Filter lists

Filter lists are either built-in and distributed via the eblocker-lists package or they are added by the user. User-defined lists can either be downloaded from a URL or manually edited.

Based on the type of the filter list there are different mechanisms for storing the lists on disk:

Domain filters

For each domain filter list two files are created for fast filtering:

• A file with extension .bloom that stores a Bloom filter. This is a very fast method to find out if a domain is not on the list. But there can be false positives.
• A file with extension .filter that stores all domains in buckets. Each domain is assigned to a bucket using a hash function. The implementation SingleFileFilter does not have false positives.

Pattern filters

Pattern filters require access to the full URL, so enabling the HTTPS feature is mandatory.

The filter lists are mainly in the EasyList format.

See also: PatternBlocker

Meta-data

The following entities are stored in Redis:

• ExternalDefinition stores meta-data for user-defined filter lists on disk. Lists were either downloaded from the given URL or manually edited. The field referenceId contains a reference to either a ParentalControlFilterMetaData object (for domain filters) or a FilterStoreConfiguration object.
• ParentalControlFilterMetaData stores meta-data for domain filters.
• FilterStoreConfiguration stores meta-data for pattern filters.

classDiagram
    ExternalDefinition --> FilterStoreConfiguration: referenceId
    ExternalDefinition --> ParentalControlFilterMetaData: referenceId (type DOMAIN)

    class FilterStoreConfiguration {
        +int id
        +String[] resources
        +FilterLearningMode learningMode
    }
    class ExternalDefinition {
        +int id
        +String file
        +String url
    }
    class ParentalControlFilterMetaData {
        +int id
        +List~String~ filenames
    }