On 1/4/2010 12:42 PM, Roland Roland wrote:
> also is there a way I could enable the PAM module which uses crack
> library to check the strength of a users password?
> any help with this is truly appreciated...
/etc/pam.d/system-auth-ac
The default is:
password requisite pam_cracklib.so try_first_pass retry=3
password sufficient pam_unix.so md5 shadow nullok try_first_pass
use_authtok
password required pam_deny.so
A stronger version is:
# See: http://www.deer-run.com/~hal/sysadmin/pam_cracklib.html
password requisite pam_cracklib.so try_first_pass retry=3
minlen=20 difok=5
password sufficient pam_unix.so md5 shadow nullok try_first_pass
use_authtok remember=36
password required pam_deny.so
(Note that the entire line that starts with "password" should be all on
one line. It'll make sense when you view the system-auth-ac file.
There are 3 password lines by default, and all you're going to do is add
options to the first two lines.)
The key changes that I made to my setup are:
- Added "minlen=20", note that cracklib gives a bonus point if you use a
number, symbol or different case character. So the minimum length is as
short as 16 characters (4 less then what you set minlen to). But that
minimum length is only achievable if you use both upper and lower case
letters along with at least one symbol and at least one number. Minimum
length should never be below 10 or 12 (in my opinion).
- The 'try_first_pass' tells cracklib to do some checking of the
supplied password against a built-in word list. I don't remember the
details behind how it works.
- Added "difok=5", which says that the new password (when the user
changes it) has to be at least 5 letters different from the old
password. In general, you'll want this to be about 1/3 to 1/4 of the
minlen value.
- Added remember=36, which tells pam_unix to remember the last 36
passwords. So if a user wants to change their password, it can't be any
of the past 36 passwords. Which is probably overkill to the Nth degree.
...
(Minor ramble about why I say minlen should be set to at least 10-12.)
My current estimate is that a $1500 PC can brute-force about 2-4 billion
MD5 password hashes per second now. (Using NVIDIA Cuda in a 4-way SLI
setup.) That's an offline attack where the attacker has a copy of your
password hash. A completely random 8-char password can be found in
about half a day. Add 2 more characters and it'll takes more like 2
years.
Things go a lot slower if they're simply doing a dictionary attack on
the SSH port (where they don't have the MD5 hash). Those attacks
typically go after low-hanging fruit using common usernames + common
passwords. Plus you can throttle the SSH logins or do other things (at
the risk of locking yourself out). Keep in mind that modern SSH
dictionary attacks ping your machine about once every few minutes from
thousands of different IP addresses. Locking out an IP address after 3
incorrect attempts only works against attackers that aren't using a
slow-attack botnet.
So in situations where you can throttle the attacker, 8 random chars is
probably enough. But if you want something a little easier to type,
you'd best go for 10-12 chars and assume that the attacker has the hash.
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