• If the system is congured to use Kerberos, the system's local time may have
drifted past the accepted variance with the Kerberos server time (this is typically
300 seconds). If NTP (network time protocol) is not working properly or local
NTP servers are not working, Kerberos authentication ceases to function because
it depends on common clock synchronization across the network.
• The system's authentication conguration is miscongured. Check the PAM
conguration les involved for any typographical errors or misordering of direc-
tives. For additional background information about PAM and the syntax of the
conguration les involved, refer to Chapter 2, Authentication with PAM (↑Secu-
rity Guide).
• The home partition is encrypted. Find more information about this topic in Sec-
tion A.2.4.3, “Login to Encrypted Home Partition Fails” (page 253).
In all cases that do not involve external network problems, the solution is to reboot
the system into single-user mode and repair the conguration before booting again
into operating mode and attempting to log in again. To boot into single-user mode:
1 Reboot the system. The boot screen appears, oering a prompt.
2
Enter 1 at the boot prompt to make the system boot into single-user mode.
3
Enter the username and password for root.
4 Make all the necessary changes.
5
Boot into the full multiuser and network mode by entering telinit 5 at the
command line.
A.2.4.2 Valid Username and Password Not Accepted
This is by far the most common problem users encounter, because there are many
reasons this can occur. Depending on whether you use local user management and
authentication or network authentication, login failures occur for dierent reasons.
Local user management can fail for the following reasons:
• The user may have entered the wrong password.
• The user's home directory containing the desktop conguration les is corrupted
or write protected.
• There may be problems with the X Window System authenticating this particular
user, especially if the user's home directory has been used with another Linux
distribution prior to installing the current one.
To locate the reason for a local login failure, proceed as follows:
1 Check whether the user remembered his password correctly before you start
debugging the whole authentication mechanism. If the user may not remember
250 Start-Up
