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Nginx module for HTTP SPNEGO auth

This module implements adds SPNEGO support to nginx(http://nginx.org). It currently supports only Kerberos authentication via GSSAPI

Prerequisites

Authentication has been tested with (at least) the following:

  • Nginx 1.2 through 1.7
  • Internet Explorer 8 and above
  • Firefox 10 and above
  • Chrome 20 and above
  • Curl 7.x (GSS-Negotiate), 7.x (SPNEGO/fbopenssl)

The underlying kerberos library used for these tests was MIT KRB5 v1.8.

Installation

  1. Download nginx source

  2. Extract to a directory

  3. Clone this module into the directory

  4. Follow the nginx install documentation and pass an --add-module option to nginx configure:

    ./configure --add-module=spnego-http-auth-nginx-module

Note that if it isn't clear, you do need KRB5 (MIT or Heimdal) header files installed. On Debian based distributions, including Ubuntu, this is the krb5-multidev, libkrb5-dev, heimdal-dev, or heimdal-multidev package depending on your environment. On other Linux distributions, you want the development libraries that provide gssapi_krb5.h.

Configuration reference

You can configure GSS authentication on a per-location and/or a global basis:

These options are required.

  • auth_gss: on/off, for ease of unsecuring while leaving other options in the config file
  • auth_gss_keytab: absolute path-name to keytab file containing service credentials

These options should ONLY be specified if you have a keytab containing privileged principals. In nearly all cases, you should not put these in the configuration file, as gss_accept_sec_context will do the right thing.

  • auth_gss_realm: Kerberos realm name. If this is specified, the realm is only passed to the nginx variable $remote_user if it differs from this default. To override this behavior, set auth_gss_format_full to 1 in your configuration.
  • auth_gss_service_name: service principal name to use when acquiring credentials.

If you would like to authorize only a specific set of users, you can use the auth_gss_authorized_principal directive. The configuration syntax supports multiple entries, one per line.

auth_gss_authorized_principal <username>@<realm>
auth_gss_authorized_principal <username2>@<realm>

Users can also be authorized using a regex pattern via the auth_gss_authorized_principal_regex directive. This directive can be used together with the auth_gss_authorized_principal directive.

auth_gss_authorized_principal <username>@<realm>
auth_gss_authorized_principal_regex ^(<username>)/(<group>)@<realm>$

The remote user header in nginx can only be set by doing basic authentication. Thus, this module sets a bogus basic auth header that will reach your backend application in order to set this header/nginx variable. The easiest way to disable this behavior is to add the following configuration to your location config.

proxy_set_header Authorization "";

A future version of the module may make this behavior an option, but this should be a sufficient workaround for now.

If you would like to enable GSS local name rules to rewrite usernames, you can specify the auth_gss_map_to_local option.

Credential Delegation

User credentials can be delegated to nginx using the auth_gss_delegate_credentials directive. This directive will enable unconstrained delegation if the user chooses to delegate their credentials. Constrained delegation (S4U2proxy) can also be enabled using the auth_gss_constrained_delegation directive together with the auth_gss_delegate_credentials directive. To specify the ccache file name to store the service ticket used for constrained delegation, set the auth_gss_service_ccache directive. Otherwise, the default ccache name will be used.

auth_gss_service_ccache /tmp/krb5cc_0;
auth_gss_delegate_credentials on;
auth_gss_constrained_delegation on;

The delegated credentials will be stored within the systems tmp directory. Once the request is completed, the credentials file will be destroyed. The name of the credentials file will be specified within the nginx variable $krb5_cc_name. Usage of the variable can include passing it to a fcgi program using the fastcgi_param directive.

fastcgi_param KRB5CCNAME $krb5_cc_name;

Constrained delegation is currently only supported using the negotiate authentication scheme and has only been testing with MIT Kerberos (Use at your own risk if using Heimdal Kerberos).

Basic authentication fallback

The module falls back to basic authentication by default if no negotiation is attempted by the client. If you are using SPNEGO without SSL, it is recommended you disable basic authentication fallback, as the password would be sent in plaintext. This is done by setting auth_gss_allow_basic_fallback in the config file.

auth_gss_allow_basic_fallback off

These options affect the operation of basic authentication:

  • auth_gss_realm: Kerberos realm name. If this is specified, the realm is only passed to the nginx variable $remote_user if it differs from this default. To override this behavior, set auth_gss_format_full to 1 in your configuration.
  • auth_gss_force_realm: Forcibly authenticate using the realm configured in auth_gss_realm or the system default realm if auth_gss_realm is not set. This will rewrite $remote_user if the client provided a different realm. If auth_gss_format_full is not set, $remote_user will not include a realm even if one was specified by the client.

Troubleshooting

Check the logs. If you see a mention of NTLM, your client is attempting to connect using NTLMSSP, which is unsupported and insecure.

Verify that you have an HTTP principal in your keytab

MIT Kerberos utilities

$ KRB5_KTNAME=FILE:<path to your keytab> klist -k

or

$ ktutil
ktutil: read_kt <path to your keytab>
ktutil: list

Heimdal Kerberos utilities

$ ktutil -k <path to your keytab> list

Obtain an HTTP principal

If you find that you do not have the HTTP service principal, are running in an Active Directory environment, and are bound to the domain such that Samba tools work properly

$ env KRB5_KTNAME=FILE:<path to your keytab> net ads -P keytab add HTTP

If you are running in a different kerberos environment, you can likely run

$ env KRB5_KTNAME=FILE:<path to your keytab> krb5_keytab HTTP

Increase maximum allowed header size

In Active Directory environment, SPNEGO token in the Authorization header includes PAC (Privilege Access Certificate) information, which includes all security groups the user belongs to. This may cause the header to grow beyond default 8kB limit and causes following error message:

400 Bad Request
Request Header Or Cookie Too Large

For performance reasons, best solution is to reduce the number of groups the user belongs to. When this is impractical, you may also choose to increase the allowed header size by explicitly setting the number and size of Nginx header buffers:

large_client_header_buffers 8 32k;

Debugging

The module prints all sort of debugging information if nginx is compiled with the --with-debug option, and the error_log directive has a debug level.

NTLM

Note that the module does not support NTLMSSP in Negotiate. NTLM, both v1 and v2, is an exploitable protocol and should be avoided where possible.

Help

If you're unable to figure things out, please feel free to open an issue on Github and I'll do my best to help you.