The Day™ has finally arrived. I've avoided IPv6 until now, but my blissful ignorance must end.

My ISP notified me that a device on my network performed a DNS lookup for one of the C&C servers taken offline in the recent law enforcement action against the Avalanche botnet. I need to find that device and deal with it, so I enabled logging on my DNS server. Finally after four days a matching DNS lookup request was made, but to my dismay the request came from the address fe80::113d:d91e:e685:943b. Crap. I'm a noob when it comes to IPv6 and I've got a machine on my 60+ node network that is part of a malware-spewing botnet.

I ran tracert and determined it's on the local link and currently online:

Tracing route to fe80::113d:d91e:e685:943b over a maximum of 30 hops

  1     9 ms    <1 ms     1 ms  fe80::113d:d91e:e685:943b

With an IPv4 device I can look at my DHCP leases to get the device name. Failing that, I'd ping it, then run arp -a to get its MAC address, which at least gives me the manufacturer. But this network doesn't have a IPv6 DHCP server and arp doesn't seem to speak IPv6.

I attempted a crash course in IPv6 and learned that the fe80 prefix means the address is link-local and I can supposedly derive the MAC address from the address. I tried that and get the MAC 13:3d:d9:85:94:3b. None of the OUI lookup tools recognize it and it doesn't appear in my IPv4 DHCP leases.

How can I determine which device on my network has this IPv6 address?

My servers and the machines where I do my troubleshooting are running Windows.


Thanks to this comment on another Super User answer, I discovered the command:

netsh int ipv6 show neighbors

To my great joy I discovered it tells me the MAC address of devices on the local link, just like arp -a does:

Interface 10: Local Area Connection

Internet Address                              Physical Address   Type
--------------------------------------------  -----------------  -----------
fe80::113d:d91e:e685:943b                     4c-72-b9-d2-b5-5d  Reachable

I was then able to compare the Physical Address to the IPv4 leases on my DHCP server and obtain the NetBIOS name of the offending device. Easy peasy.

I still don't understand why this MAC address is resulting in this particular IPv6 Address. According to this online converter, its link local IPv6 address should be fe80::4e72:b9ff:fed2:b55d. But that's another question...

  • 3
    The reason you can't convert the link local address to a MAC address is that is uses a newer algorithm for creating addresses. The original algorithm did use the MAC address and such addresses can be identified using by the ff:fe bit in the middle of the second half of the address. Using MAC addresses in that way was considered bad for privacy, so newer algorithms randomize the address. Dec 17 '17 at 7:46

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