Re: ARP requests on my net?

Les Mikesell wrote:

On Tue, 2006-04-04 at 22:22, Mike McCarty wrote:

So my Linux machine is asking for router's MAC address so it
can dump packets destined for the router? That might make sense
on a 10 Base 2, yes, because everyone would see all messages

(that didn't collide, that is :-)

Ethernet still acts the same as it did on coax.  You aren't
doing point-to-point at the hardware layer just because
the 10/100Base-T jacks work that way.

Of course. But this has nothing to do with dumping packets in
the IP layer. (I realize that you're not the one who suggested
that this was the reason.)

But the message is coming
from IP, because it knows its own IP address. Why would IP be
putting layer 2 addresses into messages?

It has to, if it wants to deliver over media that needs them.
Just like IPX or Appletalk (etc.) would have to in order to
use ethernet.

HTTP, SMTP, FTP, etc.
TCP
IP (ICMP)
LAN/PPP/Frame Relay/ATM or etc.
physical

In this case, the LAN protocol is "ethernet", which needs to
know its own MAC, and that of its gateway. Anything not matching the
MAC should be dumped. With a semi-intelligent board the board itself

should dump packets not destined for its MAC.

Yes, that's the point, packets that aren't broadcast/multicast or
destined for that MAC address are ignored efficiently.  And
unwanted multicast is usually ignored fairly efficiently.

Ok, so why is the IP layer asking the router for its MAC? So it
can send to its gateway? That makes sense, but has nothing
to do with dumping packets. Or does it?

Is it the case that
layer 1 is asking for its gateway MAC? Somehow, this looks like
mixed layers to me. It looks like IP is asking for a MAC which
it doesn't need. Or does IP need the MAC of the destination
to instruct layer 1 where to send to the gateway?

Not just it's gateway - it needs the MAC address to deliver
any packet to any specific address on the local lan.  TCP
knows about ethernet, not the other way around.  When

Presumably you mean that IP knows about ethernet.

TCP needs to send on the local subnet, it constructs an
ethernet packet, and it needs the destination MAC to
do that - and it uses ARP to get it when needed.  If it
is sending to something off the local net, then it still
needs to send to the router via ethernet so it ARP's the
gateway MAC address but constructs the packet with the

real destination IP.

What you say makes sense (except that TCP sits on top of
IP, so presumably it is IP making the requests; I do
realize that some use integrated TCP/IP stacks).

But what has this to do with dumping packets not destined for
me? IOW, what you say makes sense for routing out, but not
for dumping packets.

Is it presuming
that the gateway (router) may have gone down, and another device
with a different MAC may have taken over, and been assigned the
same IP via DHCP?

Most devices other than routers have a very short arp cache
time to allow you to change devices and addresses.  Routers
can cache up to 20 minutes.  If you aren't aware of this it
can cause trouble when you try to quickly swap in a replacment
machine or ethernet card.

Fine. My machine is querying its gateway to know how to route.

# route
Kernel IP routing table

169.254.0.0     *               255.255.0.0     U     0      0        0 eth0
172.17.0.0      *               255.255.0.0     U     0      0        0 eth0
default         router          0.0.0.0         UG    0      0        0 eth0

And it does it every two minutes, apparently.
But that has nothing to do with dumping packets, does it?

Mike
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