_ _ ____ _ ___| | | | _ \| | / __| | | | |_) | | | (__| |_| | _ <| |___ \___|\___/|_| \_\_____| 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 11.7 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 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 16. Applications 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 Transfer closed with n bytes remaining to read 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://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 1.6 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 an 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 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 2.1 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 an 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 , , and 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 , 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 , 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 the 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://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 11.3 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 11.7 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 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 13.1 --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 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 18.9 connection migration does not work https://github.com/curl/curl/issues/7695