emacs/doc/misc/url.texi

\input texinfo
@setfilename ../../info/url
@settitle URL Programmer's Manual

@iftex
@c @finalout
@end iftex
@c @setchapternewpage odd
@c @smallbook

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@dircategory Emacs lisp libraries
@direntry
* URL: (url).                   URL loading package.
@end direntry

@copying
This is the manual for the @code{url} Emacs Lisp library.

Copyright @copyright{} 1993--1999, 2002, 2004--2013 Free Software
Foundation, Inc.

@quotation
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with no
Invariant Sections, with the Front-Cover texts being ``A GNU Manual,''
and with the Back-Cover Texts as in (a) below.  A copy of the license
is included in the section entitled ``GNU Free Documentation License''.

(a) The FSF's Back-Cover Text is: ``You have the freedom to copy and
modify this GNU manual.''
@end quotation
@end copying

@c
@titlepage
@title URL Programmer's Manual
@subtitle First Edition, URL Version 2.0
@author William M. Perry @email{wmperry@@gnu.org}
@author David Love @email{fx@@gnu.org}
@page
@vskip 0pt plus 1filll
@insertcopying
@end titlepage

@contents

@node Top
@top URL

@ifnottex
@insertcopying
@end ifnottex

@menu
* Introduction::                About the @code{url} library.
* URI Parsing::                 Parsing (and unparsing) URIs.
* Retrieving URLs::             How to use this package to retrieve a URL.
* Supported URL Types::         Descriptions of URL types currently supported.
* General Facilities::          URLs can be cached, accessed via a gateway
                                and tracked in a history list.
* Customization::               Variables you can alter.
* GNU Free Documentation License:: The license for this documentation.
* Function Index::
* Variable Index::
* Concept Index::
@end menu

@node Introduction
@chapter Introduction
@cindex URL
@cindex URI
@cindex uniform resource identifier
@cindex uniform resource locator

A @dfn{Uniform Resource Identifier} (URI) is a specially-formatted
name, such as an Internet address, that identifies some name or
resource.  The format of URIs is described in RFC 3986, which updates
and replaces the earlier RFCs 2732, 2396, 1808, and 1738.  A
@dfn{Uniform Resource Locator} (URL) is an older but still-common
term, which basically refers to a URI corresponding to a resource that
can be accessed (usually over a network) in a specific way.

  Here are some examples of URIs (taken from RFC 3986):

@example
ftp://ftp.is.co.za/rfc/rfc1808.txt
http://www.ietf.org/rfc/rfc2396.txt
ldap://[2001:db8::7]/c=GB?objectClass?one
mailto:John.Doe@@example.com
news:comp.infosystems.www.servers.unix
tel:+1-816-555-1212
telnet://192.0.2.16:80/
urn:oasis:names:specification:docbook:dtd:xml:4.1.2
@end example

  This manual describes the @code{url} library, an Emacs Lisp library
for parsing URIs and retrieving the resources to which they refer.
(The library is so-named for historical reasons; nowadays, the ``URI''
terminology is regarded as the more general one, and ``URL'' is
technically obsolete despite its widespread vernacular usage.)

@node URI Parsing
@chapter URI Parsing

  A URI consists of several @dfn{components}, each having a different
meaning.  For example, the URI

@example
http://www.gnu.org/software/emacs/
@end example

@noindent
specifies the scheme component @samp{http}, the hostname component
@samp{www.gnu.org}, and the path component @samp{/software/emacs/}.

@cindex parsed URIs
  The format of URIs is specified by RFC 3986.  The @code{url} library
provides the Lisp function @code{url-generic-parse-url}, a (mostly)
standard-compliant URI parser, as well as function
@code{url-recreate-url}, which converts a parsed URI back into a URI
string.

@defun url-generic-parse-url uri-string
This function returns a parsed version of the string @var{uri-string}.
@end defun

@defun url-recreate-url uri-obj
@cindex unparsing URLs
Given a parsed URI, this function returns the corresponding URI string.
@end defun

@cindex parsed URI
  The return value of @code{url-generic-parse-url}, and the argument
expected by @code{url-recreate-url}, is a @dfn{parsed URI}: a CL
structure whose slots hold the various components of the URI@.
@xref{top,the CL Manual,,cl,GNU Emacs Common Lisp Emulation}, for
details about CL structures.  Most of the other functions in the
@code{url} library act on parsed URIs.

@menu
* Parsed URIs::           Format of parsed URI structures.
* URI Encoding::          Non-@acronym{ASCII} characters in URIs.
@end menu

@node Parsed URIs
@section Parsed URI structures

  Each parsed URI structure contains the following slots:

@table @code
@item type
The URI scheme (a string, e.g., @code{http}).  @xref{Supported URL
Types}, for a list of schemes that the @code{url} library knows how to
process.  This slot can also be @code{nil}, if the URI is not fully
specified.

@item user
The user name (a string), or @code{nil}.

@item password
The user password (a string), or @code{nil}.  The use of this URI
component is strongly discouraged; nowadays, passwords are transmitted
by other means, not as part of a URI.

@item host
The host name (a string), or @code{nil}.  If present, this is
typically a domain name or IP address.

@item port
The port number (an integer), or @code{nil}.  Omitting this component
usually means to use the ``standard'' port associated with the URI
scheme.

@item filename
The combination of the ``path'' and ``query'' components of the URI (a
string), or @code{nil}.  If the query component is present, it is the
substring following the first @samp{?} character, and the path
component is the substring before the @samp{?}.  The meaning of these
components is scheme-dependent; they do not necessarily refer to a
file on a disk.

@item target
The fragment component (a string), or @code{nil}.  The fragment
component specifies a ``secondary resource'', such as a section of a
webpage.

@item fullness
This is @code{t} if the URI is fully specified, i.e., the
hierarchical components of the URI (the hostname and/or username
and/or password) are preceded by @samp{//}.
@end table

@findex url-type
@findex url-user
@findex url-password
@findex url-host
@findex url-port
@findex url-filename
@findex url-target
@findex url-attributes
@findex url-fullness
These slots have accessors named @code{url-@var{part}}, where
@var{part} is the slot name.  For example, the accessor for the
@code{host} slot is the function @code{url-host}.  The @code{url-port}
accessor returns the default port for the URI scheme if the parsed
URI's @var{port} slot is @code{nil}.

  The slots can be set using @code{setf}.  For example:

@example
(setf (url-port url) 80)
@end example

@node URI Encoding
@section URI Encoding

@cindex percent encoding
  The @code{url-generic-parse-url} parser does not obey RFC 3986 in
one respect: it allows non-@acronym{ASCII} characters in URI strings.

  Strictly speaking, RFC 3986 compatible URIs may only consist of
@acronym{ASCII} characters; non-@acronym{ASCII} characters are
represented by converting them to UTF-8 byte sequences, and performing
@dfn{percent encoding} on the bytes.  For example, the o-umlaut
character is converted to the UTF-8 byte sequence @samp{\xD3\xA7},
then percent encoded to @samp{%D3%A7}.  (Certain ``reserved''
@acronym{ASCII} characters must also be percent encoded when they
appear in URI components.)

  The function @code{url-encode-url} can be used to convert a URI
string containing arbitrary characters to one that is properly
percent-encoded in accordance with RFC 3986.

@defun url-encode-url url-string
This function return a properly URI-encoded version of
@var{url-string}.  It also performs @dfn{URI normalization},
e.g., converting the scheme component to lowercase if it was
previously uppercase.
@end defun

  To convert between a string containing arbitrary characters and a
percent-encoded all-@acronym{ASCII} string, use the functions
@code{url-hexify-string} and @code{url-unhex-string}:

@defun url-hexify-string string &optional allowed-chars
This function performs percent-encoding on @var{string}, and returns
the result.

If @var{string} is multibyte, it is first converted to a UTF-8 byte
string.  Each byte corresponding to an allowed character is left
as-is, while all other bytes are converted to a three-character
sequence: @samp{%} followed by two upper-case hex digits.

@vindex url-unreserved-chars
@cindex unreserved characters
The allowed characters are specified by @var{allowed-chars}.  If this
argument is @code{nil}, the allowed characters are those specified as
@dfn{unreserved characters} by RFC 3986 (see the variable
@code{url-unreserved-chars}).  Otherwise, @var{allowed-chars} should
be a vector whose @var{n}-th element is non-@code{nil} if character
@var{n} is allowed.
@end defun

@defun url-unhex-string string &optional allow-newlines
This function replaces percent-encoding sequences in @var{string} with
their character equivalents, and returns the resulting string.

If @var{allow-newlines} is non-@code{nil}, it allows the decoding of
carriage returns and line feeds, which are normally forbidden in URIs.
@end defun

@node Retrieving URLs
@chapter Retrieving URLs

  The @code{url} library defines the following three functions for
retrieving the data specified by a URL@.  The actual retrieval protocol
depends on the URL's URI scheme, and is performed by lower-level
scheme-specific functions.  (Those lower-level functions are not
documented here, and generally should not be called directly.)

  In each of these functions, the @var{url} argument can be either a
string or a parsed URL structure.  If it is a string, that string is
passed through @code{url-encode-url} before using it, to ensure that
it is properly URI-encoded (@pxref{URI Encoding}).

@defun url-retrieve-synchronously url
This function synchronously retrieves the data specified by @var{url},
and returns a buffer containing the data.  The return value is
@code{nil} if there is no data associated with the URL (as is the case
for @code{dired}, @code{info}, and @code{mailto} URLs).
@end defun

@defun url-retrieve url callback &optional cbargs silent no-cookies
This function retrieves @var{url} asynchronously, calling the function
@var{callback} when the object has been completely retrieved.  The
return value is the buffer into which the data will be inserted, or
@code{nil} if the process has already completed.

The callback function is called this way:

@example
(apply @var{callback} @var{status} @var{cbargs})
@end example

@noindent
where @var{status} is a plist representing what happened during the
retrieval, with most recent events first, or an empty list if no
events have occurred.  Each pair in the plist is one of:

@table @code
@item (:redirect @var{redirected-to})
This means that the request was redirected to the URL
@var{redirected-to}.

@item (:error (@var{error-symbol} . @var{data}))
This means that an error occurred.  If so desired, the error can be
signaled with @code{(signal @var{error-symbol} @var{data})}.
@end table

When the callback function is called, the current buffer is the one
containing the retrieved data (if any).  The buffer also contains any
MIME headers associated with the data retrieval.

If the optional argument @var{silent} is non-@code{nil}, progress
messages are suppressed.  If the optional argument @var{no-cookies} is
non-@code{nil}, cookies are not stored or sent.
@end defun

@defun url-queue-retrieve url callback &optional cbargs silent no-cookies
This function acts like @code{url-retrieve}, but with limits on the
number of concurrently-running network processes.  The option
@code{url-queue-parallel-processes} controls the number of concurrent
processes, and the option @code{url-queue-timeout} sets a timeout in
seconds.

To use this function, you must @code{(require 'url-queue)}.
@end defun

@vindex url-queue-parallel-processes
@defopt url-queue-parallel-processes
The value of this option is an integer specifying the maximum number
of concurrent @code{url-queue-retrieve} network processes.  If the
number of @code{url-queue-retrieve} calls is larger than this number,
later ones are queued until ealier ones are finished.
@end defopt

@vindex url-queue-timeout
@defopt url-queue-timeout
The value of this option is a number specifying the maximum lifetime
of a @code{url-queue-retrieve} network process, once it is started.
If a process is not finished by then, it is killed and removed from
the queue.
@end defopt

@node Supported URL Types
@chapter Supported URL Types

This chapter describes functions and variables affecting URL retrieval
for specific schemes.

@menu
* http/https::                  Hypertext Transfer Protocol.
* file/ftp::                    Local files and FTP archives.
* info::                        Emacs "Info" pages.
* mailto::                      Sending email.
* news/nntp/snews::             Usenet news.
* rlogin/telnet/tn3270::        Remote host connectivity.
* irc::                         Internet Relay Chat.
* data::                        Embedded data URLs.
* nfs::                         Networked File System
* ldap::                        Lightweight Directory Access Protocol
* man::                         Unix man pages.
@end menu

@node http/https
@section @code{http} and @code{https}

The @code{http} scheme refers to the Hypertext Transfer Protocol.  The
@code{url} library supports HTTP version 1.1, specified in RFC 2616.
Its default port is 80.

  The @code{https} scheme is a secure version of @code{http}, with
transmission via SSL@.  It is defined in RFC 2069, and its default port
is 443.  When using @code{https}, the @code{url} library performs SSL
encryption via the @code{ssl} library, by forcing the @code{ssl}
gateway method to be used.  @xref{Gateways in general}.

@defopt url-honor-refresh-requests
If this option is non-@code{nil} (the default), the @code{url} library
honors the HTTP @samp{Refresh} header, which is used by servers to
direct clients to reload documents from the same URL or a or different
one.  If the value is @code{nil}, the @samp{Refresh} header is
ignored; any other value means to ask the user on each request.
@end defopt

@menu
* Cookies::
* HTTP language/coding::
* HTTP URL Options::
* Dealing with HTTP documents::
@end menu

@node Cookies
@subsection Cookies

@defopt url-cookie-file
The file in which cookies are stored, defaulting to @file{cookies} in
the directory specified by @code{url-configuration-directory}.
@end defopt

@defopt url-cookie-confirmation
Specifies whether confirmation is require to accept cookies.
@end defopt

@defopt url-cookie-multiple-line
Specifies whether to put all cookies for the server on one line in the
HTTP request to satisfy broken servers like
@url{http://www.hotmail.com}.
@end defopt

@defopt url-cookie-trusted-urls
A list of regular expressions matching URLs from which to accept
cookies always.
@end defopt

@defopt url-cookie-untrusted-urls
A list of regular expressions matching URLs from which to reject
cookies always.
@end defopt

@defopt url-cookie-save-interval
The number of seconds between automatic saves of cookies to disk.
Default is one hour.
@end defopt


@node HTTP language/coding
@subsection Language and Encoding Preferences

HTTP allows clients to express preferences for the language and
encoding of documents which servers may honor.  For each of these
variables, the value is a string; it can specify a single choice, or
it can be a comma-separated list.

Normally, this list is ordered by descending preference.  However, each
element can be followed by @samp{;q=@var{priority}} to specify its
preference level, a decimal number from 0 to 1; e.g., for
@code{url-mime-language-string}, @w{@code{"de, en-gb;q=0.8,
en;q=0.7"}}.  An element that has no @samp{;q} specification has
preference level 1.

@defopt url-mime-charset-string
@cindex character sets
@cindex coding systems
This variable specifies a preference for character sets when documents
can be served in more than one encoding.

HTTP allows specifying a series of MIME charsets which indicate your
preferred character set encodings, e.g., Latin-9 or Big5, and these
can be weighted.  The default series is generated automatically from
the associated MIME types of all defined coding systems, sorted by the
coding system priority specified in Emacs.  @xref{Recognize Coding, ,
Recognizing Coding Systems, emacs, The GNU Emacs Manual}.
@end defopt

@defopt url-mime-language-string
@cindex language preferences
A string specifying the preferred language when servers can serve
files in several languages.  Use RFC 1766 abbreviations, e.g.,
@samp{en} for English, @samp{de} for German.

The string can be @code{"*"} to get the first available language (as
opposed to the default).
@end defopt

@node HTTP URL Options
@subsection HTTP URL Options

HTTP supports an @samp{OPTIONS} method describing things supported by
the URL@.

@defun url-http-options url
Returns a property list describing options available for URL@.  The
property list members are:

@table @code
@item methods
A list of symbols specifying what HTTP methods the resource
supports.

@item dav
@cindex DAV
A list of numbers specifying what DAV protocol/schema versions are
supported.

@item dasl
@cindex DASL
A list of supported DASL search types supported (string form).

@item ranges
A list of the units available for use in partial document fetches.

@item p3p
@cindex P3P
The @dfn{Platform For Privacy Protection} description for the resource.
Currently this is just the raw header contents.
@end table

@end defun

@node Dealing with HTTP documents
@subsection Dealing with HTTP documents

HTTP URLs are retrieved into a buffer containing the HTTP headers
followed by the body.  Since the headers are quasi-MIME, they may be
processed using the MIME library.  @xref{Top,, Emacs MIME,
emacs-mime, The Emacs MIME Manual}.

@node file/ftp
@section file and ftp
@cindex files
@cindex FTP
@cindex File Transfer Protocol
@cindex compressed files
@cindex dired

The @code{ftp} and @code{file} schemes are defined in RFC 1808.  The
@code{url} library treats @samp{ftp:} and @samp{file:} as synonymous.
Such URLs have the form

@example
ftp://@var{user}:@var{password}@@@var{host}:@var{port}/@var{file}
file://@var{user}:@var{password}@@@var{host}:@var{port}/@var{file}
@end example

@noindent
If the URL specifies a local file, it is retrieved by reading the file
contents in the usual way.  If it specifies a remote file, it is
retrieved using the Ange-FTP package.  @xref{Remote Files,,, emacs,
The GNU Emacs Manual}.

  When retrieving a compressed file, it is automatically uncompressed
if it has the file suffix @file{.z}, @file{.gz}, @file{.Z},
@file{.bz2}, or @file{.xz}.  (The list of supported suffixes is
hard-coded, and cannot be altered by customizing
@code{jka-compr-compression-info-list}.)

@defopt url-directory-index-file
This option specifies the filename to look for when a @code{file} or
@code{ftp} URL specifies a directory.  The default is
@file{index.html}.  If this file exists and is readable, it is viewed.
Otherwise, Emacs visits the directory using Dired.
@end defopt

@node info
@section info
@cindex Info
@cindex Texinfo
@findex Info-goto-node

The @code{info} scheme is non-standard.  Such URLs have the form

@example
info:@var{file}#@var{node}
@end example

@noindent
and are retrieved by invoking @code{Info-goto-node} with argument
@samp{(@var{file})@var{node}}.  If @samp{#@var{node}} is omitted, the
@samp{Top} node is opened.

@node mailto
@section mailto

@cindex mailto
@cindex email
A @code{mailto} URL specifies an email message to be sent to a given
email address.  For example, @samp{mailto:foo@@bar.com} specifies
sending a message to @samp{foo@@bar.com}.  The ``retrieval method''
for such URLs is to open a mail composition buffer in which the
appropriate content (e.g., the recipient address) has been filled in.

  As defined in RFC 2368, a @code{mailto} URL has the form

@example
@samp{mailto:@var{mailbox}[?@var{header}=@var{contents}[&@var{header}=@var{contents}]]}
@end example

@noindent
where an arbitrary number of @var{header}s can be added.  If the
@var{header} is @samp{body}, then @var{contents} is put in the message
body; otherwise, a @var{header} header field is created with
@var{contents} as its contents.  Note that the @code{url} library does
not perform any checking of @var{header} or @var{contents}, so you
should check them before sending the message.

@defopt url-mail-command
@vindex mail-user-agent
The value of this variable is the function called whenever url needs
to send mail.  This should normally be left its default, which is the
standard mail-composition command @code{compose-mail}.  @xref{Sending
Mail,,, emacs, The GNU Emacs Manual}.
@end defopt

  If the document containing the @code{mailto} URL itself possessed a
known URL, Emacs automatically inserts an @samp{X-Url-From} header
field into the mail buffer, specifying that URL.

@node news/nntp/snews
@section @code{news}, @code{nntp} and @code{snews}
@cindex news
@cindex network news
@cindex usenet
@cindex NNTP
@cindex snews

The @code{news}, @code{nntp}, and @code{snews} schemes, defined in RFC
1738, are used for reading Usenet newsgroups.  For compatibility with
non-standard-compliant news clients, the @code{url} library allows
host and port fields to be included in @code{news} URLs, even though
they are properly only allowed for @code{nntp} and @code{snews}.

  @code{news} and @code{nntp} URLs have the following form:

@table @samp
@item news:@var{newsgroup}
Retrieves a list of messages in @var{newsgroup};
@item news:@var{message-id}
Retrieves the message with the given @var{message-id};
@item news:*
Retrieves a list of all available newsgroups;
@item nntp://@var{host}:@var{port}/@var{newsgroup}
@itemx nntp://@var{host}:@var{port}/@var{message-id}
@itemx nntp://@var{host}:@var{port}/*
Similar to the @samp{news} versions.
@end table

  The default port for @code{nntp} (and @code{news}) is 119.  The
difference between an @code{nntp} URL and a @code{news} URL is that an
@code{nttp} URL may specify an article by its number.  The
@samp{snews} scheme is the same as @samp{nntp}, except that it is
tunneled through SSL and has default port 563.

  These URLs are retrieved via the Gnus package.

@cindex environment variable
@vindex NNTPSERVER
@defopt url-news-server
This variable specifies the default news server from which to fetch
news, if no server was specified in the URL@.  The default value,
@code{nil}, means to use the server specified by the standard
environment variable @samp{NNTPSERVER}, or @samp{news} if that
environment variable is unset.
@end defopt

@node rlogin/telnet/tn3270
@section rlogin, telnet and tn3270
@cindex rlogin
@cindex telnet
@cindex tn3270
@cindex terminal emulation
@findex terminal-emulator

These URL schemes are defined in RFC 1738, and are used for logging in
via a terminal emulator.  They have the form

@example
telnet://@var{user}:@var{password}@@@var{host}:@var{port}
@end example

@noindent
but the @var{password} component is ignored.

To handle rlogin, telnet and tn3270 URLs, a @code{rlogin},
@code{telnet} or @code{tn3270} (the program names and arguments are
hardcoded) session is run in a @code{terminal-emulator} buffer.
Well-known ports are used if the URL does not specify a port.

@node irc
@section irc
@cindex IRC
@cindex Internet Relay Chat
@cindex ZEN IRC
@cindex ERC
@cindex rcirc

  The @code{irc} scheme is defined in the Internet Draft at
@url{http://www.w3.org/Addressing/draft-mirashi-url-irc-01.txt} (which
was never approved as an RFC).  Such URLs have the form

@example
irc://@var{host}:@var{port}/@var{target},@var{needpass}
@end example

@noindent
and are retrieved by opening an @acronym{IRC} session using the
function specified by @code{url-irc-function}.

@defopt url-irc-function
The value of this option is a function, which is called to open an IRC
connection for @code{irc} URLs.  This function must take five
arguments, @var{host}, @var{port}, @var{channel}, @var{user} and
@var{password}.  The @var{channel} argument specifies the channel to
join immediately, and may be @code{nil}.

The default is @code{url-irc-rcirc}, which uses the Rcirc package.
Other options are @code{url-irc-erc} (which uses ERC) and
@code{url-irc-zenirc} (which uses ZenIRC).
@end defopt

@node data
@section data
@cindex data URLs

  The @code{data} scheme, defined in RFC 2397, contains MIME data in
the URL itself.  Such URLs have the form

@example
data:@r{[}@var{media-type}@r{]}@r{[};@var{base64}@r{]},@var{data}
@end example

@noindent
@var{media-type} is a MIME @samp{Content-Type} string, possibly
including parameters.  It defaults to
@samp{text/plain;charset=US-ASCII}.  The @samp{text/plain} can be
omitted but the charset parameter supplied.  If @samp{;base64} is
present, the @var{data} are base64-encoded.

@node nfs
@section nfs
@cindex NFS
@cindex Network File System
@cindex automounter

The @code{nfs} scheme, defined in RFC 2224, is similar to @code{ftp}
except that it points to a file on a remote host that is handled by an
NFS automounter on the local host.  Such URLs have the form

@example
nfs://@var{user}:@var{password}@@@var{host}:@var{port}/@var{file}
@end example

@defvar url-nfs-automounter-directory-spec
@end defvar
A string saying how to invoke the NFS automounter.  Certain @samp{%}
sequences are recognized:

@table @samp
@item %h
The hostname of the NFS server;
@item %n
The port number of the NFS server;
@item %u
The username to use to authenticate;
@item %p
The password to use to authenticate;
@item %f
The filename on the remote server;
@item %%
A literal @samp{%}.
@end table

Each can be used any number of times.

@node ldap
@section ldap
@cindex LDAP
@cindex Lightweight Directory Access Protocol

The LDAP scheme is defined in RFC 2255.

@node man
@section man
@cindex @command{man}
@cindex Unix man pages
@findex man

The @code{man} scheme is a non-standard one.  Such URLs have the form

@example
@samp{man:@var{page-spec}}
@end example

@noindent
and are retrieved by passing @var{page-spec} to the Lisp function
@code{man}.

@node General Facilities
@chapter General Facilities

@menu
* Disk Caching::
* Proxies::
* Gateways in general::
* History::
@end menu

@node Disk Caching
@section Disk Caching
@cindex Caching
@cindex Persistent Cache
@cindex Disk Cache

The disk cache stores retrieved documents locally, whence they can be
retrieved more quickly.  When requesting a URL that is in the cache,
the library checks to see if the page has changed since it was last
retrieved from the remote machine.  If not, the local copy is used,
saving the transmission over the network.
@cindex Cleaning the cache
@cindex Clearing the cache
@cindex Cache cleaning
Currently the cache isn't cleared automatically.
@c Running the @code{clean-cache} shell script
@c fist is recommended, to allow for future cleaning of the cache.  This
@c shell script will remove all files that have not been accessed since it
@c was last run.  To keep the cache pared down, it is recommended that this
@c script be run from @i{at} or @i{cron} (see the manual pages for
@c crontab(5) or at(1) for more information)

@defopt url-automatic-caching
Setting this variable non-@code{nil} causes documents to be cached
automatically.
@end defopt

@defopt url-cache-directory
This variable specifies the
directory to store the cache files.  It defaults to sub-directory
@file{cache} of @code{url-configuration-directory}.
@end defopt

@defopt url-cache-creation-function
The cache relies on a scheme for mapping URLs to files in the cache.
This variable names a function which sets the type of cache to use.
It takes a URL as argument and returns the absolute file name of the
corresponding cache file.  The two supplied possibilities are
@code{url-cache-create-filename-using-md5} and
@code{url-cache-create-filename-human-readable}.
@end defopt

@defun url-cache-create-filename-using-md5 url
Creates a cache file name from @var{url} using MD5 hashing.
This is creates entries with very few cache collisions and is fast.
@cindex MD5
@smallexample
(url-cache-create-filename-using-md5 "http://www.example.com/foo/bar")
  @result{} "/home/fx/.url/cache/fx/http/com/example/www/b8a35774ad20db71c7c3409a5410e74f"
@end smallexample
@end defun

@defun url-cache-create-filename-human-readable url
Creates a cache file name from @var{url} more obviously connected to
@var{url} than for @code{url-cache-create-filename-using-md5}, but
more likely to conflict with other files.
@smallexample
(url-cache-create-filename-human-readable "http://www.example.com/foo/bar")
  @result{} "/home/fx/.url/cache/fx/http/com/example/www/foo/bar"
@end smallexample
@end defun

@defun url-cache-expired
This function returns non-nil if a cache entry has expired (or is absent).
The arguments are a URL and optional expiration delay in seconds
(default @var{url-cache-expire-time}).
@end defun

@defopt url-cache-expire-time
This variable is the default number of seconds to use for the
expire-time argument of the function @code{url-cache-expired}.
@end defopt

@defun url-fetch-from-cache
This function takes a URL as its argument and returns a buffer
containing the data cached for that URL.
@end defun

@c Fixme: never actually used currently?
@c @defopt url-standalone-mode
@c @cindex Relying on cache
@c @cindex Cache only mode
@c @cindex Standalone mode
@c If this variable is non-@code{nil}, the library relies solely on the
@c cache for fetching documents and avoids checking if they have changed
@c on remote servers.
@c @end defopt

@c With a large cache of documents on the local disk, it can be very handy
@c when traveling, or any other time the network connection is not active
@c (a laptop with a dial-on-demand PPP connection, etc).  Emacs/W3 can rely
@c solely on its cache, and avoid checking to see if the page has changed
@c on the remote server.  In the case of a dial-on-demand PPP connection,
@c this will keep the phone line free as long as possible, only bringing up
@c the PPP connection when asking for a page that is not located in the
@c cache.  This is very useful for demonstrations as well.

@node Proxies
@section Proxies and Gatewaying

@c fixme: check/document url-ns stuff
@cindex proxy servers
@cindex proxies
@cindex environment variables
@vindex HTTP_PROXY
Proxy servers are commonly used to provide gateways through firewalls
or as caches serving some more-or-less local network.  Each protocol
(HTTP, FTP, etc.)@: can have a different gateway server.  Proxying is
conventionally configured commonly amongst different programs through
environment variables of the form @code{@var{protocol}_proxy}, where
@var{protocol} is one of the supported network protocols (@code{http},
@code{ftp} etc.).  The library recognizes such variables in either
upper or lower case.  Their values are of one of the forms:
@itemize @bullet
@item @code{@var{host}:@var{port}}
@item A full URL;
@item Simply a host name.
@end itemize

@vindex NO_PROXY
The @code{NO_PROXY} environment variable specifies URLs that should be
excluded from proxying (on servers that should be contacted directly).
This should be a comma-separated list of hostnames, domain names, or a
mixture of both.  Asterisks can be used as wildcards, but other
clients may not support that.  Domain names may be indicated by a
leading dot.  For example:
@example
NO_PROXY="*.aventail.com,home.com,.seanet.com"
@end example
@noindent says to contact all machines in the @samp{aventail.com} and
@samp{seanet.com} domains directly, as well as the machine named
@samp{home.com}.  If @code{NO_PROXY} isn't defined, @code{no_PROXY}
and @code{no_proxy} are also tried, in that order.

Proxies may also be specified directly in Lisp.

@defopt url-proxy-services
This variable is an alist of URL schemes and proxy servers that
gateway them.  The items are of the form @w{@code{(@var{scheme}
. @var{host}:@var{portnumber})}}, says that the URL @var{scheme} is
gatewayed through @var{portnumber} on the specified @var{host}.  An
exception is the pseudo scheme @code{"no_proxy"}, which is paired with
a regexp matching host names not to be proxied.  This variable is
initialized from the environment as above.

@example
(setq url-proxy-services
      '(("http"     . "proxy.aventail.com:80")
        ("no_proxy" . "^.*\\(aventail\\|seanet\\)\\.com")))
@end example
@end defopt

@node Gateways in general
@section Gateways in General
@cindex gateways
@cindex firewalls

The library provides a general gateway layer through which all
networking passes.  It can both control access to the network and
provide access through gateways in firewalls.  This may make direct
connections in some cases and pass through some sort of gateway in
others.@footnote{Proxies (which only operate over HTTP) are
implemented using this.}  The library's basic function responsible for
making connections is @code{url-open-stream}.

@defun url-open-stream name buffer host service
@cindex opening a stream
@cindex stream, opening
Open a stream to @var{host}, possibly via a gateway.  The other
arguments are as for @code{open-network-stream}.  This will not make a
connection if @code{url-gateway-unplugged} is non-@code{nil}.
@end defun

@defvar url-gateway-local-host-regexp
This is a regular expression that matches local hosts that do not
require the use of a gateway.  If @code{nil}, all connections are made
through the gateway.
@end defvar

@defvar url-gateway-method
This variable controls which gateway method is used.  It may be useful
to bind it temporarily in some applications.  It has values taken from
a list of symbols.  Possible values are:

@table @code
@item telnet
@cindex @command{telnet}
Use this method if you must first telnet and log into a gateway host,
and then run telnet from that host to connect to outside machines.

@item rlogin
@cindex @command{rlogin}
This method is identical to @code{telnet}, but uses @command{rlogin}
to log into the remote machine without having to send the username and
password over the wire every time.

@item socks
@cindex @sc{socks}
Use if the firewall has a @sc{socks} gateway running on it.  The
@sc{socks} v5 protocol is defined in RFC 1928.

@c @item ssl
@c This probably shouldn't be documented
@c Fixme: why not? -- fx

@item native
This method uses Emacs's builtin networking directly.  This is the
default.  It can be used only if there is no firewall blocking access.
@end table
@end defvar

The following variables control the gateway methods.

@defopt url-gateway-telnet-host
The gateway host to telnet to.  Once logged in there, you then telnet
out to the hosts you want to connect to.
@end defopt
@defopt url-gateway-telnet-parameters
This should be a list of parameters to pass to the @command{telnet} program.
@end defopt
@defopt url-gateway-telnet-password-prompt
This is a regular expression that matches the password prompt when
logging in.
@end defopt
@defopt url-gateway-telnet-login-prompt
This is a regular expression that matches the username prompt when
logging in.
@end defopt
@defopt url-gateway-telnet-user-name
The username to log in with.
@end defopt
@defopt url-gateway-telnet-password
The password to send when logging in.
@end defopt
@defopt url-gateway-prompt-pattern
This is a regular expression that matches the shell prompt.
@end defopt

@defopt url-gateway-rlogin-host
Host to @samp{rlogin} to before telnetting out.
@end defopt
@defopt url-gateway-rlogin-parameters
Parameters to pass to @samp{rsh}.
@end defopt
@defopt url-gateway-rlogin-user-name
User name to use when logging in to the gateway.
@end defopt
@defopt url-gateway-prompt-pattern
This is a regular expression that matches the shell prompt.
@end defopt

@defopt socks-server
This specifies the default server, it takes the form
@w{@code{("Default server" @var{server} @var{port} @var{version})}}
where @var{version} can be either 4 or 5.
@end defopt
@defvar socks-password
If this is @code{nil} then you will be asked for the password,
otherwise it will be used as the password for authenticating you to
the @sc{socks} server.
@end defvar
@defvar socks-username
This is the username to use when authenticating yourself to the
@sc{socks} server.  By default this is your login name.
@end defvar
@defvar socks-timeout
This controls how long, in seconds, to wait for responses from the
@sc{socks} server; it is 5 by default.
@end defvar
@c fixme: these have been effectively commented-out in the code
@c @defopt socks-server-aliases
@c This a list of server aliases.  It is a list of aliases of the form
@c @var{(alias hostname port version)}.
@c @end defopt
@c @defopt socks-network-aliases
@c This a list of network aliases.  Each entry in the list takes the form
@c @var{(alias (network))} where @var{alias} is a string that names the
@c @var{network}.  The networks can contain a pair (not a dotted pair) of
@c @sc{ip} addresses which specify a range of @sc{ip} addresses, an @sc{ip}
@c address and a netmask, a domain name or a unique hostname or @sc{ip}
@c address.
@c @end defopt
@c @defopt socks-redirection-rules
@c This a list of redirection rules.  Each rule take the form
@c @var{(Destination network Connection type)} where @var{Destination
@c network} is a network alias from @code{socks-network-aliases} and
@c @var{Connection type} can be @code{nil} in which case a direct
@c connection is used, or it can be an alias from
@c @code{socks-server-aliases} in which case that server is used as a
@c proxy.
@c @end defopt
@defopt socks-nslookup-program
@cindex @command{nslookup}
This the @samp{nslookup} program.  It is @code{"nslookup"} by default.
@end defopt

@menu
* Suppressing network connections::
@end menu
@c * Broken hostname resolution::

@node Suppressing network connections
@subsection Suppressing Network Connections

@cindex network connections, suppressing
@cindex suppressing network connections
@cindex bugs, HTML
@cindex HTML `bugs'
In some circumstances it is desirable to suppress making network
connections.  A typical case is when rendering HTML in a mail user
agent, when external URLs should not be activated, particularly to
avoid ``bugs'' which ``call home'' by fetch single-pixel images and the
like.  To arrange this, bind the following variable for the duration
of such processing.

@defvar url-gateway-unplugged
If this variable is non-@code{nil} new network connections are never
opened by the URL library.
@end defvar

@c @node Broken hostname resolution
@c @subsection Broken Hostname Resolution

@c @cindex hostname resolver
@c @cindex resolver, hostname
@c Some C libraries do not include the hostname resolver routines in
@c their static libraries.  If Emacs was linked statically, and was not
@c linked with the resolver libraries, it will not be able to get to any
@c machines off the local network.  This is characterized by being able
@c to reach someplace with a raw ip number, but not its hostname
@c (@url{http://129.79.254.191/} works, but
@c @url{http://www.cs.indiana.edu/} doesn't).  This used to happen on
@c SunOS4 and Ultrix, but is now probably now rare.  If Emacs can't be
@c rebuilt linked against the resolver library, it can use the external
@c @command{nslookup} program instead.

@c @defopt url-gateway-broken-resolution
@c @cindex @code{nslookup} program
@c @cindex program, @code{nslookup}
@c If non-@code{nil}, this variable says to use the program specified by
@c @code{url-gateway-nslookup-program} program to do hostname resolution.
@c @end defopt

@c @defopt url-gateway-nslookup-program
@c The name of the program to do hostname lookup if Emacs can't do it
@c directly.  This program should expect a single argument on the command
@c line---the hostname to resolve---and should produce output similar to
@c the standard Unix @command{nslookup} program:
@c @example
@c Name: www.cs.indiana.edu
@c Address: 129.79.254.191
@c @end example
@c @end defopt

@node History
@section History

@findex url-do-setup
The library can maintain a global history list tracking URLs accessed.
URL completion can be done from it.  The history mechanism is set up
automatically via @code{url-do-setup} when it is configured to be on.
Note that the size of the history list is currently not limited.

@vindex url-history-hash-table
The history ``list'' is actually a hash table,
@code{url-history-hash-table}.  It contains access times keyed by URL
strings.  The times are in the format returned by @code{current-time}.

@defun url-history-update-url url time
This function updates the history table with an entry for @var{url}
accessed at the given @var{time}.
@end defun

@defopt url-history-track
If non-@code{nil}, the library will keep track of all the URLs
accessed.  If it is @code{t}, the list is saved to disk at the end of
each Emacs session.  The default is @code{nil}.
@end defopt

@defopt url-history-file
The file storing the history list between sessions.  It defaults to
@file{history} in @code{url-configuration-directory}.
@end defopt

@defopt url-history-save-interval
@findex url-history-setup-save-timer
The number of seconds between automatic saves of the history list.
Default is one hour.  Note that if you change this variable directly,
rather than using Custom, after @code{url-do-setup} has been run, you
need to run the function @code{url-history-setup-save-timer}.
@end defopt

@defun url-history-parse-history &optional fname
Parses the history file @var{fname} (default @code{url-history-file})
and sets up the history list.
@end defun

@defun url-history-save-history &optional fname
Saves the current history to file @var{fname} (default
@code{url-history-file}).
@end defun

@defun url-completion-function string predicate function
You can use this function to do completion of URLs from the history.
@end defun

@node Customization
@chapter Customization

@cindex environment variables
  The following environment variables affect the @code{url} library's
operation at startup.

@table @code
@item TMPDIR
@vindex TMPDIR
@vindex url-temporary-directory
If this is defined, @var{url-temporary-directory} is initialized from
it.
@end table

  The following user options affect the general operation of
@code{url} library.

@defopt url-configuration-directory
@cindex configuration files
The value of this variable specifies the name of the directory where
the @code{url} library stores its various configuration files, cache
files, etc.

The default value specifies a subdirectory named @file{url/} in the
standard Emacs user data directory specified by the variable
@code{user-emacs-directory} (normally @file{~/.emacs.d}).  However,
the old default was @file{~/.url}, and this directory is used instead
if it exists.
@end defopt

@defopt url-debug
@cindex debugging
Specifies the types of debug messages which are logged to
the @code{*URL-DEBUG*} buffer.
@code{t} means log all messages.
A number means log all messages and show them with @code{message}.
It may also be a list of the types of messages to be logged.
@end defopt
@defopt url-personal-mail-address
@end defopt
@defopt url-privacy-level
@end defopt
@defopt url-uncompressor-alist
@end defopt
@defopt url-passwd-entry-func
@end defopt
@defopt url-standalone-mode
@end defopt
@defopt url-bad-port-list
@end defopt
@defopt url-max-password-attempts
@end defopt
@defopt url-temporary-directory
@end defopt
@defopt url-show-status
@end defopt
@defopt url-confirmation-func
The function to use for asking yes or no functions.  This is normally
either @code{y-or-n-p} or @code{yes-or-no-p}, but could be another
function taking a single argument (the prompt) and returning @code{t}
only if an affirmative answer is given.
@end defopt
@defopt url-gateway-method
@c fixme: describe gatewaying
A symbol specifying the type of gateway support to use for connections
from the local machine.  The supported methods are:

@table @code
@item telnet
Run telnet in a subprocess to connect;
@item rlogin
Rlogin to another machine to connect;
@item socks
Connect through a socks server;
@item ssl
Connect with SSL;
@item native
Connect directly.
@end table
@end defopt

@node GNU Free Documentation License
@appendix GNU Free Documentation License
@include doclicense.texi

@node Function Index
@unnumbered Command and Function Index
@printindex fn

@node Variable Index
@unnumbered Variable Index
@printindex vr

@node Concept Index
@unnumbered Concept Index
@printindex cp

@bye