Fix ARP Table Not Updating

TL;DR

ARP spoofing attacks often rely on a slow or non-updating ARP table to function effectively. This guide shows you how to force your system to refresh its ARP cache, making it harder for attackers and helping detect malicious activity.

Understanding the Problem

The Address Resolution Protocol (ARP) maps IP addresses to MAC addresses on a local network. Your computer stores these mappings in an ARP table. If this table doesn’t update quickly when devices change their MAC address, or if entries don’t expire properly, it can be exploited by attackers using ARP spoofing.

Solution: Force ARP Table Updates

1. Check Current ARP Table
   • First, see what’s currently in your table. This gives you a baseline.

     arp -a

     On Linux/macOS, this command shows IP address and corresponding MAC address pairings.

2. Delete Specific ARP Entries (Targeted Approach)
   • If you suspect a specific entry is incorrect, remove it. Replace IP_ADDRESS with the problematic IP.

     arp -d IP_ADDRESS

     This command deletes the ARP entry associated with that IP address.

3. Clear Entire ARP Cache (Aggressive Approach)
   • This is a more forceful method, clearing all entries. Use with caution as it will temporarily disrupt network connectivity.

     sudo arp -a -d *

     On Linux/macOS, this command deletes all ARP cache entries. You’ll need administrator privileges (sudo).

4. Renew IP Address and Flush DNS Cache
   • Sometimes, an outdated IP address can cause ARP issues. Renewing your IP forces a fresh request.

     ipconfig /release

     ipconfig /renew

     These commands (Windows) release and renew your IP configuration.

5. Check ARP Timeout Settings
   • ARP entries have a timeout. If it’s too long, updates will be slow.
     • Linux: Edit /etc/sysctl.conf and adjust these values (example):

       net.ipv4.arp_time = 60

       net.ipv4.arp_gc_interval = 30

       arp_time is the time an entry stays in the cache (seconds). arp_gc_interval controls how often the kernel garbage collects stale entries.

     • After editing, apply changes:

       sudo sysctl -p

6. Monitor Network Traffic
   • Use a network monitoring tool (Wireshark is popular) to observe ARP traffic. Look for suspicious patterns like multiple MAC addresses associated with the same IP.
7. Enable Static ARP Entries (Advanced – Use Carefully)
   • For critical devices, you can create static ARP entries that won’t expire. This is a security trade-off – it prevents updates but also bypasses dynamic detection.

     arp -s IP_ADDRESS MAC_ADDRESS

     Replace IP_ADDRESS and MAC_ADDRESS with the correct values.

Important Considerations

• cyber security software (antivirus, firewalls) can often detect ARP spoofing. Ensure yours is up to date.
• Regularly check your ARP table for unexpected entries.
• Be cautious when running commands that clear the entire ARP cache, as it will briefly interrupt network connectivity.