CURLOPT_SOCKS5_AUTH: allowed methods for SOCKS5 proxy auth

If libcurl was built with GSS-API support, it unconditionally advertised
GSS-API authentication while connecting to a SOCKS5 proxy.  This caused
problems in environments with improperly configured Kerberos: a stock
libcurl failed to connect, despite libcurl built without GSS-API
connected fine using username and password.

This commit introduces the CURLOPT_SOCKS5_AUTH option to control the
allowed methods for SOCKS5 authentication at run time.

Note that a new option was preferred over reusing CURLOPT_PROXYAUTH
for compatibility reasons because the set of authentication methods
allowed by default was different for HTTP and SOCKS5 proxies.

Bug: https://curl.haxx.se/mail/lib-2017-01/0005.html
Closes https://github.com/curl/curl/pull/1454

3 files changed, 67 insertions, 0 deletions

diff --git a/docs/libcurl/curl_easy_setopt.3 b/docs/libcurl/curl_easy_setopt.3
index bf3a0f268..26f73ee14 100644
--- a/docs/libcurl/curl_easy_setopt.3
+++ b/docs/libcurl/curl_easy_setopt.3
@@ -173,6 +173,8 @@ Filter out hosts from proxy use. \fICURLOPT_NOPROXY(3)\fP
 Tunnel through the HTTP proxy. \fICURLOPT_HTTPPROXYTUNNEL(3)\fP
 .IP CURLOPT_CONNECT_TO
 Connect to a specific host and port. See \fICURLOPT_CONNECT_TO(3)\fP
+.IP CURLOPT_SOCKS5_AUTH
+Socks5 authentication methods. See \fICURLOPT_SOCKS5_AUTH(3)\fP
 .IP CURLOPT_SOCKS5_GSSAPI_SERVICE
 Socks5 GSSAPI service name. \fICURLOPT_SOCKS5_GSSAPI_SERVICE(3)\fP
 .IP CURLOPT_SOCKS5_GSSAPI_NEC
diff --git a/docs/libcurl/opts/CURLOPT_SOCKS5_AUTH.3 b/docs/libcurl/opts/CURLOPT_SOCKS5_AUTH.3
new file mode 100644
index 000000000..7b5e5822b
--- /dev/null
+++ b/docs/libcurl/opts/CURLOPT_SOCKS5_AUTH.3
@@ -0,0 +1,63 @@
+.\" **************************************************************************
+.\" *                                  _   _ ____  _
+.\" *  Project                     ___| | | |  _ \| |
+.\" *                             / __| | | | |_) | |
+.\" *                            | (__| |_| |  _ <| |___
+.\" *                             \___|\___/|_| \_\_____|
+.\" *
+.\" * Copyright (C) 1998 - 2017, Daniel Stenberg, <daniel@haxx.se>, et al.
+.\" *
+.\" * This software is licensed as described in the file COPYING, which
+.\" * you should have received as part of this distribution. The terms
+.\" * are also available at https://curl.haxx.se/docs/copyright.html.
+.\" *
+.\" * You may opt to use, copy, modify, merge, publish, distribute and/or sell
+.\" * copies of the Software, and permit persons to whom the Software is
+.\" * furnished to do so, under the terms of the COPYING file.
+.\" *
+.\" * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
+.\" * KIND, either express or implied.
+.\" *
+.\" **************************************************************************
+.\"
+.TH CURLOPT_SOCKS5_AUTH 3 "27 April 2017" "libcurl 7.55.0" "curl_easy_setopt options"
+.SH NAME
+CURLOPT_SOCKS5_AUTH \- set allowed methods for SOCKS5 proxy authentication
+.SH SYNOPSIS
+#include <curl/curl.h>
+
+CURLcode curl_easy_setopt(CURL *handle, CURLOPT_SOCKS5_AUTH, long bitmask);
+.SH DESCRIPTION
+Pass a long as parameter, which is set to a bitmask, to tell libcurl which
+authentication method(s) are allowed for SOCKS5 proxy authentication.  The only
+supported flags are \fICURLAUTH_BASIC\fP, which allows username/password
+authentication, \fICURLAUTH_GSSAPI\fP, which allows GSS-API authentication, and
+\fICURLAUTH_NONE\fP, which allows no authentication.  Set the actual user name
+and password with the \fICURLOPT_PROXYUSERPWD(3)\fP option.
+.SH DEFAULT
+CURLAUTH_BASIC|CURLAUTH_GSSAPI
+.SH PROTOCOLS
+All
+.SH EXAMPLE
+.nf
+CURL *curl = curl_easy_init();
+if(curl) {
+  curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");
+
+  /* request to use a SOCKS5 proxy */
+  curl_easy_setopt(curl, CURLOPT_PROXY, "socks5://user:pass@myproxy.com");
+
+  /* enable username/password authentication only */
+  curl_easy_setopt(curl, CURLOPT_SOCKS5_AUTH, CURLAUTH_BASIC);
+
+  /* Perform the request */
+  curl_easy_perform(curl);
+}
+.fi
+.SH AVAILABILITY
+Added in 7.55.0
+.SH RETURN VALUE
+Returns CURLE_OK if the option is supported, CURLE_UNKNOWN_OPTION if not, or
+CURLE_NOT_BUILT_IN if the bitmask contains unsupported flags.
+.SH "SEE ALSO"
+.BR CURLOPT_PROXY "(3), " CURLOPT_PROXYTYPE "(3)"
diff --git a/docs/libcurl/symbols-in-versions b/docs/libcurl/symbols-in-versions
index b1a671cd1..e141b6a57 100644
--- a/docs/libcurl/symbols-in-versions
+++ b/docs/libcurl/symbols-in-versions
@@ -17,6 +17,7 @@ CURLAUTH_ANYSAFE                7.10.6
 CURLAUTH_BASIC                  7.10.6
 CURLAUTH_DIGEST                 7.10.6
 CURLAUTH_DIGEST_IE              7.19.3
+CURLAUTH_GSSAPI                 7.55.0
 CURLAUTH_GSSNEGOTIATE           7.10.6       7.38.0
 CURLAUTH_NEGOTIATE              7.38.0
 CURLAUTH_NONE                   7.10.6
@@ -527,6 +528,7 @@ CURLOPT_SERVICE_NAME            7.43.0
 CURLOPT_SHARE                   7.10
 CURLOPT_SOCKOPTDATA             7.16.0
 CURLOPT_SOCKOPTFUNCTION         7.16.0
+CURLOPT_SOCKS5_AUTH             7.55.0
 CURLOPT_SOCKS5_GSSAPI_NEC       7.19.4
 CURLOPT_SOCKS5_GSSAPI_SERVICE   7.19.4        7.49.0
 CURLOPT_SOURCE_HOST             7.12.1        -           7.15.5