PPP Support for Microsoft's CHAP-80
Eric Rosenquist email@example.com
(updated by Paul Mackerras)
(updated by Al Longyear)
(updated by Farrell Woods)
(updated by Frank Cusack)
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:
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
rcvd [LCP ConfReq id=0x2 <asyncmap 0x0> <auth MS> <magic 0x46a3>]
MS-CHAP is enabled by default under Linux in pppd/Makefile.linux by
the line "CHAPMS=y".
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>
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'
For example, the previous example would become:
DialupNT domain\\customer47 foobar
domain\\customer47 DialupNT foobar
pppd name 'domain\\customer47' remotename DialupNT <other options>
when the Windows NT domain name is simply called 'domain'.
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
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.