@cto.af/http-headers

Parse HTTP headers from RFC 9110 (and a bunch of others) using the full ABNF.

If there is a specified and non-deprecated header you want parsed and it is not yet supported, please file an issue. I won't be tracking all of the revisions to all of the docs, but I will fix issues if they are pointed out to me.

This code was tested against the headers returned by the top 50 websites as reported by wikipedia on the day that I looked in November 2024. I made sure that all of the non-custom headers that were in use that day by 3 or more of those sites was supported here.

Installation

npm install @cto.af/http-headers

Caveats

• Check for max headers size before calling this parser. Many servers choose 8k or 16k as their maximum.
• Check the unknown property of headers. Headers that are supported, but have syntax errors, are treated as if they are unknown, un-parseable headers. They will have always have these properties:
  • kind: lowercased header name
  • name: original header name
  • value: full text of the header, to the first newline
  • unknown: true
• The option obsolete: true can be passed in to the parse function to enable a bunch of obsolete rules in processing email addresses (and a few other obs_* productions). Hopefully none of those productions have never actually been used on the web, but I have included them for completeness, and left the obsolete flag in place mostly for testing purposes.
• I've tried to stay as faithful to the ABNF for each header as possible. However, the definitions are rife with different understandings of how ABNF works. In particular, Parser Expression Grammars (PEGs) parse by trying each alternate successively until one matches. If an alternate always matches (e.g. *"foo", which matches the empty string), then none of the subsequent alternates are ever checked. Similarly, if one of two alternates is the prefix for another (e.g. "foo" and "foobar"), the longer prefix must be checked first. There are several places where look-ahead assertions were required to deal with these sorts of issues, or to ensure testability.

API

Example:

import {parse} from '@cto.af/http-headers';

// By default, the parser is strict about carriage-return/newline pairs.
const headers = parse('Date: Sun, 06 Nov 1994 08:49:37 GMT\r\n\r\n');
// [
//   {
//     kind: 'date',
//     value: 'Sun, 06 Nov 1994 08:49:37 GMT',
//     date: 1994-11-06T08:49:37.000Z,
//     name: 'Date'
//   }
// ]

// Rules named for a header (camel-case, with dashes turned into underscores)
// parse everything after the colon+whitespace for the specified header.
// Use the "startRule" option to select a specific header type.
const contentType = parse('text/html;charset=utf8', {
  startRule: 'Content_Type',
});

// {
//   kind: 'content_type',
//   value: 'text/html;charset=utf8',
//   type: 'text',
//   subtype: 'html',
//   parameters: { charset: 'utf8' }
// }

// Unknown headers, or headers that had invalid values will have 'unknown: true'
// in their parsed version.
const unknownHeader = parse('Foo: bar=baz', {startRule: 'Header'});
// {
//   kind: 'foo',
//   name: 'Foo',
//   value: 'bar=baz',
//   unknown: true
// }

See the Peggy docs for more information on the parse function.

Here are the supported values for startRule:

• 'Headers': default
• 'Headers_Loose': Accept "\r\n" or "\n" at the end of lines
• 'Header': Any single header line
• 'Accept'
• 'Accept_CH'
• 'Accept_Charset'
• 'Accept_Encoding'
• 'Accept_Language'
• 'Accept_Ranges'
• 'Access-Control-Allow-Credentials'
• 'Access-Control-Allow-Headers'
• 'Access-Control-Allow-Methods'
• 'Access-Control-Allow-Origin'
• 'Access-Control-Expose-Headers'
• 'Access-Control-Max-Age'
• 'Access-Control-Request-Headers'
• 'Access-Control-Request-Method'
• 'Age'
• 'Allow'
• 'ALPN'
• 'Alt_Svc'
• 'Authentication_Info'
• 'Authorization'
• 'Cache_Control'
• 'Connection'
• 'Content_Encoding'
• 'Content_Language'
• 'Content_Length'
• 'Content_Location'
• 'Content_Range'
• 'Content_Security_Policy'
• 'Content_Security_Policy_Report_Only'
• 'Content_Type'
• 'Cross_Origin_Embedder_Policy'
• 'Cross_Origin_Embedder_Policy_Report_Only'
• 'Cross_Origin_Opener_Policy'
• 'Cross_Origin_Opener_Policy_Report_Only'
• 'Cross_Origin_Resource_Policy'
• 'Date'
• 'ETag'
• 'Expect'
• 'Expires'
• 'From'
• 'Host'
• 'If_Match'
• 'If_Modified_Since'
• 'If_None_Match'
• 'If_Range'
• 'If_Unmodified_Since'
• 'Last_Modified'
• 'Location'
• 'Link'
• 'Max_Forwards'
• 'NEL'
• 'Permissions_Policy'
• 'Proxy_Authenticate'
• 'Proxy_Authentication_Info'
• 'Proxy_Authorization'
• 'Range'
• 'Referer'
• 'Referrer_Policy'
• 'Reporting_Endpoints'
• 'Retry_After'
• 'Server'
• 'Server_Timing'
• 'Set_Cookie'
• 'Strict_Transport_Security'
• 'TE'
• 'Trailer'
• 'Upgrade'
• 'User_Agent'
• 'Vary'
• 'Via'
• 'WWW_Authenticate'
• 'Unknown_Header'

Development

To try a rule out without having to rebuild, do a variation of this:

curl -si --head https://github.com/ | \
  tail -n +2 | \
  node_modules/.bin/peggy src/index.peggy --format es -T- -S Headers_Loose

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