----- Original Message -----
From: "Simon Goldschmidt" <[email protected]>
To: "Mailing list for lwIP users" <[email protected]>
Sent: Tuesday, November 16, 2010 8:01 PM
Subject: Re: [lwip-users] OT: Is the AutoIP range a special case IP
addressrange?
David Empson <[email protected]>
OT: Is the AutoIP range a special case IP address range?Bill Auerbach
wrote:
I just found out through testing that the PC with a NIC in the
192.168/16 subnet (netmask 255.255.255.0) will ping and connect to
devices on the
physical network in the AutoIP range (169.254/16). This is on a WinXP
PC. I
find the ARP table contains AutoIP addresses and I can ping and open
connections to the device at the AutoIP address.
Is this something that is expected to occur?
Yes, it's expected behaviour.
Is it specific to Windows?
No, it's defined in RFC 3927.
I've not come across any RFC that mentions that this 'bridging' should
or
can occur.
As David said, no bridging required. Here's an excerpt from RFC 3927:
snip >>>>
1.8. Communication with Routable Addresses
There will be cases when devices with a configured Link-Local address
will need to communicate with a device with a routable address
configured on the same physical link, and vice versa. The rules in
Section 2.6 allow this communication.
This allows, for example, a laptop computer with only a routable
address to communicate with web servers world-wide using its
globally-routable address while at the same time printing those web
pages on a local printer that has only an IPv4 Link-Local address.
<<<< snap <<<<
Windows might use a 2nd IP address in the AutoIP range, but that's not
necessary
if AutoIP is implemented correctly in all devices: IP traffic to
Link-Local address
should mainly always be sent directly by using a local ARP request.
I hadn't read that RFC in detail. It turns out I was wrong on that point.
I've now confirmed the behaviour, using my MacBook Pro running Mac OS X
10.6.5, a PC running Windows XP SP3, and an old PowerMac running Mac OS 9.1.
1. If the Mac OS X computer is configured with a static IP address, it is
able to communicate with any device using an AutoIP address. Network traffic
is sent by Mac OS X using its static IP address. The computer does not need
to allocate an AutoIP address. ARP requests look like "who has 169.254.x.y
tell 192.168.z.w".
2. If the Mac OS X computer is configured with an AutoIP address, it is able
to communicate with Windows XP using a static IP address.
3. Windows XP behaves exactly the same as Mac OS X. Communication works
whether the computer is configured with a static IP address or AutoIP
address.
4. If the Mac OS 9 computer is configured with a static IP address, it is
unable to communicate with any computer that is using an AutoIP address.
This appears to be a limitation of the Mac OS 9 TCP/IP stack, as Wireshark
shows that pings are being sent by the other computers, but the Mac OS 9
computer is not replying to them, and it doesn't send any pings.
5. If the Mac OS 9 computer is configured with an AutoIP address, everything
works, but I noticed one oddity: ARP replies sent by the Mac OS 9 computer
use an Ethernet broadcast. It doesn't do this if configured with a static IP
address.
Mac OS 9 predates RFC3927 by several years, so I expect (4) and (5) are
abnormal behaviour due to an old implementation of AutoIP.
BTW: This does *not* work for me under Windows XP, where I have to
manually
configure a 2nd address in the AutoIP range. Which version of windows were
you
using, Bill and David?
Works for me on Windows XP SP3. Behaviour appears to be identical to Mac OS
X.
The only issue I noticed with Windows XP is a delay when changing the
network configuration, before it is able to communicate (in one case it was
about 30 seconds). I waited until ipconfig /all showed valid IP addresses
before I was able to test properly.
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