152 lines
6.0 KiB
Plaintext
Executable File
152 lines
6.0 KiB
Plaintext
Executable File
PPP Support for Microsoft's CHAP-80
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===================================
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Eric Rosenquist rosenqui@strataware.com
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(updated by Paul Mackerras)
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(updated by Al Longyear)
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(updated by Farrell Woods)
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(updated by Frank Cusack)
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INTRODUCTION
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Microsoft has introduced an extension to the Challenge/Handshake
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Authentication Protocol (CHAP) which avoids storing cleartext
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passwords on a server. (Unfortunately, this is not as secure as it
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sounds, because the encrypted password stored on a server can be used
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by a bogus client to gain access to the server just as easily as if
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the password were stored in cleartext.) The details of the Microsoft
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extensions can be found in the document:
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<http://www.ietf.org/rfc/rfc2433.txt>
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In short, MS-CHAP is identified as <auth chap 80> since the hex value
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of 80 is used to designate Microsoft's scheme. Standard PPP CHAP uses
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a value of 5. If you enable PPP debugging with the "debug" option and
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see something like the following in your logs, the remote server is
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requesting MS-CHAP:
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rcvd [LCP ConfReq id=0x2 <asyncmap 0x0> <auth MS> <magic 0x46a3>]
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^^^^^^^
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MS-CHAP is enabled by default under Linux in pppd/Makefile.linux by
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the line "CHAPMS=y".
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CONFIGURATION
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If you've never used PPPD with CHAP before, read the man page (type
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"man pppd") and read the description in there. Basically, you need to
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edit the "chap-secrets" file typically named /etc/ppp/chap-secrets.
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This should contain the following two lines for each system with which
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you use CHAP (with no leading blanks):
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RemoteHost Account Secret
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Account RemoteHost Secret
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Note that you need both lines and that item 1 and 2 are swapped in the
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second line. I'm not sure why you need it twice, but it works and I didn't
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have time to look into it further. The "RemoteHost" is a somewhat
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arbitrary name for the remote Windows NT system you're dialing. It doesn't
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have to match the NT system's name, but it *does* have to match what you
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use with the "remotename" parameter. The "Account" is the Windows NT
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account name you have been told to use when dialing, and the "Secret" is
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the password for that account. For example, if your service provider calls
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their machine "DialupNT" and tells you your account and password are
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"customer47" and "foobar", add the following to your chap-secrets file:
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DialupNT customer47 foobar
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customer47 DialupNT foobar
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The only other thing you need to do for MS-CHAP (compared to normal CHAP)
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is to always use the "remotename" option, either on the command line or in
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your "options" file (see the pppd man page for details). In the case of
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the above example, you would need to use the following command line:
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pppd name customer47 remotename DialupNT <other options>
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or add:
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name customer47
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remotename DialupNT
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to your PPPD "options" file.
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The "remotename" option is required for MS-CHAP since Microsoft PPP servers
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don't send their system name in the CHAP challenge packet.
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E=691 (AUTHENTICATION_FAILURE) ERRORS WHEN YOU HAVE THE VALID SECRET (PASSWORD)
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If your RAS server is not the domain controller and is not a 'stand-alone'
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server then it must make a query to the domain controller for your domain.
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You need to specify the domain name with the user name when you attempt to
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use this type of a configuration. The domain name is specified with the
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local name in the chap-secrets file and with the option for the 'name'
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parameter.
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For example, the previous example would become:
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DialupNT domain\\customer47 foobar
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domain\\customer47 DialupNT foobar
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and
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pppd name 'domain\\customer47' remotename DialupNT <other options>
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or add:
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name domain\\customer47
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remotename DialupNT
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when the Windows NT domain name is simply called 'domain'.
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TROUBLESHOOTING
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Assuming that everything else has been configured correctly for PPP and
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CHAP, the MS-CHAP-specific problems you're likely to encounter are mostly
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related to your Windows NT account and its settings. A Microsoft server
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returns error codes in its CHAP response. The following are extracted from
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RFC 2433:
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646 ERROR_RESTRICTED_LOGON_HOURS
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647 ERROR_ACCT_DISABLED
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648 ERROR_PASSWD_EXPIRED
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649 ERROR_NO_DIALIN_PERMISSION
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691 ERROR_AUTHENTICATION_FAILURE
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709 ERROR_CHANGING_PASSWORD
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You'll see these in your pppd log as a line similar to:
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Remote message: E=649 R=0
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The "E=" is the error number from the table above, and the "R=" flag
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indicates whether the error is transient and the client should retry. If
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you consistently get error 691, then either you're using the wrong account
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name/password, or the DES library or MD4 hashing (in md4.c) aren't working
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properly. Verify your account name and password (use a Windows NT or
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Windows 95 system to dial-in if you have one available). If that checks
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out, test the DES library with the "destest" program included with the DES
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library. If DES checks out, the md4.c routines are probably failing
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(system byte ordering may be a problem) or my code is screwing up. I've
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only got access to a Linux system, so you're on your own for anything else.
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Another thing that might cause problems is that some RAS servers won't
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respond at all to LCP config requests without seeing the word "CLIENT"
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from the other end. If you see pppd sending out LCP config requests
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without getting any reply, try putting something in your chat script
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to send the word CLIENT after the modem has connected.
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STILL TO DO
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A site using only MS-CHAP to authenticate has no need to store cleartext
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passwords in the "chap-secrets" file. A utility that spits out the ASCII
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hex MD4 hash of a given password would be nice, and would allow that hash
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to be used in chap-secrets in place of the password. The code to do this
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could quite easily be lifted from chap_ms.c (you have to convert the
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password to Unicode before hashing it). The chap_ms.c file would also have
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to be changed to recognize a password hash (16 binary bytes == 32 ASCII hex
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characters) and skip the hashing stage. This would have no real security
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value as the hash is plaintext-equivalent.
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