path: root/docs/TODO

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                Things that could be nice to do in the future

 Things to do in project cURL. Please tell us what you think, contribute and
 send us patches that improve things!

 Be aware that these are things that we could do, or have once been considered
 things we could do. If you want to work on any of these areas, please
 consider bringing it up for discussions first on the mailing list so that we
 all agree it is still a good idea for the project!

 All bugs documented in the KNOWN_BUGS document are subject for fixing!

 1. libcurl
 1.2 More data sharing
 1.3 struct lifreq
 1.4 signal-based resolver timeouts
 1.5 get rid of PATH_MAX
 1.6 Modified buffer size approach
 1.7 Detect when called from within callbacks
 1.8 Allow SSL (HTTPS) to proxy
 1.9 Cache negative name resolves
 1.10 Support IDNA2008
 1.11 minimize dependencies with dynamicly loaded modules
 1.12 have form functions use CURL handle argument
 1.14 Typesafe curl_easy_setopt()
 1.15 Monitor connections in the connection pool
 1.16 Try to URL encode given URL
 1.17 Add support for IRIs
 1.18 try next proxy if one doesn't work
 1.19 Timeout idle connections from the pool
 1.20 SRV and URI DNS records
 1.21 API for URL parsing/splitting
 1.23 Offer API to flush the connection pool

 2. libcurl - multi interface
 2.1 More non-blocking
 2.2 Better support for same name resolves
 2.3 Non-blocking curl_multi_remove_handle()
 2.4 Split connect and authentication process
 2.5 Edge-triggered sockets should work

 3. Documentation
 3.1 Update date and version in man pages
 3.2 Provide cmake config-file

 4. FTP
 4.1 HOST
 4.2 Alter passive/active on failure and retry
 4.3 Earlier bad letter detection
 4.4 REST for large files
 4.5 ASCII support
 4.6 GSSAPI via Windows SSPI
 4.7 STAT for LIST without data connection

 5. HTTP
 5.1 Better persistency for HTTP 1.0
 5.2 support FF3 sqlite cookie files
 5.3 Rearrange request header order
 5.4 Use huge HTTP/2 windows
 5.5 auth= in URLs
 5.6 Refuse "downgrade" redirects
 5.7 Brotli compression
 5.8 QUIC
 5.9 Add easy argument to formpost functions

 6. TELNET
 6.1 ditch stdin
 6.2 ditch telnet-specific select
 6.3 feature negotiation debug data
 6.4 send data in chunks

 7. SMTP
 7.1 Pipelining
 7.2 Enhanced capability support
 7.3 Add CURLOPT_MAIL_CLIENT option

 8. POP3
 8.1 Pipelining
 8.2 Enhanced capability support

 9. IMAP
 9.1 Enhanced capability support

 10. LDAP
 10.1 SASL based authentication mechanisms

 11. SMB
 11.1 File listing support
 11.2 Honor file timestamps
 11.3 Use NTLMv2
 11.4 Create remote directories

 12. New protocols
 12.1 RSYNC

 13. SSL
 13.1 Disable specific versions
 13.2 Provide mutex locking API
 13.3 Evaluate SSL patches
 13.4 Cache OpenSSL contexts
 13.5 Export session ids
 13.6 Provide callback for cert verification
 13.7 improve configure --with-ssl
 13.8 Support DANE
 13.9 Support TLS v1.3
 13.10 Support SSLKEYLOGFILE
 13.11 Support intermediate & root pinning for PINNEDPUBLICKEY
 13.12 Support HSTS
 13.13 Support HPKP

 14. GnuTLS
 14.1 SSL engine stuff
 14.2 check connection

 15. WinSSL/SChannel
 15.1 Add support for client certificate authentication
 15.2 Add support for custom server certificate validation
 15.3 Add support for the --ciphers option

 16. SASL
 16.1 Other authentication mechanisms
 16.2 Add QOP support to GSSAPI authentication
 16.3 Support binary messages (i.e.: non-base64)

 17. SSH protocols
 17.1 Multiplexing
 17.2 SFTP performance

 18. Command line tool
 18.1 sync
 18.2 glob posts
 18.3 prevent file overwriting
 18.4 simultaneous parallel transfers
 18.5 provide formpost headers
 18.6 warning when setting an option
 18.7 warning when sending binary output to terminal
 18.8 offer color-coded HTTP header output
 18.9 Choose the name of file in braces for complex URLs
 18.10 improve how curl works in a windows console window
 18.11 -w output to stderr
 18.12 keep running, read instructions from pipe/socket
 18.13 support metalink in http headers
 18.14 --fail without --location should treat 3xx as a failure

 19. Build
 19.1 roffit

 20. Test suite
 20.1 SSL tunnel
 20.2 nicer lacking perl message
 20.3 more protocols supported
 20.4 more platforms supported
 20.5 Add support for concurrent connections
 20.6 Use the RFC6265 test suite

 21. Next SONAME bump
 21.1 http-style HEAD output for FTP
 21.2 combine error codes
 21.3 extend CURLOPT_SOCKOPTFUNCTION prototype

 22. Next major release
 22.1 cleanup return codes
 22.2 remove obsolete defines
 22.3 size_t
 22.4 remove several functions
 22.5 remove CURLOPT_FAILONERROR
 22.6 remove CURLOPT_DNS_USE_GLOBAL_CACHE
 22.7 remove progress meter from libcurl
 22.8 remove 'curl_httppost' from public

==============================================================================

1. libcurl

1.2 More data sharing

 curl_share_* functions already exist and work, and they can be extended to
 share more. For example, enable sharing of the ares channel and the
 connection cache.

1.3 struct lifreq

 Use 'struct lifreq' and SIOCGLIFADDR instead of 'struct ifreq' and
 SIOCGIFADDR on newer Solaris versions as they claim the latter is obsolete.
 To support IPv6 interface addresses for network interfaces properly.

1.4 signal-based resolver timeouts

 libcurl built without an asynchronous resolver library uses alarm() to time
 out DNS lookups. When a timeout occurs, this causes libcurl to jump from the
 signal handler back into the library with a sigsetjmp, which effectively
 causes libcurl to continue running within the signal handler. This is
 non-portable and could cause problems on some platforms. A discussion on the
 problem is available at https://curl.haxx.se/mail/lib-2008-09/0197.html

 Also, alarm() provides timeout resolution only to the nearest second. alarm
 ought to be replaced by setitimer on systems that support it.

1.5 get rid of PATH_MAX

 Having code use and rely on PATH_MAX is not nice:
 http://insanecoding.blogspot.com/2007/11/pathmax-simply-isnt.html

 Currently the SSH based code uses it a bit, but to remove PATH_MAX from there
 we need libssh2 to properly tell us when we pass in a too small buffer and
 its current API (as of libssh2 1.2.7) doesn't.

1.6 Modified buffer size approach

 Current libcurl allocates a fixed 16K size buffer for download and an
 additional 16K for upload. They are always unconditionally part of the easy
 handle. If CRLF translations are requested, an additional 32K "scratch
 buffer" is allocated. A total of 64K transfer buffers in the worst case.

 First, while the handles are not actually in use these buffers could be freed
 so that lingering handles just kept in queues or whatever waste less memory.

 Secondly, SFTP is a protocol that needs to handle many ~30K blocks at once
 since each need to be individually acked and therefore libssh2 must be
 allowed to send (or receive) many separate ones in parallel to achieve high
 transfer speeds. A current libcurl build with a 16K buffer makes that
 impossible, but one with a 512K buffer will reach MUCH faster transfers. But
 allocating 512K unconditionally for all buffers just in case they would like
 to do fast SFTP transfers at some point is not a good solution either.

 Dynamically allocate buffer size depending on protocol in use in combination
 with freeing it after each individual transfer? Other suggestions?

1.7 Detect when called from within callbacks

 We should set a state variable before calling callbacks, so that we
 subsequently can add code within libcurl that returns error if called within
 callbacks for when that's not supported.

1.8 Allow SSL (HTTPS) to proxy

 To prevent local users from snooping on your traffic to the proxy. Supported
 by Firefox and Chrome already:
 https://www.chromium.org/developers/design-documents/secure-web-proxy

 See this stale work in progress branch:
 https://github.com/curl/curl/tree/HTTPS-proxy based on this PR:
 https://github.com/curl/curl/pull/305

1.9 Cache negative name resolves

 A name resolve that has failed is likely to fail when made again within a
 short period of time. Currently we only cache positive responses.

1.10 Support IDNA2008

 International Domain Names are supported in libcurl since years back, powered
 by libidn. libidn implements IDNA2003 which has been superseded by IDNA2008.
 libidn2 is an existing library offering support for IDNA2008.

1.11 minimize dependencies with dynamicly loaded modules

 We can create a system with loadable modules/plug-ins, where these modules
 would be the ones that link to 3rd party libs. That would allow us to avoid
 having to load ALL dependencies since only the necessary ones for this
 app/invoke/used protocols would be necessary to load.  See
 https://github.com/curl/curl/issues/349

1.12 have form functions use CURL handle argument

 curl_formadd() and curl_formget() both currently have no CURL handle
 argument, but both can use a callback that is set in the easy handle, and
 thus curl_formget() with callback cannot function without first having
 curl_easy_perform() (or similar) called - which is hard to grasp and a design
 mistake.

 The curl_formadd() design can probably also be reconsidered to make it easier
 to use and less error-prone. Probably easiest by splitting it into several
 function calls.

1.14 Typesafe curl_easy_setopt()

 One of the most common problems in libcurl using applications is the lack of
 type checks for curl_easy_setopt() which happens because it accepts varargs
 and thus can take any type.

 One possible solution to this is to introduce a few different versions of the
 setopt version for the different kinds of data you can set.

  curl_easy_set_num() - sets a long value

  curl_easy_set_large() - sets a curl_off_t value

  curl_easy_set_ptr() - sets a pointer

  curl_easy_set_cb() - sets a callback PLUS its callback data

1.15 Monitor connections in the connection pool

 libcurl's connection cache or pool holds a number of open connections for the
 purpose of possible subsequent connection reuse. It may contain a few up to a
 significant amount of connections. Currently, libcurl leaves all connections
 as they are and first when a connection is iterated over for matching or
 reuse purpose it is verified that it is still alive.

 Those connections may get closed by the server side for idleness or they may
 get a HTTP/2 ping from the peer to verify that they're still alive. By adding
 monitoring of the connections while in the pool, libcurl can detect dead
 connections (and close them) better and earlier, and it can handle HTTP/2
 pings to keep such ones alive even when not actively doing transfers on them.

1.16 Try to URL encode given URL

 Given a URL that for example contains spaces, libcurl could have an option
 that would try somewhat harder than it does now and convert spaces to %20 and
 perhaps URL encoded byte values over 128 etc (basically do what the redirect
 following code already does).

 https://github.com/curl/curl/issues/514

1.17 Add support for IRIs

 IRIs (RFC 3987) allow localized, non-ascii, names in the URL. To properly
 support this, curl/libcurl would need to translate/encode the given input
 from the input string encoding into percent encoded output "over the wire".

 To make that work smoothly for curl users even on Windows, curl would
 probably need to be able to convert from several input encodings.

1.18 try next proxy if one doesn't work

 Allow an application to specify a list of proxies to try, and failing to
 connect to the first go on and try the next instead until the list is
 exhausted. Browsers support this feature at least when they specify proxies
 using PACs.

 https://github.com/curl/curl/issues/896

1.19 Timeout idle connections from the pool

 libcurl currently keeps connections in its connection pool for an indefinite
 period of time, until it either gets reused, gets noticed that it has been
 closed by the server or gets pruned to make room for a new connection.

 To reduce overhead (especially for when we add monitoring of the connections
 in the pool), we should introduce a timeout so that connections that have
 been idle for N seconds get closed.

1.20 SRV and URI DNS records

 Offer support for resolving SRV and URI DNS records for libcurl to know which
 server to connect to for various protocols (including HTTP!).

1.21 API for URL parsing/splitting

 libcurl has always parsed URLs internally and never exposed any API or
 features to allow applications to do it. Still most or many applications
 using libcurl need that ability. In polls to users, we've learned that many
 libcurl users would like to see and use such an API.

1.23 Offer API to flush the connection pool

 Sometimes applications want to flush all the existing connections kept alive.
 An API could allow a forced flush or just a forced loop that would properly
 close all connections that have been closed by the server already.


2. libcurl - multi interface

2.1 More non-blocking

 Make sure we don't ever loop because of non-blocking sockets returning
 EWOULDBLOCK or similar. Blocking cases include:

 - Name resolves on non-windows unless c-ares is used
 - NSS SSL connections
 - HTTP proxy CONNECT operations
 - SOCKS proxy handshakes
 - file:// transfers
 - TELNET transfers
 - The "DONE" operation (post transfer protocol-specific actions) for the
   protocols SFTP, SMTP, FTP. Fixing Curl_done() for this is a worthy task.

2.2 Better support for same name resolves

 If a name resolve has been initiated for name NN and a second easy handle
 wants to resolve that name as well, make it wait for the first resolve to end
 up in the cache instead of doing a second separate resolve. This is
 especially needed when adding many simultaneous handles using the same host
 name when the DNS resolver can get flooded.

2.3 Non-blocking curl_multi_remove_handle()

 The multi interface has a few API calls that assume a blocking behavior, like
 add_handle() and remove_handle() which limits what we can do internally. The
 multi API need to be moved even more into a single function that "drives"
 everything in a non-blocking manner and signals when something is done. A
 remove or add would then only ask for the action to get started and then
 multi_perform() etc still be called until the add/remove is completed.

2.4 Split connect and authentication process

 The multi interface treats the authentication process as part of the connect
 phase. As such any failures during authentication won't trigger the relevant
 QUIT or LOGOFF for protocols such as IMAP, POP3 and SMTP.

2.5 Edge-triggered sockets should work

 The multi_socket API should work with edge-triggered socket events. One of
 the internal actions that need to be improved for this to work perfectly is
 the 'maxloops' handling in transfer.c:readwrite_data().

3. Documentation

3.1 Update date and version in man pages

 'maketgz' or another suitable script could update the .TH sections of the man
 pages at release time to use the current date and curl/libcurl version
 number.

3.2 Provide cmake config-file

 A config-file package is a set of files provided by us to allow applications
 to write cmake scripts to find and use libcurl easier. See
 https://github.com/curl/curl/issues/885

4. FTP

4.1 HOST

 HOST is a command for a client to tell which host name to use, to offer FTP
 servers named-based virtual hosting:

 https://tools.ietf.org/html/rfc7151

4.2 Alter passive/active on failure and retry

 When trying to connect passively to a server which only supports active
 connections, libcurl returns CURLE_FTP_WEIRD_PASV_REPLY and closes the
 connection. There could be a way to fallback to an active connection (and
 vice versa). https://curl.haxx.se/bug/feature.cgi?id=1754793

4.3 Earlier bad letter detection

 Make the detection of (bad) %0d and %0a codes in FTP URL parts earlier in the
 process to avoid doing a resolve and connect in vain.

4.4 REST for large files

 REST fix for servers not behaving well on >2GB requests. This should fail if
 the server doesn't set the pointer to the requested index. The tricky
 (impossible?) part is to figure out if the server did the right thing or not.

4.5 ASCII support

 FTP ASCII transfers do not follow RFC959. They don't convert the data
 accordingly.

4.6 GSSAPI via Windows SSPI

In addition to currently supporting the SASL GSSAPI mechanism (Kerberos V5)
via third-party GSS-API libraries, such as Heimdal or MIT Kerberos, also add
support for GSSAPI authentication via Windows SSPI.

4.7 STAT for LIST without data connection

Some FTP servers allow STAT for listing directories instead of using LIST, and
the response is then sent over the control connection instead of as the
otherwise usedw data connection: http://www.nsftools.com/tips/RawFTP.htm#STAT

This is not detailed in any FTP specification.

5. HTTP

5.1 Better persistency for HTTP 1.0

 "Better" support for persistent connections over HTTP 1.0
 https://curl.haxx.se/bug/feature.cgi?id=1089001

5.2 support FF3 sqlite cookie files

 Firefox 3 is changing from its former format to a a sqlite database instead.
 We should consider how (lib)curl can/should support this.
 https://curl.haxx.se/bug/feature.cgi?id=1871388

5.3 Rearrange request header order

 Server implementors often make an effort to detect browser and to reject
 clients it can detect to not match. One of the last details we cannot yet
 control in libcurl's HTTP requests, which also can be exploited to detect
 that libcurl is in fact used even when it tries to impersonate a browser, is
 the order of the request headers. I propose that we introduce a new option in
 which you give headers a value, and then when the HTTP request is built it
 sorts the headers based on that number. We could then have internally created
 headers use a default value so only headers that need to be moved have to be
 specified.

5.4 Use huge HTTP/2 windows

 We're currently using nghttp2's default window size which is terribly small
 (64K). This becomes a bottle neck over high bandwidth networks. We should
 instead make the window size to be very big (512MB?) as we really don't do
 much flow control anyway.

5.5 auth= in URLs

 Add the ability to specify the preferred authentication mechanism to use by
 using ;auth=<mech> in the login part of the URL.

 For example:

 http://test:pass;auth=NTLM@example.com would be equivalent to specifying --user
 test:pass;auth=NTLM or --user test:pass --ntlm from the command line.

 Additionally this should be implemented for proxy base URLs as well.

5.6 Refuse "downgrade" redirects

 See https://github.com/curl/curl/issues/226

 Consider a way to tell curl to refuse to "downgrade" protocol with a redirect
 and/or possibly a bit that refuses redirect to change protocol completely.

5.7 Brotli compression

 Compression algorithms that perform better than gzip are being considered for
 use and inclusion in existing browsers. For example 'brotli'. If servers
 follow along it is a good reason for us to also allow users to take advantage
 of this. The algorithm: https://github.com/google/brotli The Firefox bug:
 https://bugzilla.mozilla.org/show_bug.cgi?id=366559

5.8 QUIC

 The standardization process of QUIC has been taken to the IETF and can be
 followed on the [IETF QUIC Mailing
 list](https://www.ietf.org/mailman/listinfo/quic). I'd like us to get on the
 bandwagon. Ideally, this would be done with a separate library/project to
 handle the binary/framing layer in a similar fashion to how HTTP/2 is
 implemented. This, to allow other projects to benefit from the work and to
 thus broaden the interest and chance of others to participate.

5.9 Add easy argument to formpost functions

 Make sure there's an easy handle passed in to `curl_formadd()`,
 `curl_formget()` and `curl_formfree()` by adding replacement functions and
 deprecating the old ones. Allows better error messages and is generally good
 API hygiene.

6. TELNET

6.1 ditch stdin

Reading input (to send to the remote server) on stdin is a crappy solution for
library purposes. We need to invent a good way for the application to be able
to provide the data to send.

6.2 ditch telnet-specific select

 Move the telnet support's network select() loop go away and merge the code
 into the main transfer loop. Until this is done, the multi interface won't
 work for telnet.

6.3 feature negotiation debug data

  Add telnet feature negotiation data to the debug callback as header data.

6.4 send data in chunks

  Currently, telnet sends data one byte at a time.  This is fine for interactive
  use, but inefficient for any other.  Sent data should be sent in larger
  chunks.

7. SMTP

7.1 Pipelining

 Add support for pipelining emails.

7.2 Enhanced capability support

 Add the ability, for an application that uses libcurl, to obtain the list of
 capabilities returned from the EHLO command.

7.3 Add CURLOPT_MAIL_CLIENT option

 Rather than use the URL to specify the mail client string to present in the
 HELO and EHLO commands, libcurl should support a new CURLOPT specifically for
 specifying this data as the URL is non-standard and to be honest a bit of a
 hack ;-)

 Please see the following thread for more information:
 https://curl.haxx.se/mail/lib-2012-05/0178.html


8. POP3

8.1 Pipelining

 Add support for pipelining commands.

8.2 Enhanced capability support

 Add the ability, for an application that uses libcurl, to obtain the list of
 capabilities returned from the CAPA command.

9. IMAP

9.1 Enhanced capability support

 Add the ability, for an application that uses libcurl, to obtain the list of
 capabilities returned from the CAPABILITY command.

10. LDAP

10.1 SASL based authentication mechanisms

 Currently the LDAP module only supports ldap_simple_bind_s() in order to bind
 to an LDAP server. However, this function sends username and password details
 using the simple authentication mechanism (as clear text). However, it should
 be possible to use ldap_bind_s() instead specifying the security context
 information ourselves.

11. SMB

11.1 File listing support

Add support for listing the contents of a SMB share. The output should probably
be the same as/similar to FTP.

11.2 Honor file timestamps

The timestamp of the transferred file should reflect that of the original file.

11.3 Use NTLMv2

Currently the SMB authentication uses NTLMv1.

11.4 Create remote directories

Support for creating remote directories when uploading a file to a directory
that doesn't exist on the server, just like --ftp-create-dirs.

12. New protocols

12.1 RSYNC

 There's no RFC for the protocol or an URI/URL format.  An implementation
 should most probably use an existing rsync library, such as librsync.

13. SSL

13.1 Disable specific versions

 Provide an option that allows for disabling specific SSL versions, such as
 SSLv2 https://curl.haxx.se/bug/feature.cgi?id=1767276

13.2 Provide mutex locking API

 Provide a libcurl API for setting mutex callbacks in the underlying SSL
 library, so that the same application code can use mutex-locking
 independently of OpenSSL or GnutTLS being used.

13.3 Evaluate SSL patches

 Evaluate/apply Gertjan van Wingerde's SSL patches:
 https://curl.haxx.se/mail/lib-2004-03/0087.html

13.4 Cache OpenSSL contexts

 "Look at SSL cafile - quick traces look to me like these are done on every
 request as well, when they should only be necessary once per SSL context (or
 once per handle)". The major improvement we can rather easily do is to make
 sure we don't create and kill a new SSL "context" for every request, but
 instead make one for every connection and re-use that SSL context in the same
 style connections are re-used. It will make us use slightly more memory but
 it will libcurl do less creations and deletions of SSL contexts.

13.5 Export session ids

 Add an interface to libcurl that enables "session IDs" to get
 exported/imported. Cris Bailiff said: "OpenSSL has functions which can
 serialise the current SSL state to a buffer of your choice, and recover/reset
 the state from such a buffer at a later date - this is used by mod_ssl for
 apache to implement and SSL session ID cache".

13.6 Provide callback for cert verification

 OpenSSL supports a callback for customised verification of the peer
 certificate, but this doesn't seem to be exposed in the libcurl APIs. Could
 it be? There's so much that could be done if it were!

13.7 improve configure --with-ssl

 make the configure --with-ssl option first check for OpenSSL, then GnuTLS,
 then NSS...

13.8 Support DANE

 DNS-Based Authentication of Named Entities (DANE) is a way to provide SSL
 keys and certs over DNS using DNSSEC as an alternative to the CA model.
 https://www.rfc-editor.org/rfc/rfc6698.txt

 An initial patch was posted by Suresh Krishnaswamy on March 7th 2013
 (https://curl.haxx.se/mail/lib-2013-03/0075.html) but it was a too simple
 approach. See Daniel's comments:
 https://curl.haxx.se/mail/lib-2013-03/0103.html . libunbound may be the
 correct library to base this development on.

 Björn Stenberg wrote a separate initial take on DANE that was never
 completed.

13.9 Support TLS v1.3

 TLS version 1.3 is about to ship and is getting implemented by TLS libraries
 as we speak. We should start to support the symbol and make sure all backends
 handle it accordingly, then gradually add support as the TLS libraries add
 the corresponding support. There may be a need to add some additional options
 to allow libcurl to take advantage of the new features in 1.3.

13.10 Support SSLKEYLOGFILE

 When used, Firefox and Chrome dumps their master TLS keys to the file name
 this environment variable specifies. This allows tools like for example
 Wireshark to capture and decipher TLS traffic to/from those clients. libcurl
 could be made to support this more widely (presumably this already works when
 built with NSS). Peter Wu made a OpenSSL preload to make possible that can be
 used as inspiration and guidance
 https://git.lekensteyn.nl/peter/wireshark-notes/tree/src/sslkeylog.c

13.11 Support intermediate & root pinning for PINNEDPUBLICKEY

 CURLOPT_PINNEDPUBLICKEY does not consider the hashes of intermediate & root
 certificates when comparing the pinned keys. Therefore it is not compatible
 with "HTTP Public Key Pinning" as there also intermediate and root certificates
 can be pinned. This is very useful as it prevents webadmins from "locking
 themself out of their servers".

 Adding this feature would make curls pinning 100% compatible to HPKP and allow
 more flexible pinning.

13.12 Support HSTS

 "HTTP Strict Transport Security" is TOFU (trust on first use), time-based
 features indicated by a HTTP header send by the webserver. It is widely used
 in browsers and it's purpose is to prevent insecure HTTP connections after
 a previous HTTPS connection. It protects against SSLStripping attacks.

 Doc: https://developer.mozilla.org/en-US/docs/Web/Security/HTTP_strict_transport_security
 RFC 6797: https://tools.ietf.org/html/rfc6797

13.13 Support HPKP

 "HTTP Public Key Pinning" is TOFU (trust on first use), time-based
 features indicated by a HTTP header send by the webserver. It's purpose is
 to prevent Man-in-the-middle attacks by trusted CAs by allowing webadmins
 to specify which CAs/certificates/public keys to trust when connection to
 their websites.

 It can be build based on PINNEDPUBLICKEY.

 Wikipedia: https://en.wikipedia.org/wiki/HTTP_Public_Key_Pinning
 OWASP: https://www.owasp.org/index.php/Certificate_and_Public_Key_Pinning
 Doc: https://developer.mozilla.org/de/docs/Web/Security/Public_Key_Pinning
 RFC: https://tools.ietf.org/html/draft-ietf-websec-key-pinning-21

14. GnuTLS

14.1 SSL engine stuff

 Is this even possible?

14.2 check connection

 Add a way to check if the connection seems to be alive, to correspond to the
 SSL_peak() way we use with OpenSSL.

15. WinSSL/SChannel

15.1 Add support for client certificate authentication

 WinSSL/SChannel currently makes use of the OS-level system and user
 certificate and private key stores. This does not allow the application
 or the user to supply a custom client certificate using curl or libcurl.

 Therefore support for the existing -E/--cert and --key options should be
 implemented by supplying a custom certificate to the SChannel APIs, see:
 - Getting a Certificate for Schannel
   https://msdn.microsoft.com/en-us/library/windows/desktop/aa375447.aspx

15.2 Add support for custom server certificate validation

 WinSSL/SChannel currently makes use of the OS-level system and user
 certificate trust store. This does not allow the application or user to
 customize the server certificate validation process using curl or libcurl.

 Therefore support for the existing --cacert or --capath options should be
 implemented by supplying a custom certificate to the SChannel APIs, see:
 - Getting a Certificate for Schannel
   https://msdn.microsoft.com/en-us/library/windows/desktop/aa375447.aspx

15.3 Add support for the --ciphers option

 The cipher suites used by WinSSL/SChannel are configured on an OS-level
 instead of an application-level. This does not allow the application or
 the user to customize the configured cipher suites using curl or libcurl.

 Therefore support for the existing --ciphers option should be implemented
 by mapping the OpenSSL/GnuTLS cipher suites to the SChannel APIs, see
 - Specifying Schannel Ciphers and Cipher Strengths
   https://msdn.microsoft.com/en-us/library/windows/desktop/aa380161.aspx

16. SASL

16.1 Other authentication mechanisms

 Add support for other authentication mechanisms such as OLP,
 GSS-SPNEGO and others.

16.2 Add QOP support to GSSAPI authentication

 Currently the GSSAPI authentication only supports the default QOP of auth
 (Authentication), whilst Kerberos V5 supports both auth-int (Authentication
 with integrity protection) and auth-conf (Authentication with integrity and
 privacy protection).

16.3 Support binary messages (i.e.: non-base64)

  Mandatory to support LDAP SASL authentication.


17. SSH protocols

17.1 Multiplexing

 SSH is a perfectly fine multiplexed protocols which would allow libcurl to do
 multiple parallel transfers from the same host using the same connection,
 much in the same spirit as HTTP/2 does. libcurl however does not take
 advantage of that ability but will instead always create a new connection for
 new transfers even if an existing connection already exists to the host.

 To fix this, libcurl would have to detect an existing connection and "attach"
 the new transfer to the existing one.

17.2 SFTP performance

 libcurl's SFTP transfer performance is sub par and can be improved, mostly by
 the approach mentioned in "1.6 Modified buffer size approach".

18. Command line tool

18.1 sync

 "curl --sync http://example.com/feed[1-100].rss" or
 "curl --sync http://example.net/{index,calendar,history}.html"

 Downloads a range or set of URLs using the remote name, but only if the
 remote file is newer than the local file. A Last-Modified HTTP date header
 should also be used to set the mod date on the downloaded file.

18.2 glob posts

 Globbing support for -d and -F, as in 'curl -d "name=foo[0-9]" URL'.
 This is easily scripted though.

18.3 prevent file overwriting

 Add an option that prevents cURL from overwriting existing local files. When
 used, and there already is an existing file with the target file name
 (either -O or -o), a number should be appended (and increased if already
 existing). So that index.html becomes first index.html.1 and then
 index.html.2 etc.

18.4 simultaneous parallel transfers

 The client could be told to use maximum N simultaneous parallel transfers and
 then just make sure that happens. It should of course not make more than one
 connection to the same remote host. This would require the client to use the
 multi interface. https://curl.haxx.se/bug/feature.cgi?id=1558595

 Using the multi interface would also allow properly using parallel transfers
 with HTTP/2 and supporting HTTP/2 server push from the command line.

18.5 provide formpost headers

 Extending the capabilities of the multipart formposting. How about leaving
 the ';type=foo' syntax as it is and adding an extra tag (headers) which
 works like this: curl -F "coolfiles=@fil1.txt;headers=@fil1.hdr" where
 fil1.hdr contains extra headers like

   Content-Type: text/plain; charset=KOI8-R"
   Content-Transfer-Encoding: base64
   X-User-Comment: Please don't use browser specific HTML code

 which should overwrite the program reasonable defaults (plain/text,
 8bit...)

18.6 warning when setting an option

 Display a warning when libcurl returns an error when setting an option.
 This can be useful to tell when support for a particular feature hasn't been
 compiled into the library.

18.7 warning when sending binary output to terminal

 Provide a way that prompts the user for confirmation before binary data is
 sent to the terminal, much in the style 'less' does it.

18.8 offer color-coded HTTP header output

 By offering different color output on the header name and the header
 contents, they could be made more readable and thus help users working on
 HTTP services.

18.9 Choose the name of file in braces for complex URLs

 When using braces to download a list of URLs and you use complicated names
 in the list of alternatives, it could be handy to allow curl to use other
 names when saving.

 Consider a way to offer that. Possibly like
 {partURL1:name1,partURL2:name2,partURL3:name3} where the name following the
 colon is the output name.

 See https://github.com/curl/curl/issues/221

18.10 improve how curl works in a windows console window

 If you pull the scrollbar when transferring with curl in a Windows console
 window, the transfer is interrupted and can get disconnected. This can
 probably be improved. See https://github.com/curl/curl/issues/322

18.11 -w output to stderr

 -w is quite useful, but not to those of us who use curl without -o or -O
 (such as for scripting through a higher level language). It would be nice to
 have an option that is exactly like -w but sends it to stderr
 instead. Proposed name: --write-stderr. See
 https://github.com/curl/curl/issues/613

18.12 keep running, read instructions from pipe/socket

 Provide an option that makes curl not exit after the last URL (or even work
 without a given URL), and then make it read instructions passed on a pipe or
 over a socket to make further instructions so that a second subsequent curl
 invoke can talk to the still running instance and ask for transfers to get
 done, and thus maintain its connection pool, DNS cache and more.

18.13 support metalink in http headers

 Curl has support for downloading a metalink xml file, processing it, and then
 downloading the target of the metalink. This is done via the --metalink option.
 It would be nice if metalink also supported downloading via metalink
 information that is stored in HTTP headers (RFC 6249). Theoretically this could
 also be supported with the --metalink option.

 See https://tools.ietf.org/html/rfc6249

 See also https://lists.gnu.org/archive/html/bug-wget/2015-06/msg00034.html for
 an implematation of this in wget.

18.14 --fail without --location should treat 3xx as a failure

 To allow a command line like this to detect a redirect and consider it a
 failure:

    curl -v --fail -O https://example.com/curl-7.48.0.tar.gz

 ... --fail must treat 3xx responses as failures too. The least problematic
 way to implement this is probably to add that new logic in the command line
 tool only and not in the underlying CURLOPT_FAILONERROR logic.


19. Build

19.1 roffit

 Consider extending 'roffit' to produce decent ASCII output, and use that
 instead of (g)nroff when building src/tool_hugehelp.c

20. Test suite

20.1 SSL tunnel

 Make our own version of stunnel for simple port forwarding to enable HTTPS
 and FTP-SSL tests without the stunnel dependency, and it could allow us to
 provide test tools built with either OpenSSL or GnuTLS

20.2 nicer lacking perl message

 If perl wasn't found by the configure script, don't attempt to run the tests
 but explain something nice why it doesn't.

20.3 more protocols supported

 Extend the test suite to include more protocols. The telnet could just do FTP
 or http operations (for which we have test servers).

20.4 more platforms supported

 Make the test suite work on more platforms. OpenBSD and Mac OS. Remove
 fork()s and it should become even more portable.

20.5 Add support for concurrent connections

 Tests 836, 882 and 938 were designed to verify that separate connections aren't
 used when using different login credentials in protocols that shouldn't re-use
 a connection under such circumstances.

 Unfortunately, ftpserver.pl doesn't appear to support multiple concurrent
 connections. The read while() loop seems to loop until it receives a disconnect
 from the client, where it then enters the waiting for connections loop. When
 the client opens a second connection to the server, the first connection hasn't
 been dropped (unless it has been forced - which we shouldn't do in these tests)
 and thus the wait for connections loop is never entered to receive the second
 connection.

20.6 Use the RFC6265 test suite

 A test suite made for HTTP cookies (RFC 6265) by Adam Barth is available at
 https://github.com/abarth/http-state/tree/master/tests

 It'd be really awesome if someone would write a script/setup that would run
 curl with that test suite and detect deviances. Ideally, that would even be
 incorporated into our regular test suite.


21. Next SONAME bump

21.1 http-style HEAD output for FTP

 #undef CURL_FTP_HTTPSTYLE_HEAD in lib/ftp.c to remove the HTTP-style headers
 from being output in NOBODY requests over FTP

21.2 combine error codes

 Combine some of the error codes to remove duplicates.  The original
 numbering should not be changed, and the old identifiers would be
 macroed to the new ones in an CURL_NO_OLDIES section to help with
 backward compatibility.

 Candidates for removal and their replacements:

    CURLE_FILE_COULDNT_READ_FILE => CURLE_REMOTE_FILE_NOT_FOUND

    CURLE_FTP_COULDNT_RETR_FILE => CURLE_REMOTE_FILE_NOT_FOUND

    CURLE_FTP_COULDNT_USE_REST => CURLE_RANGE_ERROR

    CURLE_FUNCTION_NOT_FOUND => CURLE_FAILED_INIT

    CURLE_LDAP_INVALID_URL => CURLE_URL_MALFORMAT

    CURLE_TFTP_NOSUCHUSER => CURLE_TFTP_ILLEGAL

    CURLE_TFTP_NOTFOUND => CURLE_REMOTE_FILE_NOT_FOUND

    CURLE_TFTP_PERM => CURLE_REMOTE_ACCESS_DENIED

21.3 extend CURLOPT_SOCKOPTFUNCTION prototype

 The current prototype only provides 'purpose' that tells what the
 connection/socket is for, but not any protocol or similar. It makes it hard
 for applications to differentiate on TCP vs UDP and even HTTP vs FTP and
 similar.

22. Next major release

22.1 cleanup return codes

 curl_easy_cleanup() returns void, but curl_multi_cleanup() returns a
 CURLMcode. These should be changed to be the same.

22.2 remove obsolete defines

 remove obsolete defines from curl/curl.h

22.3 size_t

 make several functions use size_t instead of int in their APIs

22.4 remove several functions

 remove the following functions from the public API:

 curl_getenv

 curl_mprintf (and variations)

 curl_strequal

 curl_strnequal

 They will instead become curlx_ - alternatives. That makes the curl app
 still capable of using them, by building with them from source.

 These functions have no purpose anymore:

 curl_multi_socket

 curl_multi_socket_all

22.5 remove CURLOPT_FAILONERROR

 Remove support for CURLOPT_FAILONERROR, it has gotten too kludgy and weird
 internally. Let the app judge success or not for itself.

22.6 remove CURLOPT_DNS_USE_GLOBAL_CACHE

 Remove support for a global DNS cache. Anything global is silly, and we
 already offer the share interface for the same functionality but done
 "right".

22.7 remove progress meter from libcurl

 The internally provided progress meter output doesn't belong in the library.
 Basically no application wants it (apart from curl) but instead applications
 can and should do their own progress meters using the progress callback.

 The progress callback should then be bumped as well to get proper 64bit
 variable types passed to it instead of doubles so that big files work
 correctly.

22.8 remove 'curl_httppost' from public

 curl_formadd() was made to fill in a public struct, but the fact that the
 struct is public is never really used by application for their own advantage
 but instead often restricts how the form functions can or can't be modified.

 Changing them to return a private handle will benefit the implementation and
 allow us much greater freedoms while still maintaining a solid API and ABI.