I was trying to rsync between two boxes that only have IPV6 link local addresses on them. This means you have to specify the ethernet port to use using the %eth0 syntax on the end of the IP. Unfortunately this confuses rsync:

:rsync -avP root@fe80::5652:ff:fe5b:d1c5%eth0:/root /tmp
rsync: getaddrinfo: fe80 873: Name or service not known
rsync error: error in socket IO (code 10) at clientserver.c(122) [Receiver=3.0.8]

To fix this, put brackets around the IP and ethernet port and rsync will know how to parse the connect string.

:rsync -avP root@[fe80::5652:ff:fe5b:d1c5%eth0]:/root /tmp

This same "fix" applies to scp and sftp file transfers as well.