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spnego-http-auth: Nginx module for HTTP SPNEGO auth

Installation

You can install this module in any RHEL-based distribution, including, but not limited to:

  • RedHat Enterprise Linux 7, 8, 9
  • CentOS 7, 8, 9
  • AlmaLinux 8, 9
  • Rocky Linux 8, 9
  • Amazon Linux 2 and Amazon Linux 2023
yum -y install https://extras.getpagespeed.com/release-latest.rpm
yum -y install nginx-module-spnego-http-auth

Enable the module by adding the following at the top of /etc/nginx/nginx.conf:

load_module modules/ngx_http_auth_spnego_module.so;

This document describes nginx-module-spnego-http-auth v1.1.1 released on Feb 26 2021.

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.

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>

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.

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.

GitHub

You may find additional configuration tips and documentation for this module in the GitHub repository for nginx-module-spnego-http-auth.