Developer Reference

Some values you look up again and again without ever memorizing them. For example: whether 302 or 307 is the redirect that preserves the request method, the MIME type for a woff2 font, or what 644 means when a deploy fails with a permission error.

Each page is a quick lookup. The HTTP status reference lists every code with what it means and when to send it, and filters as you type. The MIME type lookup works in both directions, from extension to type and back. The chmod calculator turns checkboxes into the octal number, and the octal number back into rwxr-xr-x. The user agent parser breaks a UA string into browser, engine, operating system and device, and can read your own.

Every page is static data or client-side parsing, so it loads quickly and keeps working offline once it has loaded.

Which tool for which job

  • Pick the right status code for an API response

    HTTP Status Code Reference. Filter as you type and read when each code should be sent.

  • Set the Content-Type for a file type

    MIME Type Lookup. Look up an extension to get its MIME type, or a MIME type to get its extensions.

  • Fix a permission denied error after a deploy

    Chmod Calculator. Tick read, write and execute for owner, group and others to get the octal number, or decode one like 755.

  • Work out which browser and OS a bug report came from

    User Agent Parser. Paste the user agent string to get the browser, engine, operating system and device.

Other tool categories

Specifications and references

This tool follows the specifications below. Read them when you need the exact normative wording, for example to settle an edge case between two implementations.

Frequently Asked Questions

What is the difference between 401 and 403?

401 Unauthorized means the request has no valid credentials. The client should log in or send a token, and the response should include a WWW-Authenticate header. 403 Forbidden means the server knows who is asking and still refuses, so sending the same credentials again will not help.

What do chmod 644 and 755 mean?

Each digit is the sum of read (4), write (2) and execute (1), for owner, group and others in that order. 644 lets the owner read and write and everyone else only read, which suits ordinary files. 755 adds execute for everyone, which scripts need in order to run and directories need so their contents can be opened.

Why do user agent strings all start with Mozilla?

For compatibility. In the 1990s, sites sent their richer pages only to browsers that identified as Mozilla, so each new browser added the token to get them. Chrome has since frozen and trimmed the details in its string, and sites that need the exact version should read User-Agent Client Hints.

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