2012/01/28

M, A and O flags of Router Advertizement

For some time, it has been quite confusing to me about auto config (SLAAC) in IPv6. Windows 7 can support SLAAC but no DNS resolvers can be allocated. Mac OSX Lion and Linux can support SLAAC + RDNSS which assign IPv6 address and resolvers to client hosts via Router Advertisement. In case there is a DHCPv6 and RA Server in a network segment, things will be complicated to understand. How does a host choose whether to use SLAAC or DHCPv6 or both ? Hopefully, I came across some good description in Cricket Liu’s new book “DNS and Bind on IPv6”. In fact, the A, M, and O flags in RA message have something to do with resolvers.

The “M” flag - for “Managed Address Configuration,” tells hosts that DHCPv6 is available for both address assignment and network parameters (including resolver configuration).

The “A” flag - , for “Autonomous Address Configuration,” tells hosts that SLAAC is available for address assignment and network parameters (possibly including resolver configuration).

The “O” flag - for “Other Stateful Configuration,” tells hosts that DHCPv6 is available for network parameters other than address assignment (that is, to be used together with SLAAC in the hybrid method described earlier).

I still think that Microsoft should release patches to Windows 7 such that Windows 7 can support SLAAC + RDNSS. Think it this way, if there needs a DHCPv6 for resolvers assignment working side by side with RA Server, what is the purpose of developing auto config in IPv6.

1 comment:

Revellion said...

I do agree, it's a shame that Windows 7 doesn't support both modes of operation. since i can see SLAAC + RDNSS being a very common "home/SOHO" configuration.

and SLAAC+DHCPv6 or fully Managed DHCPv6 being used more in the medium-big corporate settings.

Not to mention the way faster configuration SLAAC+RDNSS is in most cases. In my labs it's just a few milliseconds from cable plugged in until i can access IPv6 hosts by their DNS name and connect.

compared to the slow IPv4 DHCP and DNS gathering which can take a lot longer in comparison to get to the same point.