Subject: Re: IPv6 Loopback Address Range

From: Erik Kline <[email-masked]>
Date: Tue, 10 Nov 2009 02:04:36 +0000

2009/9/18 Jeroen Massar <[email-masked]>:
> Vijayrajan ranganathan wrote:
>> Hi,
>>    If I want to use more than 1 loopback IPv4 address, I can
>>    assign one from 127.0.0.0/8 address range.
>>
>>    Does IANA reserve some IPv6 address range for loopback communication?
>>    If not, what is the best address range to use for assigning such an
>> IPv6 address?
>
> As Chris mentioned there is only 1 "loopback" address, not a full /8
> anymore.
>
> Most very likely you will want these loopbacks, like everything else, to
> be globally unique. As such ULA (RFC4193) to the rescue, or just take a
> /64 or something from the address space you get from your ISP/RIR/etc.
>
> Greets,
>  Jeroen
>
> http://www.sixxs.net/tools/grh/ula/ for a handy generator/registration.
>
>
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>

As a random example of a use case where having something node-local
larger than a /128 would be helpful I offer a situation that I ran
into in software testing environments.  I've definitely seen very
useful testing code start several programs on various 127.0.0.0/8
addresses and then set a functional test in motion.  For example, one
might start a DNS server on 127.0.0.1, another one on 127.0.0.2 with
different configuration elements, and a resolver client on 127.0.0.3.
The test could then verify source-address filtering code or
rate-limiting code in the server or destination address selection code
in the resolver client or whatever.

The nice thing about this is that all these networked elements never
leave the host and the test infrastructure doesn't have to have the
ability to start these programs on other nodes (presumably relying on
ssh or something).  Being able to simulate a network within a node is
very handy.  Trying to make this work with IPv6 is much more
difficult.  One approach is to use the global scope IPv6 addresses
from other interfaces but your testing capability is then limited to
the number of appropriately-configured interfaces on a testing host.

I would actually find it very useful to have a node-local segment for
cases where simulating a network or having multiple node-local
addresses comes in handy (I'm sure there are others besides the one
I've mentioned here).  f700::/8 anyone?  ;-)