NOP - NOP Protocol Commands

NOP Protocol Commands

Many computer protocols, such as telnet, include a NOP command that a client can issue to request a response from the server without requesting any other actions. Such a command can be used to ensure the connection is still alive or that the server is responsive. A NOOP command is part of the following protocols (this is a partial list):

• telnet
• FTP
• SMTP
• X11
• POP3
• NNTP
• finger
• IMAP4
• BitTorrent
• Character encoding: the null control character

Note that unlike the other protocols listed, the IMAP4 NOOP command has a specific purpose - it allows the server to send any pending notifications to the client.

While most telnet or FTP servers respond to a NOOP command with "OK" or "+OK", some programmers have added quirky responses to the client. For example, the ftpd daemon of MINIX responds to NOOP with the message: