[LARTC] gre tunnel between networks with same subnet

[Gabriel]

            /-----------------------\
            |                       |
            |eth0                   |eth0
        |-------|               |-------|
        |       |eth1      eth1 |       |
--------    A   |____    _______|   B   |-----
        |       |    \  /       |       |
        --------|    |  |       --------|
                     |  |
                     |  |
                 -----------
                |___________|
                    switch

What you see above is my setup. Box A is connected to
Box B through a switch. Box A is connected to the
Internet through eth0, same with Box B. The link that
goes through the switch is not very reliable, so I
want to connect the two boxes using their Internet
link via a gre tunnel. The problem is that the boxes
are on the same subnet (and I can't change that). I've
read about proxy arp, about bridging, but things are
still confused. Here are some numbers: eth1 on Box A
is 192.168.1.1/24, eth1 on Box B is 192.168.1.31/24.
On Box B there are 4 NICs, 3 of them (including eth1)
are bridged, with the bridge interface being br0
(192.168.1.31 is actually assigned to br0, not eth1).
I've read the lartc howto, so I created a tun0
interface on both boxes: ip tunnel add tun0 mode gre
remote remote_ip_here local local_ip_here ttl 255; ip
link set tun0 up. The problem is what do I do from
here? Do I bridge tun0 and eth1 on Box A and add tun0
to br0 on Box B? Or do I just enable proxy_arp for
eth1 and tun0 on Box A and for br0 and tun0 on B? Are
there any routes neccesary (my guess is no, but I'm
not very sure)? And about proxy_arp: what do I have to
do to turn it on, just set
/proc/sys/net/ipv4/conf/<iface>/proxy_arp to 1 and
that's it? One last thing:
http://leaf.sourceforge.net/doc/howto/proxyarp.html#id2805973
says proxy-arp is not bridging (agreed) so DO NOT
CONFIGURE BRIDGE OPTIONS!!! Does this mean using
bridging and doing proxy-arp on the same box is not
possible?

Thanks.

(hope the ascii art comes out well)

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