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authorMystery Man <unknown@mandriva.org>2002-07-23 11:21:56 +0000
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-PPP Client Support for Microsoft's CHAP-80
-==========================================
-
-Eric Rosenquist rosenqui@strataware.com
-(updated by Paul Mackerras)
-(updated by Al Longyear)
-(updated by Farrell Woods)
-
-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:
-
- <ftp://ftp.microsoft.com/developr/rfc/chapexts.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 chap 80> <magic 0x46a3>]
- ^^^^^^^^^^^^
-
-The standard pppd implementation will indicate its lack of support for
-MS-CHAP by NAKing it:
-
- sent [LCP ConfNak id=0x2 <auth chap 05>]
-
-Windows NT Server systems are often configured to "Accept only
-Microsoft Authentication" (this is intended to enhance security). Up
-until now, that meant that you couldn't use this version of PPPD to
-connect to such a system. I've managed to get a client-only
-implementation of MS-CHAP working; it will authenticate itself to
-another system using MS-CHAP, but if you're using PPPD as a dial-in
-server, you won't be able to use MS-CHAP to authenticate the clients.
-This would not be a lot of extra work given that the framework is in
-place, but I didn't need it myself so I didn't implement it.
-
-
-BUILDING THE PPPD
-
-MS-CHAP uses a combination of MD4 hashing and DES encryption for
-authentication. You may need to get Eric Young's libdes library in
-order to use my MS-CHAP extensions. A lot of UNIX systems already
-have DES encryption available via the crypt(3), encrypt(3) and
-setkey(3) interfaces. Some may (such as that on Digital UNIX)
-provide only the encryption mechanism and will not perform
-decryption. This is okay. We only need to encrypt to perform
-MS-CHAP authentication.
-
-If you have encrypt/setkey available, then hopefully you need only
-define these two things in your Makefile: -DUSE_CRYPT and -DCHAPMS.
-Skip the paragraphs below about obtaining and building libdes. Do
-the "make clean" and "make" as described below. Linux users
-should not need to modify their Makefiles. Instead,
-just do "make CHAPMS=1 USE_CRYPT=1".
-
-If you don't have encrypt and setkey, you will need Eric Young's
-libdes library. You can find it in:
-
-ftp://ftp.funet.fi/pub/crypt/mirrors/ftp.psy.uq.oz.au/DES/libdes-3.06.tar.gz
-
-Australian residents can get libdes from Eric Young's site:
-
-ftp://ftp.psy.uq.oz.au/pub/Crypto/DES/libdes-3.06.tar.gz
-
-It is also available on many other sites (ask Archie).
-
-I used libdes-3.06, but hopefully anything newer than that will work
-also. Get the library, build and test it on your system, and install
-it somewhere (typically /usr/local/lib and /usr/local/include).
-
-
-
-You should now be ready to (re)compile the PPPD. Go to the pppd
-subdirectory and make sure the Makefile contains "-DCHAPMS" in the
-CFLAGS or COMPILE_FLAGS macro, and that the LIBS macro (or LDADD for
-BSD systems) contains "-ldes". Depending on your system and where the
-DES library was installed, you may also need to alter the include and
-library paths used by your compiler.
-
-Do a "make clean" and then a "make" to rebuild pppd. Assuming all
-goes well, install the new pppd and move on to the CONFIGURATION
-section.
-
-
-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
-Microsoft's "chapexts.txt" file referenced above:
-
- 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.
-
-If everything compiles cleanly, but fails at authentication time, then
-it might be a case of the MD4 or DES code screwing up. The following
-small program can be used to test the MS-CHAP code to see if it
-produces a known response:
-
------------------
-#include <stdio.h>
-
-#include "pppd.h"
-#include "chap.h"
-#include "chap_ms.h"
-
-int main(argc, argv)
- int argc;
- char *argv[];
-{
- u_char challenge[8];
- int challengeInt[sizeof(challenge)];
- chap_state cstate;
- int i;
-
- if (argc != 3) {
- fprintf(stderr, "Usage: %s <16-hexchar challenge> <password>\n",
- argv[0]); exit(1);
- }
-
- sscanf(argv[1], "%2x%2x%2x%2x%2x%2x%2x%2x",
- challengeInt + 0, challengeInt + 1, challengeInt + 2,
- challengeInt + 3, challengeInt + 4, challengeInt + 5,
- challengeInt + 6, challengeInt + 7);
-
- for (i = 0; i < sizeof(challenge); i++)
- challenge[i] = (u_char)challengeInt[i];
-
- ChapMS(&cstate, challenge, sizeof(challenge), argv[2], strlen(argv[2]));
- printf("Response length is %d, response is:", cstate.resp_length);
-
- for (i = 0; i < cstate.resp_length; i++) {
- if (i % 8 == 0)
- putchar('\n');
- printf("%02X ", (unsigned int)cstate.response[i]);
- }
-
- putchar('\n');
-
- exit(0);
-}
--------------
-
-This needs to link against chap_ms.o, md4.o, and the DES library. When
-you run it with the command line:
-
- $ testchap 00000000000000000000000000000000 hello
-
-it should output the following:
-
- Response length is 49, response is:
- 00 00 00 00 00 00 00 00
- 00 00 00 00 00 00 00 00
- 00 00 00 00 00 00 00 00
- F4 D9 9D AF 82 64 DC 3C
- 53 F9 BC 92 14 B5 5D 9E
- 78 C4 21 48 9D B7 A8 B4
- 01
-
-if not, then either the DES library is not working, the MD4 code isn't
-working, or there are some problems with the port of the code in
-chap_ms.c.
-
-
-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.
-
-A server implementation would allow MS-CHAP to be used with Windows NT and
-Windows 95 clients for enhanced security. Some new command-line options
-would be required, as would code to generate the Challenge packet and
-verify the response. Most of the helper functions are in place, so this
-shouldn't be too hard for someone to add.