Detecting ARP Poisoning in Network Traffic
Overview
ARP poisoning (ARP spoofing) is a Layer 2 attack where an adversary sends falsified ARP messages to associate their MAC address with the IP address of a legitimate host, enabling man-in-the-middle (MitM) interception, session hijacking, or denial of service. Since ARP has no built-in authentication mechanism, any device on a broadcast domain can forge ARP replies. Detection requires monitoring ARP traffic for anomalies such as gratuitous ARP floods, IP-to-MAC mapping changes, and duplicate IP addresses. This skill covers deploying multiple detection layers including ARPWatch, Dynamic ARP Inspection (DAI), Wireshark-based analysis, and custom Python monitoring tools.
When to Use
- When investigating security incidents that require detecting arp poisoning in network traffic
- When building detection rules or threat hunting queries for this domain
- When SOC analysts need structured procedures for this analysis type
- When validating security monitoring coverage for related attack techniques
Prerequisites
- Access to the target network segment (broadcast domain)
- Linux host for ARPWatch and custom monitoring tools
- Managed switches supporting Dynamic ARP Inspection (Cisco Catalyst, Aruba, Juniper EX)
- Wireshark or tcpdump for packet capture
- DHCP snooping configured (prerequisite for DAI)
- Network monitoring infrastructure (SIEM, syslog server)
Core Concepts
ARP Protocol Fundamentals
ARP maps IP addresses to MAC addresses on a local network segment. The protocol operates statelessly with no authentication:
Normal ARP Process:
1. Host A broadcasts: "Who has 10.0.1.1? Tell 10.0.1.100"
2. Router replies: "10.0.1.1 is at AA:BB:CC:DD:EE:01"
3. Host A caches the mapping
ARP Poisoning Attack:
1. Attacker sends unsolicited ARP reply to Host A:
"10.0.1.1 is at EV:IL:MA:CA:DD:RR" (attacker's MAC)
2. Host A updates cache, sends traffic to attacker
3. Attacker forwards to real gateway (MitM position)
Attack Indicators
| Indicator | Description | Severity |
|---|---|---|
| MAC flip-flopping | Same IP mapped to different MACs rapidly | High |
| Gratuitous ARP flood | Unsolicited ARP replies targeting multiple hosts | High |
| Duplicate IP address | Two different MACs claiming same IP | Critical |
| Unusual ARP volume | Spike in ARP packets per second | Medium |
| ARP from non-DHCP source | Static IP claims from unknown devices | Medium |
| Gateway MAC change | Default gateway MAC address changed | Critical |
Workflow
Step 1: Deploy ARPWatch for Continuous Monitoring
# Install ARPWatch
sudo apt-get install -y arpwatch
# Configure ARPWatch
sudo vi /etc/default/arpwatch
# INTERFACES="eth0"
# ARGS="-N -p -i eth0 -f /var/lib/arpwatch/arp.dat"
# Start monitoring
sudo systemctl enable arpwatch
sudo systemctl start arpwatch
# View current ARP database
cat /var/lib/arpwatch/arp.dat
# Monitor logs for changes
tail -f /var/log/syslog | grep arpwatch
ARPWatch alert types:
- new station - Previously unseen MAC address
- changed ethernet address - IP mapped to different MAC (potential poisoning)
- flip flop - MAC alternating between two addresses (active attack)
- reused old ethernet address - Previously seen mapping returned
Step 2: Configure Dynamic ARP Inspection (DAI) on Switches
Cisco Catalyst configuration:
! Enable DHCP snooping (prerequisite for DAI)
ip dhcp snooping
ip dhcp snooping vlan 10,20,30
! Configure trusted ports (uplinks, DHCP servers)
interface GigabitEthernet1/0/1
description Uplink to Distribution
ip dhcp snooping trust
interface GigabitEthernet1/0/48
description DHCP Server
ip dhcp snooping trust
! Enable Dynamic ARP Inspection
ip arp inspection vlan 10,20,30
! Configure trusted ports for DAI
interface GigabitEthernet1/0/1
ip arp inspection trust
! Set rate limits to prevent ARP flood DoS
interface range GigabitEthernet1/0/2-47
ip arp inspection limit rate 15
! Enable additional validation checks
ip arp inspection validate src-mac dst-mac ip
! Configure ARP ACL for static IP devices (servers, printers)
arp access-list STATIC-ARP-ENTRIES
permit ip host 10.0.10.100 mac host 0011.2233.4455
permit ip host 10.0.10.101 mac host 0011.2233.4456
ip arp inspection filter STATIC-ARP-ENTRIES vlan 10
! Verify DAI status
show ip arp inspection vlan 10
show ip arp inspection statistics
show ip dhcp snooping binding
Step 3: Wireshark Detection Filters
# Detect gratuitous ARP (sender and target IP are the same)
arp.src.proto_ipv4 == arp.dst.proto_ipv4
# Detect ARP replies (focus on unsolicited)
arp.opcode == 2
# Detect duplicate IP address claims
arp.duplicate-address-detected
# Detect ARP packets from specific attacker MAC
eth.src == ev:il:ma:ca:dd:rr
# Detect ARP storms (high volume)
# Use Statistics > I/O Graphs > Display filter: arp
# Detect gateway impersonation
arp.src.proto_ipv4 == 10.0.1.1 && arp.src.hw_mac != aa:bb:cc:dd:ee:01
Step 4: Custom Python ARP Monitor
#!/usr/bin/env python3
"""
Real-time ARP poisoning detection using packet capture.
Monitors ARP traffic for spoofing indicators and alerts on anomalies.
"""
import subprocess
import sys
import json
import time
from collections import defaultdict
from datetime import datetime
try:
from scapy.all import sniff, ARP, Ether, get_if_hwaddr, conf
SCAPY_AVAILABLE = True
except ImportError:
SCAPY_AVAILABLE = False
class ARPPoisonDetector:
def __init__(self, interface: str, gateway_ip: str, gateway_mac: str):
self.interface = interface
self.gateway_ip = gateway_ip
self.gateway_mac = gateway_mac.lower()
self.arp_table = {} # IP -> MAC mapping
self.arp_history = defaultdict(list) # IP -> list of (MAC, timestamp)
self.alerts = []
self.arp_count = defaultdict(int) # Source MAC -> count per interval
self.last_reset = time.time()
self.arp_rate_threshold = 50 # ARP packets per 10 seconds
def alert(self, severity: str, message: str, details: dict):
"""Generate alert for detected anomaly."""
alert_data = {
'timestamp': datetime.now().isoformat(),
'severity': severity,
'message': message,
'details': details,
}
self.alerts.append(alert_data)
print(f"\n[{severity}] {datetime.now().strftime('%H:%M:%S')} - {message}")
for key, value in details.items():
print(f" {key}: {value}")
def check_gateway_spoofing(self, src_ip: str, src_mac: str):
"""Check if someone is spoofing the gateway."""
if src_ip == self.gateway_ip and src_mac != self.gateway_mac:
self.alert('CRITICAL', 'Gateway ARP Spoofing Detected', {
'gateway_ip': self.gateway_ip,
'expected_mac': self.gateway_mac,
'spoofed_mac': src_mac,
'action': 'Potential MitM attack on default gateway',
})
return True
return False
def check_mac_change(self, src_ip: str, src_mac: str):
"""Check if IP-to-MAC mapping has changed."""
if src_ip in self.arp_table:
known_mac = self.arp_table[src_ip]
if known_mac != src_mac:
self.alert('HIGH', 'ARP Cache Poisoning Attempt', {
'ip_address': src_ip,
'previous_mac': known_mac,
'new_mac': src_mac,
'action': 'IP-to-MAC mapping changed unexpectedly',
})
return True
return False
def check_flip_flop(self, src_ip: str, src_mac: str):
"""Check for MAC address flip-flopping (active attack indicator)."""
self.arp_history[src_ip].append((src_mac, time.time()))
# Keep only last 60 seconds of history
cutoff = time.time() - 60
self.arp_history[src_ip] = [
(mac, ts) for mac, ts in self.arp_history[src_ip]
if ts > cutoff
]
unique_macs = set(mac for mac, ts in self.arp_history[src_ip])
if len(unique_macs) > 2:
self.alert('CRITICAL', 'ARP Flip-Flop Detected (Active Attack)', {
'ip_address': src_ip,
'mac_addresses': list(unique_macs),
'changes_in_60s': len(self.arp_history[src_ip]),
})
return True
return False
def check_arp_rate(self, src_mac: str):
"""Check for ARP flood (DoS or scanning)."""
self.arp_count[src_mac] += 1
# Reset counters every 10 seconds
if time.time() - self.last_reset > 10:
for mac, count in self.arp_count.items():
if count > self.arp_rate_threshold:
self.alert('MEDIUM', 'ARP Flood Detected', {
'source_mac': mac,
'arp_packets_10s': count,
'threshold': self.arp_rate_threshold,
})
self.arp_count.clear()
self.last_reset = time.time()
def process_packet(self, packet):
"""Process captured ARP packet."""
if not packet.haslayer(ARP):
return
arp = packet[ARP]
# Only process ARP replies (opcode 2) and requests (opcode 1)
if arp.op not in (1, 2):
return
src_ip = arp.psrc
src_mac = arp.hwsrc.lower()
# Run detection checks
self.check_gateway_spoofing(src_ip, src_mac)
self.check_mac_change(src_ip, src_mac)
self.check_flip_flop(src_ip, src_mac)
self.check_arp_rate(src_mac)
# Update ARP table
self.arp_table[src_ip] = src_mac
def start_monitoring(self):
"""Start real-time ARP monitoring."""
print(f"[*] Starting ARP Poison Detection on {self.interface}")
print(f"[*] Gateway: {self.gateway_ip} ({self.gateway_mac})")
print(f"[*] Monitoring... (Ctrl+C to stop)\n")
if SCAPY_AVAILABLE:
sniff(
iface=self.interface,
filter="arp",
prn=self.process_packet,
store=False,
)
else:
print("[-] Scapy not available. Install with: pip install scapy")
print("[*] Falling back to tcpdump-based monitoring...")
self._monitor_with_tcpdump()
def _monitor_with_tcpdump(self):
"""Fallback monitoring using tcpdump."""
cmd = ['tcpdump', '-i', self.interface, '-l', '-n', 'arp']
proc = subprocess.Popen(cmd, stdout=subprocess.PIPE,
stderr=subprocess.DEVNULL, text=True)
try:
for line in proc.stdout:
parts = line.strip().split()
if 'is-at' in parts:
try:
ip_idx = parts.index('is-at') - 1
mac_idx = parts.index('is-at') + 1
src_ip = parts[ip_idx]
src_mac = parts[mac_idx].lower()
self.check_gateway_spoofing(src_ip, src_mac)
self.check_mac_change(src_ip, src_mac)
self.arp_table[src_ip] = src_mac
except (IndexError, ValueError):
continue
except KeyboardInterrupt:
proc.terminate()
def generate_report(self) -> dict:
"""Generate summary report of detected anomalies."""
return {
'monitoring_interface': self.interface,
'gateway': {'ip': self.gateway_ip, 'mac': self.gateway_mac},
'total_alerts': len(self.alerts),
'arp_table_size': len(self.arp_table),
'alerts': self.alerts,
}
if __name__ == '__main__':
if len(sys.argv) < 4:
print("Usage: python process.py <interface> <gateway_ip> <gateway_mac>")
print("Example: python process.py eth0 10.0.1.1 aa:bb:cc:dd:ee:01")
sys.exit(1)
detector = ARPPoisonDetector(
interface=sys.argv[1],
gateway_ip=sys.argv[2],
gateway_mac=sys.argv[3],
)
try:
detector.start_monitoring()
except KeyboardInterrupt:
print("\n\n[*] Monitoring stopped.")
report = detector.generate_report()
print(f"[*] Total alerts generated: {report['total_alerts']}")
print(f"[*] ARP table entries: {report['arp_table_size']}")
Prevention Measures
Layer 2 Controls
- Dynamic ARP Inspection (DAI) - Validates ARP packets against DHCP snooping binding table
- DHCP Snooping - Builds trusted IP-MAC-port binding database
- Port Security - Limits MAC addresses per port
- Private VLANs - Restricts communication between hosts in the same VLAN
Network Controls
- Static ARP Entries - For critical infrastructure (gateways, DNS, DHCP)
- Network Segmentation - Reduce broadcast domain size with VLANs
- 802.1X Authentication - Authenticate devices before network access
- Encrypted Protocols - Use SSH, HTTPS, TLS to protect data even if intercepted
Best Practices
- Defense in Depth - Combine DAI, ARPWatch, and custom monitoring for comprehensive coverage
- DHCP Snooping First - Always enable DHCP snooping before DAI (DAI depends on snooping database)
- Static ARP for Gateways - Configure static ARP entries on critical servers for the default gateway
- Monitor Gratuitous ARP - Pay special attention to unsolicited ARP replies
- Small Broadcast Domains - Use VLANs to limit the scope of ARP-based attacks
- Regular Audits - Periodically compare ARP tables across devices to identify anomalies