On Tue, Jul 21, 2015 at 2:03 PM, dbextern <[email protected]> wrote:
> Hello!
>
> For our link local functionality I'm using udhcpc together with zcip out of
> busybox on a Blackfin BF-537 CPU without MMU.
>
> The base functionality is there.
> But when I connect two networks with stable IP addresses, and both networks
> had the same IP(s) in them, the conflict stays unresolved.
> My setup is one master PC or MAC and several embedded boards.
>
> What I would expect (following RFC3927
> https://tools.ietf.org/html/rfc3927#page-10[https://3c.gmx.net/mail/client/dereferrer?redirectUrl=https%3A%2F%2Ftools.ietf.org%2Fhtml%2Frfc3927%23page-10];
> chapter 2.x and 4) is that after a while the ARP messages will resolve the
> problem because the still running zcip sees ARP responses on it's own IP and
> reacts accordingly.
> What really happens is:
> * Windows only sends broadcast ARP requests as long as it never got an
> answer. After that there is only unicast. And the requested device answers
> with a unicast as well.
> * the MAC always sends broadcasts. And both embedded devices with the
> conflicting IP answer. But with a unicast ARP reply to the MAC.
>
> As I understand the specification (last paragraph in chapter 2.5) then all
> ARP messages between devices with link local addresses should be link layer
> broadcasts.
> Did I get that right? And if yes, why does zcip not follow that rule?
zcip does use bcast for all packets it sends:
# tcpdump -nliwlan0 -s0 -vv -e arp
tcpdump: listening on wlan0, link-type EN10MB (Ethernet), capture size
65535 bytes
<zcip runs>
02:49:51.078369 00:04:e2:64:23:c2 > ff:ff:ff:ff:ff:ff, ethertype ARP
(0x0806), length 42: arp who-has 169.254.194.171 tell 0.0.0.0
02:49:52.391711 00:04:e2:64:23:c2 > ff:ff:ff:ff:ff:ff, ethertype ARP
(0x0806), length 42: arp who-has 169.254.194.171 tell 0.0.0.0
02:49:54.254628 00:04:e2:64:23:c2 > ff:ff:ff:ff:ff:ff, ethertype ARP
(0x0806), length 42: arp who-has 169.254.194.171 tell 0.0.0.0
02:49:55.305731 00:04:e2:64:23:c2 > ff:ff:ff:ff:ff:ff, ethertype ARP
(0x0806), length 42: arp who-has 169.254.194.171 (00:04:e2:64:23:c2)
tell 169.254.194.171
02:49:57.307788 00:04:e2:64:23:c2 > ff:ff:ff:ff:ff:ff, ethertype ARP
(0x0806), length 42: arp who-has 169.254.194.171 (00:04:e2:64:23:c2)
tell 169.254.194.171
02:49:59.309844 00:04:e2:64:23:c2 > ff:ff:ff:ff:ff:ff, ethertype ARP
(0x0806), length 42: arp who-has 169.254.194.171 (00:04:e2:64:23:c2)
tell 169.254.194.171
> And a more general question: when I handle an ARP packet in zcip, how does
> the kernel know not to work on it as well?
I don't know...
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