README

Welcome to kdcproxy!
====================

This package contains a WSGI module for proxying KDC requests over HTTP by
following the [MS-KKDCP] protocol. It aims to be simple to deploy, with
minimal configuration.

Deploying kdcproxy
==================

The kdcproxy module follows the standard WSGI protocol for deploying Python
web applications. This makes configuration simple. Simply load up your favorite
WSGI-enabled web server and point it to the module. For example, if you wish
to use mod_wsgi, try something like this::

    WSGIDaemonProcess kdcproxy processes=2 threads=15 maximum-requests=1000 \
        display-name=%{GROUP}
    WSGIImportScript /usr/lib/python3.6/site-packages/kdcproxy/__init__.py \
        process-group=kdcproxy application-group=kdcproxy
    WSGIScriptAlias /KdcProxy /usr/lib/python3.6/site-packages/kdcproxy/__init__.py
    WSGIScriptReloading Off

    <Location "/KdcProxy">
        Satisfy Any
        Order Deny,Allow
        Allow from all
        WSGIProcessGroup kdcproxy
        WSGIApplicationGroup kdcproxy
    </Location>

[MS-KKDCP] suggests /KdcProxy as end point. For more information, see the
documentation of your WSGI server.


Configuring kdcproxy
====================

When kdcproxy receives a request, it needs to know where to proxy it to. This
is the purpose of configuration: discovering where to send kerberos requests.

One important note: where the underlying configuration does not specify TCP or
UDP, both will be attempted. TCP will be attempted before UDP, hence setting
`udp_preference_limit = 1` is not required for kdcproxy itself (though krb5
may still need it). This permits the use of longer timeouts and prevents
possible lockouts when the KDC packets contain OTP token codes (which should
preferably be sent to only one server).

Main Configuration File
-------------------------
The location of kdcproxy's main configuration file is specified by the
`KDCPROXY_CONFIG` environment variable. If not set, the default locations are
`/usr/local/etc/kdcproxy.conf` or `/etc/kdcproxy.conf`. This configuration
file takes precedence over all other configuration modules. This file is an
ini-style configuration with a special **[global]** section, wildcard realm
sections, and exact realm sections.

Exact realm sections are named after the realms that kdcproxy is expected to
receive requests for. Wildcard realm sections differ from exact realm sections
by being prefixed by a '\*' character. Such sections will match with realms
having either all or their final labels in common with the section. As an
example, **[\*EXAMPLE.COM]** will match with `EXAMPLE.COM`, `SUB.EXAMPLE.COM`,
and `SUB.SUB.EXAMPLE.COM`, but not `MYEXAMPLE.COM`.

The following parameters can be set on any of these sections, with exact realm
parameters having higher precedence, followed by wildcard realm parameters, and
then global parameters:

**use_dns** (boolean): Allows querying DNS SRV records (aka. DNS discovery) to
find KDCs associated with the requested realm in case they are not explicitly
set in the configuration (main one, or configuration module-provided). By
default (or if explicitly enabled globally), this mechanism is **activated only
for realms explicitly declared** in the main (an empty section named after the
realm, or a matching wildcard realm section is enough) or module-provided
configuration. To allow use of DNS discovery for any requested realm, see the
**dns_realm_discovery** parameter.

**silence_port_warn** (boolean): When DNS SRV records are used to discover KDC
addresses, kdcproxy will write a warning in the logs in case a non-standard
port is found in the DNS response. Setting this parameter to `true` will
silence such warnings.

The following parameters are specific to the **[global]** section:

**configs** (string): Allows you to load other configuration modules for
finding configuration in other places. The configuration modules specified in
here will have priority in the order listed. For instance, if you wished to
read configuration from MIT libkrb5, you would set the following:

    [global]
    configs = mit

**dns_realm_discovery** (boolean): When **use_dns** is not disabled globally,
kdcproxy is allowed to query SRV records to find KDCs of the realms declared in
its configuration only. This protects kdcproxy from attacks based on
server-side request forgery (CVE-2025-59088). Allowing DNS discovery for
unknown realms too is possible by also setting **dns_realm_discovery** to true,
yet heavily discouraged:

    [global]
    dns_realm_discovery = true

Exact realm sections have 2 specific parameters: **kerberos** and **kpasswd**.
These specify the locations of the remote servers for Kerberos ticket requests,
and kpasswd requests, respectively. For example:

    [EXAMPLE.COM]
    kerberos = kerberos+tcp://kdc.example.com:88
    kpasswd = kpasswd+tcp://kpasswd.example.com:464

The realm configuration parameters may list multiple servers separated by a
space. The order the realms are specified in will be respected by kdcproxy when
forwarding requests. The port number is optional. Possible schemes are:

* kerberos://
* kerberos+tcp://
* kerberos+udp://
* kpasswd://
* kpasswd+tcp://
* kpasswd+udp://

MIT libkrb5
-----------

If you load the **mit** config module in the main configuration file, kdcproxy
will also read the config using libkrb5 (usually /etc/krb5.conf). If this
module is used, kdcproxy will respect the realm configuration from the
**[realms]** section.

For more information, see the documentation for MIT's krb5.conf.

Configuration reloading
-----------------------

kdcproxy reads its configurtion files when package is imported and a global
WSGI application object is instantiated. For now kdcproxy does neither
monitor its configuration files for changes nor supports runtime updates. You
have to restart the WSGI process to make modification available. With Apache
HTTP and mod_wsgi, a reload of the server also restarts all WSGI daemons.


Configuring a client for kdcproxy
=================================

HTTPS proxy support is available since Kerberos 5 release 1.13. Some
vendors have backported the feature to older versions of krb5, too. In order
to use a HTTPS proxy, simply point the kdc and kpasswd options to the proxy URL like
explained in [HTTPS proxy] configuration guide. Your ``/etc/krb5.conf`` may
look like this::

    [libdefaults]
        default_realm = EXAMPLE.COM

    [realms]
        EXAMPLE.COM = {
            http_anchors = FILE:/etc/krb5/cacert.pem
            kdc = https://kerberos.example.com/KdcProxy
            kpasswd_server = https://kerberos.example.com/KdcProxy
    }


To debug the feature, set the environment variable ``KRB5_TRACE`` to
``/dev/stdout``. When the feature is correctly configured, you should see
two POST requests in the access log of the WSGI server and a line containing
``Sending HTTPS request`` in the debug output of kinit::

    $ env KRB5_TRACE=/dev/stdout kinit user
    [1037] 1431509096.26305: Getting initial credentials for user@EXAMPLE.COM
    [1037] 1431509096.26669: Sending request (169 bytes) to EXAMPLE.COM
    [1037] 1431509096.26939: Resolving hostname kerberos.example.com
    [1037] 1431509096.34377: TLS certificate name matched "kerberos.example.com"
    [1037] 1431509096.38791: Sending HTTPS request to https 128.66.0.1:443
    [1037] 1431509096.46387: Received answer (344 bytes) from https 128.66.0.1:443
    [1037] 1431509096.46411: Terminating TCP connection to https 128.66.0.1:443
    ...

If kinit still connects to port 88/TCP or port 88/UDP, then System Security
Services Daemon's Kerberos locator plugin might override the settings in
/etc/krb5.conf. With the environment variable ``SSSD_KRB5_LOCATOR_DEBUG=1``,
kinit and sssd_krb5_locator_plugin print out additional debug information. To
disable the KDC locator feature, edit ``/etc/sssd/sssd.conf`` and set
``krb5_use_kdcinfo`` to False:

    [domain/example.com]
    krb5_use_kdcinfo = False

Don't forget to restart SSSD!

[MS-KKDCP]: http://msdn.microsoft.com/en-us/library/hh553774.aspx

[HTTPS Proxy]: http://web.mit.edu/kerberos/krb5-current/doc/admin/https.html