Add option to not send a Negotiate headers

If negotiation was attempted but failed do not send a new Negotiate header.
Useful when only one single sign on mechanism is allowed and to avoid
misleading login prompts in some browsers.

Added a test of the GssapiDontReauth option to the test suite.
Also added SPNEGO no auth test.

[SS: reworded and fixed commit subject/comment]
[SS: fixed whitespace errors and 80 column wrappings]

Reviewed-by: Simo Sorce <simo@redhat.com>

Close #65

1 files changed, 26 insertions, 0 deletions

diff --git a/README b/README
index 72135cb..65ce17b 100644
--- a/README
+++ b/README
@@ -264,3 +264,29 @@ underscores for environment variable names.
 #### Example
     GssapiNameAttributes json
     GssapiNameAttributes RADIUS_NAME urn:ietf:params:gss:radius-attribute_1
+
+
+### GssapiNegotiateOnce
+
+When this option is enabled the Negotiate header will not be resent if
+Negotiation has already been attempted but failed.
+
+Normally when a client fails to use Negotiate authentication, a HTTP 401
+response is returned with a WWW-Authenticate: Negotiate header, implying that
+the client can retry to use Negotiate with different credentials or a
+different mechanism.
+
+Consider enabling GssapiNegotiateOnce when only one single sign on mechanism
+is allowed, or when GssapiBasicAuth is enabled.
+
+**NOTE:** if the initial Negotiate attempt fails, some browsers will fallback
+to other Negotiate mechanisms, prompting the user for login credentials and
+reattempting negotiation. This situation can mislead users - for example if
+krb5 authentication failed and no other mechanisms are allowed, a user could
+be prompted for login information even though any login information provided
+cannot succeed. When this occurs, some browsers will not fall back to a Basic
+Auth mechanism. Enable GssapiNegotiateOnce to avoid this situation.
+
+- **Enable with:** GssapiNegotiateOnce On
+- **Default:** GssapiNegotiateOnce Off
+