Internet Engineering Task Force (IETF)                         M. Koster
Request for Comments: 9309
Category: Standards Track                                      G. Illyes
ISSN: 2070-1721                                                H. Zeller
                                                              L. Sassman
                                                              Google LLC
                                                          September 2022


                       Robots Exclusion Protocol

Abstract

   This document specifies and extends the "Robots Exclusion Protocol"
   method originally defined by Martijn Koster in 1994 for service
   owners to control how content served by their services may be
   accessed, if at all, by automatic clients known as crawlers.
   Specifically, it adds definition language for the protocol,
   instructions for handling errors, and instructions for caching.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9309.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
     1.1.  Requirements Language
   2.  Specification
     2.1.  Protocol Definition
     2.2.  Formal Syntax
       2.2.1.  The User-Agent Line
       2.2.2.  The "Allow" and "Disallow" Lines
       2.2.3.  Special Characters
       2.2.4.  Other Records
     2.3.  Access Method
       2.3.1.  Access Results
         2.3.1.1.  Successful Access
         2.3.1.2.  Redirects
         2.3.1.3.  "Unavailable" Status
         2.3.1.4.  "Unreachable" Status
         2.3.1.5.  Parsing Errors
     2.4.  Caching
     2.5.  Limits
   3.  Security Considerations
   4.  IANA Considerations
   5.  Examples
     5.1.  Simple Example
     5.2.  Longest Match
   6.  References
     6.1.  Normative References
     6.2.  Informative References
   Authors' Addresses

1.  Introduction

   This document applies to services that provide resources that clients
   can access through URIs as defined in [RFC3986].  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].

   It may be inconvenient for service owners if crawlers visit the
   entirety of their URI space.  This document specifies the rules
   originally defined by the "Robots Exclusion Protocol" [ROBOTSTXT]
   that crawlers are requested to honor when accessing URIs.

   These rules are not a form of access authorization.

1.1.  Requirements Language

   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.

2.  Specification

2.1.  Protocol Definition

   The protocol language consists of rule(s) and group(s) that the
   service makes available in a file named "robots.txt" as described in
   Section 2.3:

   Rule:  A line with a key-value pair that defines how a crawler may
      access URIs.  See Section 2.2.2.

   Group:  One or more user-agent lines that are followed by one or more
      rules.  The group is terminated by a user-agent line or end of
      file.  See Section 2.2.1.  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].

    robotstxt = *(group / emptyline)
    group = startgroupline                ; We start with a user-agent
                                          ; line
           *(startgroupline / emptyline)  ; ... and possibly more
                                          ; user-agent lines
           *(rule / emptyline)            ; followed by rules relevant
                                          ; for the preceding
                                          ; user-agent lines

    startgroupline = *WS "user-agent" *WS ":" *WS product-token EOL

    rule = *WS ("allow" / "disallow") *WS ":"
          *WS (path-pattern / empty-pattern) EOL

    ; parser implementors: define additional lines you need (for
    ; example, Sitemaps).

    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

    ; UTF8 derived from RFC 3629, 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 their own name, which is called a product token, to find
   relevant groups.  The product token MUST contain only uppercase and
   lowercase letters ("a-z" and "A-Z"), underscores ("_"), and hyphens
   ("-").  The product token SHOULD be a substring of the identification
   string that the crawler sends to the service.  For example, in the
   case of HTTP [RFC9110], the product token SHOULD be a substring in
   the User-Agent header.  The identification string SHOULD describe the
   purpose of the crawler.  Here's an example of a User-Agent HTTP
   request header with a link pointing to a page describing the purpose
   of the ExampleBot crawler, which appears as a substring in the User-
   Agent HTTP header and as a product token in the robots.txt user-agent
   line:

   +==========================================+========================+
   | User-Agent HTTP header                   | robots.txt user-agent  |
   |                                          | line                   |
   +==========================================+========================+
   | User-Agent: Mozilla/5.0 (compatible;     | user-agent: ExampleBot |
   | ExampleBot/0.1;                          |                        |
   | https://www.example.com/bot.html)        |                        |
   +------------------------------------------+------------------------+

        Figure 1: Example of a User-Agent HTTP header and robots.txt
              user-agent line for the ExampleBot product token

   Note that the product token (ExampleBot) is a substring of the User-
   Agent HTTP header.

   Crawlers MUST use case-insensitive matching to find the group that
   matches the product token 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 and parsed according to
   Section 2.2.2.

    +========================================+========================+
    | Two groups that match the same product | Merged group           |
    | token exactly                          |                        |
    +========================================+========================+
    | user-agent: ExampleBot                 | user-agent: ExampleBot |
    | disallow: /foo                         | disallow: /foo         |
    | disallow: /bar                         | disallow: /bar         |
    |                                        | disallow: /baz         |
    | user-agent: ExampleBot                 |                        |
    | disallow: /baz                         |                        |
    +----------------------------------------+------------------------+

        Figure 2: Example of how to merge two robots.txt groups that
                        match the same product token

   If no matching group exists, crawlers MUST obey the group with a
   user-agent line with the "*" value, if present.

        +==================================+======================+
        | Two groups that don't explicitly | Applicable group for |
        | match ExampleBot                 | ExampleBot           |
        +==================================+======================+
        | user-agent: *                    | user-agent: *        |
        | disallow: /foo                   | disallow: /foo       |
        | disallow: /bar                   | disallow: /bar       |
        |                                  |                      |
        | user-agent: BazBot               |                      |
        | disallow: /baz                   |                      |
        +----------------------------------+----------------------+

       Figure 3: Example of no matching groups other than the "*" for
                        the ExampleBot product token

   If no group matches the product token and there is no group with a
   user-agent line with the "*" value, 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 crawler MUST match the
   paths in "allow" and "disallow" rules against the URI.  The matching
   SHOULD be case sensitive.  The matching MUST start with the first
   octet of the path.  The most specific match found MUST be used.  The
   most specific match is the match that has the most octets.  Duplicate
   rules in a group MAY be deduplicated.  If an "allow" rule and a
   "disallow" rule are equivalent, then the "allow" rule SHOULD be used.
   If no match is found amongst the rules in a group for a 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 ASCII
   coded character set, and those in the reserved range defined by
   [RFC3986], MUST be percent-encoded as defined by [RFC3986] prior to
   comparison.

   If a percent-encoded 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] 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=    | /foo/bar?baz=         | /foo/bar?baz=         |
   | https://foo.bar  | https%3A%2F%2Ffoo.bar | https%3A%2F%2Ffoo.bar |
   +------------------+-----------------------+-----------------------+
   | /foo/bar/        | /foo/bar/%E3%83%84    | /foo/bar/%E3%83%84    |
   | U+E38384         |                       |                       |
   +------------------+-----------------------+-----------------------+
   | /foo/            | /foo/bar/%E3%83%84    | /foo/bar/%E3%83%84    |
   | bar/%E3%83%84    |                       |                       |
   +------------------+-----------------------+-----------------------+
   | /foo/            | /foo/bar/%62%61%7A    | /foo/bar/baz          |
   | bar/%62%61%7A    |                       |                       |
   +------------------+-----------------------+-----------------------+

       Figure 4: Examples of matching percent-encoded URI components

   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).

   Implementors MAY bridge encoding mismatches if they detect that the
   robots.txt file is not UTF-8 encoded.

2.2.3.  Special Characters

   Crawlers MUST support the following special characters:

     +===========+===================+==============================+
     | Character | Description       | Example                      |
     +===========+===================+==============================+
     | #         | Designates a line | allow: / # comment in line   |
     |           | comment.          |                              |
     |           |                   | # comment on its own line    |
     +-----------+-------------------+------------------------------+
     | $         | Designates the    | allow: /this/path/exactly$   |
     |           | end of the match  |                              |
     |           | pattern.          |                              |
     +-----------+-------------------+------------------------------+
     | *         | Designates 0 or   | allow: /this/*/exactly       |
     |           | more instances of |                              |
     |           | any character.    |                              |
     +-----------+-------------------+------------------------------+

          Figure 5: List of special characters in robots.txt files

   If crawlers match special characters verbatim in the URI, crawlers
   SHOULD use "%" encoding.  For example:

    +============================+====================================+
    | Percent-encoded Pattern    | URI                                |
    +============================+====================================+
    | /path/file-with-a-%2A.html | https://www.example.com/path/      |
    |                            | file-with-a-*.html                 |
    +----------------------------+------------------------------------+
    | /path/foo-%24              | https://www.example.com/path/foo-$ |
    +----------------------------+------------------------------------+

                   Figure 6: Example of percent-encoding

2.2.4.  Other Records

   Crawlers MAY interpret other records that are not part of the
   robots.txt protocol -- for example, "Sitemaps" [SITEMAPS].  Crawlers
   MAY be lenient when interpreting other records.  For example,
   crawlers may accept common misspellings of the record.

   Parsing of other records MUST NOT interfere with the parsing of
   explicitly defined records in Section 2.  For example, a "Sitemaps"
   record MUST NOT terminate a group.

2.3.  Access Method

   The rules MUST be accessible in a file named "/robots.txt" (all
   lowercase) in the top-level path of the service.  The file MUST be
   UTF-8 encoded (as defined in [RFC3629]) and Internet Media Type
   "text/plain" (as defined in [RFC2046]).

   As per [RFC3986], the URI of the robots.txt file is:

   "scheme:[//authority]/robots.txt"

   For example, in the context of HTTP or FTP, the URI is:

             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 file, the
   crawler MUST follow the parseable rules.

2.3.1.2.  Redirects

   It's possible that a server responds to a robots.txt fetch request
   with a redirect, such as HTTP 301 or HTTP 302 in the case of HTTP.
   The crawlers SHOULD follow at least five consecutive redirects, even
   across authorities (for example, hosts in the case of HTTP).

   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 file is unavailable.

2.3.1.3.  "Unavailable" Status

   "Unavailable" means the crawler tries to fetch the robots.txt file
   and the server responds with status codes indicating that the
   resource in question is unavailable.  For example, in the context of
   HTTP, such status codes are in the 400-499 range.

   If a server status code indicates that the robots.txt file is
   unavailable to the crawler, then the crawler MAY access any resources
   on the server.

2.3.1.4.  "Unreachable" Status

   If the robots.txt file is unreachable due to server or network
   errors, this means the robots.txt file is undefined and the crawler
   MUST assume complete disallow.  For example, in the context of HTTP,
   server errors are identified by status codes in the 500-599 range.

   If the robots.txt file is undefined for a reasonably long period of
   time (for example, 30 days), crawlers MAY assume that the robots.txt
   file is unavailable as defined in Section 2.3.1.3 or continue to use
   a cached copy.

2.3.1.5.  Parsing Errors

   Crawlers MUST 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 [RFC9111].  Crawlers
   SHOULD NOT use the cached version for more than 24 hours, unless the
   robots.txt file is unreachable.

2.5.  Limits

   Crawlers SHOULD impose a parsing limit to protect their systems; see
   Section 3.  The parsing limit MUST be at least 500 kibibytes [KiB].

3.  Security Considerations

   The Robots Exclusion Protocol is not a substitute for valid content
   security measures.  Listing paths in the robots.txt file exposes them
   publicly and thus makes the paths discoverable.  To control access to
   the URI paths in a robots.txt file, users of the protocol should
   employ a valid security measure relevant to the application layer on
   which the robots.txt file is served -- for example, in the case of
   HTTP, HTTP Authentication as defined in [RFC9110].

   To protect against attacks against their system, implementors of
   robots.txt parsing and matching logic should take the following
   considerations into account:

   Memory management:  Section 2.5 defines the lower limit of bytes that
      must be processed, which inherently also protects the parser from
      out-of-memory scenarios.

   Invalid characters:  Section 2.2 defines a set of characters that
      parsers and matchers can expect in robots.txt files.  Out-of-bound
      characters should be rejected as invalid, which limits the
      available attack vectors that attempt to compromise the system.

   Untrusted content:  Implementors should treat the content of a
      robots.txt file as untrusted content, as defined by the
      specification of the application layer used.  For example, in the
      context of HTTP, implementors should follow the Security
      Considerations section of [RFC9110].

4.  IANA Considerations

   This document has no IANA actions.

5.  Examples

5.1.  Simple Example

   The following example shows:

   *:  A group that's relevant to all user agents that don't have an
      explicitly defined matching group.  It allows access to the URLs
      with the /publications/ path prefix, and it restricts access to
      the URLs with the /example/ path prefix and to all URLs with a
      .gif suffix.  The "*" character designates any character,
      including the otherwise-required forward slash; see Section 2.2.

   foobot:  A regular case.  A single user agent followed by rules.  The
      crawler only has access to two URL path prefixes on the site --
      /example/page.html and /example/allowed.gif.  The rules of the
      group are missing the optional space character, which is
      acceptable as defined in Section 2.2.

   barbot and bazbot:  A group that's relevant for more than one user
      agent.  The crawlers are not allowed to access the URLs with the
      /example/page.html path prefix but otherwise have unrestricted
      access to the rest of the URLs on the site.

   quxbot:  An empty group at the end of the file.  The crawler has
      unrestricted access to the URLs on the site.

               User-Agent: *
               Disallow: *.gif$
               Disallow: /example/
               Allow: /publications/

               User-Agent: foobot
               Disallow:/
               Allow:/example/page.html
               Allow:/example/allowed.gif

               User-Agent: barbot
               User-Agent: bazbot
               Disallow: /example/page.html

               User-Agent: quxbot

               EOF

5.2.  Longest Match

   The following example shows that in the case of two rules, the
   longest one is 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

6.  References

6.1.  Normative References

   [RFC2046]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part Two: Media Types", RFC 2046,
              DOI 10.17487/RFC2046, November 1996,
              <https://www.rfc-editor.org/info/rfc2046>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
              2003, <https://www.rfc-editor.org/info/rfc3629>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8288]  Nottingham, M., "Web Linking", RFC 8288,
              DOI 10.17487/RFC8288, October 2017,
              <https://www.rfc-editor.org/info/rfc8288>.

   [RFC9110]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Semantics", STD 97, RFC 9110,
              DOI 10.17487/RFC9110, June 2022,
              <https://www.rfc-editor.org/info/rfc9110>.

   [RFC9111]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Caching", STD 98, RFC 9111,
              DOI 10.17487/RFC9111, June 2022,
              <https://www.rfc-editor.org/info/rfc9111>.

6.2.  Informative References

   [KiB]      "Kibibyte", Simple English Wikipedia, the free
              encyclopedia, 17 September 2020,
              <https://simple.wikipedia.org/wiki/Kibibyte>.

   [ROBOTSTXT]
              "The Web Robots Pages (including /robots.txt)", 2007,
              <https://www.robotstxt.org/>.

   [SITEMAPS] "What are Sitemaps? (Sitemap protocol)", April 2020,
              <https://www.sitemaps.org/index.html>.

Authors' Addresses

   Martijn Koster
   Stalworthy Manor Farm
   Suton Lane
   Wymondham, Norfolk
   NR18 9JG
   United Kingdom
   Email: m.koster@greenhills.co.uk


   Gary Illyes
   Google LLC
   Brandschenkestrasse 110
   CH-8002 Zürich
   Switzerland
   Email: garyillyes@google.com


   Henner Zeller
   Google LLC
   1600 Amphitheatre Pkwy
   Mountain View, CA 94043
   United States of America
   Email: henner@google.com


   Lizzi Sassman
   Google LLC
   Brandschenkestrasse 110
   CH-8002 Zürich
   Switzerland
   Email: lizzi@google.com