disabling client use of SLAAC

Mikael Abrahamsson swmike at swm.pp.se
Fri Mar 5 10:24:54 CET 2010


We're trying out deployment scenarios and the first one was to try to 
mimic our current model with DHCPv4 but move that to DHCPv6.

We have successfully gotten a 1841 using IOS 15.0(1)M to do DHCPv6 
relaying to a wide-dhcp6-server running on a linux machine and in this 
combination handing out IPv6 addresses statefully.

On the cisco router we have:

interface FastEthernet0/1
  encapsulation dot1Q 11
  ipv6 address YY:XX::1/64
  ipv6 enable
  ipv6 nd autoconfig default-route
  ipv6 nd managed-config-flag
  ipv6 nd other-config-flag
  ipv6 nd ra interval 10
  ipv6 dhcp relay destination XX:YY::ZZ

On a linux ubuntu 9.10 machine (2.6.31 kernel) the default when running 
wide-dhcpv6-client is that it will get stateful IPv6 address from the 
dhcp6s scope and successfully set /etc/resolv.conf with information 
received via DHCPv6. It will also successfully get the on-wire prefix from 
RAs, and everything is fine and dandy apart from that the client by 
default will still do SLAAC as well, and so it gets two IPv6 addresses, 
one with SLAAC and one from DHCP. Putting 0 into 
/proc/sys/net/ipv6/conf/eth0/autconf will stop SLAAC, so then it does what 
we want, but would like to avoid changing defaults in the OS. We haven't 
tested Win7.

Looking at the RAs from the router, it has the "auto" flag set on the 
prefix info option, is this the flag that indicates if SLAAC is ok or not?

I have not been able to find any information on this and if indeed that 
flag tells clients if SLAAC should be done or not, then how do I disable 
that flag in the router?

The M and O flags says "get info via DHCP", but it seems they don't mean 
"do NOT use SLAAC" if I read http://www.ietf.org/rfc/rfc2462.txt 

So bottom line, how to make clients not use SLAAC with a Cisco router?

Mikael Abrahamsson    email: swmike at swm.pp.se

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