206 lines
8.2 KiB
Plaintext
206 lines
8.2 KiB
Plaintext
PPP Support for Microsoft's CHAP-80
|
|
===================================
|
|
|
|
Eric Rosenquist rosenqui@strataware.com
|
|
(updated by Paul Mackerras)
|
|
(updated by Al Longyear)
|
|
(updated by Farrell Woods)
|
|
(updated by Frank Cusack)
|
|
|
|
INTRODUCTION
|
|
|
|
Microsoft has introduced an extension to the Challenge/Handshake
|
|
Authentication Protocol (CHAP) which avoids storing cleartext
|
|
passwords on a server. (Unfortunately, this is not as secure as it
|
|
sounds, because the encrypted password stored on a server can be used
|
|
by a bogus client to gain access to the server just as easily as if
|
|
the password were stored in cleartext.) The details of the Microsoft
|
|
extensions can be found in the document:
|
|
|
|
<http://www.ietf.org/rfc/rfc2433.txt>
|
|
|
|
In short, MS-CHAP is identified as <auth chap 80> since the hex value
|
|
of 80 is used to designate Microsoft's scheme. Standard PPP CHAP uses
|
|
a value of 5. If you enable PPP debugging with the "debug" option and
|
|
see something like the following in your logs, the remote server is
|
|
requesting MS-CHAP:
|
|
|
|
rcvd [LCP ConfReq id=0x2 <asyncmap 0x0> <auth chap 80> <magic 0x46a3>]
|
|
^^^^^^^^^^^^
|
|
|
|
The standard pppd implementation will indicate its lack of support for
|
|
MS-CHAP by NAKing it:
|
|
|
|
sent [LCP ConfNak id=0x2 <auth chap 05>]
|
|
|
|
Windows NT Server systems are often configured to "Accept only
|
|
Microsoft Authentication" (this is intended to enhance security). Up
|
|
until now, that meant that you couldn't use this version of PPPD to
|
|
connect to such a system.
|
|
|
|
|
|
BUILDING THE PPPD
|
|
|
|
MS-CHAP uses a combination of MD4 hashing and DES encryption for
|
|
authentication. You may need to get Eric Young's libdes library in
|
|
order to use my MS-CHAP extensions. A lot of UNIX systems already
|
|
have DES encryption available via the crypt(3), encrypt(3) and
|
|
setkey(3) interfaces. Some may (such as that on Digital UNIX)
|
|
provide only the encryption mechanism and will not perform
|
|
decryption. This is okay. We only need to encrypt to perform
|
|
MS-CHAP authentication.
|
|
|
|
If you have encrypt/setkey available, then hopefully you need only
|
|
define these two things in your Makefile: -DUSE_CRYPT and -DCHAPMS.
|
|
Skip the paragraphs below about obtaining and building libdes. Do
|
|
the "make clean" and "make" as described below. Linux users
|
|
should not need to modify their Makefiles. Instead,
|
|
just do "make CHAPMS=1 USE_CRYPT=1".
|
|
|
|
If you don't have encrypt and setkey, you will need Eric Young's
|
|
libdes library. You can find it in:
|
|
|
|
ftp://ftp.funet.fi/pub/crypt/mirrors/ftp.psy.uq.oz.au/DES/libdes-3.06.tar.gz
|
|
|
|
Australian residents can get libdes from Eric Young's site:
|
|
|
|
ftp://ftp.psy.uq.oz.au/pub/Crypto/DES/libdes-3.06.tar.gz
|
|
|
|
It is also available on many other sites (ask Archie).
|
|
|
|
I used libdes-3.06, but hopefully anything newer than that will work
|
|
also. Get the library, build and test it on your system, and install
|
|
it somewhere (typically /usr/local/lib and /usr/local/include).
|
|
|
|
|
|
|
|
You should now be ready to (re)compile the PPPD. Go to the pppd
|
|
subdirectory and make sure the Makefile contains "-DCHAPMS" in the
|
|
CFLAGS or COMPILE_FLAGS macro, and that the LIBS macro (or LDADD for
|
|
BSD systems) contains "-ldes". Depending on your system and where the
|
|
DES library was installed, you may also need to alter the include and
|
|
library paths used by your compiler.
|
|
|
|
Do a "make clean" and then a "make" to rebuild pppd. Assuming all
|
|
goes well, install the new pppd and move on to the CONFIGURATION
|
|
section.
|
|
|
|
|
|
CONFIGURATION
|
|
|
|
If you've never used PPPD with CHAP before, read the man page (type
|
|
"man pppd") and read the description in there. Basically, you need to
|
|
edit the "chap-secrets" file typically named /etc/ppp/chap-secrets.
|
|
This should contain the following two lines for each system with which
|
|
you use CHAP (with no leading blanks):
|
|
|
|
RemoteHost Account Secret
|
|
Account RemoteHost Secret
|
|
|
|
Note that you need both lines and that item 1 and 2 are swapped in the
|
|
second line. I'm not sure why you need it twice, but it works and I didn't
|
|
have time to look into it further. The "RemoteHost" is a somewhat
|
|
arbitrary name for the remote Windows NT system you're dialing. It doesn't
|
|
have to match the NT system's name, but it *does* have to match what you
|
|
use with the "remotename" parameter. The "Account" is the Windows NT
|
|
account name you have been told to use when dialing, and the "Secret" is
|
|
the password for that account. For example, if your service provider calls
|
|
their machine "DialupNT" and tells you your account and password are
|
|
"customer47" and "foobar", add the following to your chap-secrets file:
|
|
|
|
DialupNT customer47 foobar
|
|
customer47 DialupNT foobar
|
|
|
|
The only other thing you need to do for MS-CHAP (compared to normal CHAP)
|
|
is to always use the "remotename" option, either on the command line or in
|
|
your "options" file (see the pppd man page for details). In the case of
|
|
the above example, you would need to use the following command line:
|
|
|
|
pppd name customer47 remotename DialupNT <other options>
|
|
|
|
or add:
|
|
|
|
name customer47
|
|
remotename DialupNT
|
|
|
|
to your PPPD "options" file.
|
|
|
|
The "remotename" option is required for MS-CHAP since Microsoft PPP servers
|
|
don't send their system name in the CHAP challenge packet.
|
|
|
|
|
|
E=691 (AUTHENTICATION_FAILURE) ERRORS WHEN YOU HAVE THE VALID SECRET (PASSWORD)
|
|
|
|
If your RAS server is not the domain controller and is not a 'stand-alone'
|
|
server then it must make a query to the domain controller for your domain.
|
|
|
|
You need to specify the domain name with the user name when you attempt to
|
|
use this type of a configuration. The domain name is specified with the
|
|
local name in the chap-secrets file and with the option for the 'name'
|
|
parameter.
|
|
|
|
For example, the previous example would become:
|
|
|
|
DialupNT domain\\customer47 foobar
|
|
domain\\customer47 DialupNT foobar
|
|
|
|
and
|
|
|
|
pppd name 'domain\\customer47' remotename DialupNT <other options>
|
|
|
|
or add:
|
|
|
|
name domain\\customer47
|
|
remotename DialupNT
|
|
|
|
when the Windows NT domain name is simply called 'domain'.
|
|
|
|
|
|
TROUBLESHOOTING
|
|
|
|
Assuming that everything else has been configured correctly for PPP and
|
|
CHAP, the MS-CHAP-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 2433:
|
|
|
|
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 R=0
|
|
|
|
The "E=" is the error number from the table above, and the "R=" flag
|
|
indicates whether the error is transient and the client should retry. If
|
|
you consistently get error 691, then either you're using the wrong account
|
|
name/password, or the DES library or MD4 hashing (in md4.c) aren't working
|
|
properly. Verify your account name and password (use a Windows NT or
|
|
Windows 95 system to dial-in if you have one available). If that checks
|
|
out, test the DES library with the "destest" program included with the DES
|
|
library. If DES checks out, the md4.c routines are probably failing
|
|
(system byte ordering may be a problem) or my code is screwing up. I've
|
|
only got access to a Linux system, so you're on your own for anything else.
|
|
|
|
Another thing that might cause problems is that some RAS servers won't
|
|
respond at all to LCP config requests without seeing the word "CLIENT"
|
|
from the other end. If you see pppd sending out LCP config requests
|
|
without getting any reply, try putting something in your chat script
|
|
to send the word CLIENT after the modem has connected.
|
|
|
|
STILL TO DO
|
|
|
|
A site using only MS-CHAP to authenticate has no need to store cleartext
|
|
passwords in the "chap-secrets" file. A utility that spits out the ASCII
|
|
hex MD4 hash of a given password would be nice, and would allow that hash
|
|
to be used in chap-secrets in place of the password. The code to do this
|
|
could quite easily be lifted from chap_ms.c (you have to convert the
|
|
password to Unicode before hashing it). The chap_ms.c file would also have
|
|
to be changed to recognize a password hash (16 binary bytes == 32 ASCII hex
|
|
characters) and skip the hashing stage. This would have no real security
|
|
value as the hash is plaintext-equivalent.
|