Internet-Security.com TCP port 873 details

TCP port 873 is the default port for the rsync protocol, used by the rsync daemon (rsyncd).
Linux/BSD distribution mirrors commonly serve content via rsync on 873, e.g., rsync://rsync.debian.org, rsync.kernel.org, rsync.archlinux.org, and rsync.freebsd.org.
NAS devices provide rsync services on this port, including Synology DSM (Rsync service in Backup settings) and QNAP Hybrid Backup Sync’s Rsync Server.
Windows implementations such as DeltaCopy Server and cwRsync use port 873 when running an rsync service.
University and open-source mirror networks expose rsync on 873, for example rsync.osuosl.org (Oregon State University Open Source Lab); some backup providers (e.g., rsync.net endpoints) also support rsync, though many prefer rsync over SSH.
Security note: attackers frequently scan port 873 for exposed rsync daemons and exploit weak/anonymous modules to list or exfiltrate files, leading to data leaks from misconfigured servers.

TCP 873