• Introduction
• Method
• Format
• Examples
• Example Code

Introduction

In 1993 and 1994 there have been occasions where robots have visited WWW servers where they weren't welcome for various reasons. Sometimes these reasons were robot specific, e.g. certain robots swamped servers with rapid-fire requests, or retrieved the same files repeatedly. In other situations robots traversed parts of WWW servers that weren't suitable, e.g. very deep virtual trees, duplicated information, temporary information, or cgi-scripts with side-effects (such as voting).

These incidents indicated the need for established mechanisms for WWW servers to indicate to robots which parts of their server should not be accessed. This standard addresses this need with an operational solution.

The Method

This approach was chosen because it can be easily implemented on any existing WWW server, and a robot can find the access policy with only a single document retrieval.

A possible drawback of this single-file approach is that only a server administrator can maintain such a list, not the individual document maintainers on the server. This can be resolved by a local process to construct the single file from a number of others, but if, or how, this is done is outside of the scope of this document.

The choice of the URL was motivated by several criteria:

• The filename should fit in file naming restrictions of all common operating systems.
• The filename extension should not require extra server configuration.
• The filename should indicate the purpose of the file and be easy to remember.
• The likelihood of a clash with existing files should be minimal.

The Format

The file consists of one or more records separated by one or more blank lines (terminated by CR,CR/NL, or NL). Each record contains lines of the form "<field>:<optionalspace><value><optionalspace>". The field name is case insensitive.

Comments can be included in file using UNIX bourne shell conventions: the '#' character is used to indicate that preceding space (if any) and the remainder of the line up to the line termination is discarded. Lines containing only a comment are discarded completely, and therefore do not indicate a record boundary.

The record starts with one or more User-agent lines, followed by one or more Disallow lines, as detailed below. Unrecognised headers are ignored.

User-agent

The value of this field is the name of the robot the record is describing access policy for.

If more than one User-agent field is present the record describes an identical access policy for more than one robot. At least one field needs to be present per record.

The robot should be liberal in interpreting this field. A case insensitive substring match of the name without version information is recommended.

If the value is '*', the record describes the default access policy for any robot that has not matched any of the other records. It is not allowed to have multiple such records in the "/robots.txt" file.

Disallow

The value of this field specifies a partial URL that is not to be visited. This can be a full path, or a partial path; any URL that starts with this value will not be retrieved. For example, Disallow: /help disallows both /help.html and /help/index.html, whereas Disallow: /help/ would disallow /help/index.html but allow /help.html.

Any empty value, indicates that all URLs can be retrieved. At least one Disallow field needs to be present in a record.

Examples

# robots.txt for http://www.example.com/

User-agent: *
Disallow: /cyberworld/map/ # This is an infinite virtual URL space
Disallow: /tmp/ # these will soon disappear
Disallow: /foo.html

# robots.txt for http://www.example.com/

User-agent: *
Disallow: /cyberworld/map/ # This is an infinite virtual URL space

# Cybermapper knows where to go.
User-agent: cybermapper
Disallow:

# go away
User-agent: *
Disallow: /

Example Code

Note: This code is no longer available. Instead I recommend using the robots exclusion code in the Perl libwww-perl5 library, available from CPAN in the LWP directory.