RE: Questions about ARP

RE: Questions about ARP

[Dave Ellis]

Steven Scholz wrote:

> Windows is checking for duplicate addresses using ARP packets
> with source _and_
> destination address set to the address in question. So of
> course Linux won't answer it!
>
> But I can't find a single word in the RFC2131
> (http://www.faqs.org/rfcs/rfc2131.html)
> saying that the
> Source Address _must_ set to 0.0.0.0!!!!

It is actually in RFC1541 "... The client may
   perform a check on the suggested address to ensure that the address
   is not already in use.  For example, if the client is on a network
   that supports ARP, the client may issue an ARP request for the
   suggested request.  When broadcasting an ARP request for the
   suggested address, the client must fill in its own hardware address
   as the sender's hardware address, and 0 as the sender's IP address,
   to avoid confusing ARP caches in other hosts on the same subnet."

Dave

Dave Ellis
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Re: Questions about ARP

[Dan Malek]

Dave Ellis wrote:

> It is actually in RFC1541 "... The client may .....

Thank you.


	-- Dan


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Re[2]: Questions about ARP

[Ricardo Scop]

It is _also_ in RFC2131:

"The client SHOULD perform a
check on the suggested address to ensure that the address is not
already in use.  For example, if the client is on a network that
supports ARP, the client may issue an ARP request for the suggested
request.  When broadcasting an ARP request for the suggested address,
the client must fill in its own hardware address as the sender's
hardware address, and 0 as the sender's IP address, to avoid
confusing ARP caches in other hosts on the same subnet."

-Scop.

Tuesday, August 12, 2003, 11:05:40 AM, you wrote:


DE> Steven Scholz wrote:

>> Windows is checking for duplicate addresses using ARP packets
>> with source _and_
>> destination address set to the address in question. So of
>> course Linux won't answer it!
>>
>> But I can't find a single word in the RFC2131
>> (http://www.faqs.org/rfcs/rfc2131.html)
>> saying that the
>> Source Address _must_ set to 0.0.0.0!!!!

DE> It is actually in RFC1541 "... The client may
DE>    perform a check on the suggested address to ensure that the address
DE>    is not already in use.  For example, if the client is on a network
DE>    that supports ARP, the client may issue an ARP request for the
DE>    suggested request.  When broadcasting an ARP request for the
DE>    suggested address, the client must fill in its own hardware address
DE>    as the sender's hardware address, and 0 as the sender's IP address,
DE>    to avoid confusing ARP caches in other hosts on the same subnet."

[]'s, Scop                            mailto:scop@digitel.com.br

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Questions about ARP

[Steven Scholz]

Hi there,

Microsoft uses "Automatic Private IP Addressing" to grab an IP address
if a DHCP request fails.

In the section "Resolving IP Conflicts" in
http://www.winnetmag.com/Articles/Index.cfm?ArticleID=7464
it says

How does the client know the IP address it's using isn't in use by
another machine? It uses a gratuitous Address Resolution Protocol
(ARP) to resolve potential conflicts. Let’s say the first client that
boots up wants to assign itself an IP address of 169.254.10.20. It
sends out a gratuitous ARP, but no one answers, so it keeps the
address ... Next, a third client boots up and picks 169.254.10.20, the
same address the first client chose. The first client tells the third
client that it's already using that IP address, so the third client
tries a different IP address and keeps it if there's no conflict.

I can see this behaviour if I connect an embedded DOS TCP/IP device to
a Windows XP computer (using a cross-linked twisted pair) and force
the IP of the DOS device to be the same as the one that the Windows XP
computer is going to claim. Fair enough.

But when I connect my MPC8xx board with Linux it seems that Linux is
not responding to the ARP requests done by the Windows machine.
Result: Both machine end up with the same IP address.

Did I miss something?
I did not find an kernel option saying something about ARP...

Any ideas?

Could someone maybe try this as well? Please?

Thanks a million,

Steven


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Re: Questions about ARP

[Dan Malek]

Steven Scholz wrote:

> But when I connect my MPC8xx board with Linux it seems that Linux is
> not responding to the ARP requests done by the Windows machine.
> Result: Both machine end up with the same IP address.

Fortunately, Linux isn't Windows......you are going to find lots of
Windows "standard" protocols that aren't documented nor supported by
anyone else.

> Did I miss something?

Well, Microsoft did.....there is RFC2131 that describes how you are
supposed to use an ARP to respond to a duplicate IPv4 used on the
network.  It is commonly used to ensure you don't have systems
manually configured or not honoring leases when DHCP is handing out
dynamic IP addresses.  Everyone else seems to implement this properly.

> I did not find an kernel option saying something about ARP...

That's because it implements the standard protocols by default.

> Any ideas?

Run DHCP or properly manually configure your Linux system.  I don't
know how old your Linux kernel is for the 8xx, but this support for
RFC2131 has been in Linux for a very long time.


Thanks.


	-- Dan


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Re: Questions about ARP

[Steven Scholz]

[-- Attachment #1: Type: text/plain, Size: 1686 bytes --]

Dan Malek wrote:

 > Steven Scholz wrote:
 >
 >> But when I connect my MPC8xx board with Linux it seems that Linux is
 >> not responding to the ARP requests done by the Windows machine.
 >> Result: Both machine end up with the same IP address.
 >
 >
 > Fortunately, Linux isn't Windows......you are going to find lots of
 > Windows "standard" protocols that aren't documented nor supported by
 > anyone else.
 >
 >> Did I miss something?
 >
 >
 > Well, Microsoft did.....there is RFC2131 that describes how you are
 > supposed to use an ARP to respond to a duplicate IPv4 used on the
 > network.  It is commonly used to ensure you don't have systems
 > manually configured or not honoring leases when DHCP is handing out
 > dynamic IP addresses.  Everyone else seems to implement this properly.

Finaly I found the point you're talking about.
Linux is answering the ARP packets with Source Address 0.0.0.0 (see net/ipv4/arp.c):

         /* Special case: IPv4 duplicate address detection packet (RFC2131) */
         if (sip == 0) {
                 if (arp->ar_op == htons(ARPOP_REQUEST) &&
                     inet_addr_type(tip) == RTN_LOCAL)
                         arp_send(ARPOP_REPLY,ETH_P_ARP,tip,dev,tip,sha,dev->dev_addr,dev->dev_addr);
                 goto out;
         }

Windows is checking for duplicate addresses using ARP packets with source _and_
destination address set to the address in question. So of course Linux won't answer it!

But I can't find a single word in the RFC2131 (http://www.faqs.org/rfcs/rfc2131.html)
saying that the Source Address _must_ set to 0.0.0.0!!!!

So the attached patch will make M$ Windows happy and APIPA working.

Thanks,

Steven

[-- Attachment #2: linux-2.4.20-APAPI.patch --]
[-- Type: text/plain, Size: 730 bytes --]

+ diff -u ./net/ipv4/arp.c.APAPI ./net/ipv4/arp.c
--- ./net/ipv4/arp.c.APAPI	Tue Aug 12 13:00:59 2003
+++ ./net/ipv4/arp.c	Tue Aug 12 14:32:40 2003
@@ -760,6 +760,18 @@
 		goto out;
 	}

+#define MAKE_MS_WINDOWS_HAPPY
+#ifdef MAKE_MS_WINDOWS_HAPPY
+	/* Special case: IPv4 duplicate address detection packet (MS Windows) */
+	if (inet_addr_type(sip) == RTN_LOCAL) {
+		printk("ARP: IPv4 duplicate address detection packet (MS Windows)\n");
+		if (arp->ar_op == htons(ARPOP_REQUEST) &&
+		    inet_addr_type(tip) == RTN_LOCAL)
+			arp_send(ARPOP_REPLY,ETH_P_ARP,tip,dev,tip,sha,dev->dev_addr,dev->dev_addr);
+		goto out;
+	}
+#endif
+
 	if (arp->ar_op == htons(ARPOP_REQUEST) &&
 	    ip_route_input(skb, tip, sip, 0, dev) == 0) {

Re: Questions about ARP

[Dan Malek]

Steven Scholz wrote:

> Windows is checking for duplicate addresses using ARP packets with
> source _and_
> destination address set to the address in question. So of course Linux
> won't answer it!
>
> But I can't find a single word in the RFC2131
> (http://www.faqs.org/rfcs/rfc2131.html)
> saying that the Source Address _must_ set to 0.0.0.0!!!!

I don't remember if it was in RFC2131 or RFC1541.  RFC2131 is an updated
version of RFC1541, so I thought the words would still be there.  In
any case it suggests as part of a DHCP the client tests to ensure the
address isn't already in use by sending a ARP using the sender hardware
address and setting the sender IP address to zero to avoid confusing
ARP caches in other hosts on the network.  I don't remember the exact
words, but RFCs tend to be terse and concise in the area where you
need to pay attention to detail :-)

I'd suggest using the linux kernel or some networking mailing lists
to discuss this.  I wouldn't apply this patch because it seems to be
wrong in the eyes of the RFCs, and I could never get it pushed into
the network portion of the source tree.

Thanks.


	-- Dan


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