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The Avahi package used in pfSense® software is a system which facilitates service discovery on a local network. This means that a laptop or computer may be connected into a network and instantly be able to view other people to chat with, find printers to print to or find files being shared.
This kind of technology is already found in Apple MacOS X (branded Rendezvous, Bonjour, and sometimes Zeroconf) and is very convenient. Avahi is mainly based on Lennart Poettering’s flexmdns mDNS implementation for Linux which has been discontinued in favour of Avahi.
You can find a list of known issues with the Avahi package on the pfSense bug tracker.