ipv6 - New subnet mask too short

Alain Williams addw at phcomp.co.uk
Fri Oct 26 18:51:34 UTC 2012

On Fri, Oct 26, 2012 at 02:29:21PM -0300, Bruce Hudson wrote:
> > I am trying to configure hdcp for IPv6 I am getting the following error:
> > 
> >     dhcpd: /etc/dhcp/dhcpd6.conf line 59: New subnet mask too short.
> >     dhcpd: subnet6 2001:4d48:ad51:2f00::5:1/112 
>     The error is telling you that you have bits set in the "host" part 
> of the CIDR address; expected when specifying an interface address but
> not the subnet address. Drop the "5:1" from your address, ending with
> the "::" (or explicitly add a "0"). 

Thanks ...

OK: that gets rid of that error, but it still does not work.

I suppose that I should have given more of a description & more of the config file.

The machine has several interfaces, I want to serve up different IP addresses to
the machines that hang off the different interfaces.  By removing the full IPv6
address DHCP does not know which interface the parameters belong to, so I get
the warning:

    Warning: subnet 2001:4d48:ad51:2f00::/32 overlaps subnet 2001:4d48:ad51:2f00::/32

and then later:

     No subnet6 declaration for eth1 (fe80::2e0:50ff:fe64:10b).
     ** Ignoring requests on eth1.  If this is not what
        you want, please write a subnet6 declaration
        in your dhcpd.conf file for the network segment
        to which interface eth1 is attached. **

With a similar message about br0.

[ also similar messages about eth0 & others where I do not want DHCP to listen
  to anyway - but it would be nice to loose the (irrelevant) warning messages. ]

This is the sort of thing that I use with the IPv4 instance of DHCP:
subnet netmask {
subnet netmask {

So the bottom bit of the config file looks like:

# eth1 has address 2001:4d48:ad51:2f00::5:1/112
subnet6 2001:4d48:ad51:2f00::/112 {

        range6 2001:4d48:ad51:2f00::5:8000 2001:4d48:ad51:2f00::5:c000;

        option dhcp6.name-servers 2001:4d48:ad51:2f00::5:1;


# Where the server is to listen for requests - on br0, address 2001:4d48:ad51:2f00::6:1/112:
subnet6 2001:4d48:ad51:2f00::/112 {
        option dhcp6.name-servers 2001:4d48:ad51:2f00::6:1;

        range6 2001:4d48:ad51:2f00::6:8000 2001:4d48:ad51:2f00::6:c000;

Alain Williams
Linux/GNU Consultant - Mail systems, Web sites, Networking, Programmer, IT Lecturer.
+44 (0) 787 668 0256  http://www.phcomp.co.uk/
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