Sams MySQL Tutorial Dec 2003 ISBN 0672325845

In Chapter 11, "Managing User Privileges," we discussed user account creation, granting and revoking privileges, and the grant tables.. For example, if there are rows in the table for te

In Chapter 11, "Managing User Privileges," we discussed user account creation, granting and revoking privileges, and the

grant tables We will begin this chapter by discussing how your MySQL server applies the privileges you have granted

There are two stages to the privilege system In the first stage, MySQL checks whether a user is allowed to connect to the

server at all The user table in the mysql database is used for this purpose MySQL looks up your username and password as entered and the host from which you are trying to connect to see whether there is a matching row If no row matches, you will not be able to connect to the server

Because the user table supports wildcards in the host column, a user/hostname combination may match more than one row MySQL determines which row is relevant by matching the most specific hostname first For example, if there are rows in the table for test from host localhost and user test from host %

(meaning any host), then the localhost row will be selected Note that these two rows can have different passwords This can cause a great deal of confusion (We will look at an example

of this in the section "Deleting Anonymous Accounts," later in this chapter.)

The second stage applies when you try to execute specific

queries or commands MySQL checks each query against the grant tables before it is executed

If the query you are trying to execute requires a global

privilegesuch as doing a LOAD DATA INFILE or trying to use

SHOW PROCESSLIST the user table will be checked For database-specific queries, the user table will be checked first If the user has the privilege on all databases, this will be sufficient If not, then the db and host tables are checked If the user does not

have the privilege at this level, then if any table- or column-level privileges are set, these will be checked last

One of MySQL's strengths is its advanced user privilege system

In this chapter, we'll discuss user account creation, the different privileges available, and how these privileges are represented within MySQL tables We'll cover the following:

Creating user accounts with GRANT and REVOKE

Privilege levels

Understanding the privilege tables

MySQL creates some anonymous accounts that require no username to log in We recommend that you delete these The reasons behind this are covered in detail in Chapter 15,

"Securing Your MySQL Installation," in the section "Deleting Anonymous Accounts." You can get rid of these accounts by typing this:

use mysql;

delete from user where User='';

delete from db where User='';

flush privileges;

Installation

In this chapter we'll discuss general security issues you should consider when running MySQL Although we cannot be

comprehensive in the space of a single chapter, we will give you

a list of the most important don'ts We will cover the following: How the privilege system works in practice

Securing accounts

Securing your installation files

Filtering user data

Other tips

There are a few general security principles that apply to the management of user accounts in MySQL We will look at these next

Setting the Password for the Root Account

When you install MySQL, the root password is not set by

default You absolutely must set this password before using

MySQL in anything other than a purely experimental

environment Without the root password set, anyone can log in and do anything he wants to your data In virtually all cases, this is a very bad thing If you have not done so already, set

this password immediately.

Deleting Anonymous Accounts

When you install MySQL on Windows, it automatically creates some accounts for you On Linux, this happens when you run the mysql_install_db script Two of these accounts are

anonymous; they represent the account you get when you don't specify a username One has a host value of localhost and the other % (any other host, so effectively any remote connection) These accounts have no passwords set by default

You can probably already see where we're going with this, but

we strongly recommend that you delete these accounts You can do this as shown here:

delete from user where User='';

You will need to follow this with a FLUSH PRIVILEGES statement

to flush the grant tables

The second reason to do this is that these accounts can cause confusion when regular users try to log in If you create an

account for, let's say username laura at any host (%), then

when laura tries to connect from localhost, the MySQL server looks for matching entries in the user table It has laura@% and

(anonymous)@localhost Because MySQL matches the most specific hostname first, the matching row is

(anonymous)@localhost Note that although laura has

supplied a username, this doesn't matter! The anonymous

accounts don't require a username This anonymous account is likely to have a different password from laura's account (by default, the password is blank, meaning the user should not supply one) This means that when laura tries to log in with her username and password from localhost, she will get an

Access Denied error for no obvious reason

Again, the best way to avoid this problem is to delete these accounts and forget about them

Dangerous Privileges

MySQL has a very fine-grained privilege system, as we

discussed in Chapter 11 You must be very careful about to

whom you grant some of these privileges The specific ones to

be most careful of are FILE, PROCESS, and WITH GRANT OPTION

The FILE privilege allows users to LOAD DATA INFILE This can

be manipulated to load in files from the server (such as the password file /etc/passwd) or even database data files,

effectively circumventing the privilege system

The PROCESS privilege allows users to SHOW PROCESSLIST This reveals the queries being executed at any given time, which may reveal confidential information about one user to another

The WITH GRANT OPTION privilege allows a user to share his privileges with others As long as you know this and understand the consequences, you can grant this privilege cautiously

Passwords and Encryption

MySQL user passwords are encrypted Before version 4.1, you could use the encrypted password as stored to log in This has now been fixed and the password and login mechanism have been made more secure

If you are writing an application that stores (non-MySQL)

usernames and passwords, we recommend that you use

something other than the PASSWORD() function to encrypt them

We recommend use of MD5() or ENCRYPT() instead See

Chapter 8, "Using MySQL Built-In Functions with SELECT," for a further discussion of these functions

Functions with SELECT

MySQL has a wide variety of built-in operators and functions that can be useful for writing queries Most of these are for use

in the SELECT and WHERE clauses There are also some special grouping functions for use in the GROUP BY clause We have already used the basic comparison operators and the count()

and max() functions A vast number of functions are available

In this chapter, we take a tour of the most useful ones This book is not trying to be a function reference by any meanswe are just trying to give you a feel for the types of functionality available

We will cover the following:

Operators

Control flow functions

String functions

Numeric functions

Date and time functions

Cast functions

Other functions

Functions for use with GROUP BY clauses

One important point to note is that, in MySQL, any expression containing NULL will evaluate to NULL, with a few exceptions we will note as we go along We will discuss this further in the

section on comparison operators

In this chapter, we will make some use of the SELECT statement without any tables We can use SELECT as a basic calculator For example, if we type

select 2+2;

we will get the result

+ -+

| 2+2 |

+ -+

| 4 |

+ -+

1 row in set (0.42 sec)

We can execute any expression without tables and have access

to a full range of math and other operators and functions

Although the capability to execute 2+2 is trivial, the capability

to do math at the SELECT level is not always so For example, this lets you perform financial analysis of values in tables and display the results in a report

In all MySQL expressions, you can use parentheses to control the order in which subexpressions are evaluated, as you would

in any programming language

We will begin by looking at the operators