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, 27 insertions, 9 deletions

diff --git a/lib/socks.c b/lib/socks.c
index 968b720b7..000cd9c2d 100644
--- a/lib/socks.c
+++ b/lib/socks.c
@@ -387,6 +387,8 @@ CURLcode Curl_SOCKS5(const char *proxy_user,
     (conn->socks_proxy.proxytype == CURLPROXY_SOCKS5) ? TRUE : FALSE;
   const size_t hostname_len = strlen(hostname);
   ssize_t len = 0;
+  const unsigned long auth = data->set.socks5auth;
+  bool allow_gssapi = FALSE;
 
   if(conn->bits.httpproxy)
     infof(conn->data, "SOCKS5: connecting to HTTP proxy %s port %d\n",
@@ -427,13 +429,24 @@ CURLcode Curl_SOCKS5(const char *proxy_user,
     return CURLE_COULDNT_CONNECT;
   }
 
+  if(auth & ~(CURLAUTH_BASIC | CURLAUTH_GSSAPI))
+    infof(conn->data,
+        "warning: unsupported value passed to CURLOPT_SOCKS5_AUTH: %lu\n",
+        auth);
+  if(!(auth & CURLAUTH_BASIC))
+    /* disable username/password auth */
+    proxy_user = NULL;
+#if defined(HAVE_GSSAPI) || defined(USE_WINDOWS_SSPI)
+  if(auth & CURLAUTH_GSSAPI)
+    allow_gssapi = TRUE;
+#endif
+
   idx = 0;
   socksreq[idx++] = 5;   /* version */
   idx++;                 /* reserve for the number of authentication methods */
   socksreq[idx++] = 0;   /* no authentication */
-#if defined(HAVE_GSSAPI) || defined(USE_WINDOWS_SSPI)
-  socksreq[idx++] = 1;   /* GSS-API */
-#endif
+  if(allow_gssapi)
+    socksreq[idx++] = 1; /* GSS-API */
   if(proxy_user)
     socksreq[idx++] = 2; /* username/password */
   /* write the number of authentication methods */
@@ -485,7 +498,7 @@ CURLcode Curl_SOCKS5(const char *proxy_user,
     ;
   }
 #if defined(HAVE_GSSAPI) || defined(USE_WINDOWS_SSPI)
-  else if(socksreq[1] == 1) {
+  else if(allow_gssapi && (socksreq[1] == 1)) {
     code = Curl_SOCKS5_gssapi_negotiate(sockindex, conn);
     if(code) {
       failf(data, "Unable to negotiate SOCKS5 GSS-API context.");
@@ -546,16 +559,12 @@ CURLcode Curl_SOCKS5(const char *proxy_user,
   }
   else {
     /* error */
-#if defined(HAVE_GSSAPI) || defined(USE_WINDOWS_SSPI)
-    if(socksreq[1] == 255) {
-#else
-    if(socksreq[1] == 1) {
+    if(!allow_gssapi && (socksreq[1] == 1)) {
       failf(data,
             "SOCKS5 GSSAPI per-message authentication is not supported.");
       return CURLE_COULDNT_CONNECT;
     }
     if(socksreq[1] == 255) {
-#endif
       if(!proxy_user || !*proxy_user) {
         failf(data,
               "No authentication method was acceptable. (It is quite likely"
diff --git a/lib/url.c b/lib/url.c
index 3b3143ab8..07bdea129 100644
--- a/lib/url.c
+++ b/lib/url.c
@@ -545,6 +545,9 @@ CURLcode Curl_init_userdefined(struct UserDefined *set)
   set->httpauth = CURLAUTH_BASIC;  /* defaults to basic */
   set->proxyauth = CURLAUTH_BASIC; /* defaults to basic */
 
+  /* SOCKS5 proxy auth defaults to username/password + GSS-API */
+  set->socks5auth = CURLAUTH_BASIC | CURLAUTH_GSSAPI;
+
   /* make libcurl quiet by default: */
   set->hide_progress = TRUE;  /* CURLOPT_NOPROGRESS changes these */
 
@@ -1541,6 +1544,11 @@ CURLcode Curl_setopt(struct Curl_easy *data, CURLoption option,
     break;
 #endif   /* CURL_DISABLE_PROXY */
 
+  case CURLOPT_SOCKS5_AUTH:
+    data->set.socks5auth = va_arg(param, unsigned long);
+    if(data->set.socks5auth & ~(CURLAUTH_BASIC | CURLAUTH_GSSAPI))
+      result = CURLE_NOT_BUILT_IN;
+    break;
 #if defined(HAVE_GSSAPI) || defined(USE_WINDOWS_SSPI)
   case CURLOPT_SOCKS5_GSSAPI_NEC:
     /*
diff --git a/lib/urldata.h b/lib/urldata.h
index 8c4a029fe..e89d1d706 100644
--- a/lib/urldata.h
+++ b/lib/urldata.h
@@ -1615,6 +1615,7 @@ struct UserDefined {
   long use_port;     /* which port to use (when not using default) */
   unsigned long httpauth;  /* kind of HTTP authentication to use (bitmask) */
   unsigned long proxyauth; /* kind of proxy authentication to use (bitmask) */
+  unsigned long socks5auth;/* kind of SOCKS5 authentication to use (bitmask) */
   long followlocation; /* as in HTTP Location: */
   long maxredirs;    /* maximum no. of http(s) redirects to follow, set to -1
                         for infinity */