ipannounce - Finding new SLAAC configured servers on an IPv6 network

Posted Sep 17, 2022

To get a better idea of what gaps and opportunities exist with IPv6 networks, I have been rolling out a lab environment that is 100% single-stack IPv6. One of the great things about IPv6 is that it makes address space planning so much simpler. The standard /64 subnet size is so large that hosts can choose random IPs with practical certainty that there will be no conflicts. This allows new hosts to self-configure without a central management service. So easy! But, how do I find these new servers, especially physical hosts, once they are provisioned? I wrote a small utility to help with that called ipannounce.

At first I figured mDNS would do the trick here. But after testing it I found that Avahi would return an interface IP at random. This was a problem since each interface was going to have a number of different IPv6 addresses that had different uses.

Every host has at least four IPs:

  • Stable Privacy GUA - Publicly routable IP in a PA (Provider Assigned) address space
  • Temporary GUA - Default source address for outbound connections to other public IPv6 addresses
  • Stable Privacy ULA - A private IPv6 address that will not change if the ISP changes my public subnet
  • Link Local - The link local address, one per physical interface

The only one I am interested in for internal connectivity is the Stable Privacy ULA on the primary network interface. The Stable GUA might be changed by the ISP. The Temporary GUA will change all the time. The link local address isn’t routable.

Also, what even is the primary network interface? Well that’s a subjective thing that depends on what the operator wants to accomplish. In this case I want the interface that has a ULA address (fd00::/8) that is connected to the network segment that is routed to the rest of my network.

The way I decided to solve this was to create a service that would reply to an interrogation with the IP that was most like the IP to which it was asked to respond. The definition of “most like” is an IP that has the most left hand bits matching the source IP of the interrogator.

The result is that I can query a network full of hosts that have a bunch of IPv6 addresses on different interfaces. One such host might look like this:

[email protected]:~$ ip ad | grep inet6
  inet6 ::1/128 scope host
  inet6 fdbc:6a5c:a49a:1005:c568:25de:87af:9c65/64 scope global dynamic mngtmpaddr stable-privacy
  inet6 2601:441:8301:83d4:4dc6:363c:fc3b:a259/64 scope global dynamic mngtmpaddr stable-privacy
  inet6 fe80::bf37:9554:b1d9:c085/64 scope link stable-privacy
  inet6 fe80::1a66:daff:fe0f:270a/64 scope link
  inet6 fdbc:6a5c:a49a:ffff::1/64 scope global tentative
  inet6 fe80::1/64 scope link tentative

And all hosts will respond with IPs that are consistently of the same address type and on the desired subnet:

thog[email protected]:~$ ipannounce -mode sol -selector fdbc:: -solport 5191
Running as solicitor using address fdbc:6a5c:a49a:1005:c568:25de:87af:9c65
Solicitor listening on [::]:5191
gurp2           fdbc:6a5c:a49a:1005:c568:25de:87af:9c65
gurp1           fdbc:6a5c:a49a:1005:184:c73e:56f:ac59
gurp3           fdbc:6a5c:a49a:1005:d61e:7b5a:47d1:a00d
horf1           fdbc:6a5c:a49a:1005:3277:7671:30bd:52d0
horf5           fdbc:6a5c:a49a:1005:b12f:26bf:a4b7:89b5
horf3           fdbc:6a5c:a49a:1005:357a:5944:5a04:cb1a
horf2           fdbc:6a5c:a49a:1005:e835:d3d9:d8de:6053
horf4           fdbc:6a5c:a49a:1005:e76f:bfeb:4ecf:4a87

GitHub project here: https://github.com/arcandspark/ipannouce