See what software UCarp is at its own home page.
Here is a patch I wrote against version 1.5.2, to:
- use the working NIC's MAC address as source of multicast or broadcast advertisement packets
- fix typo in option description string, so that "-o" argument works (from Debian)
- add a missing shutdown case (from Debian)
The patch file: ucarp-1.5.2-srcmac.patch.