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SubjectRe: ARP - hardtype 6 on ether device / [kernelpatch]
Hi!

> > I contacted cabletron again; they provided a firmware upgrade for
> > the switches to me. The release note of the new firmware says that
> > it will send to arp replies to the ethernet (one with hard type 6
> > (IEEE802)
> > and another with hard type 1 (ethernet)) to support TCP/IP
> > stacks which are not rfc1042 comlpiant, too. I installed the
> > firmware - result it doesen't work much better than the old version.
> > I can't see the two arp replies with tcpdump or such.
>
> I.e. they cheated?

Perhaps, yes

>
>
> > Again, I did some work on sniffering. I asked myself why are the
> > other UNIX machines (AIX, DEC UNIX) are having no problems.
> > And at least why are also WinNT and Win95 working with this
> > cabletron-switch behaviour.
>
> Apparently they do not verify type.

Before I patched the kernel, I did some tests on the IBM AIX-Machines.
I find out that AIX also does ARP type checking. In my tests the
machine only accepted ARP type 1 an ARP type 6 packets on Ethernet.
ARP types of i. e. 4 or 5 are discarded. IBM refers here to RFC1042.

> > the arp reply is discarded by the linux kernel but accepted by
> > the other UNIX machines.
> > Any suggestions on this?
>
> Suggestion is to remember this sad fact, not to hurry and to think a bit.
> Linux checks address type for years and it did not result in any problems.
> To change kernel only because some vendor issued one more buggy
> switch is suspicious idea.
>

I looked through some RFCs to find out something about this behaviour.
There is the host requirements rfc [rfc1122] which says a little about
ethernet and IEEE802 encapsulation in Section 2.3.3. Here is a short
cut of this section:

----> snip <----
2.3.3 Ethernet and IEEE 802 Encapsulation

The IP encapsulation for Ethernets is described in RFC-894
[LINK:3], while RFC-1042 [LINK:4] describes the IP
encapsulation for IEEE 802 networks. RFC-1042 elaborates and
replaces the discussion in Section 3.4 of [INTRO:2].

Every Internet host connected to a 10Mbps Ethernet cable:

o MUST be able to send and receive packets using RFC-894
encapsulation;

o SHOULD be able to receive RFC-1042 packets, intermixed
with RFC-894 packets; and

o MAY be able to send packets using RFC-1042 encapsulation.

An Internet host that implements sending both the RFC-894 and
the RFC-1042 encapsulations MUST provide a configuration switch
to select which is sent, and this switch MUST default to RFC-
894.

Note that the standard IP encapsulation in RFC-1042 does not
use the protocol id value (K1=6) that IEEE reserved for IP;
instead, it uses a value (K1=170) that implies an extension
(the "SNAP") which can be used to hold the Ether-Type field.
An Internet system MUST NOT send 802 packets using K1=6.

Address translation from Internet addresses to link-layer
addresses on Ethernet and IEEE 802 networks MUST be managed by
the Address Resolution Protocol (ARP).
----> snip <----

So, why not to make linux accepting ARP type 6 on ethernet?

> Probably, we should remove check for address type at all,
> but it requires thinking and digging in docs.

hm, I don't know exactly!?

-Volker-


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