1 PPP Support for Microsoft's CHAP-80
2 ===================================
4 Eric Rosenquist rosenqui@strataware.com
5 (updated by Paul Mackerras)
6 (updated by Al Longyear)
7 (updated by Farrell Woods)
8 (updated by Frank Cusack)
12 Microsoft has introduced an extension to the Challenge/Handshake
13 Authentication Protocol (CHAP) which avoids storing cleartext
14 passwords on a server. (Unfortunately, this is not as secure as it
15 sounds, because the encrypted password stored on a server can be used
16 by a bogus client to gain access to the server just as easily as if
17 the password were stored in cleartext.) The details of the Microsoft
18 extensions can be found in the document:
20 <http://www.ietf.org/rfc/rfc2433.txt>
22 In short, MS-CHAP is identified as <auth chap 80> since the hex value
23 of 80 is used to designate Microsoft's scheme. Standard PPP CHAP uses
24 a value of 5. If you enable PPP debugging with the "debug" option and
25 see something like the following in your logs, the remote server is
28 rcvd [LCP ConfReq id=0x2 <asyncmap 0x0> <auth MS> <magic 0x46a3>]
31 MS-CHAP support in pppd (along with MPPE support) can be enabled or
32 disabled at configure time using the --enable-microsoft-extensions and
33 --disable-microsoft-extensions arguments. The default is enabled.
38 If you've never used PPPD with CHAP before, read the man page (type
39 "man pppd") and read the description in there. Basically, you need to
40 edit the "chap-secrets" file typically named /etc/ppp/chap-secrets.
41 This should contain the following two lines for each system with which
42 you use CHAP (with no leading blanks):
44 RemoteHost Account Secret
45 Account RemoteHost Secret
47 Note that you need both lines and that item 1 and 2 are swapped in the
48 second line. I'm not sure why you need it twice, but it works and I didn't
49 have time to look into it further. The "RemoteHost" is a somewhat
50 arbitrary name for the remote Windows NT system you're dialing. It doesn't
51 have to match the NT system's name, but it *does* have to match what you
52 use with the "remotename" parameter. The "Account" is the Windows NT
53 account name you have been told to use when dialing, and the "Secret" is
54 the password for that account. For example, if your service provider calls
55 their machine "DialupNT" and tells you your account and password are
56 "customer47" and "foobar", add the following to your chap-secrets file:
58 DialupNT customer47 foobar
59 customer47 DialupNT foobar
61 The only other thing you need to do for MS-CHAP (compared to normal CHAP)
62 is to always use the "remotename" option, either on the command line or in
63 your "options" file (see the pppd man page for details). In the case of
64 the above example, you would need to use the following command line:
66 pppd name customer47 remotename DialupNT <other options>
73 to your PPPD "options" file.
75 The "remotename" option is required for MS-CHAP since Microsoft PPP servers
76 don't send their system name in the CHAP challenge packet.
79 E=691 (AUTHENTICATION_FAILURE) ERRORS WHEN YOU HAVE THE VALID SECRET (PASSWORD)
81 If your RAS server is not the domain controller and is not a 'stand-alone'
82 server then it must make a query to the domain controller for your domain.
84 You need to specify the domain name with the user name when you attempt to
85 use this type of a configuration. The domain name is specified with the
86 local name in the chap-secrets file and with the option for the 'name'
89 For example, the previous example would become:
91 DialupNT domain\\customer47 foobar
92 domain\\customer47 DialupNT foobar
96 pppd name 'domain\\customer47' remotename DialupNT <other options>
100 name domain\\customer47
103 when the Windows NT domain name is simply called 'domain'.
108 Assuming that everything else has been configured correctly for PPP and
109 CHAP, the MS-CHAP-specific problems you're likely to encounter are mostly
110 related to your Windows NT account and its settings. A Microsoft server
111 returns error codes in its CHAP response. The following are extracted from
114 646 ERROR_RESTRICTED_LOGON_HOURS
115 647 ERROR_ACCT_DISABLED
116 648 ERROR_PASSWD_EXPIRED
117 649 ERROR_NO_DIALIN_PERMISSION
118 691 ERROR_AUTHENTICATION_FAILURE
119 709 ERROR_CHANGING_PASSWORD
121 You'll see these in your pppd log as a line similar to:
123 Remote message: E=649 R=0
125 The "E=" is the error number from the table above, and the "R=" flag
126 indicates whether the error is transient and the client should retry. If
127 you consistently get error 691, then either you're using the wrong account
128 name/password, or the DES library or MD4 hashing (in md4.c) aren't working
129 properly. Verify your account name and password (use a Windows NT or
130 Windows 95 system to dial-in if you have one available). If that checks
131 out, test the DES library with the "destest" program included with the DES
132 library. If DES checks out, the md4.c routines are probably failing
133 (system byte ordering may be a problem) or my code is screwing up. I've
134 only got access to a Linux system, so you're on your own for anything else.
136 Another thing that might cause problems is that some RAS servers won't
137 respond at all to LCP config requests without seeing the word "CLIENT"
138 from the other end. If you see pppd sending out LCP config requests
139 without getting any reply, try putting something in your chat script
140 to send the word CLIENT after the modem has connected.
144 A site using only MS-CHAP to authenticate has no need to store cleartext
145 passwords in the "chap-secrets" file. A utility that spits out the ASCII
146 hex MD4 hash of a given password would be nice, and would allow that hash
147 to be used in chap-secrets in place of the password. The code to do this
148 could quite easily be lifted from chap_ms.c (you have to convert the
149 password to Unicode before hashing it). The chap_ms.c file would also have
150 to be changed to recognize a password hash (16 binary bytes == 32 ASCII hex
151 characters) and skip the hashing stage. This would have no real security
152 value as the hash is plaintext-equivalent.