Messages in this thread | | | Date | Fri, 16 Nov 2007 09:30:11 +0100 | From | "Jonas Danielsson" <> | Subject | Re: [PATCH] net/ipv4/arp.c: Fix arp reply when sender ip 0 |
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2007/11/16, David Miller <davem@davemloft.net>: > From: "Jonas Danielsson" <the.sator@gmail.com> > Date: Thu, 15 Nov 2007 22:40:13 +0100 > > > Is there a reason that the target hardware address isn't the target > > hardware address? >
> Because of this, in cases where a choice can be made Linux will > advertise what is most likely to result in successful communication. > > This is likely why we are changing that target address to the one of > the interface actually sending back the reply rather than the zero > value you used. > > In fact I think this information can be useful to the sender of > the DAD request. >
There seem to be some confusion about what my patch really does. It does not set the hardware address to a zero value.
An example to illustrate:
We have two computers. Computer A IP: 192.168.0.1 HW: 00:AA:00:AA:00:AA
Computer B IP: None HW: 00:BB:00:BB:00:BB
Computer B want to find out if IP 192.168.0.1 is free for use. It sends an arp-request.
Request: Opcode: request (0x0001) Sender HW: 00:BB:00:BB:00:BB Sender IP: 0.0.0.0 Target HW: 00:00:00:00:00:00 Target IP: 192.168.0.1
The reply from the Linux kernel in computer A, before the patch would look like:
Reply: Opcode: reply (0x0002) Sender HW: 00:AA.00:AA:00:AA Sender IP: 192.168.0.1 Target HW: 00:AA:00:AA:00:AA Target IP: 192.168.0.1
And the reply from the Linux kernel, in computer A, after the patch would look like:
Reply: Opcode: reply (0x0002) Sender HW: 00:AA.00:AA:00:AA Sender IP: 192.168.0.1 Target HW: 00:BB:00:BB:00:BB Target IP: 0.0.0.0
It is the fact that the Target HW address is set to computer A and not computer B that my patch wants to fix. And the Target IP: 0.0.0.0 is because OpenBSD and windows replied in that way.
I wanted to change this because, among other things, dhcpcd-2.0.3 has the following code in its arpCheck-function:
if ( memcmp(ArpMsgRecv.tHaddr,ClientHwAddr,ETH_ALEN) ) if ( DebugFlag ) syslog(LOG_DEBUG, "target hardware address mismatch:" [...]
This check will always fail when replies come from Linux machines, since the target hardware address will not match the client mac address.
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