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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 thread-safe sharing
 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 connect to multiple IPs in parallel
 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 does not 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
 1.25 Expose tried IP addresses that failed
 1.28 FD_CLOEXEC
 1.29 WebSocket read callback
 1.30 config file parsing
 1.31 erase secrets from heap/stack after use
 1.32 add asynch getaddrinfo support
 1.33 make DoH inherit more transfer properties

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
 3.1 Improve documentation about fork safety

4. FTP
 4.1 HOST
 4.4 Support CURLOPT_PREQUOTE for directories listings
 4.6 GSSAPI via Windows SSPI
 4.7 STAT for LIST without data connection
 4.8 Passive transfer could try other IP addresses

5. HTTP
 5.1 Provide the error body from a CONNECT response
 5.2 Obey Retry-After in redirects
 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 does not work
 5.7 Require HTTP version X or higher

6. TELNET
 6.1 ditch stdin
 6.2 ditch telnet-specific select
 6.3 feature negotiation debug data
 6.4 exit immediately upon connection if stdin is /dev/null

7. SMTP
 7.1 Passing NOTIFY option to CURLOPT_MAIL_RCPT
 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
 10.4 Certificate-Based Authentication

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 on non-POSIX

13. TLS
 13.1 TLS-PSK with OpenSSL
 13.2 TLS channel binding
 13.3 Defeat TLS fingerprinting
 13.4 Consider OCSP stapling by default
 13.5 Export session ids
 13.6 Provide callback for cert verification
 13.7 Less memory massaging with Schannel
 13.8 Support DANE
 13.9 TLS record padding
 13.10 Support Authority Information Access certificate extension (AIA)
 13.11 Some TLS options are not offered for HTTPS proxies
 13.13 Make sure we forbid TLS 1.3 post-handshake authentication
 13.14 Support the clienthello extension
 13.16 Share the CA cache
 13.17 Add missing features to TLS backends

14. Proxy
 14.1 Retry SOCKS handshake on address type not supported

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

17. SSH protocols
 17.1 Multiplexing
 17.2 Handle growing SFTP files
 17.3 Read keys from ~/.ssh/id_ecdsa, id_ed25519
 17.4 Support CURLOPT_PREQUOTE
 17.5 SSH over HTTPS proxy with more backends
 17.6 SFTP with SCP://

18. Command line tool
 18.1 sync
 18.2 glob posts
 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 specify which response codes that make -f/--fail return error
 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 Acknowledge Ratelimit headers
 18.14 --dry-run
 18.15 --retry should resume
 18.17 consider filename from the redirected URL with -O ?
 18.18 retry on network is unreachable
 18.19 expand ~/ in config files
 18.20 hostname 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
 18.27 -J and -O with %-encoded filenames
 18.28 -J with -C -
 18.29 --retry and transfer timeouts

19. Build
 19.2 Enable PIE and RELRO by default
 19.3 Do not use GNU libtool on OpenBSD
 19.4 Package curl for Windows in a signed installer
 19.5 make configure use --cache-file more and better

20. Test suite
 20.1 SSL tunnel
 20.2 more protocols supported
 20.3 more platforms supported
 20.4 write an SMB test server to replace impacket
 20.5 Use the RFC 6265 test suite
 20.6 Run web-platform-tests URL tests

21. MQTT
 21.1 Support rate-limiting
 21.2 Support MQTTS
 21.3 Handle network blocks

22. TFTP
 22.1 TFTP does not convert LF to CRLF for mode=netascii

23. Gopher
 23.1 Handle network blocks

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

1. libcurl

1.1 TFO support on Windows

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

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. should not 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

The share interface offers CURL_LOCK_DATA_CONNECT to have multiple easy
 handle share a connection cache, but due to how connections are used they are
 still not thread-safe when used shared.

See https://github.com/curl/curl/issues/4915 and lib1541.c

The share interface offers CURL_LOCK_DATA_HSTS to have multiple easy handle
 share an HSTS cache, but this is not thread-safe.

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) does not.

1.6 thread-safe sharing

Using the share interface users can share some data between easy handles but
 several of the sharing options are documented as not safe and supported to
 share between multiple concurrent threads. Fixing this would enable more
 users to share data in more powerful ways.

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 hostname 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
 does not have res_init() or an alternative.

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

1.13 c-ares and CURLOPT_OPENSOCKETFUNCTION

curl creates 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 is not owned by the application like other
 sockets.

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

1.14 connect to multiple IPs in parallel

curl currently implements the happy eyeball algorithm for connecting to the
 IPv4 and IPv6 alternatives for a host in parallel, sticking with the
 connection that "wins". We could implement a similar algorithm per individual
 IP family as well when there are multiple available addresses: start with the
 first address, then start a second attempt N milliseconds after and then a
 third another N milliseconds later. That way there would be less waiting when
 the first IP has problems. It also improves the connection timeout value
 handling for multiple address situations.

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 an HTTP/2 ping from the peer to verify that they are 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 does not 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.

1.25 Expose tried IP addresses that failed

When libcurl fails to connect to a host, it could offer the application the
 addresses that were used in the attempt. Source + dest IP, source + dest port
 and protocol (UDP or TCP) for each failure. Possibly as a callback. Perhaps
 also provide "reason".

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

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 WebSocket read callback

Call the read callback once the connection is established to allow sending
 the first message in the connection.

https://github.com/curl/curl/issues/11402

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

1.31 erase secrets from heap/stack after use

Introducing a concept and system to erase secrets from memory after use, it
 could help mitigate and lessen the impact of (future) security problems etc.
 However: most secrets are passed to libcurl as clear text from the
 application and then clearing them within the library adds nothing...

https://github.com/curl/curl/issues/7268

1.32 add asynch getaddrinfo support

Use getaddrinfo_a() to provide an asynch name resolver backend to libcurl
 that does not use threads and does not depend on c-ares. The getaddrinfo_a
 function is (probably?) glibc specific but that is a widely used libc among
 our users.

https://github.com/curl/curl/pull/6746

1.33 make DoH inherit more transfer properties

Some options are not inherited because they are not relevant for the DoH SSL
 connections, or inheriting the option may result in unexpected behavior. For
 example the user's debug function callback is not inherited because it would
 be unexpected for internal handles (ie DoH handles) to be passed to that
 callback.

If an option is not inherited then it is not possible to set it separately
 for DoH without a DoH-specific option. For example:
 CURLOPT_DOH_SSL_VERIFYHOST, CURLOPT_DOH_SSL_VERIFYPEER and
 CURLOPT_DOH_SSL_VERIFYSTATUS.

See https://github.com/curl/curl/issues/6605

2. libcurl - multi interface

2.1 More non-blocking

Make sure we do not 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.

- Calling curl_ws_send() from a callback

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 does not 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 do not use curl_multi_wait() or do not 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

3.1 Improve documentation about fork safety

See https://github.com/curl/curl/issues/6968

4. FTP

4.1 HOST

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

https://datatracker.ietf.org/doc/html/rfc7151

4.4 Support CURLOPT_PREQUOTE for directions listings

The lack of support is mostly an oversight and requires the FTP state machine
 to get updated to get fixed.

https://github.com/curl/curl/issues/8602

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 Passive transfer could try other IP addresses

When doing FTP operations through a proxy at localhost, the reported spotted
 that curl only tried to connect once to the proxy, while it had multiple
 addresses and a failed connect on one address should make it try the next.

After switching to passive mode (EPSV), curl could try all IP addresses for
 "localhost". Currently it tries ::1, but it should also try 127.0.0.1.

See https://github.com/curl/curl/issues/1508

5. HTTP

5.1 Provide the error body from a CONNECT response

When curl receives a body response from a CONNECT request to a proxy, it
 always just reads and ignores it. It would make some users happy if curl
 instead optionally would be able to make that responsible available. Via a
 new callback? Through some other means?

See https://github.com/curl/curl/issues/9513

5.2 Obey Retry-After in redirects

The Retry-After is said to dicate "the minimum time that the user agent is
 asked to wait before issuing the redirected request" and libcurl does not
 obey this.

See https://github.com/curl/curl/issues/11447

5.3 Rearrange request header order

Server implementers 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 hostname 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 does not 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

5.7 Require HTTP version X or higher

curl and libcurl provide options for trying higher HTTP versions (for example
 HTTP/2) but then still allows the server to pick version 1.1. We could
 consider adding a way to require a minimum version.

See https://github.com/curl/curl/issues/7980

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 does not
 work for telnet.

6.3 feature negotiation debug data

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

6.4 exit immediately upon connection if stdin is /dev/null

If it did, curl could be used to probe if there is an server there listening
 on a specific port. That is, the following command would exit immediately
 after the connection is established with exit code 0:

curl -s --connect-timeout 2 telnet://example.com:80 </dev/null

7. SMTP

7.1 Passing NOTIFY option to CURLOPT_MAIL_RCPT

Is there a way to pass the NOTIFY option to the CURLOPT_MAIL_RCPT option ?  I
 set a string that already contains a bracket. For instance something like
 that: curl_slist_append( recipients, "<foo@bar> NOTIFY=SUCCESS,FAILURE" );

https://github.com/curl/curl/issues/8232

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

10.4 Certificate-Based Authentication

LDAPS not possible with macOS and Windows with Certificate-Based Authentication

https://github.com/curl/curl/issues/9641

11. SMB

11.1 File listing support

Add support for listing the contents of an 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 does not exist on the server, just like --ftp-create-dirs.

12. FILE

12.1 Directory listing on non-POSIX

Listing the contents of a directory accessed with FILE only works on
 platforms with opendir. Support could be added for more systems, like
 Windows.

13. TLS

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 TLS channel binding

TLS 1.2 and 1.3 provide the ability to extract some secret data from the TLS
 connection and use it in the client request (usually in some sort of
 authentication) to ensure that the data sent is bound to the specific TLS
 connection and cannot be successfully intercepted by a proxy. This
 functionality can be used in a standard authentication mechanism such as
 GSS-API or SCRAM, or in custom approaches like custom HTTP Authentication
 headers.

For TLS 1.2, the binding type is usually tls-unique, and for TLS 1.3 it is
 tls-exporter.

https://datatracker.ietf.org/doc/html/rfc5929
 https://datatracker.ietf.org/doc/html/rfc9266
 https://github.com/curl/curl/issues/9226

13.3 Defeat TLS fingerprinting

By changing the order of TLS extensions provided in the TLS handshake, it is
 sometimes possible to circumvent TLS fingerprinting by servers. The TLS
 extension order is of course not the only way to fingerprint a client.

13.4 Consider OCSP stapling by default

Treat a negative response a reason for aborting the connection. Since OCSP
 stapling is presumed to get used much less in the future when Let's Encrypt
 drops the OCSP support, the benefit of this might however be limited.

https://github.com/curl/curl/issues/15483

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 does not seem to be exposed in the libcurl APIs. Could
 it be? There is so much that could be done if it were.

13.7 Less memory massaging with Schannel

The Schannel backend does a lot of custom memory management we would rather
 avoid: the repeated alloc + free in sends and the custom memory + realloc
 system for encrypted and decrypted data. That should be avoided and reduced
 for 1) efficiency and 2) safety.

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 does not 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 Some TLS options are not offered for HTTPS proxies

Some TLS related options to the command line tool and libcurl are only
 provided for the server and not for HTTPS proxies. --proxy-tls-max,
 --proxy-tlsv1.3, --proxy-curves and a few more.
 For more Documentation on this see:
 https://curl.se/libcurl/c/tls-options.html

https://github.com/curl/curl/issues/12286

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

13.16 Share the CA cache

For TLS backends that supports CA caching, it makes sense to allow the share
 object to be used to store the CA cache as well via the share API. Would
 allow multiple easy handles to reuse the CA cache and save themselves from a
 lot of extra processing overhead.

13.17 Add missing features to TLS backends

The feature matrix at https://curl.se/libcurl/c/tls-options.html shows which
 features are supported by which TLS backends, and thus also where there are
 feature gaps.

14. Proxy

14.1 Retry SOCKS handshake on address type not supported

When curl resolves a hostname, it might get a mix of IPv6 and IPv4 returned.
 curl might then use an IPv6 address with a SOCKS5 proxy, which - if it does
 not support IPv6 - returns "Address type not supported" and curl exits with
 that error.

Perhaps it is preferred if curl would in this situation instead first retry
 the SOCKS handshake again for this case and then use one of the IPv4
 addresses for the target host.

See https://github.com/curl/curl/issues/17222

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 with Schannel errors 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).

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 does 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 does not notice and does 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 Read keys from ~/.ssh/id_ecdsa, id_ed25519

The libssh2 backend in curl is limited to only reading keys from id_rsa and
 id_dsa, which makes it fail connecting to servers that use more modern key
 types.

https://github.com/curl/curl/issues/8586

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 did not 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.

17.6 SFTP with SCP://

OpenSSH 9 switched their 'scp' tool to speak SFTP under the hood. Going
 forward it might be worth having curl or libcurl attempt SFTP if SCP fails to
 follow suite.

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.

18.4 --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 does not "merge" and send complete lines fine but may send
 partial lines from several sources.

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

18.7 specify which response codes that make -f/--fail return error

Allows a user to better specify exactly which error code(s) that are fine
 and which are errors for their specific uses cases

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 is
 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 Acknowledge Ratelimit headers

Consider a command line option that can make curl do multiple serial requests
 while acknowledging server specified rate limits:
 https://datatracker.ietf.org/doc/draft-ietf-httpapi-ratelimit-headers/

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 does not have to transfer the same data again that was
 already transferred before the retry.

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

18.17 consider filename 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 filename is not extracted and used from the newly redirected-to URL
 even if the new URL may have a much more sensible filename.

This is clearly documented and helps for security since there is no surprise
 to users which filename 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 filename to use so it already
 provides the "may overwrite any file" risk.

This is extra tricky if the original URL has no filename part at all since
 then the current code path does error out with an error message, and we
 cannot *know* already at that point if curl is redirected to a URL that has a
 filename...

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 hostname sections in config files

config files would be more powerful if they could set different
 configurations depending on used URLs, hostname 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 an 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: is not supported) it lets
 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 has not stuck, but they may not appreciate the
 many-times-a-second frequency curl can end up doing it with now.

18.27 -J and -O with %-encoded filenames

-J/--remote-header-name does not decode %-encoded filenames. RFC 6266 details
 how it should be done. The can of worm is basically that we have no charset
 handling in curl and ASCII >=128 is a challenge for us. Not to mention that
 decoding also means that we need to check for nastiness that is attempted,
 like "../" sequences and the like. Probably everything to the left of any
 embedded slashes should be cut off.
 https://curl.se/bug/view.cgi?id=1294

-O also does not decode %-encoded names, and while it has even less
 information about the charset involved the process is similar to the -J case.

Note that we do not decode -O without the user asking for it with some other
 means, since -O has always been documented to use the name exactly as
 specified in the URL.

18.28 -J with -C -

When using -J (with -O), automatically resumed downloading together with "-C
 -" fails. Without -J the same command line works. This happens because the
 resume logic is worked out before the target filename (and thus its
 pre-transfer size) has been figured out. This can be improved.

https://curl.se/bug/view.cgi?id=1169

18.29 --retry and transfer timeouts

If using --retry and the transfer timeouts (possibly due to using -m or
 -y/-Y) the next attempt does not resume the transfer properly from what was
 downloaded in the previous attempt but truncates and restarts at the original
 position where it was at before the previous failed attempt. See
 https://curl.se/mail/lib-2008-01/0080.html and Mandriva bug report
 https://qa.mandriva.com/show_bug.cgi?id=22565

19. Build

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 Do not use GNU libtool on OpenBSD

When compiling curl on OpenBSD with "--enable-debug" it gives 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

19.5 make configure use --cache-file more and better

The configure script can be improved to cache more values so that repeated
 invokes run much faster.

See https://github.com/curl/curl/issues/7753

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 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).

20.3 more platforms supported

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

20.4 write an SMB test server to replace impacket

This would allow us to run SMB tests on more platforms and do better and more
 covering tests.

See https://github.com/curl/curl/issues/15697

20.5 Use the RFC 6265 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 would be good 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.

20.6 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

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.

21.2 Support MQTTS

21.3 Handle network blocks

Running test suite with
  `CURL_DBG_SOCK_WBLOCK=90 ./runtests.pl -a mqtt` makes several
  MQTT test cases fail where they should not.

22. TFTP

22.1 TFTP does not convert LF to CRLF for mode=netascii

RFC 3617 defines that an TFTP transfer can be done using "netascii"
 mode. curl does not support extracting that mode from the URL nor does it treat
 such transfers specifically. It should probably do LF to CRLF translations
 for them.

See https://github.com/curl/curl/issues/12655

23. Gopher

23.1 Handle network blocks

Running test suite with
  `CURL_DBG_SOCK_WBLOCK=90 ./runtests.pl -a 1200 to 1300` makes several
  Gopher test cases fail where they should not.

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