Re: Two ethernet interfaces puzzle

From: Craig Andersen (andersen7_at_charter.net)
Date: 08/08/04 

Date: Sun, 08 Aug 2004 11:38:26 -0500

jai wrote:
> On Sat, 07 Aug 2004 18:49:22 -0400, Gonzalo wrote:
>
>
>>I see absolutely no reason why you couldn't have two separate hardware
>>interfaces with separate IP address be part of the same subnet.
>
> I think the main reason would be DNS. If you send a packet on eth1 and you
> are registered in DNS as eth0 address the packet reply goes to the wrong
> interface which is not listening.
>
> What I do is use SuSE profiles. I have a wired and wireless profile and
> add it to my grub boot menu. In the wireless profile I set my eth0
> interface to manual. This way it never comes up hence no issues.
>
> It is also good if you switch to different wireless networks you can make
> a profile for each one.
>

Actually there is nothing incorrect about having two IPs on the same
subnet (one for say ftp and one for www), but what would be the
expectation ? Inbound connections are usually no issue as they got there
by using one or the other address. Outbound connections (traffic) is
different. Unfortunately the is no standard way to handle this, each OS
is free to innovate. Solaris, for example, tries to keep TCP connections
on the same interface (source address) but can load balance based upon a
source and destination hash. The problem is that multiple interface can
have different characteristics (Gigabit Ethernet versus 802.11b), so the
question sometimes becomes, what traffic do you want where, and how do
you accomplish this.

Most inbound services (ports) are registered for the whole machine not
just a particular interface (e.g. ssh).

tcp 0 0 *:ssh *:* LISTEN

In this case a programmer did not specify any particular interface
(address) so any interface will work (provided firewall rules permit).

As long as the source and destination address do not change,
communications should not be hampered.

Having said that it is possible where the 2 addresses are on 2 phyically
separate interfaces to also always get multicast and broadcasts for that
  subnet on both interfaces (processing 2 packets every time).

Most operations would not have wireless and wired connections on the
same subnet as each has a different security profile, and separating
them onto 2 different subnets can make things easier. The exception to
that rule is our home routers that usually have a wireless access point
and Ethernet ports on the same Ethernet segment.

So while it is not illegal to have 2 IPs on the same subnet, what are
the objectives in doing this, as it is difficult to guarentee one or the
other on outbound situations ?

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