xref: /external/curl/docs/KNOWN_BUGS

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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
 1.1 CURLFORM_CONTENTLEN in an array
 1.2 Disabling HTTP Pipelining
 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
 1.9 HTTP/2 frames while in the connection pool kill reuse
 1.10 Strips trailing dot from host name
 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 GnuTLS backend skips really long certificate fields
 2.4 DarwinSSL won't 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

 3. Email protocols
 3.1 IMAP SEARCH ALL truncated response
 3.2 No disconnect command
 3.3 SMTP to multiple recipients
 3.4 POP3 expects "CRLF.CRLF" eob for some single-line responses

 4. Command line
 4.1 -J and -O with %-encoded file names
 4.2 -J with -C - fails
 4.3 --retry and transfer timeouts
 4.4 --upload-file . hang if delay in STDIN
 4.5 Improve --data-urlencode space encoding

 5. Build and portability issues
 5.1 tests not compatible with python3
 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 Cannot compile against a static build of OpenLDAP
 5.5 can't handle Unicode arguments in Windows
 5.6 cmake support gaps
 5.7 Visual Studio project gaps
 5.8 configure finding libs in wrong directory
 5.9 Utilize Requires.private directives in libcurl.pc

 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 doesn't support password with  character
 6.6 libcurl can fail to try alternatives with --proxy-any

 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 Stick to same family over SOCKS proxy

 8. TELNET
 8.1 TELNET and time limitations don't work
 8.2 Microsoft telnet server

 9. SFTP and SCP
 9.1 SFTP doesn't do CURLOPT_POSTQUOTE correct

 10. SOCKS
 10.1 SOCKS proxy connections are done blocking
 10.2 SOCKS don't support timeouts
 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 c-ares deviates from stock resolver on http://1346569778
 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

 12. LDAP and OpenLDAP
 12.1 OpenLDAP hangs after returning results

 13. TCP/IP
 13.1 --interface for ipv6 binds to unusable IP address

 14 DICT
 14.1 DICT responses show the underlying protocol

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

1. HTTP

1.1 CURLFORM_CONTENTLEN in an array

 It is not possible to pass a 64-bit value using CURLFORM_CONTENTLEN with
 CURLFORM_ARRAY, when compiled on 32-bit platforms that support 64-bit
 integers. This is because the underlying structure 'curl_forms' uses a dual
 purpose char* for storing these values in via casting. For more information
 see the now closed related issue:
 https://github.com/curl/curl/issues/608

1.2 Disabling HTTP Pipelining

 Disabling HTTP Pipelining when there are ongoing transfers can lead to
 heap corruption and crash. https://curl.haxx.se/bug/view.cgi?id=1411

 Similarly, removing a handle when pipelining corrupts data:
 https://github.com/curl/curl/issues/2101

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.haxx.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.haxx.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 the 100-continue response.
 https://curl.haxx.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 appliction. This is tricky, but is
 ultimately a broken server.

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

1.9 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.

1.10 Strips trailing dot from host name

 When given a URL with a trailing dot for the host name part:
 "https://example.com./", libcurl will strip off the dot and use the name
 without a dot internally and send it dot-less in HTTP Host: headers and in
 the TLS SNI field. For the purpose of resolving the name to an address
 the hostname is used as is without any change.

 The HTTP part violates RFC 7230 section 5.4 but the SNI part is accordance
 with RFC 6066 section 3.

 URLs using these trailing dots are very rare in the wild and we have not seen
 or gotten any real-world problems with such URLs reported. The popular
 browsers seem to have stayed with not stripping the dot for both uses (thus
 they violate RFC 6066 instead of RFC 7230).

 Daniel took the discussion to the HTTPbis mailing list in March 2016:
 https://lists.w3.org/Archives/Public/ietf-http-wg/2016JanMar/0430.html but
 there was not major rush or interest to fix this. The impression I get is
 that most HTTP people rather not rock the boat now and instead prioritize web
 compatibility rather than to strictly adhere to these RFCs.

 Our current approach allows a knowing client to send a custom HTTP header
 with the dot added.

 In a few cases there is a difference in name resolving to IP addresses with
 a trailing dot, but it can be noted that many HTTP servers will not happily
 accept the trailing dot there unless that has been specifically configured
 to be a fine virtual host.

 If URLs with trailing dots for host names become more popular or even just
 used more than for just plain fun experiments, I'm sure we will have reason
 to go back and reconsider.

 See https://github.com/curl/curl/issues/716 for the discussion.

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've 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 isn't 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 didn't
 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 and NSS
 backends, so relying on this information in a generic app is flaky.

2.2 DER in keychain

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

2.3 GnuTLS backend skips really long certificate fields

 libcurl calls gnutls_x509_crt_get_dn() with a fixed buffer size and if the
 field is too long in the cert, it'll just return an error and the field will
 be displayed blank.

2.4 DarwinSSL won't 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 doesn't 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) never very carefully documented 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


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.haxx.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 SMTP to multiple recipients

 When sending data to multiple recipients, curl will abort and return failure
 if one of the recipients indicate failure (on the "RCPT TO"
 command). Ordinary mail programs would proceed and still send to the ones
 that can receive data. This is subject for change in the future.
 https://curl.haxx.se/bug/view.cgi?id=1116

3.4 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.haxx.se/bug/?i=740


4. Command line

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

 -J/--remote-header-name doesn't 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.haxx.se/bug/view.cgi?id=1294

 -O also doesn't 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 won't 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.haxx.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 doesn't 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.haxx.se/mail/lib-2008-01/0080.html and Mandriva bug report
 https://qa.mandriva.com/show_bug.cgi?id=22565

4.4 --upload-file . hangs if delay in STDIN

 "(echo start; sleep 1; echo end) | curl --upload-file . http://mywebsite -vv"

 ... causes a hang when it shouldn't.

 See https://github.com/curl/curl/issues/2051

4.5 Improve --data-urlencode space encoding

 ASCII space characters in --data-urlencode are currently encoded as %20
 rather than +, which RFC 1866 says should be used.

 See https://github.com/curl/curl/issues/3229

5. Build and portability issues

5.1 tests not compatible with python3

 The smb test server still needs python2.

 See https://github.com/curl/curl/issues/3289

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 Cannot compile against a static build of OpenLDAP

 See https://github.com/curl/curl/issues/2367

5.5 can't handle Unicode arguments in Windows

 If a URL or filename can't 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. And, except for Cygwin,
 Windows can't use UTF-8 as a locale.

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

5.6 cmake support gaps

 The cmake build setup lacks several features that the autoconf build
 offers. This includes:

  - use of correct soname for the shared library build

  - support for several TLS backends are missing

  - the unit tests cause link failures in regular non-static builds

  - no nghttp2 check

  - unusable tool_hugehelp.c with MinGW, see
    https://github.com/curl/curl/issues/3125

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

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 WinSSL/Schannel
 backend. The original problem was mentioned in:
 https://curl.haxx.se/mail/lib-2009-10/0024.html
 https://curl.haxx.se/bug/view.cgi?id=896

 The WinSSL/Schannel version verified to work as mentioned in
 https://curl.haxx.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.haxx.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 e-mail 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.haxx.se/bug/view.cgi?id=440 How?
 https://curl.haxx.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 doesn't 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 won't proceed to try authentication
 using NTLM.

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

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.haxx.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 isn't 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.haxx.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 doesn't detect this and
 thus fails to issue the correct command:
 https://curl.haxx.se/bug/view.cgi?id=635

7.5 ASCII FTP

 FTP ASCII transfers do not follow RFC959. They don't 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 doesn't know the state of the connection so
 it can't be reused for subsequent requests.

 With FTP however, this isn't 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 doesn't. 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 Stick to same family over SOCKS proxy

 When asked to do FTP over a SOCKS proxy, it might connect to the proxy (and
 then subsequently to the remote server) using for example IPv4. When doing
 the second connection, curl should make sure that the second connection is
 using the same IP protocol version as the first connection did and not try
 others, since the remote server will only accept the same.

 See https://curl.haxx.se/mail/archive-2018-07/0000.html

8. TELNET

8.1 TELNET and time limitations don't work

 When using telnet, the time limitation options don't work.
 https://curl.haxx.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.haxx.se/bug/view.cgi?id=649


9. SFTP and SCP

9.1 SFTP doesn't 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.haxx.se/bug/view.cgi?id=748


10. SOCKS

10.1 SOCKS proxy connections are done blocking

 Both SOCKS5 and SOCKS4 proxy connections are done blocking, which is very bad
 when used with the multi interface.

10.2 SOCKS don't support timeouts

 The SOCKS4 connection codes don't properly acknowledge (connect) timeouts.
 According to bug #1556528, even the SOCKS5 connect code does not do it right:
 https://curl.haxx.se/bug/view.cgi?id=604

 When connecting to a SOCK proxy, the (connect) timeout is not properly
 acknowledged after the actual TCP connect (during the SOCKS "negotiate"
 phase).

10.3 FTPS over SOCKS

 libcurl doesn't support FTPS over a SOCKS proxy.

10.4 active FTP over a SOCKS

 libcurl doesn't 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 c-ares deviates from stock resolver on http://1346569778

 When using the socket resolvers, that URL becomes:

     * Rebuilt URL to: http://1346569778/
     *   Trying 80.67.6.50...

 but with c-ares it instead says "Could not resolve: 1346569778 (Domain name
 not found)"

 See https://github.com/curl/curl/issues/893

11.4 HTTP test server 'connection-monitor' problems

 The 'connection-monitor' feature of the sws HTTP test server doesn't 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

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're 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

12. LDAP and OpenLDAP

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.haxx.se/mail/lib-2016-01/0101.html


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 doesn't bind to a
 locally scoped address as that is bound to fail.

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

14. DICT

14.1 DICT responses show the underlying protocol

 When getting a DICT response, the protocol parts of DICT aren't stripped off
 from the output.

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