The sys admin's daily grind: pwquality

Strong Passwords

Article from Issue 239/2020
Author(s):

Regular password changes are a thing of the past: Strong passwords for each individual service provide more protection. Charly pimped his Ubuntu accordingly with a suitable PAM module.

Changing the password regularly, about every 60 or 90 days, is now considered obsolete. It is better to use a separate strong password for each service and each login. The requirement for how strong (i.e., how complicated) a password must be is something that – at least on your own systems – you can define yourself.

On my test machine with Ubuntu, I can use almost any simple password I want – that has to change. To make sure it does, I first have to install the pwquality PAM library:

$ sudo apt install libpam-pwquality

Then I have to add a line to the /etc/pam.d/common-password configuration file. On Ubuntu 18.04 "Bionic Beaver," the default looks like this (this may be slightly different on other systems):

password [success=1 default=ignore] pam_unix.so obscure sha512

This line can remain as a fallback, but in front of it – and this is important – I need to insert the line from Listing 1. This is a single line, which I just wrapped for Listing 1 to improve readability. With the individual parameters (Table 1 breaks them down), the password requirements can be easily controlled.

Listing 1

Password Requirements

 

Table 1

pwquality Parameters

Parameter

Meaning

retry

Number of incorrect attempts

minlen

Minimum password length

difok

Number of characters that can match the old password

lcredit

Minimum number of lowercase letters

ucredit

Minimum number of uppercase letters

dcredit

Minimum number of numbers

ocredit

Minimum number of non-standard characters

reject_username

Password and username cannot be identical

enforce_for_root

Rules also apply for root

After restarting the system, the new password rule takes effect. To test it, I changed the password of the user bob (Figure 1). In doing so, I intentionally entered a password that was too short in the first round and one that can be found in common dictionaries in the second. The system categorically rejected both – and that's the way it should be.

Figure 1: After the change, the system rejects overly simple passwords.

As my third attempt, I entered a new password that complied with the modified rules: Cm1.Sya-n. This seems complicated, but it is mnemonic. It's the first letters and punctuation of the first words of Melville's Moby Dick [1], with a 1 instead of an I, because I need a digit according to the new password rule. The system accepted the password without complaint.

Infos

  1. "Call me Ishmael. Some years ago – never mind how long precisely …": http://www.online-literature.com/melville/mobydick/2/

The Author

Charly Kühnast manages Unix systems in a data center in the Lower Rhine region of Germany. His responsibilities include ensuring the security and availability of firewalls and the DMZ.

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