curl/docs/TODO.md

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Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.

SPDX-License-Identifier: curl
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# TODO intro

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](https://curl.se/docs/knownbugs.html) are subject for fixing.

# libcurl

## Consult `%APPDATA%` also for `.netrc`

`%APPDATA%\.netrc` is not considered when running on Windows. Should not it?

See [curl issue 4016](https://github.com/curl/curl/issues/4016)

## `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`SIOCGIFADDR` on newer Solaris versions as they claim the latter is obsolete.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.

## alt-svc sharing

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

See [curl issue 4476](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 [curl issue 4915](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.

## 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.

## 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.

[curl issue 2251](https://github.com/curl/curl/issues/2251)

## 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 [curl issue 2734](https://github.com/curl/curl/issues/2734)

## 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.

## 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).

[curl issue 514](https://github.com/curl/curl/issues/514)

## 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.

## 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 `PAC`.

[curl issue 896](https://github.com/curl/curl/issues/896)

## 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.

[curl issue 6743](https://github.com/curl/curl/issues/6743)

## CURLINFO_PAUSE_STATE

Return information about the transfer's current pause state, in both
directions. See [curl issue 2588](https://github.com/curl/curl/issues/2588)

## 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 + destination IP, source +
destination port and protocol (UDP or TCP) for each failure. Possibly as a
callback. Perhaps also provide reason.

[curl issue 2126](https://github.com/curl/curl/issues/2126)

## 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...

[curl issue 7268](https://github.com/curl/curl/issues/7268)

## 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 (i.e 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 [curl issue 6605](https://github.com/curl/curl/issues/6605)

# libcurl - multi interface

## 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:
  [curl issue 4852](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.
- Calling `curl_ws_send()` from a callback

## Better support for same name resolves

If a name resolve has been initiated for a given name and a second easy handle
wants to resolve that same 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
hostname when the DNS resolver can get flooded.

## 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.

## 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.

## 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()`.

## 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 [curl issue 3199](https://github.com/curl/curl/issues/3199)

## 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 [curl issue 5835](https://github.com/curl/curl/issues/5835)

## 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 [curl issue 4829](https://github.com/curl/curl/issues/4829)

# Documentation

## Improve documentation about fork safety

See [curl issue 6968](https://github.com/curl/curl/issues/6968)

# FTP

## A fixed directory listing format

Since listing the contents of a remove directory with FTP is returning the
list in a format and style the server likes without any established or even
defacto standard existing, it would be a feature to users if curl could parse
the directory listing and output a general curl format that is fixed and the
same, independent of the server's choice. This would allow users to better and
more reliably extract information about remote content via FTP directory
listings.

## GSSAPI via Windows SSPI

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

## 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 used data connection.

This is not detailed in any FTP specification.

## 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 [curl issue 1508](https://github.com/curl/curl/issues/1508)

# HTTP

## 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 [curl issue 9513](https://github.com/curl/curl/issues/9513)

## Obey `Retry-After` in redirects

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

See [curl issue 11447](https://github.com/curl/curl/issues/11447)

## 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.

## 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 [curl pull request 3581](https://github.com/curl/curl/pull/3581)

## `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.

## 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 [curl issue 4908](https://github.com/curl/curl/issues/4908)

## 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 [curl issue 7980](https://github.com/curl/curl/issues/7980)

# TELNET

## 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.

## 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.

## feature negotiation debug data

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

## 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

# SMTP

## Pass 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");`.

[curl issue 8232](https://github.com/curl/curl/issues/8232)

## Enhanced capability support

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

## 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

# POP3

## Enhanced capability support

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

# IMAP

## Enhanced capability support

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

# LDAP

## 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.

## `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.

[curl issue 4108](https://github.com/curl/curl/issues/4108)

## Paged searches on LDAP server

[curl issue 4452](https://github.com/curl/curl/issues/4452)

## Certificate-Based Authentication

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

[curl issue 9641](https://github.com/curl/curl/issues/9641)

# SMB

## Support modern versions

curl only supports version 1, which barely anyone is using anymore.

## File listing support

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

## Honor file timestamps

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

## Use NTLMv2

Currently the SMB authentication uses NTLMv1.

## 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`.

# FILE

## 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.

# TLS

## `TLS-PSK` with OpenSSL

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

[curl issue 5081](https://github.com/curl/curl/issues/5081)

## 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
- [curl issue 9226](https://github.com/curl/curl/issues/9226)

## 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.

## 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.

[curl issue 15483](https://github.com/curl/curl/issues/15483)

## Provide callback for cert verification

OpenSSL supports a callback for customized 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.

## Less memory massaging with Schannel

The Schannel backend does a lot of custom memory management we would rather
avoid: the repeated allocation + 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.

## Support DANE

[DNS-Based Authentication of Named Entities
(DANE)](https://datatracker.ietf.org/doc/html/rfc6698) is a way to provide
SSL keys and certs over DNS using DNSSEC as an alternative to the CA model.

A patch was posted on March 7 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

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

## 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 [curl issue 5398](https://github.com/curl/curl/issues/5398)

## Support Authority Information Access certificate extension (AIA)

AIA can provide various things like certificate revocation lists 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 and a serious privacy leak.

See [curl issue 2793](https://github.com/curl/curl/issues/2793)

## 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

[curl issue 12286](https://github.com/curl/curl/issues/12286)

## 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 [curl issue 5396](https://github.com/curl/curl/issues/5396)

## 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
- [curl issue 2299](https://github.com/curl/curl/issues/2299)

## 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.

## 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.

# Proxy

## 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 [curl issue 17222](https://github.com/curl/curl/issues/17222)

# Schannel

## 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://learn.microsoft.com/windows/win32/secauthn/getting-a-certificate-for-schannel)

## 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://learn.microsoft.com/windows/win32/secauthn/specifying-schannel-ciphers-and-cipher-strengths).

## 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.

[curl issue 4427](https://github.com/curl/curl/issues/4427)

# SASL

## Other authentication mechanisms

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

## 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).

# SSH protocols

## 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.

## 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...

[curl issue 4344](https://github.com/curl/curl/issues/4344)

## 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.

[curl issue 8586](https://github.com/curl/curl/issues/8586)

## Support `CURLOPT_PREQUOTE`

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

## SSH over HTTPS proxy for libssh

The SSH based protocols SFTP and SCP did not work over HTTPS proxy at all
until [curl pull request 6021](https://github.com/curl/curl/pull/6021) brought
the functionality with the libssh2 backend. Presumably, this support can/could
be added for the libssh backend as well.

## 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.

# Command line tool

## multi-threading

When asked to do transfers in parallel, the curl tool could be extended to use
a number of independent worker threads. This would allow faster transfers in
situations where curl becomes CPU bound.

Ideally, curl would (with permission) fire up new threads on demand when it
deems that it might be helpful. Perhaps, if it has more transfers to add and
the existing transfers make the CPU busy enough and there are more cores
available.

## 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.

## glob posts

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

## `--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 [curl issue 4941](https://github.com/curl/curl/issues/4941)

## 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.

[curl issue 1888](https://github.com/curl/curl/issues/1888)

## 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.

[curl issue 5175](https://github.com/curl/curl/issues/5175)

## 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

## 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 [curl issue 221](https://github.com/curl/curl/issues/221)

## improve how curl works in a Windows console window

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

## 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 [curl issue 3354](https://github.com/curl/curl/issues/3354)

## 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.

## 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 [curl issue 5406](https://github.com/curl/curl/issues/5406)

## `--dry-run`

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

See [curl issue 5426](https://github.com/curl/curl/issues/5426)

## `--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 [curl issue 1084](https://github.com/curl/curl/issues/1084)

## 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?

[curl issue 1603](https://github.com/curl/curl/issues/1603)

## 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.

## 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 [curl issue 5462](https://github.com/curl/curl/issues/5462)

## 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 [curl issue 5768](https://github.com/curl/curl/issues/5768)

## 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 [curl issue 5774](https://github.com/curl/curl/issues/5774)

## 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 [curl issue 6150](https://github.com/curl/curl/issues/6150)

## `-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. See
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.

## `-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

## `--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

# Build

## 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.

## 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 [curl issue 5862](https://github.com/curl/curl/issues/5862)

## Package curl for Windows in a signed installer

See [curl issue 5424](https://github.com/curl/curl/issues/5424)

## 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 [curl issue 7753](https://github.com/curl/curl/issues/7753)

# Test suite

## 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

## 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).

## more platforms supported

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

## 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 [curl issue 15697](https://github.com/curl/curl/issues/15697)

## Use the RFC 6265 test suite

A test suite made for HTTP cookies (RFC 6265) by Adam Barth [is
available](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 deviance. Ideally, that would even be
incorporated into our regular test suite.

## 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 [curl issue 4477](https://github.com/curl/curl/issues/4477)

# MQTT

## 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.

## Support MQTTS

## 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.

## large payloads

libcurl unnecessarily allocates heap memory to hold the entire payload to get
sent, when the data is already perfectly accessible where it is when
`CURLOPT_POSTFIELDS` is used. This is highly inefficient for larger payloads.
Additionally, libcurl does not support using the read callback for sending
MQTT which is yet another way to avoid having to hold large payload in memory.

# TFTP

## 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 [curl issue 12655](https://github.com/curl/curl/issues/12655)

# Gopher

## 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.

# Signals

## SIGPIPE

Since we control the IO functions for most protocols and disable
SIGPIPE on sends, libcurl could skip the special SIGPIPE ignore
handling for those transfers.