Bridge to a used eth1 card without losing connection via that card

1. Connect to the machine via a different interface.
2. Connect to the machine at the console.
3. Set up the bridge in the system's startup scripts (you didn't specify what sort of system) and then reboot.

This works:

tunctl -t tap0 ; brctl addbr br0 ; brctl addif br0 tap0 ; \
ip addr del 192.168.115.200/24 dev eth0 ; \
ip addr add 192.168.115.200/24 dev br0 ; \
brctl addif br0 eth0 ; ip link set dev br0 up

This is executed so fast that the connection didn't break (not even a LAN connection). Even safer would be to do this within screen (which I did first but which turned out not to be necessary). The safest solution would be (within screen, of course) to drop everything in the firewall during these commands. So insert after the first line (before ip addr del) iptables -I INPUT 1 -j DROP and add at the end (after ip link set) iptables -D INPUT 1

I was getting the same problem, but running first the ip link set dev br0 up command was enough to keep the connection.