I'm running debian lenny. Suddenly when I rebooted my system I was unable to login. I type root and the password and it tells me the login is incorrect.

If I boot into single user mode, it accepts the password just fine. Looking in the /etc/shadow file, it appears to be fine as well.

Running passwd to set a new password sets a new password just fine, i can login to single user mode with the new password, however, I cannot login at a higher runlevel with the password.

If I delete a password in the /etc/shadow file, then try to login as that user, it simply says "Login Incorrect" without even asking for a password.

HELP!

are you trying to login as root into gui? because if i'm not mistaken either pam or selinux explicitely forbids logging into the desktop as root, however you could modify the appropriate config file to allow this but otherwise it is set to simply fail login if username is root (or maybe even uid 0) even if the password is correct, try logging in as normal user
it's usually not recomended to log in as root anyways, but rather log in as normal user then use sudo or su to accomplish root tasks

This system is command line only. It fails login on all user accounts including root. The problem hasn't happened until today. I've been using this system for 6 months.

I'm no linux expert, so help me a bit...
What's the difference between the login method in single user mode and init 2+?
Is it just that in single user mode PAM is not enabled?

If you normally log in as root (bad idea) and now can't, you may have edited /etc/securetty and blocked yourself

cheers

Nope. Root is set to login fine on nearly every terminal you could ever think of.

It's not just root that can't login. It's every user, super user or not.

Have a look at /var/log/messages .. you may see some relevant errors. Have you changed anything to do with pam ?

Nothing in there to do with PAM. Hmmm...
I have a theory...
I think somehow the passwords are being encrypted in a format that PAM isn't set to read.

Before uploading this, I used passwd to change the root password to the letter a so no one can steal my password.
Take a loot at /etc/passwd... heres the root line:
root:x:0:0:root:/root:/bin/bash
everything okay there? I'm not sure what the 0's mean.

Now heres /etc/shadow
root:$1$aOZ7l1FI$LaOg/Oozn.XT7LrDXi1d90:14690:0:99999:7:::

First, does that line look okay?
Second, what encryption format is that?

Also interesting...

debian:/etc# pwck
user lp: directory /var/spool/lpd does not exist
user news: directory /var/spool/news does not exist
user uucp: directory /var/spool/uucp does not exist
user list: directory /var/list does not exist
user irc: directory /var/run/ircd does not exist
user gnats: directory /var/lib/gnats does not exist
user nobody: directory /nonexistent does not exist
pwck: no changes

Those passwd and shadow entries look fine, the '0's are the uid and gid respectively for root, the hashing algo looks like md5 going by the '$1$' prefix, nothing unusual

The problem may not be pam related, is there anything else in /var/log/messages ?

I think that's it... in the configuration file for pam it says the default encryption is DES and the line which manually sets it to anything else is commented out. I'm now nowhere near the box. I'll give this a shot tomorrow.

Have you run 'authconfig' recently ?

Not recently, no. The most recent thing I did was compile mono. It had a significant number of dependencies which needed to be satisfied, so any one of them could have screwed with the login settings.

Tonight I'll take another whack at this.

If you are having problems logging in as anyone but root, you should check your disk space.

When your / partition gets full, strange things happen.

Fortunately, a default of 5% of the filesystem allocated to / is reserved exclusively for the user root, so you can still login as root and fix things up, before there is not even enough space free for root to login.

So, please login (no GUI) as root and tell us the output of df -h

Root also cannot login in runlevel 2 and up. Root can only login when booting into single user mode. There is no GUI on this machine. It is a server and command line only. That may just mean that I've used up all the space possible and root can't even login anymore. I'll look into it.