Wrong prefix length is advertised

Simon Hobson dhcp1 at thehobsons.co.uk
Tue Mar 29 19:09:06 UTC 2016

Juergen Bachsteffel <juebac at web.de> wrote:

> Thank you for yourexplanations. Yes, that helped a bit!

Ah good, it's a relief that it made sense ;-)

> On my Linux-Client with Network-Manager I have two options for IPv6.
> "Automatic" and "Automatic, DHCP only".
> With 'Automatic' the client gets a IP-address from radvd

Being pedantic, it'll get a prefix from radvd - but it doesn't get an address from it. The address will come from autoconfiguration or a managed process like DHCP.

> and there is an
> entry when I type: ip -f inet6 route
> fda0:65da:e55a:1::/64 dev enp0s25  proto kernel  metric 256  expires
> 86375sec

Right, so it's got a local prefix.

> With 'Automatic, DHCP only' the client gets an IP-address from the
> DHCP-Server and the entry is missing! (See the first output of ip -6 route).

OK, so it's not configured any local prefixes. To me, "DHCP only" implies just that - so perhaps it's ignoring radvd with that setting.

Now, bearing in mind that I'm very much not an expert, and have quite limited experience ...
From memory there's a flag in RAs which if set tells the clients that the network is managed - ie there's an address management process (eg DHCP) running and that the client shouldn't autoconfigure it's address.
Just checking an radvd server I have running, I have these in the config file (I'm using auto config on that network) :
#		AdvManagedFlag on;
#		AdvOtherConfigFlag on;
#		AdvAutonomous off;

I'd suggest checking the manual for those options.

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