---
title: NEURLFilterPrefilter
framework: networkextension
role: symbol
role_heading: Structure
path: networkextension/neurlfilterprefilter
---

# NEURLFilterPrefilter

A structure containing a prefilter returned by a filter control provider.

## Declaration

```swift
struct NEURLFilterPrefilter
```

## Overview

Overview You return this type from the fetchPrefilter(existingPrefilterTag:) method of NEURLFilterControlProvider. The returned NEURLFilterPrefilter must contain a Bloom filter built with the 32-bit FNV-1a and 32-bit MurmurHash3 hash functions, with double hashing. Depending on the size of the Bloom filter data, it can be passed as data or the path of a file containing the data.

## Topics

### Creating a prefilter

- [init(data:tag:bitCount:hashCount:murmurSeed:)](networkextension/neurlfilterprefilter/init(data:tag:bitcount:hashcount:murmurseed:).md)

### Working with prefilter properties

- [data](networkextension/neurlfilterprefilter/data.md)
- [NEURLFilterPrefilter.PrefilterData](networkextension/neurlfilterprefilter/prefilterdata.md)
- [tag](networkextension/neurlfilterprefilter/tag.md)
- [bitCount](networkextension/neurlfilterprefilter/bitcount.md)
- [hashCount](networkextension/neurlfilterprefilter/hashcount.md)
- [murmurSeed](networkextension/neurlfilterprefilter/murmurseed.md)

## See Also

### Fetching a prefilter

- [fetchPrefilter(existingPrefilterTag:)](networkextension/neurlfiltercontrolprovider/fetchprefilter(existingprefiltertag:).md)