Re: [fw-wiz] Layer 2 (stealth) firewalls - PBR?



On Tue, 8 Apr 2008, Patrick Darden wrote:

I've been doing networking since the broadband/baseband LAN thing a long
time ago, and I'm pretty cognizant of how it all works...


Chest thumping. Gotchya.


No, just saying that I'm (a) aware of the differences in layers and (b)
aware of when those differences are not treated as true boundaries.



Layer 2 devices like switches have to forwrd layer 3 multicast packets out
ports for the multicast group, so they in essence have to peek up a layer
even though they're not "routers, firewalls, etc." They also have to
forward layer 3 broadcasts out all ports in a LAN or VLAN, once again
without being "routers, firewalls, etc."

Layer 2 devices forward based on MAC addresses. End of story. They do
NOT peak up the stack. ARP/RARP bridges the gap between layer 2 and 3,

I will refer you to RFC 4541, Considerations for Internet Group Management
Protocol (IGMP) and Multicast Listener Discovery (MLD) Snooping Switches.
(May 2006)

Which says in part:

In recent years, a number of commercial vendors have introduced
products described as "IGMP snooping switches" to the market. These
devices do not adhere to the conceptual model that provides the
strict separation of functionality between different communications
layers in the ISO model, and instead utilize information in the upper
level protocol headers as factors to be considered in processing at
the lower levels. This is analogous to the manner in which a router
can act as a firewall by looking into the transport protocol's header
before allowing a packet to be forwarded to its destination address.

In the case of IP multicast traffic, an IGMP snooping switch provides
the benefit of conserving bandwidth on those segments of the network
where no node has expressed interest in receiving packets addressed
to the group address. This is in contrast to normal switch behavior
where multicast traffic is typically forwarded on all interfaces.

Many switch datasheets state support for IGMP snooping, but no
recommendations for this exist today. It is the authors' hope that
the information presented in this document will supply this
foundation.

...

The suggestions in this document are based on IGMP, which applies
only to IPv4. For IPv6, Multicast Listener Discovery [MLD] must be
used instead. Because MLD is based on IGMP, we do not repeat the
entire description and recommendations for MLD snooping switches.
Instead, we point out the few cases where there are differences from
IGMP.

but layer 2 devices such as NICs, hubs, bridges, and layer 2 switches do
not rely on IP or any other layer 3 protocol whatsoever for forwarding.


So, you see switch vendors really are looking into layer 3 information for
multicast traffic. Enough so that someone thought "Hey, we should have an
RFC to cover this!"

You seem to be conflating layer 3 multicast/broadcast/unicast Packets with
broadcast/unicast Frames. To begin with, packets are not frames, and
layer 2 devices cannot interpret packets.

Perhaps I crossed frame and packet, I tend to do that from time to time,
doesn't change the fact that the vendors are a' shipping it.


You state "They also have to forward layer 3 broadcasts out all ports in a
LAN" which is patently false--if a 128 port layer 2 switch has 64 ports on
10.0.0.0/24 and the other 64 ports on 10.1.0.0/24, then a broadcast sent
to 10.0.0.0/24 will only hit the correct 64 ports. The switch decides

That's two LANs the way I've always counted it in terms of addressing
unless your'e supernetting on some devices and not on others, in which
case you can count it several ways. A dumb switch doesn't always know
your mask either. I think the algorithm for a dumb switch actually tends
to be "if I don't know the destination MAC address, send it out all the
ports," but I'd have to get some playtime to test it effectively.

I think this is the problem. You are confusing layer 2 unicast/broadcast
frames with layer 3 unicast/multicast/broadcast packets. Certainly layer
2 devices do unicast and broadcast, but again NOT based on IP or any other
layer 3 protocol. Layer 2 Unicast and Broadcast are all in relation to

No, I'm talking about both types, you're simply missing the case where the
switch vendors peeking up the stack. Your refusal to acknowlege this blinds
you, and causes you to misinterpret.

IPv6 has nothing to do with layer 2. I am going to completely ignore this
statement.

Again, I'll point you to MLD snooping. Again, I'll admit my term of
"peeking" isn't the common "snooping" that seems to be vogue, but it's
still there and it's still a factor in shipping hardware.

Paul
-----------------------------------------------------------------------------
Paul D. Robertson "My statements in this message are personal opinions
paul@xxxxxxxxxxxx which may have no basis whatsoever in fact."
http://www.fluiditgroup.com/blog/pdr/
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