On Tue, 31 May 2011, Markus Hanauska wrote:
On 2011-05-30, at 22:05 , Ray Hunter wrote:
Which source address (SLAAC/DHCPv6) would be used by the client for an
outbound session if a SLAAC address and a DHCPv6 were both configured
on the same link and with the same prefix, in the absence of a flag?
As I already said in my previous mail: The DHCP address should win by
default; that is, of course, unless the admin/user of the node has
configured anything else. If a network runs a DHCP server that hands out
addresses, network admins probably do so for a reason. The SLAAC one is
valid and inbound traffic might arrive for it, but for outgoing traffic,
some RFC should simply have given an order of preference:
1. Manual configured IP
2. DHCP
3. SLAAC with Privacy Extension
4. SLAAC with Interface ID
I don't see the exact argument behind this ordering.
What about the ordering, if you get more than one DHCP addresses?
From operating system point of view all the addresses are the same. How do
you select which one to use? The RFC 4941 states, that usage of Privacy
enhanced addresses can be selected based on the destination addresses.
If you want consistent behaviour, the application should bind() to a
specific address.
There are a lot of operational people who currently rely on having
predictable IP addresses for accounting, audit, scripting, firewall
rules, neighbor filtering, fault tracing, reverse DNS, policy based
routing, setting DSCP in QoS, any other number of ACL's ....
I can also totally agree with your statement above - but there is one
thing you are missing: It is not only important, to have predictable IP
addresses in some scenarios, it is also important to have "constant" IP
addresses, since even if the address is predictable, it will be a
problem if it is going to change every couple of months. SLAAC addresses
based on interface IP are in fact predictable; but they not constant! On
the other hand, when using DHCP, it takes me 2 minutes to change the
stored interface ID of a DHCP client in the server config to assure,
that even after an interface ID change, the client still gets the same
IP address as before. How will you do this with SLAAC?
Not with SLAAC, but static addresses. Anyway usually the requirement is
not the "constant" IP, but accountability of the user/server.
Best Regards,
Janos Mohacsi
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