How to automatically accept the remote key when rsyncing?

You can add this host's key to known_hosts beforehand like this:

ssh-keyscan $someip >> ~/.ssh/known_hosts

If they genuinely are new hosts, and you can't add the keys to known_hosts beforehand (see York.Sar's answer), then you can use this option:

-e "ssh -o StrictHostKeyChecking=no"

I know that this was asked 3 years ago, but this was at the top of my google search and I was unable to get either of these solutions in the middle of a Vagrant script to work correctly for me. So I wanted to put here the method that I found somewhere else.

The solution there talks about updating the ~/.ssh/config or /etc/ssh/ssh_config file with the following blocks of code.

To disable host key checking for a particular host (e.g., remote_host.com):

Host remote_host.com    StrictHostKeyChecking no

To turn off host key checking for all hosts you connect to:

Host *    StrictHostKeyChecking no

To avoid host key verification, and not use known_hosts file for 192.168.1.* subnet:

Host 192.168.0.*    StrictHostKeyChecking no    UserKnownHostsFile=/dev/null

I hope this helps someone else who runs into this issue.