rsync takes long time to connect

rsync stuck at connect, usually one or two minutes.

Debug Method：

Add -vvvv to rsync command to get ssh command used. Add -vvv to ssh command to see connection details.

Solution:

rync tries ipv6 connection first, and there is problem with my firewall and dropbear configuration (ipv6 connection is direct (original port, not forwarded port) remote connection to the ssh server but also forwarded by router firewall). Resolve firewall/dropbear issues OR simply add --ipv4 to rsync command.