This is the manual for the
url Emacs Lisp library.
Copyright © 1993–1999, 2002, 2004–2015 Free Software Foundation, Inc.
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.”
|Introduction||About the |
|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.|
A 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 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):
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
This manual describes the
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.)
2 URI Parsing
A URI consists of several components, each having a different meaning. For example, the URI
specifies the scheme component ‘http’, the hostname component ‘www.gnu.org’, and the path component ‘/software/emacs/’.
The format of URIs is specified by RFC 3986. The
provides the Lisp function
url-generic-parse-url, a (mostly)
standard-compliant URI parser, as well as function
url-recreate-url, which converts a parsed URI back into a URI
- Function: url-generic-parse-url uri-string
This function returns a parsed version of the string uri-string.
- Function: url-recreate-url uri-obj
Given a parsed URI, this function returns the corresponding URI string.
The return value of
url-generic-parse-url, and the argument
url-recreate-url, is a parsed URI: a CL
structure whose slots hold the various components of the URI.
See the CL Manual in GNU Emacs Common Lisp Emulation, for
details about CL structures. Most of the other functions in the
url library act on parsed URIs.
|• Parsed URIs:||Format of parsed URI structures.|
|• URI Encoding:||Non-ASCII characters in URIs.|
2.1 Parsed URI structures
Each parsed URI structure contains the following slots:
The URI scheme (a string, e.g.,
http). See Supported URL Types, for a list of schemes that the
urllibrary knows how to process. This slot can also be
nil, if the URI is not fully specified.
The user name (a string), or
The user password (a string), or
nil. The use of this URI component is strongly discouraged; nowadays, passwords are transmitted by other means, not as part of a URI.
The host name (a string), or
nil. If present, this is typically a domain name or IP address.
The port number (an integer), or
nil. Omitting this component usually means to use the “standard” port associated with the URI scheme.
The combination of the “path” and “query” components of the URI (a string), or
nil. If the query component is present, it is the substring following the first ‘?’ character, and the path component is the substring before the ‘?’. The meaning of these components is scheme-dependent; they do not necessarily refer to a file on a disk.
The fragment component (a string), or
nil. The fragment component specifies a “secondary resource”, such as a section of a webpage.
tif the URI is fully specified, i.e., the hierarchical components of the URI (the hostname and/or username and/or password) are preceded by ‘//’.
These slots have accessors named
part is the slot name. For example, the accessor for the
host slot is the function
accessor returns the default port for the URI scheme if the parsed
URI’s port slot is
The slots can be set using
setf. For example:
(setf (url-port url) 80)
2.2 URI Encoding
url-generic-parse-url parser does not obey RFC 3986 in
one respect: it allows non-ASCII characters in URI strings.
Strictly speaking, RFC 3986 compatible URIs may only consist of ASCII characters; non-ASCII characters are represented by converting them to UTF-8 byte sequences, and performing percent encoding on the bytes. For example, the o-umlaut character is converted to the UTF-8 byte sequence ‘\xD3\xA7’, then percent encoded to ‘%D3%A7’. (Certain “reserved” ASCII characters must also be percent encoded when they appear in URI components.)
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.
- Function: url-encode-url url-string
This function return a properly URI-encoded version of url-string. It also performs URI normalization, e.g., converting the scheme component to lowercase if it was previously uppercase.
To convert between a string containing arbitrary characters and a
percent-encoded all-ASCII string, use the functions
- Function: url-hexify-string string &optional allowed-chars
This function performs percent-encoding on string, and returns the result.
If 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: ‘%’ followed by two upper-case hex digits.
The allowed characters are specified by allowed-chars. If this argument is
nil, the allowed characters are those specified as unreserved characters by RFC 3986 (see the variable
url-unreserved-chars). Otherwise, allowed-chars should be a vector whose n-th element is non-
nilif character n is allowed.
- Function: url-unhex-string string &optional allow-newlines
This function replaces percent-encoding sequences in string with their character equivalents, and returns the resulting string.
If allow-newlines is non-
nil, it allows the decoding of carriage returns and line feeds, which are normally forbidden in URIs.
3 Retrieving URLs
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 url argument can be either a
string or a parsed URL structure. If it is a string, that string is
url-encode-url before using it, to ensure that
it is properly URI-encoded (see URI Encoding).
- Function: url-retrieve-synchronously url
This function synchronously retrieves the data specified by url, and returns a buffer containing the data. The return value is
nilif there is no data associated with the URL (as is the case for
- Function: url-retrieve url callback &optional cbargs silent no-cookies
This function retrieves url asynchronously, calling the function callback when the object has been completely retrieved. The return value is the buffer into which the data will be inserted, or
nilif the process has already completed.
The callback function is called this way:
(apply callback status cbargs)
where 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:
This means that the request was redirected to the URL redirected-to.
(:error (error-symbol . data))
This means that an error occurred. If so desired, the error can be signaled with
(signal error-symbol data).
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 silent is non-
nil, progress messages are suppressed. If the optional argument no-cookies is non-
nil, cookies are not stored or sent.
- Function: url-queue-retrieve url callback &optional cbargs silent no-cookies
This function acts like
url-retrieve, but with limits on the number of concurrently-running network processes. The option
url-queue-parallel-processescontrols the number of concurrent processes, and the option
url-queue-timeoutsets a timeout in seconds.
To use this function, you must
- User Option: url-queue-parallel-processes
The value of this option is an integer specifying the maximum number of concurrent
url-queue-retrievenetwork processes. If the number of
url-queue-retrievecalls is larger than this number, later ones are queued until earlier ones are finished.
- User Option: url-queue-timeout
The value of this option is a number specifying the maximum lifetime of a
url-queue-retrievenetwork process, once it is started. If a process is not finished by then, it is killed and removed from the queue.
4 Supported URL Types
This chapter describes functions and variables affecting URL retrieval for specific schemes.
|• 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.|
http scheme refers to the Hypertext Transfer Protocol. The
url library supports HTTP version 1.1, specified in RFC 2616.
Its default port is 80.
https scheme is a secure version of
transmission via SSL. It is defined in RFC 2069, and its default port
is 443. When using
url library performs SSL
encryption via the
ssl library, by forcing the
gateway method to be used. See Gateways in general.
- User Option: url-honor-refresh-requests
If this option is non-
nil(the default), the
urllibrary honors the HTTP ‘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
nil, the ‘Refresh’ header is ignored; any other value means to ask the user on each request.
|• HTTP language/coding:|
|• HTTP URL Options:|
|• Dealing with HTTP documents:|
- Function: url-cookie-list
This command creates a *url cookies* buffer listing the current cookies, if there are any. You can remove a cookie using the C-k (
- User Option: url-cookie-file
The file in which cookies are stored, defaulting to cookies in the directory specified by
- User Option: 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 http://www.hotmail.com.
- User Option: url-cookie-trusted-urls
A list of regular expressions matching URLs from which to accept cookies always.
- User Option: url-cookie-untrusted-urls
A list of regular expressions matching URLs from which to reject cookies always.
- User Option: url-cookie-save-interval
The number of seconds between automatic saves of cookies to disk. Default is one hour.
4.1.2 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 ‘;q=priority’ to specify its
preference level, a decimal number from 0 to 1; e.g., for
"de, en-gb;q=0.8, en;q=0.7". An element that has no ‘;q’ specification has
preference level 1.
- User Option: url-mime-charset-string
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. See Recognizing Coding Systems in The GNU Emacs Manual.
- User Option: url-mime-language-string
A string specifying the preferred language when servers can serve files in several languages. Use RFC 1766 abbreviations, e.g., ‘en’ for English, ‘de’ for German.
The string can be
"*"to get the first available language (as opposed to the default).
4.1.3 HTTP URL Options
HTTP supports an ‘OPTIONS’ method describing things supported by the URL.
- Function: url-http-options url
Returns a property list describing options available for URL. The property list members are:
A list of symbols specifying what HTTP methods the resource supports.
A list of numbers specifying what DAV protocol/schema versions are supported.
A list of supported DASL search types supported (string form).
A list of the units available for use in partial document fetches.
The Platform For Privacy Protection description for the resource. Currently this is just the raw header contents.
4.1.4 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. See Emacs MIME in The Emacs MIME Manual.
4.2 file and ftp
file schemes are defined in RFC 1808. The
url library treats ‘ftp:’ and ‘file:’ as synonymous.
Such URLs have the form
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. See Remote Files in The GNU Emacs Manual.
When retrieving a compressed file, it is automatically uncompressed
if it has the file suffix .z, .gz, .Z,
.bz2, or .xz. (The list of supported suffixes is
hard-coded, and cannot be altered by customizing
- User Option: url-directory-index-file
This option specifies the filename to look for when a
ftpURL specifies a directory. The default is index.html. If this file exists and is readable, it is viewed. Otherwise, Emacs visits the directory using Dired.
info scheme is non-standard. Such URLs have the form
and are retrieved by invoking
Info-goto-node with argument
‘(file)node’. If ‘#node’ is omitted, the
‘Top’ node is opened.
mailto URL specifies an email message to be sent to a given
email address. For example, ‘mailto:email@example.com’ specifies
sending a message to ‘firstname.lastname@example.org’. 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
mailto URL has the form
where an arbitrary number of headers can be added. If the
header is ‘body’, then contents is put in the message
body; otherwise, a header header field is created with
contents as its contents. Note that the
url library does
not perform any checking of header or contents, so you
should check them before sending the message.
- User Option: url-mail-command
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
compose-mail. See Sending Mail in The GNU Emacs Manual.
If the document containing the
mailto URL itself possessed a
known URL, Emacs automatically inserts an ‘X-Url-From’ header
field into the mail buffer, specifying that URL.
snews schemes, defined in RFC
1738, are used for reading Usenet newsgroups. For compatibility with
non-standard-compliant news clients, the
url library allows
host and port fields to be included in
news URLs, even though
they are properly only allowed for
nntp URLs have the following form:
Retrieves a list of messages in newsgroup;
Retrieves the message with the given message-id;
Retrieves a list of all available newsgroups;
Similar to the ‘news’ versions.
The default port for
news) is 119. The
difference between an
nntp URL and a
news URL is that an
nttp URL may specify an article by its number. The
‘snews’ scheme is the same as ‘nntp’, except that it is
tunneled through SSL and has default port 563.
These URLs are retrieved via the Gnus package.
- User Option: 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,
nil, means to use the server specified by the standard environment variable ‘NNTPSERVER’, or ‘news’ if that environment variable is unset.
4.6 rlogin, telnet and tn3270
These URL schemes are defined in RFC 1738, and are used for logging in via a terminal emulator. They have the form
but the password component is ignored.
To handle rlogin, telnet and tn3270 URLs, a
tn3270 (the program names and arguments are
hardcoded) session is run in a
Well-known ports are used if the URL does not specify a port.
irc scheme is defined in the Internet Draft at
was never approved as an RFC). Such URLs have the form
and are retrieved by opening an IRC session using the
function specified by
- User Option: url-irc-function
The value of this option is a function, which is called to open an IRC connection for
ircURLs. This function must take five arguments, host, port, channel, user and password. The channel argument specifies the channel to join immediately, and may be
The default is
url-irc-rcirc, which uses the Rcirc package. Other options are
url-irc-erc(which uses ERC) and
url-irc-zenirc(which uses ZenIRC).
data scheme, defined in RFC 2397, contains MIME data in
the URL itself. Such URLs have the form
media-type is a MIME ‘Content-Type’ string, possibly including parameters. It defaults to ‘text/plain;charset=US-ASCII’. The ‘text/plain’ can be omitted but the charset parameter supplied. If ‘;base64’ is present, the data are base64-encoded.
nfs scheme, defined in RFC 2224, is similar to
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
A string saying how to invoke the NFS automounter. Certain ‘%’ sequences are recognized:
The hostname of the NFS server;
The port number of the NFS server;
The username to use to authenticate;
The password to use to authenticate;
The filename on the remote server;
A literal ‘%’.
Each can be used any number of times.
The LDAP scheme is defined in RFC 2255.
man scheme is a non-standard one. Such URLs have the form
and are retrieved by passing page-spec to the Lisp function
5 General Facilities
|• Disk Caching:|
|• Gateways in general:|
5.1 Disk Caching
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. Currently the cache isn’t cleared automatically.
- User Option: url-automatic-caching
Setting this variable non-
nilcauses documents to be cached automatically.
- User Option: url-cache-directory
This variable specifies the directory to store the cache files. It defaults to sub-directory cache of
- User Option: 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
- Function: url-cache-create-filename-using-md5 url
(url-cache-create-filename-using-md5 "http://www.example.com/foo/bar") ⇒ "/home/fx/.url/cache/fx/http/com/example/www/b8a35774ad20db71c7c3409a5410e74f"
- Function: url-cache-create-filename-human-readable url
Creates a cache file name from url more obviously connected to url than for
url-cache-create-filename-using-md5, but more likely to conflict with other files.
(url-cache-create-filename-human-readable "http://www.example.com/foo/bar") ⇒ "/home/fx/.url/cache/fx/http/com/example/www/foo/bar"
- Function: url-cache-expired
This function returns non-
nilif a cache entry has expired (or is absent). The arguments are a URL and optional expiration delay in seconds (default url-cache-expire-time).
- User Option: url-cache-expire-time
This variable is the default number of seconds to use for the expire-time argument of the function
- Function: url-fetch-from-cache
This function takes a URL as its argument and returns a buffer containing the data cached for that URL.
5.2 Proxies and Gatewaying
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
protocol is one of the supported network protocols (
ftp etc.). The library recognizes such variables in either
upper or lower case. Their values are of one of the forms:
- A full URL;
- Simply a host name.
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:
says to contact all machines in the ‘aventail.com’ and
‘seanet.com’ domains directly, as well as the machine named
NO_PROXY isn’t defined,
no_proxy are also tried, in that order.
Proxies may also be specified directly in Lisp.
- User Option: url-proxy-services
This variable is an alist of URL schemes and proxy servers that gateway them. The items are of the form
(scheme . host:portnumber), says that the URL scheme is gatewayed through portnumber on the specified host. An exception is the pseudo scheme
"no_proxy", which is paired with a regexp matching host names not to be proxied. This variable is initialized from the environment as above.
(setq url-proxy-services '(("http" . "proxy.aventail.com:80") ("no_proxy" . "^.*\\(aventail\\|seanet\\)\\.com")))
5.3 Gateways in General
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.1 The library’s basic function responsible for
making connections is
- Function: url-open-stream name buffer host service
Open a stream to host, possibly via a gateway. The other arguments are as for
open-network-stream. This will not make a connection if
- Variable: url-gateway-local-host-regexp
This is a regular expression that matches local hosts that do not require the use of a gateway. If
nil, all connections are made through the gateway.
- Variable: 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:
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.
This method is identical to
telnet, but uses
rloginto log into the remote machine without having to send the username and password over the wire every time.
Use if the firewall has a SOCKS gateway running on it. The SOCKS v5 protocol is defined in RFC 1928.
This method uses Emacs’s builtin networking directly. This is the default. It can be used only if there is no firewall blocking access.
The following variables control the gateway methods.
- User Option: 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.
- User Option: url-gateway-telnet-parameters
This should be a list of parameters to pass to the
- User Option: url-gateway-telnet-password-prompt
This is a regular expression that matches the password prompt when logging in.
- User Option: url-gateway-telnet-login-prompt
This is a regular expression that matches the username prompt when logging in.
- User Option: socks-server
This specifies the default server, it takes the form
("Default server" server port version)where version can be either 4 or 5.
- Variable: socks-password
If this is
nilthen you will be asked for the password, otherwise it will be used as the password for authenticating you to the SOCKS server.
- Variable: socks-username
This is the username to use when authenticating yourself to the SOCKS server. By default this is your login name.
- Variable: socks-timeout
This controls how long, in seconds, to wait for responses from the SOCKS server; it is 5 by default.
|• Suppressing network connections:|
5.3.1 Suppressing Network Connections
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.
- Variable: url-gateway-unplugged
If this variable is non-
nilnew network connections are never opened by the URL library.
The library can maintain a global history list tracking URLs accessed.
URL completion can be done from it. The history mechanism is set up
url-do-setup when it is configured to be on.
Note that the size of the history list is currently not limited.
The history “list” is actually a hash table,
url-history-hash-table. It contains access times keyed by URL
strings. The times are in the format returned by
- Function: url-history-update-url url time
This function updates the history table with an entry for url accessed at the given time.
- User Option: url-history-track
nil, the library will keep track of all the URLs accessed. If it is
t, the list is saved to disk at the end of each Emacs session. The default is
- User Option: url-history-file
The file storing the history list between sessions. It defaults to history in
- User Option: url-history-save-interval
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
url-do-setuphas been run, you need to run the function
- Function: url-history-parse-history &optional fname
Parses the history file fname (default
url-history-file) and sets up the history list.
- Function: url-history-save-history &optional fname
Saves the current history to file fname (default
- Function: url-completion-function string predicate function
You can use this function to do completion of URLs from the history.
The following environment variables affect the
operation at startup.
The following user options affect the general operation of
- User Option: url-configuration-directory
The value of this variable specifies the name of the directory where the
urllibrary stores its various configuration files, cache files, etc.
The default value specifies a subdirectory named url/ in the standard Emacs user data directory specified by the variable
user-emacs-directory(normally ~/.emacs.d). However, the old default was ~/.url, and this directory is used instead if it exists.
- User Option: url-debug
Specifies the types of debug messages which are logged to the *URL-DEBUG* buffer.
tmeans log all messages. A number means log all messages and show them with
message. It may also be a list of the types of messages to be logged.
- User Option: url-confirmation-func
The function to use for asking yes or no functions. This is normally either
yes-or-no-p, but could be another function taking a single argument (the prompt) and returning
tonly if an affirmative answer is given.
- User Option: url-gateway-method
A symbol specifying the type of gateway support to use for connections from the local machine. The supported methods are:
Run telnet in a subprocess to connect;
Rlogin to another machine to connect;
Connect through a socks server;
Connect with SSL;
Appendix A GNU Free Documentation License
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This License is a kind of “copyleft”, which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software.
We have designed this License in order to use it for manuals for free software, because free software needs free documentation: a free program should come with manuals providing the same freedoms that the software does. But this License is not limited to software manuals; it can be used for any textual work, regardless of subject matter or whether it is published as a printed book. We recommend this License principally for works whose purpose is instruction or reference.
- APPLICABILITY AND DEFINITIONS
This License applies to any manual or other work, in any medium, that contains a notice placed by the copyright holder saying it can be distributed under the terms of this License. Such a notice grants a world-wide, royalty-free license, unlimited in duration, to use that work under the conditions stated herein. The “Document”, below, refers to any such manual or work. Any member of the public is a licensee, and is addressed as “you”. You accept the license if you copy, modify or distribute the work in a way requiring permission under copyright law.
A “Modified Version” of the Document means any work containing the Document or a portion of it, either copied verbatim, or with modifications and/or translated into another language.
A “Secondary Section” is a named appendix or a front-matter section of the Document that deals exclusively with the relationship of the publishers or authors of the Document to the Document’s overall subject (or to related matters) and contains nothing that could fall directly within that overall subject. (Thus, if the Document is in part a textbook of mathematics, a Secondary Section may not explain any mathematics.) The relationship could be a matter of historical connection with the subject or with related matters, or of legal, commercial, philosophical, ethical or political position regarding them.
The “Invariant Sections” are certain Secondary Sections whose titles are designated, as being those of Invariant Sections, in the notice that says that the Document is released under this License. If a section does not fit the above definition of Secondary then it is not allowed to be designated as Invariant. The Document may contain zero Invariant Sections. If the Document does not identify any Invariant Sections then there are none.
The “Cover Texts” are certain short passages of text that are listed, as Front-Cover Texts or Back-Cover Texts, in the notice that says that the Document is released under this License. A Front-Cover Text may be at most 5 words, and a Back-Cover Text may be at most 25 words.
A “Transparent” copy of the Document means a machine-readable copy, represented in a format whose specification is available to the general public, that is suitable for revising the document straightforwardly with generic text editors or (for images composed of pixels) generic paint programs or (for drawings) some widely available drawing editor, and that is suitable for input to text formatters or for automatic translation to a variety of formats suitable for input to text formatters. A copy made in an otherwise Transparent file format whose markup, or absence of markup, has been arranged to thwart or discourage subsequent modification by readers is not Transparent. An image format is not Transparent if used for any substantial amount of text. A copy that is not “Transparent” is called “Opaque”.
Examples of suitable formats for Transparent copies include plain ASCII without markup, Texinfo input format, LaTeX input format, SGML or XML using a publicly available DTD, and standard-conforming simple HTML, PostScript or PDF designed for human modification. Examples of transparent image formats include PNG, XCF and JPG. Opaque formats include proprietary formats that can be read and edited only by proprietary word processors, SGML or XML for which the DTD and/or processing tools are not generally available, and the machine-generated HTML, PostScript or PDF produced by some word processors for output purposes only.
The “Title Page” means, for a printed book, the title page itself, plus such following pages as are needed to hold, legibly, the material this License requires to appear in the title page. For works in formats which do not have any title page as such, “Title Page” means the text near the most prominent appearance of the work’s title, preceding the beginning of the body of the text.
The “publisher” means any person or entity that distributes copies of the Document to the public.
A section “Entitled XYZ” means a named subunit of the Document whose title either is precisely XYZ or contains XYZ in parentheses following text that translates XYZ in another language. (Here XYZ stands for a specific section name mentioned below, such as “Acknowledgements”, “Dedications”, “Endorsements”, or “History”.) To “Preserve the Title” of such a section when you modify the Document means that it remains a section “Entitled XYZ” according to this definition.
The Document may include Warranty Disclaimers next to the notice which states that this License applies to the Document. These Warranty Disclaimers are considered to be included by reference in this License, but only as regards disclaiming warranties: any other implication that these Warranty Disclaimers may have is void and has no effect on the meaning of this License.
- VERBATIM COPYING
You may copy and distribute the Document in any medium, either commercially or noncommercially, provided that this License, the copyright notices, and the license notice saying this License applies to the Document are reproduced in all copies, and that you add no other conditions whatsoever to those of this License. You may not use technical measures to obstruct or control the reading or further copying of the copies you make or distribute. However, you may accept compensation in exchange for copies. If you distribute a large enough number of copies you must also follow the conditions in section 3.
You may also lend copies, under the same conditions stated above, and you may publicly display copies.
- COPYING IN QUANTITY
If you publish printed copies (or copies in media that commonly have printed covers) of the Document, numbering more than 100, and the Document’s license notice requires Cover Texts, you must enclose the copies in covers that carry, clearly and legibly, all these Cover Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on the back cover. Both covers must also clearly and legibly identify you as the publisher of these copies. The front cover must present the full title with all words of the title equally prominent and visible. You may add other material on the covers in addition. Copying with changes limited to the covers, as long as they preserve the title of the Document and satisfy these conditions, can be treated as verbatim copying in other respects.
If the required texts for either cover are too voluminous to fit legibly, you should put the first ones listed (as many as fit reasonably) on the actual cover, and continue the rest onto adjacent pages.
If you publish or distribute Opaque copies of the Document numbering more than 100, you must either include a machine-readable Transparent copy along with each Opaque copy, or state in or with each Opaque copy a computer-network location from which the general network-using public has access to download using public-standard network protocols a complete Transparent copy of the Document, free of added material. If you use the latter option, you must take reasonably prudent steps, when you begin distribution of Opaque copies in quantity, to ensure that this Transparent copy will remain thus accessible at the stated location until at least one year after the last time you distribute an Opaque copy (directly or through your agents or retailers) of that edition to the public.
It is requested, but not required, that you contact the authors of the Document well before redistributing any large number of copies, to give them a chance to provide you with an updated version of the Document.
You may copy and distribute a Modified Version of the Document under the conditions of sections 2 and 3 above, provided that you release the Modified Version under precisely this License, with the Modified Version filling the role of the Document, thus licensing distribution and modification of the Modified Version to whoever possesses a copy of it. In addition, you must do these things in the Modified Version:
- Use in the Title Page (and on the covers, if any) a title distinct from that of the Document, and from those of previous versions (which should, if there were any, be listed in the History section of the Document). You may use the same title as a previous version if the original publisher of that version gives permission.
- List on the Title Page, as authors, one or more persons or entities responsible for authorship of the modifications in the Modified Version, together with at least five of the principal authors of the Document (all of its principal authors, if it has fewer than five), unless they release you from this requirement.
- State on the Title page the name of the publisher of the Modified Version, as the publisher.
- Preserve all the copyright notices of the Document.
- Add an appropriate copyright notice for your modifications adjacent to the other copyright notices.
- Include, immediately after the copyright notices, a license notice giving the public permission to use the Modified Version under the terms of this License, in the form shown in the Addendum below.
- Preserve in that license notice the full lists of Invariant Sections and required Cover Texts given in the Document’s license notice.
- Include an unaltered copy of this License.
- Preserve the section Entitled “History”, Preserve its Title, and add to it an item stating at least the title, year, new authors, and publisher of the Modified Version as given on the Title Page. If there is no section Entitled “History” in the Document, create one stating the title, year, authors, and publisher of the Document as given on its Title Page, then add an item describing the Modified Version as stated in the previous sentence.
- Preserve the network location, if any, given in the Document for public access to a Transparent copy of the Document, and likewise the network locations given in the Document for previous versions it was based on. These may be placed in the “History” section. You may omit a network location for a work that was published at least four years before the Document itself, or if the original publisher of the version it refers to gives permission.
- For any section Entitled “Acknowledgements” or “Dedications”, Preserve the Title of the section, and preserve in the section all the substance and tone of each of the contributor acknowledgements and/or dedications given therein.
- Preserve all the Invariant Sections of the Document, unaltered in their text and in their titles. Section numbers or the equivalent are not considered part of the section titles.
- Delete any section Entitled “Endorsements”. Such a section may not be included in the Modified Version.
- Do not retitle any existing section to be Entitled “Endorsements” or to conflict in title with any Invariant Section.
- Preserve any Warranty Disclaimers.
If the Modified Version includes new front-matter sections or appendices that qualify as Secondary Sections and contain no material copied from the Document, you may at your option designate some or all of these sections as invariant. To do this, add their titles to the list of Invariant Sections in the Modified Version’s license notice. These titles must be distinct from any other section titles.
You may add a section Entitled “Endorsements”, provided it contains nothing but endorsements of your Modified Version by various parties—for example, statements of peer review or that the text has been approved by an organization as the authoritative definition of a standard.
You may add a passage of up to five words as a Front-Cover Text, and a passage of up to 25 words as a Back-Cover Text, to the end of the list of Cover Texts in the Modified Version. Only one passage of Front-Cover Text and one of Back-Cover Text may be added by (or through arrangements made by) any one entity. If the Document already includes a cover text for the same cover, previously added by you or by arrangement made by the same entity you are acting on behalf of, you may not add another; but you may replace the old one, on explicit permission from the previous publisher that added the old one.
The author(s) and publisher(s) of the Document do not by this License give permission to use their names for publicity for or to assert or imply endorsement of any Modified Version.
- COMBINING DOCUMENTS
You may combine the Document with other documents released under this License, under the terms defined in section 4 above for modified versions, provided that you include in the combination all of the Invariant Sections of all of the original documents, unmodified, and list them all as Invariant Sections of your combined work in its license notice, and that you preserve all their Warranty Disclaimers.
The combined work need only contain one copy of this License, and multiple identical Invariant Sections may be replaced with a single copy. If there are multiple Invariant Sections with the same name but different contents, make the title of each such section unique by adding at the end of it, in parentheses, the name of the original author or publisher of that section if known, or else a unique number. Make the same adjustment to the section titles in the list of Invariant Sections in the license notice of the combined work.
In the combination, you must combine any sections Entitled “History” in the various original documents, forming one section Entitled “History”; likewise combine any sections Entitled “Acknowledgements”, and any sections Entitled “Dedications”. You must delete all sections Entitled “Endorsements.”
- COLLECTIONS OF DOCUMENTS
You may make a collection consisting of the Document and other documents released under this License, and replace the individual copies of this License in the various documents with a single copy that is included in the collection, provided that you follow the rules of this License for verbatim copying of each of the documents in all other respects.
You may extract a single document from such a collection, and distribute it individually under this License, provided you insert a copy of this License into the extracted document, and follow this License in all other respects regarding verbatim copying of that document.
- AGGREGATION WITH INDEPENDENT WORKS
A compilation of the Document or its derivatives with other separate and independent documents or works, in or on a volume of a storage or distribution medium, is called an “aggregate” if the copyright resulting from the compilation is not used to limit the legal rights of the compilation’s users beyond what the individual works permit. When the Document is included in an aggregate, this License does not apply to the other works in the aggregate which are not themselves derivative works of the Document.
If the Cover Text requirement of section 3 is applicable to these copies of the Document, then if the Document is less than one half of the entire aggregate, the Document’s Cover Texts may be placed on covers that bracket the Document within the aggregate, or the electronic equivalent of covers if the Document is in electronic form. Otherwise they must appear on printed covers that bracket the whole aggregate.
Translation is considered a kind of modification, so you may distribute translations of the Document under the terms of section 4. Replacing Invariant Sections with translations requires special permission from their copyright holders, but you may include translations of some or all Invariant Sections in addition to the original versions of these Invariant Sections. You may include a translation of this License, and all the license notices in the Document, and any Warranty Disclaimers, provided that you also include the original English version of this License and the original versions of those notices and disclaimers. In case of a disagreement between the translation and the original version of this License or a notice or disclaimer, the original version will prevail.
If a section in the Document is Entitled “Acknowledgements”, “Dedications”, or “History”, the requirement (section 4) to Preserve its Title (section 1) will typically require changing the actual title.
You may not copy, modify, sublicense, or distribute the Document except as expressly provided under this License. Any attempt otherwise to copy, modify, sublicense, or distribute it is void, and will automatically terminate your rights under this License.
However, if you cease all violation of this License, then your license from a particular copyright holder is reinstated (a) provisionally, unless and until the copyright holder explicitly and finally terminates your license, and (b) permanently, if the copyright holder fails to notify you of the violation by some reasonable means prior to 60 days after the cessation.
Moreover, your license from a particular copyright holder is reinstated permanently if the copyright holder notifies you of the violation by some reasonable means, this is the first time you have received notice of violation of this License (for any work) from that copyright holder, and you cure the violation prior to 30 days after your receipt of the notice.
Termination of your rights under this section does not terminate the licenses of parties who have received copies or rights from you under this License. If your rights have been terminated and not permanently reinstated, receipt of a copy of some or all of the same material does not give you any rights to use it.
- FUTURE REVISIONS OF THIS LICENSE
The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. See http://www.gnu.org/copyleft/.
Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License “or any later version” applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation. If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free Software Foundation. If the Document specifies that a proxy can decide which future versions of this License can be used, that proxy’s public statement of acceptance of a version permanently authorizes you to choose that version for the Document.
“Massive Multiauthor Collaboration Site” (or “MMC Site”) means any World Wide Web server that publishes copyrightable works and also provides prominent facilities for anybody to edit those works. A public wiki that anybody can edit is an example of such a server. A “Massive Multiauthor Collaboration” (or “MMC”) contained in the site means any set of copyrightable works thus published on the MMC site.
“CC-BY-SA” means the Creative Commons Attribution-Share Alike 3.0 license published by Creative Commons Corporation, a not-for-profit corporation with a principal place of business in San Francisco, California, as well as future copyleft versions of that license published by that same organization.
“Incorporate” means to publish or republish a Document, in whole or in part, as part of another Document.
An MMC is “eligible for relicensing” if it is licensed under this License, and if all works that were first published under this License somewhere other than this MMC, and subsequently incorporated in whole or in part into the MMC, (1) had no cover texts or invariant sections, and (2) were thus incorporated prior to November 1, 2008.
The operator of an MMC Site may republish an MMC contained in the site under CC-BY-SA on the same site at any time before August 1, 2009, provided the MMC is eligible for relicensing.
ADDENDUM: How to use this License for your documents
To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page:
Copyright (C) year your name. 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, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled ``GNU Free Documentation License''.
If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the “with…Texts.” line with this:
with the Invariant Sections being list their titles, with the Front-Cover Texts being list, and with the Back-Cover Texts being list.
If you have Invariant Sections without Cover Texts, or some other combination of the three, merge those two alternatives to suit the situation.
If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.
Command and Function Index
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Proxies (which only operate over HTTP) are implemented using this.