Fix ARP Requests to Non-Existent Hosts

TL;DR

You’re seeing ARP requests for a MAC address that doesn’t respond, indicating a ‘ghost host’. This usually means stale ARP cache entries. We’ll clear the cache and investigate potential causes like misconfigured devices or network loops.

What’s happening?

ARP (Address Resolution Protocol) maps IP addresses to MAC addresses on your local network. When a device tries to talk to an IP address it doesn’t know, it sends out an ARP request. If no one responds, and the requests keep coming, you have a problem – likely a ghost host. This can slow down your network or cause connection issues.

How to fix it: Step-by-Step

1. Identify the Problem IP/MAC

First, find out which IP address is causing the ARP requests. Use a packet sniffer like Wireshark or a network monitoring tool. Look for frequent ARP requests to an IP that isn’t responding.

Alternatively, on Linux you can use:

arp -a

On Windows:

arp -a

Note the IP address and associated MAC address that’s causing issues.

2. Clear Your ARP Cache

Clearing your local ARP cache is the first thing to try. This forces your device to re-request the mapping, which might resolve the issue if it’s just a stale entry.

• Windows: Open Command Prompt as Administrator and run:

  arp -d *

• Linux (most distributions): Use:

  sudo ip neigh flush all

• macOS: Use:

  sudo arp -d 

  Replace <IP_ADDRESS> with the problematic IP.

3. Check for Network Loops

Network loops happen when data packets get stuck circulating endlessly. This can cause ARP storms and ghost host issues.

• Spanning Tree Protocol (STP): If you have managed switches, ensure STP is enabled and configured correctly. It prevents loops by blocking redundant paths.
• Check Switch Logs: Look for STP-related messages in your switch logs that might indicate a loop.
4. Investigate Misconfigured Devices

A device with an incorrect static IP address or gateway can cause ARP problems.

• Check Static IPs: Review any devices on your network that have static IP addresses. Make sure they are unique and within the correct range.
• DHCP Server Conflicts: If you’re using DHCP, ensure only one server is active on your network to avoid conflicts.
5. Examine Rogue Devices

Sometimes a device might be sending out incorrect ARP responses.

• Network Scan: Use a network scanner (like Nmap) to identify all devices on your network. Look for unexpected or unknown devices.
• MAC Address Lookup: Try to determine the manufacturer of the problematic MAC address using an online MAC address lookup tool. This might give you a clue about the device’s identity.
6. Firewall/Security Software

Occasionally, overly aggressive firewall rules or security software can interfere with ARP communication.

• Temporarily Disable: As a test, temporarily disable your firewall and any security software to see if the problem resolves. Be careful when doing this!
• Review Rules: If disabling fixes it, review your firewall rules for anything that might be blocking ARP traffic.

If you’ve tried these steps and are still having problems, the issue could be more complex. Consider consulting a cyber security professional or network administrator.