# IPv6: Duplicate Address Detection (DAD) failed for fe80::1/64 on eth0, address in use by 00:1a:2b:3c:4d:5e

- **ID:** `networking/ipv6-ndp-dad-failed`
- **Domain:** networking
- **Category:** layer2
- **Verification:** ai_generated
- **Fix Rate:** 85%

## Root Cause

Another host on the same link-local network segment is already using the IPv6 address fe80::1, causing the Duplicate Address Detection (DAD) mechanism (RFC 4862) to reject the address assignment and leave the interface in a tentative state.

## Version Compatibility

| Version | Status | Introduced | Deprecated |
|---------|--------|------------|------------|
| Linux kernel 6.1.0-17-amd64 | active | — | — |
| Windows Server 2022 | active | — | — |
| Cisco IOS XE 17.9.1 | active | — | — |

## Workarounds

1. **Identify and reconfigure the conflicting host by checking the neighbor cache: ip -6 neigh show; then change the duplicate address on one host to a unique link-local address (e.g., fe80::2/64).** (90% success)
   ```
   Identify and reconfigure the conflicting host by checking the neighbor cache: ip -6 neigh show; then change the duplicate address on one host to a unique link-local address (e.g., fe80::2/64).
   ```
2. **Temporarily disable DAD on the interface to force address assignment: sysctl -w net.ipv6.conf.eth0.accept_dad=0; then manually add the address with ip addr add fe80::1/64 dev eth0. Re-enable DAD after resolving the conflict.** (80% success)
   ```
   Temporarily disable DAD on the interface to force address assignment: sysctl -w net.ipv6.conf.eth0.accept_dad=0; then manually add the address with ip addr add fe80::1/64 dev eth0. Re-enable DAD after resolving the conflict.
   ```
3. **Use a privacy extension address (RFC 4941) for the interface to avoid link-local conflicts: sysctl -w net.ipv6.conf.eth0.use_tempaddr=2** (70% success)
   ```
   Use a privacy extension address (RFC 4941) for the interface to avoid link-local conflicts: sysctl -w net.ipv6.conf.eth0.use_tempaddr=2
   ```

## Dead Ends

- **** — The kernel will still perform DAD and reject the assignment if the address is already in use, even with manual configuration. (90% fail)
- **** — This breaks all IPv6 connectivity and is an overreaction if only one address is duplicated. (75% fail)
- **** — The link-local address is derived from the MAC address by default, but the conflict is with a specific address, not the MAC; changing MAC may not resolve the duplication if the address is manually configured on the other host. (60% fail)