about this script at https://jcutrer.com/python/scapy-dhcp-listener

scapy-dhcp-listener.py

#!/usr/bin/env python3
"""scapy-dhcp-listener.py

Listen for DHCP packets using scapy to learn when LAN 
hosts request IP addresses from DHCP Servers.

Copyright (C) 2018 Jonathan Cutrer

License Dual MIT, 0BSD

"""

from __future__ import print_function
from scapy.all import *
import time

__version__ = "0.0.3"

# Fixup function to extract dhcp_options by key
def get_option(dhcp_options, key):

    must_decode = ['hostname', 'domain', 'vendor_class_id']
    try:
        for i in dhcp_options:
            if i[0] == key:
                # If DHCP Server Returned multiple name servers 
                # return all as comma seperated string.
                if key == 'name_server' and len(i) > 2:
                    return ",".join(i[1:])
                # domain and hostname are binary strings,
                # decode to unicode string before returning
                elif key in must_decode:
                    return i[1].decode()
                else: 
                    return i[1]        
    except:
        pass


def handle_dhcp_packet(packet):

    # Match DHCP discover
    if DHCP in packet and packet[DHCP].options[0][1] == 1:
        print('---')
        print('New DHCP Discover')
        #print(packet.summary())
        #print(ls(packet))
        hostname = get_option(packet[DHCP].options, 'hostname')
        print(f"Host {hostname} ({packet[Ether].src}) asked for an IP")


    # Match DHCP offer
    elif DHCP in packet and packet[DHCP].options[0][1] == 2:
        print('---')
        print('New DHCP Offer')
        #print(packet.summary())
        #print(ls(packet))

        subnet_mask = get_option(packet[DHCP].options, 'subnet_mask')
        lease_time = get_option(packet[DHCP].options, 'lease_time')
        router = get_option(packet[DHCP].options, 'router')
        name_server = get_option(packet[DHCP].options, 'name_server')
        domain = get_option(packet[DHCP].options, 'domain')

        print(f"DHCP Server {packet[IP].src} ({packet[Ether].src}) "
              f"offered {packet[BOOTP].yiaddr}")

        print(f"DHCP Options: subnet_mask: {subnet_mask}, lease_time: "
              f"{lease_time}, router: {router}, name_server: {name_server}, "
              f"domain: {domain}")


    # Match DHCP request
    elif DHCP in packet and packet[DHCP].options[0][1] == 3:
        print('---')
        print('New DHCP Request')
        #print(packet.summary())
        #print(ls(packet))

        requested_addr = get_option(packet[DHCP].options, 'requested_addr')
        hostname = get_option(packet[DHCP].options, 'hostname')
        print(f"Host {hostname} ({packet[Ether].src}) requested {requested_addr}")


    # Match DHCP ack
    elif DHCP in packet and packet[DHCP].options[0][1] == 5:
        print('---')
        print('New DHCP Ack')
        #print(packet.summary())
        #print(ls(packet))

        subnet_mask = get_option(packet[DHCP].options, 'subnet_mask')
        lease_time = get_option(packet[DHCP].options, 'lease_time')
        router = get_option(packet[DHCP].options, 'router')
        name_server = get_option(packet[DHCP].options, 'name_server')

        print(f"DHCP Server {packet[IP].src} ({packet[Ether].src}) "
              f"acked {packet[BOOTP].yiaddr}")

        print(f"DHCP Options: subnet_mask: {subnet_mask}, lease_time: "
              f"{lease_time}, router: {router}, name_server: {name_server}")

    # Match DHCP inform
    elif DHCP in packet and packet[DHCP].options[0][1] == 8:
        print('---')
        print('New DHCP Inform')
        #print(packet.summary())
        #print(ls(packet))

        hostname = get_option(packet[DHCP].options, 'hostname')
        vendor_class_id = get_option(packet[DHCP].options, 'vendor_class_id')

        print(f"DHCP Inform from {packet[IP].src} ({packet[Ether].src}) "
              f"hostname: {hostname}, vendor_class_id: {vendor_class_id}")

    else:
        print('---')
        print('Some Other DHCP Packet')
        print(packet.summary())
        print(ls(packet))

    return

if __name__ == "__main__":
    sniff(filter="udp and (port 67 or 68)", prn=handle_dhcp_packet)

Gives this error

File "scapy-dhcp-listener.py", line 45
print(f"Host {hostname} ({packet[Ether].src}) asked for an IP")
^
SyntaxError: invalid syntax

Gives this error

File "scapy-dhcp-listener.py", line 45
print(f"Host {hostname} ({packet[Ether].src}) asked for an IP")
^
SyntaxError: invalid syntax

This is because the script above uses f strings which were introduced in Python 3.6 - if your python is <3.6 you will get an error.