Image analyzer

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Overview

The Imgfilter section scores image/* response and upload bodies and blocks or debug-replaces content when the engine score is at or above a threshold. Blocked images can be replaced with a template (default checkeredgif). It exists to keep unwanted image content (for example, adult material) off the network — useful both for blocking what users download and for stopping the same kind of content from being uploaded through the proxy.

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Core Mechanics (C++ Source Validation)

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First match wins

Walk enabled filtering policy rows top to bottom; first profile match sets threshold, bypass, debug, and template.

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MIME and size gates

Only parts whose MIME starts with image/ are candidates. Images smaller than 50×50 pixels are skipped — too small to meaningfully score, and excluding them avoids wasted work on tracking pixels and icons.

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Threshold scale

Roughly -10 (unlikely inappropriate) through 0 (very likely). Block when engine score ≥ threshold.

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Debug mode

On responses with Debug enabled, images at or above threshold are blurred and annotated in-place; connection still marked blocked but block template not sent.

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Bypass

Bypass Image Scanning on a matching row skips buffering and scoring entirely.

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Processing flow

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Schema Fields

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Global fields

• Enabled (enabled) — Master switch; requires library init success.
• Default template (dtempl) — Fallback when row Template blank; default checkeredgif.
• Library path (libpath) — Path to imgfilter engine module.

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Policy rows

• Profiles (profiles) — Limit to tagged connections. Blank matches all.
• Threshold (threshold) — Block when score ≥ this value. A lower Threshold number is stricter (blocks more); a higher one is more permissive — for example, a Threshold of -2 blocks less aggressively than a Threshold of 0.
• Bypass Image Scanning (bypass) — Skip scan for matching profiles.
• Debug (debug) — Blur/annotate on response instead of block template. Has no equivalent effect on the upload path, which always uses the block template.
• Template (template) — Replacement on block. Falls back through the matching entry’s own Template, then the section’s Default template, then the built-in checkered placeholder — whichever is first non-blank.

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Examples

Open Configure → Real time content security → Image analyzer → Filtering policies. Row fields are Enabled, Comment, Bypass Image Scanning, Profiles, Threshold, Template, and Debug.

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Block for students

• Configuration: Profiles STUDENT, Threshold 0, Template checkeredgif.
• Result: student image responses scoring ≥ 0 replaced with checkered template.

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Staff bypass

• Configuration: Row A Profiles STAFF Bypass on; Row B blank Threshold -2.
• Result: staff skip imgfilter; others scored via row B.

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Debug tuning

• Configuration: Debug on, Threshold -5.
• Result: borderline response images blurred with score annotation; still logged blocked.

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How to verify

1. Fetch image through profile with image-filter tag and imgfilter enabled.
2. Enable imgfilter log level for score lines.
3. Detailed logs show filter name imgfilter on block. If a request you expected to be scored is missing, check whether an earlier entry’s Bypass Image Scanning claimed it first.