[ale] IPv6 interface identifiers

[Derek Atkins]

Hi,

Michael Trausch <mike at trausch.us> writes:

> Yes, this network is a single physical segment. Link local addresses are
> limited to a single segment and I don't think that there is a way to route
> them.

Correct, you cannot route them.  Linux took that to mean that you must
specify which link to use because using the routing table could allow
them to be routed ala source routing.  And even if you only have a
single physical link, you may have multiple virtual links and still need
to specify which virtual link to use.

> I have been using IPv6 for a long time as well, but I have never really had a
> need to connect to a link local address before. I have tried to ping such
> addresses in the past as well, though, and an interface identifier is required
> to do so.

Ah, but have you been using it on Linux?  Honestly, I don't know when
this went into Linux, but it is definitely something that seems to be
limited to Linux.  Not all OSes use the interface identifier.  It's
certainly fouled up many-a-piece-of-software.   C.f. my issues with
postfix a month or so ago.

If you want to know *why* this is the case, I suggest you take it up
with the linux-networking guys.

-derek

> On Aug 17, 2012 9:03 AM, "Jeremy T. Bouse" <jeremy.bouse at undergrid.net> wrote:
>
>     On 08/16/2012 01:41 PM, mike at trausch.us wrote:
>     > Perhaps someone could explain something to me:  why are interface
>     > identifiers required for link-local communications, particularly on a
>     > system that has only a single network interface?
>     >
>     > e.g., I have a router that is using the address fe80::c9f:e0ff:fef1:ae2a
>     > on my network.  The system I am on uses fe80::6e62:6dff:fe53:103.  But
>     > if I want to reach the router by SSH, I must either SSH to its public
>     > address, or I must do this:
>     >
>     > ssh fe80::c9f:e0ff:fef1:ae2a%eth0
>     >
>     > Or I get an "invalid argument" error.
>     >
>     > The fact that I have only one interface shouldn't matter.  I thought
>     > IPv6 ND made it possible to hide exactly this sort of detail.
>     >
>     >       --- Mike
>     >
>             Are both machines on the same physical network segment? I mean are
>     the
>     both wired or both wifi? the LL IPv6 interface addresses are meant for
>     communication only on the same physical layer. My entire network here is
>     dual-stacked and I've never had to specify the interface identifier.
>     I've been working with IPv6 since '01 when I was working for NTT MCL
>     back in CA.
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-- 
       Derek Atkins, SB '93 MIT EE, SM '95 MIT Media Laboratory
       Member, MIT Student Information Processing Board  (SIPB)
       URL: http://web.mit.edu/warlord/    PP-ASEL-IA     N1NWH
       warlord at MIT.EDU                        PGP key available