8 Headers: Parsing and Constructing

The net/head module provides utilities for parsing and constructing RFC 822 headers [RFC822], which are used in protocols such as HTTP, SMTP, and NNTP.

A header is represented as a string or byte string containing CRLF-delimited lines. Each field within the header spans one or more lines. In addition, the header ends with two CRLFs (because the first one terminates the last field, and the second terminates the header).

8.1 Functions

value
empty-header : string?

The string "\r\n\r\n", which corresponds to the empty header. This value is useful for building up headers with insert-field and append-headers.

procedure
(validate-header candidate) → void?
candidate : (or string? bytes?)

Checks that candidate matches RFC 822. If it does not, an exception is raised.

procedure
(extract-field field header) → (or/c string? bytes? false/c)
field : (or/c string? bytes?)
header : (or/c string? bytes?)

Returns the header content for the specified field, or #f if the field is not in the header. The field string should not end with ":", and it is used case-insensitively. The returned string will not contain the field name, color separator, or CRLF terminator for the field; however, if the field spans multiple lines, the CRLFs separating the lines will be intact.

The field and header arguments must be both strings or both byte strings, and the result (if not #f) is of the same type.

Example:

> (extract-field "TO" (insert-field "to" "me@localhost"
empty-header))
"me@localhost"

procedure
(extract-all-fields header)
→
(listof (cons/c (or/c string? bytes?)
(or/c string? bytes?)))
header : (or/c string? bytes?)

Returns an association-list version of the header; the case of the field names is preserved, as well as the order and duplicate uses of a field name.

The result provides strings if header is a string, byte strings if header is a byte string.

procedure
(remove-field field header) → (or/c string? bytes?)
field : (or/c string? bytes?)
header : (or/c string? bytes?)

Creates a new header by removing the specified field from header (or the first instance of the field, if it occurs multiple times). If the field is not in header, then the return value is header.

The field and header arguments must be both strings or both byte strings, and the result is of the same type.

procedure
(insert-field field value header) → (or/c string? bytes?)
  field : (or/c string? bytes?)
  value : (or/c string? bytes?)
  header : (or/c string? bytes?)

Creates a new header by prefixing the given header with the given field-value pair. The value string should not contain a terminating CRLF, but a multi-line value (perhaps created with data-lines->data) may contain separator CRLFs.

The field, value, and header arguments must be all strings or all byte strings, and the result is of the same type.

procedure
(replaces-field field value header) → (or/c string? bytes?)
  field : (or/c string? bytes?)
  value : (or/c string? bytes? false/c)
  header : (or/c string? bytes?)

Composes remove-field and (if value is not #f) insert-field.

procedure
(append-headers header1 header2) → (or/c string? bytes?)
header1 : (or/c string? bytes?)
header2 : (or/c string? bytes?)

Appends two headers.

The header1 and header2 arguments must be both strings or both byte strings, and the result is of the same type.

procedure
(standard-message-header from
to
cc
bcc
subject) → string?
  from : string?
  to : (listof string?)
  cc : (listof string?)
  bcc : (listof string?)
  subject : string?

Creates a standard mail header given the sender, various lists of recipients, a subject. A "Date" field is added to the header automatically, using the current time.

The BCC recipients do not actually appear in the header, but they’re accepted anyway to complete the abstraction.

procedure
(data-lines->data listof) → string?
listof : string?

Merges multiple lines for a single field value into one string, adding CRLF-TAB separators.

procedure
(extract-addresses line kind)
→
(or/c (listof string?)
      (listof (list/c string? string? string?)))
  line : string?
   kind :
(one-of/c 'name 'address
          'full 'all)

Parses string as a list of comma-delimited mail addresses, raising an exception if the list is ill-formed. This procedure can be used for single-address strings, in which case the returned list contains only one address.

The kind argument specifies which portion of an address should be returned:

'name — the free-form name in the address, or the address itself if no name is available.

Examples:

> (extract-addresses "John Doe <doe@localhost>" 'name)
'("John Doe")
> (extract-addresses "doe@localhost (Johnny Doe)" 'name)
'("Johnny Doe")
> (extract-addresses "doe@localhost" 'name)
'("doe@localhost")
> (extract-addresses " \"Doe, John\" <doe@localhost>, jane"
'name)
'("\"Doe, John\"" "jane")

'address — just the mailing address, without any free-form names.

Examples:

> (extract-addresses "John Doe <doe@localhost>" 'address)
'("doe@localhost")
> (extract-addresses "doe@localhost (Johnny Doe)" 'address)
'("doe@localhost")
> (extract-addresses "doe@localhost" 'address)
'("doe@localhost")
> (extract-addresses " \"Doe, John\" <doe@localhost>, jane"
'address)
'("doe@localhost" "jane")

'full — the full address, essentially as it appears in the input, but normalized.

Examples:

> (extract-addresses "John Doe   < doe@localhost >" 'full)
'("John Doe <doe@localhost>")
> (extract-addresses "  doe@localhost  (Johnny Doe)" 'full)
'("doe@localhost (Johnny Doe)")
> (extract-addresses "doe@localhost" 'full)
'("doe@localhost")
> (extract-addresses " \"Doe, John\" <doe@localhost>, jane"
                     'full)
'("\"Doe, John\" <doe@localhost>" "jane")

'all — a list containing each of the three possibilities: free-form name, address, and full address (in that order).

Examples:

> (extract-addresses "John Doe <doe@localhost>" 'all)
'(("John Doe" "doe@localhost" "John Doe <doe@localhost>"))
> (extract-addresses "doe@localhost (Johnny Doe)" 'all)
'(("Johnny Doe" "doe@localhost" "doe@localhost (Johnny Doe)"))
> (extract-addresses "doe@localhost" 'all)
'(("doe@localhost" "doe@localhost" "doe@localhost"))
> (define r
(extract-addresses " \"John\" <doe@localhost>, jane"
'all))
> (length r)
2
> (car r)
'("\"John\"" "doe@localhost" "\"John\" <doe@localhost>")
> (cadr r)
'("jane" "jane" "jane")

procedure
(assemble-address-field addrs) → string?
addrs : (listof string?)

Creates a header field value from a list of addresses. The addresses are comma-separated, and possibly broken into multiple lines.

Example:

> (assemble-address-field '("doe@localhost"
"Jane <jane@elsewhere>"))
"doe@localhost, Jane <jane@elsewhere>"

8.2 Header Unit

head@ and head^ are deprecated. They exist for backward-compatibility and will likely be removed in the future. New code should use the net/head module.

(require net/head-unit)

value
head@ : unit?

Imports nothing, exports head^.

8.3 Header Signature

(require net/head-sig)

signature
head^ : signature

Includes everything exported by the net/head module.

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1	URLs and HTTP
2	URI Codec: Encoding and Decoding URIs
3	Web Socket
4	FTP: Client
5	Send URL: Opening a Web Browser
6	SMTP: Sending E-Mail
7	sendmail: Sending E-Mail
8	Headers: Parsing and Constructing
9	IMAP: Reading Mail
10	POP3: Reading Mail
11	MIME: Decoding Internet Data
12	Base 64: Encoding and Decoding
13	Quoted-Printable: Encoding and Decoding
14	DNS: Domain Name Service Queries
15	NNTP: Newsgroup Protocol
16	TCP: Unit and Signature
17	TCP Redirect: tcp^ via Channels
18	SSL Unit: tcp^ via SSL
19	CGI Scripts
20	Cookie: HTTP Client Storage
	Bibliography
	Index