Saturday, September 22, 2012

IPv6 support in debian-installer, take 2

The IPv6 patch set for netcfg (part of debian-installer) has landed in Debian unstable. In follow-up uploads I diverged from the Ubuntu patch set a little bit:
  • I dropped the use of DUID-LL as the client identifier used by the DHCPv6 client. While basically a good idea, because it's predictable, it's also against the RFC to do that. We simply don't know if the network interface is permanently attached to the device. If the NIC that's used for DUID-LL generation is reused for installation in another box, the same identifier would be generated and stored on the machine. That's what DUID-LLT (the default in other software) avoids. As much as I loathe the replacement of the current MAC address-based scheme of DHCPv4 with DUID-LLT, it seems to be more correct to use that instead of DUID-LL, because that identifier is designed to be decoupled from the presence of the NIC. If you want predictable addressing, just enable SLAAC in your broadcast domain and use the EUI-64 address, which already contains the current MAC address.
  • If SLAAC is used, netcfg will activate IPv6 privacy extensions in the installed system by default. They can be turned off by editing /etc/network/interfaces post-installation. This will only affect outgoing traffic, incoming traffic can be addressed at the stable EUI-64 address. (We don't use a randomized interface identifier like Windows does.)
Thanks to Michael Tokarev's recent upload of busybox, we now also have a more featureful ping and ping6 in debian-installer's environment.

Known bugs:
  • As far as I can see, the DUID generated in the debian-installer environment is not copied into the installed system. Given the low prevalence of stateful DHCPv6 for servers I don't consider this a blocker, though.
  • If stateful DHCPv6 is requested by the router advertisement (other config flag being set), the DHCPv6 client will not time out and continuously retry to get an address.
  • According to Debian bug #688273 preseeding of netcfg/use_autoconfig does not work correctly. netcfg/disable_dhcp needs to be re-added as a deprecated preseeding option and mapped onto the use_autoconfig value until somebody comes up with a better scheme.
  • We should state in the installation guide that ftp.ipv6.debian.org and http.debian.net can both be used for IPv6-only installation. The installer does not show in the mirror list if a mirror is IPv6-capable.
If you could take another look at a current d-i daily, install a system with it and look if the installed system has a correct network configuration, that would still be appreciated. I also need somebody to test the GNU/kFreeBSD image. I fixed it up to build there but the result might be entirely wrong.

My sincere thanks go to Bernhard Schmidt and Karsten Merker, who successfully completed installations over IPv6 in various environments!

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