xref: /third_party/curl/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
 1.1 TFO support on Windows
 1.2 Consult %APPDATA% also for .netrc
 1.3 struct lifreq
 1.4 alt-svc sharing
 1.5 get rid of PATH_MAX
 1.6 native IDN support on macOS
 1.7 Support HTTP/2 for HTTP(S) proxies
 1.8 CURLOPT_RESOLVE for any port number
 1.9 Cache negative name resolves
 1.10 auto-detect proxy
 1.11 minimize dependencies with dynamically loaded modules
 1.12 updated DNS server while running
 1.13 c-ares and CURLOPT_OPENSOCKETFUNCTION
 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 provide timing info for each redirect
 1.20 SRV and URI DNS records
 1.21 netrc caching and sharing
 1.22 CURLINFO_PAUSE_STATE
 1.23 Offer API to flush the connection pool
 1.24 TCP Fast Open for windows
 1.25 Expose tried IP addresses that failed
 1.27 hardcode the "localhost" addresses
 1.28 FD_CLOEXEC
 1.29 Upgrade to websockets
 1.30 config file parsing

 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
 2.6 multi upkeep
 2.7 Virtual external sockets
 2.8 dynamically decide to use socketpair

 3. Documentation
 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
 4.5 ASCII support
 4.6 GSSAPI via Windows SSPI
 4.7 STAT for LIST without data connection
 4.8 Option to ignore private IP addresses in PASV response

 5. HTTP
 5.1 Better persistency for HTTP 1.0
 5.2 Set custom client ip when using haproxy protocol
 5.3 Rearrange request header order
 5.4 Allow SAN names in HTTP/2 server push
 5.5 auth= in URLs
 5.6 alt-svc should fallback if alt-svc doesn't work

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

 7. SMTP
 7.2 Enhanced capability support
 7.3 Add CURLOPT_MAIL_CLIENT option

 8. POP3
 8.2 Enhanced capability support

 9. IMAP
 9.1 Enhanced capability support

 10. LDAP
 10.1 SASL based authentication mechanisms
 10.2 CURLOPT_SSL_CTX_FUNCTION for LDAPS
 10.3 Paged searches on LDAP server

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

 12. FILE
 12.1 Directory listing for FILE:

 13. SSL
 13.1 TLS-PSK with OpenSSL
 13.2 Provide mutex locking API
 13.4 Cache/share OpenSSL contexts
 13.5 Export session ids
 13.6 Provide callback for cert verification
 13.8 Support DANE
 13.9 TLS record padding
 13.10 Support Authority Information Access certificate extension (AIA)
 13.11 Support intermediate & root pinning for PINNEDPUBLICKEY
 13.13 Make sure we forbid TLS 1.3 post-handshake authentication
 13.14 Support the clienthello extension
 13.15 Support mbedTLS 3.0

 14. GnuTLS
 14.2 check connection

 15. Schannel
 15.1 Extend support for client certificate authentication
 15.2 Extend support for the --ciphers option
 15.4 Add option to allow abrupt server closure

 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 Handle growing SFTP files
 17.3 Support better than MD5 hostkey hash
 17.4 Support CURLOPT_PREQUOTE
 17.5 SSH over HTTPS proxy with more backends

 18. Command line tool
 18.1 sync
 18.2 glob posts
 18.3 prevent file overwriting
 18.4 --proxycommand
 18.5 UTF-8 filenames in Content-Disposition
 18.6 Option to make -Z merge lined based outputs on stdout
 18.7 at least N milliseconds between requests
 18.8 Consider convenience options for JSON and XML?
 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 Windows: set attribute 'archive' for completed downloads
 18.12 keep running, read instructions from pipe/socket
 18.13 Ratelimit or wait between serial requests
 18.14 --dry-run
 18.15 --retry should resume
 18.16 send only part of --data
 18.17 consider file name from the redirected URL with -O ?
 18.18 retry on network is unreachable
 18.19 expand ~/ in config files
 18.20 host name sections in config files
 18.21 retry on the redirected-to URL
 18.23 Set the modification date on an uploaded file
 18.24 Use multiple parallel transfers for a single download
 18.25 Prevent terminal injection when writing to terminal
 18.26 Custom progress meter update interval

 19. Build
 19.1 roffit
 19.2 Enable PIE and RELRO by default
 19.3 Don't use GNU libtool on OpenBSD
 19.4 Package curl for Windows in a signed installer

 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
 20.7 Support LD_PRELOAD on macOS
 20.8 Run web-platform-tests url tests
 20.9 Bring back libssh tests on Travis

 21. MQTT
 21.1 Support rate-limiting

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

1. libcurl

1.1 TFO support on Windows

 TCP Fast Open is supported on several platforms but not on Windows. Work on
 this was once started but never finished.

 See https://github.com/curl/curl/pull/3378

1.2 Consult %APPDATA% also for .netrc

 %APPDATA%\.netrc is not considered when running on Windows. Shouldn't it?

 See https://github.com/curl/curl/issues/4016

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 alt-svc sharing

 The share interface could benefit from allowing the alt-svc cache to be
 possible to share between easy handles.

 See https://github.com/curl/curl/issues/4476

1.5 get rid of PATH_MAX

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

 Currently the libssh2 SSH based code uses it, 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 native IDN support on macOS

 On recent macOS versions, the getaddrinfo() function itself has built-in IDN
 support. By setting the AI_CANONNAME flag, the function will return the
 encoded name in the ai_canonname struct field in the returned information.
 This could be used by curl on macOS when built without a separate IDN library
 and an IDN host name is used in a URL.

 See initial work in https://github.com/curl/curl/pull/5371

1.7 Support HTTP/2 for HTTP(S) proxies

 Support for doing HTTP/2 to HTTP and HTTPS proxies is still missing.

 See https://github.com/curl/curl/issues/3570

1.8 CURLOPT_RESOLVE for any port number

 This option allows applications to set a replacement IP address for a given
 host + port pair. Consider making support for providing a replacement address
 for the host name on all port numbers.

 See https://github.com/curl/curl/issues/1264

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 auto-detect proxy

 libcurl could be made to detect the system proxy setup automatically and use
 that. On Windows, macOS and Linux desktops for example.

 The pull-request to use libproxy for this was deferred due to doubts on the
 reliability of the dependency and how to use it:
 https://github.com/curl/curl/pull/977

 libdetectproxy is a (C++) library for detecting the proxy on Windows
 https://github.com/paulharris/libdetectproxy

1.11 minimize dependencies with dynamically 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 updated DNS server while running

 If /etc/resolv.conf gets updated while a program using libcurl is running, it
 is may cause name resolves to fail unless res_init() is called. We should
 consider calling res_init() + retry once unconditionally on all name resolve
 failures to mitigate against this. Firefox works like that. Note that Windows
 doesn't have res_init() or an alternative.

 https://github.com/curl/curl/issues/2251

1.13 c-ares and CURLOPT_OPENSOCKETFUNCTION

 curl will create most sockets via the CURLOPT_OPENSOCKETFUNCTION callback and
 close them with the CURLOPT_CLOSESOCKETFUNCTION callback. However, c-ares
 does not use those functions and instead opens and closes the sockets
 itself. This means that when curl passes the c-ares socket to the
 CURLMOPT_SOCKETFUNCTION it isn't owned by the application like other sockets.

 See https://github.com/curl/curl/issues/2734

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 provide timing info for each redirect

 curl and libcurl provide timing information via a set of different
 time-stamps (CURLINFO_*_TIME). When curl is following redirects, those
 returned time value are the accumulated sums. An improvement could be to
 offer separate timings for each redirect.

 https://github.com/curl/curl/issues/6743

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 netrc caching and sharing

 The netrc file is read and parsed each time a connection is setup, which
 means that if a transfer needs multiple connections for authentication or
 redirects, the file might be reread (and parsed) multiple times. This makes
 it impossible to provide the file as a pipe.

1.22 CURLINFO_PAUSE_STATE

 Return information about the transfer's current pause state, in both
 directions. https://github.com/curl/curl/issues/2588

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.

1.24 TCP Fast Open for windows

 libcurl supports the CURLOPT_TCP_FASTOPEN option since 7.49.0 for Linux and
 Mac OS. Windows supports TCP Fast Open starting with Windows 10, version 1607
 and we should add support for it.

1.25 Expose tried IP addresses that failed

 When libcurl fails to connect to a host, it should be able to offer the
 application the list of IP addresses that were used in the attempt.

 https://github.com/curl/curl/issues/2126

1.27 hardcode the "localhost" addresses

 There's this new spec getting adopted that says "localhost" should always and
 unconditionally be a local address and not get resolved by a DNS server. A
 fine way for curl to fix this would be to simply hard-code the response to
 127.0.0.1 and/or ::1 (depending on what IP versions that are requested). This
 is what the browsers probably will do with this hostname.

 https://bugzilla.mozilla.org/show_bug.cgi?id=1220810

 https://tools.ietf.org/html/draft-ietf-dnsop-let-localhost-be-localhost-02

1.28 FD_CLOEXEC

 It sets the close-on-exec flag for the file descriptor, which causes the file
 descriptor to be automatically (and atomically) closed when any of the
 exec-family functions succeed. Should probably be set by default?

 https://github.com/curl/curl/issues/2252

1.29 Upgrade to websockets

 libcurl could offer a smoother path to get to a websocket connection.
 See https://github.com/curl/curl/issues/3523

 Michael Kaufmann suggestion here:
 https://curl.se/video/curlup-2017/2017-03-19_05_Michael_Kaufmann_Websocket_support_for_curl.mp4

1.30 config file parsing

 Consider providing an API, possibly in a separate companion library, for
 parsing a config file like curl's -K/--config option to allow applications to
 get the same ability to read curl options from files.

 See https://github.com/curl/curl/issues/3698

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 or the threaded resolver is used.

 - The threaded resolver may block on cleanup:
 https://github.com/curl/curl/issues/4852

 - file:// transfers

 - TELNET transfers

 - GSSAPI authentication for FTP transfers

 - The "DONE" operation (post transfer protocol-specific actions) for the
 protocols SFTP, SMTP, FTP. Fixing multi_done() for this is a worthy task.

 - curl_multi_remove_handle for any of the above. See section 2.3.

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().

2.6 multi upkeep

 In libcurl 7.62.0 we introduced curl_easy_upkeep. It unfortunately only works
 on easy handles. We should introduces a version of that for the multi handle,
 and also consider doing "upkeep" automatically on connections in the
 connection pool when the multi handle is in used.

 See https://github.com/curl/curl/issues/3199

2.7 Virtual external sockets

 libcurl performs operations on the given file descriptor that presumes it is
 a socket and an application cannot replace them at the moment. Allowing an
 application to fully replace those would allow a larger degree of freedom and
 flexibility.

 See https://github.com/curl/curl/issues/5835

2.8 dynamically decide to use socketpair

 For users who don't use curl_multi_wait() or don't care for
 curl_multi_wakeup(), we could introduce a way to make libcurl NOT
 create a socketpair in the multi handle.

 See https://github.com/curl/curl/issues/4829

3. Documentation

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.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.

4.5 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: https://www.nsftools.com/tips/RawFTP.htm#STAT

 This is not detailed in any FTP specification.

4.8 Option to ignore private IP addresses in PASV response

 Some servers respond with and some other FTP client implementations can
 ignore private (RFC 1918 style) IP addresses when received in PASV responses.
 To consider for libcurl as well. See https://github.com/curl/curl/issues/1455

5. HTTP

5.1 Better persistency for HTTP 1.0

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

5.2 Set custom client ip when using haproxy protocol

 This would allow testing servers with different client ip addresses (without
 using x-forward-for header).

 https://github.com/curl/curl/issues/5125

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 Allow SAN names in HTTP/2 server push

 curl only allows HTTP/2 push promise if the provided :authority header value
 exactly matches the host name given in the URL. It could be extended to allow
 any name that would match the Subject Alternative Names in the server's TLS
 certificate.

 See https://github.com/curl/curl/pull/3581

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 alt-svc should fallback if alt-svc doesn't work

 The alt-svc: header provides a set of alternative services for curl to use
 instead of the original. If the first attempted one fails, it should try the
 next etc and if all alternatives fail go back to the original.

 See https://github.com/curl/curl/issues/4908

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.


7. SMTP

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.se/mail/lib-2012-05/0178.html


8. POP3

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.

10.2 CURLOPT_SSL_CTX_FUNCTION for LDAPS

 CURLOPT_SSL_CTX_FUNCTION works perfectly for HTTPS and email protocols, but
 it has no effect for LDAPS connections.

 https://github.com/curl/curl/issues/4108

10.3 Paged searches on LDAP server

 https://github.com/curl/curl/issues/4452

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. FILE

12.1 Directory listing for FILE:

 Add support for listing the contents of a directory accessed with FILE. The
 output should probably be the same as/similar to FTP.


13. SSL

13.1 TLS-PSK with OpenSSL

 Transport Layer Security pre-shared key ciphersuites (TLS-PSK) is a set of
 cryptographic protocols that provide secure communication based on pre-shared
 keys (PSKs). These pre-shared keys are symmetric keys shared in advance among
 the communicating parties.

 https://github.com/curl/curl/issues/5081

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.

13.4 Cache/share 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.

 Technically, the "caching" is probably best implemented by getting added to
 the share interface so that easy handles who want to and can reuse the
 context specify that by sharing with the right properties set.

 https://github.com/curl/curl/issues/1110

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!

13.8 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.se/mail/lib-2013-03/0075.html) but it was a too simple
 approach. See Daniel's comments:
 https://curl.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 TLS record padding

 TLS (1.3) offers optional record padding and OpenSSL provides an API for it.
 I could make sense for libcurl to offer this ability to applications to make
 traffic patterns harder to figure out by network traffic observers.

 See https://github.com/curl/curl/issues/5398

13.10 Support Authority Information Access certificate extension (AIA)

 AIA can provide various things like CRLs but more importantly information
 about intermediate CA certificates that can allow validation path to be
 fulfilled when the HTTPS server doesn't itself provide them.

 Since AIA is about downloading certs on demand to complete a TLS handshake,
 it is probably a bit tricky to get done right.

 See https://github.com/curl/curl/issues/2793

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 themselves out of their servers".

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

13.13 Make sure we forbid TLS 1.3 post-handshake authentication

 RFC 8740 explains how using HTTP/2 must forbid the use of TLS 1.3
 post-handshake authentication. We should make sure to live up to that.

 See https://github.com/curl/curl/issues/5396

13.14 Support the clienthello extension

 Certain stupid networks and middle boxes have a problem with SSL handshake
 packets that are within a certain size range because how that sets some bits
 that previously (in older TLS version) were not set. The clienthello
 extension adds padding to avoid that size range.

 https://tools.ietf.org/html/rfc7685
 https://github.com/curl/curl/issues/2299

13.15 Support mbedTLS 3.0

 Version 3.0 is not backwards compatible with pre-3.0 versions, and curl no
 longer builds due to breaking changes in the API.

 See https://github.com/curl/curl/issues/7385

14. GnuTLS

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. Schannel

15.1 Extend support for client certificate authentication

 The existing support for the -E/--cert and --key options could be
 extended by supplying a custom certificate and key in PEM format, see:
 - Getting a Certificate for Schannel
   https://msdn.microsoft.com/en-us/library/windows/desktop/aa375447.aspx

15.2 Extend support for the --ciphers option

 The existing support for the --ciphers option could be extended
 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

15.4 Add option to allow abrupt server closure

 libcurl w/schannel will error without a known termination point from the
 server (such as length of transfer, or SSL "close notify" alert) to prevent
 against a truncation attack. Really old servers may neglect to send any
 termination point. An option could be added to ignore such abrupt closures.

 https://github.com/curl/curl/issues/4427

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 Handle growing SFTP files

 The SFTP code in libcurl checks the file size *before* a transfer starts and
 then proceeds to transfer exactly that amount of data. If the remote file
 grows while the transfer is in progress libcurl won't notice and will not
 adapt. The OpenSSH SFTP command line tool does and libcurl could also just
 attempt to download more to see if there is more to get...

 https://github.com/curl/curl/issues/4344

17.3 Support better than MD5 hostkey hash

 libcurl offers the CURLOPT_SSH_HOST_PUBLIC_KEY_MD5 option for verifying the
 server's key. MD5 is generally being deprecated so we should implement
 support for stronger hashing algorithms. libssh2 itself is what provides this
 underlying functionality and it supports at least SHA-1 as an alternative.
 SHA-1 is also being deprecated these days so we should consider working with
 libssh2 to instead offer support for SHA-256 or similar.

17.4 Support CURLOPT_PREQUOTE

 The two other QUOTE options are supported for SFTP, but this was left out for
 unknown reasons!

17.5 SSH over HTTPS proxy with more backends

 The SSH based protocols SFTP and SCP didn't work over HTTPS proxy at
 all until PR https://github.com/curl/curl/pull/6021 brought the
 functionality with the libssh2 backend. Presumably, this support
 can/could be added for the other backends as well.

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 --proxycommand

 Allow the user to make curl run a command and use its stdio to make requests
 and not do any network connection by itself. Example:

   curl --proxycommand 'ssh pi@raspberrypi.local -W 10.1.1.75 80' \
        http://some/otherwise/unavailable/service.php

 See https://github.com/curl/curl/issues/4941

18.5 UTF-8 filenames in Content-Disposition

 RFC 6266 documents how UTF-8 names can be passed to a client in the
 Content-Disposition header, and curl does not support this.

 https://github.com/curl/curl/issues/1888

18.6 Option to make -Z merge lined based outputs on stdout

 When a user requests multiple lined based files using -Z and sends them to
 stdout, curl will not "merge" and send complete lines fine but may very well
 send partial lines from several sources.

 https://github.com/curl/curl/issues/5175

18.7 at least N milliseconds between requests

 Allow curl command lines issue a lot of request against services that limit
 users to no more than N requests/second or similar. Could be implemented with
 an option asking that at least a certain time has elapsed since the previous
 request before the next one will be performed. Example:

    $ curl "https://example.com/api?input=[1-1000]" -d yadayada --after 500

 See https://github.com/curl/curl/issues/3920

18.8 Consider convenience options for JSON and XML?

 Could we add `--xml` or `--json` to add headers needed to call rest API:

 `--xml` adds -H 'Content-Type: application/xml' -H "Accept: application/xml" and
 `--json` adds -H 'Content-Type: application/json' -H "Accept: application/json"

 Setting Content-Type when doing a GET or any other method without a body
 would be a bit strange I think - so maybe only add CT for requests with body?
 Maybe plain `--xml` and ` --json` are a bit too brief and generic. Maybe
 `--http-json` etc?

 See https://github.com/curl/curl/issues/5203

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 Windows: set attribute 'archive' for completed downloads

 The archive bit (FILE_ATTRIBUTE_ARCHIVE, 0x20) separates files that shall be
 backed up from those that are either not ready or have not changed.

 Downloads in progress are neither ready to be backed up, nor should they be
 opened by a different process. Only after a download has been completed it's
 sensible to include it in any integer snapshot or backup of the system.

 See https://github.com/curl/curl/issues/3354

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 Ratelimit or wait between serial requests

 Consider a command line option that can make curl do multiple serial requests
 slow, potentially with a (random) wait between transfers. There's also a
 proposed set of standard HTTP headers to let servers let the client adapt to
 its rate limits:
 https://www.ietf.org/id/draft-polli-ratelimit-headers-02.html

 See https://github.com/curl/curl/issues/5406

18.14 --dry-run

 A command line option that makes curl show exactly what it would do and send
 if it would run for real.

 See https://github.com/curl/curl/issues/5426

18.15 --retry should resume

 When --retry is used and curl actually retries transfer, it should use the
 already transferred data and do a resumed transfer for the rest (when
 possible) so that it doesn't have to transfer the same data again that was
 already transferred before the retry.

 See https://github.com/curl/curl/issues/1084

18.16 send only part of --data

 When the user only wants to send a small piece of the data provided with
 --data or --data-binary, like when that data is a huge file, consider a way
 to specify that curl should only send a piece of that. One suggested syntax
 would be: "--data-binary @largefile.zip!1073741823-2147483647".

 See https://github.com/curl/curl/issues/1200

18.17 consider file name from the redirected URL with -O ?

 When a user gives a URL and uses -O, and curl follows a redirect to a new
 URL, the file name is not extracted and used from the newly redirected-to URL
 even if the new URL may have a much more sensible file name.

 This is clearly documented and helps for security since there's no surprise
 to users which file name that might get overwritten. But maybe a new option
 could allow for this or maybe -J should imply such a treatment as well as -J
 already allows for the server to decide what file name to use so it already
 provides the "may overwrite any file" risk.

 This is extra tricky if the original URL has no file name part at all since
 then the current code path will error out with an error message, and we can't
 *know* already at that point if curl will be redirected to a URL that has a
 file name...

 See https://github.com/curl/curl/issues/1241

18.18 retry on network is unreachable

 The --retry option retries transfers on "transient failures". We later added
 --retry-connrefused to also retry for "connection refused" errors.

 Suggestions have been brought to also allow retry on "network is unreachable"
 errors and while totally reasonable, maybe we should consider a way to make
 this more configurable than to add a new option for every new error people
 want to retry for?

 https://github.com/curl/curl/issues/1603

18.19 expand ~/ in config files

 For example .curlrc could benefit from being able to do this.

 See https://github.com/curl/curl/issues/2317

18.20 host name sections in config files

 config files would be more powerful if they could set different
 configurations depending on used URLs, host name or possibly origin. Then a
 default .curlrc could a specific user-agent only when doing requests against
 a certain site.

18.21 retry on the redirected-to URL

 When curl is told to --retry a failed transfer and follows redirects, it
 might get a HTTP 429 response from the redirected-to URL and not the original
 one, which then could make curl decide to rather retry the transfer on that
 URL only instead of the original operation to the original URL.

 Perhaps extra emphasized if the original transfer is a large POST that
 redirects to a separate GET, and that GET is what gets the 529

 See https://github.com/curl/curl/issues/5462

18.23 Set the modification date on an uploaded file

 For SFTP and possibly FTP, curl could offer an option to set the
 modification time for the uploaded file.

 See https://github.com/curl/curl/issues/5768

18.24 Use multiple parallel transfers for a single download

 To enhance transfer speed, downloading a single URL can be split up into
 multiple separate range downloads that get combined into a single final
 result.

 An ideal implementation would not use a specified number of parallel
 transfers, but curl could:
 - First start getting the full file as transfer A
 - If after N seconds have passed and the transfer is expected to continue for
   M seconds or more, add a new transfer (B) that asks for the second half of
   A's content (and stop A at the middle).
 - If splitting up the work improves the transfer rate, it could then be done
   again. Then again, etc up to a limit.

 This way, if transfer B fails (because Range: isn't supported) it will let
 transfer A remain the single one. N and M could be set to some sensible
 defaults.

 See https://github.com/curl/curl/issues/5774

18.25 Prevent terminal injection when writing to terminal

 curl could offer an option to make escape sequence either non-functional or
 avoid cursor moves or similar to reduce the risk of a user getting tricked by
 clever tricks.

 See https://github.com/curl/curl/issues/6150

18.26 Custom progress meter update interval

 Users who are for example doing large downloads in CI or remote setups might
 want the occasional progress meter update to see that the transfer is
 progressing and hasn't stuck, but they may not appreciate the
 many-times-a-second frequency curl can end up doing it with now.

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

19.2 Enable PIE and RELRO by default

 Especially when having programs that execute curl via the command line, PIE
 renders the exploitation of memory corruption vulnerabilities a lot more
 difficult. This can be attributed to the additional information leaks being
 required to conduct a successful attack. RELRO, on the other hand, masks
 different binary sections like the GOT as read-only and thus kills a handful
 of techniques that come in handy when attackers are able to arbitrarily
 overwrite memory. A few tests showed that enabling these features had close
 to no impact, neither on the performance nor on the general functionality of
 curl.

19.3 Don't use GNU libtool on OpenBSD
 When compiling curl on OpenBSD with "--enable-debug" it will give linking
 errors when you use GNU libtool. This can be fixed by using the libtool
 provided by OpenBSD itself. However for this the user always needs to invoke
 make with "LIBTOOL=/usr/bin/libtool". It would be nice if the script could
 have some magic to detect if this system is an OpenBSD host and then use the
 OpenBSD libtool instead.

 See https://github.com/curl/curl/issues/5862

19.4 Package curl for Windows in a signed installer

 See https://github.com/curl/curl/issues/5424

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.

20.7 Support LD_PRELOAD on macOS

 LD_RELOAD doesn't work on macOS, but there are tests which require it to run
 properly. Look into making the preload support in runtests.pl portable such
 that it uses DYLD_INSERT_LIBRARIES on macOS.

20.8 Run web-platform-tests url tests

 Run web-platform-tests url tests and compare results with browsers on wpt.fyi

 It would help us find issues to fix and help us document where our parser
 differs from the WHATWG URL spec parsers.

 See https://github.com/curl/curl/issues/4477

20.9 Bring back libssh tests on Travis

 In https://github.com/curl/curl/pull/7012 we remove the libssh builds and
 tests from Travis CI due to them not working. This should be remedied and
 libssh builds be brought back.


21. MQTT

21.1 Support rate-limiting

 The rate-limiting logic is done in the PERFORMING state in multi.c but MQTT
 is not (yet) implemented to use that!