| Internet-Draft | cbcp | July 2025 |
| Illyes & Kuehlewind | Expires 8 January 2026 | [Page] |
This document describes best pratices for web crawlers.¶
This note is to be removed before publishing as an RFC.¶
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Automatic clients, such as crawlers and bots, are used to access web resources, including indexing for search engines or, more recently, for new artificial intelligence (AI) applications like training models. As crawling activity increases, automatic clients must behave appropriately and respect the constraints of the resources they access. This includes clearly documenting how they can be identified and how their behavior can be influenced. Therefore, crawler operators are asked to follow the best practices for crawling outlined in this document.¶
To further assist website owners, it should also be considered to create a central registry where website owners can look up well-behaved crawlers. Note that while self-declared research crawlers, including privacy and malware discovery crawlers, and contractual crawlers are welcome to adopt these practices, due to the nature of their relationship with sites, they may exempt themselves from any of the Crawler Best Practices with a rationale.¶
The following best practices should be followed and are already applied by a vast majority of large-scale crawlers on the Internet:¶
Crawlers must support and respect the Robots Exclusion Protocol.¶
Crawlers must be easily identifiable through their user agent string.¶
Crawlers must not interfere with the regular operation of a site.¶
Crawlers must support caching directives.¶
Crawlers must expose the IP ranges they are crawling from in a standardized format.¶
Crawlers must expose a page that explains how the crawled data is used and how it can be blocked.¶
All well behaved-crawlers must support the REP as defined in Section 2.2.1 of [REP] to allow site owners to opt out from crawling.¶
Especially if the website chooses not to use a robots.txt file as defined
by the REP, crawlers further need to respect the X-robots-tag in the HTTP header.¶
As outlined in Section 2.2.1 of [REP] (Robots Exclusion Protocol; REP), the HTTP request header 'User-Agent' should clearly identify the crawler, usually by including a URL that hosts the crawler's description. For example:¶
User-Agent: Mozilla/5.0 (compatible; ExampleBot/0.1; +https://www.example.com/bot.html).¶
This is already a widely accepted practice among crawler operators. To remain compliant, crawler operators must include unique identifiers for their crawlers in the case-insensitive User-Agent, such as "contains 'googlebot' and 'https://url/...'". Additionally, the name should clearly identify both the crawler owner and its purpose as much as reasonably possible.¶
Depending on a site's setup (computing resources and software efficiency) and its size, crawling may slow down the site or even take it offline altogether. Crawler operators must ensure that their crawlers are equipped with back-out logic that relies on at least the standard signals defined by Section 15.6 of [HTTP-SEMANTICS], preferably also additional heuristics such as a change in the relative response time of the server.¶
Therefore, crawlers should log already visited URLs, the number of requests sent to each resource, and the respective HTTP status codes in the responses, especially if errors occur, to prevent repeatedly crawling the same source.¶
Generally, crawlers should avoid sending multiple requests to the same resources at the same time and should limit the crawling speed to prevent server overload, if possible, following the limits outlined in the REP protocol. Additionally, resources should not be re-crawled too often. Ideally, crawlers should restrict the depth of crawling and the number of requests per resource to prevent loops.¶
Crawlers should not attempt to bypass authentication or other access restrictions, such as when login is required, CAPTCHAs are in use, or content is behind a paywall, unless explicitly agreed upon with the website owner.¶
Crawlers should primarily access resources using HTTP GET requests, resorting to other methods (e.g., POST, PUT) only if there is a prior agreement with the publisher or if the publisher's content management system automatically makes those calls when JavaScript runs. Generally, the load caused by executing JavaScript should be carefully considered or even avoided whenever possible.¶
[HTTP-CACHING] HTTP caching removes the need of repeated access from crawlers to the same URL.¶
To complement the REP, crawler operators should publish the IP ranges they have allocated for crawling in a standardized, machine-readable format, and keep this information reasonably up-to-date (i.e., should not be outdated for more than 7 days).¶
The object containing the IP addresses must be linked from the page describing the crawler, and it must also be referenced in the page's metadata for machine readability. For example:¶
<link rel="help" href="https://example.com/crawlerips.json">
¶
Crawlers must be easily identifiable through their user-agent string, and they
should explain how the data they collect will be used. In practice, this is usually
done via the documentation page linked in the crawler's user agent. Additionally,
the documentation page should include a contact address for the crawler owner.¶
The webpage should also provide an example REP file to block the crawler and a method for verifying REP files.¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
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