Well go figure, re-configuring the sync interface to use igb4 instead of igb5, and then swapping the firewall rules assigned to the interface and hey presto, a working XMLRPC setup, so devs…bug here hey?!
tcpdump -i igb4 results:
08:19:47.313327 IP 172.16.0.3 > 172.16.0.2: PFSYNCv5 len 280
update compressed count 3
eof count 1
08:19:47.758196 IP 172.16.0.2 > 172.16.0.3: PFSYNCv5 len 280
update compressed count 3
eof count 1
08:19:48.377325 IP 172.16.0.3 > 172.16.0.2: PFSYNCv5 len 196
update compressed count 2
eof count 1