what is a plugin in wireshark?

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A broad subject, I'll try to cover the bases.

In general a plugin is an extension to the build-in capabilities of a program. For Wireshark this usually means adding a protocol dissector for a protocol that Wireshark doesn't know about (yet). But there are other plugin types to Wireshark: capture file format plugins and audio codec plugins.

The protocol dissector plugins for Wireshark come in two forms, either as a C module compiled for a specific Wireshark version, or as a Lua script, interpreted by the build-in Lua interpreter in Wireshark.