up vote 70 down vote favorite
19

When I install the Ubuntu, I set a short password(<4). Now I want to change the other short password by "passwd" or change passphrase on "Password and Keys" program, it needs a password >4 char.

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2  
Why do u want to set a short password? That is not safe, try you change the password from root account even if you forced to do. – karthick87 Aug 25 '12 at 18:48
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@karthick87 because my home computer is not Fort Knox. If strangers are messing with my desktop then I have bigger concerns than my computer. – Torben Gundtofte-Bruun Sep 18 '13 at 18:24
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@karthick87 Why a short password? Ubuntu might run as a virtual machine and your virtualization software lacks copy and paste functionality for the command line of Linux guests (like Parallels desktop for Mac 8.0 does miss), then you want to access Ubuntu with a short password, or even better without a password at all. – Pro Backup Feb 12 '14 at 10:08
    
@karthick87 Fixing the parameters of what a password should be like, makes a password easier to guess and therefore less secure. – DustWolf Dec 9 '16 at 13:16
up vote 116 down vote

Use following command in Terminal:

sudo passwd <user>

Replace <user> with the username whose password you wish to change.

This works because passwd suppresses all checks for length or entropy when you use it as the root user.

Warning: if the target user has an encrypted home directory, this will cause problems! (see comments below)

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1  
This does not work. It does not provide the information required. – NlightNFotis Aug 25 '12 at 19:17
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It does work. If you are root it will not force you to fallow the password strength requirements. – user72421 Aug 25 '12 at 19:56
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This works fine for me. I'm able to set a user's password to a using this method, on an Ubuntu 12.04 LTS system. – Eliah Kagan Sep 8 '12 at 1:35
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This will cause problems when you have an encrypted home directory, as it breaks the automatic decryption of the ecryptfs passphrase. – guntbert Jan 16 '14 at 22:37
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@guntbert is right: Forcing the password this way the user won't be able to login again if his/her home directory is encrypted, so this is not the right solution in these cases. – fuenfundachtzig Aug 28 '14 at 8:55
up vote 32 down vote

By default, Ubuntu requires a minimum password length of 6 characters, as well as some basic entropy checks. These values are controlled in the file /etc/pam.d/common-password, which is outlined below.

password        [success=2 default=ignore]      pam_unix.so obscure sha512

If you would like to adjust the minimum length to 4 characters, add the appropriate variable (minlen=4) to the end of the line. The modification is outlined below.

password        [success=2 default=ignore]      pam_unix.so obscure sha512 minlen=4

Source.

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1  
I couldn't get min= to work and it doesn't match the man page, despite the Wiki page. Seems to be minlen= – John S Gruber Aug 25 '12 at 19:42
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remove "obscure" for also disable complexity check – Pisu Jul 3 '13 at 8:38
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sudo passwd user seems more useful – gyozo kudor Mar 3 '14 at 14:50
up vote 21 down vote

Bring up a terminal and edit /etc/pam.d/common-password

Change this line:

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

to:

password    [success=1 default=ignore]  pam_unix.so obscure sha512 minlen=4

Password also need a certain amount of complexity, as specified by the obscure parameter above.

password    [success=1 default=ignore]  pam_unix.so minlen=2 sha512

removes that check also.

This all presupposes that you think this is wise.

See man pam_unix

These work on my system.

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this works for ubuntu12.04 – David May 23 '13 at 14:37
    
valid for 13.04 too – Torben Gundtofte-Bruun Sep 18 '13 at 18:33
    
valid for 15.10 (Wily), too. – kmonsoor Jul 22 '16 at 13:29
up vote 0 down vote

To set up a simple password, I tried the simple sudo passwd username method, but it failed on my Ubuntu Server 12.04 LTS.

So I tried to remove the obscure option from /etc/pam.d/common-passwd config file, but it still failed.

So I also removed the obscure option from /usr/share/pam-configs/unix config file. And then it worked :-)

I do agree that it should be simplier, when acting as su to set up a weak password, whatever the reason why one wants to do it! A warning saying "weak password, confirm?" would be perfect...

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Narrow minded people have given negative votes on this excellent answer. The reference to "obscure" in /usr/share/pam-configs/unix is very important and relevant to some situations. Just because it doesn't work for you doesn't mean it's not a good answer. Lighten up on the negative votes guys and welcome someone with a viewpoint that's different than yours! You might be better informed at the end of the day. – LMSingh Mar 8 '17 at 3:21
    
In support of user171987's answer read the page at wiki.ubuntu.com/PAMConfigFrameworkSpec#config_file_format . It shows how the PAM configuration works and explains the profiles and their usage. The folder pam-configs holds various configurations and "unix" is just one of them. – LMSingh Mar 8 '17 at 3:30
    
On Ubuntu 18, it looks like it's sufficient to simply remove the 'obscure' option from /etc/pam.d/common-passwd -- there's no need to overly complicate things by also editing /usr/share/pam-configs/unix – Gino Dec 17 '17 at 19:45

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