created_at2022-03-19 11:58:23.580392
updated_at2023-03-29 12:23:36.349666
descriptionTexting Robots: A Rust native `robots.txt` parser with thorough unit testing.
Stephen Merity (Smerity)



Texting Robots

Workflow Status Crate API

Crate texting_robots is a library for parsing robots.txt files. A key design goal of this crate is to have a thorough test suite tested against real world data across millions of sites. While robots.txt is a simple specification itself the scale and complexity of the web teases out every possible edge case.

To read more about the robots.txt specification a good starting point is How Google interprets the robots.txt specification.

This library cannot guard you against all possible edge cases but should give you a strong starting point from which to ensure you and your code constitute a positive addition to the internet at large.


You can install the library by adding this entry:

texting_robots = "0.2"

to your Cargo.toml dependency list.

Overview of usage

This crate provides a simple high level usage through the Robot struct.

The Robot struct is responsible for consuming the robots.txt file, processing the contents, and deciding whether a given URL is allow for your bot or not. Additional information such as your bot's crawl delay and any sitemaps that may exist are also available.

Given the many options and potential preferences Texting Robots does not perform caching or a HTTP GET request of the robots.txt files themselves. This step is up to the user of the library.

use texting_robots::{Robot, get_robots_url};

// If you want to fetch a URL we'll find the URL for `robots.txt`
let url = "https://www.rust-lang.org/learn";
let robots_url = get_robots_url(url);
// Then we fetch `robots.txt` from robots_url to parse as below

// A `robots.txt` file in String or byte format.
let txt = r"User-Agent: FerrisCrawler
Allow: /ocean
Disallow: /rust
Disallow: /forest*.py
Crawl-Delay: 10
User-Agent: *
Disallow: /
Sitemap: https://www.example.com/site.xml";

// Build the Robot for our friendly User-Agent
let r = Robot::new("FerrisCrawler", txt.as_bytes()).unwrap();

// Ferris has a crawl delay of one second per limb
// (Crabs have 10 legs so Ferris must wait 10 seconds!)
assert_eq!(r.delay, Some(10.0));

// Any listed sitemaps are available for any user agent who finds them
assert_eq!(r.sitemaps, vec!["https://www.example.com/site.xml"]);

// We can also check which pages Ferris is allowed to crawl
// Notice we can supply the full URL or a relative path?
assert_eq!(r.allowed("https://www.rust-lang.org/ocean"), true);
assert_eq!(r.allowed("/ocean"), true);
assert_eq!(r.allowed("/ocean/reef.html"), true);
// Sadly Ferris is allowed in the ocean but not in the rust
assert_eq!(r.allowed("/rust"), false);
// Ferris is also friendly but not very good with pythons
assert_eq!(r.allowed("/forest/tree/snake.py"), false);

Crawling considerations

Obtaining robots.txt

To obtain robots.txt requires performing an initial HTTP GET request to the domain in question. When handling the HTTP status codes and how they impact robots.txt the suggestions made by Google are recommended.

  • 2xx (success): Attempt to process the resulting payload
  • 3xx (redirection): Follow a reasonable number of redirects
  • 4xx (client error): Assume there are no crawl restrictions except for:
    • 429 "Too Many Requests": Retry after a reasonable amount of time (potentially set by the "Retry-After" header)
  • 5xx (server errors): Assume you should not crawl until fixed and/or interpret with care

Even when directed to "assume no crawl restrictions" it is likely reasonable and polite to use a small fetch delay between requests.

Always set a User Agent

For crawling robots.txt (and especially for crawling in general) you should include a user agent in your request. Most crawling libraries offer adding the user agent in a single line.

ClientBuilder.new().user_agent("FerrisCrawler/0.1 (https://ferris.rust/about-this-robot)")...

Beyond respecting robots.txt providing a good user agent provides a line of communication between you and the web master.

Beyond the robots.txt specification and general suggestions

texting_robots provides much of what you need for safe and respectful crawling but is not a full solution by itself.

As an example, the HTTP error code 429 (Too Many Requests) must be tracked when requesting pages on a given site. When a 429 is seen the crawler should slow down, even if obeying the Crawl-Delay set in robots.txt, and potentially using the delay set by the server's Retry-After header.

An even more complex example is that multiple domains may back on to the same backend web server. This is a common scenario for specific products or services that host thousands or millions of domains. How you rate limit fairly using the Crawl-Delay is entirely up to the end user (and potentially the service when using HTTP error code 429 to rate limit traffic).

To protect against adverse input the user of Texting Robots is also suggested to follow Google's recommendations and limit input to 500 kibibytes. This is not yet done at the library level in case a larger input may be desired but may be revisited depending on community feedback.

Usage of Texting Robots in other languages

While not yet specifically supporting any languages other than Rust the library was designed to support language integrations in the future. Battle testing this intepretation of the robots.txt specification against the web is easier done testing with friends!

A C API through Rust FFI should be relatively easy to provide given Texting Robots only relies on strings, floats, and booleans. The lack of native fetching abilities should ensure the library is portable across platforms, situations, and languages.

A proof of concept was performed in WASI, the "WebAssembly System Interface", showing that the library compiles happily and only experiences a 50% or 75% speed penalty when used with the Wasmer (LLVM backend) and Wasmtime runtimes respectively. No optimizations have been done thus far and there's likely low hanging fruit to reap.

See wasi_poc.sh for details.


To run the majority of core tests simply execute cargo test.

Unit and Integration Tests

To check Texting Robot's behaviour against the robots.txt specification almost all unit tests from Google's C++ robots.txt parser and Moz's reppy have been translated and included.

Certain aspects of the Google and Moz interpretation disagree with each other. When this occurred the author deferred to as much common sense as they were able to muster.

For a number of popular domains the robots.txt of the given domain was saved and tests written against them.

Common Crawl Test Harness

To ensure that the robots.txt parser will not panic in real world situations over 34 million robots.txt responses were passed through Texting Robots. While this test doesn't guarantee the robots.txt files were handled correctly it does ensure the parser is unlikely to panic during practice.

Many problematic, invalid, outrageous, and even adversarial robots.txt examples were discovered in this process.

For full details see the Common Crawl testing harness.

Fuzz Testing

In the local fuzz directory is a fuzz testing harness. The harness is not particularly sophisticated but does utilize a low level of structure awareness through utilizing dictionary guided fuzzing. The harness has already revealed one low level unwrap panic.

To run:

cargo fuzz run fuzz_target_1 -- -max_len=512 -dict=keywords.dict


  • cargo fuzz requires nightly (i.e. run rustup default nightly in the fuzz directory)
  • If you have multiple processors you may wish to add --jobs N after cargo run

Code Coverage with Tarpaulin

This project uses Tarpaulin to perform code coverage reporting. Given the relatively small surface area of the parser and Robot struct the coverage is high. Unit testing is more important for ensuring behavioural correctness however.

To get line numbers for uncovered code run:

cargo tarpaulin --ignore-tests -v


Licensed under either of

at your option.


Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.

Commit count: 71

cargo fmt