Room Full of Mirrors

… and all I could see was me

Adding A Static IPv6 Address To A Void Linux Server

First, a little background. At home, I am stuck with Comcast for an ISP. Probably the only redeeming quality of this is that they have fully rolled out dual stack IPv4+IPv6 to all of their customers. Everyone on their network gets personal access to more IPv6 addresses than ever existed in the whole IPv4 space. They assign a /128 to your WAN interface, and from there you can request a /64, or in my case a /60 to handle my multiple VLANs, for all of your internal devices. So, last weekend I took the time to get IPv6 enabled on my home network. Primarily to get my Xbox One X directly connected to the Internet, but also, because it was something to do.

This weekend, on a lark, I checked with my VPS provider, Vultr, to see if they provide an IPv6 address for my server. Answer: yep! they provide a full /64. The result is a couple hours of me adding IPv6 to the host that served this website to you and this article detailing the things I learned.

Implementation §

Void Linux provides a very basic plaform upon which you can build whatever system you like. For me, one particularly attractive aspect is that I get to define my network configuration simply with /etc/rc.local and the iproute2 tools. So, while this article is techincally specific to Void, the information should be easily applicable to any situation where one is managing everything themselves instead of through higher level tooling.

My previous IPv4 only rc.local:

ip addr add brd scope global dev eth0
ip link set up dev eth0
ip route add default via dev eth0

My rc.local after adding my IPv6 configuration:

ip addr add brd scope global dev eth0
ip addr add 2001:19f0:5:23de:5400:00ff:fe1e:54e2/64 scope global dev eth0
ip link set up dev eth0
ip route add default via dev eth0

A couple of things to notice here:

  1. The difference is only one line: adding the IPv6 address to the interface.
  2. IPv4 requires manual configuration of the gateway.

It’s item #2 that makes things tricky. Whereas IPv4 reserves specific IPs for specific purposes (broadcast and gateway) in each subnet, IPv6 does not. Instead, it uses the Neighbor Discovery Protocol (NDP) to advertise, and learn about, routers available on the network. Thus, only adding an IPv6 address to the network interface will not accomplish anything useful. We need to listen for router advertisement messages and use them to build our route rules.

First, we need to be able to hear said advertisements. So we need to tweak the system’s firewall to allow ICMPv6 traffic:

ip6tables -I INPUT -j ACCEPT -p icmpv6

In the IPv4 world we are used to simply denying all ICMP traffic, or allowing only very specific types of ICMP traffic. This is due to various remote attacks via ICMP messages, but IPv6 relies on several ICMP messages to operate correctly. Thus, we allow ICMPv6 traffic to pass through our firewall. Ultimately, my /etc/iptables/ip6tables.rules file looks like:

-A INPUT -j ACCEPT -p tcp -m tcp --dport 80
-A INPUT -j ACCEPT -p tcp -m tcp --dport 443
-A INPUT -j ACCEPT -p icmpv6
-A INPUT -j REJECT --reject-with icmp6-adm-prohibited
-A FORWARD -j REJECT --reject-with icmp6-adm-prohibited

The finaly piece of this puzzle is actually listening for the router advertisement messages. This is accomplished through the ndppd daemon:

  1. xbps-install -y ndppd
  2. echo "proxy eth0 {}" > /etc/ndppd.conf
  3. ln -s /etc/sv/ndppd /var/service

Done! At this point all that is left is to update service configurations to listen on the IPv6 address in addition to the IPv4 address. For example, the HTTP frontend in my HAProxy config required one new line:

frontend proxy
   bind 2001:19f0:5:23de:5400:ff:fe1e:54e2:80 v6only
   use_backend default-server

Well, there is one more thing: don’t forget to add a AAAA record in your DNS.

Conclusion §

Getting IPv6 enabled is really not difficult. The difficult part was tracking down all of the information. Pretty much every article I could find assumed the underlying distribution was already taking care of the details for you. None of them actually outlined the constituent parts those distributions use to make things work. The key was researching how router advertisements are handled by clients on an IPv6 network (routers would be using radvd instead of ndppd). I hope this article clears things up for others looking to do implement things on their own.