Re: DNS discovery

[Iljitsch van Beijnum <iljitsch@muada.com>]

On 12-nov-03, at 14:22, Ted Lemon wrote:

>> Reading RFC 2462 will clear that confusion right up. There is an 
>> explicit bit in router advertisements that tells hosts whether they 
>> should use a stateful mechanism

> The stateful vs. stateless bit refers to address configuration in the 
> sense of getting an IP address for the node, not getting an IP address 
> to which to send DNS requests.   When the stateful bit is set, you are 
> supposed to use some stateful address configuration mechanism to get 
> an IP address to use when communicating on the network.

This is from RFC 2461, [ADDRCONF] refers to RFC 2462:

4.2.  Router Advertisement Message Format

    Routers send out Router Advertisement message periodically, or in
    response to a Router Solicitation.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Type      |     Code      |          Checksum             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Cur Hop Limit |M|O|  Reserved |       Router Lifetime         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Reachable Time                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          Retrans Timer                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   Options ...
      +-+-+-+-+-+-+-+-+-+-+-+-

[...]

       M              1-bit "Managed address configuration" flag.  When
                      set, hosts use the administered (stateful) protocol
                      for address autoconfiguration in addition to any
                      addresses autoconfigured using stateless address
                      autoconfiguration.  The use of this flag is
                      described in [ADDRCONF].

       O              1-bit "Other stateful configuration" flag.  When
                      set, hosts use the administered (stateful) protocol
                      for autoconfiguration of other (non-address)
                      information.  The use of this flag is described in
                      [ADDRCONF].

> Your error may be in thinking that DHCPv6 is always stateful, and 
> perhaps in not understanding what stateful means

Whether or not DHCPv6 is stateful doesn't make a lot of difference with 
regard to the problem we're facing. If we assume that hosts will be 
using router advertisements for autoconfiguration, then adding DHCP for 
DNS configuration is exactly that: adding another protocol. This is bad 
because there is now an additional mechanism that can fail and there is 
additional delay in configuring.

Now obviously doing this using a heavy version of DHCP is worse than 
doing this using a light version of DHCP, but the fundamental problem 
is the same in both cases. The only reason I use the words stateful and 
stateless during this discussion is because this is what's in the RFCs.

While I can relate to the slippery slope and implementing certain 
functionality just once arguments, I strongly believe that pragmatic 
operational arguments (not having an additional protocol on the wire) 
should carry the most weight in this very particular case.

> I've heard you repeat several times that the stateless bit in the 
> router advertisement can be used to say which mechanism to use, but 
> that is completely wrong,

I beg to differ. I believe the above excerpt from RFC 2461 proves my 
case.

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