Difference between revisions of "Projects/HTTP Transport"

Revision as of 16:30, 21 April 2014

Overview

This project intends to add an HTTPS transport which can be used to send Kerberos and kpasswd protocol requests to an HTTPS proxy which will then send the requests to a KDC or kpasswd server and relay back their response. This change is useful especially for firewall configurations that allow traffic on port 443 but not on port 88.

Precedent

Both Heimdal and Microsoft Kerberos have such a technology.

Heimdal

Heimdal has such a mechanism as seen here. It uses a GET request with a base64-encoded version of the UDP traffic. Leaving aside questions of idempotence and RESTfulness, Apache has a URL length for GET of about 4000 characters, and requests of this nature have been measured as close to that limit and may exceed it in practice. It uses a separate field in krb5.conf for specification of the http_proxy to be used. There is almost no evidence of this in use in active deployment.

Microsoft

Microsoft has documented their mechanism, MS-KKDCP, here. It uses POST requests which is much more in keeping with the HTTP specification than GET, and it also specifies HTTPS to be used in all cases, as Microsoft's implementation does not work over plain HTTP.

Implementation Process

An HTTPS transport will be implemented. Initial portions of the patch series will focus on refactoring the client library's internal send-to-kdc logic to make the addition of new transports (such as a possible HTTP transport to be added later) less disruptive going forward. The HTTPS transport implementation will follow Microsoft's specification. We will implement it using OpenSSL initially and may add optional support for building using NSS at a later date.

Implementation Design

• We will expand the definition of the `kdc` field (and related server location fields including kpasswd_server and admin_server) in krb5.conf to take a URL (optionally including page) in order to allow HTTP or HTTPS transport. The new syntax will look like this:

 kdc = https://<proxy.addr>[:<port>][/<page>]

Note that the syntax (and parser) does not change in the non-HTTPS cases. In order to facilitate this change, we will need to stop carrying a socktype around in the code that is either SOCK_DGRAM or SOCK_STREAM, and instead carry around our own protocol designator, an enumerated type called k5_transport. Because the contents of a proxy request also incorporate the realm's name and the `page' portion of the URL, we'll need to start to carry them around, too.

• We will need to build against a cryptography library, and to add options to the build system for such. We will include an option to disable HTTPS support (i.e., by building against no cryptographic library).

Test Plan

Due to the nature of the changes, it will be extremely difficult to write test cases for the new code. However, the code can be tested by standing up Microsoft's implementation and running against that.

Review

This section documents the review of the project according to Project policy. It is divided into multiple sections. First, approvals should be listed. To list an approval type

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(hash mark followed by four tilde characters) on its own line. The next section is for summarizing discussion, which should take place on krbdev@mit.edu. Provide links to the archive at http://mailman.mit.edu/pipermail/krbdev/ if appropriate. Blocking objections can be noted with {{project-block}}.

Approvals

Discussion

The first version had comments from mail from ghudson, which we attempted to address.