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SubjectRE: [2.4 PATCH] bugfix: ARP respond on all devices
David S. Miller wrote:
> It means that systems (like Linux) that make IP addresses owned by the
> host instead of specific interfaces cannot correctly interoperate with
> such remote systems.
This makes sense for replies, but not for requests.

When a HOST sends out an ARP request, it's NOT associated with a
single connection, it's associated with the host. Why should it pick a
"random" IP number to send as the source address?

The way the network code is currently, you're reducing your
connectivity to chance. There should be a defined process for making a
connection to another host. As it stands, this process is simply not

If you insist that an ARP request IS directly associated with a
connection, then you are required to have one ARP cache per source IP
address. It'd be predictable again ... but I don't think anyone wants to go

> It is also the case that a host cannot possibly be aware of all
> subnets present on a given LAN, therefore is should be liberal in it's
> replies to ARP requests.
Well, actually, I know exactly which IP subnets are on which LAN
segments - they're defined by the IP address and subnet of the interface. I
think you'll find that this a pretty basic feature of most hosts.

> Finally, it violates the most basic rule of IP networking:
> "Be liberal in what you accept, and conservative in what you send"
> -Jon Postel
I'm sorry, but Linux simply isn't being conservative in what it
sends. It's being bloody awkward.

Look at it this way - when a host sends out an ARP request, it WANTS
a reply, it's not doing it for fun. If it uses the IP number of the
interface it's sending the ARP request on, it will ALWAYS get a reply
(assuming there's one to get.) If it uses the IP number of another
interface, it MAY get a reply, but it MAY NOT.

Are there any cases when this is reversed? I don't think so! Linux
is being intentionally difficult, and as far as I can tell, for no good

> In general, when a host posses the information necessary to allow
> other hosts to communicate, it should provide that information
> whenever possible.
No, it should follow the rules for letting traffic pass through it.
Just because a host can see two networks, doesn't mean it should route
between them - it possesses information, but there have to be rules to
determine how this information is used.

Compare it to IP: If a firewall sees a packet come in on an
interface it shouldn't, it'll probably drop it - it's called anti-spoofing.
Should the firewall forward the packet on just because it can?

So at the lower layer, a router sees an ARP packet with what looks
like a "spoofed" source address. Should it trust it implicitly and place it
in its cache, or should it drop it?

No one yet has given one single example of a network that relies on
Linux's current behaviour. I've given two examples of networks that break
because of it. I would kindly suggest that the default should be changed.


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