Loading...
Who Service
The WHO service on UDP port 513 is a legacy Unix command networking service designed to remotely return information about users logged into a host system. Often bundled with early UNIX systems, it provides details such as usernames, terminal locations, and login times. Due to its age and security concerns, its usage has largely declined with modern alternatives offering enhanced security..
The WHO service on port 513 uses UDP to facilitate quick and simple retrieval of currently logged-in users on a remote Unix server. This command-line utility (who) queries the remote daemon which responds with session details.
Historically, it formed part of a suite of remote management and information services on UNIX systems alongside others like rwho and finger. WHO is a stateless protocol relying on UDP's connectionless delivery, making it lightweight but inherently unreliable.
The output typically includes details such as usernames, terminals, hostnames, and login timestamps. Due to the simplicity of the protocol and typical lack of authentication or encryption, it is now largely obsolete, replaced by secure alternatives (such as SSH) or disabled by default.