DHCPv6 and prefix length

"Petr Morávek [Xificurk]" xificurk at gmail.com
Sun Aug 22 22:23:58 UTC 2010

I'm running dhclient from dhcp package version 4.1.1_p1 for IPv6
configuration and I'm having trouble to get the right configuration - my
ISP uses subnet with 117 prefix length, but dhclient
has hardcoded value 64.

I have found these lines in client/dhc6.c:

/* Current practice is that all subnets are /64's, but
 * some suspect this may not be permanent.
client_envadd(client, prefix, "ip6_prefixlen",
	      "%d", 64);

This results in a wrong address configuration.

I've submitted a bug report regarding this issue to dhcp-bugs. It was
suggested to me to ask here for help, apparently there might be some
workarounds for this problem.
Currently I've implemented a nasty hack to the dhclient-script allowing
me to set the prefix length via the environment variable, but then it's
not much of a Dynamic Host Configuration.

The prefix length information comes in Router Advertisement and dhclient
does not try to dug out the information out of the system, nor request
the information on its own. As I understand it, there are currently no
plans for fixing this issue systematically in the main tree of dhclient.
I would really appreciate any advice on this matter leading to a correct
dynamic IPv6 configuration.

Petr Morávek

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