I run into this crap all the time with the /etc/network/interfaces file. Maybe your static IPv6 addresses aren't getting added to your network interface, or maybe Ubuntu gets stuck during booting at “Waiting for network configuration” for a few minutes.

Usually a problem like this is due to a misconfiguration in /etc/network/interfaces. Here are some tips I've learned about the interfaces file:

• Test it out by doing ifup -v eth0
  • This will show the commands being run and might give you a hint about what's broken
  • You may have to manually do what it takes to bring the interface down first, especially if there's a problem where the interface only goes halfway up and trying to bring it up repeats commands that have already been executed but wouldn't cause an error on the first boot.
• I've seen SolusVM generate an incorrect interfaces file with two problems:
  • “iface eth0 inet static” and “iface eth0:0 inet static” both had a gateway supplied. Multiple gateways don't work, so remove it from one. This can also be a problem if both eth0 and eth1 have a gateway, for example.
  • Under “iface eth0 inet6 static”, there was a /64 at the end of the “address” line, but the /64 is already automatically added because of the “netmask 64” line. Thus, the setup command had /64/64 at the end of the IPV6 address, causing a failure.
• /etc/network/interfaces is very picky about indentation and spaces–possibly only when Network Manager is involved. I think other mechanisms that use the file aren't picky, but Network Manager seems to be. This has caused problems on my desktop computers if I indent a line with four spaces instead of tabs, for example.