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Re: Network routing methods

On Wednesday, July 23, 2003, at 08:35 AM, John Clark wrote:

I don't do this but a customer has done the following and gotten the
described behavior.

The customer has a Mac OS X system with two network interfaces attached
to the same physical network, and associated two successive IP addresses
in the same IP network. One adapter is a slow speed built-in 10/100 and
the other a 1 Gigabit.

As I say, I would not do this. The behavior the custerm observes is
that when Mac OS 9 based machines use the 'chooser' and the under
lying Appletalk namebinding-to-ip association the ip address is resolved
to the 'slow' speed interface. When the Mac OS 9 user explicitly uses
the Gigabit address, packets sent, according to the customer, are
taken up by the 'highspeed' connection, but replies are sent via the
slow speed connection.

I have not duplicated the customer's set up because I no longer
have Mac OS 9 systems setup convienently in my usual networking
environment.

The question is, is it possible to have such a split on the transfers?

This setup is not a supportable configuration. I'm not aware of any OS that will work (reliably) with this setup.

My immediate response to the customer was to place the two cards
on logically different networks, even though physically on the
same network 'cable'.

Yup.

The thought as to why such a thing may occur is on the Mac 9 to
Mac OS X transmissions the Mac OS 9 system uses given IP
address. On replies from the Mac OS X system, in looking up the ethernet
MAC address, the network stack uses the 'first connection found', which
is the low speed built-in interface. Since the Mac OS 9 system has
'arp'ed both interfaces have received that arp and then associated
the MAC address given with an IP address.

Look in xnu/bsd/netinet for ip_output.c, and follow the tracks there.

Routing in IP stacks works, typically, by determining the "best match" (longest bit-substring) between the destination address and the set of known routes in the system. The "default" route is that which has the lowest score possible (0); if there is no other match, the default route is chosen (not the "first"). If the destination is on the same subnet as an address assigned to the system, the interface to which that address is assigned is the one used (and therefore, if you have two addresses on the same subnet assigned to different addresses, the "first found" is used). Depending on your configuration, this may not work.

Regards,

Justin

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References: 
 >Network routing methods (From: John Clark <email@hidden>)



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