curl / Docs / Project / Known Bugs

Known Bugs

This is the document 'docs/KNOWN_BUGS' from the curl release archive.

HTTP

1.3 STARTTRANSFER time is wrong for HTTP POSTs
1.4 multipart formposts file name encoding
1.5 Expect-100 meets 417
1.8 DoH is not used for all name resolves when enabled
1.11 CURLOPT_SEEKFUNCTION not called with CURLFORM_STREAM

TLS

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
2.13 CURLOPT_CERTINFO results in CURLE_OUT_OF_MEMORY with Schannel
2.14 Secure Transport disabling hostname validation also disables SNI
2.15 Renegotiate from server may cause hang for OpenSSL backend

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

Command line

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

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
5.5 cannot handle Unicode arguments in non-Unicode builds on Windows
5.6 make distclean loops forever
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
5.13 long paths are not fully supported on Windows
5.14 Windows Unicode builds use homedir in current locale

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
6.9 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
6.13 Negotiate against Hadoop HDFS

FTP

7.3 FTP with NOBODY and FAILONERROR
7.4 FTP with ACCT
7.5 ASCII FTP
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
7.12 FTPS directory listing hangs on Windows with Schannel

TELNET

8.2 Microsoft telnet server

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
9.4 libssh blocking and infinite loop problem

SOCKS

10.3 FTPS over SOCKS
10.4 active FTP over a SOCKS

Internals

11.1 Curl leaks .onion hostnames in DNS
11.2 error buffer not set if connection to multiple addresses fails
11.4 HTTP test server 'connection-monitor' problems
11.5 Connection information when using TCP Fast Open
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
11.15 CURLOPT_OPENSOCKETPAIRFUNCTION is missing
11.16 libcurl uses renames instead of locking for atomic operations

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

TCP/IP

13.2 Trying local ports fails on Windows

DICT

14.1 DICT responses show the underlying protocol

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 libpsl 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

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

HTTP/3

18.1 If the HTTP/3 server closes connection during upload curl hangs
18.2 Transfer closed with n bytes remaining to read
18.4 timeout when reusing an http3 connection
18.9 connection migration does not work


1. HTTP

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://datatracker.ietf.org/doc/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

1.8 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

When using libcurl to POST form data using a FILE* with the CURLFORM_STREAM option of curl_formadd(). I notice 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

2.3 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.

2.13 CURLOPT_CERTINFO results in CURLE_OUT_OF_MEMORY with Schannel

https://github.com/curl/curl/issues/8741

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

5.5 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

5.8 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

5.13 long paths are not fully supported on Windows

curl on Windows cannot access long paths (paths longer than 260 characters). However, as a workaround, the Windows path prefix \\?\ which disables all path interpretation may work to allow curl to access the path. For example: \\?\c:\longpath.

See https://github.com/curl/curl/issues/8361

5.14 Windows Unicode builds use homedir in current locale

The Windows Unicode builds of curl use the current locale, but expect Unicode UTF-8 encoded paths for internal use such as open, access and stat. The user's home directory is retrieved via curl_getenv in the current locale and not as UTF-8 encoded Unicode.

See https://github.com/curl/curl/pull/7252 and

 https://github.com/curl/curl/pull/7281

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

6.9 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/TLS 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

6.13 Negotiate authentication against Hadoop HDFS

https://github.com/curl/curl/issues/8264

7. FTP

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.

7.9 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

7.12 FTPS directory listing hangs on Windows with Schannel

https://github.com/curl/curl/issues/9161

8. TELNET

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 the CURLOPT_FTP_CREATE_MISSING_DIRS option is set.

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

9.4 libssh blocking and infinite loop problem

In the SSH_SFTP_INIT state for libssh, the ssh session working mode is set to blocking mode. If the network is suddenly disconnected during sftp transmission, curl will be stuck, even if curl is configured with a timeout.

https://github.com/curl/curl/issues/8632

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://datatracker.ietf.org/doc/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

11.4 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

11.7 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

11.15 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 expect and want 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

13.2 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 libpsl 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

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 Transfer closed with n bytes remaining to read

HTTP/3 transfers with the Jetty HTTP/3 server seem to not work.

https://github.com/curl/curl/issues/8523

18.4 timeout when reusing an http3 connection

HTTP/3 with quiche seems to not work and always timeout a subsequent transfer that reuses an already established connection

https://github.com/curl/curl/issues/8764

18.9 connection migration does not work

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