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Although Kea offers a lot of flexibility, there may be cases where its behavior needs customisation. To accommodate this possibility, Kea includes the idea of "Hooks". This feature lets Kea load one or more dynamically-linked libraries (known as "hooks libraries") and, at various points in its processing ("hook points"), call functions in them. Those functions perform whatever custom processing is required. Hooks libraries are attached to individual Kea processes, not to Kea as a whole. This means (for example) that it is possible to associate one set of libraries with the DHCP4 server and a different set to the DHCP6 server. Another point to note is that it is possible for a process to load multiple libraries. When processing reaches a hook point, Kea calls the hooks library functions attached to it. If multiple libraries have attached a function to a given hook point, Kea calls all of them, in the order in which the libraries are specified in the configuration file. The order may be important: consult the documentation of the libraries to see if this is the case. The next section describes how to configure hooks libraries. If you are interested in writing your own hooks library, information can be found in the Kea Developer's Guide.

The hooks libraries for a given process are configured using the hooks-libraries keyword in the configuration for that process. (Note that the word "hooks" is plural). The value of the keyword is an array of map structures, each structure corresponding to a hooks library. For example, to set up two hooks libraries for the DHCPv4 server, the configuration would be: "Dhcp4": { : "hooks-libraries": [ { "library": "/opt/charging.so" }, { "library": "/opt/local/notification.so", "parameters": { "mail": "spam@example.com", "floor": 13, "debug": false, "users": [ "alice", "bob", "charlie" ], "languages": { "french": "bonjour", "klingon": "yl'el" } } } ] : } This is a change to the syntax used in Kea 0.9.2 and earlier, where hooks-libraries was a list of strings, each string being the name of a library. The change has been made in Kea 1.0 to facilitate the specification of library-specific parameters, a feature that will be added to a future version of Kea. The library reloading behavior has changed in Kea 1.1. Libraries are reloaded, even if their list hasn't changed. Kea does that, because the parameters specified for the library (or the files those parameters point to) may have changed. Libraries may have additional parameters. Those are not mandatory in the sense that there may be libraries that don't require them. However, for specific library there is often specific requirement for specify certain set of parameters. Please consult the documentation for your library for details. In the example above, the first library has no parameters. The second library has five parameters, specifying mail (string parameter), floor (integer parameter), debug (boolean parameter) and even lists (list of strings) and maps (containing strings). Nested parameters could be used if the library supports it. This topic is explained in detail in the Hooks Developer's Guide in Configuring Hooks Libraries section. Notes: The full path to each library should be given. As noted above, order may be important - consult the documentation for each library. An empty list has the same effect as omitting the hooks-libraries configuration element all together. There is one case where this is not true: if Kea is running with a configuration that contains a hooks-libraries item, and that item is removed and the configuration reloaded, the removal will be ignored and the libraries remain loaded. As a workaround, instead of removing the hooks-libraries item, change it to an empty list. This will be fixed in a future version of Kea. At the present time, only the kea-dhcp4 and kea-dhcp6 processes support hooks libraries.