Network Working Group M. Koster Internet-Draft Stalworthy Computing, Ltd. Intended status: Informational G. Illyes Expires: June 5, 2021 H. Zeller L. Harvey Google December 08, 2020 Robots Exclusion Protocol draft-koster-rep-04 Abstract This document standardizes and extends the "Robots Exclusion Protocol" method originally defined by Martijn Koster in 1996 for service owners to control how content served by their services may be accessed, if at all, by automatic clients known as crawlers. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. 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Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Koster, et al. Expires June 5, 2021 [Page 1] Internet-Draft I-D July 2019 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 2 2. Specification . . . . . . . . . . . . . . . . . . . . . . . . 3 2.1. Protocol definition . . . . . . . . . . . . . . . . . . . 3 2.2. Formal syntax . . . . . . . . . . . . . . . . . . . . . . 3 2.2.1. The user-agent line . . . . . . . . . . . . . . . . . 4 2.2.2. The Allow and Disallow lines . . . . . . . . . . . . 4 2.2.3. Special characters . . . . . . . . . . . . . . . . . 5 2.2.4. Other records . . . . . . . . . . . . . . . . . . . . 6 2.3. Access method . . . . . . . . . . . . . . . . . . . . . . 6 2.3.1. Access results . . . . . . . . . . . . . . . . . . . 7 2.4. Caching . . . . . . . . . . . . . . . . . . . . . . . . . 8 2.5. Limits . . . . . . . . . . . . . . . . . . . . . . . . . 8 2.6. Security Considerations . . . . . . . . . . . . . . . . . 8 2.7. IANA Considerations . . . . . . . . . . . . . . . . . . . 8 3. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3.1. Simple example . . . . . . . . . . . . . . . . . . . . . 8 3.2. Longest Match . . . . . . . . . . . . . . . . . . . . . . 9 4. References . . . . . . . . . . . . . . . . . . . . . . . . . 9 4.1. Normative References . . . . . . . . . . . . . . . . . . 9 4.2. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10 1. Introduction This document applies to services that provide resources that clients can access through URIs as defined in RFC3986 [1]. For example, in the context of HTTP, a browser is a client that displays the content of a web page. Crawlers are automated clients. Search engines for instance have crawlers to recursively traverse links for indexing as defined in RFC8288 [2]. It may be inconvenient for service owners if crawlers visit the entirety of their URI space. This document specifies the rules that crawlers MUST obey when accessing URIs. These rules are not a form of access authorization. 1.1. Terminology 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. Koster, et al. Expires June 5, 2021 [Page 2] Internet-Draft I-D July 2019 2. Specification 2.1. Protocol definition The protocol language consists of rule(s) and group(s): o *Rule*: A line with a key-value pair that defines how a crawler may access URIs. See section The Allow and Disallow lines. o *Group*: One or more user-agent lines that is followed by one or more rules. The group is terminated by a user-agent line or end of file. See User-agent line. The last group may have no rules, which means it implicitly allows everything. 2.2. Formal syntax Below is an Augmented Backus-Naur Form (ABNF) description, as described in RFC5234 [3]. robotstxt = *(group / emptyline) group = startgroupline ; We start with a user-agent *(startgroupline / emptyline) ; ... and possibly more ; user-agents *(rule / emptyline) ; followed by rules relevant ; for UAs startgroupline = *WS "user-agent" *WS ":" *WS product-token EOL rule = *WS ("allow" / "disallow") *WS ":" *WS (path-pattern / empty-pattern) EOL ; parser implementors: add additional lines you need (for ; example Sitemaps), and be lenient when reading lines that don't ; conform. Apply Postel's law. product-token = identifier / "*" path-pattern = "/" *UTF8-char-noctl ; valid URI path pattern empty-pattern = *WS identifier = 1*(%x2D / %x41-5A / %x5F / %x61-7A) comment = "#" *(UTF8-char-noctl / WS / "#") emptyline = EOL EOL = *WS [comment] NL ; end-of-line may have ; optional trailing comment NL = %x0D / %x0A / %x0D.0A WS = %x20 / %x09 Koster, et al. Expires June 5, 2021 [Page 3] Internet-Draft I-D July 2019 ; UTF8 derived from RFC3629, but excluding control characters UTF8-char-noctl = UTF8-1-noctl / UTF8-2 / UTF8-3 / UTF8-4 UTF8-1-noctl = %x21 / %x22 / %x24-7F ; excluding control, space, '#' UTF8-2 = %xC2-DF UTF8-tail UTF8-3 = %xE0 %xA0-BF UTF8-tail / %xE1-EC 2UTF8-tail / %xED %x80-9F UTF8-tail / %xEE-EF 2UTF8-tail UTF8-4 = %xF0 %x90-BF 2UTF8-tail / %xF1-F3 3UTF8-tail / %xF4 %x80-8F 2UTF8-tail UTF8-tail = %x80-BF 2.2.1. The user-agent line Crawlers set a product token to find relevant groups. The product token MUST contain only "a-zA-Z_-" characters. The product token SHOULD be part of the identification string that the crawler sends to the service (for example, in the case of HTTP, the product name SHOULD be in the user-agent header). The identification string SHOULD describe the purpose of the crawler. Here's an example of an HTTP header with a link pointing to a page describing the purpose of the ExampleBot crawler which appears both in the HTTP header and as a product token: +-------------------------------------------------+-----------------+ | HTTP header | robots.txt | | | user-agent line | +-------------------------------------------------+-----------------+ | user-agent: Mozilla/5.0 (compatible; | user-agent: | | ExampleBot/0.1; | ExampleBot | | https://www.example.com/bot.html) | | +-------------------------------------------------+-----------------+ Crawlers MUST find the group that matches the product token exactly, and then obey the rules of the group. If there is more than one group matching the user-agent, the matching groups' rules MUST be combined into one group. The matching MUST be case-insensitive. If no matching group exists, crawlers MUST obey the first group with a user-agent line with a "*" value, if present. If no group satisfies either condition, or no groups are present at all, no rules apply. 2.2.2. The Allow and Disallow lines These lines indicate whether accessing a URI that matches the corresponding path is allowed or disallowed. To evaluate if access to a URI is allowed, a robot MUST match the paths in allow and disallow rules against the URI. The matching SHOULD be case sensitive. The most specific match found MUST be used. The most specific match is the match that has the most octets. If an allow and disallow rule is equivalent, the allow SHOULD be used. If no match is found amongst the rules in a group for a Koster, et al. Expires June 5, 2021 [Page 4] Internet-Draft I-D July 2019 matching user-agent, or there are no rules in the group, the URI is allowed. The /robots.txt URI is implicitly allowed. Octets in the URI and robots.txt paths outside the range of the US- ASCII coded character set, and those in the reserved range defined by RFC3986 [1], MUST be percent-encoded as defined by RFC3986 [1] prior to comparison. If a percent-encoded US-ASCII octet is encountered in the URI, it MUST be unencoded prior to comparison, unless it is a reserved character in the URI as defined by RFC3986 [1] or the character is outside the unreserved character range. The match evaluates positively if and only if the end of the path from the rule is reached before a difference in octets is encountered. For example: +-------------------+-----------------------+-----------------------+ | Path | Encoded Path | Path to match | +-------------------+-----------------------+-----------------------+ | /foo/bar?baz=quz | /foo/bar?baz=quz | /foo/bar?baz=quz | | | | | | /foo/bar?baz=http | /foo/bar?baz=http%3A% | /foo/bar?baz=http%3A% | | ://foo.bar | 2F%2Ffoo.bar | 2F%2Ffoo.bar | | | | | | /foo/bar/U+E38384 | /foo/bar/%E3%83%84 | /foo/bar/%E3%83%84 | | | | | | /foo/bar/%E3%83%8 | /foo/bar/%E3%83%84 | /foo/bar/%E3%83%84 | | 4 | | | | | | | | /foo/bar/%62%61%7 | /foo/bar/%62%61%7A | /foo/bar/baz | | A | | | +-------------------+-----------------------+-----------------------+ The crawler SHOULD ignore "disallow" and "allow" rules that are not in any group (for example, any rule that precedes the first user- agent line). Implementers MAY bridge encoding mismatches if they detect that the robots.txt file is not UTF8 encoded. 2.2.3. Special characters Crawlers SHOULD allow the following special characters: Koster, et al. Expires June 5, 2021 [Page 5] Internet-Draft I-D July 2019 +-----------+--------------------------------+----------------------+ | Character | Description | Example | +-----------+--------------------------------+----------------------+ | "#" | Designates an end of line | "allow: / # comment | | | comment. | in line" | | | | | | | | "# comment at the | | | | end" | | | | | | "$" | Designates the end of the | "allow: | | | match pattern. A URI MUST end | /this/path/exactly$" | | | with a $. | | | | | | | "*" | Designates 0 or more instances | "allow: | | | of any character. | /this/*/exactly" | +-----------+--------------------------------+----------------------+ If crawlers match special characters verbatim in the URI, crawlers SHOULD use "%" encoding. For example: +------------------------+------------------------------------------+ | Pattern | URI | +------------------------+------------------------------------------+ | /path/file- | https://www.example.com/path/file- | | with-a-%2A.html | with-a-*.html | | | | | /path/foo-%24 | https://www.example.com/path/foo-$ | +------------------------+------------------------------------------+ 2.2.4. Other records Clients MAY interpret other records that are not part of the robots.txt protocol. For example, 'sitemap' [4]. 2.3. Access method The rules MUST be accessible in a file named "/robots.txt" (all lower case) in the top level path of the service. The file MUST be UTF-8 encoded (as defined in RFC3629 [5]) and Internet Media Type "text/ plain" (as defined in RFC2046 [6]). As per RFC3986 [1], the URI of the robots.txt is: "scheme:[//authority]/robots.txt" For example, in the context of HTTP or FTP, the URI is: http://www.example.com/robots.txt Koster, et al. Expires June 5, 2021 [Page 6] Internet-Draft I-D July 2019 https://www.example.com/robots.txt ftp://ftp.example.com/robots.txt 2.3.1. Access results 2.3.1.1. Successful access If the crawler successfully downloads the robots.txt, the crawler MUST follow the parseable rules. 2.3.1.2. Redirects The server may respond to a robots.txt fetch request with a redirect, such as HTTP 301 and HTTP 302. The crawlers SHOULD follow at least five consecutive redirects, even across authorities (for example hosts in case of HTTP), as defined in RFC1945 [7]. If a robots.txt file is reached within five consecutive redirects, the robots.txt file MUST be fetched, parsed, and its rules followed in the context of the initial authority. If there are more than five consecutive redirects, crawlers MAY assume that the robots.txt is unavailable. 2.3.1.3. Unavailable status Unavailable means the crawler tries to fetch the robots.txt, and the server responds with unavailable status codes. For example, in the context of HTTP, unavailable status codes are in the 400-499 range. If a server status code indicates that the robots.txt file is unavailable to the client, then crawlers MAY access any resources on the server or MAY use a cached version of a robots.txt file for up to 24 hours. 2.3.1.4. Unreachable status If the robots.txt is unreachable due to server or network errors, this means the robots.txt is undefined and the crawler MUST assume complete disallow. For example, in the context of HTTP, an unreachable robots.txt has a response code in the 500-599 range. For other undefined status codes, the crawler MUST assume the robots.txt is unreachable. If the robots.txt is undefined for a reasonably long period of time (for example, 30 days), clients MAY assume the robots.txt is unavailable or continue to use a cached copy. Koster, et al. Expires June 5, 2021 [Page 7] Internet-Draft I-D July 2019 2.3.1.5. Parsing errors Crawlers SHOULD try to parse each line of the robots.txt file. Crawlers MUST use the parseable rules. 2.4. Caching Crawlers MAY cache the fetched robots.txt file's contents. Crawlers MAY use standard cache control as defined in RFC2616 [8]. Crawlers SHOULD NOT use the cached version for more than 24 hours, unless the robots.txt is unreachable. 2.5. Limits Crawlers MAY impose a parsing limit that MUST be at least 500 kibibytes (KiB). 2.6. Security Considerations The Robots Exclusion Protocol MUST NOT be used as a form of security measures. Listing URIs in the robots.txt file exposes the URI publicly and thus making the URIs discoverable. 2.7. IANA Considerations. This document has no actions for IANA. 3. Examples 3.1. Simple example The following example shows: o *foobot*: A regular case. A single user-agent token followed by rules. o *barbot and bazbot*: A group that's relevant for more than one user-agent. o *quxbot:* Empty group at end of file. Koster, et al. Expires June 5, 2021 [Page 8] Internet-Draft I-D July 2019 User-Agent : foobot Disallow : /example/page.html Disallow : /example/disallowed.gif User-Agent : barbot User-Agent : bazbot Allow : /example/page.html Disallow : /example/disallowed.gif User-Agent: quxbot EOF 3.2. Longest Match The following example shows that in the case of a two rules, the longest one MUST be used for matching. In the following case, /example/page/disallowed.gif MUST be used for the URI example.com/example/page/disallow.gif . User-Agent : foobot Allow : /example/page/ Disallow : /example/page/disallowed.gif 4. References 4.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 2119, May 2017. 4.2. URIs [1] https://tools.ietf.org/html/rfc3986 [2] https://tools.ietf.org/html/rfc8288 [3] https://tools.ietf.org/html/rfc5234 [4] https://www.sitemaps.org/index.html [5] https://tools.ietf.org/html/rfc3629 [6] https://tools.ietf.org/html/rfc2046 Koster, et al. Expires June 5, 2021 [Page 9] Internet-Draft I-D July 2019 [7] https://tools.ietf.org/html/rfc1945 [8] https://tools.ietf.org/html/rfc2616 Authors' Address Martijn Koster Stalworthy Manor Farm Suton Lane, NR18 9JG Wymondham, Norfolk United Kingdom Email: m.koster@greenhills.co.uk Gary Illyes Brandschenkestrasse 110 8002, Zurich Switzerland Email: garyillyes@google.com Henner Zeller 1600 Amphitheatre Pkwy Mountain View, CA 94043 USA Email: henner@google.com Lizzi Harvey 1600 Amphitheatre Pkwy Mountain View, CA 94043 USA Email: lizzi@google.com Koster, et al. Expires June 5, 2021 [Page 10]