Stop ARP Spoofing: Why the Constant Replies?

TL;DR

An attacker sends constant ARP replies to maintain a false association between MAC addresses and IP addresses on your network. This allows them to intercept traffic intended for other devices (a ‘Man-in-the-Middle’ attack). Stopping it involves detecting these rogue replies, identifying the attacker, and preventing future attacks with static ARP entries or dynamic ARP inspection.

Understanding the Problem

ARP (Address Resolution Protocol) maps IP addresses to MAC addresses. Normally, when a device wants to talk to another on the network, it asks ‘Who has this IP address?’ and gets a reply with the corresponding MAC address. ARP spoofing exploits this by sending false replies.

The attacker doesn’t just send one reply; they keep sending them repeatedly. Here’s why:

• ARP entries expire: ARP caches aren’t permanent. Entries time out after a few minutes (typically around 5-10 minutes). The attacker needs to refresh the cache with their false information before it disappears.
• Traffic redirection: If another device asks for the target’s MAC address, and the attacker doesn’t respond *immediately*, the correct MAC address will be used, breaking the attack. Constant replies ensure they remain ‘the answer’.

Solution Steps

1. Detect ARP Spoofing
   • ARP Table Inspection: Use a tool to view your ARP table and look for inconsistencies. Common tools include arp -a (Windows, Linux, macOS).
   • Example Output & What to Look For: You’ll see IP addresses paired with MAC addresses. If you see the same IP address listed multiple times with different MAC addresses, that’s a strong indicator of spoofing. Also, look for MAC addresses associated with IPs they shouldn’t be on (e.g., your router’s MAC address being used by another device).
   • ARP Monitoring Tools: Consider using dedicated ARP monitoring tools like arpspoof (Linux) or Wireshark to capture and analyse ARP traffic in real-time. Wireshark lets you filter for ARP packets specifically.
2. Identify the Attacker
   • MAC Address Lookup: Once you suspect a rogue MAC address, try to identify the device it belongs to. You might be able to find the manufacturer from the first three bytes of the MAC address (using an online MAC address lookup tool).
   • Port Mapping: If possible, investigate which port on your switch is associated with the suspicious MAC address. This will tell you where the attacker’s device is physically connected. (Requires access to managed switch configuration.)
3. Mitigation Techniques
   • Static ARP Entries: Manually configure static ARP entries for critical devices (e.g., your router, servers). This prevents the ARP cache from being poisoned for those IPs.

     arp -s 192.168.1.1 AA:BB:CC:DD:EE:FF

     (Replace with actual IP and MAC address.) Be careful – incorrect static entries can disrupt network connectivity!

   • Dynamic ARP Inspection (DAI): Enable DAI on your managed switch. This feature validates ARP packets and drops invalid ones, preventing spoofing attacks.

     Note: Configuration varies significantly between switch manufacturers. Consult your switch documentation for specific instructions.

   • Port Security: Configure port security on your switch to limit the MAC addresses allowed on each port. This can prevent an attacker from connecting a rogue device.

     Note: Configuration varies significantly between switch manufacturers. Consult your switch documentation for specific instructions.

   • Network Segmentation: Divide your network into smaller segments (VLANs) to limit the scope of potential attacks.
4. Further Security Measures
   • Regular Firmware Updates: Keep your router and switch firmware up-to-date to patch security vulnerabilities.
   • Strong Passwords: Use strong, unique passwords for all network devices.
   • Network Monitoring: Implement a comprehensive network monitoring solution to detect suspicious activity.