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Known Bugs

These are problems and bugs known to exist at the time of this release. Feel
free to join in and help us correct one or more of these. Also be sure to
check the changelog of the current development status, as one or more of these
problems may have been fixed or changed somewhat since this was written.

1. HTTP
1.2 Multiple methods in a single WWW-Authenticate: header
1.3 STARTTRANSFER time is wrong for HTTP POSTs
1.4 multipart formposts file name encoding
1.5 Expect-100 meets 417
1.6 Unnecessary close when 401 received waiting for 100
1.7 Deflate error after all content was received
1.8 DoH is not used for all name resolves when enabled
1.11 CURLOPT_SEEKFUNCTION not called with CURLFORM_STREAM

2. TLS
2.1 CURLINFO_SSL_VERIFYRESULT has limited support
2.2 DER in keychain
2.3 Unable to use PKCS12 certificate with Secure Transport
2.4 Secure Transport will not import PKCS#12 client certificates without a password
2.5 Client cert handling with Issuer DN differs between backends
2.6 CURL_GLOBAL_SSL
2.7 Client cert (MTLS) issues with Schannel
2.8 Schannel disable CURLOPT_SSL_VERIFYPEER and verify hostname
2.9 TLS session cache does not work with TFO
2.10 Store TLS context per transfer instead of per connection
2.11 Schannel TLS 1.2 handshake bug in old Windows versions
2.12 FTPS with Schannel times out file list operation
2.14 Secure Transport disabling hostname validation also disables SNI
2.15 Renegotiate from server may cause hang for OpenSSL backend

3. Email protocols
3.1 IMAP SEARCH ALL truncated response
3.2 No disconnect command
3.3 POP3 expects "CRLF.CRLF" eob for some single-line responses
3.4 AUTH PLAIN for SMTP is not working on all servers

4. Command line
4.1 -J and -O with %-encoded file names
4.2 -J with -C - fails
4.3 --retry and transfer timeouts

5. Build and portability issues
5.1 OS400 port requires deprecated IBM library
5.2 curl-config --libs contains private details
5.3 curl compiled on OSX 10.13 failed to run on OSX 10.10
5.4 Build with statically built dependency
5.5 cannot handle Unicode arguments in non-Unicode builds on Windows
5.6 make distclean loops forever
5.7 Visual Studio project gaps
5.8 configure finding libs in wrong directory
5.9 Utilize Requires.private directives in libcurl.pc
5.10 curl hangs on SMB upload over stdin
5.11 configure --with-gssapi with Heimdal is ignored on macOS
5.12 flaky Windows CI builds

6. Authentication
6.1 NTLM authentication and unicode
6.2 MIT Kerberos for Windows build
6.3 NTLM in system context uses wrong name
6.4 Negotiate and Kerberos V5 need a fake user name
6.5 NTLM does not support password with § character
6.6 libcurl can fail to try alternatives with --proxy-any
6.7 Do not clear digest for single realm
6.8 RTSP authentication breaks without redirect support
6.9 SHA-256 digest not supported in Windows SSPI builds
6.10 curl never completes Negotiate over HTTP
6.11 Negotiate on Windows fails
6.12 cannot use Secure Transport with Crypto Token Kit

7. FTP
7.1 FTP without or slow 220 response
7.2 FTP with CONNECT and slow server
7.3 FTP with NOBODY and FAILONERROR
7.4 FTP with ACCT
7.5 ASCII FTP
7.6 FTP with NULs in URL parts
7.7 FTP and empty path parts in the URL
7.8 Premature transfer end but healthy control channel
7.9 Passive transfer tries only one IP address
7.10 FTPS needs session reuse
7.11 FTPS upload data loss with TLS 1.3

8. TELNET
8.1 TELNET and time limitations do not work
8.2 Microsoft telnet server

9. SFTP and SCP
9.1 SFTP does not do CURLOPT_POSTQUOTE correct
9.2 wolfssh: publickey auth does not work
9.3 Remote recursive folder creation with SFTP

10. SOCKS
10.3 FTPS over SOCKS
10.4 active FTP over a SOCKS

11. Internals
11.1 Curl leaks .onion hostnames in DNS
11.2 error buffer not set if connection to multiple addresses fails
11.3 Disconnects do not do verbose
11.4 HTTP test server 'connection-monitor' problems
11.5 Connection information when using TCP Fast Open
11.6 slow connect to localhost on Windows
11.7 signal-based resolver timeouts
11.8 DoH leaks memory after followlocation
11.9 DoH does not inherit all transfer options
11.10 Blocking socket operations in non-blocking API
11.11 A shared connection cache is not thread-safe
11.12 'no_proxy' string-matches IPv6 numerical addresses
11.13 wakeup socket disconnect causes havoc
11.14 Multi perform hangs waiting for threaded resolver
11.15 CURLOPT_OPENSOCKETPAIRFUNCTION is missing
11.16 libcurl uses renames instead of locking for atomic operations

12. LDAP
12.1 OpenLDAP hangs after returning results
12.2 LDAP on Windows does authentication wrong?
12.3 LDAP on Windows does not work
12.4 LDAPS with NSS is slow

13. TCP/IP
13.1 --interface for ipv6 binds to unusable IP address
13.2 Trying local ports fails on Windows

14. DICT
14.1 DICT responses show the underlying protocol

15. CMake
15.1 use correct SONAME
15.2 support build with GnuTLS
15.3 unusable tool_hugehelp.c with MinGW
15.4 build docs/curl.1
15.5 build on Linux links libcurl to libdl
15.6 uses -lpthread instead of Threads::Threads
15.7 generated .pc file contains strange entries
15.8 libcurl.pc uses absolute library paths
15.9 cert paths autodetected when cross-compiling
15.10 libspsl is not supported
15.11 ExternalProject_Add does not set CURL_CA_PATH
15.12 cannot enable LDAPS on Windows
15.13 CMake build with MIT Kerberos does not work

16. Applications
16.1 pulseUI VPN client

17. HTTP/2
17.1 Excessive HTTP/2 packets with TCP_NODELAY
17.2 HTTP/2 frames while in the connection pool kill reuse
17.3 ENHANCE_YOUR_CALM causes infinite retries
17.4 Connection failures with parallel HTTP/2
17.5 HTTP/2 connections through HTTPS proxy frequently stall

18. HTTP/3
18.1 If the HTTP/3 server closes connection during upload curl hangs
18.2 Uploading HTTP/3 files gets interrupted at certain file sizes
18.3 HTTP/3 download is 5x times slower than HTTP/2
18.4 Downloading with HTTP/3 produces broken files
18.5 HTTP/3 download with quiche halts after a while
18.6 HTTP/3 multipart POST with quiche fails
18.7 HTTP/3 quiche upload large file fails
18.8 HTTP/3 does not support client certs
18.9 connection migration does not work

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

1. HTTP

1.2 Multiple methods in a single WWW-Authenticate: header

The HTTP responses headers WWW-Authenticate: can provide information about
multiple authentication methods as multiple headers or as several methods
within a single header. The latter way, several methods in the same physical
line, is not supported by libcurl's parser. (For no good reason.)

1.3 STARTTRANSFER time is wrong for HTTP POSTs

Wrong STARTTRANSFER timer accounting for POST requests Timer works fine with
GET requests, but while using POST the time for CURLINFO_STARTTRANSFER_TIME
is wrong. While using POST CURLINFO_STARTTRANSFER_TIME minus
CURLINFO_PRETRANSFER_TIME is near to zero every time.

https://github.com/curl/curl/issues/218
https://curl.se/bug/view.cgi?id=1213

1.4 multipart formposts file name encoding

When creating multipart formposts. The file name part can be encoded with
something beyond ascii but currently libcurl will only pass in the verbatim
string the app provides. There are several browsers that already do this
encoding. The key seems to be the updated draft to RFC2231:
https://tools.ietf.org/html/draft-reschke-rfc2231-in-http-02

1.5 Expect-100 meets 417

If an upload using Expect: 100-continue receives an HTTP 417 response, it
ought to be automatically resent without the Expect:. A workaround is for
the client application to redo the transfer after disabling Expect:.
https://curl.se/mail/archive-2008-02/0043.html

1.6 Unnecessary close when 401 received waiting for 100

libcurl closes the connection if an HTTP 401 reply is received while it is
waiting for the 100-continue response.
https://curl.se/mail/lib-2008-08/0462.html

1.7 Deflate error after all content was received

There's a situation where we can get an error in a HTTP response that is
compressed, when that error is detected after all the actual body contents
have been received and delivered to the application. This is tricky, but is
ultimately a broken server.

See https://github.com/curl/curl/issues/2719

1.8 DoH is not used for all name resolves when enabled

Even if DoH is specified to be used, there are some name resolves that are
done without it. This should be fixed. When the internal function
`Curl_resolver_wait_resolv()` is called, it does not use DoH to complete the
resolve as it otherwise should.

See https://github.com/curl/curl/pull/3857 and
https://github.com/curl/curl/pull/3850

1.11 CURLOPT_SEEKFUNCTION not called with CURLFORM_STREAM

I'm using libcurl to POST form data using a FILE* with the CURLFORM_STREAM
option of curl_formadd(). I have noticed that if the connection drops at just
the right time, the POST is reattempted without the data from the file. It
seems like the file stream position is not getting reset to the beginning of
the file. I found the CURLOPT_SEEKFUNCTION option and set that with a
function that performs an fseek() on the FILE*. However, setting that did not
seem to fix the issue or even get called. See
https://github.com/curl/curl/issues/768

2. TLS

2.1 CURLINFO_SSL_VERIFYRESULT has limited support

CURLINFO_SSL_VERIFYRESULT is only implemented for the OpenSSL, NSS and
GnuTLS backends, so relying on this information in a generic app is flaky.

2.2 DER in keychain

Curl does not recognize certificates in DER format in keychain, but it works
with PEM. https://curl.se/bug/view.cgi?id=1065

2.3 Unable to use PKCS12 certificate with Secure Transport

See https://github.com/curl/curl/issues/5403

2.4 Secure Transport will not import PKCS#12 client certificates without a password

libcurl calls SecPKCS12Import with the PKCS#12 client certificate, but that
function rejects certificates that do not have a password.
https://github.com/curl/curl/issues/1308

2.5 Client cert handling with Issuer DN differs between backends

When the specified client certificate does not match any of the
server-specified DNs, the OpenSSL and GnuTLS backends behave differently.
The github discussion may contain a solution.

See https://github.com/curl/curl/issues/1411

2.6 CURL_GLOBAL_SSL

Since libcurl 7.57.0, the flag CURL_GLOBAL_SSL is a no-op. The change was
merged in https://github.com/curl/curl/commit/d661b0afb571a

It was removed since it was

A) never clear for applications on how to deal with init in the light of
different SSL backends (the option was added back in the days when life
was simpler)

B) multissl introduced dynamic switching between SSL backends which
emphasized (A) even more

C) libcurl uses some TLS backend functionality even for non-TLS functions (to
get "good" random) so applications trying to avoid the init for
performance reasons would do wrong anyway

D) not documented carefully so all this mostly just happened to work
for some users

However, in spite of the problems with the feature, there were some users who
apparently depended on this feature and who now claim libcurl is broken for
them. The fix for this situation is not obvious as a downright revert of the
patch is totally ruled out due to those reasons above.

https://github.com/curl/curl/issues/2276

2.7 Client cert (MTLS) issues with Schannel

See https://github.com/curl/curl/issues/3145

2.8 Schannel disable CURLOPT_SSL_VERIFYPEER and verify hostname

This seems to be a limitation in the underlying Schannel API.

https://github.com/curl/curl/issues/3284

2.9 TLS session cache does not work with TFO

See https://github.com/curl/curl/issues/4301

2.10 Store TLS context per transfer instead of per connection

The GnuTLS `backend->cred` and the OpenSSL `backend->ctx` data and their
proxy versions (and possibly other TLS backends), could be better moved to be
stored in the Curl_easy handle instead of in per connection so that a single
transfer that makes multiple connections can reuse the context and reduce
memory consumption.

https://github.com/curl/curl/issues/5102

2.11 Schannel TLS 1.2 handshake bug in old Windows versions

In old versions of Windows such as 7 and 8.1 the Schannel TLS 1.2 handshake
implementation likely has a bug that can rarely cause the key exchange to
fail, resulting in error SEC_E_BUFFER_TOO_SMALL or SEC_E_MESSAGE_ALTERED.

https://github.com/curl/curl/issues/5488

2.12 FTPS with Schannel times out file list operation

"Instead of the command completing, it just sits there until the timeout
expires." - the same command line seems to work with other TLS backends and
other operating systems. See https://github.com/curl/curl/issues/5284.

2.14 Secure Transport disabling hostname validation also disables SNI

SNI is the hostname that is sent by the TLS library to the server as part of
the TLS handshake. Secure Transport does not send SNI when hostname validation
is disabled. Servers that host multiple websites may not know which
certificate to serve without SNI or which backend server to connect to. The
server may serve the certificate of a default server or abort.

If a server aborts a handshake then curl shows error "SSL peer handshake
failed, the server most likely requires a client certificate to connect".
In this case the error may also have been caused by lack of SNI.

https://github.com/curl/curl/issues/6347

2.15 Renegotiate from server may cause hang for OpenSSL backend

A race condition has been observed when, immediately after the initial
handshake, curl has sent an HTTP request to the server and at the same time
the server has sent a TLS hello request (renegotiate) to curl. Both are
waiting for the other to respond. OpenSSL is supposed to send a handshake
response but does not.

https://github.com/curl/curl/issues/6785
https://github.com/openssl/openssl/issues/14722

3. Email protocols

3.1 IMAP SEARCH ALL truncated response

IMAP "SEARCH ALL" truncates output on large boxes. "A quick search of the
code reveals that pingpong.c contains some truncation code, at line 408, when
it deems the server response to be too large truncating it to 40 characters"
https://curl.se/bug/view.cgi?id=1366

3.2 No disconnect command

The disconnect commands (LOGOUT and QUIT) may not be sent by IMAP, POP3 and
SMTP if a failure occurs during the authentication phase of a connection.

3.3 POP3 expects "CRLF.CRLF" eob for some single-line responses

You have to tell libcurl not to expect a body, when dealing with one line
response commands. Please see the POP3 examples and test cases which show
this for the NOOP and DELE commands. https://curl.se/bug/?i=740

3.4 AUTH PLAIN for SMTP is not working on all servers

Specifying "--login-options AUTH=PLAIN" on the command line does not seem to
work correctly.

See https://github.com/curl/curl/issues/4080

4. Command line

4.1 -J and -O with %-encoded file names

-J/--remote-header-name does not decode %-encoded file names. RFC6266 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 will not add decoding to -O without the user asking for it with
some other means as well, since -O has always been documented to use the name
exactly as specified in the URL.

4.2 -J with -C - fails

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 file name (and thus its
pre-transfer size) has been figured out.
https://curl.se/bug/view.cgi?id=1169

4.3 --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 will truncate and restart 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

5. Build and portability issues

5.1 OS400 port requires deprecated IBM library

curl for OS400 requires QADRT to build, which provides ASCII wrappers for
libc/POSIX functions in the ILE, but IBM no longer supports or even offers
this library to download.

See https://github.com/curl/curl/issues/5176

5.2 curl-config --libs contains private details

"curl-config --libs" will include details set in LDFLAGS when configure is
run that might be needed only for building libcurl. Further, curl-config
--cflags suffers from the same effects with CFLAGS/CPPFLAGS.

5.3 curl compiled on OSX 10.13 failed to run on OSX 10.10

See https://github.com/curl/curl/issues/2905

5.4 Build with statically built dependency

The build scripts in curl (autotools, cmake and others) are primarily done to
work with shared/dynamic third party dependencies. When linking with shared
libraries, the dependency "chain" is handled automatically by the library
loader - on all modern systems.

If you instead link with a static library, we need to provide all the
dependency libraries already at the link command line.

Figuring out all the dependency libraries for a given library is hard, as it
might also involve figuring out the dependencies of the dependencies and they
may vary between platforms and even change between versions.

When using static dependencies, the build scripts will mostly assume that
you, the user, will provide all the necessary additional dependency libraries
as additional arguments in the build. With configure, by setting LIBS/LDFLAGS
on the command line.

We welcome help to improve curl's ability to link with static libraries, but
it is likely a task that we can never fully support.

5.5 cannot handle Unicode arguments in non-Unicode builds on Windows

If a URL or filename cannot be encoded using the user's current codepage then
it can only be encoded properly in the Unicode character set. Windows uses
UTF-16 encoding for Unicode and stores it in wide characters, however curl
and libcurl are not equipped for that at the moment except when built with
_UNICODE and UNICODE defined. And, except for Cygwin, Windows cannot use UTF-8
as a locale.

https://curl.se/bug/?i=345
https://curl.se/bug/?i=731
https://curl.se/bug/?i=3747

5.6 make distclean loops forever

Due to an issue (probably) in automake, "make distclean" can end up in a
never-ending loop.

See https://github.com/curl/curl/issues/7716

5.7 Visual Studio project gaps

The Visual Studio projects lack some features that the autoconf and nmake
builds offer, such as the following:

- support for zlib and nghttp2
- use of static runtime libraries
- add the test suite components

In addition to this the following could be implemented:

- support for other development IDEs
- add PATH environment variables for third-party DLLs

5.8 configure finding libs in wrong directory

When the configure script checks for third-party libraries, it adds those
directories to the LDFLAGS variable and then tries linking to see if it
works. When successful, the found directory is kept in the LDFLAGS variable
when the script continues to execute and do more tests and possibly check for
more libraries.

This can make subsequent checks for libraries wrongly detect another
installation in a directory that was previously added to LDFLAGS by another
library check.

A possibly better way to do these checks would be to keep the pristine LDFLAGS
even after successful checks and instead add those verified paths to a
separate variable that only after all library checks have been performed gets
appended to LDFLAGS.

5.9 Utilize Requires.private directives in libcurl.pc

https://github.com/curl/curl/issues/864

5.10 curl hangs on SMB upload over stdin

See https://github.com/curl/curl/issues/7896

5.11 configure --with-gssapi with Heimdal is ignored on macOS

... unless you also pass --with-gssapi-libs

https://github.com/curl/curl/issues/3841

5.12 flaky Windows CI builds

We run many CI builds for each commit and PR on github, and especially a
number of the Windows builds are flaky. This means that we rarely get all CI
builds go green and complete without errors. This is unfortunate as it makes
us sometimes miss actual build problems and it is surprising to newcomers to
the project who (rightfully) do not expect this.

See https://github.com/curl/curl/issues/6972

6. Authentication

6.1 NTLM authentication and unicode

NTLM authentication involving unicode user name or password only works
properly if built with UNICODE defined together with the Schannel
backend. The original problem was mentioned in:
https://curl.se/mail/lib-2009-10/0024.html
https://curl.se/bug/view.cgi?id=896

The Schannel version verified to work as mentioned in
https://curl.se/mail/lib-2012-07/0073.html

6.2 MIT Kerberos for Windows build

libcurl fails to build with MIT Kerberos for Windows (KfW) due to KfW's
library header files exporting symbols/macros that should be kept private to
the KfW library. See ticket #5601 at https://krbdev.mit.edu/rt/

6.3 NTLM in system context uses wrong name

NTLM authentication using SSPI (on Windows) when (lib)curl is running in
"system context" will make it use wrong(?) user name - at least when compared
to what winhttp does. See https://curl.se/bug/view.cgi?id=535

6.4 Negotiate and Kerberos V5 need a fake user name

In order to get Negotiate (SPNEGO) authentication to work in HTTP or Kerberos
V5 in the email protocols, you need to provide a (fake) user name (this
concerns both curl and the lib) because the code wrongly only considers
authentication if there's a user name provided by setting
conn->bits.user_passwd in url.c https://curl.se/bug/view.cgi?id=440 How?
https://curl.se/mail/lib-2004-08/0182.html A possible solution is to
either modify this variable to be set or introduce a variable such as
new conn->bits.want_authentication which is set when any of the authentication
options are set.

6.5 NTLM does not support password with § character

https://github.com/curl/curl/issues/2120

6.6 libcurl can fail to try alternatives with --proxy-any

When connecting via a proxy using --proxy-any, a failure to establish an
authentication will cause libcurl to abort trying other options if the
failed method has a higher preference than the alternatives. As an example,
--proxy-any against a proxy which advertise Negotiate and NTLM, but which
fails to set up Kerberos authentication will not proceed to try authentication
using NTLM.

https://github.com/curl/curl/issues/876

6.7 Do not clear digest for single realm

https://github.com/curl/curl/issues/3267

6.8 RTSP authentication breaks without redirect support

RTSP authentication broke in 7.66.0. A work-around is to enable RTSP in
CURLOPT_REDIR_PROTOCOLS. Authentication should however not be considered an
actual redirect so a "proper" fix needs to be different and not require users
to allow redirects to RTSP to work.

See https://github.com/curl/curl/pull/4750

6.9 SHA-256 digest not supported in Windows SSPI builds

Windows builds of curl that have SSPI enabled use the native Windows API calls
to create authentication strings. The call to InitializeSecurityContext fails
with SEC_E_QOP_NOT_SUPPORTED which causes curl to fail with CURLE_AUTH_ERROR.

Microsoft does not document supported digest algorithms and that SEC_E error
code is not a documented error for InitializeSecurityContext (digest).

https://github.com/curl/curl/issues/6302

6.10 curl never completes Negotiate over HTTP

Apparently it is not working correctly...?

See https://github.com/curl/curl/issues/5235

6.11 Negotiate on Windows fails

When using --negotiate (or NTLM) with curl on Windows, SSL/TSL handshake
fails despite having a valid kerberos ticket cached. Works without any issue
in Unix/Linux.

https://github.com/curl/curl/issues/5881

6.12 cannot use Secure Transport with Crypto Token Kit

https://github.com/curl/curl/issues/7048

7. FTP

7.1 FTP without or slow 220 response

If a connection is made to a FTP server but the server then just never sends
the 220 response or otherwise is dead slow, libcurl will not acknowledge the
connection timeout during that phase but only the "real" timeout - which may
surprise users as it is probably considered to be the connect phase to most
people. Brought up (and is being misunderstood) in:
https://curl.se/bug/view.cgi?id=856

7.2 FTP with CONNECT and slow server

When doing FTP over a socks proxy or CONNECT through HTTP proxy and the multi
interface is used, libcurl will fail if the (passive) TCP connection for the
data transfer is not more or less instant as the code does not properly wait
for the connect to be confirmed. See test case 564 for a first shot at a test
case.

7.3 FTP with NOBODY and FAILONERROR

It seems sensible to be able to use CURLOPT_NOBODY and CURLOPT_FAILONERROR
with FTP to detect if a file exists or not, but it is not working:
https://curl.se/mail/lib-2008-07/0295.html

7.4 FTP with ACCT

When doing an operation over FTP that requires the ACCT command (but not when
logging in), the operation will fail since libcurl does not detect this and
thus fails to issue the correct command:
https://curl.se/bug/view.cgi?id=635

7.5 ASCII FTP

FTP ASCII transfers do not follow RFC959. They do not convert the data
accordingly (not for sending nor for receiving). RFC 959 section 3.1.1.1
clearly describes how this should be done:

The sender converts the data from an internal character representation to
the standard 8-bit NVT-ASCII representation (see the Telnet
specification). The receiver will convert the data from the standard
form to his own internal form.

Since 7.15.4 at least line endings are converted.

7.6 FTP with NULs in URL parts

FTP URLs passed to curl may contain NUL (0x00) in the RFC 1738 <user>,
<password>, and <fpath> components, encoded as "%00". The problem is that
curl_unescape does not detect this, but instead returns a shortened C string.
From a strict FTP protocol standpoint, NUL is a valid character within RFC
959 <string>, so the way to handle this correctly in curl would be to use a
data structure other than a plain C string, one that can handle embedded NUL
characters. From a practical standpoint, most FTP servers would not
meaningfully support NUL characters within RFC 959 <string>, anyway (e.g.,
Unix pathnames may not contain NUL).

7.7 FTP and empty path parts in the URL

libcurl ignores empty path parts in FTP URLs, whereas RFC1738 states that
such parts should be sent to the server as 'CWD ' (without an argument). The
only exception to this rule, is that we knowingly break this if the empty
part is first in the path, as then we use the double slashes to indicate that
the user wants to reach the root dir (this exception SHALL remain even when
this bug is fixed).

7.8 Premature transfer end but healthy control channel

When 'multi_done' is called before the transfer has been completed the normal
way, it is considered a "premature" transfer end. In this situation, libcurl
closes the connection assuming it does not know the state of the connection so
it cannot be reused for subsequent requests.

With FTP however, this is not necessarily true but there are a bunch of
situations (listed in the ftp_done code) where it *could* keep the connection
alive even in this situation - but the current code does not. Fixing this would
allow libcurl to reuse FTP connections better.

7.9 Passive transfer tries only one IP address

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 should 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

7.10 FTPS needs session reuse

When the control connection is reused for a subsequent transfer, some FTPS
servers complain about "missing session reuse" for the data channel for the
second transfer.

https://github.com/curl/curl/issues/4654

7.11 FTPS upload data loss with TLS 1.3

During FTPS upload curl does not attempt to read TLS handshake messages sent
after the initial handshake. OpenSSL servers running TLS 1.3 may send such a
message. When curl closes the upload connection if unread data has been
received (such as a TLS handshake message) then the TCP protocol sends an
RST to the server, which may cause the server to discard or truncate the
upload if it has not read all sent data yet, and then return an error to curl
on the control channel connection.

Since 7.78.0 this is mostly fixed. curl will do a single read before closing
TLS connections (which causes the TLS library to read handshake messages),
however there is still possibility of an RST if more messages need to be read
or a message arrives after the read but before close (network race condition).

https://github.com/curl/curl/issues/6149

8. TELNET

8.1 TELNET and time limitations do not work

When using telnet, the time limitation options do not work.
https://curl.se/bug/view.cgi?id=846

8.2 Microsoft telnet server

There seems to be a problem when connecting to the Microsoft telnet server.
https://curl.se/bug/view.cgi?id=649

9. SFTP and SCP

9.1 SFTP does not do CURLOPT_POSTQUOTE correct

When libcurl sends CURLOPT_POSTQUOTE commands when connected to a SFTP server
using the multi interface, the commands are not being sent correctly and
instead the connection is "cancelled" (the operation is considered done)
prematurely. There is a half-baked (busy-looping) patch provided in the bug
report but it cannot be accepted as-is. See
https://curl.se/bug/view.cgi?id=748

9.2 wolfssh: publickey auth does not work

When building curl to use the wolfSSH backend for SFTP, the publickey
authentication does not work. This is simply functionality not written for curl
yet, the necessary API for make this work is provided by wolfSSH.

See https://github.com/curl/curl/issues/4820

9.3 Remote recursive folder creation with SFTP

On this servers, the curl fails to create directories on the remote server
even when CURLOPT_FTP_CREATE_MISSING_DIRS option is set.

See https://github.com/curl/curl/issues/5204

10. SOCKS

10.3 FTPS over SOCKS

libcurl does not support FTPS over a SOCKS proxy.

10.4 active FTP over a SOCKS

libcurl does not support active FTP over a SOCKS proxy

11. Internals

11.1 Curl leaks .onion hostnames in DNS

Curl sends DNS requests for hostnames with a .onion TLD. This leaks
information about what the user is attempting to access, and violates this
requirement of RFC7686: https://tools.ietf.org/html/rfc7686

Issue: https://github.com/curl/curl/issues/543

11.2 error buffer not set if connection to multiple addresses fails

If you ask libcurl to resolve a hostname like example.com to IPv6 addresses
only. But you only have IPv4 connectivity. libcurl will correctly fail with
CURLE_COULDNT_CONNECT. But the error buffer set by CURLOPT_ERRORBUFFER
remains empty. Issue: https://github.com/curl/curl/issues/544

11.3 Disconnects do not do verbose

Due to how libcurl keeps connections alive in the "connection pool" after use
to potentially transcend the life-time of the initial easy handle that was
used to drive the transfer over that connection, it uses a *separate* and
internal easy handle when it shuts down the connection. That separate
connection might not have the same settings as the original easy handle, and
in particular it is often note-worthy that it does not have the same VERBOSE
and debug callbacks setup so that an application will not get the protocol
data for the disconnect phase of a transfer the same way it got all the other
data.

This is because the original easy handle might have already been freed at that
point and the application might not at all be prepared that the callback
would get called again long after the handle was freed.

See for example https://github.com/curl/curl/issues/6995

11.4 HTTP test server 'connection-monitor' problems

The 'connection-monitor' feature of the sws HTTP test server does not work
properly if some tests are run in unexpected order. Like 1509 and then 1525.

See https://github.com/curl/curl/issues/868

11.5 Connection information when using TCP Fast Open

CURLINFO_LOCAL_PORT (and possibly a few other) fails when TCP Fast Open is
enabled.

See https://github.com/curl/curl/issues/1332 and
https://github.com/curl/curl/issues/4296

11.6 slow connect to localhost on Windows

When connecting to "localhost" on Windows, curl will resolve the name for
both ipv4 and ipv6 and try to connect to both happy eyeballs-style. Something
in there does however make it take 200 milliseconds to succeed - which is the
HAPPY_EYEBALLS_TIMEOUT define exactly. Lowering that define speeds up the
connection, suggesting a problem in the HE handling.

If we can *know* that we are talking to a local host, we should lower the
happy eyeballs delay timeout for IPv6 (related: hardcode the "localhost"
addresses, mentioned in TODO). Possibly we should reduce that delay for all.

https://github.com/curl/curl/issues/2281

11.7 signal-based resolver timeouts

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

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

11.8 DoH leaks memory after followlocation

https://github.com/curl/curl/issues/4592

11.9 DoH does not inherit all transfer options

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

11.10 Blocking socket operations in non-blocking API

The list of blocking socket operations is in TODO section "More non-blocking".

11.11 A shared connection cache is not thread-safe

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

11.12 'no_proxy' string-matches IPv6 numerical addresses

This has the downside that "::1" for example does not match "::0:1" even
though they are in fact the same address.

See https://github.com/curl/curl/issues/5745

11.13 wakeup socket disconnect causes havoc

waking an iPad breaks the wakeup socket pair, triggering a POLLIN event and
resulting in SOCKERRNO being set to ENOTCONN.

This condition, and other possible error conditions on the wakeup socket, are
not handled, so the condition remains on the FD and curl_multi_poll will
never block again.

See https://github.com/curl/curl/issues/6132 and
https://github.com/curl/curl/pull/6133

11.14 Multi perform hangs waiting for threaded resolver

If a threaded resolver takes a long time to complete, libcurl can be blocked
waiting for it for a longer time than expected - and longer than the set
timeouts.

See https://github.com/curl/curl/issues/2975 and
https://github.com/curl/curl/issues/4852

11.15 CURLOPT_OPENSOCKETPAIRFUNCTION is missing

When libcurl creates sockets with socketpair(), those are not "exposed" in
CURLOPT_OPENSOCKETFUNCTION and therefore might surprise and be unknown to
applications that expects and wants all sockets known beforehand. One way to
address this issue is to introduce a CURLOPT_OPENSOCKETPAIRFUNCTION callback.

https://github.com/curl/curl/issues/5747

11.16 libcurl uses renames instead of locking for atomic operations

For saving cookies, alt-svc and hsts files. This is bad when for example the
file is stored in a directory where the application has no write permission
but it has permission for the file.

https://github.com/curl/curl/issues/6882
https://github.com/curl/curl/pull/6884

12. LDAP

12.1 OpenLDAP hangs after returning results

By configuration defaults, openldap automatically chase referrals on
secondary socket descriptors. The OpenLDAP backend is asynchronous and thus
should monitor all socket descriptors involved. Currently, these secondary
descriptors are not monitored, causing openldap library to never receive
data from them.

As a temporary workaround, disable referrals chasing by configuration.

The fix is not easy: proper automatic referrals chasing requires a
synchronous bind callback and monitoring an arbitrary number of socket
descriptors for a single easy handle (currently limited to 5).

Generic LDAP is synchronous: OK.

See https://github.com/curl/curl/issues/622 and
https://curl.se/mail/lib-2016-01/0101.html

12.2 LDAP on Windows does authentication wrong?

https://github.com/curl/curl/issues/3116

12.3 LDAP on Windows does not work

A simple curl command line getting "ldap://ldap.forumsys.com" returns an
error that says "no memory" !

https://github.com/curl/curl/issues/4261

12.4 LDAPS with NSS is slow

See https://github.com/curl/curl/issues/5874

13. TCP/IP

13.1 --interface for ipv6 binds to unusable IP address

Since IPv6 provides a lot of addresses with different scope, binding to an
IPv6 address needs to take the proper care so that it does not bind to a
locally scoped address as that is bound to fail.

https://github.com/curl/curl/issues/686

13.2 Trying local ports fails on Windows

This makes '--local-port [range]' to not work since curl can't properly
detect if a port is already in use, so it'll try the first port, use that and
then subsequently fail anyway if that was actually in use.

https://github.com/curl/curl/issues/8112

14. DICT

14.1 DICT responses show the underlying protocol

When getting a DICT response, the protocol parts of DICT are not stripped off
from the output.

https://github.com/curl/curl/issues/1809

15. CMake

15.1 use correct SONAME

The autotools build sets the SONAME properly according to VERSIONINFO in
lib/Makefile.am and so should cmake to make comparable build.

See https://github.com/curl/curl/pull/5935

15.2 support build with GnuTLS

15.3 unusable tool_hugehelp.c with MinGW

see https://github.com/curl/curl/issues/3125

15.4 build docs/curl.1

The cmake build does not create the docs/curl.1 file and therefore must rely on
it being there already. This makes the --manual option not work and test
cases like 1139 cannot function.

15.5 build on Linux links libcurl to libdl

... which it should not need to!

See https://github.com/curl/curl/issues/6165

15.6 uses -lpthread instead of Threads::Threads

See https://github.com/curl/curl/issues/6166

15.7 generated .pc file contains strange entries

The Libs.private field of the generated .pc file contains -lgcc -lgcc_s -lc
-lgcc -lgcc_s

See https://github.com/curl/curl/issues/6167

15.8 libcurl.pc uses absolute library paths

The libcurl.pc file generated by cmake contains things like Libs.private:
/usr/lib64/libssl.so /usr/lib64/libcrypto.so /usr/lib64/libz.so. The
autotools equivalent would say Libs.private: -lssl -lcrypto -lz

See https://github.com/curl/curl/issues/6169

15.9 cert paths autodetected when cross-compiling

The autotools build disables the ca_path/ca_bundle detection when
cross-compiling. The cmake build keeps doing the detection.

See https://github.com/curl/curl/issues/6178

15.10 libspsl is not supported

See https://github.com/curl/curl/issues/6214

15.11 ExternalProject_Add does not set CURL_CA_PATH

CURL_CA_BUNDLE and CURL_CA_PATH are not set properly when cmake's
ExternalProject_Add is used to build curl as a dependency.

See https://github.com/curl/curl/issues/6313

15.12 cannot enable LDAPS on Windows

See https://github.com/curl/curl/issues/6284

15.13 CMake build with MIT Kerberos does not work

Minimum CMake version was bumped in curl 7.71.0 (#5358) Since CMake 3.2
try_compile started respecting the CMAKE_EXE_FLAGS. The code dealing with
MIT Kerberos detection sets few variables to potentially weird mix of space,
and ;-separated flags. It had to blow up at some point. All the CMake checks
that involve compilation are doomed from that point, the configured tree
cannot be built.

https://github.com/curl/curl/issues/6904

16. Applications

16.1 pulseUI VPN client

This application crashes at startup with libcurl 7.74.0 (and presumably later
versions too) after we cleaned up OpenSSL initialization. Since this is the
only known application to do this, we suspect it is related to something they
are doing in their setup that is not kosher. We have not been able to get in
contact with them nor got any technical details to help us debug this
further.

See
https://community.pulsesecure.net/t5/Pulse-Desktop-Clients/Linux-Pulse-Client-does-not-work-with-curl-7-74/m-p/44378
and https://github.com/curl/curl/issues/6306

17. HTTP/2

17.1 Excessive HTTP/2 packets with TCP_NODELAY

Because of how curl sets TCP_NODELAY by default, HTTP/2 requests are issued
using more separate TCP packets than it would otherwise need to use. This
means spending more bytes than it has to. Just disabling TCP_NODELAY for
HTTP/2 is also not the correct fix because that then makes the outgoing
packets to get delayed.

See https://github.com/curl/curl/issues/6363

17.2 HTTP/2 frames while in the connection pool kill reuse

If the server sends HTTP/2 frames (like for example an HTTP/2 PING frame) to
curl while the connection is held in curl's connection pool, the socket will
be found readable when considered for reuse and that makes curl think it is
dead and then it will be closed and a new connection gets created instead.

This is *best* fixed by adding monitoring to connections while they are kept
in the pool so that pings can be responded to appropriately.

17.3 ENHANCE_YOUR_CALM causes infinite retries

Infinite retries with 2 parallel requests on one connection receiving GOAWAY
with ENHANCE_YOUR_CALM error code.

See https://github.com/curl/curl/issues/5119

17.4 Connection failures with parallel HTTP/2

See https://github.com/curl/curl/issues/5611

17.5 HTTP/2 connections through HTTPS proxy frequently stall

See https://github.com/curl/curl/issues/6936

18. HTTP/3

18.1 If the HTTP/3 server closes connection during upload curl hangs

See https://github.com/curl/curl/issues/6606

18.2 Uploading HTTP/3 files gets interrupted at certain file sizes

See https://github.com/curl/curl/issues/6510

18.3 HTTP/3 download is 5x times slower than HTTP/2

See https://github.com/curl/curl/issues/6494

18.4 Downloading with HTTP/3 produces broken files

See https://github.com/curl/curl/issues/7351

18.5 HTTP/3 download with quiche halts after a while

See https://github.com/curl/curl/issues/7339

18.6 HTTP/3 multipart POST with quiche fails

https://github.com/curl/curl/issues/7125

18.7 HTTP/3 quiche upload large file fails

https://github.com/curl/curl/issues/7532

18.8 HTTP/3 does not support client certs

aka "mutual authentication".

https://github.com/curl/curl/issues/7625

18.9 connection migration does not work

https://github.com/curl/curl/issues/7695

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