i got problem here,
when i'm using the normal user login when i want 2 switch su account
i type
[user@ftp1 ~] $ su
Password: (i'm keyin root password)
su: incorrect password (but the root password is correct i keyin)
afterthat i close it, using root to login
and try to change su password
[root@ftp ~] $ passwd su
passwd:Unknown user name 'su'.

what problem with this?

su is not a user, su is the command you can use to switch to another user (including root).

this 1 i know, i want to know to way to solve this problem..

Do you want to change the password of the root user? Please use capitalization and the spell checker. Maybe we don't understand what the question is due to the irc speak.

If you use Fedora Core, and use sudo, you need to enter in your own password instead of that of root.

If you are using Ubuntu, then use sudo and not su. The root account is disabled.

Also, please put your distro in the profile so that we know what the best advice might be.

but i'm using centos5

Try logging into a virtual console as root. If you can't then you probably forgot the password.

i'm using the the root user login is no problem. i suspect the root password and su password is different. but i'heard somebody say su is totally root account.

Please capitalize the personal pronoun "I".

You might need to make yourself a member of the wheel group. You may not have permissions to su to root rather than root having a different password. You might try logging out of kde or gnome and log into one of the virtual terminals as root. Then use the passwd program to change the root password.

Also check the files in /etc/security. There may be a configuration file like access.conf that could be restricting access to the root account.

As said, su is a command to switch to another user. You can use it to become any user (without a username it will 'make' you root). Read man su.

As you can login as root under normal circumstances, changing the root password will not help. See jschiwal's advise in the posts.

i got it, su is stand for switch user.
but i have root password, how come i login locally can, but i'm using putty to login, then try to su root account, the error it come out.but it's correct password

system accept it when login and

when doing any thing required password

wht is the solution

I don't know, but jschiwal gave some pointers in the earlier posts.

Are you accessing an Ubuntu machine? Root logins are disabled in that case. You need to be a member of the wheel group and use "sudo" instead. The /etc/security/access.conf settings can deny root access on remote terminals, but I don't know if that would effect "su". If you can login as root directly, try the "passwd" command to change the password, even if you change it to the same password (although there may be a PAM setting to deny repeating passwords). There may also be a policy that only allows certain members or groups to use "su". After logging in as root, enter "passwd" by itself to change the root passwd.

On some systems like Fedora Core, the "sudo" password is the same as your regular password. This allows you to give others permission to execute certain commands as root without having to reveal the root password.