IPv6 on the WiFi

Yes yes yes… We all know that we ran out of IPv4 addresses long ago and IPv6 has been around for 300 years and is the solution to all our problems.

But, we’re still not using it. So why not?

“IPv6 is haaard”

For people who are used to an IPv4 world, with 32 bit addresses you can remember like 178.79.163.251 and broadcast domains with a few hundred addresses, IPv6 can be a terrifying prospect with 128 bit addresses that look like 2a01:7e00::f03c:91ff:fe92:c52b, umpteen billion addresses per subnet, a whole new reliance on ICMP and… well, it’s just different.

I never fail to be slightly surprised how conservative and resistant to change some folk in IT can be. It’s just human nature of course, to keep things working and try to avoid too much rapid change, but still.

Dabbling in IPv6 can result in some odd behaviour as it will generally be used in preference to IPv4. So if you can resolve a IPv6 address but can’t route to it, things break. There are ways to address this and most web browsers do, but people have been caught out and that led to IT departments disabling IPv6 on all Windows systems, for example.

Truth be told, IPv6 isn’t all that hard, so why haven’t we deployed it on our WiFi?

She doesn’t have the capacity Jim!

The problem on our network is address table capacity. Previously I’ve talked about a problem we encountered of filling the arp table on the core routers. Keen not to be burned twice by the same problem, IPv6 presents new challenges.

Firstly there’s the obvious issue of each address being four times the size and therefore needing more memory. Then there’s the issue of just how many addresses you’re dealing with.

With IPv4 a client on our network requests an address via DHCP, is given one, and that’s the end of it.

With IPv6 clients come up with a link-local address (starting FE80:) which can be used to talk to other devices on the same subnet without any configuration being done at all. Then, using SLAAC (Stateless Address Autoconfiguration), the router advertises it’s address and by virtue of that the local subnet. Clients then come up with their own IPv6 address based on their hardware MAC address, which will (probably) be a globally routable address. But this address, built out of the MAC address of the client, can be used to track individual machines as they move around different networks. To avoid this most operating systems will also come up with a privacy address. This is another IPv6 address that’s valid in the subnet but is not based on the MAC address of the hardware. Because there are so many addresses in an IPv6 subnet, the chance of a conflict is… well, you don’t have to worry about it.

These privacy addresses are what the client uses to talk to the world, but the client will also respond on its SLAAC address. Privacy addresses are also changed periodically. When a privacy address changes, the client may hold on to the old one in case there’s any incoming traffic still trying to use it.

Long and short of all this is you have to ensure your network equipment can handle the number of IPv6 addresses required for the number of clients being supported.

In our testing so far we’ve seen an average 2.6 IPv6 addresses per client. We think our routers could just handle that for our regular concurrent client count but with no room for growth it’s asking for trouble.

It’s worth mentioning that whilst we really do want to provide globally routed IPv6 addresses to the WiFi clients, this isn’t something we actually need to do right now, but do expect it will be required in the foreseeable future. And we do have options available if we had to make this work right now, the easiest being to spread the subnets over a few routers so as to avoid the need to replace the core routers. We could also just buy some hardware with enough capacity to handle just the WiFi traffic routing.

This situation though presents an opportunity to look again at our whole network and maybe simplify some of it, using technologies such as VXLAN that weren’t available on the hardware we were using previously.

Suffice to say our WiFi is ready for IPv6, the firewall rules are built and tested, the radius accounting all works… we just need the rest of the network to catch up.

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