160 lines
6.6 KiB
Plaintext
160 lines
6.6 KiB
Plaintext
PPP Client Support for Microsoft's CHAP-80
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==========================================
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Eric Rosenquist rosenqui@strataware.com
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(NOTE: ipppd now supports MS-CHAP out of the box -- Paul Slootman 2002-06-25)
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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 eliminates the need for them to have
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access to cleartext passwords. The details of the Microsoft extensions can
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be found in the document:
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<ftp://ftp.microsoft.com/developr/rfc/chapexts.txt>
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In short, MS-CHAP is identified as <auth chap 80> since the hex value of 80
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is used to designate Microsoft's scheme. Standard PPP CHAP uses a value of
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5. If you enable PPP debugging with the "debug" option and see something
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like the following in your logs, the remote server is requesting MS-CHAP:
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rcvd [LCP ConfReq id=0x2 <asyncmap 0x0> <auth chap 80> <magic 0x46a3>]
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^^^^^^^^^^^^
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The standard PPP implementation will indicate its lack of support for
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MS-CHAP by NAKing it:
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lcp_reqci: rcvd AUTHTYPE
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(c223)
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(NAK)
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Windows NT Server systems are often configured to "Accept only Microsoft
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Authentication" to enhance security. Up until now, that meant that you
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couldn't use this version of PPPD to connect to such a system. I've
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managed to get a client-only implementation of MS-CHAP working; it will
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authenticate itself to another system using MS-CHAP, but if you're using
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PPPD as a dial-in server, you won't be able to use MS-CHAP to authenticate
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the clients. This would not be a lot of extra work given that the
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framework is in place, but I didn't need it myself so I didn't implement
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it.
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BUILDING THE PPPD
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MS-CHAP uses a combination of MD4 hashing and DES encryption for
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authentication. You'll need to get Eric Young's DES library in order to
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use my MS-CHAP extensions. You can find it in:
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<ftp://ftp.psy.uq.oz.au/pub/Crypto/DES/>
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I used libdes-3.06, but hopefully anything newer than that will work also.
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Get the library, build and test it on your system, and install it somewhere
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(typically /usr/local/lib and /usr/local/include).
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You should now be ready to (re)compile the PPPD. Go to the ppp-X.Y/pppd
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directory and make sure the Makefile contains "-DUSE_MSCHAP" in the
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COMPILE_FLAGS macro, and that the LIBS macro contains "-ldes". Depending
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on your system and where the DES library was installed, you may also need
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to alter the include and library paths used by your compiler.
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Do a "make clean" and then a "make" to rebuild the PPPD. Assuming all goes
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well, install the new PPPD and move on to the CONFIGURATION section.
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CONFIGURATION
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If you've never used PPPD with CHAP before, read the man page (type "man
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pppd") and read the description in there. Basically, you need to edit the
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"chap-secrets" file typically named /etc/ppp/chap-secrets. This should
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contain the following two lines for each system with which you use CHAP
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(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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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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Microsoft's "chapexts.txt" file referenced above:
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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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I can generate an MD4 test program if necessary in order to verify proper
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MD4 operation.
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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.
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A server implementation would allow MS-CHAP to be used with Windows NT and
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Windows 95 clients for enhanced security. Some new command-line options
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would be required, as would code to generate the Challenge packet and
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verify the response. Most of the helper functions are in place, so this
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shouldn't be too hard for someone to add.
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