rsyncd

rsyncd is the standalone daemon component of the rsync utility, providing a daemon-mode server that serves files over the rsync protocol via TCP, typically on port 873. It is used for centralized file synchronization and backups, allowing clients to access predefined named modules configured on the server. rsyncd operates independently of a shell and can be run as a background service on Unix-like systems or Windows, often managed through system services or init scripts.

The server’s behavior is controlled by a configuration file, commonly rsyncd.conf. The file contains global settings and one or more module definitions. Global options may specify the user and group under which the daemon runs, whether to chroot, log and pid file locations, and access controls. Each module defines a share with parameters such as path (the directory exposed by the module), read only or write permissions, whether the module is listed to clients, allowed or denied hosts, and authentication requirements. Authentication is achieved through an auth users directive and a secrets file that maps usernames to passwords. Clients access modules using the rsync://host/module syntax or via a push/pull operation when allowed.

Security considerations: rsyncd authentication provides access control but the rsync protocol itself does not encrypt data by default, so transmissions can be intercepted. To ensure confidentiality, rsync over SSH or the use of tunneling/TLS is recommended. Access can be further restricted by hosts allow/deny and by limiting modules to specific directories with appropriate permissions.

Usage: run rsync --daemon with an appropriate rsyncd.conf, or start it as a service. Clients connect using rsync rsync://host/module/path or rsync -a options module-based source/destination.