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.

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

Implementation Steps

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

1. Dynamic ARP Inspection (DAI) - Validates ARP packets against DHCP snooping binding table
2. DHCP Snooping - Builds trusted IP-MAC-port binding database
3. Port Security - Limits MAC addresses per port
4. Private VLANs - Restricts communication between hosts in the same VLAN

Network Controls

1. Static ARP Entries - For critical infrastructure (gateways, DNS, DHCP)
2. Network Segmentation - Reduce broadcast domain size with VLANs
3. 802.1X Authentication - Authenticate devices before network access
4. 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

References

• NIST SP 800-54 - Border Gateway Protocol Security
• Cisco DAI Configuration Guide
• Comparitech ARP Poisoning Guide
• Okta ARP Poisoning Techniques