Windows Messenger service
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This legacy technology, although it has a similar name, is not related in any way to the modern, Internet-based Microsoft Messenger service for instant messaging or the Windows Messenger and Windows Live Messenger (formerly named MSN Messenger) client software.
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. WinPopup works by means of the NetBEUI protocol.
There is also a port to Linux with an extended feature called LinPopUp, which allows adding Linux computers to the set. Linpopup is an X Window graphical port of Winpopup, and a package for Debian linux. It runs over Samba. Linpopup does not have to run all the time, can run minimized, and its messages are encrypted with a strong cypher.
The Messenger Service was originally designed for use by system administrators to notify Windows users about their networks. 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.
The Messenger Service, unlike many other network utilities included with Windows, uses the NetBIOS protocol. The service waits for a message, then it displays it onscreen. The alternative way to send a message is to write it to a MailSlot named messngr.
- Official Microsoft sources