Checking User Acccounts with the dscl Command Utility

On an Apple OS X system, a user's account is distinguished from other accounts by a User Identifier (UID), which is a unique number that identifies a particular user on a system having a particular login ID. A UID identifies the owner of a file and controls users' access to files.

OS X assigns some UIDs for special purposes

Number	Use	Comment
UID 0	Reserved for the root user	Should not be deleted or modified except to change the password of the root user.
UIDs below 100	Reserved for system use	Should not be deleted or modified.
UIDs 500 - 2,147,483,648	Users	Should be unique on the system. If modified, the ownership of files and directories for the user must be changed.

A user's name and UID can be viewed with the dscl command utility.

To list users, within the terminal type:

dscl . list /users

To read a user account, within the terminal type:

dscl . read /users/

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Services That Should Normally be Disabled

To increase security on an Apple OS X system, the following services should normally be disabled, unless you have a definite need for them:

1. Windows File Sharing (SMB) - allows Windows™ users to access shared folders on your computer
2. Personal Web Sharing / Hypertext Transfer Protocol (HTTP) - lets users of other computers view web pages in the sites folder on your computer
3. Remote Login (SSH) - lets users of other computers access your computer using Secure Shell (SSH) and allows connection with Simple File Transfer Protocol (SFTP)
4. File Transfer Protocol (FTP Access) - lets users of other computers exchange files with your computer using FTP applications and provides users access to all files on the Mac for which they have privileges. FTP transmits userids and passwords as cleartext, so can could allow someone else on the network on which your system resides to learn a userid and password for your system.
5. Remote Apple Events - allows applications on other Mac OS X computers to send Apple Events to your computer
6. Printer Sharing / Line Printer Request (LPR) - lets other people use printers connected to your computer

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Changing Firewall Settings

The firewall settings can be chaned on a MAC OS X system by taking the following steps:

1. Select the Apple menu
2. Select the System Preferences option
3. Select the Security option
4. Select the Firewall tab
5. Review and select options

Turning on a service automatically reconfigures the built-in firewall to open the appropriate port(s) necessary for that service.

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Sudo on OS X

The sudo command is used in the Terminal to execute a command with the privileges of another user, such as root. On Mac OS X, those with administrative privileges are allowed to use the sudo command.

On Unix and Linux systems, the su command is used to assume the identity of another user, typically root. Since the root account is normally disabled on Mac OS X systems, su will not work. As an alternative to enabling the root account, you can use sudo to run individual commands as root, one at a time. If you need a root shell, you can get one by running sudo -s. The sudo command requires an administrator password.

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