| 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 MS> <magic 0x46a3>] |
| ^^^^^^^ |
| |
| MS-CHAP is enabled by default under Linux in pppd/Makefile.linux by |
| the line "CHAPMS=y". |
| |
| |
| 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. |