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Re: IPv6 RA vs DHCPv6 - The chosen one?
From: Tomas Podermanski <tpoder () cis vutbr cz>
Date: Thu, 22 Dec 2011 21:46:17 +0100


On 12/22/11 12:18 AM, Owen DeLong wrote:
The long answer is:

I completely disagree with opinion that both DHCPv6 and RA (SLAAC)
should be supported. It is easy to say that both have place but it has
some consequences. I and my colleagues have been working on deploying
IPv6 for a few years and from the operation perspective we conclude into
a quite clear opinion in that area. Both SLAAC and DHCPv6 uses a
opposite principles although the goal is just one. DHCPv6 is based on a
central DHCPv6 server that assigns addresses. SLAAC does opposite -
leaves the decision about the address on a client side. However we have
to run both of them in a network to provide all necessary pieces of
information to the clients (default route and DNS). This brings many
implementation and operational complications.

I agree that the requirement to run both is broken. I don't agree that this
means we should remove the option of using SLAAC in environments
where it makes sense.

- Clients have to support both SLAAC and DHCPv6 to be able to work in
 both environments

It makes the client side more difficult to implement (=more expensive). 
What worse SLAAC and DHCPv6 are differed protocols, so there is bigger
probability for attacks (overflow, flood etc.). For example in UNIX-like
systems autoconfiguration have to be solved by 3 parts of the system:

1. some SLAAC options are usually processed by a kernel (address
selection, MTU) and behavior of that process can be changed via sysctl
2. some SLAAC options are processed by rdnssd daemon (processing DNS
3. DHCPv6 options are processed byt dhcpv6-client

All those parts have to cooperate together. At the first sight it is
obvious that there is pretty good probability that something can go
wrong. Troubleshooting then is really piece of cake. For example in IPv4
environment we have following scenario:

1. DHCP options are processed by dhcp-client

- There must be solved a situation what to do when SLAAC and DHCPv6
 provides some conflict information (quite long thread with various
 can be found at 
SLAAC and DHCPv6 can't really provide conflicting information unless
the router is misconfigured. Even if a host gets different answers for the
same prefixes from SLAAC and DHCP, it should be able to use both
host addresses. There's the question of source address selection, but,
the answer to that question at the IETF level should only be a matter
of what the default answer is. There are configuration options for setting
host source address selection priorities.

I am not thinking about address. It is the easier part - we can use all
provided. There are other options like DNS servers, search list, NTP
servers, ...

- The first hop security have to be solved twice - for SLAAC and for
DHCPv6. Both
 of then uses a differed communication way. SLAAC is part of ICMPv6,
but DHCPv6
 uses "own" UDP communication what does not make things easier.
Solved for SLAAC -- SEND.
Allegedly, there is Secure DHCPv6 for DHCP, but, I haven't seen any
actual implementations yet.

Right, very easy to write but pretty difficult (impossible) to use
today. None of operating systems supports SEND  and some will probably
never be:

as we can find http://technet.microsoft.com/en-us/library/bb726956.aspx
However, Microsoft does not support SEND in any version of Windows.

I have found only one implementation for Linux
(http://amnesiak.org/NDprotector/) that is not ready for production. So
we can not think seriously about SEND today. SEND also brings another
set of problems like certificate management, etc., but is a little
differed story.

- SLAAC is usually processed in a kernel, DHCPv6 is usually run as a
 process in the user space. Diagnostic and troubleshooting is more
That seems like an argument for SLAAC, if anything.

- DHCPv6 is currently tied with SLAAC (M,O flags), what means that
 a DHCPv6 client have to wait until some RA message arrives to start DHCPv6
 discovery. That unnecessary prolongs whole autoconfiguration process.
While I agree with you that the standard is broken in this regard, there is at
least one OS vendor that already violates that rule anyway.
Some other issues are also described in [1].

I personally prefer to bury SLAAC once forever for several reasons:
- In fact SLAAC does nothing more what DHCPv6 can do.
Yes, but, it does it in a much simpler way with a lot less overhead which
can be a benefit in some environments.

I have to admit that less overhead is one of benefit of SLAAC. But
having experience with DHCP(v4) all devices that we have today (phones,
cameras, etc.) do not have a problem to process DHCPv4 packets, so there
is no reason why same devices could not do it for DHCPv6. The sensor
networks mentioned in one mail before is a very special case of use. I
believe SLAAC might be useful there but is not typical case.

- SLAAC is quite difficult to secure. One (really only ONE)  RA packet
can destroy
 the IPv6 connection for all clients in a local network just in a few
This is what RA-Guard is for and it's quite simple to deploy. SEND makes
it even better, but is a bit more complicated.

I can not agree. Access switches usually do not support RA-Guard or
IPv6-ACLs. For example looking at Cisco portfolio only  4900, 4500,
6500, 3750 series supports RA-Guard. In HP 54xx, 82xx, 66xx, 35xx series
(all with K software). All this series are not suitable (means too
expensive) in a role of access switches. What worse RA-Guard can be
easily bypassed
So even if you buy 3 times more expensive switches it does not help you
:-(. As I wrote before - SEND is not an option today.

 It also happens accidentally by "connection sharing" in Vista, Win7

This is an argument for burying Windows, not an argument for burying
SLAAC. It's not like ICS in IPv4 didn't create rogue DHCP servers. If you
were to bury SLAAC, Micr0$0ft would simply switch to breaking DHCPv6
instead of breaking SLAAC.

- The full protection against that behavior it's impossible today.
RA-Guard or
 PACL can be bypassed using extension headers or fragmentation
Yes and no. RA Guard implementations are getting better at addressing
those issues.

How that is getting better. Can you provide an example.

- With SLAAC is difficult to implement security features like ARP/ND
 protection/inspection, IP source guard/Dynamic lock down, because
 all this techniques are based on a MAC-IP database created during
 a communication with a DHCP server. There are some attempts (ND
protection, SAVI)
 but it does not provide the same level of security.
Most sites don't need that level of security. I agree there should be a
way to disable SLAAC reliably at a site as a policy matter, but, frankly
the techniques you're talking about come in one of two flavors:

      1.      They dynamically enable the switch to accept packets from
              a client, in which case, SLAAC based clients would be blocked
              until they registered with DHCP anyway.
      2.      They don't effectively block an attacker who cobbles his own
              address even without SLAAC.

In the former case, you get the security you want and force DHCP anyway,
so I don't see a problem. In the latter case, you only had the illusion of
security to begin with, so, SLAAC just makes it easy to disillusion you.

Agree. The firs option is the answer but you have to have DHCPv6 only

- Just the same technique was introduced in IPv4 as Router Discovery
(RFC 1256).
 Nobody uses it today. Do we really need go through same death way again?
 (Oh right, we are already going :-( )
Not a fair comparison. There were a number of additional issues with 1256 that
prevented it from gaining acceptance in IPv4.

Comparing to SLAAC, DHCPv6 have several advantages:
- DHCPv6 is very similar to DHCP(v4) and many people are used to using it.
That just makes it familiar, not necessarily better for all environments.

- DHCPv6 can potentially do much more - for example handle an information
 for PXE, options for IP phones, prefix delegation.
True, but, that comes at a cost of complexity and overhead which may not be
desirable in all environments.

As I wrote before. I do not think that overhead is an issue today.

- DHCPv6 allows handle an information only for some hosts or group of
 hosts (differed lease time, search list, DNS atc.). With SLAAC it is
 impossible and all host on a subnet have to share the same set of
 the configuration information.
Which is not an issue in 99+% of environments.

- Frankly said, I have not found any significant benefit that SLAAC brings.
Perhaps you have not, but, others have. I have seen environments where
SLAAC is much more useful than DHCPv6. I've seen environments where
DHCPv6 is needed.

It is true today, because not all operating systems supports DHCPv6. In
many cases DHCPv6 is not an option.

Unfortunately there is another issue with DUIDs in DHCPv6. But it is a
little bit differed tale.

At the beginning the autoconfiguration was meant as easy to use and easy
to configure but the result turned out into kind of nightmare. For those
who do not know what I am talking about I prepared two images. The first
one shows necessary communication before first regular packet can be
send over IPv4 (http://hawk.cis.vutbr.cz/~tpoder/tmp/autoconf/IPv4.png)
and just the same thing in IPv6
(http://hawk.cis.vutbr.cz/~tpoder/tmp/autoconf/IPv4.png). In IPv4 we
have very simple answer if somebody asks for autoconfiguration  = use
DHCP. In IPv6 the description how things work have to be written into
more than 10 pages [1]. I believe that is not what we really wanted.

That's no really a fair characterization. Yes, DHCPv6 is more complex
than DHCPv4, but, not significantly so.

In reality it can be summed up relatively quickly:

1.    Choose link local address (fe80::EUI64)
2.    Send RS packet to all-routers multicast address
3.    Receive one or more RA packets.
      a. if Packet contains prefix information:
              i.      Set timers, apply addresses to interfaces
                      (first regular packet can be sent at this point)
      b. If packet has O bit set:
              i.      Send DHCPv6 request to DHCP server
              ii.     Get response
              iii.    Configure accordingly.
                      (If a was false (a and b are not mutually exclusive), then
                      you can now send your first regular packet).

Yes, there are a few corner cases not completely addressed above,
but, unless you're building the software to implement the standards,
they are mostly irrelevant. Even if you add them in (interactions between
the M, A, and O bits), you can still describe it in about a page, not

And when we compare it with IPv4

- send DHCP request to DHCP server
- get response
- configure accordingly

No waiting for RA packets, no additional "IFs", no additional conditions
and all corner cases are solved. Why it can not work similar in IPv6?  I
know, there maybe is many  reasons :-).


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