GSSAPI Negotiate module for Apache
C Python ApacheConf M4 Makefile Shell
Latest commit cfb4b77 Feb 11, 2017 @alejandro-perez alejandro-perez committed with simo5 Add cleanup function for mag_conn->name_attributes
Reviewed-by: Simo Sorce <>
Closes #127



This module has been built as a replacement for the aging mod_auth_kerb. Its aim is to use only GSSAPI calls and be as much as possible agnostic of the actual mechanism used.


A modern version of MIT's Krb5 distribution or any GSSAPI implementation that supports the credential store extension is necessary to achieve full functionality. Reduced functionality is provided without these extensions.

krb5 (>=1.11)
Apache (>=2.4)


To run tests, you also need:

  • The Kerberos 5 Key-Distribution-Center (krb5-kdc package on Debian, krb5-server on Fedora)
  • Packages mod_session, krb5-workstation, python-requests-kerberos, and python-gssapi on Fedora
  • nss_wrapper, packaged in Fedora
  • socket_wrapper, packaged in Fedora


autoreconf -fi
make install


Apache authentication modules are usually configured per location, see the mod_authn_core documentation for the common directives

Basic configuration

The simplest configuration scheme specifies just one directive, which is the location of the keytab.


<Location /private>
    AuthType GSSAPI
    AuthName "GSSAPI Single Sign On Login"
    GssapiCredStore keytab:/etc/httpd.keytab
    Require valid-user

Your Apache server need read access to the keytab configured. If your Kerberos implementation does not support the credential store extensions you can also simply set the KRB5_KTNAME environment variable in the Apache init script and skip the GssapiCredStore option completely.

Configuration Directives


Forces the authentication attempt to fail if the connection is not being established over TLS


GssapiSSLonly On


Tries to map the client principal to a local name using the gss_localname() call. This requires configuration in the /etc/krb5.conf file in order to allow proper mapping for principals not in the default realm (for example a user coming from a trusted realm). See the 'auth_to_local' option in the [realms] section of krb5.conf(5)

When this options is used the resolved name is set in the REMOTE_USER variable however the complete client principal name is also made available in the GSS_NAME variable.


GssapiLocalName on


When using GSS mechanisms that require more than one round-trip to complete authentication (like NTLMSSP) it is necessary to bind to the authentication to the connection in order to keep the state between round-trips. With this option enable incomplete context are store in the connection and retrieved on the next request for continuation.


GssapiConnectionBound On


For clients that make use of Persistent-Auth header, send the header according to GssapiConnectionBound setting.


GssapiSignalPersistentAuth On


In order to avoid constant and costly re-authentication attempts for every request, mod_auth_gssapi offers a cookie based session method to maintain authentication across multiple requests. GSSAPI uses the mod_sessions module to handle cookies so that module needs to be activated and configured. GSSAPI uses a secured (encrypted + MAC-ed) payload to maintain state in the session cookie. The session cookie lifetime depends on the lifetime of the GSSAPI session established at authentication. NOTE: It is important to correctly set the SessionCookieName option. See the mod_sessions documentation for more information.


GssapiUseSessions On
Session On
SessionCookieName gssapi_session path=/private;httponly;secure;


When GssapiUseSessions is enabled a key use to encrypt and MAC the session data will be automatically generated at startup, this means session data will become unreadable if the server is restarted or multiple servers are used and the client is load balanced from one to another. To obviate this problem the admin can choose to install a permanent key in the configuration so that session data remain accessible after a restart or by multiple servers sharing the same key.

Two schemes to read persistent keys are provided, 'key' and 'file'.

  • 'key' A key is read from the configuration directive. The key must be a base64 encoded raw key of 32 bytes of length.

  • 'file' A file on the file system is used to store the key. If the file does not exists one is created with a randomly generated key during the first execution.


GssapiSessionKey key:VGhpcyBpcyBhIDMyIGJ5dGUgbG9uZyBzZWNyZXQhISE=
GssapiSessionKey file:/var/lib/httpd/secrets/session.key


The GssapiCredStore option allows to specify multiple credential related options like keytab location, client_keytab location, ccache location etc.


GssapiCredStore keytab:/etc/httpd.keytab
GssapiCredStore ccache:FILE:/var/run/httpd/krb5ccache


If delegation of credentials is desired credentials can be exported in a private directory accessible by the Apache process. The delegated credentials will be stored in a file named after the client principal and a request environment variable (KRB5CCNAME by default) will be set to point to that file.


GssapiDelegCcacheDir /var/run/httpd/clientcaches

A user foo@EXAMPLE.COM delegating its credentials would cause the server to create a ccache file named /var/run/httpd/clientcaches/foo@EXAMPLE.COM


Enables using unique ccache names for delegation. ccache files will be placed in GssapiDelegCcacheDir and named using the principal and a six-digit unique suffix.

Note: Consuming application must delete the ccache otherwise it will litter the filesystem if sessions are used. An example sweeper can be found in the contrib directory.


GssapiDelegCcacheUnique On


Set the name of the request environment variable that will receive the credential cache name. If unspecified, defaults to KRB5CCNAME.


GssapiDelegCcacheEnvVar AJP_KRB5CCNAME


Enables the use of the s4u2Proxy Kerberos extension also known as constrained delegation This option allows an application running within Apache to operate on behalf of the user against other servers by using the provided ticket (subject to KDC authorization). This options requires GssapiDelegCcacheDir to be set. The ccache will be populated with the user's provided ticket which is later used as evidence ticket by the application.

Note: This flag has no effect when Basic-Auth is used since user's credentials are delegated anyway when GssapiDelegCcacheDir is set.


GssapiUseS4U2Proxy On
GssapiCredStore keytab:/etc/httpd.keytab
GssapiCredStore client_keytab:/etc/httpd.keytab
GssapiCredStore ccache:FILE:/var/run/httpd/krb5ccache
GssapiDelegCcacheDir /var/run/httpd/clientcaches

NOTE: The client keytab is necessary to allow GSSAPI to initiate via keytab on its own. If not present an external mechanism needs to kinit with the keytab and store a ccache in the configured ccache file.


Allows the use of Basic Auth in conjunction with Negotiate. If the browser fails to use Negotiate it will instead fallback to Basic and the username and password will be used to try to acquire credentials in the module via GSSAPI. If credentials are acquired successfully then they are validated against the server's keytab.

  • Enable with: GssapiBasicAuth On
  • Default: GssapiBasicAuth Off


<Location /gssapi>
  AuthType GSSAPI
  AuthName "Login"
  GssapiBasicAuth On
  GssapiCredStore keytab:/etc/httpd/http.keytab
  Require valid-user


List of allowed mechanisms. This is useful to restrict the mechanism that can be used when credentials for multiple mechanisms are available. By default no mechanism is set, this means all locally available mechanisms are allowed. The recognized mechanism names are: krb5, iakerb, ntlmssp


GssapiAllowedMech krb5
GssapiAllowedMech ntlmssp


List of mechanisms against which Basic Auth is attempted. This is useful to restrict the mechanisms that can be used to attempt password auth. By default no mechanism is set, this means all locally available mechanisms are allowed, unless GssapiAllowedMech is set, in which case those are used. GssapiBasicAuthMech always takes precedence over GssapiAllowedMech. The recognized mechanism names are: krb5, iakerb, ntlmssp


GssapiBasicAuthMech krb5


Enables the module to source Name Attributes from the client name (authorization data associated with the established context) and exposes them as environment variables.


This option can be specified multiple times, once for each attribute to expose. The Special value "json" is used to expose all attributes in a json formatted string via the special environment variable GSS_NAME_ATTRS_JSON The environment variable GSS_NAME_ATTR_ERROR is set with the Gssapi returned error string in case the inquire name function fails to retrieve attributes, and with the string "0 attributes found", if no attributes are set.

Note: These variables are NOT saved in the session data stored in the cookie so they are available only on the first authenticated request when GssapiUseSessions is used.

Note: It is recommended but not required to use only capital letters and underscores for environment variable names.


GssapiNameAttributes json
GssapiNameAttributes RADIUS_NAME urn:ietf:params:gss:radius-attribute_1


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


This option can be used even if AuthType GSSAPI is not used for given Location or LocationMatch, to obtain service ticket for a user that was already authenticated by different module.

The principal of the user is retrieved from the internal r->user identifier which typically holds the username from the authentication results.

Make sure the server principal is set to allow to acquire forwardable tickets to itself from arbitrary users, for use with constrained delegation, for example with the option +ok_to_auth_as_delegate.

  • Enable with: GssapiImpersonate On
  • Default: GssapiImpersonate Off


This option is used to set alternative ownership and permission for delegated ccache files stored in the GssapiDelegCcacheDir location. It is a multivalue configuration directive that can accept the following three settings:

  • mode
  • uid
  • gid If a setting is not present the relative file property will not be modified and the default owners and/or mode will be retained.


This option allows to set the file mode, the format used is a numeric mode
with the same semantics of the chmod unix command for mapping numbers to


A user id number or name, an attempt to change the user owner of the file
to the uid number specified will be made. If a user name has been
specified, it will be resolved at startup time and the user's id number
stored internally for all subsequent operations.


A group id number or name, an attempt to change the group owner of the
file to the gid number specified will be made. If a group name has been
specified, it will be resolved at startup time and the group's id number
stored internally for all subsequent operations.


GssapiDelegCcachePerms mode:0660 gid:webuiworkers