With the development of LanManager and Windows NT compatible password encryption for Samba, it is now able to validate user connections in exactly the same way as a LanManager or Windows NT server.
This document describes how the SMB password encryption algorithm works and what issues there are in choosing whether you want to use it. You should read it carefully, especially the part about security and the "PROS and CONS" section.
LanManager encryption is somewhat similar to UNIX password encryption. The server uses a file containing a hashed value of a user's password. This is created by taking the user's plaintext password, capitalising it, and either truncating to 14 bytes or padding to 14 bytes with null bytes. This 14 byte value is used as two 56 bit DES keys to encrypt a 'magic' eight byte value, forming a 16 byte value which is stored by the server and client. Let this value be known as the "hashed password".
Windows NT encryption is a higher quality mechanism, consisting of doing an MD4 hash on a Unicode version of the user's password. This also produces a 16 byte hash value that is non-reversible.
When a client (LanManager, Windows for WorkGroups, Windows 95 or Windows NT) wishes to mount a Samba drive (or use a Samba resource), it first requests a connection and negotiates the protocol that the client and server will use. In the reply to this request the Samba server generates and appends an 8 byte, random value - this is stored in the Samba server after the reply is sent and is known as the "challenge". The challenge is different for every client connection.
The client then uses the hashed password (16 byte values described above), appended with 5 null bytes, as three 56 bit DES keys, each of which is used to encrypt the challenge 8 byte value, forming a 24 byte value known as the "response".
In the SMB call SMBsessionsetupX (when user level security is selected) or the call SMBtconX (when share level security is selected), the 24 byte response is returned by the client to the Samba server. For Windows NT protocol levels the above calculation is done on both hashes of the user's password and both responses are returned in the SMB call, giving two 24 byte values.
The Samba server then reproduces the above calculation, using its own stored value of the 16 byte hashed password (read from the smbpasswd file - described later) and the challenge value that it kept from the negotiate protocol reply. It then checks to see if the 24 byte value it calculates matches the 24 byte value returned to it from the client.
If these values match exactly, then the client knew the correct password (or the 16 byte hashed value - see security note below) and is thus allowed access. If not, then the client did not know the correct password and is denied access.
Note that the Samba server never knows or stores the cleartext of the user's password - just the 16 byte hashed values derived from it. Also note that the cleartext password or 16 byte hashed values are never transmitted over the network - thus increasing security.
The unix and SMB password encryption techniques seem similar on the surface. This similarity is, however, only skin deep. The unix scheme typically sends clear text passwords over the network when logging in. This is bad. The SMB encryption scheme never sends the cleartext password over the network but it does store the 16 byte hashed values on disk. This is also bad. Why? Because the 16 byte hashed values are a "password equivalent". You cannot derive the user's password from them, but they could potentially be used in a modified client to gain access to a server. This would require considerable technical knowledge on behalf of the attacker but is perfectly possible. You should thus treat the smbpasswd file as though it contained the cleartext passwords of all your users. Its contents must be kept secret, and the file should be protected accordingly.
Ideally we would like a password scheme which neither requires plain text passwords on the net or on disk. Unfortunately this is not available as Samba is stuck with being compatible with other SMB systems (WinNT, WfWg, Win95 etc).
In order for Samba to participate in the above protocol it must be able to look up the 16 byte hashed values given a user name. Unfortunately, as the UNIX password value is also a one way hash function (ie. it is impossible to retrieve the cleartext of the user's password given the UNIX hash of it), a separate password file containing this 16 byte value must be kept. To minimise problems with these two password files, getting out of sync, the UNIX /etc/passwd and the smbpasswd file, a utility, mksmbpasswd.sh, is provided to generate a smbpasswd file from a UNIX /etc/passwd file.
To generate the smbpasswd file from your /etc/passwd file use the following command :
$ cat /etc/passwd | mksmbpasswd.sh > /usr/local/samba/private/smbpasswd
If you are running on a system that uses NIS, use
$ ypcat passwd | mksmbpasswd.sh > /usr/local/samba/private/smbpasswd
The mksmbpasswd.sh program is found in the Samba source directory. By default, the smbpasswd file is stored in :
The owner of the /usr/local/samba/private/ directory should be set to root, and the permissions on it should be set to 0500 (chmod 500 /usr/local/samba/private).
Likewise, the smbpasswd file inside the private directory should be owned by root and the permissions on is should be set to 0600 (chmod 600 smbpasswd).
The format of the smbpasswd file is (The line has been wrapped here. It should appear as one entry per line in your smbpasswd file.)
username:uid:XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX:XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX: [Account type]:LCT-<last-change-time>:Long name
Although only the username, uid, XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX, [Account type] and last-change-time sections are significant and are looked at in the Samba code.
It is VITALLY important that there by 32 'X' characters between the two ':' characters in the XXX sections - the smbpasswd and Samba code will fail to validate any entries that do not have 32 characters between ':' characters. The first XXX section is for the Lanman password hash, the second is for the Windows NT version.
When the password file is created all users have password entries consisting of 32 'X' characters. By default this disallows any access as this user. When a user has a password set, the 'X' characters change to 32 ascii hexadecimal digits (0-9, A-F). These are an ascii representation of the 16 byte hashed value of a user's password.
To set a user to have no password (not recommended), edit the file using vi, and replace the first 11 characters with the ascii text "NO PASSWORD" (minus the quotes).
For example, to clear the password for user bob, his smbpasswd file entry would look like :
bob:100:NO PASSWORDXXXXXXXXXXXXXXXXXXXXX:XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX:[U ]:LCT-00000000:Bob's full name:/bobhome:/bobshell
If you are allowing users to use the smbpasswd command to set their own passwords, you may want to give users NO PASSWORD initially so they do not have to enter a previous password when changing to their new password (not recommended). In order for you to allow this the smbpasswd program must be able to connect to the smbd daemon as that user with no password. Enable this by adding the line :
null passwords = yes
to the [global] section of the smb.conf file (this is why the above scenario is not recommended). Preferably, allocate your users a default password to begin with, so you do not have to enable this on your server.
Note : This file should be protected very carefully. Anyone with access to this file can (with enough knowledge of the protocols) gain access to your SMB server. The file is thus more sensitive than a normal unix /etc/passwd file.
The smbpasswd command maintains the two 32 byte password fields in the smbpasswd file. If you wish to make it similar to the unix passwd or yppasswd programs, install it in /usr/local/samba/bin/ (or your main Samba binary directory).
Note that as of Samba 1.9.18p4 this program MUST NOT BE INSTALLED setuid root (the new smbpasswd code enforces this restriction so it cannot be run this way by accident).
smbpasswd now works in a client-server mode where it contacts the local smbd to change the user's password on its behalf. This has enormous benefits - as follows.
To run smbpasswd as a normal user just type :
Old SMB password: <type old value here - or hit return if there was no old password>
New SMB Password: <type new value>
Repeat New SMB Password: <re-type new value
If the old value does not match the current value stored for that user, or the two new values do not match each other, then the password will not be changed.
If invoked by an ordinary user it will only allow the user to change his or her own Samba password.
If run by the root user smbpasswd may take an optional argument, specifying the user name whose SMB password you wish to change. Note that when run as root smbpasswd does not prompt for or check the old password value, thus allowing root to set passwords for users who have forgotten their passwords.
smbpasswd is designed to work in the same way and be familiar to UNIX users who use the passwd or yppasswd commands.
For more details on using smbpasswd refer to the man page which will always be the definitive reference.
This is a very brief description on how to setup samba to support password encryption.
Note that you can test things using smbclient.