Messenger Service is a network-based system notification service included in some versions of Microsoft Windows. This service, although it has a similar name, is not related in any way to the .NET Messenger Service or the Windows Messenger and MSN Messenger/Windows Live Messenger instant messaging clients.

Net Send

The Net Send command allows users to use the Messenger Service under Windows XP Professional and Home editions. It only requires that the Messenger Service be started. The syntax of the command is:

For example:

The above example uses a computer name, not an IP address. Either way, the message will be sent as long as the target computer has the Messenger Service running.

This example will actually send the message to every user and every computer on the workgroup or domain.

This method of instant messaging requires no Internet connection; rather, just a local network. This is due to the architecture of the MailSlot protocol, which is what the Messenger Service is based on.

WinPopup

WinPopup sends messages from one Windows computer to another on the same LAN. It is available in all Windows versions from Windows for Workgroups 3.1 to Windows Me, but has never been included with Windows NT-based operating systems.

There is also a port to Linux called LinPopup, which allows you to send and receive messages on Linux.

Abuse

The Messenger Service was originally designed for use by system administrators to notify Windows users about their networks1. It has been used maliciously to present pop-up advertisements to users over the Internet (by using mass-messaging systems which sent a desired message to a specified range of IP addresses). Even though Windows XP includes a firewall it is not enabled by default. Because of this, many users received such messages. As a result of this abuse, the Messenger Service has been disabled by default in Windows XP Service Pack 2, a change perhaps redundant as the same service pack also enabled the firewall by default.

In order to use the Messenger Service functionality of Windows through the NET SEND command, either by itself or with the use of a third party NetBIOS messaging application, it is recommended that the NetBIOS ports cannot be reached from sources external to the local network. The ports that can be used by the Messenger service are 135, 137, 138, and 139. Many Internet service providers currently block access to these NetBIOS ports over the Internet, helping to prevent spamming even for older unfirewalled clients.