Sorry, the "backfeed queue" :)
See README.backfeed: https://sourceforge.net/p/xymon/code/HEAD/tree/branches/4.x-master/README.ba...
The short of it is that it provides an alternate way of submitting messages to xymond for local processes, using SysV-IPC instead of localhost TCP connections. It's much more efficient for injecting lots of messages, but sort of obviates the testing of xymond submissions over IPv6 :) I see.
Looking into this more, I believe we'll be able to use the existing "conn" framework for handling multiple IP addresses, but we may need a default set of flags for how to handle priority and fallbacks on DNS lookup.
I'd envision something along the lines of: IPv4 Only IPv4 Mandatory/IPv6 Optional IPv4 Optional/IPv6 Mandatory IPv6 Only I agree with Adam and even think IPv4 Mandatory/IPv6 Mandatory would be the default in our case.
I'll be ready to perform more tests after the Easter break.
cheers, -d