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Re: default gateway on different subnet

James Chacon wrote:

On Nov 15, 2008, at 7:02 AM, Johnny Billquist wrote:

James Chacon skrev:
On Nov 14, 2008, at 5:30 PM, Rhialto wrote:
On Fri 14 Nov 2008 at 06:26:07 +0100, Michael Kell Jensen wrote:
Well i dont understand why some say it is broken apparently some of my
network guys, friends do it all the time.

Well the reasoning is this. I tried to find it in the original RFCs
which define the Internet Protocol, but presumably the authors found it
so obvious that there was no need to mention it explicitly; at least I
couldn't find it so far. The closest I could find it was a passage in
RFC 950, about subnetting, which I will quote below, but it uses the
weasel-word "Generally" instead of stating that it is the only thinkable

That's because it's not specifying an implementation but just showing possible logic to do so. Nothing in that RFC disallows an interface "knowing" it can directly connect to more than one network out a given interface.

True. It is perfectly fine to configure an interface with two addresses.

However, that was not the scenario described in the original question.

To quote rfc 1122:

    3.3.1  Routing Outbound Datagrams

        The IP layer chooses the correct next hop for each datagram it
        sends.  If the destination is on a connected network, the
        datagram is sent directly to the destination host; otherwise,
        it has to be routed to a gateway on a connected network.

Note the operative word "connected network".

Show a standard requiring "connected network" to mean I have to have an actual interface + IP configured for that network vs just physically being able to see it in some way from the host. I don't believe such a specification exists (that I could find) thereby allowing implementations to easily allow this sort of thing at their choosing.

Ok. Continuing with rfc 1122 then:

***  Local/Remote Decision

            To decide if the destination is on a connected network, the
            following algorithm MUST be used [see IP:3]:

            (a)  The address mask (particular to a local IP address for
                 a multihomed host) is a 32-bit mask that selects the
                 network number and subnet number fields of the
                 corresponding IP address.

            (b)  If the IP destination address bits extracted by the
                 address mask match the IP source address bits extracted
                 by the same mask, then the destination is on the
                 corresponding connected network, and the datagram is to
                 be transmitted directly to the destination host.

            (c)  If not, then the destination is accessible only through
                 a gateway.  Selection of a gateway is described below


You know, reading rfc's aren't at all that difficult.


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