OpenSSH for remote communication when possible. This enables ControlPersist (a performance feature), Kerberos, and options in ~/.ssh/config such as Jump Host setup. However, when using Enterprise Linux 6 operating systems as the control machine (Red Hat Enterprise Linux and derivatives such as CentOS), the version of OpenSSH may be too old to support ControlPersist. On these operating systems, Ansible will fallback into using a high-quality Python implementation of
OpenSSH called 'paramiko'. If you wish to use features like Kerberized SSH and more, consider using Fedora, OS X, or Ubuntu as your control machine until a newer version of OpenSSH is available for your platform -- or engage 'accelerated mode' in Ansible. See :doc:`playbooks_acceleration`.
In releases up to and including Ansible 1.2, the default was strictly paramiko. Native SSH had to be explicitly selected with the -c ssh option or set in the configuration file.
Occasionally you'll encounter a device that doesn't support SFTP. This is rare, but should it occur, you can switch to SCP mode in :doc:`intro_configuration`.
When speaking with remote machines, Ansible by default assumes you are using SSH keys. SSH keys are encouraged but password authentication can also be used where needed by supplying the option ``--ask-pass``. If using sudo features and when sudo requires a password, also supply ``--ask-become-pass`` (previously ``--ask-sudo-pass`` which has been depricated).
While it may be common sense, it is worth sharing: Any management system benefits from being run near the machines being managed. If you are running Ansible in a cloud, consider running it from a machine inside that cloud. In most cases this will work better than on the open Internet.
As an advanced topic, Ansible doesn't just have to connect remotely over SSH. The transports are pluggable, and there are options for managing things locally, as well as managing chroot, lxc, and jail containers. A mode called 'ansible-pull' can also invert the system and have systems 'phone home' via scheduled git checkouts to pull configuration directives from a central repository.
If a host is reinstalled and has a different key in 'known_hosts', this will result in an error message until corrected. If a host is not initially in 'known_hosts' this will result in prompting for confirmation of the key, which results in an interactive experience if using Ansible, from say, cron. You might not want this.
Ansible will log some information about module arguments on the remote system in the remote syslog, unless a task or play is marked with a "no_log: True" attribute. This is explained later.
To enable basic logging on the control machine see :doc:`intro_configuration` document and set the 'log_path' configuration file setting. Enterprise users may also be interested in :doc:`tower`. Tower provides a very robust database logging feature where it is possible to drill down and see history based on hosts, projects, and particular inventories over time -- explorable both graphically and through a REST API.
