Date: Thu, 02 Jan 2003 09:42:13 -0500 From: "Bill Moran" <bill_moran2@hotmail.com> To: y.grossel@hexanet.fr Cc: freebsd-questions@freebsd.org Subject: Re: promiscuous mode / strange ethernet packets duplication problem Message-ID: <F20D3VCr8yKnTR5q83A00023bd0@hotmail.com>
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>From: Yann GROSSEL <y.grossel@hexanet.fr> >On Thu, 02 Jan 2003 08:56:42 -0500 >"Bill Moran" <bill_moran2@hotmail.com> wrote: > > > There's your answer. Any machine with forwarding turned on will resend > > a packet that isn't destin for it. That's by design. > > It doesn't make much sense to me that you'd have a lot of machines with > > forwarding turned on. Usually only gateways use this. Honestly, I > > can't thing of any reason to have forwarding on if your machine only > > has 1 IP address. > > > > >As several boxes have this problem, they resend packets to each others > > >very quickly, generating a flood on the network. This flood only stop > > >when all TTL of packets reach 0 or when the switch finally re-learn > > >on what port is located the interface with the target MAC address. > > > > > >Does anybody have any clue about what this kind of problem may be ? > > > > Turn forwarding of on all but your gateways. > >Mhhh. > >Gateways are designed to forward packets from network to network. If a >machine wants to send a packet to a remote network, it will send that >packet to the gateway by putting the gateway interface MAC address in the >destination field of the ethernet packet. The gateway will know that it >must forward the packet because of that. And it will know where to forward >the packet by looking to the destination IP address field of the packet. > >Here the machines are "forwarding" ethernet packets with a destination >MAC address field set to ANOTHER machine of our network. In other words, >these packets are NOT targetted to the "gateways", neither from their >MAC address destination field nor from their IP address destination field. > >So why are these packets "forwarded" ? Well, this is getting into internals that are a little beyond me, but I would say that it's because forwarding occurs at the IP level. You seem to be confusing the behaviour your expecting with a bridge, which forwards at the MAC level. I'd bet the kernel logic that handles forwarding knows nothing about MAC addresses (based on the network stack model) and thus can't make decisions based on them. IP forwarding would have nothing to do with MAC addresses, if it did, how could you forward across a PPP or serial link (or any other media that doesn't have a MAC addy)? Is there a reason that forwarding should be on for these machines? -Bill _________________________________________________________________ Help STOP SPAM: Try the new MSN 8 and get 2 months FREE* http://join.msn.com/?page=features/junkmail To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-questions" in the body of the message
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