# NDP: Duplicate Address Detection failed for fe80::1 on eth0, address already in use by 00:1a:2b:3c:4d:5e

- **ID:** `networking/ipv6-ndp-duplicate-address`
- **Domain:** networking
- **Category:** layer2
- **Verification:** ai_generated
- **Fix Rate:** 79%

## Root Cause

IPv6 Duplicate Address Detection (DAD) detected that the link-local address fe80::1 is already claimed by another host on the same link, preventing the interface from using that address.

## Version Compatibility

| Version | Status | Introduced | Deprecated |
|---------|--------|------------|------------|
| Linux kernel 6.2.0 | active | — | — |
| FreeBSD 13.2 | active | — | — |
| Windows 11 IPv6 stack | active | — | — |

## Workarounds

1. **Change the link-local address on one host by modifying the interface identifier: `ip link set dev eth0 address 02:00:00:00:00:01` (EUI-64 based), then reboot or restart networking.** (85% success)
   ```
   Change the link-local address on one host by modifying the interface identifier: `ip link set dev eth0 address 02:00:00:00:00:01` (EUI-64 based), then reboot or restart networking.
   ```
2. **Use a static link-local address with a unique suffix on each host: `ip -6 addr add fe80::1/64 dev eth0` but ensure no conflict by checking with `ndp -a`.** (90% success)
   ```
   Use a static link-local address with a unique suffix on each host: `ip -6 addr add fe80::1/64 dev eth0` but ensure no conflict by checking with `ndp -a`.
   ```
3. **Reconfigure the network to use globally routable IPv6 addresses instead of link-local for communication, and disable DAD for link-local only: `sysctl -w net.ipv6.conf.eth0.dad_transmits=0`.** (75% success)
   ```
   Reconfigure the network to use globally routable IPv6 addresses instead of link-local for communication, and disable DAD for link-local only: `sysctl -w net.ipv6.conf.eth0.dad_transmits=0`.
   ```

## Dead Ends

- **** — Disabling DAD leads to network instability and packet loss as two hosts respond to the same address. (95% fail)
- **** — Both hosts will conflict; only one can respond to neighbor solicitations, causing intermittent failures. (90% fail)
- **** — Flushing caches does not resolve the underlying address duplication; the conflict persists. (80% fail)