ARP spoofing (also called ARP poisoning) is a network-layer technique that can cause devices on a local network to associate the wrong hardware (MAC) address with an IP address. In this article we explain the concept at a high level, show how it can appear in packet captures, and — importantly — cover safe detection and mitigation strategies for administrators and students.
Table of Contents
What is ARP Spoofing?
ARP (Address Resolution Protocol) maps IPv4 addresses to MAC addresses on a local network. ARP spoofing occurs when incorrect ARP replies are injected into the network so that devices update their ARP tables with wrong mappings. This can cause traffic to be redirected, monitored, or disrupted.
How ARP Works (High Level)
When one device wants to communicate with another on the same LAN, it checks its ARP table for the destination IP's MAC address. If it's unknown, the device sends an ARP request asking "Who has IP x.x.x.x?" The owner replies with its MAC address and devices cache this mapping for a short time.
How ARP Spoofing Appears on a LAN
On a compromised network, a device may send forged ARP replies that claim a different MAC address for a given IP. In packet captures this can show as multiple ARP replies, sudden ARP table changes, or the same IP associated with different MAC addresses. Network monitoring tools and packet analyzers can surface these anomalies.
Risks & Impacts
Possible impacts include interception of unencrypted traffic (confidentiality risk), session hijacking, traffic tampering, or denial of service on the local segment. The level of impact depends on network configuration, encryption in use, and the attacker’s capabilities.
Detecting ARP Spoofing (Non-actionable)
Administrators typically look for indicators such as frequent ARP table changes, duplicate IP/MAC pairings, unusual amounts of ARP traffic, and mismatches between known device inventories and observed ARP mappings. Network monitoring, endpoint logs, and IDS/IPS systems can be configured to alert on such anomalies. Always follow internal policies when investigating suspected incidents.
Mitigation & Best Practices
- Use strong encryption: Protocols like HTTPS, SSH, and VPNs protect the content even if traffic is redirected.
- Network segmentation: Reduce flat networks where possible so attacks have limited scope.
- Static mappings: Where feasible, configure static ARP entries for critical infrastructure (manage at scale carefully).
- Switch features: Enable Dynamic ARP Inspection, port security, and DHCP snooping if your switch supports them.
- Monitoring & alerting: Deploy tools to detect sudden ARP anomalies and investigate through proper incident response channels.
Further Resources
For administrators and students: consult official vendor docs for your network equipment, peer-reviewed security guides, and reputable network security blogs. If you need to test detection or defenses, always use an isolated lab environment or obtain explicit written authorization before testing on production networks.
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