🤖 Overview

Sleipnir is a web crawler operated by Fenrir Inc., a Japanese software company best known for the Sleipnir web browser. This automated agent is primarily used to pre-fetch and cache web pages for the browser's Smart Search and Quick Access features, enabling faster page loads and offline reading for users. It also feeds data into Fenrir's proprietary search engine and AI-based content summarization services.

🌐 Technical Behavior

According to Fenrir's official documentation, the Sleipnir crawler employs a breadth-first crawling strategy with a default crawl delay of 1 second between requests to the same domain. It uses both HTTP/1.1 and HTTPS/2 protocols and respects the Cache-Control and Expires headers. The bot issues requests from a pool of IP addresses belonging to Fenrir's AS in Japan (AS2518). Observed requests originate from IPv4 blocks such as 202.238.x.x and 203.104.x.x. The crawler does not support the If-Modified-Since header by default, but it does accept gzip encoding. It identifies itself via the User-Agent string SleipnirBot/2.0 (see http://www.fenrir.co.jp/sleipnir/bot.html).

📋 robots.txt Compliance

Fenrir explicitly states that the Sleipnir crawler honors robots.txt directives. The bot checks for a Disallow rule before crawling any URL. If the file is unreachable, it defaults to allowed. This is documented on their official bot information page at http://www.fenrir.co.jp/sleipnir/bot.html.

🔍 Detection Indicators

The primary identifier is the User-Agent string SleipnirBot/2.0. Additionally, the bot sends a From header containing an email address (e.g., [email protected]) and a Referer header often set to the page being crawled. Server logs show requests with a consistent interval of 1 second and no parallel connections to the same host.

📊 Data Usage

Collected content is used for three primary purposes: (1) building a local cache for Sleipnir browser's offline reading mode, (2) generating search indexes for the Sleipnir Smart Search feature, and (3) training Fenrir's AI models for content summarization and recommendation. The data is stored on Fenrir's servers in Japan and is not shared with third parties according to their privacy policy.

⚙️ Rate Limiting Policy

Although legitimate, the Sleipnir crawler can be aggressive due to its pre-fetching behavior, which may generate high volumes of requests during initial site indexing. Rate limiting is recommended to prevent server overload; a threshold of 10 requests per minute from a single IP is a common defensive measure that does not block legitimate navigation.

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