Today I set out to figure out how to get outbound IPv6 connectivity working with the Docker setup for Mastodon.
So, as we all know Ubuntu 14.04 was released today. I downloaded the server ISO to test in VirtualBox.
Let us see what we have here:
ss@trusty-testing:~$ cat /etc/lsb-release DISTRIB_ID=Ubuntu DISTRIB_RELEASE=14.04 DISTRIB_CODENAME=trusty DISTRIB_DESCRIPTION="Ubuntu 14.04 LTS" ss@trusty-testing:~$ ip -6 addr show dev eth0 2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qlen 1000 inet6 2001:470:1d:96b:70bb:7393:2071:faa2/64 scope global temporary dynamic valid_lft 597675sec preferred_lft 78675sec
Wait what? Am I going blind or is that an IPv6 temporary address  on a supposedly server image?
ss@trusty-testing:~$ sudo sysctl -a | grep tempaddr net.ipv6.conf.all.use_tempaddr = 2 net.ipv6.conf.default.use_tempaddr = 2 net.ipv6.conf.eth0.use_tempaddr = 2 net.ipv6.conf.lo.use_tempaddr = 2
What the hell? Not only did they leave temporary addresses turned on, they set the sysctl value at 2 which means that the system will prefer temporary addresses over standard ones for making connections. 
I asked around and apparently this is the case on Ubuntu 12.04 server as well.
ss@ubuntu-testing:~$ cat /etc/lsb-release DISTRIB_ID=Ubuntu DISTRIB_RELEASE=12.04 DISTRIB_CODENAME=precise DISTRIB_DESCRIPTION="Ubuntu 12.04.4 LTS" ss@ubuntu-testing:~$ sudo sysctl -a | grep tempaddr net.ipv6.conf.all.use_tempaddr = 2 net.ipv6.conf.default.use_tempaddr = 2 net.ipv6.conf.eth0.use_tempaddr = 2 net.ipv6.conf.lo.use_tempaddr = 2
So that is two LTS server releases with IPv6 temporary addresses turned on and set to 2.
Why are temporary addresses bad on a server?
Unpredictability – anything depending on source address validation. Even SLAAC addresses are more predictable because they can be calculated given the MAC address of the NIC.
Ideally, you should be configuring your server’s addresses statically. Leaving temporary addresses turned on on a server image is just a bad default.
 – https://tools.ietf.org/html/rfc4941
 – http://ipv6int.net/systems/linux-ipv6.html#privacy