README.MSCHAP81

PPP Support for Microsoft's CHAP-81
===================================

Frank Cusack		frank@google.com

Some text verbatim from README.MSCHAP80,
by Eric Rosenquist, rosenqui@strataware.com

INTRODUCTION

First, please read README.MSCHAP80; almost everything there applies here.
MS-CHAP was basically devised by Microsoft because rather than store
plaintext passwords, they (Microsoft) store the md4 hash of passwords.
It provides no advantage over standard CHAP, since the hash is used
as plaintext-equivalent.  (Well, the Change-Password packet is arguably
an advantage.)  It does introduce a significant weakness if the LM hash
is used.  Additionally, the format of the failure packet potentially
gives information to an attacker.  The weakness of the LM hash is partly
addressed in RFC 2433, which deprecates its use.

MS-CHAPv2 adds 2 benefits to MS-CHAP.  (1) The LM hash is no longer
used.  (2) Mutual authentication is required.  Note that the mutual
authentication in MS-CHAPv2 is different than the case where both PPP
peers require authentication from the other; the former proves that
the server has access to the client's password, the latter proves that
the server has access to a secret which the client also has -- which
may or may not be the same as the client's password (but should not be
the same, per RFC 1994).  Whether this provides any actual benefit is
outside the scope of this document.  The details of MS-CHAPv2 can be
found in the document:

    <http://www.ietf.org/rfc/rfc2759.txt>


BUILDING THE PPPD

In addition to the requirements for MS-CHAP, MS-CHAPv2 uses the SHA-1
hash algorithm.  A public domain implementation is provided with pppd.


TROUBLESHOOTING

Assuming that everything else has been configured correctly for PPP and
CHAP, the MS-CHAPv2-specific problems you're likely to encounter are mostly
related to your Windows NT account and its settings.  A Microsoft server
returns error codes in its CHAP response.  The following are extracted from
RFC 2759:

 646 ERROR_RESTRICTED_LOGON_HOURS
 647 ERROR_ACCT_DISABLED
 648 ERROR_PASSWD_EXPIRED
 649 ERROR_NO_DIALIN_PERMISSION
 691 ERROR_AUTHENTICATION_FAILURE
 709 ERROR_CHANGING_PASSWORD

You'll see these in your pppd log as a line similar to:

   Remote message: E=649 No dialin permission

Previously, pppd would log this as:

   Remote message: E=649 R=0

Now, the text message is logged (both for MS-CHAP and MS-CHAPv2).